United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
EPA/600/R-92/064
June 1992
Preliminary Risk
Assessment for Viruses in
Municipal Sewage Sludge
Applied to Land

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                                  EPA/600/R-92/064
                                       June 1992
Preliminary Risk Assessment for
  Viruses in Municipal Sewage
     Sludge Applied to Land
   Environmental Criteria and Assessment Office
   Office of Health and Environmental Assessment
       Office of Research and Development
      U.S. Environmental Protection Agency
            Cincinnati, OH 45268
                                Printed on Recycled Paper

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                                 DISCLAIMER

      This document has been reviewed in accordance with U.S. Environmental Protection
Agency policy and approved for publication.   Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.      .,./..
                                      n

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                                     PREFACE

       Section 405 of the Clean Water Act requires the U.S. Environmental Protection
Agency to develop and issue regulations that identify: (1) uses for sludge including disposal;
(2) specific factors (including costs) to be taken into account in determining the measures
and practices applicable for each use or disposal; and (3) concentrations of pollutants that
interfere with each use or disposal.  To  comply with this mandate, the U.S. EPA has
embarked on a program to develop four  major technical regulations:  land application,
including distribution and marketing; landfilling;  incineration and surface disposal.  The
development of these technical regulations requires a consideration of pathogens as well as
chemical constituents of sludge.   Public  concern related to  the reuse and disposal  of
municipal sludge  often focuses on the issue of pathogenic organisms.
       This report is one of a series whose purpose is to use the methodology described in
Pathogen Risk Assessment for Land Application of Municipal Sludge to develop preliminary
assessments of risk to human health posed by parasites, bacteria and viruses in municipal
sewage sludge  applied  to  land  as fertilizer or soil conditioner.   The  preliminary risk
assessment includes a description of the most critical data gaps that must be filled before
development of a definitive risk assessment can be accomplished and recommends research
priorities.
                                         in

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                          DOCUMENT DEVELOPMENT

 Cynthia Sonich-Mullin, Project Officer
 Norman E. Kowal, Technical Project Manager
 Environmental Criteria and Assessment Office
 U.S. Environmental Protection Agency
 Cincinnati, OH

 Authors

 Marialice Wilson, Project Manager
 Charles T. Hadden
 Mary C. Gibson
 Science Applications International Corporation
 Oak Ridge, TN

 Reviewers

 Joseph B. Farrell                 ;.
 Risk Reduction Engineering Laboratory
 Andrew W. Breidenbach Environmental Research Center
 U.S. Environmental Protection Agency
 Cincinnati, OH

 Mary-Ellen Morris                <
 Virology Branch, Microbiol. Res. Div.
 Environmental Monitoring Systems Laboratory
 U.S. Environmental Protection Agency
 Cincinnati, OH

 Babasaheb Sonawane
 Reproductive and Developmental Toxicology Branch
 Human Health Assessment Group
 U.S. Environmental Protection Agency
 Washington, D.C.                 ;

 Richard Walentowicz
 Exposure Assessment Group
 U.S. Environmental Protection Agency
 Washington, D.C.                 :

Judith Olsen, Editorial Review
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH
                                       IV

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                           TABLE OF CONTENTS
1. EXECUTIVE SUMMARY	. ..'	  1-1

2. INTRODUCTION 	  2-1

3. LITERATURE REVIEW OF VIRUSES  	  3-1

      3.1.   SIGNIFICANCE OF PATHOGENIC VIRUSES	  3-1

            3.1.1.   Transmission/Exposure Routes  .	  3-3
            3.1.2   Occurrence of Viruses in Sludge	  3-6
            3.1.3.   Infective Dose	  3-6
            3.1.4.   Epidemiology	  3-8

      3.2.   ENTERIC VIRUSES IN TREATED SLUDGE  	  3-12

            3.2.1.   Effects of Treatment Processes  	   3-12
            3.2.2.   Density of Viruses in Treated Sludge	   3-16

      3.3.   OCCURRENCE OF VIRUSES IN NATURAL MEDIA	   3-21

            3.3.1.   Persistence in Soil	   3-21
            3.3.2.   Persistence in Water	   3-29
            3.3.3.   Persistence in Aerosols	Y.	   3-41
            3.3.4.   Persistence in Agricultural Products	   3-45

      3.4.   TRANSPORT	   3-47

            3.4.1.   Transport in  Soil		   3-47
            3.4.2.   Transport in  Surface Runoff  	   3-53
            3.4.3.   Transport in  the Subsurface and in Groundwater	   3-54
            3.4.4.   Transport by Wind	   3-56

4. PARAMETERS FOR MODEL  RUNS	  4-1

      4.1.   RATIONALE FOR PARAMETER SELECTION	  4-1
      4.2.   PARAMETER VALUES  	. . .	  4-5

            4.2.1.   Main Program Parameters	  4-5
            4.2.2.   Parameters for  Subroutine RISK	   4-21
            4.2.3.   Parameters for  Subroutine GRDWTR	   4-21

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                       TABLE OF CONTENTS (continued)

            4.2.4.    Parameters for Subroutine RAINS	   4-21

5.  SITES FOR MODEL RUNS  ..;		  5-1

      5.1.   SITE 1: ANDERSON COUNTY, TN	  5-1

            5.1.1.    Description of Soil	  5-1
            5.1.2.    Narrative Climatologic Summary	  5-1
            5.1.3.    Temperature	  5-2
            5.1.4.    Rainfall		  5-2
            5.1.5.    Parameters for Subroutine RAINS	  5-3

      5.2.   SITE 2: CHAVES COUNTY, NM	  5-4

            5.2.1.    Description of Soil	  5-4
            5.2.2.    Narrative Climatologic Summary	  5-5
            5.2.3.    Temperature  :		  5-5
            5.2.4.    Rainfall		  5-5
            5.2.5.    Parameters for Subroutine RAINS	'..	  5-6

      5.3.   SITE 3: CLINTON COUNTY, IA	  5-6
                                 I
            5.3.1.    Description of Soil	  5-6
            5.3.2.    Narrative Climatologic Summary	  5-7
            5.3.3.    Temperature	  5-7
            5.3.4.    Rainfall	.;		  5-7
            5.3.5.    Parameters for Subroutine RAINS	  5-8

      5.4.    SITE 4: HIGHLANDS  COUNTY, FL	  5-8

            5.4.1.    Description of Soil	  5-9
            5.4.2.    Narrative Climatologic Summary	  5-9
            5.4.3.    Temperature  .;	  5-9
            5.4.4.    Rainfall	   5-10
            5.4.5.    Parameters for Subroutine RAINS	   5-10
                                 i
      5.5.    SITE 5: KERN COUNTY, CA		   5-H

            5.5.1.   Description of Soil	   5-H
            5.5.2.   Narrative Climatologic Summary	   5-11
            5.5.3.   Temperature  .	'....'	   5-12
            5.5.4.   Rainfall	   5-12
                                      VI

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     5.6.
          TABLE OF CONTENTS (continued)

5.5.5.   Parameters for Subroutine RAINS	   5-12

SITE 6: YAKIMA COUNTY, WA  	.......	   5-13
           5.6.1.    Description of Soil	  5-13
           5.6.2.    Narrative Climatologic Summary	  5-13
           5.6.3.    Temperature	 •	  5~14
           5.6.4.    Rainfall	  J-14
           5.6.5.    Parameters for Subroutine RAINS	  5-14

6. RESULTS	•	•.•••'	  6-1

     6.1.   ALGORITHM FOR INFECTIVE DOSE  ... ... r.	•  6-1
     6.2.   SENSITIVITY TO VARIABLES 	  6-3
     6.3.   ONSITE EXPOSURES	  6-14
     6.4.   OFFSITE EXPOSURES	*.....	•	  6-16
     6.5.   EXPOSURE FROM CONTAMINATED FOOD		  6-16
     6.6.   EXPOSURE FROM CONTAMINATED GROUNDWATER	  6-17
     6.7.   SURFACE WATER EXPOSURE	  6-20


7.  CONCLUSIONS	• • • •	  7~1

     7.1.   LITERATURE REVIEW		 .	  7-1
     7.2.   MODELING RESULTS		  7-1

           7.2.1.   Sensitivity Analysis	  7-2
           7.2.2.   Onsite Exposures	  7-2
           7.2.3.   Sediment Transport and Surface Runoff	  7-3
           7.2.4.   Offsite Exposures	  7-3
           7.2.5.   Waiting Period	  7-4


8  RESEARCH NEEDS	•	  S'1

      8.1.   INFORMATION NEEDS FOR VIRUSES 	  8-1
      8.2.   MODEL DEVELOPMENT			  8-3

9. REFERENCES		• •  9'1

APPENDIX A. Model Overview	  A-l
                                   vn

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                                 LIST OF TABLES


                                       Title                                  Page

 2-1    Compartments Included in the Sludge Management Practices	   2-2

 2-2    Sludge Management Practices and Descriptions in Pathogen Risk
        Assessment Model	      2-4

 3-1    Computer Search Strategy	-	   3.3

 3-2    Human Viruses in Sludge and Wastewater	   3.4

 3-3    Virus Densities in Sludge	          3.17

 3-4    Virus Inactivation in Soil	.              3_22


 3-5   Virus Inactivation in Aquatic Systems	   3.30

 3-6   Virus Inactivation in Aerosols  	          3.44


 4-1   Temperature Algorithm for Inactivation of Viruses	  4.4

 4-2   Main Program Parameters 	.	  4.5

 4-3   Parameters for Subroutine RISK	   4.^9

 4-4   Parameters for Subroutine GRDWTR  		          4-20

 4-5   Parameters for Subroutine RAINS	           4_22

 6-1    Results of Model Runs, Baseline Conditions	  6-6


 6-2    Maximum Probability of Infection by Site and Practice	  6-8

 6-3    Effect of Process Functions on Probability of Infection	    6-11

 6-4    Effect of Distribution of Crop Types on Risk To Consumers of
       Garden Vegetables	    6_18


6-5    Effect of Variables for Subroutine GRDWTR on Infection via Groundwater  6-21

8-1    Parameters for Temperature-Dependent Inactivation of Viruses in
      Aquatic Systems	;                g_4


                                       viii

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3-1



6-1



6-2



6-3



6-4




6-5



6-6



6-7



6-8



6-9
                         LIST OF FIGURES



                                Title                                   Page



Inactivation Rates of Viruses in Water as a Function of Temperature ....   3-36



Effect of MID on Cumulative Infection, Poisson Distribution Model -------   6-2



Effect of Dispersed Population, Poisson Distribution Model ............   6-4



Time Course of Risk from Viruses, Site 1, Practice I  ---- .............   6-9



Effect of Inactivation Rates ONSITE, Site 1, Practice I  ..............    6-12



Effect of Inactivation Rates on SWIMMER, Site 1, Practice I  .........    6-13



Time Course of ONSITE Risk,  Site 1, All Practices .................    6-15



Time Course of Risk to DRINKER, Site 1, Practices I-HI ............    6-19



Probability of SWIMMER Infection, All Sites, Practice I .............    6-22



Time Course of Risk to SWIMMER, Site 1, Practices I-IH ............    6-24
                                   IX

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                        ABBREVIATIONS AND SYMBOLS
 AGI        Acute gastrointestinal illness
 CEC        Cation exchange capacity
 CPU        Colony-forming units
 D&M       Distribution and marketing
 dia         Diameter
 ffu         Focus-forming units
 g           Gram
 ha          Hectare
 HAV        Hepatitis A Virus
 HCV        Human coronavirus
 HID        Human infective dose
 hr          Hour
 HVCS       High volume cyclone scrubber
 ID          Infective dose
 MID        Minimum infective dojse
 min         Minute
 MPN        Most probable number
 NOAA      National Oceanic and Atmospheric Administration
 PFRP        Processes to Further Reduce Pathogens
 PFU        Plaque-forming Units
 PSRP        Processes to Significantly Reduce Pathogens
 RH         Relative humidity
 sec          Second
 SRV        Small round viruses
 TCID        Tissue Culture Infective Dose
 TDS         Total dissolved solids
 TPB         Tryptose phosphate broth
 USDA       U.S. Department of Agriculture
wt           Weight

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                            1.  EXECUTIVE SUMMARY

      This preliminary risk assessment study focuses on the probability of human infection
from  enteric viral pathogens in municipal  sludge  applied to land.  It is based  on the
Pathogen Risk Assessment computer model and methodology described in Pathogen Risk
Assessment for Land Application of Municipal Sludge (U.S. EPA, 1989a).
      This document reports (1) the results of a literature review designed to find the data
on pathogenic viruses  required  by  the  pathogens  methodology,  and  (2) the results of
numerous site-specific computer simulations, running the Pathogen Risk Assessment Model
with a wide range of values for the  parameters required.  The parameters required for
viruses are (1) density of infective viruses in treated sludge destined for land application;
(2) minimum infective dose; (3)  inactivation rates in soil, dry particulates, liquid aerosols
and water; and (4) dispersion in the environment, i.e., transport in water, soil and air.
      Literature values  for virus  density in treated sludge were  so variable, both by
treatment methodology and by virus type, that no single number could be selected as typical.
However, 2000 virus particles/kg was chosen as representative of viral density in composted
sludge and  100,000  particles/kg in  digested  sludge.  Infective doses, while  varying by
detection method and by virus type, have been reported to be as low as 1 infective particle.
As a conservative assumption, this minimum value was used for the model runs. Reported
inactivation rates  range  from T.lxlO'5 to 1.6X10'1  logs/hour in soil,  1.6X10"4  to  1.4X10'1
logs/hour in water, and 4.9xlO'5 to 8xlO'7 logs/second in aerosols.  Like the density values,
these rates are quite variable. Information  on dispersion of viruses in the environment is
limited in its applicability to generating  a rate of transport in environmental media.
Development of a variety  of  transport models has been an attempt to quantify the
movement of viruses, especially in the subsurface and in groundwater.
       Six sites were chosen to provide diversity in geographic location, topography, soil
type, rainfall pattern and temperature. Locations selected for site-specific application of the
model include Anderson County, TN; Chaves  County, NM; Clinton County, IA; Highlands
County,  FL; Kern County, CA; and Yakima County, WA.
                                        1-1

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         An initial sensitivity analysis was performed using site-specific parameters for Site 1,
  Anderson County, TN. Main program variables used in the model run were varied over a
  range of values to determine the sensitivity of the model to variations in conditions.  In
  general, the default value of a given parameter was compared with a reasonable higher and
  a reasonable lower value, where the  high and low values were  taken from  available
  literature or estimated when literature values were not available.
        In this analysis, it is assumed that viruses are transported into subsurface soil and
  subsequently into groundwater and are included in any droplet aerosols formed by spray
  application, as well as  in any particulate aerosols formed by disturbance of the soil by wind
  or by cultivation.  It is also assumed that the viruses are inactivated at a characteristic rate
  that depends on the ambient temperature and the medium in which they are found.
        Using baseline parameters at Site 1, the maximum probabilities of infection in each
 practice were evaluated.  Infection ONSITE was similar for all practices, between 1% and
 7%. The probability of infection to the OFFSITE receptor was calculated as zero in every
 case.  Risk of infection via contaminated food products (EATER) was; shown only in
 Practice IV, and risk via offsite wellwater (DRINKER) was shown in Practice HI.  In
 contrast, infection by contact with onsite surface water (SWIMMER) was significant in all
 three practices (Practices I-IH) that include the pond, the risk level being dependent on site-
 specific as well as practice-specific variables.
       The effects of  site-specific and  practice-specific differences in parameters and
 assumptions are illustrated by comparing the outcome of baseline model runs. The second
 set of model runs, in which inactivation rates were  decreased in soil, water, and droplet
 aerosols, showed  higher  probabilities of infection  at all sites and for most exposure
 compartments. The results of these model runs, using baseline parameters except for the
 more conservative inactivation  rates,  showed the maximum calculated probabilities of
 infection ONSITE were similar for each site, and again  no OFFSITE; infection was
 predicted.  Infection via contaminated food products was calculated to be significant only
 in Practices I and IV,  whereas infection via contaminated  wellwater was  indicated in
Practices I-IH at all sites. Infection to the SWIMMER was predicted at significantly higher
levels than with the default inactivation parameters.
                                        1-2

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      Results show that the inactivation rate of virus particles is extremely important in
determining whether a  groundwater  well  is  likely  to become  contaminated  and in
determining how long surface  soils or surface water are likely to remain infectious.  The
results also demonstrate the importance of accurate characterization of inactivation rate for
viruses of different kinds in the various transport and exposure media.
      Using reference values including the conservative inactivation rates, the baseline
maximum probability of infection was  0.270 for Practice I, 0.046 for Practices H and ffl,
0.0055 for Practice IV, and 0.0028 for Practice V. Generally, site-specific variables did not
have a significant  effect on the probability of ONSITE infection, because the site-specific
variables alter temperature-dependent inactivation rates and rainfall-dependent runoff and
sediment transport, none  of which  exerts major effects  on the  ONSITE  exposure
compartment before the time of maximum infection. Significant impacts on the probability
of infection were observed in all  application practices with changes in pathogen density in
the  applied sludge [ASCRS, P(l)] and sludge application rate [APRATE, P(2)], both of
which determine the number of viral particles applied. Because of the exponential nature
of the probability algorithm, the changes in probability were not directly proportional to the
change in parameter values, but varied as would be expected for a proportional change in
exposure.
       In all model runs the probability of infection OFFSITE was  calculated as zero,
indicating that although the inactivation of viruses in aerosols  may be less than initially
expected, the calculated quantities of liquid and dry particulate aerosols and concentrations
of viruses in the aerosols were too low to provide an infective dose to the modeled receptor.
       Consumption of contaminated vegetable crops was shown by model calculations to
be a potential source of human infection, provided that inactivation rates were sufficiently
 low or harvesting times were sufficiently close to application of the sludge.  Infection via
 food crops was  sensitive not only to infectious dose, inactivation rates, and the parameters
 that directly affect the number  of pathogens applied to the soil, but also to the relative
 fractions of pathogens transferred among surface soil, subsurface soil, and crop surface and
 to the type of crop or fraction of the total crop grown aboveground, below-ground, or on the
 ground.
                                          1-3

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        Contamination of meat or milk by viruses from sewage sludge did not appear to pose
 a significant risk to human health.
        Transport of viruses via groundwater to an offsite well was not shown by this model
 to be a major risk, but exposure by contaminated groundwater was shown to be likely if the
 rate of inactivation of viruses in water was less than the default values.  The probability of
 infection was related to the periodic introduction of pathogens to groundwater by  the
 infiltration of rainwater. The most important parameter related to subsurface transport of
 viruses appeared to be the inactivation rate of viruses in water.  The results also showed an
 increase in probability of infection at the  offsite well whenever the  time required for  the
 viruses to reach the well was decreased.
       Contaminated surface water, represented by the SWIMMER in an onsite pond, was
 the most significant source of exposure. A peak in probability of infection occurred after
 each rainfall, when additional contaminated surface water and soil were washed into the
pond.
       The following information is needed to improve the usefulness of the Pathogen Risk
Assessment Model and to allow for a bore reliable risk assessment of the land application
of sewage sludge:
       •  Simple and accurate standardized methods for detecting and quantifying, by type,
         pathogenic viruses in treated sludge destined for land application, in final D&M
         sludge products, and in environmental media;
      •  Improved understanding of minimum infective doses, particularly low-dose effects
         and MIDs for sensitive subjects;
      •  More accurate persistence and transport data on all pathogenic viruses  of major
         concern in sludge;
      • Development of an  index of soil types that would correlate capacity for solute
        transport and suitability for sludge application (also valuable for onsite waste
        disposal or solid waste disposal);
     • Research on subsurface injection of sludge and the relative probability of virus
        transport in groundwater; and
     •  Epidemiologic studies evaluating enteric viral transmission.
                                       1-4

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      The following revisions would improve the accuracy of the model:
      •  Revision of default parameter values, especially for inactivation rates in aerosols
•  • •'      and temperature-dependent inactivation rates in soil arid water;
      •  Revision of temperature-dependent inactivation algorithms;
      •  Incorporation of factors for humidity and temperature in inactivation equations
         for aerosols;
      •  Incorporation of subroutines for subsurface transport under conditions of transient
         flow; and
      •  Incorporation of factors  to  allow for subsurface transport through solution
         channels, cracks, etc.
In addition, field validation of the model's predictions is necessary before the Pathogen Risk
Assessment  Model can be considered an accurate predictor of health risk.
                                          1-5

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                              2. INTRODUCTION

      This preliminary risk assessment study focuses on the probability of human infection
from enteric viral pathogens in municipal sludge applied to land. Sludge, a byproduct of
sewage treatment, is the mixture of solids and liquids remaining after treatment processes
remove solids from municipal  or  domestic wastewater.  Secondary and tertiary sludges
contain biomass resulting from microbial digestion of the sewage.  Derived from human
sanitary wastes, sludge contains microorganisms  that colonize humans and can  cause
infection and disease.
      This risk assessment is based on the Pathogen Risk Assessment computer model and
methodology described in the U.S.  Environmental Protection Agency's (U.S. EPA's)
Pathogen Risk Assessment for Land Application of Municipal Sludge (U.S. EPA, 1989a).
Appendix A provides an overview of the model. The purpose of the model is to determine
the probability of infection of a human receptor from pathogens in land-applied sludge.  The
model consists of a series of compartments (Table 2-1) representing discrete points in the
application pathway.  The compartments are the  various locations, states or activities in
which sludge or sludge-associated pathogens exist; they vary to some extent among practices.
Compartments representing sources of human exposure are designated with an asterisk in
Table 2-1.  In each compartment, pathogens increase,  decrease or remain the same in
number with time, as specified by "process functions" (growth, die-off or no population
changes) and "transfer functions" (movement between compartments). Infection rather than
disease is used to measure risk in the methodology, since exposures to pathogenic viruses
may lead to no infection, human infection that is  asymptomatic  or subclinical (no illness),
or human infection with illness (Kowal, 1985).  The outputs produced by running the model
 are numerical values for the  probability of a human receptor receiving an exposure
 exceeding  the minimum infective dose (MID) in a 24-hr period. The MID for humans is
 considered to be as low as 1 virus particle, although infective doses can vary depending on
 the virus and susceptibility of the human receptor (Kbwal, 1985). The model will run until
 the day specified or until  the number of pathogens in each compartment decreases to < 1,
 at which point the number is rounded to zero.
                                        2-1

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                     TABLE 2-1




Compartments Included in the Sludge Management Practices
Compartment
Name and Number

Application
Incorporation
Application/Tilling
Emissions
Soil Surface
Particulates
Surface Runoff
Direct Contact
Subsurface Soil
Groundwater
Irrigation Water
Soil Surface Water
Offsite Well
Aerosols
Crop Surface
Harvesting
(Commercial) Crop
Animal Consumption
Meat
Manure
Milk
Hide
Udder
aSource: U.S. EPA, 1989a
Asterisk indicates exposure
Liquid Sludge Dried/Composted
Management Practices Sludge Management
Practices
T
1
2
3*b
4
5*
6*
T
8
9
10
11
12*
13*
14
15
16*






compartments.
IT
1
2
3*
4.
5*
6*
7*
8
9
10
11
12*
13*
14


17
18*
19
20*
21
22

— m — • . iv.
i '•• i
2
3* '• 3*
4 4 '
5* ' 5* ' '
6*
7* 7*
8 8 ,
9
10
11 11
12*
13*
14 14
15 . 15 •
16*
17 :
18*
19
20*
21
22

V
1

3*
4
5*

7*
8


11


14





1 '•



                     2-2

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      Two categories of land application are employed in the methodology: (1) agricultural
utilization and (2) distribution and marketing (D&M). The source of pathogenic viruses is
either liquid or dried/composted municipal sewage sludge.  The five municipal  sewage-
sludge management practices (Table 2-2) included in the model are application of liquid
treated sludge to (I) commercial crops for human consumption, (n) grazed pastures, and
(III) crops to be processed before being consumed by animals; and application of dried or
composted sludge to (IV) residential vegetable gardens and (V) residential lawns. Practices
III and V, while ostensibly limited to hay fields and residential lawns, respectively, can be
modified by selection of appropriate parameters to represent sludge application to golf
courses, reclaimed strip mines or logged sites, parks, roadsides, etc.  Although Practice V
does not include an onsite pond, the risk to a human swimming in a pond (SWIMMER) can
be modeled by using appropriate parameters in Practice HI.
      Risk assessment for pathogens in  land-applied municipal sludges requires  the
following input data:
      •  Types of pathogens and their concentrations in the sludge, their survivabilities, and
         their infective doses;
      •  The  sludge reuse/disposal option used and the conditions of sludge application
         (quantities, frequencies, application method);
      •  The  fate of the pathogens in the environment, i.e., the inactivation rate under
         different conditions including moist soil, dry particulates, droplet aerosols and
         water; and
      •  The  level of exposure of human receptors to the applied sludge.
      In general,  these  data  are sparse, and in many cases parameter values  must be
selected by using best scientific judgement. In addition, no field experimental work has been
done to validate the predictions of the model.  The purpose of this study is not to provide
a definitive health risk assessment for viruses in municipal sewage sludge applied to land,
but  rather to  identify parameters  most in need of further  research and  validation.
Therefore, this document serves as an evaluation of data needs for use of the model. The
recommendations  for  further research are intended  to provide  those  data, but  field
validation of the model will still be necessary for application of the model.
                                        2-3

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                                  TABLE 2-2
 PRACTICE
                  Sludge Management Practices and Descriptions in
                         Pathogen,Risk Assessment Model
                  DESCRIPTION"
    n
   m

   rv

   v
                  Application of Liquid Treated Sludge for Production of
                  Commercial Crops for Human Consumption
Application of Liquid Treated Sludge to Grazed Pastures

Application of Liquid Treated Sludge for Production of
Crops Processed before Animal Consumption

Application of Dried or Composted Sludge to Residential
Vegetable Gardens

Application of Dried or Composted Sludge to Residential
Lawns
 'Source:  U.S. EPA, 1989a
"Two types of sludge are used in this model - liquid and dried/composted.  The extent
of treatment or conditioning prior to application is variable and must be determined
 for each case.
                                      2-4

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      This document reports the results of a literature review designed to find the viral
data required by the pathogens methodology, and  the  results of numerous computer
simulations, i.e., running the Pathogen Risk Assessment Model with a wide range of values
for the parameters required.  Six sites, chosen to provide  diversity in geographic location,
topography, soil type,  rainfall pattern and temperature, were selected for site-specific
applications of the model: Anderson County, TN; Chaves County, NM; Clinton County, IA;
Highlands County, FL; Kern County, CA; and Yakima County, WA. Since the number of
possible sites was essentially unlimited, the final selections, although somewhat arbitrary,
were based on an attempt to represent different geographic regions and to ensure a variety
of weather patterns.
      Exposure pathways, i.e., migration routes of viruses from or within the application site
to a receptor, for sludge  applied to land include the following:
      •  Inhalation and  ingestion of aerosols from the application of liquid sludge or
         wastewater;
      •  Inhalation and ingestion of windblown or mechanically generated particulates;
      •  Swimming in a pond fed by surface water runoff;
      •  Direct  contact  with  sludge-contaminated  soil or crops  (including grass,
         vegetables, or forage crops);
      •  Drinking water from an offsite well;
      •  Consumption of vegetables grown in sludge-amended soil; and
      •  Consumption of meat or milk from cattle grazing on or consuming forage from
         sludge-amended fields.
      Because the focus of the model is enteric pathogens, this methodology assumes that
exposure to  viruses  will not result in  infection unless  the  virus particles are actually
swallowed. Risks due to inhalation of enteric pathogens will be considered only because the
infectious agents can be subsequently swallowed. However, disease can result through
routes of exposure other than  the  alimentary tract; risks from such  exposures  can be
modeled by choice of the appropriate virus-specific parameter values.
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        The model calculates the probability of infection by viruses for the following human
 receptors:
        •  Onsite person (ONSITE): who is exposed  by ingestion  (includes  pica in
          children) of soil, vegetables  or  forage, or by inhalation and subsequent
          ingestion of aerosols (particulates or liquid);
        •  Offsite person (OFFSITE) who is exposed to  particulate or liquid aerosols
          carried by wind;
        •  Food consumer (EATER) who eats vegetable crops, meat or milk produced on
          sludge-amended soil;
        •  Groundwater drinker (DRINKER) who consumes water from a well near but
          not on the sludge application site;
       *  Pond  swimmer (SWIMMER)  who ingests a small amount of water while
          swimming in the pond that receives the surface runoff from the application
          site.
 The model conceptualization (U.S. EPA, 1989a)  specifies that workers engaged in the
 transportation,  handling and application of liquid sludge are not included as exposed
 individuals because such activity is an occupational exposure.
       The U.S. EPA (1986, 1988) has  provided  extensive information relevant to the
 conceptual risk assessment framework for land application of sludge.  These key studies
 address the pathogens associated with sewage sludge, as well as exposure pathways and the
 potential risks to humans from each of the pathways. Most of that information will not be
 repeated here.   Additional  information about the computer  model and methodology,
 including basic assumptions,  model limitations and sources of uncertainty, is available in
 Volumes I and  II of Pathogen Risk Assessment for Land Application of Municipal Sludge
 (U.S. EPA, 1989a) and in Wilson et al. (1989); a brief overview of the model is included as
Appendix A.
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                     3. LITERATURE REVIEW OF VIRUSES

      A literature search was performed to find the most current information available for
the parameters required by the model for simulating land application of sludge.  This
literature was reviewed for  data on which to base the ranges of values for each of those
parameters. The parameters required for viruses are (1) minimum infective dose; (2)
density of infective viruses in treated sludge destined for land application; (3) inactivation
rates  in soil,  dry particulates, liquid  aerosols and water;  and, (4) dispersion  in the
environment, Le., transport  in air,  soil and water.
      Appropriate codes and keyword truncation were used to produce the most effective
search strategy for query of each  data base. Table 3-1 lists the computerized data bases
queried and the keywords used. The three columns of keywords were "anded" together to
produce a set in which at  least one keyword in each column was  a descriptor  or was
contained in a retrieved record.
      References in reviews and in relevant articles retrieved by the computer search were
also evaluated, and names of pertinent authors were searched to find recent  papers.
Because the scope  of  this literature review is limited  to  information satisfying the
parameters required by the model, the reader is directed to  cited  references for  more
comprehensive background information.

3.1.   SIGNIFICANCE OF PATHOGENIC VIRUSES
       The presence of pathogenic viruses in sewage sludge has been well-researched and
documented.  Several reviews include information on specific  types  of viruses present in
municipal sewage, their persistence and density in sludge, their pathogenicity, and potential
health risks associated with  land application (WHO, 1981; Kowal, 1985; Carnow et al, 1979;
Loehr et al., 1979; IAWPRC, 1983; U.S. EPA, 1986, 1988; Lund,  1978; Burge and Marsh,
 1978; Pedersen, 1981; Elliot and Ellis,  1977; Feachem et al., 1983).
       From the Latin word meaning poison, the word virus was  originally used to mean any
poison or noxious agent.  Organisms that could pass through bacteriologic filters and be
 transferred serially from one animal to another were termed "filterable viruses" and later,
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                               TABLE 3-1
                         Computer Search Strategy
        Data Bases
                                            Keyword Groups
AORICOLA
AGRIS
WOSIS
GAB ABSTRACTS
CRIS/USDA
ENYIROLINE
FSTA
NTIS
POLLUTION ABSTRACTS
TQXLINE
WATER RESOUR. ABS
ZOOLOGICAL RECORD
VIRUS
ENTEROVIRUS
ENTERIC VIRUS
POtlOVIRUS
COXSACKIE(VIRUS)
ECHOVIRUS
HEPATITIS A
ADENOVIRUS
REOVIRUS
ROTAVIRUS
PARVOVIRUS
CORONAVIRUS
NORWALK/
NORWALK-LIKE
PAPOVAVIRUS
ASTROVIRUS
CALICIVIRUS
SURVIVAL
DISPOSAL
TRANSPORT
FATE
VIABILITY/
VIABLE
DIE-OFF
MOVEMENT
SEWAGE
SOIL
AIR
AEROSOL
WATER
SLUDGE
GROUND-
WATER
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viruses. The complete infectious virus particle is called the virion, and it consists of nucleic
acid or nucleoid (DNA or RNA), a protein shell (capsid), and, for some viruses, an envelope
derived from  the host cell membrane.  As  obligate intracellular parasites, viruses lack
independent metabolism and can replicate only within living host cells (Borland's Illustrated
Medical Dictionary, 1981; Kucera,' 1983). As a group, the viruses are the smallest agents
of infectious diseases, ranging in size from 20 nm (e.g., poliovirus) to  200-300 nm (Berk,
1983; Yates and Yates, 1988).
       Although viruses cannot multiply outside a host cell (unlike bacteria, which can grow
extracellularly in a suitable nutrient-rich medium if conditions are favorable) and will always
decrease in numbers following excretion, the enteric (relating to the  intestines or, more
generally, the alimentary tract) viruses can  persist in the environment for many weeks,
particularly if temperatures are cool (<15°C) (Feachem et al., 1983).
       Table 3-2 lists the major viruses found in wastewater and therefore those most likely
to be of concern from the land application of municipal sewage sludge.
3.1.1.  Transmission/Exposure Routes. Many viruses infect the intestinal tract, are excreted
in the feces, and can infect new human hosts, following either their direct ingestion or their
mucociliary translocation and subsequent ingestion after being inhaled by  the host (Slote,
 1976).  While the concentration of viruses in the feces of an uninfected person is normally
zero (Kowal et al. 1981), concentrations of > 106 to 109 infectious virus particles maybe shed
in 1 g of human feces from an infected individual, even if that person does not  exhibit frank
 disease (Feachem et al., 1983).
        Transmission of enteric viruses is by the fecal-oral route, with water- and food-borne
 outbreaks  being  of major importance;  by  direct  personal contact or  contact  with
 contaminated surfaces or fomites; by contact with recreational water; and possibly by the
 airborne route.  Although the fecal-oral route of transmission of enterovirus infections is
 well-established, it is not fully understood.  Ingestion of contaminated food or water or
 swimming  in contaminated water are important routes of viral infections (Cliver,  1987).
 Human exposure  to viruses in surface water is possible through deliberate  or accidental
 ingestion as well as through contamination of facial mucosal surfaces during recreational
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                                     TABLE 3-2

                       Human Viruses in Sludge and Wastewater
              Virus
                Disease or Symptoms
  Enteroviruses
    Poliovirus
    Coxsackievirus A
    CoxsacMevirus B
 poliomyelitis, meningitis, fever
 herpangina, respiratory disease, meningitis, fever
 myocarditis, congenital heart anomalies, meningitis,
 respiratory disease, pleurodynia, rash, fever
    Echovirus
 meningitis, respiratory disease, rash, diarrhea, fever
   New Enteroviruses
 Hepatitis A Virus
 acute hemorrhagic conjunctivitis, meningitis,
 encephalitis, respiratory disease, fever
 infectious hepatitis
 Rotavirus
 acute gastroenteritis with severe diarrhea, vomiting
 Norwalk-like Agents (or Small
 Round Viruses, SRVs)
 epidemic gastroenteritis with diarrhea, vomiting,
 abdojminal pain, headache, myalgia
 Adenovirus
                                 respiratory and eye infection
 Reovirus
 possibly fever, diarrhea and upper respiratory
 disease, but relationship to clinical disease in
 humans is not clear
 Papovaviras
 may be associated with progressive multifocal
 leukpencephalopathy
 Astrovirus
may be associated with gastroenteritis, diarrhea
 Calicivirus
gastroenteritis
Coronavirus-like Particles
respiratory tract infections
Parvovirus and Parvovirus-like
Agents
gastroenteritis, aplastic anemia, fever, rash, fetal
death or damage including hydrops fetalfs
Non-A non-B Hepatitis
hepatitis
Snow Mountain Agent
gastroenteritis
Pararotavirus
gastroenteritis
Sources: Kowal, 1985; Feachem et al., 1983; Kucera, 1983; Akin et al  1978- US
EPA, 1986; Rao et al., 1986; Levy and Read, 1990
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water use (Hurst et al., 1989). Following sludge application, these viruses can find their way
into surface  waters  either  by overland runoff or infiltration into the subsurface and
subsequent transport to an intersection with the surface.
      Although it is unknown whether animals serve as reservoirs for viruses likely to be
transmitted from land-applied sludge via soil or water (Kowal, 1985), it has been proven that
consumption of virus-contaminated shellfish, especially  raw shellfish, from seawater or
                                                                           \
estuarine habitats is a persistent problem. Shellfish can serve as a virus reservoir in nature
because  of the possible bioaccumulation of viruses associated with particulates  and the
persistence of those viruses for up to months if shellfish are dormant (Metcalf,  1987).
      Viral  contamination of water supplies may lead to localized  epidemics  of viral
gastroenteritis. More than 5000 persons suffered from gastroenteritis caused by Norwalk-
like virus in a multistate  outbreak traced to commercially-produced ice made  from
contaminated well water.  That well was flooded by a creek during heavy rains.  Nearby
residents with private wells flooded by the same creek were believed to have been infected
(Levine and  Craun, 1990).
      Acute gastrointestinal illness (AGI) of  unknown etiology comprised 48% of the
outbreaks of waterborne disease reported by the Waterborne Outbreak Surveillance System
in 1986-1988. Characteristics of these outbreaks suggest that many were caused by Norwalk-
like viruses.   These  reported outbreaks were associated only with water intended  for
drinking and probably represent only  a small percentage of all  illnesses associated with
waterborne-disease agents.  Additional reports of illness associated with recreational water
use during 1986-1988 included 41 cases of gastroenteritis caused by a Norwalk-like agent in
a lake.  Hundreds of other cases of AGI were attributed to recreational water use, and at
least some  of these were  undoubtedly caused  by viruses.   The  authors indicate that
improving the availability  of testing for viral serology and detection of viral antigen in stool
would aid in determining the etiology of these outbreaks of AGI (Levine and Craun, 1990).
      Transmission of viruses by the airborne route has been well established (Sattar and
Ijaz, 1987).   A number of environmental factors affect the survival of viruses in aerosols,
particularly temperature and relative humidity, as discussed in Section 3.3.3 (Spendlove and
Fannin, 1982; Sattar and Ijaz, 1987). Foster et al. (1980),  reporting on a rotavirus epidemic
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  on Truk Island, suggested that in addition to the fecal-oral route, human rotavirus infection
  may have been spread by the respiratory route.  Prince et al. (1986) indicate that the
  airborne route could be a pathway for the spread of gastrointestinal tract infections caused
  by rotavirus.   Sattar and Ijaz (1987) emphasize that the  potential exists for airborne
  transmission of all viruses that can survive the aerosolization process, even if the typical or
  dominant route of transmission is direct person-to-person contact or some other vehicle such
  as food, water or fomites.
  3.1.2.  Occurrence of Viruses in Sludge.  Although viruses do not normally inhabit the
  gastrointestinal tract of uninfected persons, the incidence of viral infection is high enough
  that viruses are ubiquitous in sewage.  Many of the viruses in wastewater  adsorb to
  suspended particles and are removed in sewage treatment processes, becoming concentrated
 in the sludge product.  Excreted viruses are described by Feachem et al. (1983), and their
 occurrence and persistence in the environment, their inactivation in treatment processes, and
 their transmission routes  and epidemiology are discussed.  U.S. EPA (1988), Pedersen
 (1981), Hurst (1989), Englande (1983) and Ward et al. (1984) review viral concentrations
 in sludge and  the  effectiveness  of sludge  treatment processes  in  inactivating viruses.
 Information on the densities of viruses in treated sludge is summarized in Section 3.2.2.
 3.13.  Infective Dose. The ability of pathogenic viruses to cause infection a,nd disease in the
 human exposed to them depends on the number and virulence (infectivity or pathogenic
 potential)  of the virions and the susceptibility of the host.  Host factors include the route
 of entry, the mode of virus spread, and the resistance and immunity factors of the human
 receptor (Menna and Soderberg, 1983). Consequently, infection is often considered a dose-
 response relationship in which the dose is the number of virions  to which the  human is
 exposed and the response is the level of infection, i.e., no infection, subclinical infection with
 no disease, or infection with disease (Kowal, 1985).
      Estimating infective dose can involve exposing volunteers to known doses of the virus,
 inferring from epidemiologic data the probable levels of exposure associated with observed
 frequencies of infection or disease, or measuring cytopathic  effect by infecting  tissue
 cultures. When virus concentrations are determined by infecting an indicator cell line, the
information gained is a ratio of infective activities.  The most quantitative determination,
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the plaque assay, yields a concentration of infective virus units, but the dose response of
plaque formation must also be known because, in some instances, more than one infectious
virus particle may be required to initiate  plaque formation.  When infection can only be
measured by cytopathic effect, it may be necessary to calculate a tissue culture infective dose
(usually TCID50 or TCD50, i.e., dose required to infect 50% of the cultures).  In this case,
also, the dose response of the indicator cell line to infection must be known for an accurate
calculation of dose response in humans (U.S. EPA, 1989a).
       The MID, or minimum infective dose, is generally considered to be the dose that will
infect 50% of the population (U.S. EPA, 1988). It is now thought that a few virus particles
can produce an infection if conditions are favorable, and data suggest that the infective dose
of enteroviruses to humans is possibly 10 or fewer virus particles. Reported infective doses
may vary widely, however.  For example,  the oral  infective dose to  humans for poliovirus
ranges from 1 to IxlO7'6 TCID50, and the range of reported plaque-forming units  (PFU) is
0.2 to 5.5xl06 (Kowal, 1985).  In a review of the issue of minimum human infectious dose
(IAWPRC, 1983),  the evidence suggests that while the minimum dose of enteric viruses
required to produce infection in healthy adults may be much larger than 1 PFU, susceptible
individuals are much more likely to be infected by a single PFU. Ward and Akin (1983),
in their review of minimum infective doses of animal viruses, urge more studies using larger
numbers of subjects and employing  the most current techniques to answer some of the
questions regarding viral infective doses.
       Ward et al. (1986) exposed volunteers to human rotavirus in doses ranging from
9xlO"3 to 9xl04 focus-forming units (ffu). The human 50% infectious dose was -10 ffu, and
it was estimated that -25% of susceptible adults would be  infected by 1 ffu. Since the
occurrence of illness was not dose-related, it appeared that the same dose that could cause
infection could also  cause illness.
       Schiff et al.  (1984) found  that the oral 50% human  infective dose (HID50) of
echovirus-12 in volunteers was 919 PFU, and by  statistical analysis they  predicted a 1%
infective dose (HIDj) of 17 PFU.  Their results indicated that previous infection did not
provide lasting protection of volunteers against reinfection.
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  3.1.4.  Epidemiology. According to Yates (1990), "viruses may be responsible for one-third
  of all the waterborae disease outbreaks that occur in this country." Wellings (1987) suggests
  that as  many as 60%  of waterborne  disease  outbreaks may be  caused by  viruses.
  Historically, viruses have been difficult to detect and isolate from environmental media and
  from clinical samples. As detection methods have improved, there has been an increase in
  the percentage of waterborne diseases characterized as virus-caused (Gerba, 1984c).
        Several of the viruses associated  with  sewage sludge and possibly with human
  gastroenteritis are poorly understood, such as papovaviruses, astroviruses, caliciviruses and
  coronavirus-like particles (Kowal et al., 1981).   Some  of these have not been well-
  characterized  because they  cannot be grown in vitro in cell cultures (Righthand, 1983).
 Reliable, comparable information on others is limited by inadequate detection methods.
 In fact, Rao et al.  (1986)  characterize  the extraction, concentration and enumeration
 techniques for viruses in soils as "marginally efficient" for some enterovirases  and "totally
 inadequate" for detecting HAV, Norwalk virus, and human rotaviruses.  They conclude that
 detecting any enteric virus in soil implies that many other types may be present.
       Epidemiologic evidence of adverse health effects associated with exposure to viruses
 from land application of sludge or wastewater has been mixed. Katzenelson et al. (1976)
 found sewage-associated disease  incidence 2-4 times higher in Israeli kibbutzim using
 wastewater for spray irrigation than in  those not using  wastewater.   However,  the
 researchers did note the possibility that pathogens could expose the affected population by
 other routes, such as on the clothes of irrigation workers returning from the fields.  The
 microbiologic quality of the wastewater was similar to that of raw wastewater and the
 likelihood of disease transmission  is probably increased by  the closed nature  of the
 kibbutzim, communities.
       Fattal et al. (1986a), in a subsequent retrospective study, noted a 2-fold excess risk
 only in the 0-4 year old age group during summer irrigation months but no significant risk
 on a year-round basis. In a prospective epidemiologic morbidity and serology study (Fattal
 et al.,  1986b), no excess  rate of enteric disease  was seen in kibbutzim using wastewater
aerosols compared with those using no wastewater or using wastewater but not exposed to
aerosols. There was a significant excess of ECHO 4 virus antibodies found, in the 0-5 and
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6-17 year old groups, compared with all other categories of those exposed to wastewater
aerosols from nearby towns, but there was no morbidity excess. The authors conclude that
little or no wastewater-associated health risks were detected in spite of the poor water
quality  of the effluent used, but they caution that the possibility of aerosol transmission of
viruses  "under  extraordinary circumstances is supported by the circumstantial evidence
provided by the serological study." Ward et al. (1989), as part of the Lubbock Infection
Surveillance Study, found that wastewater spray irrigation had no detectable effect on the
incidence of rotavirus infection in a community surrounding a spray irrigation site.
       Johnson et al. (1980b) found no significant health hazards to residents near a new
sewage treatment plant in Schaumburg, IL, and they were unable  to distinguish levels of
microorganisms in the air of residential areas from background levels.
       Recent  and  current epidemiologic  studies  are  providing important additional
information on the survival and transport of viruses in aerosol.  Currently, however,  the
preponderance of evidence suggests that, even though it is thought aerosolized pathogens
may have a lower infective dose than when ingested (Loehr et al., 1979), public health risks
from aerosolized enteric viruses do not appear to be of major concern.
        Clark et al. (1980) did not demonstrate an increased risk of infection from viruses
in wastewater  workers, although there was evidence of increased minor gastrointestinal
illnesses in inexperienced sewage-exposed workers compared with experienced workers.
However, the illnesses, occurring during the second quarter of the year, did not correspond
to enteroviral infections.                                                         .
        Loehr et al.  (1979)  list  normal  precautions  that should be  observed  at land
 application sites to protect workers, but they also point out that continual low-level exposure
 to pathogens can be beneficial to workers by building their immunity to infectious diseases.
        Several viruses, including Norwalk agent in the United States (Dolin et al.,  1971),
 the W agent in Britain (Clarke et al.,  1972), and rotaviruses in many countries (Davidson
 et  al.,  1975), have been associated with gastroenteritis.  Astroviruses were isolated from
 feces of 17 of 27 symptomatic children suffering from gastroenteritis in a pediatric ward in
 England (Kurtz  et al.,   1977).    Human  calicivirus  (HCV),  an  important cause of
 gastroenteritis in day care centers (Matson et al., 1989), was detected in 32% of symptomatic
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  cases of gastroenteritis in a day care center in Australia, but the mode of transmission could
  not be identified (Grohmann et  al., 1991).  The  authors suggest that  HCV may be  a
  common cause of gastroenteritis that is underrecognized because of insensitive detection
  methods.
        Lew et al. (1990) reviewed 6 years of viral gastroenteritis data in which  16% of
  specimens were positive for a virus. The most commonly observed agent was rotavirus (26-
  83%), followed by adenoviruses (8-27%, including respiratory and enteric) and small round
  viruses (SRVs) (0-40%).  Rotavirus and astrovirus detections were most common in winter.
  Lew et al. (1990) attribute insensitive Screening methods to the underestimation of disease
  prevalence cause by astroviruses, caliciviruses and SRVs.
       Payne et al. (1986) studied viral agents associated with gastroenteritis over an 8-year
  period.   Of the stool specimens submitted, 41% were positive for viruses  or virus-like
  particles belonging to 7 groups: coronavirus-like particles were present in 69.8% of positive
  specimens, rotavirus in 17%, adenovirus in 4.5%, picornavirus/parvovirus agents in 2.9%,
 Norwalk-like agents in 2.9%, astrovirus  in 1.9%,  and calicivirus  in 0.5%, with 0.5%
 unclassified SRVs.  Excretion of all  viruses  except  coronavirus-like particles exhibited a
 seasonal distribution, with the majority of viruses  being  identified in the cooler, drier
 months.
       The Norwalk agents  cause -36% of the outbreaks  of infectious, nonbacterial
 gastroenteritis; they have been-transmitted by ingestion of contaminated  food and water
 (Righthand, 1983). Keswick et al. (1985) suggest that the Norwalk agent is responsible for
 -23% of all reported waterborne outbreaks.
       The total reported cases of hepatitis  A was  28,507 in 1988, the highest number
 reported since 1980.  Total poliomyelitis cases were  9 in 1988 with a range of 6-34 cases
 annually over the past 20  years (CDC, 1989).
       In children between 6 and  24 months, rotavirus is the most  frequent cause  of
 nonbacterial gastroenteritis, but frequently neonatal infections are asymptomatic. Rotavirus
 infections occur less frequently among children 5 years old or older because by age  5 they
have acquired circulating antibody to rotavirus (Estes  et al.,  1983). The  spread of rotaviral
infections is poorly understood, but it is thought that air may be a vehicle for these viral
                                       3-10

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outbreaks (Ijaz et al., 1985c).  Viral survival was greatest at 50% relative humidity, and
reducing the temperature from 20°C to 6°C enhanced that survival.  Even after 24 hours,
there was only a 30% reduction in infectivity of the virus. In addition, rotavirus suspended
in fecal matter from an infant with rotaviral diarrhea survived longer as an aerosol than
rotavirus suspended in tryptose phosphate broth (TPB). After 24 hours, the fecal suspension
rotavirus lost <20% infectiviry compared with a 50% reduction in infectivity under the same
conditions for the TPB suspension of rotavirus (Ijaz et al., 1985c). These results support the
suggestion of Brandt et  al. (1982)  that a low outdoor  temperature, low indoor relative
humidity and indoor crowding may contribute to rotavirus survival and spread  of rotavirus
infections. Likewise, epidemiologic observations seem to correlate rotaviral gastroenteritis
outbreaks with climate, i.e., outbreaks in cool, dry weather of tropical climates and during
winter in temperate zones (Ijaz et al., 1985c; Brandt et al., 1982).
       Ward et al. (1986), in their study of infectious dose of rotavirus in adults, suggest
several problems with epidemiologic understanding of adult rotavirus infections. They point
out that many cases of adult gastroenteritis may be caused by rotavirus because the dose
required to cause infection and illness is very small (1-10 ffu).  The lack of evidence of high
incidence of adult rotavirus illness could be because infected adults may manifest the disease
differently than children or because the amount of virus shed by infected adults is below
detection limits of current assay methods.
       Most of the well-known viruses that are dangerous to fetuses of women infected with
systemic viral illness during pregnancy, such  as rubella, cytomegalovirus, and  herpesvirus
(Gold and Nankervis, 1989; Nahmias et al.,  1989), are not typically found in sewage sludge.
It should be recognized, however, that some viruses that may be present in sludge have been
found to be associated with risk of adverse  effects on the fetus or neonate.  These include
human parvovirus B19, which can result in fetal hydrops and death (Levy and Read, 1990)
and enteroviruses, such as echovirus and coxsackievirus B, associated with fetal heart effects
and  neonatal disease ranging  from  asymptomatic infection to death (Modlin,  1988;
Rosenberg, 1987).
       It appears that the difficulties  inherent in epidemiologic studies are  particularly
significant in determining the incidence and distribution of viral infections and diseases.
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 More research  is needed in  this area to  characterize the  potential public health risks
 associated with  transmission of viruses by application of sludge and wastewater.

 3.2.   ENTERIC VIRUSES IN TREATED SLUDGE
       The frequency of isolation and the quantity of virus recovered from sewage depend
 upon water use in a community, the  day  and season of sample collection, the level of
 infection in the population and the efficiency of recovery procedures (Rao et al., 1986).
 Concentrations  of viruses in wastewater in the United States vary greatly, reflecting the
 infection and carrier status of the population.  The larger the contributing population, the
 more uniform the viral concentrationbecom.es. Concentrations tend to peak in late summer
 and early fall when enteric viral infections increase (Kowal,  1985).
 3.2.1.  Effects of Treatment Processes.  Wastewater treatment processes are less effective
 in removing viruses than in removing bacteria (Sobsey et al.,  1980). Viruses in wastewater
 tend to  become adsorbed to suspended  particles;  consequently,  during primary, and
 secondary sewage treatment processes, viruses will be removed from the wastewater and
 concentrated in  the sludge. Chlorination is not as effective in destroying viruses in effluent
 wastewater as it  is in killing bacteria (Feachem et al.,  1983). Viruses in wastewater may be
 protected by particulates so that longer exposure times or higher chlorine concentrations
 may be required to destroy them (Sproul, 1978).  According to Hurst (1989), wastewater
 sludges cannot be readily disinfected by chlorine because of their solid nature and high
 organic content.
       The most important factors affecting the stability of viruses in wastewater sludges are.
 temperature, loss of moisture, and the presence of aerobic microorganisms. Other factors
 affecting viruses  in sludges are pH levels; presence of detergents, ammonia and certain salts;
 and the type of virus (Hurst, 1989).
       In his review of density levels  of pathogenic organisms in municipal  wastewater
sludges, Pedersen (1981) concludes that levels of enteroviruses in primary and secondary
sludges are similar; few data were available on levels of enteric viruses in  mixed sludge.
Primary sludge has  received primary treatment such  as screening and settling; secondary
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 sludge is produced by biologic waste treatment, or secondary treatment; primary and
 secondary sludge are combined to produce mixed sludge (U.S. EPA, 1986).
       Conventional sludge treatment processes designed to stabilize sludge and reduce
 volatile  solids also lower densities  of pathogens in sludge  to varying degrees.  These
 processes are: (1) anaerobic digestion, the microbiologic degradation of the organic matter
 in sludge in the absence of oxygen; (2) aerobic digestion, the biochemical oxidation of
 organic matter; (3) composting, the natural, aerobic microbiologic process of decomposing
 organic matter to produce humus; (4) lime stabilization, the application of lime to sludge
 to raise the pH; and (5)  air drying,  exposure of a layer of sludge to air to drain or dry.
 These sludge treatment processes reduce but do not eliminate viruses (Melnick, 1987). U.S.
 EPA has established regulations in 40 CFR 257 for these processes to qualify as Processes
 to Significantly Reduce Pathogens  (PSRP).   Additional processes, either singly or  in
 combination with PSRPs, have been defined as Processes to Further Reduce Pathogens
 (PFRPs).
       Anaerobic Digestion. Following high-rate anaerobic digestion at 35°C, Jewell et al.
 (1980) and Berg and Berman (1980) report log reductions for enteroviruses of 1.36 and 1.05,
 respectively.  Eisenhardt et al.  (1977) and Sanders et al. (1979) found that  by raising the
 temperature 3°C during mesophilic anaerobic digestion, two- to four-fold increases in viral
 inactivation rates were achieved.  In full-scale anaerobic digesters, Ohara and Colbaugh
 (1975) observed that mesophilic digestion achieved a 1 log reduction in virus content while
 thermophilic digestion achieved a 3 log reduction.  Although virus reductions  of 1 log
 (90%)/day at 30°C have been demonstrated in studies of anaerobic digestion with seeded
 enterovirus, Moore et al.  (1978) reported a reduction of 1 log/week at 30°C for poliovirus
when it was bound to the  sludge solids.
       During sludge treatment processes, temperature is not always the most important
factor responsible for virus inactivation; the relative effects of heat are dependent on the
temperature range (Traub et al., 1986).  When the effects of mesophilic and thermophilic
anaerobic digestion and aerobic thermophilic fermentation were examined, heat accounted
for  19% of bacteriophage f2 inactivation at 34.5°C  (mesophilic anaerobic digestion), for
32% at 54.5°C (thermophilic anaerobic digestion) and, in combination with pressure, for
                                        3-13

-------
 100% of inactivation at 60°C (aerobic thermophilic fermentation). The authors of this study
 suggest that enteric viruses would be inactivated by 1 day of thermophilic anaerobic
 digestion (54-55°C) and that virus recovery after thermophilic  digestion, as reported by
 Lund (1971) and Berg and Berman (1980), probably resulted from short circuiting in the
 sludge digesters.
       Inactivation rates will vary with the type of virus.  Bertucci et  al.  (1977)  report
 significant differences in inactivation rates  among viruses during mesophilic anaerobic
 digestion.  Poliovirus I (Sabin) was 98.8% inactivated after 2 days of digestion at 35°C.
       According to Spilhnann et al. (1987), mesophilic anaerobic digestion achieved only
 minor inactivation of a human rotavirus, coxsackievirus B5, and bovine parvovirus.  With
 thermophilic digestion, the rotavirus and  coxsackievirus were  rapidly inactivated; the
 parvovirus was heat stable.   Temperature was  the predominant contributor  to total
 inactivation only  for processes above  54°C.  The  authors  classify  these  viruses as:
 thermolabile/comparatively   chemoresistant  (rotaviruses   and   enteroviruses),   and
 thermostable/chemolabile  (parvoviruses).   "Chemoresistant"  is  defined  as  relatively
 insensitive to ammonia, detergents arid microbial factors in the sludge.
       The presence of aqueous ammonia enhanced the inactivation of poliovirus types 1
 and 2, coxsackievirus A13 and Bl and echovirus 11 during anaerobic digestion, according
 to Ward (1977).  However, reovirus was  insensitive  to the concentration of ammonia.
 Cationic detergents present in wastewaters increase susceptibility  of  reoviridae to heat
inactivation; these same detergents protect enteroviruses from heat  (Goddard et al., 1981;
Ward et al., 1976).
       Although sludges with undetectable viral levels were not produced by any  of the
treatments examined, anaerobic mesophilic digestion with subsequent thickening and aerobic
thermophilic digestion were  both found  to be  efficient  in reducing infectious  virus
concentrations  (Goddard  et al.,  1981).  Anaerobic mesophilic digestion alone  was not
reliable in  reducing virus levels in sludge.
      Large quantities of sludge that had been anaerobically digested and lagooned were
applied to  a 15,000-acre site in central Illinois by the Metropolitan Sanitary District of
Greater Chicago during 1971-1978 (MSDGC and IIT Research Institute, 1979). Water, soil,
                                        3-14

-------
and sludge sampling detected no viruses in the sludge product applied or sprayed and no
effects on the surface or groundwater at the site.
       Aerobic Digestion.  In laboratory bench studies of mesophilic aerobic digestion,
Scheuerman (1984) observed log reductions of 0.219-0.779 for poliovirus type 1, 0.189-0.59
for echovirus, 0.439-0.449 for rotavirus  SA-11, and 0.469 for coxsackievirus.  Ward et al.
(1984) conclude that standard aerobic digestion probably achieves pathogen reductions equal
to or greater than those achieved in mesophilic anaerobic digestion.
       Composting.  Although hepatitis A has been shown to withstand temperatures of
80°C, most enteric viruses succumb to composting temperatures (U.S. EPA, 1988; Pedersen,
1981).  Inactivation depends on equal  distribution of heat  throughout the compost pile
(Englande,  1983).  In seeding experiments with bacteriophage £2,  which is  more heat
resistant than most enteric viruses, Burge et al. (1978) studied the effects of windrow and
static pile composting methods  on viral inactivation.  Virus concentrations in windrows
decreased 90% every 4-7 days  in dry  weather, and 50% of this  rate  in wet weather.
Reductions were greater in aerated piles. According to Ward et al. (1984), enteric viruses
would be inactivated at greater rates; but some viruses, such  as hepatitis A and parvoviras
(Spillman et al., 1987), may be more heat resistant.
       Lime Treatment and Air Drying. According to Ward et al. (1984), it has not been
shown that viruses in sludge are inactivated by lime  treatment, but viruses have been shown
to be destroyed rapidly at high pH values. Pedersen (1981) reports a single laboratory-scale
study (Sattar et al., 1976) in which  a >4-log reduction of poliovirus type 1 was achieved in
the sludge product resulting from injection of the virus into raw sewage that was limed to
pH 11.5, settled and centrifuged.
      Ward and Ashley (1977) observed that the air drying process  inactivates enteric
viruses in sludge.  Inactivation of enteric viruses seeded into raw sludge was proportional
to water loss until the sludge reached -70% solids; viral concentrations decreased 3 orders
of magnitude between 70 and 90% solids. The drying process itself apparently causes the
inactivation  since  storage of viruses in  sludge at low moisture levels has little effect on
persistence.  Similar results were  observed for indigenous viruses in air-dried sludge by
Brashear and Ward (1983).
                                       3-15

-------
       Conclusions. Ward et al. (1984), U.S. EPA (1988), Hurst (1989), Rao et al. (1986),
 Englande (1983) and Yanko (1988) discuss the potential for reduction of enteric viruses by
 conventional sludge treatment processes. Ward et al. (1984) summarize viral reductions in
 sludge treatment processes as  follows:
             anaerobic digestion (mesophilic)  0.5-2 log reductions
             aerobic digestion                0.5-2
             composting                     2->4
             air drying                       0.5-> 4
             lime stabilization                >4
 They conclude that composting is the best of conventional sewage treatment methods since
 the high temperatures destroy most sludge pathogens. The results of Yanko (1988) support
 the conclusions of Ward et al. (1984).
 322.  Density of Viruses in Treated Sludge.  Viral densities in raw sewage will vary with
 season, region, and the nature and sizeiof the population. Akin and Hoff (1978) report that
 the concentration of viruses in raw wastewater is most often < 1000 units/L; however, Sorber
 and Guter (1975) estimate 7000 PFU/fL. Gerba (1983a) reports a range of 2-215 enteric
 virus units/g of raw sludge in the United States.  According to Booz-Allen and Hamilton,
 Inc. (1983), the  density of viruses in raw sludge is several hundred PFU/L, and Melnick
 (1987) reports virus concentrations of 5000-28,000 PFU/L in raw sludge. Ward et al. (1984)
 report densities of 102-104 enteric viruses/g dry weight  in primary  sludges  and SxlO2
 organisms/g dry weight in secondary sludges. U.S. EPA (1988) and Pedersen (1981) report
 average geometric mean values for enteric viruses of 3.9xl02 PFU/g dry weight in primary
 sludge, 3.2X102 PFU/g dry weight in secondary sludge, and 3.6X102 TCID50 in mixed sludge.
 Due to the limitations of recovery procedures, the actual numbers of viruses in sludges and
wastewater could be 1-2 logs higher thdm those reported (Akin et al., 1978). Table 3-3 lists
virus densities in treated sludge.
      Anaerobic Digestion. Limited data are available on the effects of sludge treatment
processes on indigenous sewage viruses. Mesophilic anaerobic digestion results in significant
decreases (90%) in viral concentrations, and thermophilic digestion reduces concentrations
by at least 99% (Berg, 1978). The results of Jewell et al. (1980), Berg and Berman (1980)
                                   ;    3-16

-------
      TABLE 3-3




Virus Densities in Sludge
Virus
Enteroviruses
Enteroviruses
Enteroviruses
Enteroviruses
Enteroviruses
Enteroviruses
Enteroviruses
Enteroviruses
Enteric Viruses
Enteric Viruses
Enteric Viruses"
Sludge
Treatment
Process
Anaerobic
Digestion
High Rate
Anaerobic
Digestion
High Rate
Anaerobic
Digestion
High Rate
Anaerobic
Digestion
Aerobic
Digestion
Aerobic
Digestion
Aerobic
Digestion
Aerobic
Digestion
None
Anaerobic
Digestion
Anaerobic
Digestion
Mean
Density
no/100 mL
5.9 PFU/100
mL, range 13-
17 PFU/100
mL
19 PFU/100
mL, range
4-100
138 PFU/100
mL, range 30-
410
<3.3 PFU/100
mL, range
< 1.4-16.7
0.3-1.2
TCID50/g dry
wt
ND
53TCID50/g
dry wt, range
14-260
TCID50/g dry
wt
3.35 PFU/g
dry wt
2-215 units/g
0.04-17 units/g
0.007 PFU/mg
TSS
Detention
Time
(days)
12,8
21
20
20
90
180
30



3
Conditions
30°C,
7-10°C
35°C
35°C
49°C
mesophilic
mesophilic
mesophilic
mesophilic



Reference
Cliver, 1975
Jewell et al.,
1980
Berg and
Berman, 1980
Berg and
Berman, 1980
Bitton et al.,
1980
Bitton et al.,
1980
Bitton et al.,
1980
Hurst et al.,
1978
Gerba, 1983a
Gerba, 1983a
Moore et al.,
1978
         3-17

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TABLE 3-3 (continued)
Virus
Enteric Viruses
Enteric Viruses
Enteric Viruses
Enteric Viruses
Picornavirus
(ECHO)
Picornavirus
(ECHO)
Reoviruses
Echovirus type 7
Viruses
Viruses
Viruses
Sludge
Treatment
Process
3 stage
Anaerobic
digestion
Anaerobic
Digestion
Anaerobic
Digestion
Aerobic
Digestion
Windrow
Composting
Static Pile
Composting
Anaerobic
Digestion
Sand Drying
Mesophilic
Digestion
Thermophilic
Digestion
Anaerobic
Digestion
Mean
Density
no/100 mL
12.
MPNCU/100
mL, range 0-19
MPNCU/100
mL
0.007 PFU/mg
TSS
0.007-0.04
PFU/mg TSS
0-260 units/g
<2.3 PFU/g
<2.3 PFU/g
8 PFU/100
mL, range 6-17
PFU/100 mL
0.1 PFU/g
50-360
PFU/100 mL
1.7-16.7
PFU/100 ml.
2.1 PFU/g
0.03 PFU/g
Detention
Time
(days)
40
5


40-90
21
composting,
30 curing
12,8
!3
20
20

Conditions
33°C (for 10
days)





30°C,
7-10°C

~35°C
49°C
mesophilic
thermophilic
TCED50 « Tissue culture infectious dose for 50% response
ND « Not detected
MPNCU = Most probable number of cytopathogenic units
TSS - Total suspended solids
Sources: Pedersen, 1981; U.S. EPA, 1988; Kowal, 1985.
Reference
Palfi, 1972
Moore et al.,
1978
Moore et al.,
1978
Gerba, 1983a
Yanko, 1988
Yanko, 1988
Cliver, 1975
Wellings et al.,
1976
Berg, 1978
Berg, 1978
Ohara and
Colbaugh,
1975

         3-18

-------
and Oliver (1975) suggest a range of mean densities for enteroviruses of 5.9-138 PFU/100
mL following mesophilic digestion, and Berg and Berman (1980) report a mean density of
<3.3 PFU/100 mL after anaerobic digestion under thermophilic conditions.
      Aerobic Digestion. Little research has been done on the effects of mesophilic aerobic
digestion on viral pathogens in sludge (U.S. EPA, 1988). Bitton et al. (1980) tested sludges
for enteroviruses at 30, 90, and 180 days following mesophilic aerobic digestion. No viruses
were detected after 180 days, and mean concentrations were greatest (53 TCID50/g dry
weight) with the shortest digester time (30 days). Kabrick et al. (1979) report that virus
concentrations (in total PFUs) are below detection levels when the digestion temperature
is >40°C and pH is >7.
        Composting.  Most studies of viral inactivation during composting have  involved
seeding of viruses prior to mixing the piles. A recent U.S. EPA study looked at indigenous
pathogens in sludge following composting. An aerated static pile composting facility and a
windrow composting facility designed to meet PFRP criteria were sampled weekly for a year
by Yanko (1988). Elution/concentration techniques proven effective for high-solid samples
were used in viral testing, and, in addition to several procedures on two  cell lines in the
project laboratory, samples were tested by different procedures in two other laboratories.
However, during the testing period, indigenous viruses (untypable picornavirus) were found
in only two  samples, one from each facility. Isolates from "blind seeds" were found in six
other samples. The picornavirus was found at a level below the quantitative limit for the
plaque assay, <2.3 PFU/g.  During the bimonthly sampling for a year  of 24 treatment
facilities (including anaerobic and aerobic digestion, heat drying, and composting facilities),
viruses were detected in  only one sample and were probably  laboratory contaminants,
according to the author.  Yanko (1988) concludes that none of the treated sludges in this
study produced viral  health hazards since only two  low-level isolations were made from
many samples.
       Lime Stabilization and Air  Drying.   Little  research has been done on the
effectiveness of the  lime stabilization process  in destroying  indigenous sludge viruses,
although viruses are  known to be destroyed by high pH  (Ward et al., 1984). Koch and
                                        3-19

-------
  Strauch (1981) report inactivation of poliovirus in raw and digested sludges at pH levels
  resulting from lime treatment.
        Several studies have reported the detection of indigenous enteric viruses in lime
  sludges at pH 10 or 10.5 (Pancorbo et al., 1988). Pancorbo et al. (1988) suggest that the pH
  achieved  and maintained determines  the  effectiveness  of this  treatment.   In  then-
  experiments, almost complete inactivation of seeded poliovirus type 1 was attained when
  alum sludge was treated with lime at pH 11.5.  Similarly, when Sattar et al. (1976) treated
  raw sewage with lime at pH 11.5, only 0.005% of the poliovirus type 1 input was recovered
 from the lime sludge.  In the  United Kingdom, Goddard et al. (1982) did not detect
 indigenous enteroviruses in sludge conditioned with lime at pH 11.
       Laboratory studies  and seeding experiments suggest that air drying is an effective
 method of inactivating enteric viruses (Ward et al., 1984).   Wellings et al. (1976)  reported
 a 0.1 PFU/g concentration of echovirus type 7 from a full-scale sand drying bed facility for
 sludge  treatment.  Dewatering  of sludge  by evaporation  inactivated human poliovirus,
 coxsackieviruses and reoviruses (Ward and Ashley,  1977).
       Conclusions. According to Feachem et al. (1983), "any sludge treatment process that
 involves temperatures of 50°C or above should yield a virus-free product if the process is
 well controlled and carried out for sufficiently long to ensure that all parts of the mass are
 heated." However, results reported in a more recent review (Rao et al., 1986) suggest that
 higher temperatures may be necessary to ensure virus inactivation. They report that HAV
 was not inactivated after heating to 60°C for 12 hours and was infective  at  80°C in the
 presence of high concentrations of some salts.
      Other factors affect virus persistence in relation to temperature. Ward et al. (1976)
 report that an ionic detergent protected poliovirus from heat in raw sludge.  Detergents
 from wastewater  become  associated with  solids  and are  concentrated  in  sludges.
 Composting causes degradation of the detergents; poliovirus is more readily inactivated
 during composting, but the survival of reovirus is enhanced  (Ward and Ashley, 1978).
      Aqueous ammonia, which is formed during anaerobic  digestion, added to raw sludge
speeds  inactivation,  allowing moderate  heat  treatment  to inactivate  enteroviruses.
                                       3-20

-------
Adsorption of virus particles to sludge particles may protect them from heat inactivation and
from inactivation by ammonia (Ward and Ashley, 1978).
      After monitoring a human rotavirus, a coxsackievirus B5, and a bovine parvovirus
during sludge treatment processes, Spillman et al. (1987) conclude that the best treatment
to eliminate viruses  from sludge would  be  thermal (60°C)  treatment to inactivate
thermolabile viruses, followed by anaerobic mesophilic digestion to eliminate thermostable
viruses that are sensitive to chemicals and microbes.
3.3.    OCCURRENCE OF VIRUSES IN NATURAL MEDIA
3.3.1.  Persistence in Soil. Persistence of viruses in the subsurface is dependent upon the
type of virus, the nature of the soil, and the climate of the environment (Yates and Yates,
1988). Although specific factors that control the fate of viruses in soil have been identified,
interactions between factors make consideration of separate effects difficult. Factors known
to affect the fate of viruses in the environment include: temperature, microbial activity,
moisture content, pH, salt species and concentration, soil properties, virus association with
soil, virus aggregation, virus type and organic matter. In many cases, the  mechanisms by
which these factors influence viral inactivation or protection are not clear.  Table 3-4
summarizes data on virus persistence in soils.
       Temperature.   Temperature is probably the most  important factor  influencing
persistence and inactivation of viruses in the environment (Yates and Yates, 1988; Gerba
and Bitton, 1984; Bitton, 1978).  Most enteric viruses are inactivated at temperatures of
60°C or above (Morris and Darlow, 1971); however, some types, such as hepatitis A, have
been shown to withstand higher temperatures (U.S. EPA,  1988).   Hurst et al. (1980a)
observed that in vials of soil the inactivation rate for poliovirus increased as temperature
increased, and Lefler and Kott (1974) made a similar observation with poliovirus in a sandy
soil in Israel. This relationship of viruses to soil temperatures has been confirmed by other
studies (Gerba and Bitton, 1984).  Viruses persisted up to 170 days in waste-treated soil at
3-10°C, according to Bagdasar'yan (1964). After irrigation of crops with sewage effluent,
the time for 99% inactivation of viruses in soil was 2 months during winter but 2-3 days in
 summer (Larkin et al., 1976a; Tierney et  al., 1977).
                                        3-21

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      TABLE 3-4




Virus Inactivation in Soil
Virus

—
Poliovirus
typel































Inactivation
Rate in
Days
T90 T99




455

152


588

323


31

10


30

33


3.0

1.4


1.7

3.5

Inactivation
Rate
Constant
(Iog10 day'1)




0.0022
(0.000092/hr)
0.0066
(0.00027/hr)

0.0017
(0.000071/hr)
0.0031
(0.00013/hr)

0.0323
(0.00135/hr)
0.1035
(0.0043/hr)

0.0338
(0.00141/hr)
0.0304
(0.00126/hr)

0.3331
(0.0139/hr)
0.7077
(0.0294/hr)

0.5809
(0.0242/hr)
0.2884
(0.0120/hr)
Sludge
Treatment/
Application
Inoculated viruses
applied to vials of
soil in:
Aerobic:
Sterile media

Nonsterile media

Anaerobic:
Sterile media

Nonsterile media

Aerobic:
Sterile media

Nonsterile media

Anaerobic:
Sterile media

Nonsterile media

Aerobic:
Sterile media

Nonsterile media

Anaerobic:
Sterile media

Nonsterile media

Conditions


Sandy loam
soil, pH 7.8

1°C




1°C




23°C




23°C




37°C




37°C




Reference


Hurst, 1988b
































      3-22

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TABLE 3-4 (continued)
Virus
Poliovirus

Poliovirus


Viruses

Viruses









Inactivation
Rate in
Days
90 99
Not given

Not given


Not given

Not given









Inactivation
Rate
Constant
(Iog10 day'1)
0.10
(0.0042/hr)
0.09
(0.0038/hr)
0.04
(0.0017/hr)
0.16
(0.0068/hr)
1.45 avg.
(0.060/hr)
(0.04 min.
(0.0017/hr),
3.69 max.
(0.154/hr)}
0.26
(0.0108/hr)
0.2171
(0.0091/hr)
0.057
(0.0024/hr)
0.2796
(0.0116/hr)
0.328
(O.OB7/hr)
Sludge
Treatment/
Application
Soil flooded with
inoculated
secondary
effluent
Soil treated with
sewage sludge
In forest soil


Soil/water/plant
system

Aerobically
digested sludge,
indigenous
viruses









Conditions
Not given

4°C
20°C

Not given

Late summer
(20-31°C daily
temp)
No rain







Moderate rain

Reference
Larkin et al.,
1976a

Duboise et
al., 1974


Reddy et al.,
1981

Hurst et al.,
1978









         3-23

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        Inactivation at higher temperatures may result from protein denaturation of the viral
  capsid (Yates and Yates, 1988).  Dimmock (1967) suggests that at temperatures >44°C,
  viral inactivation is associated with structural changes in the viral capsid; but at temperatures
  <44°C, inactivation depends on the inactivation rate of viral nucleic acid.  Temperature
  also has an indirect effect on virus persistence due to its effect on the growth of aerobic
  bacteria (Lance and Gerba, 1982).
        Soil Moisture Content.  According to Bitton et al. (1987) and Gerba and Bitton
  (1984), temperature and soil desiccation synergistically influence the fate of viruses in the
  soil environment.  In their review of virus survival in nature, Bitton et al. (1987) report that
  temperature and moisture are the  primary controls of viral persistence in sludge-amended
  soil, and Bitton et al. (1981) report that soil drying is a major detrimental factor in viral
 persistence in sludge-soil mixtures. During development of methods for the  detection of
 enteroviruses in sludges, Hurst et al. (1978) applied aerobically digested sludges to land and
 subsequently found reductions of naturally occurring enteroviruses at a rate of 2 Iog10/week.
 After 3 months in the field, no viruses were detected in sludge solids. The authors suggest
 that virus inactivation in the sludge solids was directly related to desiccation.
       When columns of sand treated with sludge were exposed to warm, dry  fall weather
 (<0.13 cm cumulative rainfall), infectivity of seeded echovirus and poliovims declined more
 rapidly than when the columns were exposed to wet summer weather (> 13 cm cumulative
 rainfall) (Bitton et al., 1981,1984). Bagdasar'yan (1964) reports that enteroviruses, including
 poliovirus 1, coxsackievirus B3, and echoviruses 7 and 9 persisted 2-3 months in soil with
 10% moisture compared with 15-25  days in air-dried soils. Poliovirus type 1 inactivation was
 much more rapid in drying soil (1 week for 99% inactivation as moisture decreased from
 13% to 0.6%) than in soils maintained at higher moisture levels (7-8 and  10-11 weeks for
 99% inactivation at levels of 25 and 15%, respectively) (Sagik et al., 1978).  During rapid
 infiltration of wastewater in a field  study, Hurst et al. (1980b) found that viral  inactivation
 rates were higher in rapidly drying soil. Periodic drying followed by aeration was found to
 enhance viral inactivation.
      Comparing the inactivation rate of poliovirus in eight soils saturated ¥/ith  river water,
groundwater, or septic wastewater  with the  same soils allowed to dry out, Yeager and
                                        3-24

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O'Brien (1979a) noted a sharp increase in inactivation rate at 1.2% soil moisture compared
with the rate at 2.9%. Hurst et al. (1980a) found that viral survival did not correlate linearly
with soil moisture, but decreased with increasing moisture up to the saturation point, then
increased with moisture beyond that point.  Possible explanations for this enhanced survival
at both high and low soil moisture levels include differences in extent of viral adsorption to
soil, in mechanisms of adsorption,  and in microbial growth rates at various soil moisture
levels.
       Yeager  and O'Brien (1979a) used radiolabeled viruses to show that viruses did not
become irreversibly bound to soil particles but were inactivated during the drying process.
Yeager and O'Brien (1979b) suggest that the mechanisms for viral inactivation in moist soils
are different from the mechanisms in drying soils.
       Microbial Activity. Microbial activity may play a role in the inactivation of viruses
in soil. Although some studies (Gerba and Bitton, 1984) have not observed any difference
in viral decline in sterile and nonsterile soils, others noted greater inactivation in nonsterile
soil. The inactivation rates of poliovirus and reovirus were found to be greater in nonsterile
soil suspensions than in sterile  soil suspensions (Sobsey et al.,  1980).  Comparing four
different combinations of aerobic/anaerobic and sterile/honsterile conditions on poliovirus
type 1, Hurst (1988b) observed that aerobic microorganisms exerted a statistically significant
effect on the rate of viral inactivation in sandy loam soil at three incubation temperatures
 (1,  23,  and 379C).   He concludes  that  microbial  antagonism appears to be a major
 determinant of viral stability in soil and suggests that the antagonism results from metabolic
products released  from bacteria or from interference with adsorption onto soil particles.
 Mechanisms of virus inactivation by microbes in soil have not been clarified at this point.
       Sobsey et al. (1986) report a temperature effect on the antiviral activity of microbes.
 Sterile and nonsterile soil samples gave similar survival rates for HAV, poliovirus type 1,
 and echovirus type 1 at 5°C, but at 25°C the time for inactivation of all three viruses was
 shorter in the nonsterile samples.
        p'H. Bitton (1978) asserts that, in general, enteric viruses will not be affected by the
 pH values of the natural environment. The direct effects of pH on viral persistence in soil
 have  not been studied extensively.  Reported results indicate that pH can influence virus
                                         3-25

-------
  inactivation, but the extent of this influence is not clear (Sobsey and Shields, 1987).  Hurst
  et al. (1980a) observed that poliovirus 1 and bacteriophages MS-2 and T2 persisted longer
  at lower soil saturation pH values.  The results of Salo and Oliver (1976) with aqueous
  solutions indicate that inactivation by pH varies with the type of virus. Various mechanisms
  have been suggested for the direct effects of pH on virus persistence, including alterations
  in the viral capsid and increase in sensitivity of the nucleic acids to DNase or ribonuclease
  (Yates  and Yates, 1988).
        In addition, pH may have indirect effects on viral persistence by affecting adsorption.
  Above pH 7, the net charge on the virus particle is negative; and sand, clay minerals, and
  organic matter are also negatively charged at pH>7 (Gerba and Bitton, 1984). Although
  adsorption would appear to be at a minimum at alkaline pH values, conflicting reports in
  the literature indicate that this relationship is not clear-cut (Gerba and Bitton, 1984).
       Salt Species and Concentration. Inactivation of enteroviruses in the environment can
 be influenced by salt species and  their concentrations.  Several investigators have reported
 that poliovirus, echoviruses, and bacteriophages were less susceptible to thermal inactivation
 in the presence of certain cations (e.g., Ca and Mg) in the media (Yates and Yates, 1988).
 According to  Gerba and Bitton (1984), this phenomenon may be  significant  to viral
 persistence in soil. Several studies have indicated that the type and concentration of salts
 in the soil affect virus adsorption to soil, adsorption increasing with increasing ionic strength
 (Yates and Yates, 1988).
       Organic Matter.  Organic matter may have a protective effect on the persistence of
 viruses in soil.   Although Hurst et al. (1980a) did not  find that virus persistence was
 significantly related to the amount of soil organic matter, Lefler and Kott (1974) found that
 poliovirus persisted longer in sand watered with waste pond effluent than with distilled
 water.
       Organic matter has been found to decrease virus adsorption by competing for sites
 on soil particles  (Yates and Yates, 1988).  Moore  et al. (1981) and Bitton et al. (1976)
report that organic matter interfered with viral adsorption in soil, indirectly affecting viral
persistence; and other investigators report that organic material acts as an eluting agent,
desorbing viruses from  the soil (Gerba, 1984a). Humic  and fulvic acids have also been
                                        3-26

-------
reported to prevent adsorption and to cause loss of virus infectivity (Yates and Yates, 1988).
The effects of organic matter on soil properties such as pH,  moisture content, and ion
exchange capacity may indirectly influence the persistence of enteric viruses in soils (Sobsey
and Shields, 1987).
       Adsorption. Depending upon the sorbent, the survival of viruses may be enhanced
or reduced by adsorption to soils or other materials (Yates and Yates, 1988).  Significant,
rapid inactivation of poliovirus type 1 occurred in soil after  its adsorption to manganese,
aluminum, and copper oxide particles but not  after adsorption  to silica and iron oxide,
according  to  Murray and Laband  (1979).   Moore et al.  (1982) report that reovirus
adsorption to  organic  muck, montmorillonite, dolomite,  and Ottawa sand resulted in
considerable inactivation.  Sobsey et al. (1980) found that survival was not prolonged in
every  case when poliovirus  type 1  and reovirus type 3 adsorbed to eight different soil
materials.
       Recent studies indicate that adsorption of virus particles by the soil is a major factor
in virus persistence (Gerba, 1985). In their experiments, Hurst et al. (1980a) found that soil
adsorption was one of the most important of the factors  that significantly affected viral
survival, with survival increasing with greater adsorption. The enhanced survival of viruses
in soil with a high adsorption capacity presents a dilemma for land application of wastes.
Although adsorption of viruses prevents their movement to the groundwater, soils with this
capacity are likely to enhance virus  survival (Hurst et al., 1980a).
       Gerba and Schaiberger (1975) suggest several mechanisms by which adsorption of
virus particles to various solids may enhance or reduce their  survival.  Among these are
interference with the action of virucides, increased stability of the protein capsid, prevention
of aggregate formation, and adsorption of enzymes and other inactivating substances.
       Formation of Aggregates. Although there are no studies on the  relationship of
aggregate formation to viral persistence in soil,  the  fact that aggregates affect  virus
persistence  in  water  suggests  that aggregates  might  protect  viral  particles  from
environmental factors in the soil (Yates and Yates, 1988; Sobsey and Shields, 1987).
        Soil Characteristics.  Soil properties probably influence viral persistence by affecting
the degree of adsorption of virions to soil particles (Yates and Yates, 1988),  According to
                                         3-27

-------
 Bitton et al. (1987), the relationship of soil type to virus persistence is difficult to determine
 because of the influence of other environmental factors.  Mineral and organic content of
 soil, moisture level, pH, and cation exchange capacity (CEC) affect viral persistence in soil
 and are related to soil type.
        Hurst et al. (1980a) found that virus survival was significantly correlated to adsorption
 and to soil saturation pH but not to other soil characteristics.  However, stepwise multiple
 regression analysis  of virus survival and  19  soil characteristics revealed  that extractable
 phosphorus and exchangeable aluminum were the next highest ranking of soil characteristics
 affecting survival.  Hurst et al. (1980a) suggest that the relationship between survival and
 aluminum  is due to the increase in virus adsorption at  high aluminum levels, and the
 increase in virus survival with decrease in level of resin-extractable phosphorus is due to
 increased adsorption at lower  levels of resin-extractable phosphorus.
       Fine-textured soils that  contain clay remove more viruses by adsorption than coarse-
 textured soils (Yates and Yates, 1988; IGerba and Bitton, 1984). Gerba and Bitton (1984)
 observe that humic materials and clay minerals are the two most active components of the
 soil. Clays increase viral adsorption to soil as  well as influencing survival of microbial
 populations in the soil; survival of microbes could be influenced by water that may be tightly
 bound to clay. Also, clay can be protective to the viral genome.
       The soil CEC may indirectly affect persistence of viruses by influencing adsorption.
 The  work  of  Sobsey  (1983)  and  others indicates  that virus  adsorption increases  with
 increasing CEC, as well as with clay content, exchangeable aluminum and low flow  rate.
       In examining the retention of poliovirus by 34 soils and minerals, Moore et al. (1981)
 found that adsorption was negatively correlated with soil organic matter and with available
 negative surface charge. Adsorption was not  significantly correlated with soil pH, surface
 area or elemental composition. Their results  indicate that soils are potentially efficient at
 binding viruses since 106 viruses adsorbed to a gram of Ottawa sand with  only 1% of the
 surface covered.  Sobsey and Shields (1987) report  that minerals were better adsorbents
 than soils in some studies.  According to Gerba and Bitton  (1984), soil iron oxides have
been shown to increase the retention of viruses in soil.  Magnetite sand adsorbed 99.99%
of virus particles while muck soil retained 16-79%.
                                        3-28

-------
      Van der Waals forces, double-layer interactions, and hydrophobic interaction are
discussed by Gerba (1984a) as mechanisms for viral adsorption to soil particles.
      Type of Virus. Susceptibility to inactivation in soil may vary with the type of virus
and the particular strain.  According to Lefler and Kott (1974), bacteriophage £2 survived
longer than poliovirus in saturated and in dry sand.  Hurst et al. (1980a) found different
inactivation rates for seven viruses under the same conditions.  The inactivation rates of
poliovirus, reovirus, echovirus and HAV in several types of soil material at 25°C were quite
different, although all three survived well in soil suspensions at 5°C (Sobsey et al.,  1986).
Goyal and Gerba (1979), studying a number of viruses including human enteroviruses, found
strain and type differences in adsorption to soil.
3.3.2. Persistence in Water.  Viruses persist longer in groundwater than hi surface waters;
Keswick et al. (1982) attributes this to lower temperatures, protection from sunlight, and
lack of microbial antagonism in groundwater. For poliovirus type 1, the decay rate reported
by Bitton et al. (1983a) was 0.0019 hr'1 in groundwater, and O'Brien and Newman (1977)
report a decay rate of 0.031 hr"1 in river water.  A field study by  Wellings et al. (1975)
suggests that viruses persist  for up to 28 days in groundwater, and Sattar (1981) reports
persistence of 560 days in surface water  in  the laboratory.   In natural waters, several
interacting factors,  including  temperature,  chemicals,  pH, light, biologic factors  and
suspended particulate matter, affect virus persistence (Melnick and Gerba, 1980).
       Table 3-5 summarizes data on virus inactivation in water. Virus inactivation follows
apparent first-order kinetics (O'Brien and Newman, 1977).  Virus inactivation curves in
aquatic systems range from linear to S-shaped, with variations being a result of clumping of
the viruses (Berg et al., 1967).  O'Brien and Newman (1977) attributed S-shaped curves to
higher initial virus titers (> 106 PFU/mL) and noted that linear inactivation curves (straight
lines on semilog plots) were  associated with initial virus concentrations of 105 PFU/mL or
less.
       Temperature. The length of time  that viruses persist and remain infective in surface
waters  depends to a great extent upon  temperature, with lower temperatures enhancing
survival and infectivity (Yates and Yates, 1988). In river water, enteroviruses persisted 5-20
days at 20°C in the laboratory (Clarke et al., 1964), and in farm pond water, enteroviruses
                                         3-29

-------
           TABLE 3-5




Virus Inactivation in Aquatic Systems
System
Description

Organisms
inoculated in lab
into groundwater
from 475-ft deep
well (Bitton et al.,
1983b)
Organisms
inoculated in lab
into McFeters' type
survival chambers
with groundwater
from 275-ft deep
well (Keswick et al.,
1982)
McFeters type
survival chambers in
situ in river
(O'Brien and
Newman, 1977)







Organism

Poliovirus 1





Poliovirus 1






Poliovirus 1



Poliovirus 3

Coxsackievirus
A-13
Coxsackievirus
Bl

ptt

7.6





7.8

















Season or
Temperature

22°C





3-15°C






23-27°C
12-20°C
7-17°C
4-8°C
23-27°C
12-20°C
23-27°C
12-20°C
12-20°C
7-1TC
4-8°C
Length of
Study














1 year










k
(hr-1)

0.0019





0.0088






0.031
0.040
0.032
0.028
0.022
0.053
0.042
0.14
0.083
0.034
0.023
0.017
Inactivation
Rate
(Iog10 days"1)
0.046





0.21






0.77
T9025hr

TgoSehr

Tgol9hr
T^24hr
T9o7hr
y\j
TgolZhr
T9029hr
T9044hr
T^SShr
             3-30

-------
TABLE 3-5 (continued)
System
Description
Organisms
inoculated into
groiindwater in lab
(Yates et al., 1985)










Organisms
inoculated into
groiindwater in lab
(Yates et al., 1985)











Organisms
inoculated in lab
into McFeters type
survival chambers
with groundwater
from 275-ft deep
well (Keswick et al.,
1982)
Organism
Poliovirus 1
Wisconsin
Arizona
North
Carolina 1
North
Carolina 2
Univ. of
Arizona
New York 1
New York 2
Texas 1
Texas 2
California 1
California 2
Echovirus 1
Wisconsin
Arizona
North
Carolina 1
North
Carolina 2
Univ. of
Arizona
New York 1
New York 2
Texas 1
Texas 2
California 1
California 2
Coxsackievirus







PH





























7.8







Season or
Temperature

12°
23°C
12°C

12°C

23°C
12°C
12°C
13°C
13°C
18°C
17°C

12°C
23°C

12°C

12°C

23°C
12°C
12°C
13°C
13°C
18°C
ire
3-15°C







Length of
Study





































k
(hr-1)

0.0025
0.015
0.0057

0.0047

0.028
0.0015
0.0021
0.0015
0.0057
0.0077
0.0034

0.0028
0.0078

0.0077

0.0072

0.0026
0.0022
0.0021
0.0057
0.0033
0.0063
0.0038
0.0079







Inactivation
Rate
(Iog10 days"1)

0.060
0357
0.138

0.114

0.676
0.035
0.051
0.036
0.137
0.185
0.081

0.066
0.188

0.186

0.174

0.628
0.054
0.051 ,
0.138
0.079
0.151
0.091
0.19







        3-31

-------
TABLE 3-5 (continued)
System
Description

Organisms
inoculated into
McFeters type
survival chambers
with groundwater
from 275-ft deep
well (Keswick et al.,
1982)
Summary data from
review sources
(Kutz and Gerba,
1988)
polluted river
unpolluted river
impounded
groundwater
Surface freshwater
from 5 sites (Hurst
et al., 1989)






Natural surface
waters (Haas, 1986)



Organism

Rotavirus
SA11






Enteric viruses
and coliphage



Coxsackie-
virus B3
Echovirus 7


Poliovirus 1


Viruses



pH

7.8






















Season or
Temperature
111 • ^—
T 	 "
3-15°C










5-28°C
4-37°C
5-22.5°C
4-30.5°C
-20°C
1°C
22°C
-20°C
1°C
22°C
-20°C
rc
22°C
3-5°C
22-25°C

37°C
Length of
Study

I











12 wk
12 wk
8wk
12 wk
12 wk
8wk
12 wk
12 wk
8wk




k
(hr-1)

0.015










0.013
0.010
0.016
0.007
0.00016
0.0023
0.0102
0.00016
0.0023
0.0062
0.00031
0.0021
0.0093
0.0032-
0.0071
0.032-
0.12
0.058
Iriactivation
Rate
(Iog10 days'1)
=======
0.36










0.325
0.250
0.374
0.174
0.0039
0.0475
0.2455
0,0039
0.0544
0.1498
0.0075
0.0498
0.2232
0.077-0.17
0.76-2.8

1.4
     3-32

-------
TABLE 3-5 (continued)
System
Description

(McDaniels et al.,
1983)
Distilled Water



Secondary
Wastewater


(McDaniels et al.,
1983)
Distilled Water



Secondary
Wastewater


River (Haas, 1986)

River water, in the
presence or absence
of sunlight
with low or high
turbidity (Hurst,
1988a based on data
in Cubbage et al.,
1979)
Organism


Calf Rotavirus

CPE assay
IFA assay
CPE assay
IFA assay
CPE assay
IFA assay
CPE assay
IFA assay
Calf Reovirus

CPE assay
IFA assay
CPE assay
IFA assay
CPE assay
IFA assay
CPE assay
IFA assay
Viruses

Poliovirus
sunlight
low turbid.
hi turbid.
no sunlight
low turbid.
hi turbid.

PH















f















Season or
Temperature



8°C

26°C

8°C

26°C



8°C

26°C

8°C

26°C

Moderate








Length of
Study





















20-70 hr








k
4
(hr-1)



0.00057
0.00022
0.0055
0.0050
0.00050
0.00018
0.0060
0.0053


0.00032
0.00027
0.0061
0.0063
0.00016
0.000066
0.0031
0.0049
0.014-
0.050

0.099
0.056

0.034
0.030

Inactivation
Rate
(Iog10 days'1)


Tgo 73 days
Tgo 185 days
Tgo 7.6 days
Tgo 8.4 days
Tgo 84 days
T^ 236 days
Tgo 7.0 days
Tgo 7.8 days


Tgo 130 days
Tgo 154 days
Tgo 6.8 days
Tgo 6.6 days
Tgo 262 days
Tgo 630 days
Tgo 133 days
Tgo 8.5 days
1-3


2.383
1.334

0.808
0.710

        3-33

-------
 persisted 84  days  at 20°C and 91 days at 4°C (Joyce and Weiser, 1967).   O'Brien and
 Newman  (1977) found  that  the rates  of inactivation of polioviruses  1 and 3  and
 coxsackieviruses A-13 and B-l in membrane dialysis chambers in the Rio Grande River
 were affected principally by the water temperature.
       In laboratory experiments, human rotavirus persisted longer in river water at 4°C
 than at 20°C (Raphael et al., 1985).  Since virus concentrations in filtered river water were
 essentially the same irrespective of temperature, the effect of the higher temperature may
 be indirect, with higher temperature promoting growth of bacteria and other microorganisms
 with antiviral activity.
       Groundwater samples from 11 locations were  analyzed  for various chemical and
 physical factors; inoculated with poliovirus 1, echovirus 1, and MS-2 coliphage; held at the
 in situ temperature; and  examined at intervals for virus persistence (Yates et al., 1985).
 Temperature  was significantly correlated with the inactivation rates of the three viruses.
       In their study of HAV in soils, groundwater and wastewater, Sobsey et al. (1986)
 report that at 5°C, HAV, poliovirus type 1  and  echovirus  type 1 persisted  (<90%
 inactivation) for at least 12 weeks in groundwater.  However, in 12 weeks at 25°C, HAV
 was somewhat less affected than the other two viruses, with 90-99% inactivation compared
 with 99.9% inactivation of poliovirus and echovirus.
       Hurst et al.  (1989), analyzing; viral inactivation rates, found that the statistically
 significant factors affecting persistence of three human enterovirus serotypes (coxsackievirus
 B3,  echovirus  7 and poliovirus 1) were incubation temperature, viral serorype and water
 source. Tgo, the number of days required for 90% inactivation or a 1-log reduction, varied
 with temperature for the three  viruses studied (Hurst et al., 1989):
                                -20°C        1°C          22°C      ;
       Coxsackievirus B3         255 days     21 days      4  days
       Echovirus 7               196 days     18 days      6.7 days
       Poliovirus  1               133 days     20 days      4.5 days
       Through its   effect on  the  chemical and biologic reactions  in  natural  waters,
temperature may have an indirect effect on virus persistence that is reflected in seasonable
                                        3-34

-------
variations (Niemi, 1976).  Kapuscinski and Mitchell (1980) also suggest that temperature
may not directly affect virus inactivation but may control other inactivation mechanisms.
       The inactivation rates for viruses in aquatic systems (Table 3-5) were examined to
determine whether they could be  used to generate a general temperature-dependence
equation like those used for die-off of bacteria and parasites in soil arid Water and for
inactivation of viruses in soil. These data are presented in Figure 3-1, which demonstrates
a relationship between temperature and inactivation rate for all three viruses examined, as
well as a difference between the viruses in inactivation rates as a function of temperature.
The figure  also shows the extensive scatter  observed among the  different studies and
different viruses.  This scatter makes it difficult to predict viral inactivation rates with a high
degree of confidence.
       Water Characteristics. Hurst et al. (1989) analyzed viral inactivation rates in relation
to surface water characteristics. The average viral inactivation ifi five surface water samples
from different sites (expressed in Iog10 units of viral loss/day of incubation) was 6.5-7.0 logs
over 8 weeks at  22°C,  4-5 logs over 12 weeks  at 1°C, and 0.4^0.8  log over  12 weeks at
-20°C.  Hardness and conductivity, strongly correlated  with each other; turbidity and
suspended solids content, strongly correlated with each other; and the number of generations
of bacterial  growth supported in the sample, also correlated with hardness and conductivity,
were the apparent water characteristics affecting viral persistence (Hurst et al., 1989).
       In their study of groundwater samples collected in the United States, however, Yates
et al.  (1985) found that water characteristics (pH, nitrate, ammonia, sulfate, iron,  total
dissolved  solids (TDS), hardness,  and  turbidity) Were not  significantly correlated  with
inactivation of three viruses held at in situ temperatures.  However, the decay rate of one
virus, MS-2 coliphage, was significantly correlated to calcium concentration.
       Jansons et al. (1989) examined the inactivation rates of enteroviruses (echoviruses
6,11, and 24; coxsackievirus type B5; and poliovirus type 1) in dialysis bags lowered into the
groundwater. Survival was variable and was influenced by temperature arid dissolved oxygen
concentrations, with the inactivation rate increasing as dissolved oxygen increased.  They
suggest that dissolved oxygen may indirectly affect enteroviruses by influencing the activity
                                         3-35

-------
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                3-36

-------
of antagonistic  organisms or  may have  a direct  effect by  oxidation  of virus  capsid
components.
       Light. At wavelengths <370 nm, light has been shown to be detrimental to viruses
in water (Bitton et al., 1987). Ultraviolet light inactivated virus particles, and visible sunlight
inactivated poliovirus type 1  suspended in clean water; inactivation decreased with depth
and addition of clay particles (Bitton et al., 1979a).  Algae that may shade viruses from
inactivation by light are controlled by light, indirectly affecting virus persistence (Bitton et
al., 1979a); and naturally occurring  chemicals such  as humic and fulvic  acids  may
photosensitize viruses in aquatic environments (Kapuscinski and Mitchell, 1980).
       Hurst (1988a) reviewed six  studies examining enteric virus persistence in systems
using natural surface fresh waters. Rotaviruses appeared to be more stable in fresh surface
water than enteroviruses.  High turbidity levels decreased the inactivation rate of poliovirus
in river water, possibly by reducing the inactivating effects of UV radiation from sunlight
(Cubbage et al., 1979). Inactivation rates associated with low turbidity in the presence of
sunlight were 2.383 Iog10 units/day  compared with 1.334 Iog10 units/day for high turbidity.
When  sunlight was  absent, the inactivation rate was 0.808 logs/day with high turbidity (36
nephelometric turbidity units [NTU]) and 0.710 logs/day with low turbidity (<2.5 NTU).
       Microbial Activity. Investigations  on the antiviral activity of microorganisms in
natural waters have yielded inconsistent results.  According to the results of O'Brien and
Newman (1977), raw river water and filter-sterilized river water had comparable viracidal
activity, but virus inactivation rates were lower in autoclaved river water. They suggest that
the water  may contain  a heat-labile  or  volatile inactivating  factor.  With lake  water,
Herrmann et al. (1974) observed more rapid enterovirus inactivation in untreated water in
dialysis bags in situ than in filtered water under similar laboratory conditions, suggesting that
a biologic factor was involved in inactivation.
       In a study of >30 groundwater samples, Yates (1984) found that indigenous bacteria
had an inconsistent effect on virus  inactivation.  In some cases, viruses survived longer in
sterile samples,  but with other samples,  survival was  greater in the nonsterile portion.
Filtered and unfiltered groundwater samples from several states were  analyzed by Yates et
al. (1990).  Temperature was the only water characteristic that was consistently, significantly
                                         3-37

-------
 correlated  to  decay rates  of coliphage  MS-2 and poliovirus  type  1, which had  been
 inoculated into the samples.  Although there was no consistent trend associated with the
 presence or absence of bacteria, differences in inactivation rates were observed among some
 water samples incubated at the same temperature. In another experiment in which bacteria
 were enumerated  throughout the experiment, the coliphage MS-2  inactivation  rate in
 unfiltered samples was significantly correlated to an increase in bacterial numbers in the first
 24 hours, suggesting that the bacteria produced some substance that inactivated viruses.
 Yates et al.  (1990)  summarize  suggested  mechanisms  by which organisms may affect
 inactivation:  production of enzymes that destroy the virus protein  coat, production of
 substances that increase virus susceptibility to photodynamic inactivation, and production
 of oxidizing and reducing agents.
       Jansons et al. (1989), examining the inactivation rates of enterovinises (echoviruses
 6, 11, and 24; coxsacMevirus type B5;  and poliovirus type 1) in groundwater, found no
 association between rate of virus inactivation and bacterial numbers. However, they suggest
 that the presence of large numbers of specific microorganisms in some bores may have
 contributed to the more rapid inactivation in those bores.
       pH.  In general, viruses survive well at the pH level  of natural waters  (pH 5-9)
 (Bitton et al., 1987).  The pH of the water  affects virus adsorption  to surfaces, virus
 sensitivity to enzymes, and their heat stability (Bitton et al., 1987). According to Salo and
 Cliver (1976), virus persistence relative to pH in the aqueous environment varies with the
 type of virus. Poliovirus 1 survived best at near-neutral values (5 and 7), but coxsackievirus
 A9 was inactivated more rapidly at pH 5 than at higher and lower values.  Pancorbo et al.
 (1987) found that for human rotavirus type 2 (strain Wa), inactivation was significantly
 correlated with the water pH; inactivation increased with increasing pH.
       Salt Species and Concentration. Various salts affect the inactivation rates of viruses
 in aqueous solutions. With increasing Concentrations of NaCl in solutions, inactivation rates
 of poliovirus type  1 increased (Salo and Cliver, 1976).  Specific ion effects are indicated
since different inorganic salts cause different inactivation rates under  conditions of equal
ionic strength.  However, Cords  et al. (1975) found that low ionic strength solutions
                                        3-38

-------
inactivated type A coxsackieviruses more rapidly than high ionic strength solutions. This did
not hold true for group B coxsackieviruses or poliovirus type 1.
      Studies have shown an enhanced thermal  stabilization of enteroviruses  in the
presence of high concentrations of some salts (Yates and Yates, 1988).  Burnet and McKie
(1930) observed that bacteriophage inactivation at 60°C was partially prevented with low
concentrations of CaCl2 and BaCl2, but thermal inactivation was increased with a higher
concentration.
      In their analysis of groundwater samples from locations within the United States,
Yates et al. (1985) found that the decay rate for MS-2 coliphage was significantly correlated
with the concentration of calcium in the 11 samples examined; decay rate increased with
increasing  calcium  concentrations.  This was not true for poliovirus  1 and  echovirus 1.
However, Yates (1984) found that calcium concentration was not significantly correlated
with MS-2 inactivation rate when concentrations of calcium in the same water sample were
varied.  Yates et al.  (1985) suggest that, in their experiment, some property of the water was
correlated  with calcium concentration and was involved in the observed inactivation rate.
      Formation of Aggregates.  According to Bitton  et al. (1980), virus persistence in
natural  water is affected by the formation of aggregates.  The formation of aggregates in
water has been found by Young and Sharp (1977) to influence the inactivation of viruses
by chemicals such as bromine.  Bitton et al. (1980) suggest that virus particles within the
aggregate are more protected from environmental factors.
      In a discussion of experiments on the inactivation rates of human rotavirus type 2
(strain Wa) and poliovirus type 1, Pancorbo et al. (1987) observed that inactivation rates
measured with single virus  particles may not  reflect the  inactivation rates of indigenous
viruses, which  are  often aggregated in water.  They review  several studies  in which
aggregated viruses (human and simian rotaviruses and polioviruses) were more resistant to
chlorine inactivation than those in single-particle suspensions.
           .!"          '                    '           '                '
      Adsorption.  The association of viruses with organic and inorganic particles in water
may enhance survival;  survival is enhanced when these solids-associated viruses settle into
the sediments  (Bitton et al., 1987).  Gerba and  Bitton (1984) report  that in  aquatic
                                        3-39

-------
 environments, clays have been shown to protect viruses from light, heat,  and biologic
 degradation.
        By monitoring pollution of two rivers in Germany, Walter et al. (1989) determined
 that, in general, virus levels in surface water depend upon virus input from domestic sewage,
 dilution by dispersion and typical metabolic interactions that have not been fully elucidated.
 The authors suggest that the industrial wastes in one river may have had a toxic effect on
 the virus  capsid  and that the oxygen content may have stimulated growth of a virus
 inactivating microflora.  They suggest that virus adsorption to solid particles was promoted
 in the second  river and that adsorption protected  viruses against other inactivating
 mechanisms.
       Virus Type. Yates et al. (1985) found no overall significant differences in  survival
 of poHoviras, echovirus  and coliphage MS-2 in different groundwater samples, but there
 were differences in individual samples.  HAV has been shown to persist longer than polio
 and echovirus at 25°C in groundwater, wastewater, and soil suspensions, although all three
 did well at 5°C (Sobsey et al.,  1986). Human enteric virus in survival chambers exposed to
 a flow of groundwater from a 275-foot deep well persisted for >24 days (Keswick et al.,
 1982); coxsackievirus B3 and poliovirus type 1 persisted longer than rotaviras SA-11.
       Pancorbo et al. (1987) found that inactivation rates for human rotavirus type 2 (strain
 Wa) were significantly different from that of poliovirus type 1 (strain CHAT) when seeded
 into vials of mountain lake water, groundwater, wastewater effluent, or polluted creek water.
 Both viruses  persisted longest in the lake water.  Human rotavirus persisted longer than
 poUovirus in the polluted samples (creek water and wastewater effluent), but poliovirus
 persisted longer in the unpolluted waters (groundwater and lake water).
       Hydrostatic Pressure.  Although  elevated  hydrostatic pressures affect microbial
 persistence in seawater, Bitton et al. (1983a) found that pressures ranging from 500-4000 psi
 did not inactivate poliovirus in groundwater. According to Bitton et al. (1987), hyperbaric
pressure is not expected  to have a significant impact on virus persistence in grbundwater.
       Sediments.  In their summary  of data on the isolation of viruses from estuarine and
freshwater sediments, Rao and Bitton (^987) observed that there are few studies On virus
detection in freshwater sediments.  Representatives of most groups of enteric viruses were
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isolated from the sediments examined.  Viruses adsorb readily to solids in water; in the
laboratory, virus persistence has been shown to be prolonged by association with solids (Rao
et al., 1986).
3.3.3. Persistence in Aerosols. The particle size range for aerosols is -0.01-50 nm (10-
50,000 nm), and infection by inhalation of these biologic particles depends on the depth of
respiratory penetration, which is greatest for particles in the 1-2 nm range or for those
below 0.25 Mm (Sorber et al., 1979).
      The potential for human exposure and associated health risk from viruses in aerosols
is derived from the concentration of pathogens in the wastewater or liquid sludge and the
aerosolization efficiency of the spray irrigation; the effect of the impact factor or aerosol
shock on the viruses within the first fraction of a second as they become aerosolized; and
the inactivation rate or decay rate, which, like aerosol shock, is influenced by meteorologic
factors and whether the viruses are  released by day or night (Camann et al., 1978; Sorber
et al., 1979).  The potential exists for adverse human health effects from wastewater  or
sludge aerosols because viruses are at the low end of the size range for aerosol particles and
can be inhaled and subsequently ingested and because the infective dose can be as small as
one virus particle or PFU.
      Sorber et al. (1984) studied microbiologic aerosols sampled near liquid sludge spray
application sites.  Liquid sludge was applied by tank trucks or by high-volume spray guns.
No enteric viruses were detected  in pooled samples representing 1470 m3 of air sampled.
Converted to < 0.0016 PFU/ni3 of air,  this value implies that virus aerosolization from land
application of liquid sludge was not a significant problem.
      Spray irrigation  of wastewater, however, did produce levels  of enteroviruses  in
ambient air at 50 m downwind of the  spray site (Johnson et al.,  1978). A geometric mean
concentration of 0.076 of poliovirus/ml wastewater resulted in a geometric mean aerosol
concentration of 0.002/m3 of air. For other enteroviruses, the geometric mean concentration
of 0.12/ml wastewater resulted in a geometric mean aerosol concentration of 0.014/m3 of
air.  The data indicate that the viruses detected were very hardy. The authors suggest that
because of the extraordinary methods  required to monitor viruses in air near the spray site,
a  more feasible  approach would  be to measure the concentration of viruses  in the
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 wastewater and use a predictive model to estimate virus concentrations in air, Such a model
 was developed by Camann et al.  (1978) to predict pathogen concentrations in sprayed
 wastewater aerosols downwind from the spray site.  Use of the model predicted a nighttime
 enterovirus concentration of 0.01 PFU/m3 -650 m from the spray site, a value similar to
 the measured enterovirus aerosol concentrations of 0.011 PFU/m3 and 0.017 PFU/m3 at 50
 m downwind from the same spray site described in Johnson et al. (1978).  There were too
 few aerosol runs and virus concentrations were measured only at 50 m downwind, so it was
 not possible  to generate age decay rate estimates for enteroviruses. The authors noted,
 however, that the enteroviruses appeared to have impact factor values in the median range
 even higher than the hardy bacteria, i.e., the enteroviruses survive better during entry into
 the aerosolized state and during the initial travel in the aerosol (Camann et al., 1978). This
 feature is  particularly significant since  the indicator organisms that are often  used to
 measure aerosols had much lower impact  factors  than the enteroviruses, implying that
 failure to detect bacteria or indicator organisms does not mean a human health risk from
 exposure to viruses in aerosols is diminished or absent.
       Camann et al. (1980) monitored school attendance and its relationship to generation
 of wastewater aerosols from an adjacent wastewater treatment plant.  No enteroviruses were
 recovered in the total air volume (1980 m3) sampled 30 m downwind of the aeration basin,
 giving a calculated enterovirus aerosol concentration of < 0.002 PFU/m3.  There was no
 adverse effect on communicable disease  incidence in the school at this exposure level.
      A more recent study by some of the same authors (Camann et al., 1988) investigated
 the  possibility of adverse human health effects from spray irrigation of municipal
 wastewater in Lubbock, TX.  Enteroviruses were recovered at a  geometric mean level of
 0.05 PFU/m3 and a maximum level of 16 PFU/m3 at 44-60 m downwind. These levels were
 higher than those observed at other wastewater aerosol sites in the United States (Fannin
 et al., 1985; Johnson et al., 1980a,b; Camann et al., 1980) and Israel (Katzenelson et al.,
 1976), characterizing the irrigation site as a source of infectious microbial aerosols.
      Moore et al. (1988) evaluated the virus levels in irrigation wastewater in the Lubbock
study.  Prior to reservoir storage, the wastewater levels of viruses ranged from 100-1000
PFU/L.  Following impoundment, viral levels were lowered to < 10 PFU/L.  Because of
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variable sample concentration processes and virus enumerating systems, there were daily and
seasonal variations in human enterovirus levels and serotypes.
       Shuval et al. (1989) completed field investigations on the spread of enteric viruses
by wastewater sprinkler irrigation at kibbutzim in Israel. Of 152 air samples taken at 30-730
m downwind of irrigation sites, 15 (~10%) were presumptively positive for enteric viruses.
Even at the distant 730-m station, 3 were positive out of 24 samples taken.  Samples were
taken using High-Volume Cyclone Scrubbers (HVCS) for viable microbial aerosols. The
fact that 31% of positive virus samples were negative for bacterial indicators suggests that
aerosolized enteric viruses are better able to withstand hostile environmental conditions than
are the indicators. In this study, the authors did not determine whether the very low virus
concentrations (0.03-2 PFU/m3) measured could lead to infection and  disease in exposed
human receptors.
       Sorber et al. (1979) conclude that, although microorganisms can  be aerosolized and
transported at spray irrigation sites, the risk of public health impacts from use of treated and
disinfected wastewater is probably minimal.
       Ijaz et al. (1985a,b,c) have investigated survival characteristics of several viruses in
aerosols.   Table 3-6 summarizes  the effects of temperature and relative humidity on
persistence and illustrates the complexity of the factors determining persistence of viruses
in aerosols.   For example,  when aerosolized  viruses  were tested at 20-24°C, human
coronavirus persisted significantly longer at medium  (50%) relative humidities (RH) than
at high or low RHs (Ijaz et al., 1985a); but  at the mid-range of 45-55% RH, there  was a
pronounced decline in infectivity of reovirus particles compared with high levels of infectivity
at high and low RHs (Adams et al., 1982).
       Generally speaking, lipid-containing viruses are more stable in aerosols and persist
better at low humidity than lipid-free viruses.   They are typically more contagious  as
aerosols and are more likely to be infective in winter when indoor relative humidity is
<50%, unlike lipid-free viruses that survive better in moist air and infect more frequently
in summer (de Jong et al., 1973). The authors caution, however, that these epidemiologic
generalizations are based on insufficient data and, in fact, contradict the higher frequency
of winter infections with rhinoviruses and adenoviruses.
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        TABLE 3-6



Virus Inactivation in Aerosols
Organism
Human
coronavirus
229E




Poliovirus
typel
(Sabin)
Human
rotavirus
(subgroup 2,
strain Wa)


Calf
rotavirus

Poliovirus
typel
(Sabin)
Reovirus
particles
(infectious,
IV, and
potentially
infectious,
PIV)
Temperature
(°C)
20±1
. ,* >. $ -

6tlb



20±1
20±1
6±1


20±1

20±1
21-24
Relative
Humidity
>5%)
30%
50%
80%
30%
50%
80%

30%
50%
80%
30%
50%
80%
30%
50%
80%
30%
50%
80%
30%
50%
80%
25-35%
45-55%
65-75%
85-95%
% Virus
Survival











36.5±5.2
70.0±1.5
27.3±1.8
0
0
100.7±9.0

tV2 0"-)
26.76±6.21
67.33±8.24
3.34±0.16
34.46±3.21
102.53:19.3
8
86.01±5.28
NRC
NR
9.07±1.82
24.5±3.5
44.2±63
3.8±1.0
21.6±4.0
57.4*7.2
1.71±0.7



avg. decay
rate:
3.2%/min
2.85%/min
3.25%/min
2.0%/min
Inactivation
Rate8
(x 10-6)
(sec'1)
3.1
1.2
25.0
2.4
0.8
0.97

9.2
3.4
1.9
22
3.9
1.5
48.9



6.67
Reference
Ijaz et al.,
1985a





Ijaz et al.,
1985a
Ijaz et al.,
1985c



Ijaz et al.,
1985b

Ijaz et al.,
1985b
Adams et
al., 1982
aCalculated; standard deviation not included in values.
bHalf-life values were predicted by regression analysis of the 24-hour survival results.
CNR, no virus recovered.
            3-44

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      Adams et al. (1982) found reovirus particles to be relatively stable as aerosols. At
high humidities (90-100%), reovimses had < 10-fold loss after 12 hours, but at lower relative
humidities the aerosolized virus decayed more quickly.  Table 3-6 illustrates the effect of
RH on reovirus decay during equilibration; average decay rates range from a high of 3.2-
3.25%/min at both 25-35% and 65-75% RH to 2.85% at 45-55% RH, but decay rates at 85-
95% RH were much lower, averaging 2%/min.  Overall decay rates averaged ~0.1%/min
over a 12-hour period,  with  rates of 0.6%/min for  infectious particles held at 45-55%
relative humidity and 0.3%/min for potentially infectious particles held at 85-95% humidity
over a 2.5-hour period.
      Brandt et  al. (1982) emphasize that one possible result of low humidity is that
rotavirus-laden dust would be more likely to form from  fecally contaminated clothes and
bedding, tending to stay suspended in air and thus reaching susceptible individuals.
3.3.4. Persistence in Agricultural Products.  Because of the concentration of viruses hi
sludges  during treatment processes, land application of sludges  may present a severe
problem in contamination of fruits and vegetables.  Adsorbed onto the sludge solids, viruses
may be protected from thermal inactivation.  Enteroviruses have persisted for 10-15 days
on vegetables at  refrigerator temperatures and up to 36 days on vegetables  after spray
irrigation (Hurst,  1989).
      Parsons et al. (1975)  have indicated  that enteroviruses persist  for 4-6 days on
vegetables. Grigor'eva et al. (1965) reported enterovirus survival times of 4, 12 and 18 days
on artificially contaminated cabbage, pepper and tomato plants, respectively. Survival times
taken from  Kowal (1985) suggest ranges of 4-23 days for  aboveground  crops such as
tomatoes and lettuce and > 60 days for below-ground crops like radishes. Konowalchuk and
Speirs (1975a,b)  studied virus  persistence  on  vegetables and  fruits stored  at  4°C,
determining that most were undetectable after 4-6 days, although viruses inoculated in feces
persisted longer than those inoculated in water.  The same authors (Konowalchuk and
Speirs, 1977) reported a 99% reduction in poliovirus  1 and coxsackievirus B5 after 5 days
on bunches of grapes hung indoors at 22°C.  Feachem et al. (1983), summarizing a number
of survival studies, conclude that practically complete elimination of viruses will occur in <5
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 days, with  negligible  survival of  enteroviruses,  on  crops  exposed  to  >2 weeks of
 temperatures above 25°C.
       Larkin et al. (1976b) spray-irrigated lettuce and radishes with sewage sludge and
 effluent and found that although polipvirus persisted on the vegetables for 36 days, there
 was a 99% loss in detectable viruses within the first 5-6 days. On a second crop planted the
 following year, the virus persisted only 14 days.  When the same authors (Tierney et al.,
 1977) investigated poliovirus survival under natural field conditions in plots flooded 1 inch
 deep with inoculated sewage, they determined that the longest survival (96 days) occurred
 in winter compared with 11-day survival in summer.  Poliovirus was recovered from the
 mature vegetables 23 days after cessation of flooding, supporting the idea of a minimum 1-
 month waiting period following final sewage sludge or wastewater application before harvest
 (Kowal, 1985).  Bagdasar'yan (1964) showed that enterovirus levels were reduced 90% in
 10 days at 3-8°C and 99% in 10 days at 18-21°C.
       LasowsM  and  Kott  (1990)  introduced  poliovirus LSC  1,  coxsackievirus  A9,
 coxsackievirus B5 and echovirus 6 into secondary wastewater, some chlorinated and some
 not, then sprayed on parsley, kohlrabi, lettuce, onion leaves, tomatoes and grapevine leaves.
 With or without chlorination, poliovirus persisted longest on parsley leaves and consistently
 persisted longer than coxsackie strains.   When applied at a concentation of 163,000
 PFU/ml wastewater, >500 PFU poliovirus/cm2 of surface adhered to the tomato. Natural
 (no chlorination) inactivation occurred in 3-7 days. Chlorination enhanced the inactivation
 process resulting in no viable enteric viruses within  1 day.
       Ward and Irving (1987) spray-irrigated field-grown vegetables with stored wastewater,
 which was seeded with poliovirus or  adenovirus  at  concentrations typically found in
 secondary effluent (S.lxlO2 - 2.6X105 infectious units/L). Poliovirus was inactivated within
 48 hours on field crops, but there was low-level persistence of the virus for -13 days.
 Adenovirus was inactivated  even more quickly, as early as 24 hours after irrigation.
 Poliovirus persisted for 76 days on celery and for 55 days on spinach that had been irrigated
with wastewater and then refrigerated at 4°C.
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       A few studies have investigated the uptake of viruses by the root systems of plants
(Kowal, 1985), but Katzenelson and Mills (1984) find little evidence that viruses penetrate
the roots or stem.
       Wallis et al. (1984) applied sludge to hayfields and pasture, but no enteric viruses
were detected throughout the study.  They caution that techniques for detection of viruses
in sludge may be limiting.
       The variability in environmental conditions, crops and methods of virus introduction
(spraying, flooding, etc.) makes any reliable  comparison of inactivation rates  on crops
difficult. Generally, the exposure of viruses on aboveground crops to desiccation, sunlight,
high temperatures, or  rainfall limits their persistence.  Viruses on below-ground or on-
ground crops will have inactivation rates more like those in soil, suggesting a safe waiting
period might be close to 100 days (Kowal, 1985).

3.4.    TRANSPORT
       Transport of viruses in soil, water and air is influenced by many of the same factors
that affect their persistence, discussed in Section 3.3 and reviewed by Yates (1990), Sattar
and Ijaz (1987), Rao et al. (1986), Duboise et al. (1979), Keswick and Gerba (1980), Gerba
and Bitton (1984), Goyal and Gerba (1979), Yates and Yates (1988) and others.
3.4.1. Transport in Soil.   According to Sobsey and Shields (1987), small particles can be
removed or retained in soils by  straining or filtering action,  by  sedimentation, or by
adsorption to soil  surfaces.  Adsorbed virions or those associated  with  each other in
aggregates may be removed by sedimentation or by straining as they move through the soil.
However, for free virions, adsorption is the primary mode of removal in soil. Adsorption
to soil particles retards virus movement through soil; desorption allows further transport
through the soil (Yates and Yates, 1988).
       In addition to adsorption, soil moisture, hydraulic conditions, pH, salt species and
concentrations, virus aggregation, virus type, soil type and properties, and organic matter
affect and interact to influence the transport or movement of viruses through the soil (Yates
and Yates, 1988; Gerba and Bitton, 1984).  Many of these factors influence virus adsorption
by soil (Rao et al.,  1986). Low pH and high-ionic-strength water, for instance,  contribute
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to the retention of viruses by soils (Keswick and Gerba, 1980). Unsaturated soil, which also
restricts movement of viruses, may enhance adsorption of viruses by holding them in close
proximity to soil surfaces (Lance and Gerba, 1984).
       Although enteric viruses bound to sludge particles are not easily released, Bitton et
al. (1978) note that unbound virions in liquid sludge may penetrate the soil.  Most studies
have not  found viruses  in groundwater beneath land application sites, but a few have
reported isolating viruses. Jorgensen and Lund (1985) found enteroviruses In water samples
3 m below the surface of a forest 11 weeks after municipal sludge application. Gerba (1987)
summarizes information  on virus isolation from groundwater near sites of land application
of sewage and concludes  that "...this information clearly demonstrates that if enteric viruses
are present in sewage being applied to the land, at least some of the viruses can be expected
to penetrate the subsurface and gain entrance to the underlying groundwater." Sobsey and
Shields (1987) caution that migration of enteric viruses through soil to groundwater at
application sites is a significant public health concern.
       Adsorption. Initially, most viruses applied to soil are retained in the upper soil layers
(Rao et al., 1986).  When soils containing 7.6-81% sand were studied for their ability to
adsorb coliphage £2 from septic tank effluent,  most viruses were found in the first 15 cm,
although several isolations were found below 85  cm.  Similar results were found by Hurst
et al. (1980b) and Landry et al. (1980) with seeded poliovirus in soil.  Lance et al. (1976)
found that flooding a soil column with a sewage virus mixture for 27 days did not saturate
the surface layer of soil with viruses. Increasing the concentration of viruses in the sewage
water increased the  numbers adsorbed at  various  soil levels but did  not  change the
maximum depth of penetration of the virus (Lance and Gerba, 1980).
       Vaughn et al. (1981) suggest that formation of a surface mat of sewage solids explains
the greater removal of poliovirus found at low application rates through a coarse sand-fine
gravel soil. However, Lance and Gerba (1980) found that the concentration of viruses was
greatest near the soil surface when soil  columns that had not previously been exposed to
sewage water were flooded with a virus-enriched sewage, suggesting that build-up of organic
matter near the surface was not responsible for the high concentrations  of virus particles
detected there.
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       Lance and Gerba (1982) suggest that viruses applied in sludge will be less mobile
that those in sewage water because they will be adsorbed to sludge solids.  Bitton et al.
(1979b) found no viruses in groundwater after spread of sludge on agricultural land in
Florida.
       pH.  The influence of pH on virus transport is related to its effects on virus
adsorption to  soil.  Studying poliovirus adsorption in 34 minerals and soils, Moore et al.
(1981) found  that adsorption was strong by  most of the neutral  and acidic materials.
However, because of the great variation in viral adsorption in alkaline materials, substrate
pH was not significantly correlated to viral  adsorption in this study. Sobsey and Shields
(1987) report on a number of studies that indicate that virus retention by soils increases with
lower pH levels.  With poliovirus, Taylor et  al. (1981) found a characteristic pH region of
transition from strong to weak adsorption for each adsorbent studied (three soils, a sand,
and a clay mineral).
       In soil suspension experiments on the effects of 7 soil properties on the adsorption
of 15 viruses to 9 soils,  Goyal and Gerba (1979) observed that a  soil  saturation pH<5
provided good viral adsorption. Gerba et al. (1981) divided these 15 viruses into two groups
based on  their adsorption behavior in soils.  For the poorly adsorbed viruses, (including
coxsackie  B4 viruses, echo 1 viruses, and phages 0x174 and MS-2), viral  adsorption to soil
was greatly affected by pH,  as  well as by CEC  and prganic matter; but for the highly
adsorbed viruses (including polio 1, echo 7, coxsackie B3, and phages T4 and T2), pH and
other soil characteristics  were not correlated with soil adsorption.  Adsorption varies not
only with virus type but also with isolates within the same type; this explains the conflicting
results from various  virus studies (Gerba and Bitton, 1984).
       Because of the strong repulsive forces that will result between viruses and  soil
particles at higher pH values, virus desorptipn and therefore virus migration will occur if
high pH values are induced in the soil environment.  However, except under unusual
circumstances, these pH  values are not  expected to be found in the normal environment
(Rao et al.,  1986).
       Mechanisms for  pH influence on adsorption of viruses to soils have not been
thoroughly studied.  Murray and Parks (1980) suggest that  van der Waals forces  are
                                        3-49

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 responsible for adsorption and that electrostatic repulsion inhibits adsorption. Sobsey and
 Shields (1987) report that recent  studies indicate hydrophobic interactions may also  be
 important in adsorption of viruses  to soils.
       Ionic Strength. Cations reduce the repulsive forces on both virus and soil particles,
 allowing  adsorption to occur  (Gerba  and  Bitton, 1984).   Several  studies using soil
 suspensions or soil columns have  shown  enhanced adsorption of viruses to a variety of
 materials with increasing ionic concentrations (Sobsey and Shields,  1987).  Taylor et  al.
 (1981) observed that both type and concentration of electrolyte affected the adsorption of
 poliovirus 2 to soils, and Sobsey et al. (1980) report that addition of divalent cations such
 as Mg2* caused some viruses to adsorb to poor sorbents.
       Since increasing the concentration of ionic salts  increases virus adsorption to soil
 particles, virus transport in the soil is retarded by increasing concentrations of these salts
 (Sobsey, 1983). During rainfall, the salt concentration decreases, and thus the ionic strength
 of the soil water; desorption and redistribution of viruses within the soil may occur with the
 potential for groundwater contamination (Gerba, 1983b). This remobilization of soil-bound
 viruses has been shown to be more pronounced in sandy than in clay soils and depends on
 virus type and strain (Gerba and Bitton, 1984). After a heavy rain at a land application site,
 Wellings et al. (1975) detected viruses hi wells that had been virus-free.
       Virus "type.   Overall virion  electronegativity,  which  is  type-dependent,  affects
 adsorption of viruses to  soils and hence desorption and migration in the soil (Rao et al.,
 1986). According to Sobsey and Shields (1987), virus-specific differences in soil adsorption
 are probably due to physicochemical  differences in virus capsid surfaces.
       Studying adsorption and subsequent elution with  rainwater from columns of intact
 cores of sandy soil dosed with wastewater,  Landry et al. (1979) found  differences in extent
 of adsorption and elution among enterovirus types and among strains of poliovirus  type 1.
This variation in ability to adsorb to soils has been confirmed in several studies (Sobsey and
Shields, 1987). Goyal and  Gerba  (1979) report differences in adsorption efficiencies by
enterovirus types  and strains.  Comparing adsorptive  capacities of  various strains  of
poliovirus type 1, echoviruses 1,7 and 29, and coxsackieviruses B4 and B3 in the laboratory,
Gerba et al. (1980) found that adsorption was both type- and strain-dependent.
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      On the other hand, when comparing movement of echo 1 and echo 29 viruses with
polio 1 in soil columns, Lance et al. (1982) observed similar movement of the viruses in soils
and suggest poliovirus as a model for virus movement in soil. Although echo 1 did not
adsorb well near the surface, leaching patterns were similar for echo 1, echo 29, and polio
1 below the 40-cm depth.
      Sobsey et al. (1986) studied virus adsorption and persistence in soil suspensions, and
transport in soil columns of poliovirus  type  1, echovirus type 1, and HAV.  Poliovirus
adsorbed more extensively to soils than HAV, which adsorbed better than echovirus. In soil
columns, echovirus was transported through the column  to  the effluent  in  greatest
concentrations, and poliovirus concentrations were lowest in the effluent. The authors
suggest that neither poliovirus type 1 nor echovirus type  1 is suitable as a model for
adsorption, persistence,  and transport of HAV in soil.
       Soil Moisture and Flow Rate. Movement of viruses through soil is affected by soil
moisture. In laboratory  studies with deionized water, Lance et al. (1976) report that viruses
near the surface desorb  and migrate through soil columns; they suggest that the viruses will
continue to travel vertically through the soil with periodic rainfall.  Periods of drying
between application of  water reduced desorption, suggesting that virus movement in soil
would not  be great unless rainfall occurred within the first few hours after application of
sewage.  In a study of  a land application site, Wellings et al. (1974) report that rainfall
influences  penetration into the soil  depths. No viruses were detected in wells 3  and 6 m
below the soil surface until after periods of heavy rainfall.
       Lance and Gerba (1984) found that with unsaturated flow conditions, poliovirus did
not move below 40 cm in a column  of loamy sand; but with saturated conditions,  the virus
penetrated at least 160  cm. Gerba and Bitton (1984)  suggest that with unsaturated-flow
conditions, water fills  only the smaller soil pores or remains as a film around soil particles,
allowing viruses to get closer to particle surfaces. Under saturated soil conditions, coliphage
0X174 was found to move laterally -350 m/day (Noonan and McNabb, 1979a).
       When soil columns of Red  Bay sandy loam were treated with chemical sludge or with
anaerobically-treated, polyelectrolyte-conditioned, dewatered sludge and leached with
natural rainwater,  none of the seeded poliovirus type 1  were found in the leachates during
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  saturated flow conditions for a sustained period (Pancorbo et al.,  1988).  According to
  Pancorbo et al. (1988), association with the solids in the sludges may have immobilized the
  viruses in the top portion of the soil columns.  The high level of cations in these sludges
  probably enhanced the adsorption of viruses to sludge and soil particles.  However, they
  caution that poliovirus adsorbs more readily to soils and does not migrate  through soil as
  easily as other enteric viruses.
        Virus removal by the soil depends on the rate of application of water or effluent
  (Yates and Yates, 1988). Thus, the application rate of wastewater and sludges wUl affect
  the number of viruses passing through the soil and entering the groundwater (Rao et al.,
  1986).  Increasing the application rate from 0.6 m/day to 1.2 m/day increased the number
  of virus particles moving through the  soil column in the effluent, but increasing the flow
 rates up to  12 m/day gave  no further increase in movement (Lance  and  Gerba, 1980).
 Lance and Gerba (1980) suggest that virus adsorption is not affected by increases in flow
 rate up to a breakthrough rate point that corresponds to the rate at which water begins to
 move only through the large son pores with  little or no contact between, viruses and soil
 particles. Viruses have not generally been detected in well samples  or lysimeter samples
 with infiltration rates below 1 m/day  (Lance and Gerba, 1982).  Lance and Gerba (1982)
 state that both column and field studies suggest that water flow velocity is possibly the most
 important soil  characteristic affecting virus movement.
       Soil Type.  Soil properties have an important effect on virus transport; migration is
 promoted by coarse-textured soils that  do not adsorb well (Yates and Yates, 1988). Virus
 aggregates or paniculate-associated viruses may be strained or filtered out by the  smaller
 soU pores (Yates and Yates,  1988). Wang et al. (1980) report that adsorption is inversely
 proportional to the  permeability of  soil.   In field  studies, movement  of viruses to
 groundwater was  a problem primarily when the soil contained  coarse sands or  gravels
 (Lance and Gerba, 1982). According to Gerba (1987), viruses can travel long distances in
sandy and gravel soils, but studies have|not been done in the field to determine just how far
viruses move from application sites.
      Clay is most active in virus adsorption because of its high CEC and large surface area
(Yates and Yates, 1988). Using soil columns flushed with virus-laden settled sewage,  Sobsey
                                        3-52

-------
et al. (1980) report that four soil types varied in retention of poliovirus type 1, sandy clay
loam retaining more than sandy or organic soils.  Funderburg et al. (1981) flooded soil
columns with simulated rain after application of poliovirus 1 and reovirus 3 in wastewater
and  found that of the eight different soils tested, those with  a high CEC demonstrated
stronger retention. However, Goyal and Gerba (1979) did not find a significant correlation
between CEC and enterovirus adsorption to soil.
       Studying five different soils, Burge and Enkiri (1978) found that bacteriophage 0X174
adsorption differed among soils and, in most cases, was correlated with soil CEC, specific
surface area, and organic matter content.  Organic compounds reduce virus adsorption since
they compete with viruses for adsorption sites (Gerba and Lance, 1978).  In their review of
virus persistence and transport in soils, Sobsey and Shields (1987) report evidence that virus
association with organic substances may protect them from inactivation by preventing their
adsorption to  soil, thereby enhancing persistence and mobility hi soils.  Because of their
influence  on virus adsorption,  humic and fulvic acids increase  virus transport through the
soil  (Bixby and O'Brien, 1979).
       In  their study of the  adsorption of enteroviruses to soils, Goyal and Gerba (1979)
found that the nine soils examined differed in their abilities to adsorb several enteric viruses.
Although the  most important soil property influencing retention was pH, exchangeable
aluminum correlated with high adsorption for some viruses. Minerals such as iron oxides
have also been shown to increase the retention of viruses in the soil (Sobsey and Shields,
1987).
3.4.2.   Transport in Surface Runoff.  Virus particles adsorbed to solids in natural water
may settle in the bottom sediments.  According to Rao et al. (1986),  when associated with
large particles (>6 /zm),  viruses settle into sediments; but when adsorbed onto smaller
particles (<3  AHU), they may remain suspended for a longer  time.  Accumulated solids-
associated viruses settle  into a  loose,  flurry  layer that is easily  resuspended  by mild
turbulence and transported to distant locations.  Increased stream velocity or seasonal
turnover may also resuspend the sediments, releasing the viruses into the water (Bitton,
1978).
                                        3-53

-------
        There have been few studies on the transport of microorganisms in runoff from
  sludge-amended fields.  MSDGC and HT (1979) analyzed water samples from streams,
  reservoirs, wells and runoff for viruses for 15 months during a 7-year reclamation project in
  which an aerobically digested and lagooned sludge was applied in large quantities to 15,000
  acres. Of the 68 water samples, only 3 contained viruses (an echovirus and 2 unidentified
  virus isolates).  These isolates  were found in surface waters.  No viruses were found in
  runoff water from fields, in groundwater or in sludge and soil samples.
        Both human and nonhuman enteric viruses were found in large numbers in waters
  from sites along the  Assomption River  and its tributaries (Payment, 1989).   Untreated
  wastewaters from the two major cities in the area are discharged into surface water, and
  runoff occurs from heavy land disposal of untreated farm animal wastes. Concentrations of
  all viruses varied with season, increasing in early fall.
  3.4.3.  Transport in the Subsurface and in  Groundwater.  In  sandy  gravel aquifers,
  groundwater flows largely through pores at rates of < 1 m/day to a few m/day; in hard rock
  aquifers, transport is  through fissures at 0.3-8000 m/day, or ^26,000 m/day  in karstic
  aquifers (Matthess and Pekdeger, 1985).  The larger diameter flow paths In the  hard  rock
 and karstic aquifers permit rapid passage of suspended microorganisms. Due to the small
 diameter of viruses, the filtering action of the porous aquifers is not very effective. Since
 viruses are subject to adsorption on underground particles, passage through loamy aquifers
 with high cation concentrations can effectively remove viruses, especially those that adsorb
 well.  The  sorptive small particles and microbial slime  at the boundary of water  and
 sediment is very effective in reducing virus transport.  Desorption may occur with decrease
 in cation concentration, as in heavy rainfall, with further virus transport.  The continuous
 adsorption/desorption reactions  retard'movement of viruses relative to groundwater flow,
 providing time for inactivation processes to affect viruses.
       Mack et al. (1972) observed that poliovirus traveled at least  90 m underground.
 Keswick and Gerba (1980)  report that viruses have penetrated to depths of 67 m  and
 travelled horizontally for distances as great as 408 m.  When wastewater was applied by
rapid infiltration at a land application site, tracer phage and pathogenic animal viruses were
found in groundwater at a horizontal distance of 183 m from the site (Scha.ub and Sorber,
                                        3-54

-------
1977).  The tracer virus penetrated to the groundwater at the same rate as the effluent and
was isolated from an 18.3-m well beneath the application site within 48 hours.
       Coliphage, used as a tracer for groundwater movement in carbonate rock terrain in
Missouri, traveled 1600 m in 16 hours (Fletcher and Meyers, 1974; Gerba, 1984b).  Noonan
and McNabb (1979b) observed a rate of movement of -300 m/day (covering -920 m) for
phages in groundwater at a land disposal site in New Zealand.  In shallow groundwater in
South Wales, phage used as a tracer moved at a velocity of 36-180 m/day to be isolated in
monitoring wells 690 m from the site (Martin and Thomas, 1974).
       A number of models have been developed to predict the fate/survival and transport
of microorganisms in the subsurface (Grosser,  1984; Vilker et al., 1978; Corapcioglu and
Haridas, 1985; Yates and Yates, 1988; and Matthess and Pekdeger, 1985).  Most of these
models address the transport of viruses under conditions of steady-state flow and saturated
soil. Tim and Mostaghimi (1991) developed a model that predicts the transport of viruses
with soil water in transient flow conditions in unsaturated soil. The model incorporates
mass conservation equations for simultaneous transport of water  and viruses through
variably saturated media, an equilibrium relationship representing the rapid, instantaneous,
and reversible adsorption of virus by the soil  matrix and a first-order reaction describing
viral inactivation in the subsurface environment (Tim and Mostaghimi, 1991).  There is a
phase distribution of virus particles-between the liquid phase, or suspended virus particles
associated with water moving through the soil, and a solid phase, the adsorbed particles in
the soil matrk-that determines the mass available for transport in groundwater. Thus, the
model represents the three key elements describing viral transport in the subsurface: (1)
transport process, including convection and hydrodynamic dispersion; (2) phase distribution
of the viruses between soil and water; and (3) inactivation of the viruses, which ultimately
determines whether viruses persist long enough in the subsurface  to become a problem by
entering the groundwater or reaching surface water.
       Tim and Mostaghimi (1991)  point out that models addressing virus transport in the
subsurface have typically been limited  to conditions of steady-state flow in saturated soil,
whereas their model addresses  transient flow conditions of the  unsaturated  zone.  In  a
transient flow system, changes  in  the system can alter the equilibrium.  Therefore, to
                                        3-55

-------
 determine the long-term risk of virus accumulation in soil, those parameters affecting virus
 interactions in soils must be identified and the resulting fate and transport of the viruses
 modeled in variably saturated media under the condition of transient, instead of steady-state,
 flow.
 3.4.4.  Transport by Wind. Although the dispersion of aerosol-borne viruses is partially
 dependent on the size of the particle, the time the particles can remain suspended and the
 distance they can travel are influenced by the airflow, or wind speed, and turbulence (Sattar
 and Ijaz, 1987).
       There is significant overlap in the information on persistence and on transport of
 viruses in  aerosols.  For that reason, the studies discussed in Section 3,3.3. will only be
 summarized with respect to virus transport.
       Johnson et al. (1978) found that spray irrigation of wastewater produced a geometric
 mean aerosol concentration of poliovirus of 0.002/m3 of air at 50 m downwind of the spray
 site and a geometric mean concentration of other enteroviruses of 0.014/m3 of air. Camann
 et al. (1980) recovered no  enteroviruses in the total air volume (1980 m3) sampled 30 m
 downwind  of a wastewater aeration basin, giving an enterovirus aerosol concentration of
 < 0.002 PFU/m3.  As part of the Lubbock  study, enteroviruses were recovered  at a
 geometric mean  level of 0.05 PFU/m3 and a maximum level of 16 PFU/m3  at 44-60 m
 downwind from a spray irrigation site using municipal wastewater (Camann et al., 1988).
 Shuval et al. (1989), performing field investigations on the spread of enteric  viruses by
wastewater sprinkler irrigation at kibbutzim in Israel, reported that of  15/152 air samples
taken at 30-730 .m downwind  of  irrigation sites were presumptively positive  for enteric
viruses. Even at the distant 730-m station, 3/24 were positive. It is evident from these
results that viruses can be transported in aerosols, but quantifying the health risk from these
airborne viruses requires more information  than is currently available.
                                       3-56

-------
                      4. PARAMETERS FOR MODEL RUNS

4.1.   RATIONALE FOR PARAMETER SELECTION
      The assessment of human health risk from pathogenic viruses as a result of land
application of sewage sludge requires a realistic description of the fate and transport of the
pathogens.  The  preceding chapter describes information that has been  found in the
published literature describing infectious doses, viral density in treated sludge, and survival
and transport of viruses in soil, surface water, groundwater, and aerosols. Limited data were
found for several  of the many viruses capable of causing disease. Generally, the ranges of
reported inactivation rates in different media were wide, varying among reports on the same
virus as well as among media. In addition, descriptions of transport in soil were not easily
converted to parameters useful in the Pathogen Risk Assessment Model.  For this analysis,
the most conservative values observed were included among the test runs to determine then-
effect on the modeled outcome.
      Earlier reports in this series  (U.S. EPA, 1990;  1991) identified  a  number of
alterations that would improve the operation of the model. However, for this study, the
model was used without changes. Therefore, the computations made during model runs are
comparable to those in the previous studies. In cases where default values presented in the
initial description of  the model were judged to be unrealistic, the default values  were
replaced on the basis  of best scientific judgement
      In these model runs, it is assumed that viruses are transported into subsurface soil
and subsequently into  groundwater and are included in any droplet aerosols formed by spray
application, as well as in  any particulate aerosols formed by disturbance of the soil by wind
or by cultivation. It is also assumed that they will die at a characteristic rate that depends
on the  ambient temperature and the medium in which they are found; thus,  there are
different inactivation  rates for the same organism in moist soil, dry particulates, droplet
aerosols and water. Default parameter values are included in the program's code.  If it is
necessary to update these values as indicated by new information or to revise them to
conform to specific conditions, new values are entered during the initiation of the model run
(U.S. EPA, 1989a).  In a number of cases, the default  values used are known to be

                                        4-1

-------
  unreasonable for the average case but are chosen to be protective; in some cases, no data
  are available to support more than a best scientific judgement; in other cases, modeling and
  research are available to document the values used.  More detailed discussion of the choice
  of parameter values can be found in U.S. EPA (1989a).
        The most significant parameters for risk of infection have been shown to be density
  of pathogens in sludge, inactivation or die-off rates, transfers among exposure media, and
  infectious dose (U.S. EPA,  1989a;  1990;  1991).  The values  chosen for  initial density
  represent a reasonable upper  bound for density  of viruses in treated sludge  for the
  applications indicated (Table 3-3). Because of new information, the base value for viruses
 in composted sludge, 2500/kg dry weight, is markedly larger than the value recommended
 in the model conceptualization documents  (U.S. EPA, 1989a). In some tests of the limits
 of sensitivity of the model, virus  concentration was set at 500/kg.  The other values remain
 the same as described previously (U.S. EPA, 1989a). Transfers among exposure media are
 not well characterized and are estimates only. The infectious dose of many viruses may not
 be known, and even for well-characterized viral types, infectivity may depend on many
 conditions.  Therefore, because  experimental infections have indicated that  a single virus
 may cause infection, an infectious dose of 1 was used as  the default value.  Reported
 inactivation rates vary widely, depending on both the virus and the physical  and chemical
 conditions; additional research is necessary to characterize inactivation rates accurately. In
 the test model runs, each of the crucial parameters was varied, usually above and below the
 default value, to determine the importance of that parameter in model outcome. Parameter
 values were  tested  extensively  for the first and second  sites,  and only  the sensitive
parameters so identified were tested in subsequent model runs for the other sites.    :
      The temperature-specific rate used in the model for inactivation of viruses in moist
soil is calculated by an algorithm derived from a line fitted  to a logarithmic transform of
survival data found in published literature (U.S. EPA, 1989a).  This equation is as follows:
         RHO =  10
               _    10(A*TEMP2-B)
                                       4-2

-------
      Where:
        RHO
        A
        TEMP
        B
=  Fractional survival
=  SLOPES [P(37)]
=  Temperature of soil (°C)
=  NTRCPS [P(38)]
To generate alternative values for the present sensitivity analysis, the slope was increased
or decreased by a factor of two, and the intercept was adjusted by trial until the inactivation
rate at 0°C remained approximately the same. Alternatively, values were chosen by trial
to fit information presented in Hurst (1989). The relative logarithmic inactivation per hour
at various temperatures for moist soil was calculated; the results are shown in Table 4-1.
It is clear from this table that the slope and intercept of the inactivation curve are important
in determining the extent of inactivation by temperature.
      The lowest temperature reported for observations of inactivation of viruses in water
was -20°C. This temperature is unrealistically low to be used throughout the operation of
a model designed to represent the growing season.  However, it could be used to calibrate
an algorithm for temperature-dependent inactivation similar to the one described above for
soil.   The model does not provide for temperature-dependent inactivation of viruses in
water, because there were insufficient data to parameterize the relevant algorithm.
      As alternatives to these values, temperature-insensitive values were substituted during
some of the model runs.   Reported inactivation rates  range from T.lxlO"5 to 1.6X10"1
logs/hour in soil (Table 3-4), 1.6x10"* to 1.4X10'1 logs/hour in water (Table 3-5), and 4.9xlO'5
to 8xlO"7 logs /second in  aerosols (Table 3-6); these values were substituted to test the
inactivation algorithm.
      A number of parameters are interdependent. For example, the  rate and depth of
irrigation should not combine to exceed the infiltration rate and moisture-holding capacity
of the soil; otherwise, runoff of irrigation water will occur. However, in the present test, no
attempt was made to prevent runoff by irrigation. For parameters directly affecting specific
crops or animal feeding practices, the relevant practices must be  specified when those
parameters are varied. For example, when testing FCROP1 [P(46)] and  FCROP4 [P(49)],
                                        4-3

-------
                 TABLE 4-1




Temperature Algorithm for Inactivation of Viruses

A=
B=
Temp.
0
4
8
12
16
20
24
28
32
36
Standard
0.00145
2.957

0.0006
2.957

0.002
2.957
Log10 Inactivation per
-0.00110
-0.00116
-0.00136
-0.00178
-0.00259
-0.00419
-0.00755
-0.01512
-0.03371
-0.08359
-0.00110
-0.00112
-0.00120
-0.00134
-0.00157
-0.00191
-0.00244
-0.00326
-0.00454
-0.00661
-0.00110
-0.00118
-0.00148
-0.00214
-0.00358
-0.00696
-0.01566
-0.04083
-0.12331
-0.43151
Test Values
0.0007
3.0
Hour
-0.00100
-0.00102
-0.00110
-0.00126
-0.00151
-0.00190
-0.00253
-0.00353
-0.00520
-0.00807

0.0029
3.0

-0.00100
-0.00111
-0.00153
-0.00261
-0.00552
-0.01445
-0.04681
-0.18775
-0.93239
-5.73323

0.0015
3.3 •

-0.00050
-0.00052
-0.00062
-0.00082
-0.00121
-0.00199
-0.00366
-0.00751
-0.01721
-0..04405
                    4-4

-------
both of which determine transfers to an aboveground crop, the aboveground crop option
(CROP [P(66>] = 1) must be used.  Otherwise, the exposures affected by FCROP1 and
FCROP4 will not be calculated. Similarly, some transfers are not relevant to all practices,
so the parameters governing them need not be tested in all practices.

4.2.   PARAMETER VALUES
      The values of the main program parameters used in the initial study are given in
Table 4-2.  The values of the parameters are based on the model description (U.S. EPA,
1989a), which explains their meaning and use in  more detail.  The parameters include a
number of transfer factors, which regulate the fraction of the pathogens  in a particular
compartment that are transferred to another compartment each hour or at specific tunes.
For example, parameter 46 (FCROP1) specifies the fraction of viruses in the soil surface
that are transferred to the surface of the aboveground crop each hour the crop is present.
Other parameters may be flags to indicate that a specific subroutine should be included.
In cases in which the parameter is a number with a unit, the unit is indicated in the table.
       Values for  Subroutine RISK, which calculates  direct  onsite  exposures  and
modifications related to processing of crops and meat, are given in Table 4-3.  Parameters
for Subroutine GRDWTR are given in Table 4-4. Site-specific descriptive parameters for
rainfall are given in Section 5, Sites for Model Runs. All five practices were  tested with
appropriate combinations of these variables to determine which ones are significant for
infection.  In addition, application of sludge to municipal parks and  golf courses was
modeled by use of Practice V combined with characteristic parameters for lawns in Practice
III to assess runoff to  an onsite pond. Parameters shown to have no effect on the outcome
of model runs at the first site were not tested extensively in model runs for the other sites.
4.2.1.  Main Program Parameters. The names, definitions, and values of the main program
parameters are listed  in Table 4-2. The default values are listed in bold-face type, and the
rationale for choice of each value is summarized. Each value  of each parameter was used
in model runs for all practices at Site 1. Some variables were then eliminated from the
sensitivity analysis because they seemed to cause no significant effect on the model outcome.
In one set of model runs for Site 2, more conservative inactivation rates were used; they
                                        4-5

-------

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subsurface s

B
C/2

fe
cs
Tl-







Calculated by the program.



1
1
2
1
3
en
1— 1
'o
en
ef
o
'•§ «ti
Transfer fra
surface runo

§
oi ^
fri
CO
"*







Very low transfer to subsurface.
Default value.
Very high transfer to subsurface.


"to*^ t>
333
2
1
t!
3
en
f—4
"o
en
cf
.° — <
tj 'S
Transfer fra
subsurface s

O
oo
PQ
^3
to
^.
"*
 o
T3 en
Vary to determine sensitivity of mo
resuspension from surface soil into
water.


CS=>^ to
333
o
1
t-l
3
en
^_
"S
_J> l-H
C CD
O **
"'S ^
C M
r"^ en
Q
1
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m
^

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•73
42
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T3

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3
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rH
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3 TO
13 T3
> (D
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t?5 ^ "^
4) "e3 13
Q O "Si,


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en

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s
















                                             4-12

-------
 (U

.1
3
S




*
J3
13
^^
t-i
,o
CD
1
^O

Cfl
 °
"o ^
M S
s .I?
Calculated as 0.
yield.
Calculated as 0.
yield.
Default value.


fc - fc
TJ in 'Ti
^S « . l^i
1-1 CS ^O
o
8
*g
en .
;^3
O
jj-
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c2 3

o *^
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^ °3
cT ^
I t OH
u S
"*^**. W
s« "S
S »0
Calculated as 0.
yield.
Calculated as 2.
yield.
Default value.


•fe - T*
% a 4s
2
1
s
CA
r^i
O .
ef

'•g
«s.
V-l
II
ST5
O-i
§
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oo










1 1
Arbitrary lower
Default value.
Arbitrary upper


Q^5 S^ C^^

=
^j
on
0
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OH
'O

c
.2
ts
c3
5Q y
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FCROP4
5










I 1
Arbitrary lower
Default value.
Arbitrary uppei


«>)
^D 'S ^
1— 1 T^H 1— 1
..-<
O
on
0
•4-*
O
OH
O
&
c"
.2-
"'S
2
tM
E^ "1 '
FCROP5
0










3
1
Default value.
Arbitrary uppei


^5
T™^

"o
on
O
i— i
i
OH
O
o
£
.2
1
43
1 §
rt tl
£ S
FCROP6
i— i



5
X>
_C/J
OH
Q
fci
<-M
0
1
• Vs
Arbitrary value
subsurface.
Default value.


^^j ^»
^^ ^^^
a 3
"o

-------
T3
 (U
 g

1
-s-

3

a


en
CD
i>
U
0)
^3
a
_o

en
3
?




a
.2
^••-j
S

O rt
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^
*







ci
3
1
0)
> 

"O
o
I
en
1
Transfer fraction,
during mowing

O
2
00
PH
10
10







CO
3
1
1 Arbitrary uppei



o

















co 1)
3 ^
1 5
> O CX
ll§-
&* ^*
21— i CQ
*- 3 ~
•1-H Cv "f*
III


^•o-o
333
o

1
t3
S
C/J
DH
O
U
Transfer fraction,
soil surface

00
\r*
%
&>







CD
3
1
is "3
•IM Ctf


'tote
33

o
1
1
en
Transfer fraction,
crop surface

CO
p
00
00
fc







(5
3
1
Arbitrary uppei


^
3













i— i
1
H
en
ft) *>
3 *r**
1 §
Arbitrary lowei
Default value, ]



s §
o
•S
1
3
en
1
Transfer fraction,
soil surface water

^
co
P
B
oo


>'
T3
>
t-T.

O 4)
"-J3 3
§1
Default value, ]
Arbitrary uppei


wji
	 »
^^ H™"'.



c3
Transfer fraction, i

S
U
S
ON
1 *
> •&
+*> ^J
jg !S
1 ^J ^^*
i ^
B ^
22
en en
cii .52
•g 3
1*^ »^"» a
"S*"8'!
1 i--
|||



O T-"



fl)
consumption to m<





                                               4-14

-------
I
3
a
1


en

«2
(U
1
.0

un
V
•i
^






C3
s
.g
fl\
Q


4) c^
^-H C
cd *7
I
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|
o
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M
1
i-i
I
0




o




13
.
^
cf
o
1
43
i_
«§
E§
«
i
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a
^o
5
S
1
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O
1
S-a
o w
"S tn
C3 0)
^ bO
C3 cl




T-H



:


3
c
o
1
CL
consum










J —
^ ^
Arbitrary lower
Default value.
Arbitrary uppei



^
O O r-t

rS
cx>
0
2
d
9
a
cf
o
1
l_c
. •§ 8
§ 43
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C/2

r— i
SO






^
1 1
Cd F— i Cd
*"* s
18!



rM
000

1)
T3
"^
2
5
§
a
cf
,0
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43
1 _t
1

H
ts
SO







. i
Type of crop is
consumption ol




*? 0
T3~
a
o
bb
JD
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44-1
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1
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1
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0
43



                                            4-15

-------
T3
 OJ
       13
                                            a-
•£•4  JTJ
                                 -
                                 O
                                          2!
                                           •§•
O    TS ••£
      en  52
Q^     Q)  C
a     >  ?u
                    .
                        i
                                    •O    -TH
3    "M    5
                                   "O    _4    713
                                   »—H    T3    i-'-i
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                                                                       a    5
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                                                                                    00
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                              0)
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                             u
                             oo
                                                                 bfl
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                                                                        bo
                                                     bo
                        (D
                                                            
-------
*o
 (U
 g
1
3
S




en
O
^









.•^
p- H
Default value.
Arbitrary uppei



 GO;.;,,
0 0 :













&••














•I "i

Arbitrary lowei
Default value.
Arbitrary uppe:



rH f*^ OO
O O O


J

£
13
J3

ja
d
•g
§)
"S |

.2 "^
£ 1)

P
PH
vd
r~-








d ,
cd
2
Calculated by j




!

^J
^J

_d

d
o
"i
»H
V
g
o 'So
d ^~

en
1
I

^^ '

Default value.
Store for (arbil


o
o -
Q
:
'
)
<
i
^
)
>
5
                                                4-17

-------
                                                      (D
       v 


S   K-
                     "e3   "?3
                          
                                           t
                                               
o o o
I™™1 m oo
         3

         P
                  O ^5 
      
-------
         TABLE 4-3




Parameters for Subroutine RISK
Parameter
# Name
5 DRECTC
(g/day)
6 DRECTS
(g/day)
8 ICAN
11 IFREE
29 TSTM2
(hours)
32 TSTR7
(hours)
33 VOLPND
(m3/ha)
34 XDIST
(m)
35 YDIST
(m)
Value*

0.02
0.1
1.0
0.02
0.1
1.0
0
1
0
1
360
720
2880
360
720
2880
2X101
IxlO2
IxlO3
100
200
0
10
30
Rationale for Choice of Values

Determine effect of amount of contaminated crop
surface ingested during routine daily work in the
field.
Determine effect of amount of contaminated soil
ingested during routine daily work.
Default, for fresh vegetables.
Flag for canning sequence.
Default, for fresh vegetables.
Flag for freezing sequence.
Reduced storage time of meat before processing.
Default value.
Increased storage time of meat before processing.
Reduced storage time of vegetables after
processing.
Default value.
Increased storage time of vegetables after
processing.
Very small onsite pond.
Default value.
Increased value for pond.
Determine effect of distance to receptor of offsite
aerosol.
Determine effect of lateral distance of receptor
from centerline of aerosol plume.
*Default values are in bold-face type.
             4-19

-------
           TABLE 4-4



Parameters for Subroutine GRDWTR
Parameter
# Name
2 V

3 D

4 R

9 XI

10 DX

11 XM

Definition

Velocity
1 '(cm/hr)

Dispersion
coefficient

Retardation
coefficient

Starting
distance (m)

Distance
increment (m)

Maximum
distance (m)

Value*

0.9
3.6
10.8
20
60
100
0.2
1.0
2.0
20
50
200
20
50
200
20
50
200
Rationale for
Choice of Values
Arbitrary lower value.
Default value.
Arbitrary upper value,.
Arbitrary lower value.
Default value.
Arbitrary upper value,
Arbitrary lower value.
Default value.
Arbitrary upper value.
Arbitrary lower value.
Default value.
Arbitrary upper value.
Arbitrary lower value.
Default value.
Arbitrary upper value.
Arbitrary lower value.
Default value.
Arbitrary upper value.
*Default values are in bold-face type.
             4-20

-------
were 0.0017 logs/hour for moist soil, 0.0015 logs/hour for water, and 8xlO"7 logs/second for
aerosols.
422.  Parameters for Subroutine RISK.  Subroutine RISK is used to calculate exposures
in various exposure  compartments,  using  data for pathogen  concentration  in each
compartment as calculated by the model.  Parameters for Subroutine RISK were varied to
simulate differences in  exposure to viral pathogens by direct contact with soil or crop
surfaces, viruses in processed foods, the volume of the onsite pond (which had a direct effect
on concentration of viral particles in surface water), or location of the receptor of offsite
aerosols.  Values of the parameters for Subroutine RISK are given in Table 4-3. Default
values are printed in bold-face type.
42.3.  Parameters for Subroutine GRDWTR. Values for this subroutine are difficult to find
in the published literature.  Ongoing research and development of groundwater transport
models for pathogens should yield valuable new information to allow more realistic choices
of parameter values for Subroutine GRDWTR.  Values used in the study are given in Table
4-4.
42.4.  Parameters for Subroutine RAINS.   Because  the Modified  Universal Soil Loss
Equation, which is the basis for Subroutine RAINS, depends on soil type, topography and
land use practices, parameters for Subroutine RAINS are influenced strongly by the choice
of site.  The sites suggested for trial runs of the Pathogen Risk Assessment Model include
one location each chosen from potential farming areas of TN, CA, FL, NM, IA and WA.
       Parameters  for Subroutine  RAINS  are defined  in  Table 4-5.   Values for the
parameters were chosen to be appropriate  for soil type,  topography and meteorologic
patterns for these locations  (see Chapter 5). Although the model is limited in its ability to
represent the rainfall pattern of any location because of its restriction to <10 rainfall events,
inclusion of these events early in the model run ensures that the effects of rainfall on surface
runoff/sediment transport are maximized.
                                       4-21

-------
         TABLE 4-5



Parameters for Subroutine RAINS
Parameter Definition
# Name
2
3
4
5
6
7
8
9
10
11
PDUR
PTOT
BTLAG
, CN
AMC
STAD
USLEK
USLEL
USLES
USLEC
Duration of rainfall (hr)
Total rainfall (cm)
Basin time lag (hr)
Curve number
Antecedent moisture conditions
Storm advancement coefficient
USLE K value (soil credibility factor)
USLE L value (slope length factor)
USLE S value (slope steepness factor)
USLE C value (cover management factor)
            4-22

-------
                          5. SITES FOR MODEL RUNS

      Six sites were chosen to provide a variety of soil types, topography and meteorologic
patterns.  Other than Anderson County, TN, for which more detailed meteorologic data
were  available  to  the  authors, specific sites were chosen  arbitrarily with the goal of
geographic diversity.  Data on soil properties were taken from U.S. Soil Conservation
Service  soil surveys,  which have been developed for each county in the  United States.
Meteorologic data were taken from the National Oceanic and Atmospheric Administration
Local Climatological Data Annual Summaries for 1981 (NOAA, 1981).  The sites chosen
for the model runs are  described below.
5.1.    SITE 1: ANDERSON COUNTY, TN
       Values of site-specific variables were chosen to reflect conditions at an agricultural
location in the Clinch River Valley of East Tennessee.
5.1.1.  Description of Soil. The soil chosen for the model run is the Claiborne series, which
comprises fine-loamy, siliceous,  mesic Typic Paleudults. It is further described as follows
(USDA, 1981a):
             The Claiborne series consists of deep, well drained soils that formed
       in sediment deposited by water or in residuum of dolomite.  These soils are
       on ridgetops, on hillsides, and at the base of slopes. The slope range is 5 to
       45 percent, but in most areas the gradient is 12 to 30 percent....
             The solum is more than 60 inches thick.  Depth to dolomite bedrock
       is more  than 72 inches.   The soil  is strongly acid or  very strongly acid
       throughout except for the surface layer where limed. The content of coarse
       chert  fragments  ranges  from 5 to  25  percent in  each horizon.   These
       fragments commonly increase in size and abundance with increasing depth.
Claiborne soils are of hydrologic group B, characterized by moderately low runoff potential,
moderate infiltration rates and moderate rates  of water transmission.
       For this analysis, it was assumed that  sites with slopes > 10% (6°) would not be used
because of the likelihood of excessive runoff.
5.1.2.  Narrative Climatologic Summary.  The following climatologic summary for Oak
Ridge, Anderson County, TN, was taken from NOAA (1981):
                                        5-1

-------
       Oak Ridge is located in a broad valley between the Cumberland Mountains,
       which lie to the northwest of the area, and the Great Smoky Mountains, to
       the southeast. These mountain ranges are oriented northeast-southwest and
       the valley between is corrugated by broken ridges 300 to 500 feet high and
       oriented parallel to the  main valley.  The local climate is noticeably
       influenced by topography.  Prevailing winds are usually either up-valley, from
       west to southwest, or down-valley, from east to northeast. During periods of
       light winds daytime winds are usually southwesterly, nighttime winds usually
       northeasterly. Wind velocities are somewhat decreased by the mountains and
       ridges. Tornadoes rarely occur in the valley between the Cumberlands and
       the Great Smokies. In winter the Cumberland Mountains have a moderating
       influence on the local climate by retarding the flow of cold air from the north
       and west.

       The coldest month is normally January but differences between the mean
       temperatures of the three winter months of December, January, and February
       are comparatively small. The lowest mean monthly temperature of the winter
       has occurred in  each of the months December, January, or February in
       different years.  The lowest  temperature recorded  during  the  year has
       occurred in each  of the months November, December, January,  or February
       in various years. July is usually the hottest month but differences between the
       mean temperatures of the summary months of June, July, and August are also
       relatively small. The highest mean monthly temperature may occur in either
       of the months June, July, or August and the highest temperature of the year
       has occurred in the months of June, July, August, and September in different
       years. Mean temperatures of the spring and fall months progress orderly from
       cooler to warmer and warmer to cooler, respectively, without a secondary
       maximum or minimum. Temperatures of 100° [38°C] or higher are unusual,
       having occurred during less  than one-half of the years of the period of record,
       and temperatures of zero or below are rare. The average number of days
       between the last freeze of spring and the first freeze of fall is approximately
       200.  The average daily temperature range is about 22°  [12°C]  with the
       greatest average range in spring and fall and the smallest in winter. Summery
       nights are seldom oppressively hot and humid.   Low level temperature
       inversions occur during approximately 57 percent of the hourly observations.
       Fall is usually the season with the greatest  number of hours of low level
       inversion with the number decreasing progressively through spring and winter
       to a summertime  minimum but seasonal differences are small.

5.1.3.  Temperature. The monthly average temperatures  at this location ranged between a
low of 2.8° C and a high of 24.8° C.

5.1.4.  Rainfall. An hourly rainfall record for April and May, 1989, was obtained from the

Atmospheric Turbulence and Diffusion Laboratory, National  Oceanic and Atmospheric

                                       5-2

-------
Administration, Oak Ridge, TN. Profiles of the first ten rain events beginning April 1, the
time the model run is initiated, were constructed from this record. Profiles consisted of the
duration of the event (PDUR), the total amount of precipitation in the event (PTOT) and
the storm advancement coefficient (STAD), which was  determined by inspection of the
hourly precipitation.  The resulting parameters were as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
77
174
726
826
924
1180
1340
1549
1590
1650
PDUR
5
8
11
7
4
5
12
2
9
14
PTOT
1.60
1.52
1.55
3.30
1.50
2.31
4.06
1.63
2.52
3.48
STAD
0.65
0.36
0.12
0.27
0.56
0.19
0.52
0.46
0.52
0.45
5.1.5.  Parameters for Subroutine RAINS.   Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions for Anderson County, TN.  Values
were calculated as described in Appendix B of U.S. EPA (1989a).  The values used in the
model run were based on a field with dimensions 500 m by 200 m, sloping at an angle of
6° (10.5%).  It was  assumed  for Practice I that  before a crop was present,  the cover
management factor was not modified, whereas after the crop was present, a canopy cover
of 30%, a canopy height of 0.5  m and a relative root network factor of 30% were provided;
for Practices II and HI, the canopy cover was taken to be 90%, the canopy height was taken
to be <0.5 m and a relative root network factor of 90% was assumed. The resulting values
were:
                                        5-3

-------
Parameter
No. Name
4
5
6

8
9
10
11

BTLAG
CN
AMC

USLEK
USLEL
USLES
USLEC

PRACTICE NUMBER
I IT TTT
0.2
78
3 (TCROP)
0.32
4.76
1.25
0.45 (
-------
5.2.2. Narrative Climatologic Summary.
      The climate at Roswell conforms to the basic trend of the four seasons, but
      shows certain deviations related to geography.  A location south and west of
      the main part of major weather activity affords a degree of climatic seclusion.
      There are also topographic effects that are inclined to alter the course of the
      weather in this area. Higher landmasses almost surround the valley location,
      with a long, gradual descent from points southwest through west  and north.
      The topography acts to modify air masses, especially the cold  outbreaks in
      wintertime.  Downslope warming of air, as well as air interchange  within a
      tempering environment,  often  prevents  sharp  cooling.   Moreover, the
      elevation of 3,600 feet in common with the geographic situation, discourages
      a significant part of the heat and humidity that originates in  the  south in
      summer. In winter, subfreezing at night is tempered by considerable warming
      during the day.  Zero [°F] or lower temperatures occur as a rule a time or
      two each winter. Subzero cold spells are of short duration. Whiter is the
      season of least precipitation (NOAA, 1981).

5.2.3. Temperature. The monthly average temperatures at this location  ranged between a

low of 4.2° C and a high of 26.2° C.
5.2.4. Rainfall.  Times, duration and total rainfall for each rainfall event were constructed
                     *                                •
from the rainfall record for 1980 at the location (NOAA, 1981).  The record provided the
date and amount of the largest rainfall during a 24-hour period each month, as well as the

total amount of rainfall each month.  The largest rainfall (greater than the subroutine's

lower limit of 1 cm) was  always used, and the remaining rainfall during  the period was

divided  into events placed at arbitrary times.  The  storm  advancement  coefficient was
chosen to reflect the nature of rainfall  in the region; the low number  used reflects a
preponderance of thunderstorms and sudden showers, whereas larger numbers were used
for some  other sites to reflect  a more gradual buildup  of the rainstorm.  The resulting

parameters were as follows:
                                        5-5

-------
Event
No.
1
2
3
4
5
6
7
8
9
10
START
ftirt
328
784
966
1280
1830
2174
2366
2800
3328
3518
PDUR
fhr)
5
8
6
3
8
10
10
5
10
6
PTOT
(cni)
1.17
4.5
4.55
1.5
3.05
7.75
12.47
3.45
4.19
3.0
5.2.5.  Parameters for Subroutine RAINS.   Parameters  for  Subroutine  RAINS were
modified to describe local rainfall and soil conditions.  The slope value used in the model
run was 1 degree (1.7%). The resulting values were:
Parameter
No. Name
4
5
7
8
9
10
11
BTLAG
CN
STAD
USLEK
USLEL
USLES
USLEC
PRACTICE NUMBER
I H TTT
0.32
89
0.25
0.32
2.54
0.16
0.45(TCROP)
0.38
84
0.25
0.32
2.54
0.16
0.05
0.38
84
0.25
0.32
2.54
0.16
0.05
S3.   SITE 3: CLINTON COUNTY, IA
      Values for site-specific variables for Site 3 were chosen to represent an agricultural
area in eastern IA in a county that borders on the Mississippi River.
5.3.1. Description of Soil. The soil chosen for the model run is the Fayette Series, which
comprises fine-silty, mixed, mesic type Hapludalfs. It is further described as follows (USDA,
1981b):
      The Fayette series consists of well drained, moderately permeable  soils on
      loess-covered uplands. These soils formed in loess that is more than 40 inches
      thick. Slope ranges from 2 to 40 percent.
      The solum ranges from 40 to 60 inches in thickness. There are no carbonates
      to a depth of 40 inches to 60 inches.
                                       5-6

-------
Fayette soils are of hydrologic group B, characterized by moderately low runoff potential,

moderate infiltration rates, and moderate rates of water transmission.
532.  Narrative Climatologic Summary. Because a meteorologic report for Clinton County
was not included in NOAA (1981), the climatologic summary and data reported for nearby

Dubuque, IA, (NOAA, 1981) were used:
       The principal feature of the climate in Dubuque is its variety.  Standing, as
       it  does, at the crossroads of the various air masses that cross the continent,
       the Dubuque area is  subject to weather ranging from that of the cold, dry,
       arctic air masses in the winter with readings  as low as 32° below [-36°C],
       when the ground is snow covered, to the hot, dry weather of the air masses
       from the desert southwest in the summer when the temperatures reach as high
       as 110° [43°C].  More often the area is covered by mild Pacific air that has
       lost considerable moisture in crossing  the mountains far to  the west, or by
       cool, dry Canadian air, or by warm, moist air from the Gulf regions. Most of
       the year the latter three types of air masses dominate Dubuque weather, with
       the invasions of Gulf air rarely occurring in the winter.

       The seasons  vary  widely  from year  to  year at Dubuque;  for example,
       successive invasions of cold air from the north may just reach this far one
       winter and bring a long, cold winter with snow-covered ground from mid-
       November  until March, and many days of sub-zero temperatures, while
       another season the cold air may not reach quite this far arid the winter can
       be mild with bare ground most  of the season, and only  a  few  sub-zero
       readings. The summers, too, may vary from hot and humid with considerable
       thunderstorm activity when the Gulf air prevails, to  relatively cool,  dry
       weather when air of northerly origin dominates the season.

 5.3.3.  Temperature. The monthly average temperatures at this location ranged between a

 low of-7.5° C and a high of 23.2° C.
 5.3.4.  Rainfall. Times, duration and total rainfall for each rainfall event were constructed
 from the rainfall record for 1980 (NOAA, 1981).  The resulting rainfall parameters were as

 follows:
                                         5-7

-------
Event
No.
1
2
3
4
5
6
7
8
9
10
START
flirt
180
231
396
636
970
1264
1791
1834
1934
2080
PDUR
flirt
8
6
8
6
6
8
10
10
6
4
PTOT
(cml
2.84
1.0
1.6
1.2
1.0
1.27
6.12
4.56
3.0
2.5
 53.5.  Parameters for Subroutine RAINS.  Parameters for Subroutine  RAINS were
 modified to describe soil conditions for Clinton County, IA, and rainfall for Dubuque, IA,
 the nearest reporting station. The slope value used in the model run was 4.6° (8%). The
 resulting values were:
Parameter
No. Name
4
5
6

7
8
9
10
11

BTLAG
CN
AMC

STAD
USLEK
USLEL
USLES
USLEC

PRACTICE NUMBER
I TT m
0.17
78
3 (TCROP)
0.375
0.37
4.76
0.85
0.45 (
-------
5.4.1. Description of Soil. The soil chosen for the model run is the Archbold Series, which
comprises hyperthermic,  uncoated Typic Quartzipsomments.  It is further described as

follows (USDA, 1989):
      The Archbold series consists of nearly level to gently sloping, moderately well
      drained, droughty soils that formed in marine and eolian deposits. These soils
      are on moderately high ridges in the ridge part  of the county.  The slopes
      range from 0 to 5  percent.

      Typically, the surface layer is gray sand about 4 inches thick. The underlying
      material to a depth of 80 niches or more is white sand.

      The soil reaction is slightly acid to extremely acid. The texture is sand or fine
      sand. The content of silt plus clay in the 10- to 40-inch control section is less
      than 2 percent.

Archbold soils are of hydrologic group A, characterized by having a high infiltration rate

(low runoff potential) when thoroughly wet.  They consist mainly of deep, well drained to

excessively drained sands or  gravelly sands.  These  soils  have a high  rate of Water

transmission.
5.4.2. Narrative Climatologic Summary.  Because meteorologic information was not given

in NOAA (1981) for Highlands County, the summary and data for nearby Orlando, FL, were

used.
      Orlando, by virtue of  its location in the central section of the  Florida
      peninsula (which is abounding with lakes), is almost surrounded by water and,
      therefore, relative humidities remain high here the year round, with values
      hovering near 90 percent at night and dipping  to 40 to 50 percent in  the
      afternoon (sometimes to 20 percent in the winter).

      The rainy season extends from June through September (sometimes  through
       October when  tropical storms  are  near).   During this  period, scattered
       afternoon thundershowers are an almost daily occurrence, and these bring a
       drop in temperature to make the climate bearable (although, most summers,
       temperatures above 95° [35°C]  are rather rare).  Too, a breeze is usually
       present, and this also contributes towards general comfort.

 5.4.3.  Temperature.  The monthly average temperatures at this location ranged between a

 low of 15.8° C and a high of 28.0° C.
                                         5-9

-------
  5.4.4.  Rainfall. Times, duration and total rainfall for each rainfall event were constructed
  from the rainfall record for 1980 at Orlando (NOAA, 1981). The resulting parameters were
  as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
fhrt
1043
1166
1667
1789
1958
2536
2918
3301
3547
4025
PDU
flirt
7
8
10
9
6
10
6
6
7
10
PTOT
(cm) 	
2.1
3.12
11.17
5.7
3.0
3.17
2.0
1.6
2.44
9.93
       In model runs from Practice I, Subroutine RAINS returned a floating-point error
 during computations for rainfall event 9. This error did not occur for Practices H and HI,
 so it was probably related to both the number of organisms and the long time over which
 the subroutine operated.  To complete the model run, it was necessary to delete rainfall
 events 9 and 10 for Practice I.
 5.4.5. Parameters for Subroutine RAINS.   Parameters  for Subroutine RAINS were
 modified to  describe local rainfall reported for Orlando, FL,  and soil  conditions  for
Highlands County, FL.   The slope value  used in the model run was 1.2°  (2%).  The
resulting values were:
Parameter
No. Name
4
5
1
8
9
10
11

BTLAG
CN
STAD
USLEK
USLEL
USLES
USLEC

PRACTICE NUMBER
I TT TTT
0.45
67
0.2
0.1
2.54
0.26
0.45 (TCROP)
0.8
39
0.2
0.1
2.54
0.26
0.05

0.8
39
+j s
0.2
0 1
\J» JL
254
****s~
026
\jt£*\j
0.05

                                      5-10

-------
5.5.   SITE 5: KERN COUNTY, CA
      Values for site-specific variables for Site 5 were chosen to represent a soil near

Bakersfield, CA, which is located in southern CA.
5.5.1. Description of Soil.  The soil chosen for the model run is the Arvin series, which

comprises coarse-loamy, mixed, nonacid, thermic Mollic Xerofluvents. It is further described

as follows (USDA, 1981c):
      The Arvin series consist of very deep, weU drained soils on alluvial fan,
      stream flood  plains, and stream terraces.   These soils formed in mixed
      alluvium derived from granitic rock. Slope ranges from 2 to 9 percent.

      Clay content ranges from 5 to  18 percent in the  control section.  Organic
      matter content  is 0.9 percent or less.  Reaction  is slightly  acid to mildly
      alkaline throughout.

Arvin soils are of hydrologic group B, characterized by moderately low runoff potential,

moderate infiltration rates  and moderate rates of water transmission.

5.5.2. Narrative Climatologic Summary.

      Bakersfield, situated in the extreme south end of the great San Joaquin
      Valley, is partially surrounded by a horseshoe-shaped rim of mountains with
      an open  side to the northwest  and the  crest at an average distance of 40
      miles.

      The Sierra Nevadas to the northeast shut out most of the cold air that flows
      southward over the continent during winter. They  also catch and store snow,
      which provides irrigation water for use during the dry months.  The Tehachapi
      Mountains, forming the southern boundary, act as an obstruction to northwest
      wind, causing heavier precipitation on the windward slopes, high wind velocity
       over the  ridges and, at times, prevailing cloudiness in the south end of the
      valley after skies have cleared elsewhere. To the  west are the coast ranges,
       and the ocean shore lies at a distance of 75 to 100 miles.

       Because of the nature of the surrounding topography, there are large climatic
       variations within relatively short distances. These zones of variation may be
       classified as  Valley,  Mountain, and Desert areas.  The  overall climate,
       however, is warm and semi-arid. There is only one wet season during the
       year,  as  90 percent of all  precipitation falls from October through April,
       inclusive. Snow in the valley is infrequent, with only a trace occurring in
                                        5-11

-------
        about one year out of seven. Thunderstorms also seldom occur in the vallev
        (NOAA, 1981).                                                      y

 5.53.  Temperature. The monthly average temperatures at this location ranged between a
 low of 8.5° C and a high of 28.8° C.
 5.5.4.  Rainfall. Times, duration and total rainfall for each rainfall event were constructed
 from the rainfall record for 1980 at the location (NOAA, 1981).  The resulting parameters
 were as follows:
Event
No.
1
2
3
4
5
6
7
8
9
10
START
flirt
16
4378
7256
7530
8016
8320
8606
8782
13164
16022
PDUR
fhr>
8
10
9
6
6
5
8
8
10
9
PTOT
(cml
1.0
1.78
1.47
1.07
1.0
1.0
1.63
1.0
1.78
1.47
5.5.5. Parameters for Subroutine RAINS.   Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions. The slope value used in the model
run was 1.7° (3%). The resulting values were:
Parameter
No. Name
4
5
6

7
8
9
10
11

BTLAG
CN
AMC

STAD
USLEK
USLEL
USLES
USLEC

PRACTICE NUMBER
	 !_ 	 	 H TTT
0.3
78
3 (TCROP)
0.4 :
0.32
2.54
0.26
0.45(TCROP)
0.45
61
2

0.4
0.32
2.54
0.26
0.05

0.45
61
•-• 2


0.32
2.54
0.26
0.05

                                      5-12

-------
5.6.   SITE 6: YAKIMA COUNTY, WA

      Values for site-specific variables for Site 6 were chosen to  represent a soil near

Yakima, WA, which is located in south-central WA along the Yakima River.  This is a

region of fairly low rainfall, but which is successfully farmed by irrigation.

5.6.1. Description of Soil. The soil chosen for the model run is the Kittitas Series, which

comprises fine-silty,  mixed (calcareous), mesic Fluvaquentic Haplaquolls.  It is further

described as follows (USDA,  1985):

      The Kittitas series consists of very deep, somewhat poorly drained soils on
      flood plains.  These soils formed in mixed alluvium.  Slopes range from 0 to
      2 percent.

Kittitas  soils are of  hydrologic group C,  characterized by a slow infiltration rate when

thoroughly wet.  They consist chiefly of soils having a layer that impedes the downward

movement of water or soils of moderately fine texture or fine texture. These soils have a

slow rate of water transmission.

5.6.2. Narrative Climatologic Summary.

      Yakima is located in a small east-west valley in the upper (northwestern) part
      of the irrigated Yakima Valley.  Local topography is complex with a number
      of minor valleys and ridges giving a local relief of as much as 500 feet.  This
      complex topography results in marked variations in air drainage, winds, and
      minimum temperatures within short distances.

      The climate  of the Yakima  Valley  is relatively mild and dry.   It has
      characteristics of both maritime and continental climates, modified by the
      Cascade and the Rocky Mountains, respectively.  Summers are dry and rather
      hot,  and winters cool with only  light snowfall.   The maritime influence is
      strongest in winter when the prevailing westerlies are the strongest and most
      steady.  The Selkirk and Rocky  Mountains in British Columbia  and Idaho
      shield the area from most  of the  very cold air masses that sweep down from
      Canada into the Great Plains and eastern United States. Sometimes a strong
      polar high pressure area over western Canada will occur at the same time that
      a low pressure area covers the southwestern  United  States.   On  these
      occasions, the  cold arctic air will pour through the passes and down the river
      valleys of British Columbia, bringing very cold temperatures to Yakima.  That
      this happens infrequently is shown by the occurrence  of temperatures of 0
      degrees [F] or below on only 4 days a winter on the average. On about 21
      days during the winter  the temperature will fail to rise to the freezing point.
      In January and February 1950, there were 4 consecutive days colder than -20°
                                       5-13

-------
       [-29°C], including -25° [-32°C] On February 1. However, over one-half of the
       winters remain above 0 degrees [F (-18°C)] (NOAA, 1981).
5.6.3.  Temperature. The monthly average temperatures at this location ranged between a
low of-1.5° C and a high of 22.3° C.
5.6.4.  Rainfall.  Times, duration and total rainfall for each rainfall event were constructed
from the rainfall record for 1980 at the location (NOAA,  1981). The resulting parameters
were:                                                                  ,
Event
No.
1
2
3
4
5
6
7
8
9
10
START
fhrt
1628
4290
4506
5490
5722
5966
7002
7212
7498
7816
PDUR
flirt
6
8
10
6
6
6
10
8
8
8
PTOT
(cm)
1.0
1.25
2.06
1.14
1.0
1.0
2.65
1.5
i.2
1.0
5.6.5. Parameters for Subroutine RAINS.   Parameters for Subroutine RAINS were
modified to describe local rainfall and soil conditions. The slope value used in ithe model
run was 0.6° (1%). The resulting values were:
Parameter
No. Name
4
5
6
7
8
9
10
11
BTLAG
CN
AMC
STAD
USLEK
USLEL
USLES
USLEC
PRACTICE NUMBER
I II III
0.4
85
2
0.4
0.43
2.54
0.12
0.45 (TCROP)
0.5
74
2
0.4
0.43
2.54
0.12
0.05
0.5
74


0.43
2.54
0.12
0.05
                                       5-14

-------
                                   6. RESULTS

6.1. ALGORITHM FOR INFECTIVE DOSE
      During the  development of the algorithm  for infection, it was assumed that the
probability of infection could be described by the Poisson distribution (U.S. EPA, 1989a).
According to this assumption, the probability of infection with a pathogen (whose infectious
dose is M)  is given by the sum of the  probabilities of being exposed to M or more
pathogens. While it is not feasible to calculate this number directly, an indirect method of
calculation is well  suited to the computer. Briefly, when the average exposure level is X,
the probability of being exposed to N pathogens is given by the Poisson distribution:
                                 P(N)  = e-xXN/N!
In the exposure algorithm, the value of e'x is found and multiplied by successive values of
X/N as N is incremented from 1 to M-l.  All values of P(N) for N
-------
                                                 O
                                                 E
                                             n
NOHO3JNI jo AiniavaoHd
                6-2

-------
      The model algorithm also exhibited  the  expected behavior with  a simulated
heterogeneous receptor population. It is expected that a population that is heterogeneous
in its sensitivity to a pathogen will be described by a more disperse infection response than
a homogeneous population, that is, the curve will rise less steeply and cover a wider range
of exposure values.  To test the applicability of the  model's infection algorithm to this
assumption, a uniform population with an MID. of 10 was compared with a heterogeneous
population in which the mean MID was 10. Assuming a standard deviation of 2.0 and using
for guidance a table of areas of the normal distribution, the calculated distribution of MID
values was:  5 and 15,1% each; 6 and 14, 2.8% each; 7 and 13, 6.5% each; 8 and 12, 12.1%
each; 9 and 11, 17.5% each; and 10,19.7%. The infectivity ratio at each modeled dose was
determined for uniform populations with these MID values, and the ratios were weighted
by the population distribution and summed to yield the more disperse curve shown in Figure
6-2.   This figure  demonstrates that the model's  infection algorithm  is responsive to
heterogeneity of sensitivity in the hypothetical exposed population. The disperse curve is
not markedly different from the curve for a uniform population, but the entire range of
sensitivity was ±50% of the MID. The range of sensitivity of >75% of the population was
only ±20% of the MID, whereas the variability of sensitivity to pathogens in a typical human
population is likely to be at least a few  orders of magnitude.  While the model could be
revised to include an allowance for heterogeneity in the receptor population, such a revision
is not likely to improve the accuracy  of the model significantly.

6.2. SENSITIVITY TO VARIABLES
      The effect of specific parameter values was tested by varying each parameter singly,
using the parameter values described  in Chapter 4. This required a method to compare the
outcome of many model runs and express the comparison in quantitative terms.  No specific
day after application can be used as  a good indicator of the probability  of infection.  The
model assumes that the impact of each day's exposure is independent of any other day's
exposure, despite the  fact that  infectious viruses could persist and accumulate, or that
chronic exposure at low levels could induce immunity. The time course of exposures could
be different in each compartment because of the timing of transfers of pathogens. As an
                                        6-3

-------
NOHD3JNI jo
     3Aiivinwro
6-4

-------
example, in the ONSITE compartment, the maximum exposure to viruses in soil occurs
immediately after  incorporation.  The greatest exposure to groundwater  (DRINKER)
depends  on the rate of subsurface transport, the distance to the well, and the rate of
inactivation of the  viruses.  Transport by surface runoff to the onsite pond (SWIMMER)
cannot occur without rainfall, which determines the timing of the transfer; other parameters
modify the amount of transport.  Since no specific day after application can be used as a
good indicator of the probability of infection, for each model run the maximum probability
of infection hi each exposure compartment was chosen as an appropriate indicator.
      In a preliminary assessment of risk from viruses in sewage sludge, site-specific data
for Site 1 and the ranges of parameters listed in Chapter 4 were tested.  Approximately 1200
model runs were  made, and the effects on maximum probability of infection were
determined.  The maximum observed probabilities of infection in each practice at Site 1,
using baseline parameters,  are given in Table 6-1.  This table shows that the maximum
probability of infection ONSITE was similar for all practices, between 1% and 7%. The
probability of infection to the OFFSITE receptor was calculated as zero in every case. Risk
of infection via contaminated food products (EATER) was shown only in Practice IV, and
risk via offsite wellwater (DRINKER) was shown in Practice HI.  In contrast, infection by
contact with onsite surface water (SWIMMER) was significant in all three practices that
include  the pond  (Practices I-III),  the risk level depending  on site-specific as well as
practice-specific variables.
       In an additional detailed assessment, the inactivation rate parameters were altered
as described in Section 4.2.1, and an additional  set of model runs was made using site-
specific data for Site 2. The parameters used as a baseline for comparisons were the default
values, except that the fractional transfer from soil surface to aboveground crops [FCROP1,
P(46)] was increased to SxlO"6, the time at which the crop is present  [TCROP, P(68)] was
reduced to 240 hours, and the time of harvesting [THARV,  P(69)] was reduced to 300
hours.
       On the basis of these model runs, parameters showing  no  effect on maximum
probability of infection were eliminated from further consideration.  As reported for
parasites (U.S. EPA, 1990)  and bacterial pathogens (U.S. EPA,  1991),  a large number of the
                                        6-5

-------
              TABLE 6-1



Results of Model Runs, Baseline Conditions
Practice

I
n
m
IV
V
Maximum Probability of
ONSITE
7.02X10'2
l.lOxlO-2
l.lOxlO'2
5.54xl(T2
4.67xlO'2
OFFSITE
0.00
0.00
0.00
0.00
0.00
EATER
0.00
0.00
0.00
1.49xlO'2
• —
Infection
DRINKER
0.00
0.00
1.32xlO'7
•*•
~

SWIMMER
2.39xlO-2
4.13xlO'3
4.14xlO'3

•
                6-6

-------
 parameters had no effect on the maximum calculated probability of infection.  Increasing
 the fraction of viruses transferred from surface soil to another compartment would not be
 expected to affect the maximum probability of infection ONSITE if the transfer occurred
 late in the model run,  because the maximum probability of infection ONSITE typically
 occurs  within the first  few days after sludge application.  Similarly in other exposure
 pathways,  changes in the fraction of viruses transferred would have no effect on the
 probability of infection  if the transfer occurred after the time of maximum infection and
 inactivation had reduced the virus population more than the transfer increased it.  In some
 cases, the altered parameters governed processes that were insignificant compared to the
 main determinants of exposure, and so were not able to change the maximum probability
 of  infection.  In other  cases, the exposure in a given compartment was so  small that
 changing the tune or amount of virus transfer to or from that compartment had no effect
 on the probability of infection.
       The effects of site-specific  and practice-specific differences in parameters and
 assumptions are illustrated by comparing the outcome of baseline model runs.  The final set
 of  model runs, in which inactivation rates were decreased  in  soil, water,  and  droplet
 aerosols, showed  higher  probabilities of infection at all  sites  and for most exposure
 compartments. The results of these model runs, using baseline parameters except for the
 more conservative inactivation rates, are summarized  in Table 6-2.   The  maximum
 calculated  probabilities  of infection ONSITE were similar for each  site, and again no
 OFFSITE infection was predicted. Infection via contaminated food products was calculated
 to be significant only in Practices I and IV, whereas infection via contaminated wellwater
 was indicated in Practices I-III at all sites.  Infection to the SWIMMER was  predicted at
 significantly higher levels than with the default inactivation parameters. The time course
 of infection probability in each exposure compartment in Practice I (Site 1) is illustrated in
Figure 6-3.
       The probability  of infection by consumption  of crops was proportional to  the
concentration of viruses in the applied sludge, but decreasing  the concentration yielded a
less than proportional decrease in probability of infection ONSITE and to the  SWIMMER
in the onsite pond.  For example, reducing the concentration of viruses in sludge by a factor
                                        6-7

-------
          TABLE 6-2
Maximum Probability of Infection
      by Site and Practice
SITE
1




2




3




4




5




6




PRACTICE
I

m
IV
V
I
n
m
IV
V
i
ii
ni
iv
V
i
n
m
IV
V
i
n
m
IV
V
i
ii
ni
IV
V
ONSITE
2.70X10'1
4.63X10'2
4.63xlO'2
5.54xlO-3
2.79xlO-3
2.70X10-1
4.63xlO-2
4.63X10'2
5.53xlO-3
2.79xlO'3
2.70X10-1
4.63xlO'2
4.63X10'2
5.54xlO'3
2.79xlO'3
2.70X10-1
4.63xlO-2
4.63xlO-2
5.54xlO'3
2.79xlO-3
2.70X10'1
4.63xlO'2
4.63xlO'2
5.54xlO-3
2.79xlO'3
2.70X10'1
4.63xlO'2
4.63xlO'2
5.54xlO'3
2.79X10'3
OFFsrrE
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
EATER
5.58X10-4
0.0
0.0
9.27X10'1
0.0
S.llxlO-4
0.0
0.0
9.28X10'1
0.0
s.eixio-4
0.0
0.0
9.28X10'1
0.0
5.11x10"
0.0
0.0
9.25X10'1
0.0
S.llxlO"1
0.0
0.0
9.25X10'1
0.0
5.11x10"
. 0.0
0.0
9.25X10'1
0.0
DRINKER
2.71X10-6
3.21x10-*
6.44X10"6
0.0
0.0
2.80X10-6
3.20X10"6
6.41X10"6
0.0
0.0
4.28X10-6
5.04X10-6
l.OlxlO'5
0.0
0.0
2.81X10"6
3.20X10"6
6.42X10"6
0.0
0.0
2.00X10-6
3.20X10"6
6.42X10"6
0.0
0.0
2.81x10-*
3.20X10"6
6.42X10"6
0.0
0.0
SWIMMER
7.40X10'1
2-lOxlO-1.
2.12X10'1
0.0
0.0
1.94X10'1
2.88xlO'2
2.90xlO'2
0.0
0.0
S.OSxlO'1
S.lSxlO'1
3.15X10'1
0.0
0.0
3.71xlO'3
S.OSxlO-4
8.07X10"4
0.0
0.0
0.0
4.73xlO-2
4.73x10-2
0.0
0.0
3.15xlO'5
4.93X10"6
4.94X10-6
0.0
0.0
              6-8

-------
                                                       D
                                                             *?
                                                             vo
                                                             W

                                                             §
                                                             E
U01JD3JUJ jo Xjijiqeqoaj

              6-9

-------
of 200 (from 100,000/kg to 500/kg) reduced the probability of infection via crops from
S.llxlO"4 to 2.55X10"6 (also a factor of 200), whereas ONSITE infection wias reduced from
0.27 to 0.0016 (a factor of 172), and infection to the SWIMMER was reduced from 0.194
to 0.0011 (a factor of 180).  This is consistent with the conclusions for infection by parasites
(U.S. EPA, 1990). When the infectious dose is 1, the probability of infection is proportional
to dose at low doses. However, because of the exponential basis of the Poisson distribution,
as the dose increases the probability of infection  increases less rapidly.
       To test the effect of the rate of loss of infectivity or viability, a constant logarithmic
inactivation rate was substituted for the temperature-dependent inactivation rates described
in Table 4-1.  The results for Practice  HI  (and for EATER in Practice  IV) at Site 1 are
given in Table 6-3 (similar effects were seen in model runs for the other practices, but
Practice m was used in this illustration because of its greater groundwater exposure). All
values for inactivation rates in soil, dry particulates and water gave higher probabilities of
infection than the default temperature-dependent algorithm. This suggests that the slope
and intercept values  used in  the algorithm may be incorrect.   Substitute values for
inactivation of viral particles in aerosols had no effect on the probabilities of infection; the
probability of infection for offsite aerosol exposure was calculated to be zero in every model
run.
       The  effect of  inactivation  rate  on  the time  course  of ONSITE  exposure is
demonstrated in Figure 6-4. With the inactivation rates described above, the probability of
infection fell rapidly as infectious particles were inactivated. However, when the inactivation
rates were set to zero, the decrease in probability of infection was gradual, as virus particles
in surface soil were gradually transferred to surface  water, subsurface soil and groundwater.
Superimposed on this pattern was a biweekly increase for one day of 4-5% in the probability
of onsite infection as the farmer makes a closer inspection of the crops. Accompanying the
decrease in viral particles hi surface soil, there was an increase in the probability of infection
to the pond swimmer as each rainfall added more  runoff to the pond (Figure 6-5). At an
inactivation rate of 0.0015 log/hr, the SWIMMER exposure rapidly decreased, whereas with
no inactivation, the probability of infection increased to 1.0. The probability of infection by
 consumption of contaminated well water increased gradually, beginning 51 days after initial
                                          6-10

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 application of the sludge. These results show that the inactivation rate of virus particles is
 extremely important in determining whether a groundwater well  is likely to become
 contaminated and in determining how long surface soil or surface water is likely to remain
 infectious.  The results also demonstrate the importance of accurate characterization of
 inactivation rate for viruses of different kinds in the various transport and exposure media.

 <5.3. ONSITE EXPOSURES
        In each model run for Practice I, in which the liquid sludge must be allowed to dry
 for 24 hours before it is tilled, the maximum probability of infection ONSnnE occurred on
 day 3  and decreased as the sludge-borne viruses were inactivated and as they were
 transferred into other compartments. In Practices II and HI, which do not require the 24-
 hour waiting period, the maximum probability of infection occurred on day 2. In Practice
 V the maximum probability of infection ONSITE occurred on day 2; in Practices IV and V,
 composted sludge is assumed to be spread by hand, providing a high level of exposure to
 the user during the  application process, followed by a lower exposure as the composted
 sludge is incorporated and thus  diluted with soil.  However, exposure during subsequent
 tilling in Practice IV led to a maximum risk of infection at 15 days.  Onsite exposures
 decreased rapidly as the viral particles in the soil were inactivated or transferred into other
 compartments. Figure 6-6 presents the time courses of ONSITE infection probabilities for
 Practices I-V at Site 1.
       Using  reference values including the conservative inactivation rates, the baseline
 maximum probability of infection was 0.270 for Practice I, 0.046 for Practices II and III,
 0.0055 for Practice IV, and 0.0028 for Practice V.  Generally, site-specific variables did not
 have a significant effect on the probability of ONSITE infection, because the site-specific
variables alter temperature-dependent inactivation rates and rainfall-dependent runoff and
sediment  transport,  none  of which exerts major effects on  the  ONSITE  exposure
compartment before  the time of maximum infection. Significant impacts on the probability
of infection were observed in all  application practices with changes in pathogen density in
the applied sludge [ASCRS, P(l)] and sludge application rate [APRATE,  P(2)], both of
                                        6-14

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which determine the number of viral particles applied.  Because of the exf nonential nature
of the probability algorithm, the changes in probability were not directly proportional to the
change in parameter values, but varied as would be expected for a proportional change in
exposure.
       The method of application was significant in Practices I-in, subsurface application
removing all probability of ONSITE infection. Increasing the infective dose [MID, P(12)]
markedly reduced the probability of infection  in  all  practices.  The concentrations of
particulates, as determined by the soil composition (EPSMLT [P(26)] and ESDLT [P(27)]>
and the height of the participate cloud (EHT [P(28)]),  had an effect on the probability of
infection  ONSITE.  Changing the inactivation rates,  either by varying the parameters
SLOPES  [P(37)] and NTRCPS [P(38)]  or  the process functions PROC1-PROC3 and
HCRTT, had significant effects on the probability of infection, as illustrated by .a comparison
of Tables 6-1 and 6-2. The RISK parameter DRECTS, which describes the amount of soil
ingested daily, also had a significant effect on probability of infection, as would be expected
for a parameter directly governing exposure by contact with a contaminated medium,

6A. OFFSITE EXPOSURE
       In all model runs; the probability of infection OFFSITE was zero,, indicating that
although the inactivation of viruses in aerosols may be less than initially expected (compare
Table 3-6 with default value of 0.002 log/sec (U.S. EPA, 1989a», the calculated quantities
of liquid and dry particulate aerosols and concentrations of viruses in the aerosols were too
low to provide an infective dose to the modeled receptor.

6.5. EXPOSURE FROM CONTAMINATED FOOD
       Consumption of contaminated vegetable crops was shown by model calculations to
be a potential source of human infection, provided that inactivation rates were sufficiently
low or harvesting times were sufficiently close to application of the sludge.  The maximum
probability of infection by consumption of crops in Practices I and IV at each  site is shown
in Table 6-2.  Infection via food crops  was sensitive  not only to infectious dose (MID
[P(12)], inactivation rates,  and the parameters described in Section 6.3 that directly affect
                                       6-16

-------
the number of pathogens applied to the soil (ASCRS [P(l>] and APRATE [P(2)]), but also
to the relative fractions of pathogens transferred among surface soil, subsurface soil, and
crop surface, and to the type of crop or fraction of the total crop grown aboveground, below-
ground, or on-ground (variables P(44)-P(58)). The probability of infection by consumption
of vegetable crops was weakly correlated with the crop yield and strongly correlated with
crop type, the fraction of garden crops as on-ground crops being most strongly positively
correlated with infection and the fraction as aboveground crops being negatively correlated
with infection.  The first group of rows in Table 6-4 shows the effect of changes in fraction
of on-ground crops when the fraction of below-ground crops was held constant.  As the
fraction of on-ground crops increased, the calculated probability of infection increased. In
the second and third groups, the fraction of aboveground crops was held constant while the
fraction of on-ground crops increased. In each case, the calculated probability of infection
increased. When the fraction of on-ground crops was held constant (0.1), the calculated
probability of infection increased as the fraction of below-ground  crops increased.  The
fraction of aboveground crops had a negative influence on the probability of infection.
       Contamination of meat or milk by viruses from sewage sludge did not appear to pose
a significant risk to human health.  No evidence was found in the literature search to
substantiate infection of cattle, poultry or swine by human enteric viruses and subsequent
transmission of those viruses to humans by ingestion of dairy or meat products.  In addition,
no condition tested resulted in a calculated probability of infection > IxlO"16 for ingestion
of meat or milk.

6.6. EXPOSURE FROM CONTAMINATED GROUNDWATER
       Transport of viruses via groundwater to an offsite well was not shown by this model
to be a major risk, but exposure by contaminated groundwater was shown to be likely if the
rate of inactivation of viruses in water was less than the default values.  The probability of
infection was related to the periodic introduction  of pathogens to groundwater by the
infiltration of rainwater (Figure 6-7).  The most important parameter related to subsurface
transport of viruses appeared to be the inactivation rate of viruses in water. The results also
                                       6-17

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               TABLE 6-4

    Effect of Distribution of Crop Types
on Risk to Consumers of Garden Vegetables3
Value of Variable
PLNT1" PLNT2C
P(74) P(75)
0.8 0.1
0.4 0.5
0.1 0.8
0.4 0.1
0.4 0.3
0.1 0.1
0.1 0.4
"Results for Site 1, Practice IV,
bAboveground crops
°On-ground crops
dBelow-ground crops
PLNT3d
P(76)
0.1
0.1
0.1
0.5
0.3
0.8
0.5
Probability
of EATER
Infection
0.0014
0.0067
0.0108
0.0109
0.0149
0.0229
0.0417
default inactivation rates
                  6-18

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 showed an increase in probability of infection at the offsite well whenever the time required
 for the viruses to reach the well was decreased. Thus, increasing groundwater velocity or
 reducing the dispersion or retardation coefficients or the distance to the well moderately
 increased the probability of infection in Practices II and III.  Conversely, exposures were
 reduced when these parameters were changed in the opposite direction.  No effect was
 observed in Practice I using the default inactivation rates because the concentration of viral
 particles  entering the groundwater was too small to be significant.  However, when  the
 reduced, more conservative inactivation rates were used, the number of pathogens reaching
 the offsite well increased and the effects of GRDWTR parameters decreased. The effects
 of differences in GRDWTR parameters in model runs for Site 1 are summarized in Table
 6-5.

 6.7. SURFACE WATER EXPOSURE
        Contaminated surface water, represented by the SWIMMER in an onsite pond, was
 the most significant source of exposure. A peak in probability of infection occurred after
 each rainfall, when additional contaminated soil surface water and soil were washed into the
 pond.  The successive pulses of pathogens released  to the onsite pond  are illustrated in
 Figure 6-3. Site-specific parameters for Subroutine RAINS reflect differences in timing and
 amount of rainfall  and in properties  of the soil that affect surface runoff  and sediment
 transport.  Sites at which rainfall is slow and steady would be expected to have less runoff
 and sediment transport than sites at which rainfall is infrequent but heavy. Similarly, sites
 that are nearly level and  have  a good soil and extensive ground cover  should have less
 runoff than steeply sloped sites with poor soil and little ground cover. A comparison of the
 effects of site-specific parameters for Subroutine RAINS on  probability of infection are
 demonstrated  in Figure 6-8, which presents results  for Practice  I at all sites.  Risk of
infection decreases  with time before the first rainfall because of inactivation of the viral
pathogens in soil.  Table 6-2 summarizes the maximum risk of infection to the pond
swimmer at each site for each of the three practices that includes an onsite pond.
                                       6-20

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               TABLE 6-5

Effect of Variables for Subroutine GRDWTR
       on Infection via Groundwater
Variable
# Name
2 V


3D


4 R


11 XM


Definition
Velocity
(cm/far)

Dispersion
coefficient

Retardation
coefficient

Maximum
distance (m)

Value
0.9
3.6
10.8
0
60
100
0.2
1.0
2.0
20
50
200
Maximum Infection Risk
Probability Day
1.20X10-6
2.71X10-6
4.15x10-*
2.79x10-*
2.71X10-6
2.63x10-*
3.99X10"6
2.71X10-6
2.46X10-6
2.78X10-6
2.71X10"6
2.47X10"6
254
77
39
77
77
78
31
77
139
42
77
255
                   6-21

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                                                 6-22

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       Practice-specific differences in runoff can also occur. Figure 6-9 shows the calculated
probability of infection at Site  1 for practices I,  II and HI.  This figure shows that the
maximum probability of infection is less and the highest probability of infection occurs later
in Practices II and HI, in which there is more ground cover and therefore less surface runoff.
                                         6-23

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                                7. CONCLUSIONS

7.1.  LITERATURE REVIEW
       Literature values for virus density in  treated sludge were so variable, both by
treatment methodology and by virus type, that no single number could be selected as typical.
However, 2000 virus particles/kg was chosen as representative of viral density in composted
sludge and  100,000 particles/kg in digested sludge.  Infective doses,  while varying by
detection method and by virus type, have been reported to be as low as 1 infective particle.
As a conservative assumption, this minimum value was used for the model runs. Reported
inactivation rates range from T.lxlO'5 to 1.6X10'1  logs/hour in soil,  1.6X10"4 to 1.4X10"1
logs/hour in water, and 4.9xlO"5 to  8xlO"7 logs per second in aerosols.   Like the density
values,  these rates are quite variable.   Information on dispersion of viruses  in the
environment is limited in its applicability to generating a rate of transport in environmental
media.  Development of a variety of transport models has been an attempt to quantify the
movement of viruses,  especially in the subsurface and in groundwater.
7.2.  MODELING RESULTS
       Although detailed data on survival and transport of viruses in soil are limited, the
model appears to confirm the general observations in the literature that viruses in treated
sewage sludge present a potential health risk, justifying land-use restrictions.  However,
model runs implied that restrictions may be overly conservative. Reports of offsite infection
by viruses in sludge-amended soil or in aerosols from liquid treated sludge were not found;
model runs confirm the low probability of offsite infection except by uncontrolled surface
runoff.
       A test  of the  infection  algorithm  yielded  results that  are  consistent  with
experimentally and epidemiologically  observed responses  of populations to pathogen
exposure. The algorithm could be revised to include allowances for individual variation in
sensitivity in the population, but because of the uncertainties inherent in the model and in
the data used in its operation, it was concluded that the additional complexity was not
warranted.
                                        7-1

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 72.1. Sensitivity Analysis.  Model runs showed that the probability of infection by viruses
 as a  result of exposure  to soil contaminated with  sewage sludge is related  to the
 concentration of organisms in the sludge, the amount of sludge applied ;and the amount of
 contaminated soil to which the individual is exposed, either by casual contact or by ingestion
 of food grown in the contaminated soil.  The method of application was also significant.
 Subsurface application resulted in no exposure to any individual onsite, either  by direct
 contact or in the onsite pond, although exposure via groundwater was not eliminated.
        Direct proportionality of response to exposure level was not observed, because the
 probability of infection is calculated by a Poisson distribution,  which is an exponential
 function of exposure rather than a proportional one.  It was shown previously (U.S. EPA,
 1990) that when the MID = 1, variations in the probability of infection can be extrapolated
 from variations in virus concentrations up to  a probability of about 0.1.  Many of the
 parameters of the  model seemed to have little bearing on the  probability of infection,
 apparently because they ultimately had no  effect on the number of viral particles to which
 the human receptor was exposed in each exposure compartment,  or because they exerted
 their effect on survival or transport after the maximum probability of infection had occurred.
        The probability of infection was sensitive to the rate of inactivation of the viruses.
 This was the most significant property of the viruses themselves,  the other most sensitive
 properties being practice-specific or related to host response as well,   i   .
 122.  Onsite Exposures. Significant onsite exposures were calculated in all practices. The
 greatest ONSITE risk, -0.27 per day, was associated with Practice I» application of sludge
 for production of commercial crops.  These calculations imply that the field worker who
 inadvertently ingests soil during daily activity on the sludge application site is at significant
 risk of infection by viruses.  The results imply that there should be a waiting period before
 routine daily activity on the site. The length of the waiting period should depend on the
 initial  application rate and pathogen concentration as well as  the inactivation rate of the
virus. A benchmark risk of IxlO"4 per day was suggested previously  (U.S. EPA, 1990) on the
basis of proposed U.S.  EPA pathogen reduction regulations (U.S.EPA,  1989b).   In the
model runs reported here,  the  probability of  infection ONSITE in  Practice I, using a
conservative inactivation rate, was > IxlO'2 for 37 days and  > 1x10^ for 82 days.
                                         7-2

-------
       The maximum risks  of infection calculated in domestic applications were lower
(0.0055 for Practice IV and  0.0028 for Practice V), but not low enough to be protective.
However, it must be noted that the concentration used in the model runs, 2500 viruses/kg,
is a maximum value derived  from a limit of detection reported in the literature, and lower
levels may routinely be found in composted sludge. It is assumed that incorporation in these
practices is done by hand or  with power tools rather than by farm machinery; however, the
calculated exposures do not  include direct exposure to non-incorporated sludge.
       In summary,  it  appears  that a  probability of infection greater than the arbitrary
benchmark value of  IxlO"4 is likely during application and incorporation of liquid treated
sludge for agricultural practices.  If the initial viral concentrations in composted sludge are
>50/kg,  the user is likely to be at risk of  infection. A person engaged in these activities
could probably reduce the risk by wearing a protective mask and washing thoroughly before
handling food.
7.2.3.  Sediment Transport and Surface Runoff. The most significant potential source of
infection was exposure to runoff water and transported sediment after rainfall.  Rainfall
events were modeled as being able to transport contaminated soil from the field to the
onsite pond, where the suspended or particle-bound viruses accumulated. A swimmer in the
pond was therefore exposed to the viruses, by ingestion of either contaminated water or
sediments. Model runs indicated that it would be prudent to limit access to runoff water
and sediment from a sludge-amended field, either by mulching to reduce runoff, ditching
and/or diking to contain the runoff or restricting access to any onsite ponds receiving runoff.
7.2.4. Offsite Exposures. No health hazard was indicated as a result of offsite transport of
viruses by droplet aerosols or by wind-blown dust. This may be a model limitation caused
by over-simplification of the  Gaussian-plume aerosol transport subroutine in the model, or
it may reflect a very low concentration or probability of transport of infective viruses in
aerosols.   Modeling results  were  consistent  with abundant  literature reports that
groundwater can be a significant source of viral transport. However, because of dilution and
inactivation during subsurface transport, the highest probabilities of infection predicted for
consumption of groundwater at an offsite well were < IxlO"5 (Table 6-2).  Variations in the
parameters used for operation  of the groundwater  subroutine had little  effect on the
                                        7-3

-------
 maximum probability of infection when conservative inactivation rates were used, although
 the time at which the maximum probability was observed did depend strongly on these
 parameters (Table 6-5). Because infiltration to groundwater is treated by the subroutine as
 a point source rather than as a large area (U.S.  EPA,  1989a), the subroutine probably
 greatly overestimates the  concentration of viruses reaching the offsite well.
 7.2.5.  Waiting Period.  Practice-specific waiting periods are proposed in the U.S. EPA
 Pathogen Reduction Requirements (U.S. EPA, 1989b) before access to sludge-amended land
 or consumption  of crops grown thereon.  For exposure  comparisons, a probability of
 infection of IxlO"4 was tentatively chosen as a benchmark for sufficient protection of human
 health (U.S. EPA, 1990).  Using this benchmark value, the default values for application
 rate and inactivation rate, and a virus density of Ixl05/kg in Practice I, the initial maximum
 probability of infection for aboveground crops harvested 13 days after sludge application was
 S-fixlO"4 (Table 6-2); at the conservative inactivation rate for viruses in soil, a waiting period
 of at least 45 days would be required to reduce the probability of infection below IxlO"4 for
 aboveground crops contaminated with 0.1  g soil per crop unit. A waiting period of 5  months
 appeared to be adequate (probability of infection 90% when  the crops were harvested beginning at 13 days; a 4-month waiting period
 reduced the probability of infection below IxlO"4.
       In all cases, the probability of infection depended  on the amount of soil consumed
with the crop. The default value for fraction of  soil adhering to the aboveground  crop is
very low; using this value (-20 mg/crop unit), the calculated probability of infection of the
food consumer was <10'16 in Practice I, indicating that  a waiting period would  not be
required for a low level of surface contamination.  Similarly, the probability of infection
depends on the concentration of pathogens in the sludge when it is applied.  Therefore, the
appropriate waiting period should probably be variable, depending not only on intended
land use,  as  is  currently true, but  also on  sludge application  rate  and  pathogen
                                        7-4

-------
concentration.  In calculating a safe waiting period, conservative assumptions should be
made about amounts of soil ingested with crops.
       U.S. EPA restrictions (U.S. EPA, 1989b) on growing food crops in sludge-amended
soil, while necessary for protection against potential health hazards from parasites, appear
to be more stringent than required by typical or even worst-case inactivation rates for viruses
on crops.
                                         7-5

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-------
                              8.  RESEARCH NEEDS
8.1.    INFORMATION NEEDS FOR VIRUSES
       Major requirements  to  improve the understanding of viruses are standardized
methods for detecting and enumerating viruses and for studying their survival. Ideally, any
standard method for testing virus occurrence and infectivity should be able to (1) detect all
pathogenic viruses; (2) allow differentiation among the viruses in the sample; (3) provide
a correlation between assay and infective dose to humans; (4) accurately characterize
infective plaques, i.e., neither overestimate  nor underestimate  the number of infective
plaque forming units (Wellings, 1987; Bertucci et al, 1983).
       Unfortunately, standardized methods for detecting and characterizing viruses in soil,
sediments, and both groundwater and surface water are not yet available (Rao and Melnick,
1987). For example, many of the methods used for concentrating and assaying viruses in
water do not recover that portion of the viruses adsorbed on suspended solids (Rao, 1987)
or adhering to  sample containers (Ward and Winston,  1985).  Wellings (1987) describes
methods for recovering viruses from different soil types (Berg and Berman, 1984; Farrah and
Bitton,  1984; Goyal, 1984) but emphasizes the absence of an efficient, standard method.
       The recent publication (ASTM, 1990) of a standard method for recovery of viruses
from wastewater sludge is a significant improvement in methodology, although Cliver (1987)
points out that  there is no single recovery method that is optimal for all viruses in all types
of sludges.  Likewise, the development by Ijaz et al. (1987) of a methodology for studying
the aerobiology of viruses is an important contribution to laboratory techniques, because the
lack of standardization has  been a problem in comparing  research results.  Sobsey et al.
 (1985) have developed a method for recovery and quantitation of hepatitis A virus (HAV)
from water, and Payment and Armon (1989) have reviewed detection methods for viruses
 in drinking water.
        Citing the need to develop more reliable techniques for recovering more types of
 enteroviruses from contaminated water, Jansons and Bucens (1986) describe a method for
 concentrating rotavirus by  hollow fiber  ultrafiltration, thereby overcoming some of the
 drawbacks  of filter adsorption-elution methods. Guttman-Bass et al. (1987) compared
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  methods for detecting rotavirus in water but found that none of the methods was sensitive
  enough to detect rotaviruses in Jerusalem wastewater.  Hurst et al. (1988) compare the
  efficiency of three methods for detecting adenoviruses.
         Gerba et aL (1989) suggest that perhaps one of the most promising techniques for
  detecting viruses in water and other environmental samples may be the use of gene probes,
  a methodology that is rapid, inexpensive, sensitive, does not rely on viral cultivation, and
  could  allow  development of field  test kits.  Limitations are that the  technique  cannot
  distinguish between infectious and noninfectious viruses without use of cell culture, and it
  currently requires radioactive labels for increased sensitivity. More research is needed to
  evaluate the sensitivity and reliability of this technique in environmental media.
        Another major information need is a better understanding of the  epidemiology and
 relative infectivity under varied environmental conditions of enteric viruses, particularly
 rotaviruses, coronaviruses, picornavirases, and the other viruses that have been less studied
 than poliovirus and HAV.   Although infection by droplet aerosols has been studied
 extensively for many enteroviruses, much less is known about infection via dry particulate
 aerosols, which appear to be of more concern for sludge application practices.
        Additional data are needed on persistence and transport of viruses,  particularly those
 that have not been as well characterized, in  soil, surface water and groundwater. A  better
 understanding of  viral transport and  persistence in  relation to physical conditions  and
 predatory microorganisms would provide necessary data for improved modeling. Likewise,
 development of predictive parameters for densities of viruses in treated sludge would be of
value in determining  initial concentrations for adjusting application rates,,
       In summary, the following information is needed to improve the usefulness of the
Pathogen Risk Assessment Model and to allow for a more reliable risk assessment of land
application of sewage sludge:
       • Simple and accurate standardized methods for detecting and quantifying,  by type,
         pathogenic  viruses  in treated sludge  destined for land application, in final
         composted sludge products, and in environmental media;
      •  Improved understanding of minimum infective doses, particularly low-dose effects
          and MIDs for sensitive subjects;
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       • More accurate persistence and transport data on all pathogenic viruses of major
         concern in sludge;
       • Development of an index of soil types that would correlate capacity for solute
         transport and suitability for sludge application (also valuable  for onsite waste
         disposal or solid waste disposal);
       • Research on subsurface injection of sludge and the relative probability of virus
         transport in groundwater; and
       • Epidemiologic studies evaluating enteric virus transmission.

8.2.  MODEL DEVELOPMENT
       The literature review yielded a considerable body of information that invalidated
some of the recommended starting parameters for the model. Among these are density of
viruses in treated sludge, inactivation rates in aerosols, temperature-dependent inactivation
rates in soil  and water, and fractional transfers among compartments.   Many of these
parameters are represented by default values included in the model code (U.S. EPA, 1989a).
An update of the model should include revision of default parameters for parasites (U.S.
EPA, 1990)  and bacterial pathogens (U.S. EPA, 1991) as well as viruses.  Exposure
equations for Practices IV and V should be revised to allow  for exposure to undiluted
composted sludge during application.
       Equations describing temperature-dependent inactivation of viruses in environmental
media should be revised in light of new information.  The slope and intercept of each
inactivation function is fitted empirically to literature values. Regression analysis of the data
in Table 3-5 resulted in a correlation coefficient of 0.32. A log transform of the inactivation
rates was used  in derivation of other temperature-dependent rate equations  (U.S. EPA,
1989a). When the  data of Table 3-5 were analyzed similarly, a correlation coefficient of
0.67 was found.  The data were then grouped by virus type and analyzed separately. The
results of this analysis are shown in Table 8-1. The correlation coefficient for poliovirus and
echovirus are not good enough to justify use of the parameters derived from the data. The
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                       TABLE 8-1

          Parameters for Temperature-Dependent
         Inactivation of Viruses in Aquatic Systems*
Virus
Type
Polio
Coxsackie
Echo
Number of
Samples
18
20
11
Slope
(m)
-2.44
-2.61
-2.51
Intercept
(b)
0.076
0.049
0.009
Correlation
Coefficient
0.28
0.87
0.17
"Fitted to the equation Y = mX + b, where
      X = Temperature
      Y = Log10 of the inactivation rate
      m = Slope
      b = Intercept
                          8-4

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data for coxsackie viruses  represent  several studies, although  in  many of them the
temperature was given as a range rather than a specific value (the mid-point of the range
was used in Figure 3-1 and in the regression analysis). Therefore, the slope and intercept
derived from the data may be less valid than indicated.  More data are necessary for
derivation of reliable virus-specific slope and intercept factors.
       The model  has no  temperature  and  relative humidity parameters linked with
inactivation of viruses in air.  As Ijaz et al. (1985a,b,c; 1987) have  demonstrated, these
factors are significant determinants in virus persistence. Viruses have been shown to survive
longer than bacterial indicators under hostile environmental conditions (Shuval et al., 1989),
and some viruses live significantly longer than  others (Ijaz et al., 1985b).
       Likewise, according to Yates and Yates (1988), subsurface transport models do not
incorporate effects  of temperature, pH,  moisture content, organic matter, etc. on viral
inactivation, and  it  appears  no research is in process to  incorporate such environmental
effects into subsurface transport models.  Some researchers, however, have  developed
alternate methods for model solute transport that implicitly include such factors but avoid
the input of environmental data needed by more  explicit models (Jury, 1982, 1983; Jury et
al., 1982, 1986; Sposito et al., 1986).
       Tim and Mostaghimi (1991) have developed a numerical model for predicting virus
fate and transport  through  variably saturated porous medium in the subsurface under
transient flow conditions.   Simulation of the vertical movement of viruses  applied in
wastewater  effluents  and  sewage  sludges  is accomplished by  combining expressions
describing  transient water flow and subsurface  solute transport of virus  particles.  The
current model does not  now  have an adequate component for describing  subsurface
transport.   Consequently, continued  model  development  suggests the  importance of
evaluating existing aerosol and  subsurface transport models to determine if improvements
based on recent modeling efforts  can be applied  to the Pathogen Risk Assessment Model.
       As Gerba (1987) points out, the existing  models for predicting virus transport and
survival should be field-validated. Likewise, the  Pathogen Risk Assessment Model should
also be field-validated.  Perhaps  it is  some  combination  of the most useful and effective
portions of several models that will ultimately prove  to be the best predictor of pathogen
                                         8-5

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 risk. However, the size limitations of a PC-based model make it imperative to refine the
 selection of the parameters of most significance in assessing pathogen risk from land
 application of sludge.
        In summary, the following revisions would improve the accuracy of the model:
        •  Revision of default parameter values, especially for inactivation rates in aerosols
          and temperature-dependent inactivation rates in soil and water;
        •  Revision of temperature-dependent inactivation algorithms;
        •  Incorporation of factors for humidity and temperature in inactivation equations
          for aerosols;
        •  Incorporation of subroutines  for subsurface transport  under  conditions of
          transient flow; and
        •  Incorporation of factors  to allow for subsurface transport  through solution
          channels, cracks, etc.
In addition, field validation of the model's predictions is necessary before the model can be
considered an accurate predictor of health risk. The model would be easier to use if it were
revised  to operate from a menu rather than the current lengthy questionnaire format.
                                       8-6

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 Sobsey, M.D., P.A. Shields, F.H. Hauchman, R.L. Hazard and L.W. Caton. 1986. Survival
 and transport of hepatitis  A virus in soils, groundwater,  and wastewater.   Water Sci.
Technol. 18(10): 97-106.

Sorber,  C.A. and K.J. Outer. 1975.  Health and hygiene aspects of spray  irrigation. Am.
J. Public Health 65(1):  47-52.

Sorber,  C.A., B.P. Sagik and B.E. Moore.  1979. Aerosols from municipal wastewater spray
irrigation. Jn: Utilization of Municipal Sewage Effluent and Sludge on Forest and Disturbed
Land, W.E. Sopper and S.N. Kerr, Ed. University Park, Pennsylvania State University Press
p. 255-263.
                                       9-17

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Sorber, C.A., B.E.  Moore,  D.E. Johnson,  HJ.  Harding  and R.E.  Thomas.   1984.
Microbiological aerosols from the application of liquid sludge to land.  J. Water Pollut.
Control Fed.  56(7): 830-836.

Spendlove, J.C. and K.F. Fannin.  1982. Methods of characterization of virus aerosols. In:
Methods in Environmental Virology, C.P. Gerba and S.M. Goyal, Ed. Marcel Dekker, Inc.,
NY. p. 261-329.  (Cited in Sattar and Ijaz, 1987)

Spillmann, S.K., F. Traub, M. Schwyzer and R. Wyler.  1987.  Inactivation of animal viruses
during sewage sludge treatment. Appl. Environ. Microbiol. 53(9): 2077-2081.

Sposito, G., R.E. White, P.R. Darrah and W.A. Jury.  1986.  A transfer function model of
solute transport through soil:  3. The convection-dispersion equation. Water Resour. Res.
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Sproul, O J.   1978.  The efficiency of wastewater unit processes in risk  reduction.  In:
Proceedings of the Conference on Risk Assessment and Health Effects of Land Application
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Research and Technology, The  University of Texas at San Antonio, TX. p. 282-296.

Taylor, D.H.,  R.S. Moore, and L.S. Sturman.  1981.  Influence of pH and electrolyte
composition on adsorption of poliovirus by soils and minerals. Appl. Environ. Microbiol.
42(6): 976-984.

Tierney, J.T., R. Sullivan and E.P. Larkin. 1977. Persistence of poliovirus 1 in soil and on
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Environ. Microbiol. 33(1): 109-113.

Tim, U.S. and S. Mostaghimi.  1991.  Model for predicting virus  movement through soils.
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Traub, F., S.K. Spillmann and R. Wyler. 1986. Method for determining virus inactivation
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USDA (U.S. Department of Agriculture), Soil Conservation Service. 1980, Soil Survey of
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USDA (U.S. Department of Agriculture), Soil Conservation Service.  1981a.  Soil Survey of
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USDA (U.S. Department of Agriculture), Soil Conservation Service. 198 Ib. Soil Survey
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                                       9-18

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USD A (U.S. Department of Agriculture), Soil Conservation Service.  198 Ic. Soil Survey of
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USDA (U.S. Department of Agriculture), Soil Conservation Service.  1985. Soil Survey of
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USDA (U.S. Department of Agriculture), Soil Conservation Service.  1989. Soil Survey of
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U.S. EPA  1989b.  Standards for the Disposal of Sewage Sludge; Proposed Rule. Federal
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U.S. EPA.  1990.  Preliminary Risk Assessment for Parasites in Municipal Sewage  Sludge
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Vaughn, J.M., E.F. Landry, C.A. Beckwith and M.Z. Thomas.  1981.  Virus removal during
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Vilker, V.L., L.H. Frommhagen, R. Kamdar and S. Sundarum.  1978. Applications of ion
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                                      9-19

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Ward, R.L. and E.W. Akin. 1984. Minimum infective dose of animal viruses. CRC Crit.
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Ward, R.L. and C.S. Ashley.  1977.  Inactivation of enteric viruses in wastewater sludge
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Ward, R.L. and C.S.  Ashley.  1978. Heat inactivation of enteric viruses in dewatered
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Ward, B.K. and L.G. Irving.  1987.  Virus survival  on vegetables spray-irrigated with
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Ward, R.L. and P.E. Winston.  1985. Development of methods to measure virus inactivation
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Ward R.L., C.S. Ashley and  R.H. Moseley.  1976.  Heat inactivation of poliovirus in
wastewater sludge. Appl. Environ. Microbiol. 33(3): 339-346.

Ward, R.L., G.A. McFeters and J.G. Yeager.  1984.  Pathogens in Sludge: Occurrence,
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                                       9-20

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Wellings, P.M. 1987. Methods of enterovirus recovery from different types of soils. Jn:
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Wellings, P.M., A.L. Lewis and C.W. Mountain.  1976. Demonstration of solids-associated
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Yanko, W.A  1988.  Occurrence of Pathogens  in Distribution and Marketing Municipal
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Yates, M.V.  1984. Virus Persistence  in Ground Water, Ph.D. dissertation; University of
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Yates, M.V.   1990.   The Use of Models for Granting Variances from Mandatory
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Yates, M.V., C.P. Gerba and L.M. Kelley. 1985.  Virus persistence in groundwater.  Appl.
Environ. Microbiol. 49(4): 778-781.

Yates, M.V., L.D. Stetzenbach, C.P. Gerba and N.A  Sinclair.   1990.  The effect of
indigenous bacteria on virus survival in ground water. J. Environ. Sci. Health A25(l): 81-
100.
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Yeager, J.G. and R.T. O'Brien.  1979a.  Enterovirus inactivation in soil, Appl. Environ.
Microbiol. 38(4): 694-701.

Yeager, J.G. and R.T. O'Brien.  1979b.  Structural changes  associated with poliovirus
inactivation in soil. Appl. Environ. Microbiol. 38(4): 702-709.

Young, D.C. and D.G. Sharp.  1977.  Poliovirus aggregates and their survival in water.
Appl. Environ. Microbiol. 33(1): 168-177.
                                        9-22

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  APPENDIX A



MODEL OVERVIEW
      A-l

-------

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                              MODEL OVERVIEW
      Five  sludge  management  practices,  representing land  application and  D&M
management options, are included in the present model and are numbered I-V. They are
listed in Table A-l and illustrated in Figures A-l through A-5.  Two of the practices use
heat-dried or composted sludge for residential purposes and three use liquid sludge for
commercial farming operations.  Since each of these two types of sludge represents a wide
range of sludge treatment possibilities, the extent of treatment or conditioning prior to land
application must be approximated for each  case (i.e., the pathogen concentration in the
applied sludge must be specified).  The computer model represents the compartments and
transfers  among compartments of the five management practices. The compartments are
the various locations, states or activities in which sludge or sludge-associated pathogens exist;
they vary to some extent among practices. In each compartment, pathogens either increase,
decrease  or remain the same in number with time, as specified by "process functions"
(growth,  dieoff or no population changes) and "transfer functions"  (movement between
compartments). The population in each compartment, therefore, generally varies with time
and is determined by a combination of initial pathogen input,  "transfer functions" and
"process functions." The populations of pathogens in the compartments representing human
exposure locations (designated with an asterisk in Figures A-l through A-5 and in Table A-
2), together  with appropriate intake and infective dose data, are used to estimate human
health risk.
      Although each practice listed in Table A-l is  different, all  five practices share
common  characteristics.  All compartments that appear in one or more of the five sludge
management practices are listed in Table A-2.  Those compartments  with an asterisk
represent exposure sites  for the human receptor:
      •     3* inhalation or ingestion of emissions from application of sludge or
            tilling  of sludge/soil;
      •     5* inhalation or ingestion of windblown or mechanically generated
            particulates;
      •     6* swimming in  a pond fed by surface water runoff;
                                       A-3

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                                  TABLE A-l
                 Sludge Management Practices and Descriptions in
                        Pathogen Risk Assessment Model
PRACTICE
                 DESCRIPTION"
    n

   m

   IV


    V
                  Application of Liquid Treated Sludge for Production of
                  Commercial Crops for Human Consumption
Application of Liquid Treated Sludge to Grazed Pastures

Application of liquid Treated Sludge for Production of
Crops Processed before Animal Consumption

Application of Dried or Composted Sludge to Residential
Vegetable Gardens

Application of Dried or Composted Sludge to Residential
Lawns
"Source:  U.S. EPA, 1989a
'Two types of sludge are used in this model - liquid and dried/composted.  The extent
of treatment or conditioning prior to application is variable and must be, determined
for each case.
                                     A-4

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Subsurface
   Sol!
Groundwater
  Offsite
   Well
                                Application
                               Incorporation
                                  Soil
                                 Surface
                                  Crop
                                 Surface
                                Harvesting
Commercial
   Crop
                                 Irrigation
                                  Water
                                                        Application/Tilling
                                                           Emissions
                                 External
                                  Source
                                FIGURE A-1
  Input/Output Diagram for Practice I - Application of Liquid Sludge
    for Production of Commercial Crops for Human Consumption
                                    A-5

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                           Application
Application/Tilling
   Emissions
                              Soil
                             Surfaca
                              Crop
                             Surfaca
                               v    17
                             Animal
                           Consumption
                             Manure
                            Irrigation
                             Water
                            External
                             Sourca
                           FIGURE A-2

Input/Output Diagram for Practice II — Application of Liquid
                  Sludge to Grazed Pastures
                               A-6

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                                                  Application/Tilling
                                                     Emissions
                              External
                              Sourca
                              FIGURE A-3
Input/Output Diagram for Practice 111 - Application of Liquid Sludge
  for Production of Crops Processed before Animal Consumption
                                A-7

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Subsurfaca
  Soli
                                                   Application/Tilling
                                                      Emissions
                            FIGURE A-4

    Input/Output Diagram for Practice IV - Application of Dried
     or Composted Sludge to Residential Vegetable Gardens
                               A-8

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Subsurfaca
  Soil
                                                   Application/Tilling
                                                      Emissions
                            FIGURE A-5

    Input/Output Diagram for Practice V - Application of Dried
            or Composted Sludge to Residential Lawns
                                A-9

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                    TABLE A-2




Compartments Included in the Sludge Management Practices
Compartment
Name and Number

Application
Incorporation
Application/Tilling
Emissions
Soil Surface
Participates
Surface Runoff
Direct Contact
Subsurface Soil
Groundwater
Irrigation Water
Soil Surface Water
Offsite Well
Aerosols
Crop Surface
Harvesting
(Commercial) Crop
Animal Consumption
Meat
Manure
Milk
Hide
Udder
aSource: U.S. EPA, 1989a
Asterisk indicates exposure
Liquid Sludge D lied/Composted
Management Practices Sludge Management
Practices
1
1
2
2*b
4
5*
6*
7*
8
9
10
11
12*
13*
14
15
16*






compartments.
11
1
2
3*
4
5*
6*
7*
8
9
10
11
12*
13*
14


17
18*
19
20*
21
22

111 IV
1 1
2 !
3* 3*
4 4
5* 5'"
6*
7* 7'"
8 8
9
10
11 11
12*
13*
14 14
15 15
16'"
17
18*
19
20*
21
22

V
1

3*
4
5*

7*
8


11


14









                          A-10

-------
      •      7* direct contact with sludge-contaminated soil or crops (including
             grass, vegetables, or forage crops);
      •      12* drinking water from an offsite well;
      •      13* inhalation and subsequent ingestion of aerosols from irrigation;
      •      16* consumption of vegetables grown in sludge-amended soil;
      •      18* consumption of meat or
      •      20* consumption of milk from cattle grazing on or consuming forage
             from sludge-amended fields.
      The first 14 compartments, most of which are common to all practices, are described
below.
      APPLICATION (1) represents the application of sludge to a field (default size 10 ha)
or to a yard or garden of specified size.  Liquid sludge may be applied by spread-flow
techniques,  by  spray, or by subsurface  injection.   The application  rate and pathogen
concentrations are variables to be entered by the user of the model.  During spread-flow
and spray application, sludge will be spread thinly on the soil, where it will be subject to
drying, heating  and solar radiation,  thus losing the protective benefits provided by bulk
sludge. It is assumed, therefore, that inactivation will occur at a rate characteristic of the
organism in soil at 5°C above the ambient temperature (Brady, 1974; USDA, 1975). It is
also assumed that liquid sludge is absorbed by the upper 5 cm of soil surface during this
time.  The default time period for transfer from APPLICATION (1) to INCORPORATION
(2) is 24 hours, which allows a field treated with liquid sludge to dry sufficiently to plow or
cultivate. If the injection option is chosen, the liquid sludge goes directly to SUBSURFACE
SOIL (8)  at hour  10.  During spray application of liquid sludge or application of dry
composted sludge, droplets or loose particulates may become airborne.  Liquid aerosols are
modeled  by a  Gaussian-plume air dispersion  model that calculates  the  downwind
concentration of airborne particulates.  Dry particulate emissions are calculated using
models for  generation of dust by tilling or mechanical disturbance of  soil.  Both are
represented  as transfers  from APPLICATION  (1)  to  APPLICATION/TILLING
EMISSIONS (3).
                                       A-ll

-------
       INCORPORATION (2) involves the mixing, by plowing or cultivation, of the sludge
 and sludge-associated pathogens evenly throughout the upper 15  cm of soil.   Process
 functions associated with this compartment are the same as for the relevant pathogen type
 in soil. Particulate emissions generated by cultivation are represented by a transfer from
 INCORPORATION (2) to APPLICATION/TILLING EMISSIONS (3) beginning at hour
 24, extending for enough time to cultivate the field (at a rate of 5 ha/hr) or till the garden
 or lawn (at a rate of 0.002 ha/hr).  At the end of this  time, all remaining pathogens are
 transferred to SOIL SURFACE (4).
       APPLICATION/TILLING EMISSIONS  (3*) is an exposure compartment that
 receives the dust, or suspended particulates, generated by application or by the  tilling of
 dried sludge or sludge-soil mixture. It also receives aerosols generated by spray application
 of liquid sludge.  All process functions associated with this compartment are incorporated
 in the aerosol subroutines. Exposure in this compartment is by inhalation but,  as in all
 inhalation exposures, model simplification limits the exposure to the pathogens assumed to
 be ingested after the inhaled dust or aerosol spray is trapped in the upper respiratory tract,
 swept back to the mouth by ciliary action and swallowed.
       SOIL SURFACE (4) describes the processes occurring in the upper 15 cm (Practices
 I, IV and V) or upper 5 cm (Practices II and HI) of the soil layer. Microbes are inactivated
 at rates characteristic for moist soil at 5°C above the chosen ambient temperature (Crane
 and Moore, 1986; Kibbey et al., 1978). Transfers  from SOIL SURFACE (4) occur by wind
 to WIND-GENERATED PARTICULATES (5), at a time chosen by the user, by surface
 runoff and sediment transport after rainfall events to SURFACE RUNOFF (6), by a person
 walking through the field or contacting soiled implements or clothing or by other casual
 contact to  DIRECT CONTACT  (7), by leaching  after  irrigation, or  rainfall  to
 SUBSURFACE SOIL (8), by resuspension during irrigation or rainfall to SOIL SURFACE
 WATER (11), or at harvest to CROP SURFACE (14).
      WIND-GENERATED PARTICULATES (5*) describes the  airborne particulates
 generated by wind.  Process functions are the same as for the organism in air-dried soil at
 the ambient temperature (Crane  and Moore, 1986; Kibbey et al.,  1978).  The  exposed
individual is standing in the field or at a user-specified  distance downwind  from  the field
                                     A-12

-------
  during a windstorm.  The wind-generated exposure is calculated from user-specified values
  for duration and severity of the windstorm (default values, 6 hr at 18 m/sec (40 mph)).
        SURFACE RUNOFF (6*) is an exposure compartment describing an onsite pond
  containing pathogens transferred from SOIL SURFACE (4) by surface runoff and sediment
  transport after  rainfall.  These processes  are described by  a  separate  subroutine.
  Inactivation rates in this compartment are characteristic of microbes in water and are much
  lower than rates for soil. Water is removed from the pond by infiltration and recharge of
  the groundwater aquifer, but it is assumed that no microbes are transferred by this process.
  The human receptor is an individual who incidentally ingests 0.1 L of contaminated water
  while swimming in the pond. This compartment is also an exposure compartment for cattle
  drinking 20 L of water daily from the pond (Practice II).
        DIRECT CONTACT (7*) is the exposure compartment for a worker or a child less
  than 5 years old who  plays in or walks  through the field, yard or garden, incidentally
  ingesting 0.1 g of soil or vegetation at the daily geometric mean concentration of pathogens.
  This  human receptor  represents the worst-case example of an  individual  contacting
  contaminated soil  or soiled clothing or implements. No process functions are associated
  with this compartment because it is strictly an exposure compartment.
        SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
  subsurface soil between 5 or  15  cm depth and  the water table.  It also serves as the
  incorporation site  for  subsurface injection of liquid sludge.   Process  functions in
  SUBSURFACE SOIL (8) are the same as for moist soil at the ambient temperature. The
  transfer from SOIL SURFACE (4) occurs after each rain or irrigation event as a result of
  leaching from the soil surface. The time of transfer is calculated by dividing the depth of
  rainfall or  irrigation by the infiltration  rate.   Transfer  to GROUNDWATER (9) is
  arbitrarily set at one hour later. At present, the relation between unsaturated water flow
  and subsurface transport has not been well-established.  Thus, this model lacks a satisfactory
  subroutine to describe pathogen transport from the subsurface soil to groundwater. Instead,
~ user-specified variables are used to describe the fraction of pathogens transferred from SOIL
  SURFACE  (4) to  SUBSURFACE  SOIL  (8) and from SUBSURFACE  SOIL to
  GROUNDWATER.
                                        A-13

-------
      GROUNDWATER (9)  describes  the flow of pathogens in the  saturated zone.
Process functions are the same as for other water  compartments. Transfers occur to
IRRIGATION WATER (10) if the water is needed for irrigation or to OFFSITE WELL
(12*) if the  water is used  for drinking.   The number of pathogens  transferred  to
IRRIGATION WATER (10) is based on the concentration of pathogens in the groundwater
compartment and the total depth of irrigation.  The transfer to OFFSITE WELL (12) is
described by a modification of the subsurface solute transport model of van Genuchten and
Alves (van Genuchten and Alves,  1982).   Because microbes in suspension are passively
transported by bulk water flow and interact with soil particles by adsorption and desorption,
they behave similarly enough to dissolved chemicals that existing solute transport models can
be used to describe their fate in the saturated zone (Gerba, 1988).
      IRRIGATION WATER (10) describes the transfers for pathogen-contaminated water
used for irrigation.   No processes  are associated with this compartment because it is
intended as a transition compartment.  Irrigation of the field, lawn or garden takes place a
user-specified number of tunes each week. This irrigation water may come from either an
onsite well fed by GROUNDWATER (9) or from an outside source erf treated, liquid
sludge.  The  default conditions vary by practice.  In either  case, AEROSOLS (13) are
generated unless a non-spray option is chosen. Spray irrigation is the default since it would
be most likely to  cause a significant exposure to workers or offsite persons. In addition to
aerosol  emissions, irrigation transfers pathogens to CROP SURFACE (14) and to SOIL
SURFACE WATER (11).
      SOIL SURFACE WATER (11) represents any irrigation water or rainfall in contact
with the ground prior to infiltration. This compartment describes the  temporary suspension
of pathogens in such a water layer and their subsequent transfer to CROP SURFACE (14)
or to SOIL  SURFACE (4).   Process  functions  are the same  as for other water
compartments.
      OFFSITE WELL (12*) is the exposure site for a human receptor drinking 2 L/day
of contaminated  water whose pathogens  have been transported through grpundwater.
Process  functions are the same as for groundwater.  The groundwater transport subroutine
supplies the concentration of pathogens in the well at a user-specified distance from the
                                      A-14

-------
 source.  No transfers out of the compartment are specified because it is an exposure
 compartment only.
       AEROSOLS (13*) describes fugitive emissions from spray irrigation, which occurs
 at  a default rate of 0.5 cm/hr for 5 hr.   The source  of irrigation water producing
 AEROSOLS can be an onsite well (i.e., GROUNDWAIER) or liquid sludge. A Gaussian-
 plume model is used to calculate concentrations of airborne microbes downwind.  The
 human receptor is an onsite worker or a person offsite who is exposed during the time of
 irrigation.
       CROP SURFACE (14) describes contamination of vegetable or forage crops by
 transfer of user-specified amounts to or from SOIL SURFACE (4),  from IRRIGATION
 WATER (10), or to or from SOIL SURFACE WATER (11). Process functions are not well
 characterized but are assumed to be influenced by drying,  thermal inactivation and solar
 radiation; they are thus most characteristic of pathogens in surface soil.

       These preceding 14 compartments are common to most of the five practices modeled.
 The following descriptions of the five management  practices help clarify the differences
 among the practices.

 Practice I:  Application of Liquid Treated Sludge for Production of Commercial Crops for
 Human Consumption.
      Liquid sludge may be applied as fertilizer/soil conditioner for the production of
 agricultural crops for human consumption or for animal forage or prepared feed.  Both
 existing (CFR, 1988) and proposed (U.S. EPA, 1989b) regulations prohibit direct application
 of sewage sludge to crop surfaces.  Therefore, this model practice is designed for a single
 application of liquid sludge, which is incorporated into the soil before the crop is planted.
 Regulations also require various waiting periods before  the planting of crops that will be
 consumed uncooked by humans. These restrictions, however, are optional in the model and
 can be tested.
      Vegetables can be grown aboveground, on-ground  or below-ground.  These are
represented by tomatoes, zucchini and carrots, respectively.  At HARVESTING (15) time,
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all pathogens remaining on CROP SURFACE (14) are transferred to HARVESTING (15),
which represents a single harvest of all of the crop. The same process functions apply as
in CROP SURFACE (14).  The crop is held for 24 hours before being processed.  The
number  of pathogens  is  then transferred  to COMMERCIAL CROPS  (16*),  the
compartment in which further processing takes place. The number of pathogens/crop unit
following processing is calculated  in this compartment and  is the figure used  hi  the
vegetable-exposure  risk calculations.   A  24-hour  pathogen  exposure  is computed by
Subroutine VEG. Pathogen concentrations are determined as number/crop unit for each
sludge management practice. Before being consumed, vegetables normally are processed
in some way. Included in the program is a series of user-selectable processing steps. The
user has the option of choosing any or all processing steps and of specifying some conditions
within processing steps. The human receptor is a person who consumes minimally prepared
vegetables (washed, but not peeled or cooked) at a rate of 81 g tomatoes, 80 g zucchini or
43 g carrots per eating occasion (Pao et al., 1982).

Practice II:  Application of Liquid Sludge to Grazed Pastures.
      In this practice, liquid sludge is applied as fertilizer, soil conditioner and irrigation
water for the production of forage crops for pasture. This model practice is designed for
repeated applications of liquid sludge, initially on a field with a standing forage crop used
for pasture.  It is assumed that spray irrigation will be used because this method is effective
for delivering large amounts of sludge to a large area.  In this way, the pasture is also used
as a final treatment and disposal system for the treated sludge. The irrigation rate, the total
weekly depth and the number of times per week can be specified by the user. A sludge
solids concentration of 5% is assumed.
      The model assumes that each hectare of pasture supports 12 head of cattle, although
both area and herd size may be  varied.  This may be a higher density than is the common
practice for fields that receive no irrigation, but with adequate  irrigation, sufficient forage
is  expected to be produced.  Current and proposed regulations require various waiting
periods before animals can be grazed.  These requirements can also be tested by the model.
                                       A-16

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      ANIMAL CONSUMPTION (17) describes the ingestion of CROP SURFACE (14)
by cattle grazing, in the pasture. Transfers from ANIMAL CONSUMPTION (17) are to
MEAT (18*),,MANURE (19) and MILK (20*).
      MEAT (18*) is the compartment describing transfer of pathogens from ANIMAL
CONSUMPTION (17) to meat. The human receptor is assumed to consume 0.256 kg of
meat daily (U.S. FDA, 1978).  Contamination of meat by gut contents during slaughter or
by systemic infection by sludge-borne pathogens can be modeled.  The  model allows for
inactivation  of pathogens in meat by  cooking, assuming reasonable cooking times and
temperatures.
      The production and consumption of milk from cattle pastured on the sludge-amended
field are modeled  when the dairy cattle option is chosen.  The default condition is for
consumption of raw milk because commercial production of milk poses an extremely small
hazard of exposure to pathogens.  In  the model, contamination from dirty utensils and
careless handling  are combined as a transfer from the  manure-contaminated udder
[MANURE (19)], which occurs at each  milking.  All three pathogens can enter milk by this
route. MILK (20*) is the compartment describing production and consumption of milk from
cattle pastured on  the sludge-amended field when the dairy cattle option is chosen.  The
default condition models the consumption of raw milk that has been stored for 24 hours.
In exposure  calculations, it is assumed  that the human receptor consumes 2 kg milk/day,
roughly three times the national average milk consumption (U.S. FDA, 1978).

Practice III:  Apph'cation of Liquid Treated Sludge for Production  of  Crops Processed
before Animal Consumption.
      In this practice, liquid sludge is applied as fertilizer, soil conditioner and irrigation
water for the production of forage crops to be processed and stored for animal feed. This
model practice is designed for repeated applications of liquid sludge, initially on a field with
a standing forage crop.  It is assumed that spray irrigation will be used for the application
of liquid sludge, because this method is effective for delivering large amounts of sludge to
a large area. In this way, the field is also used as a final treatment and disposal system for
the treated sludge.  The rate, the total weekly depth and the number of irrigations per week
                                      A-17

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can be changed by the user.  A sludge solids concentration of 5% is assumed. The risks to
the human receptor are similar to those for the preceding practice, i.e., exposure through
meat or milk, in addition to direct contact with the forage grown in the field.

Practice IV:  Application of Dried or Composted Sludge to Residential Vegetable Gardens.
      Dried or composted treated sludge may be sold or given away to the public as a bulk
or bagged product for use as fertilizer or soil conditioner for the production of domestic
garden crops for human consumption. Although  some studies have shown that composting
is highly effective in removing pathogens from sludge (Wiley and Westerberg, 1969), other
studies have shown that bacterial pathogens may grow in dried or composted sludge to
concentrations of IxlO6 organisms/kg dry weight (U.S. EPA, 1988). Exposure of individuals
to materials used in home gardening would be expected to be more frequent than exposure
in a commercial agricultural setting. Therefore,  this practice would be expected to pose a
greater risk of infection. This model practice is designed to describe the application of dried
or composted  treated sludge, which is incorporated into the soil before the  crops are
planted.
      Vegetables can be grown aboveground,  on-ground  or below-ground.  These are
represented by tomatoes, zucchini and carrots,  respectively. The user may specify the
proportions of above-ground,  on-ground and below-ground crops in the garden.  At
HARVESTING (15) tune,  all pathogens  remaining  on  CROP  SURFACE (14) are
transferred to HARVESTING (15).  The same process  functions  apply  as  in  CROP
SURFACE (14). The crop is held for 24 hours before  being processed.  The number of
pathogens is then transferred to CROP (16*), the compartment in which further processing
takes place.  The number of pathogens/crop unit following processing is calculated in this
compartment and is the figure used in the vegetable-exposure risk calculations. A 24-hour
pathogen  exposure is  computed by Subroutine VEG.   Pathogen concentrations are
determined  as number/crop  unit for each  sludge  management  practice.  Pathogen
concentrations are determined  as number/crop unit.
      Before being consumed, vegetables normally are  processed in some way. Included
in the program is a series of user-selectable processing steps. The  user hjis the option of
                                      A-18

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 choosing any or all processing steps and of specifying some conditions within processing
 steps.  In the default condition, the human receptor is a person who consumes minimally
 prepared vegetables (washed, but not peeled or cooked) at a rate of 81 g tomatoes, 80 g
 zucchini or 43 g carrots per eating occasion (Pao et al., 1982).

 Practice V:  Application of Dried or Composted Sludge to Residential Lawns.
       Dried or composted treated sludge may be made available to the public as a bulk or
 bagged product to be sold or given away for use as fertilizer or soil conditioner for the
 preparation of a seed bed for domestic lawns. Individuals engaged in preparing a seed bed
 for a lawn are likely to come into contact with the soil and any additives used to improve
 the seed bed. If the soil or the additives contain pathogens, this practice would be expected
 to pose a risk of infection. This model practice is designed to describe  the application of
 dried or composted treated sludge, which is incorporated into the soil before the lawn is
 seeded.
       The main exposure in this practice is for the lawn worker or for a child younger than
 5 years old who plays in or walks through the lawn site, incidentally ingesting soil or crop
surface at the daily geometric mean concentration of pathogens.  This human receptor
represents the worst-case example of an individual contacting contaminated soil or soiled
 clothing or implements. Before all pathogens have been transferred to  SOIL SURFACE
 (4), exposure is at the pathogen concentration found in undiluted sludge whereas, after the
transfer, the concentration is that calculated for the soil-sludge mixture.
       After 840 hours, the time assumed necessary for the lawn to require mowing, the
lawn is mowed weekly, and a fraction of the pathogens associated with CROP SURFACE
(14) are transferred to DIRECT CONTACT (7). ,It is assumed that the person mowing the
lawn is exposed by inhalation and/or ingestion at each mowing.
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                                 REFERENCES

Brady, N.C.  1974. The Nature and Properties of Soil. Macmillan Publishing Co., Inc., New
York. p. 266-276.

CFR (Code of Federal Regulations). 1988.  Disease, 40 CFR 257.3-6.  In:  U.S. EPA,
Criteria for classification of solid waste disposal facilities and practices, 40 CFR 257.3.

Crane, S.R. and J.A. Moore.  1986. Modeling enteric bacterial die-off: A review. Water
Air Soil Pollut. 27:411-439.

Gerba, C.P.  1988.  Alternative Procedures for Predicting Viral and Bacterial Transport
from the Subsurface into Groundwater.  Draft.  U.S. Environmental Protection Agency,
Cincinnati, OH.

Kibbey,  HJ., C. Hagedorn and E.L. McCoy. -1978. Use of fecal streptococci as indicators
of pollution in soil.  Appl. Environ. Microbiol. 35:711-717.

Pao, E.M., Fleming, K.H., Guenther, P.M. and Mickle, S.J. 1982. Foods commonly eaten
by individuals: Amount per day and per eating occasion. U.S. Department of Agriculture,
Economics Report No. 44.
             r                                                              f-
USDA (U.S. Department  of Agriculture).   1975.  Soil taxonomy: A basic system of soil
classification for making and  interpreting soil surveys.   Soil  Conservation Service.
Agriculture Handbook No. 436.

U.S. EPA.  1988.   Occurrence of Pathogens in Distribution and Marketing Municipal
Sludges.  Prepared by County Sanitation Districts of Los Angeles County for the Health
Effects Research Laboratory, Office of Research and Development, Research Triangle Park,
NC EPA/600/1-87/014.  NTTS PB88 154273.

U.S. EPA. 1989a.  Pathogen Risk Assessment for Land Application of Municipal Sludge,
Volumes I and II. Office of Health and Environmental Assessment, Environmental Criteria
and Assessment Office, Cincinnati, OH.  EPA/600/6-90/002A,B. NTIS PB90-171901/AS
and PB90-171919/AS.

U.S. EPA. 1989b.  Standards  for the Disposal of Sewage Sludge; Proposed Rule. Federal
Register 54(23): 5886-5887.

U.S. FDA (U.S. Food and Drug Administration). 1978. FY78 Total Diet Studies.

van Genuchten, M.T. and W.J. Alves. 1982. Analytical solutions on the one-dimensional
convective-dispersive solute transport equation.  USDA Technical Bulletin No.1661.

Wiley, B.B. and S.C. Westerberg. 1969. Survival of human pathogens in composted sewage.
Appl. Microbiol.  18: 994-1001.

                                       A-20
                         4U.S. GOVERNMENT PRINTING OFFICE: 19 92 -61(8 -003/ 
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