EPA/600/R-94/110
                                                September 1993
PRELIMINARY RISK ASSESSMENT FOR PATHOGENS
  IN LANDFILLED MUNICIPAL SEWAGE SLUDGE
     Environmental Criteria and Assessment Office
     Office of Health and Environmental Assessment
     Office of Research and Development
     U.S. Environmental Protection Agency
     Cincinnati, Ohio 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.
                                         11

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                                       PREFACE

        Section 405 of the Clean Water Act requires the U.S. Environmental Protection Agency
 (U.S. EPA) to develop and issue regulations that identify:   (1)  uses  for sludge, including
 disposal; (2) 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
xof pathogenic organisms.
        This  report is one of a series  whose purpose  is to assess the potential risk to human
 health posed by parasites, bacteria and viruses hi municipal sludge and to develop preliminary
 risk assessments for each of these types of pathogens.  This document evaluates human health
 risks from pathogens in landfilled municipal sludge.
                                             m

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                          DOCUMENT DEVELOPMENT
Dr. Norman E. Kowal, Work Assignment Manager
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, OH 45268

Authors

Marialice Wilson, Project Manager
Dr. Charles T. Hadden
Mary C. Gibson
Environmental Analysis Division
Science Applications International Corporation
Oak Ridge, TN 37831
Internal SAIC Reviewers

Dr. Barney W. Cornaby
Dr. Elizabeth D. Caldwell
Environmental Analysis Division
Science Applications International Corporation
Oak Ridge, TN  37831
U.S. EPA Reviewers

Dr. Arthur Chiu
Dr. Sheila L. Rosenthal
Human Health Assessment Group
Office of Health and Environmental Assessment
U.S. Environmental Protection Agency
Washington, D.C.  20460

Walter Jakubowski
Microbiology Research Division
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH  45268
                                        IV

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

2.     INTRODUCTION	  2-1

3.     LITERATURE REVIEW	  3-1

      3.1.   SIGNIFICANCE OF ENTERIC PATHOGENS	  3-1
            3.1.1.  Persistence and Density in Treated Sludge	  3-4
            3.1.2.  Transmission/Exposure Routes	  3-8
            3.1.3.  Epidemiology  	  3-10
            3.1.4.  Infective Dose	3-13
      3.2.   PERSISTENCE/SURVrVABILITY IN MEDIA	  3-15
            3.2.1.  Persistence/Survivability in Soil	  3-15
            3.2.2.  Persistence/Survivability in Water	3-21
      3.3    TRANSPORT	3-24
            3.3.1.  Transport in Soil  . .  .	  3-25
            3.3.2.  Transport in Groundwater	3-29

4.     DESCRIPTION OF THE SLDGFILL MODEL AND SITES FOR MODEL
      RUNS	  4-1
      4.1.   OVERVIEW OF THE SLDGFILL MODEL AND METHODOLOGY  .  4-1
      4.2.   TEST SITES FOR MODEL RUNS	  4-4
            4.2.1.  Site 1: Anderson County, TN	  4-4
            4.2.2.  Site 2: Chaves County, NM	  4-5
            4.2.3.  Site 3: Clinton County, LA	  4-7
            4.2.4.  Site 4: Highlands County, FL	  4-8
            4.2.5.  Site 5: Kern County, CA	  4-9
            4.2.6.  Site 6: Yakima County, WA	4-10
      4.3.   USE OF SOIL AND CLIMATOLOGICAL DATA	  4-11

5.     RATIONALE FOR PARAMETER VALUES	  5-1
      5.1.   ASSUMPTIONS	  5-1
      5.2.   INPUT PARAMETERS   	  5-2
            5.2.1.  Site-Specific Parameters	  5-3
            5.2.2.  Bulk Sludge Parameters	  5-3
            5.2.3.  Pathogen-Specific Data  	  5-4
            5.2.4.  Groundwater Transport Parameters	  5-6
            5.2.5.  Parameters  for Evaluation of Test Sites  	  5-6

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                      TABLE OF CONTENTS (continued)

6.     RESULTS  	  6-1
      6.1.   MODEL RESULTS FOR BACTERIA	6-1
            6.1.1.  Site-Specific Parameters	  6-1
            6.1.2.  Bulk Sludge Parameters	  6-3
            6.1.3.  Pathogen-Specific Data  	  6-3
            6.1.4.  Groundwater Transport Parameters	  6-7
      6.2.   MODEL RESULTS FOR VIRUSES	  6-7
            6.2.1.  Site-Specific Parameters	  6-11
            6.2.2.  Bulk Sludge Parameters	6-11
            6.2.3.  Pathogen-Specific Parameters	6-11
            6.2.4.  Groundwater Transport Parameters	6-15
      6.3.   RESULTS FOR TEST SITES	6-15

7.     CONCLUSIONS   	-	  7~1
      7.1.   RESULTS OF MODEL CALCULATIONS	  7-1
      7.2.   UNCERTAINTIES	  7'2
            7.2.1.  Model Parameters .	  7"3
            7.2.2.  Environmental Media  	  7-3
            7.2.3.  Infectivity	  7'3

8.     RESEARCH NEEDS  .	  8-1

9.     REFERENCES 		  9-1
                                      VI

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                                 LIST OF TABLES




No.                                     Title                                   Page




2-1    Pathogens of Concern in Landfilled Municipal Sludges  	  2-3




2-2    Infective Doses  	,	  2-5




5-1    Site-Specific Parameters	  5-8




5-2    Sludge Loading Parameters	  5-9




5-3    Pathogen-Specific Parameters   	  5-10




5-4    Groundwater Transport Parameters  	  5-11




5-5    Site-Specific Parameters for Example Sites   	  5-12




6-1    Site-Specific Results for Bacteria  	  6-4




6-2    Bulk Sludge and Pathogen-Specific Results for Bacteria	  6-5




6-3    Groundwater Transport Results for Bacteria   	  6-8



6-4    Site-Specific Results for Viruses	6-13



6-5    Bulk Sludge and Pathogen-Specific Results for Viruses	'.	  6-14




6-6    Groundwater Transport Results for Viruses  	  6-16



6-7    Well Setback Distances for Protection of Wellwater Consumers from Viruses   .  6-17
                                          vu

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                                 LIST OF FIGURES

No.                                     Title

2-1    Migration Pathways for Pathogens from Municipal Sludge Landfills	  2-7

4-1    Probability of Infection from a Representative Pathogen, Graphed as a Function
       of Exposure Concentration at Various Minimum Infective Doses (MIDs)  ....  4-3

5-1    Offsite Migration Pathways for Pathogens from Sewage Sludge Landfills  ....  5-2

6-1    Kinetics of Bacterial Transport	  6-2

6-2    Effect of INACTB [P(13)] on Kinetics of Bacterial Transport to Wellwater  ...  6-6

6-3    Effect of VGW [P(22>] on Bacteria Transported to Wellwater	  6-9
6-4   Graphical Determination of Well Setback Distances for Bacterial
      Pathogens	
6-10
6-5   Kinetics of Virus Transport	6-12

6-6   Comparison of Bacterial Concentrations in Wellwater at Test Sites	6-18
                                         Vlll

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                             1.  EXECUTIVE SUMMARY
       A methodology and accompanying model, SLDGFILL (sludge monofill), have been
developed to assess  the  risk to human health posed by parasites,  bacteria and viruses  in
municipal sewage  sludge disposed of hi  sludge-only landfills (monofills).  The following
information is required for risk assessment of pathogens hi sludges: (1) nature and amount of
sludge; (2) physical attributes of the monofill site; (3) properties  of the pathogens hi sludge,
including their concentration and survival  capabilities (i.e., inactivation rates); (4) minimum
infective dose to  the  receptor; and (5) fate and transport  of  the  pathogens  hi  soil and
groundwater.  The SLDGFILL model assumes an exposure pathway  of infiltration  from the
monofill to groundwater and subsequent ingestion from a well by a human receptor.  SLDGFILL
determines the probability of infection of the human receptor by pathogens hi wellwater.
       SLDGFILL calculations indicate that there are risks to people drinking wellwater 50 m
downgradient from a sludge landfill. Under default conditions, the risk from both bacterial and
viral pathogens exceeds the U.S. EPA risk  target of not more than one excess infection hi
10,000people annually (risk of 1 x IQr4 annually or approximately 2.57 x Ifr7 per day). Computer
simulations using  conservative default  values provided in the model result hi  a calculated
probability of bacterial infection for a person, drinking wellwater 50 m downgradient from a
sludge monofill, of  5.5xlO'2 per day  when the minimum infective dose (MID) is  10,  or
2.9 x 1Q-6 per day when the MID is 20.  Compared with the U.S. FJ*A's benchmark probability
of infection (approximately 2.57 xlO'7 per day) from pathogens hi groundwater, the projected
probability of infection from bacteria in the 50-m distant well ranges from 200,000-fold above
the benchmark when MID=10 to only ten-fold above when MID=20.  Computer simulations
for viruses showed that the maximum daily  risk of infection from consumption of wellwater
from a similar well is 1X10"2, using field-derived inactivation data combined with other default
values.
       Based on sensitivity analyses, the parameters to which the model is most sensitive are
infective dose, pathogen density hi sludge, and inactivation rate in water.  The most sensitive
parameters for determining pathogen concentration hi the wellwater describe inactivation and
retention during subsurface transport.
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       Six representative test sitesin California, Florida, Iowa, New Mexico, Tennessee and
Washingtonwere selected to provide diversity in geographic location,  topography,  soil type,
rainfall pattern and temperature. Computer simulations varying site-specific parameters showed
little difference among the six sites in pathogen risk to groundwater consumers.  However, depth
to groundwater significantly  influenced pathogen transport, indicating that pathogen movement
from the monofill to downgradient wells is more likely hi areas with a  shallow water table.
       Because pathogens are inactivated and retained by soil during subsurface transport,  the
concentration of pathogens hi the well can be reduced by increasing the vertical distance from
the sludge trench to groundwater and the lateral distance of the well from the sludge trench.  By
using the SLDGFILL model  iteratively, it is possible to calculate a setback distance for the well
that is protective of human  health.  For example, using default values for bacterial density,
infective dose and inactivation rates, a safe  setback distance  of 110 m was calculated.  That
distance was reduced to 60 m when an MID of 20 was used hi place of MID=10.  The
calculated setback distance for viruses, using the field-derived  inactivation rate constant of 0.13
logio/day with other default values and an MID of 1, was 165 m. The disparity between these
results emphasizes the importance of accurate field-determined rates of inactivation and retention
of representative pathogenic  organisms.  In other words, the difficulty inherent hi producing a
risk assessment model representative of real  conditions is the  limited information available  for
the most important parameters.
       To better satisfy the  information requirements of pathogen risk assessment modeling,
carefully controlled field and laboratory research is needed hi the following subject areas:
             methods  that are rapid, accurate and standardized for detection of pathogens hi
              sludge and hi environmental media;
             understanding of retention/inactivation of pathogens hi the subsurface and reliable
              inactivation rates, at least for  representative pathogenic microorganisms;
             data on  subsurface transport  and retention  rates under  varying  environmental
              conditions;
             epidemiological or field data for verifying and  validating the model; and
             data on  infectivity of  representative pathogens  hi  drinking  water  at  low
              concentrations.
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                                 2. INTRODUCTION

       This preliminary risk assessment uses the computer model, SLDGFILL (sludge monofill),
to determine the probability of human infection from pathogenic microorganisms following
municipal sludge landfilling.  The SLDGFILL model, described in Pathogen Risk Assessment
Methodology for Municipal Sludge Landfilling (U.Sr. EPA, 1993), is under development by the
U.S. Environmental Protection Agency (U.S.EPA) for use as a tool hi evaluating the potential
risk  to humans from  microorganisms in municipal sludge.   The  major  purposes  of this
preliminary risk assessment are  (1) to evaluate the risk sludge pathogens pose to a human
receptor, using available literature data to satisfy  the parameters needed by the model for
simulating  sludge landfilling and (2) to identify areas that need further research in order to
                      %
produce meaningful health risk assessments.  This document reports the results of a literature
review designed to find existing data on the various parameters and the results of running the
SLDGFILL model using these data for a broad range of realistic parameters. The document also
describes the use of the model to calculate a setback distance for a well that is protective of the
human receptor ingesting wellwater.
       Risk assessment is "the characterization of the potential adverse health effects of human
exposures to environmental hazards" (NRC, 1983).  It entails hazard identification,  dose
response assessment, exposure assessment and risk characterization. Epidemiological methods,
which  evaluate incidence, distribution and control of pathogens and their associated diseases,
may be used to predict adverse outcomes  from exposure to pathogens.  However, when these
methods are not sensitive enough, or sufficient epidemiological data are not available, risk
modeling can  be used as a tool to estimate potential health risks  from pathogens (Rose and
Gerba, 1991).
       Bacterial, viral, parasitic (protozoan and helminthic) and fungal pathogens hi municipal
sewage sludge have been identified as potential hazards to human health (WHO, 1981;  Kowal,
1982, 1985; U.S. EPA 1988a,b). Many factors affect the type and concentrations of pathogens
found hi municipal sludge.  As a result, the levels of pathogens reported hi surveys of treated
municipal sludge vary greatly (Reimers et al.,  1981, 1986; Pedersen,  1981; U.S. EPA, 1988a,
1991a,b,c; Yanko, 1988).  Gastroenteric  disease is the most common result of infection with
                                          2-1

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these pathogens (U.S. EPA, 1988b, 1993). Some pathogens may invade other tissues, resulting
in more serious diseases such as poliomyelitis, meningitis, encephalitis, hepatitis and myocarditis
(enteric viruses) (Kowal, 1985); visceral larval migrans and amebiasis (parasites) (Kowal, 1985);
and septicemia, typhoid or paratyphoid fever, meningitis, abortion and septic arthritis (bacteria)
(Domingue,1983).  The most significant pathogenic microorganisms in municipal  sludge are
listed in Table 2-1.  Fungi have been omitted from this listing because they are generally not
pathogens of concern hi landfilled sludge.
       Because the number of species of pathogens found in sludge is large, the U.S. EPA has
identified representative pathogens for use hi the risk assessment process. This selection is based
on the pathogens' known presence hi sludge; their ability to cause human disease; the adequacy
of available data on minimum infective  dose, hardiness outside the human host and routes of
                                                                >
infection; and survivability typical of other group members. Of these representative  pathogens,
the SLDGFILL model examines Salmonella spp. representing enteric bacteria and enteroviruses
representing enteric viruses. The U.S. EPA selected Ascaris lumbricoides to represent parasites,
including helminths and protozoa. Because Ascaris is so large that its subsurface transport for
any significant distance is unlikely, results from modeling these parasites are not included hi this
document.   However, smaller parasites such as the protozoan cysts of Cryptosporidium and
Giardia could pass through the soil,  and they should be modeled as transport and inactivation
data required by the SLDGFILL  model become available.
       The number of viable organisms  to which a host is exposed is the "dose" of pathogens.
The dose response can be no infection,  subclinical infection (no apparent  illness) or  infection
with illness (Kowal, 1985).  For a given species or strain, there are no clearly defined exposure
levels that always result in infection, as evidenced by the wide range of infective doses for
Salmonella (102 -  1010) shown hi Table 2-2.  Both  the  susceptibility of the host and the
pathogen's ability to overcome theliost defenses contribute to the virulence of a pathogen. By
avoiding infection in susceptible subpopulations,  such as infants or immunocompromised
individuals, disease may be avoided in the total .population (Regli et al., 1991).
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         Table 2-1.  Pathogens of Concern in LandflUed Municipal Sludges
                                                     Organism
Bacteria
                                Campylobacter jejuni
                                Escherichia coli pathogenic strains)
                                Leptospira spp.
                                Salmonella spp.
                                Shigella spp.
                                VZfcrio cholerae
                                Yersinia enterocolitica
                                Yersinia pseudotuberculosis
Viruses
                                Adenovirus
                                Astrovirus
                                Calicivirus
                                Coronavirus
                                Enteroviruses
                                       Coxsackievirus A
                                       Coxsackievirus B
                                       Echovirus
                                       Hepatitis A virus
                                       New enteroviruses
                                       Poliovirus
                                Hepatitis E
                                Norwalk virus and other small round structured viruses
                                Parvovirus  and parvovirus-like agents
                                Reovirus
                                Rotavirus
                                         2-3

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Table 2-1. (continued)
Type
Protozoans
Helminths
Organism
Blastocystis hominis
Cryptosporidium spp.
Dientamoeba fragilis
Entamoeba coli
Entameoba histotytica
Giardia lamblia
Ancylostoma duodenale
Ascaris lumbricoides
Clonorchis sinensis
Enterobius vermicularis
Hymenolepis nana
Strongyloides stercoralis
Taenia spp.
Tax.oca.ra spp.
Trichuris spp.
Source: U.S. EPA, 1988b; Thurn, 1988; Hurst, 1989; CDC, 1991.
2-4

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Table 2-2. Infective Doses3
Organism
Infective Dose
Range
Bacteria
Escherichia coli
Salmonella (pathogenic
strains)
Shigella
Vibrio cholerae
104
1Q2b
10-102
103
10M010
lOMO10"
10-109
lOMO11
Viruses
Echovirus 12
Poliovirus '
Rotavirus
HIDso 919 PFU
HID! 17 PFU predicted
1 TCIDjo
< 1PFU
HIDso ~10fmc
fflDzs 1 ffu estimated
17-919 PFU
1-1 X 107 6 TCID5o (infants)
0.2-5. 5 X106 PFU (infants)
9xlO-1-9xl04ffuc
Parasites
Entamoeba coli
Cryptosporidium spp.
Giardia lambtia
Helminths
1-10 cysts
10 cystsd
1 cyst (estimated)
legg
1-10 cysts
10-100 cystsd
NA
NA
a Kowal, 1985.
b Metro, 1983a.
0 Ward etal., 1986.
d Casemore, 1991.
HID = Human infective dose
TCIDso = Tissue culture infectious dose for 50% response.
PFU = Plaque forming units.
ffu = Focus-forming units.
NA = Not applicable.
2-5

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       This risk assessment uses infection rather than disease as the detection endpoint.  The
lowest dose that will infect any exposed individual, the minimum infective dose (MID), has been
estimated for a few microbial pathogens (Table 2-2).  A single virus particle or Giardia cyst may
be sufficient to cause infection (U.S. EPA, 1992). However, as Table 2-2 shows, the range of
infective doses  for each organism is broad.
       The sludge disposal practice  determines the exposure pathways.  Sludge-only landfilling
(monofilling) by trench, area fill or diked containment is the disposal option examined in this
assessment. Landfilling is defined as burying sludge beneath a soil cover that exceeds the depth
of the plow zone (Walsh, 1978). Sludges disposed by this method have undergone primary and
secondary treatment processes, which may concentrate pathogenic organisms in the sludge, and
usually some degree of stabilization  (such as aerobic or anaerobic digestion, composting or lime
treatment), which generally reduces  pathogen levels.  Potential migration pathways, or exposure
routes, of pathogens from the disposal site to  a target receptor, include groundwater, surface
water  runoff and  suspended particulates (aerosols)  (Figure 2-1).  In this risk assessment,
groundwater is the only pathway of  concern because both surface water runoff and aerosols can
be controlled by good landfill management practices (U.S. EPA, 1988a; 1989a).
       A description of  the SLDGFILL  model can be  found in Section  4.   Additional
information about the risk assessment methodology and model, including basic assumptions,
model limitations and sources of uncertainty, is available  hi Pathogen Risk Assessment
Methodology for Municipal Sludge Landfilling (U.S. EPA,  1993).
                                           2-6

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2-7

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                             3. LITERATURE REVIEW

3.1.   SIGNIFICANCE OF ENTERIC PATHOGENS
       Enteric pathogens are excreted by infected individuals. These pathogens have been found
hi municipal sewage and hi sewage sludges.  The U.S. EPA is charged with regulating the
disposal of these sludges so that humans are protected from any adverse effects resulting from
exposure to sludge pathogens.   If the pathogens present  hi sludge persist or move in the
environment once they are placed hi a sludge monofill, the potential for human exposure exists.
In order to estimate the probability of the risk of human infection after sludge is monofilled,
enteric pathogens (bacteria, viruses and parasites) must be studied to determine the numbers of
pathogens that would typically be found hi monofilled sludge, the routes by which they may be
transmitted  to humans,  evidence  of actual  transmission by these routes and the number of
organisms necessary to cause an adverse effect hi humans.
       Bacteria.  The families Enterobacteriaceae and Vibrionaceae,  often referred to as enteric
bacteria, are found  hi  municipal wastewater and sewage sludge  (Elliott and Ellis,  1977;
Pedersen, 1981; Kowal, 1985; Pahren,  1987; U.S. EPA, 1988b, 1991a). Salmonella,  Shigella
and Campylobacter  spp.  are major causes of such  gastrointestinal problems as diarrhea,
dysentery,  vomiting and cramps (Feachem et al., 1983).  The severity of the infection depends
on the health and age of the patient,  the serotype of the organism and  the infective dose.
Invasion of other tissues by some of these organisms may result  hi septicemia, typhoid or
paratyphoid fever, meningitis, abortion and septic arthritis (Domingue, 1983).  These diseases
are not prevalent hi areas with high standards for public health and sanitation; treatment with
appropriate drugs keeps mortality rates low hi developed countries.
       Bacteria are intermediate hi size (200 - 10,000 nm hi dia), larger than viruses but smaller
than helminth eggs and  most protozoan cysts (Yates and Yates, 1988).  They may be  removed
by filtration through and adsorption to soil and are therefore not expected to be as mobile hi the
environment as viruses.  Many enteric bacteria are also poorly able to maintain metabolism hi
soil and are, therefore,  likely to die during prolonged periods hi soil.
       Viruses.  Many  sources document the presence of enteric viruses hi sewage sludge and
 then- potential health risks (Pedersen,  1981; Feachem et al., 1983; IAWPRC, 1983; Kowal,
                                           3-1

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1985; Sorter and Moore, 1987; Rao and Melnick, 1987; Yates and Yates,  1988; U.S.  EPA,
1988a,b, 1991c; Hurst, 1989).  Following excretion, the number of viruses.will not increase
because viruses can only replicate within living host cells.  However, viruses  have the ability
to persist in the environment for many weeks, particularly with cool temperatures (<15C)
(Feachemetal., 1983).
       Most enteric viruses cause gastroenteritis or influenza-like disease. Viruses may cause
more severe disease with the possibility of disability or death if spread to other organs, such as
the liver or central  nervous system.  A significant waterborne health threat from viruses
throughout the world is hepatitis A because of the severity of the disease and because the virus
is more resistant to disinfection than many other pathogens  (Sobsey et al., 1991).
       Viruses are the smallest infectious disease agents found hi sludge, ranging hi size from
20 run to 200-300 nm (Berk,  1983;  Yates and Yates,  1988). Because of their  small size, they
are more mobile in the environment  (U.S. EPA, 1991c).  They have the potential to travel great
distances (-920 m reported by Noonan and McNabb,  1979)  if not retarded by filtration and
adsorption (U.S. EPA, 1993).
       Bacteriophage have teen used extensively hi studies of viral adsorption  and inactivation
because the assay for bacteriophage is fast and inexpensive. Then: environmental behavior is
presumed to be similar to human pathogenic viruses  (Hurst, 1989),  and their release into the
environment for research purposes is presumed safe.  Although they are not  enteric pathogens,
bacteriophage have also proved useful hi transport studies of enteric pathogenic viruses for these
same reasons.  Because the available data on human viruses  hi the environment are limited,
experimental results that include bacteriophage are reported hi this document hi order to indicate
the range and diversity of viral adsorption, transport and inactivation.  However, bacteriophage
are not used hi this preliminary risk assessment to calculate human risk from enteric pathogens.
       Parasites. Pathogenic helminths and protozoa are discussed extensively  hi Kowal (1985)
and U.S. EPA (1988a,b; 1990).  Protozoan cysts and oocysts, dormant structures resistant to
adverse environmental conditions, and helminth ova are present hi sewage and sludge (U.S.
EPA,  1991b). Reimers et al.  (1986) report that the most commonly found helminths in sludge
in the southeastern  and  northern United States  are Ascaris,  Toxocara  and  Trichuris.
Cryptosporidium spp., Entamoeba spp., Giardia lamblia andDientamoebafragilis are currently
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the most prevalent protozoa in human stool specimens in the United States (CDC, 1991). The
types and levels  of the parasites  in sludges are determined by the degree  of non-human
contribution to the sewage system and the levels of disease in the contributing human population
(Kowal, 1985).
       Protozoa colonize the  gastrointestinal tract of humans and other mammals. Although
severe cases of such protozoan diseases as amebiasis may produce organ damage (liver, lung,
brain) and death, most cases produce mild to severe gastroenteritis that is  rarely fatal in
developed countries. However, cryptosporidosis can be life-threatening hi immunocompromised
individuals (Casemore, 1991).
       The pathogenic helminths, including nematodes (hookworms, pinworms, roundworms and
whipworms), trematodes (flukes) and cestodes (tapeworms), frequently require more than one
host to complete  their life cycles.  Some helminth pathogens are only incidental parasites to
humans (Kowal,  1985).
       After ingestion, eggs of some species of hehninths (e.g., pinworms) develop into larvae
and colonize the gut. With other species  (e.g., Ascaris), eggs hatch in the intestine, and the
migration of larval  stages through the body can cause serious tissue and organ damage before
maturation hi the  gut. These adult forms that reside hi the gut of the host cause malnutrition and
anemia.
       Helminth  eggs and most protozoan cysts are relatively large,  ~ 15,000 - 80,000 nm dia
and  -5,000 - 25,000 nm dia, respectively.  When compared with 200 - 10,000 nm bacteria and
20 - 300 nm viruses (U.S. EPA, 1988b; Yates and Yates, 1988), helminth eggs and the larger
protozoan cysts are greater by up to three orders of magnitude.  Therefore,  these eggs and
protozoan cysts are more likely to be physically prohibited from passing from sludges, through
the soil, to groundwater (Kowal, 1985; Sorber and Moore, 1987; Reneau et al., 1989). Because
groundwater is the significant exposure pathway from sludge landfilling and helminth ova do not
typically  reach  groundwater,  modeling results for Ascaris are not included  hi this risk
assessment.  However, the smaller protozoan oocysts, such as Cryptosporidium (-5,000 nm)
 (Daly, 1983) and Giardia (7,000 -15,000 nm) (Jakubowski et al., 1991), which are comparable
 hi size to some bacteria, have the potential to be transported to the  groundwater.  The lack of
 information on the behavior of these cysts and oocysts  hi the environment limits accurate
                                          3-3

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modeling  of protozoan risks (Smith, 1992).  But their  small size,  low infective dose and
relatively  long survival suggest possible risk and the need to model the behavior of these
organisms as data become available.
3.1.1.   Persistence and  Density in Treated Sludge.  The incidence  of infection  in the
community  and the type of sludge treatment are two  of the  major determinants  of the
concentration and  types of  pathogens  in final sludges destined  for  landfilling.   Secondary
treatment  processes  such  as the activated  sludge process  and  stabilization processes  (lime
treatment or thermophilic digestion) reduce the numbers of enteric microbial pathogens (U.S.
EPA, 1988a). Reduction depends on the type of waste  treatment and the conditions (e.g.,
moisture,  temperature) of the treatment methods.
       Comparisons of study results are difficult because of the variations in reported units and
                                f
the differences hi concentration techniques used, as well as  the lack of sensitivity of enumeration
methods.  Results would be more meaningful if standard methods for enumeration of pathogens
in sludges and soil were developed and adopted.   Units  are reported here as in  the original
research papers, and only comparable units are compared.  However, this does not ensure that
concentration or detection techniques were the same or even the same sensitivity.
       Bacteria. Persistence and density of bacteria hi sludge are discussed hi Pedersen (1981),
Rao et al. (1986b), Yanko (1988) and U.S.  EPA (1988b;  1991a).
       Organisms  become concentrated hi sludges during wastewater treatment.   Prior to
monofilling, sludges are  subjected  to further treatment that will reduce  the numbers of
organisms.    Conventional  sludge treatment processes  (aerobic  and anaerobic digestion,
composting, air drying and lime stabilization) lower the concentrations of pathogenic bacteria
in sludges (Ward et al., 1984; U.S. EPA, 1988b).  Lime  stabilization and  composting achieve
reductions of 2->4 orders of magnitude.  Bacterial reductions during air drying depend on the
time the sludge is  held at various moisture  levels; at 50% solids, reductions of 2-3 orders of
magnitude per month can be achieved.
       Yanko (1988) found  detectable levels of Salmonella in roughly 20% of 144 samples
collected from 24 facilities representing many different treatment processes.  In most cases the
densities were low (0.1 - 810 MPN/g).  Yanko (1988) reports toxigenic Escherichia  coli in
 <1% of the samples from various treatment facilities and no Campylobacter hi any sample.
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The sensitivity of Campylobacter to oxygen and its susceptibility to drying (Doyle and Roman,
1982) make it unlikely to persist through any composting or sludge drying process  (Yanko,
1988).
       Metro  (1983b) reported high  levels (106 -  109  organisms/g wet wt) of  Yersinia
enterocolitica in anaerobically digested  sludges.  Dudley et al. (1980) reported 2  x 10s CFU/g
total suspended solids for Y. enterocolitica in a digested sludge sample. Yanko (1988) reported
detectable levels of Yersinia in 10% of samples from various treatment facilities.
       Inadequate or incomplete sludge treatment or recontamination may result hi regrowth of
some bacteria.   The bacteria that cause salmonellosis  are capable of significant regrowth in
sludges (Ward  et al., 1984).  However, bacterial regrowth is not likely hi a sludge monofill
because of the competition and antagonism of other microbes.
       Reported densities of pathogenic bacteria in treated sludge vary widely because of the
variation hi concentrations of organisms in raw wastes, the sludge treatment process used, and
the lack of standardization and sensitivity of detection methods.   Yanko (1988) notes that
bacterial densities  hi sludge products vary  greatly  between sludge treatment facilities and
between samples of products from the same facility.  Temperature, retention time, moisture,
solids content,  pH and the interaction of these factors may affect concentrations of bacteria hi
sludge products.
       According to Brigmon et al. (1992), accurate detection of Salmonella in environmental
samples is difficult and time consuming.  Among several methods that have been developed in
recent years, they report that the enzyme-linked immunosorbent assay is both reliable, sensitive
and fast hi detecting Salmonella enteritidis in environmental-samples.
       In summary, the pathogen content of treated sludges is highly  variable.  The  mean
density of only one bacterial species in treated sludges ranged from 0.1  MPN/g dry  wt   -
 10,000 MPN/g dry wt of sludge, according to U.S. EPA (1991a).  The model default value for
the density of bacteria hi treated sludges is 5 X104 pathogens/kg dry wt, an approximate median
value between the highest densities reported and those too low to be significant.
       Viruses.  Additional information on the persistence and density of viruses in sludge may
be found in Pedersen (1981), Kowal (1985) and U.S. EPA (1988b; 1991c).
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       Conventional sludge treatment processes designed to stabilize sludge and reduce volatile
solids also reduce virus levels but do not eliminate them (Melnick, 1987).  Sludge treatment
processes may be less effective in removing viruses than in removing bacteria (Sobsey et al.,
1980) because viruses may be protected in sludge by adsorption onto sludge solids.  According
to U.S. EPA (1988a), reports indicate that viral inactivation in sludge is slow, citing recoveries
of viruses up to 6 months after sludge injection into soil.
       Temperature, loss of moisture and the presence of competing aerobic microorganisms are
the most important of the factors affecting the stability of viruses hi wastewater sludges. Other
contributing factors are pH levels; presence of detergents, ammonia and certain salts; and the
type of virus (Hurst, 1989).  Municipal sludges vary greatly, and one or more of these factors
may be operable under landfill conditions.
       According to Scheuerman et al. (1991), the single most important factor influencing
inactivation in sludge is temperature. They found that virus inactivation was lower hi anaerobic
digestion than hi aerobic despite the higher temperature in anaerobic digesters.  They suggest
that components of aerobic digestion may accelerate inactivation or that components of anaerobic
digesters may protect viruses.
       Feachem et al. (1983) conclude  that  ..."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 periods to ensure that all parts of the mass are heated."
Because hepatitis A virus (HAV) was infective at 80C hi the presence of high concentrations
of some salts, Rao et al. (1986a)  suggest that higher temperatures  may be  necessary for
disinfection of sludge. Spillman et al. (1987) conclude that thermal treatment (60C) of sludge
to inactivate thermolabile viruses  (those that  are inactivated by heat)  should be followed by
anaerobic mesophilic digestion to eliminate thermostable viruses (those that are not affected
greatly by changes of temperature) that are sensitive to chemicals and microbes.
        For enteric viruses, U.S. EPA (1991c) reports levels of 0 - 260 units/g of sludge and for
all viruses 1.7 - 360 PFU/100 mL of sludge. The model default value for the density of viruses
in sludge is IXIO5  pathogens/kg dry wt, an approximate median value between the highest
densities reported and those too low to be significant.
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       Parasites.  Information on persistence and density of parasites in sludge may be found
in Reimers et al. (1981), Kowal (1982, 1985), and U.S. EPA (1988a,b; 19P:b).
       Protozoans secrete a tough membrane around a rounded precyst, forming thick-walled,
environmentally resistant, dormant cysts  or oocysts that are excreted in the feces of the host
(Kowal, 1985).  A wide range of densities of cysts and oocysts has been reported in treated
sewage sludge, from none to as many as 38,700 oocysts/g dry wt (Cryptosporidium) (Kayed and
Rose,  1987).  According to Reimers et al.  (1981)  and Leftwich et al. (1981), 89% of final
municipal sludges from southern states contained large numbers of viable parasite cysts and ova.
Yanko (1988) found no protozoan cysts when he examined the  final sludge products from 24
treatment  facilities distributed throughout the United States. Regional variations as well as
differences in detection techniques may account for this apparent discrepancy.  According to
Packham (1990), there are no routine, reliable methods to differentiate protozoan species and
assess the viability of oocysts.
       Jakubowski et al. (1991) and Sykora et al. (1991) report direct counts of Giardia cysts
in 11 municipal sludges ranging from 70 - 30,000 cysts/L.  The highest average concentration
was ~ 1,723 cysts/L from a plant where sludge was dewatered but not digested. Using special
filters and direct immunofluorescent assays with monoclonal antibodies for Cryptosporidium.,
Crawford and  Vermund (1988) reported an average  of 5,180  cysts/L hi raw sewage hi the
Tucson area and 1,300 cysts/L hi treated sewage.
       Protozoan oocysts may survive several months  hi water (Barer and Wright, 1990; Payer
and Ungar, 1986).  The limits of environmental survival have not been established,  but  hi
laboratory experiments, both temperature and moisture level have affected inactivation.
       According to Craun (1986) and West (1991), physical removal by filtration of drinking
water is probably the best method for preventing waterborne giardiasis.  However, outbreaks
of waterborne disease attributed to protozoans have been reported where filtration of the water
supply occurred  (Rose,  1988;  Hayes et  al., 1989).   Karanis et al. (1992) suggest  that small
protozoan cysts, such as Cryptosporidius and Giardia, evade filter barriers.  Processes used for
disinfection of water, such as the use of free chlorine, chloramines, chlorine dioxide, ozone and
ultraviolet light, are not as effective for inactivating protozoans  such as Giardia as they are for
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some bacteria and viruses (Sobsey, 1989; West,  1991; Karanis et al., 1992).  Korich et al.
(1990) report that Cryptosporidium parvum oocysts are 14 times more resistant to chlorine
dioxide and 30 times more resistant to ozone disinfectants than Giardia cysts exposed under the
same conditions to these disinfectants.
       Fertilized helminth ova, shed free or in proglottids hi the feces of the final host,  are the
resistant  stage found  in sewage sludge (U.S. EPA, 1991b).  Reimers et al. (1986) report
geometric mean densities of 565, 265, 270, 370 eggs/kg dry wt (one or more ofAscaris spp.,
Trichuris trichiura,  Trichuris vulpis or Toxocara spp.) hi municipal sludge samples from four
northern states; 89% of the sludge samples contained eggs.
       Although parasite  eggs become  concentrated hi sludges during ordinary wastewater
treatment,  high temperatures (>55C) during such  stabilization  processes as aerobic and
particularly anaerobic digestion have been reported to inactivate the eggs (Reimers et al., 1986).
Lulling and caustic stabilization increase inactivation (Mbela et al.,  1990).  Inactivation varies
both with the type of digestion process and the helminth species (Black et al., 1982).  According
to Pike et al. (1988),  viable ova were not destroyed by heat alone at temperatures <51C for
1 hour, but heat at 55C for 15 minutes was lethal.  Drying beds are most effective (100% kill)
if moisture is <5% (Leftwich et al., 1981).
       U.S. EPA (1991b) reviewed densities of helminths hi treated sludges and reports mean
levels of  <10 -  11,000  ova of one species/kg dry wt of sludge.   They report densities  of
protozoan cysts of 0 - 38,700 oocysts/g dry wt of sludge.
3.1.2. Transmission/Exposure Routes.
       Bacteria.  Enteric bacteria are transmitted by the direct fecal-oral route under conditions
of poor sanitation (Shigelld) or by ingestibn of contaminated food and water (Salmonella spp.)
(Domingue, 1983). Shigella sonnet was the most commonly reported bacterial pathogen causing
disease transmitted by water hi the United States during the period 1986-1988  (Levine et al.,
1990).   Outbreaks of disease  from  water  contaminated  with  other  bacterial pathogens
(Campylobacter,  Vibrio  and Yersinid)  have also  been reported  (Stelzer and Jacob,  1991;
Feachemetal., 1983).
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       Some bacteria persist as reservoirs in infected animals.  According to the World Health
Organization Working Group (WHO, 1981),  the major reservoir of human salmonellosis in
Great Britain was contaminated poultry, meat and dairy products resulting from inadequate food
hygiene. Domingue (1983) reported that animals are the greatest single source of salmonellosis
in the United States.  Although animals may contribute to sewage sludge, animal contamination
probably occurs more frequently in surface water than in the  groundwater receiving leachate
from sludge monofills.
       Viruses.  Enteric  viruses are primarily transmitted from person to  person by the
fecal/oral  route; transmission may also occur by direct personal  contact or contact with
contaminated surfaces, by contact with water during swimming or other water activities, possibly
by the airborne route and by ingestion  of contaminated food or water.   Concentrations of
viruses in the  feces of infected individuals can be quite  high  (>106 -  109 infectious virus
particles/g feces) even if the individual does not have symptoms of disease (Feachem et al.,
1983).  If these viruses find then- way into the groundwater following sludge  monofilling,
localized epidemics of gastroenteric illness may occur. Animal reservoirs of the pathogenic
viruses likely to be found hi sludge have not been shown to be significant, but a few cases have
been reported suggesting the transmission of HAV from nonhuman primates (Feachem et al.,
1983).  These animals are not likely to be contributing to municipal sewage.  Transmission of
HAV by water  has been reported.
       Parasites.  The transmission or exposure route of enteric parasites to humans includes
ingestion of contaminated food or water;  direct contact with the parasite hi feces, soil or water;
and consumption  of undercooked flesh of an infected animal.  After sludge  monofilling,
contaminated groundwater is expected to  be  the most significant exposure pathway.  Other
pathways  can be eliminated by good management practices.
       Cryptosporidium is transmitted by the fecal-oral route (Packham, 1990; Casemore, 1991),
by direct person-to-person contact and by  water (Rose, 1988;  Crawford and Vermund, 1988;
Hayes et al., 1989; Smith, 1992).  the oocyst is stable in the environment and is infective to
a number of animal species, including mammals,  birds and fish (Casemore, 1991; Payer and
Ungar, 1986;  Rose,  1988).  C. parvum, which is probably  responsible  for  most human
                                          3-9

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infections, is also infectious to livestock animals (Casemore, 1991), thereby creating another
source of human infection when infected animal wastes are included hi landfilled sewage sludge.
      Entamoeba histofytica, Giardia lamblia and Balantidium coli may be transmitted through
contaminated water (Kowal, 1985; Jakubowski et al., 1991).  Giardia cysts are environmentally
stable and Giardia infections have been reported in a number of species of animals.  Cross-
species transmission may occur (Jakubowski,  1990).
      Many of the cestodes and nematodes of concern hi sludge can be contracted by ingestion
of water, food or soil that has  been contaminated with the ova.  However, hookworm and
threadworm are usually contracted by direct human contact with the organism in soil.
3.1.3.  Epidemiology.
      Bacteria.   The Centers  for Disease Control  (CDC) reported -49,000  cases  of
salmonellosis,  > 30,000 cases  of shigellosis,  >400 cases of typhoid fever, 54 cases  of
leptospirosis  and  8 cases of cholera hi the United States during 1988  (CDC, 1989), and the
actual incidence of enteric bacterial disease is probably much higher than reported. According
to Domingue (1983), the estimated incidence of sahnonellosis infection hi the  United States
resulting from Salmonella-contaminated food or drink approaches 2 million cases per year.  A
family outbreak of yersiniosis attributed to contaminated well water (Thompson and Gravel,
1986) suggests that Yersinia enterocolitica can seep into groundwater, surviving sufficiently long
hi the environment to cause disease (Chao et al., 1988).
      Epidemiological studies  that  consider all the evidence for human  health risk from
landfilling are  difficult to find, but land application results are probably conservative when
compared with the impact of landfilling on groundwater.  Epidemiologic evidence has failed to
link enteric bacterial illness with land application of treated sludge (Kowal, 1985; WHO, 1981;
Farber and Losos, 1988).  Although Reddy et al. (1985) note  a number of instances hi which
land application of sewage sludge resulted in cases of human sahnonellosis, they found  no
significant health risk to human or animal health from Salmonella spp. when they applied sludge
at 2-10 metric tons/ha to eight farms. Based on the low frequency of  isolation of Salmonella
spp. hi the sludge and on the prevalence of antibodies hi members of the families,  Ottolenghi
and Hamparian (1987) reported no apparent risk to farm families when anaerobically digested
                                          3-10

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sludges known to contain Salmonellae were  used for agricultural  applications.   Although
Salmonella was detected on school playing fields inadvertently spread with untreated sludge, no
incidences of disease were reported; the modeled average risk of Salmonella infection was 1.2
x 10~2 (Rose, 1993).  This risk was based on the conservative assumption of ingesting 480
mg/day of soil and dust.
       Viruses.  Yates (1990) concludes that viruses may be responsible for one-third of the
waterborne disease outbreaks hi this country.  Wellings  (1987) suggests as  many as 60% of
waterbome disease outbreaks may be caused by viruses. The CDC reports that there were
28,507 cases of hepatitis A reported hi 1988 (the highest since 1980) and 9 cases of poliomyelitis
(CDC, 1989).  The percentage of reported waterborne disease attributed to viruses has been
increasing as detection methods have unproved (Gerba,  1984b).  However, the IAWPRC Study
Group on  Health Related Water Microbiology (IAWPRC, 1991)  concludes  that with the
exception of HAV, epidemiological evidence does not support transmission of enteroviruses by
water "to any significant extent."  Outbreaks of infectious hepatitis have been linked to HAV
hi drinking water (IAWPRC, 1991).
       Norwalk agent, rotaviruses, astrovirus andcalicivirus have been associated with outbreaks
of gastroenteritis.  Grohmann et al. (1991) suggest that human calicivirus (HCV)  may be a
                                                                                       *>
common cause of gastroenteritis that is under-recognized  because of insensitive detection
methods.  According to Keswick et al. (1985), the  Norwalk agent is responsible for  ~23% of
all reported waterborne disease outbreaks, and Righthand (1983) suggests that  -36% of the
infectious,  nonbacterial gastroenteritis outbreaks result  from  Norwalk agent transmitted by
contaminated food and water.   IAWPRC  (1991)  states  that there is strong epidemiological
evidence of the potential for water transmission of Norwalk virus and other small round viruses
if these agents are present hi public water.
       In children between 6 and 24 months of age,  rotavirus is the most frequent cause of
nonbacterial gastroenteritis (Estes et al., 1983).  Suflita et al.  (1992) report that the average
       
annual rate of rotavirus gastroenteritis in the general population of the United States is 10.4%,
with 10% of children under  two years of age infected at any given tune.  The low reported
incidence for adult rotavirus infection may be because infected adults manifest the disease
                                          3-11

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differently than children or because the amount of virus  shed by infected adults is below
detection limits of current assay methods (Ward et al., 1986)
       Most of the well-known viruses that are dangerous to fetuses of women infected during
pregnancy (rubella, cytomegalovirus and herpesvirus) are not typically found in sewage sludge
(Gold and Nankervis, 1989; Nahmias et aL, 1989). However, some of the viruses that may be
present in sludge have been associated with risk to the fetus or neonate, for example, human
parvovirus B19 (fetal hydrops and death) (Levy and Read, 1990) or echovirus and coxsackievirus
B (fetal heart effects, infection and death) (Modlin, 1988; Rosenberg, 1987). Because of their
small size and perhaps their under-developed system,  fetuses may be extremely susceptible to
these agents.
       Parasites.  Epidemiological studies considering the evidence for the risk to human health
from parasites in municipal sludge or wastewater applied to land suggest that the risk is small.
The filtering action of the soil beneath a sludge landfill  should prevent that movement of
helminth  eggs and large protozoan cysts into the.groundwater.  Because of their small size,
Giardia and Cryptosporidium may have the potential to enter the groundwater beneath a landfill.
       Giardia is now one of the most commonly reported pathogens responsible for waterborne
illness hi the United States (Craun, 1988; West,  1991).  Cryptosporidium is emerging as an
important cause of gastroenteritis worldwide (Casemore, 1991; West, 1991).  The potential for
waterborne disease transmission of Cryptosporidium may equal or exceed that of Giardia (Rose,
1988;  Current,  1987).  Both have been the cause of several outbreaks of waterborne disease.
       Parasite  eggs and most cysts and oocysts are large enough to be filtered out of properly
managed public water supplies.  According to Karanis et al. (1992), the small protozoan cysts,
such as Cryptosporidium and Giardia, may evade filter barriers.  The failure of water treatment
filtration systems, as well as unfiltered water and treated water contaminated with sewage, was
found to be a common factor associated with community epidemics of waterborne giardiasis
(Craun, 1988; Hayes et al., 1989; West,  1991).  Meyer and Radulescu (1979) attribute the
frequency of giardiasis to ingestion of Giardia cysts hi public water supplies or in surface water.
Animal reservoirs are believed to be a source of protozoan contamination of freshwater streams
(Daly, 1983).
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       Because of the limited evidence linking landfilling to transmission of parasitic diseases,
examination of some of the studies of sludge that has been spread on land may provide some
clues to the potential transmission of parasites once they are in the soil.  Humans and animals
on Ohio farms that received yearly applications of municipal  sewage sludge did not differ
significantly from controls hi respiratory and digestive illness (Dorn et al.,  1985).  However,
higher concentrations  of disease organisms hi the sludge, higher application rates or increases
hi treated acreage might produce significant health risks,  according to the authors.
        According to Rose (1993), the modeled risk of infection from Giardia was < 9.5 x 10"3
based on levels of organisms detected on a baseball field that had been inadvertently spread with
untreated sludge.  No incidences of disease were reported.
3.1.4.  Infective  Dose.
       Bacteria.  Estimates of minimum infective doses (MIDs) for bacteria vary appreciably
depending on several  important factors: route or exposure;  timing of exposure (e.g., acute or
chronic); resistance mechanisms of the host, including immune responses or barriers to infection
such as stomach acidity or leukocyte activity; general health and age of the host; treatment with
antibiotics, which reduces competition and,  thus,  the number  of bacteria  required to cause
infection; and virulence of the strain or serotype of bacteria.  Based on reviews of information
on oral infective doses of bacteria (Kowal, 1985; Pahren, 1987),  103 would be a conservative
infective dose  because neither infection nor illness occurred at that level. U.S. EPA (1988b)
reports infective doses for bacteria hi the range of 102 -108.  Metro (1983a)  reports a range of
102 - 105 for Salmonella.  Blaser and Newman (1982) concluded that the  infective dose for
Salmonella may be fewer than 103 bacteria.  Keswick (1984) reported infective doses of 101 -102
for Shigella dysenteriae, 106 - 108 for Escherichia coli and 106 - 108 for Vibrio choleras.  In the
methodology and computer model used hi this preliminary risk assessment, the conservative
default MID for Salmonella bacteria is 101 units, one order of magnitude less than the lowest
dose generally reported to cause infection. This selection is based on the premise that this dose
would probably protect sensitive individuals, such as older people, small children, or those with
low immunological defenses. However, it may be overly conservative.
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       Viruses.  Both the pathogenic potential or infectivity of the virions and the susceptibility
of the host affect the MID of viruses (Menna and Soderberg, 1983).  Reported infective doses
vary widely.  The oral infective dose for poliovirus ranges from 1 to 1 x 107 6 TCID50 and 0.2 -
 5.5 XlO6 PFU  (Kowal, 1985). Schiff et al. (1984) predict a 1% human oral infective dose
(HEDi) for echovirus of 17 plaque forming units (PFUs).  Regli et al. (1991) suggest an MID
for rotavirus of ~3 units.  IAWPRC (1983) suggests that the MID of enteric viruses in healthy
adults may be larger than 1 PFU, but a single PFU may be infective to susceptible individuals.
The default MID for enteroviruses hi the SLDGFILL model is 1 virus unit.
       As demonstrated by these values, it is clear that there are  no standard  methods of
reporting  virus  concentrations.  For example, infection  measured  by cytopathic effects  is
reported as PFUs,  the number of particles causing cytopathogenic effects as measured by areas
of clearing on a cell culture sheet; as most probable number of cytopathogenic units (MPNCU),
the most probable number of particles capable of causing cytopathic effects;  and as tissue culture
infective dose (TCID) or TCID50, the dose required to infect 50% of the cultures. Comparisons
of reported values  are hampered by the lack of standard methods.
       Parasites.  A number of authors have suggested that the low infective doses of protozoan
oocysts and their resistance to disinfection contribute to their hazard to public health (Karanis
et al.,  1992; West, 1991; Packham, 1990; Sobsey, 1989).  Molbak et al. (1990) suggest that a
small infective dose may explain the peak prevalence of cryptosporidiosis  hi small infants,
prevalence decreasing with age despite the fecal-oral transmission route.  The development of
protective immunity was also suggested to partly explain this decrease.
       In a study  of cryptosporidiosis  using infant macaques as a model  for young  children,
Miller et al. (1990) found that inoculation with as few as 10 oocysts produced clinical enteritis
and shedding of large numbers of oocysts.   Blewett et al. (1993) report that the  minimum
infectious dose may be as low  as one oocyst based on experiments with gnotobiotic  lambs on
a  diet  artificially  contaminated  with a predetermined  level  of  Cryptosporidium oocysts.
According to Casemore (1991), the infectious dose of protozoans has been determined to be as
few as one cyst of Giardia or 10-100 oocysts of Cryptosporidium.
       A single helminth egg may produce human infection; however, because infection is dose-
responsive, many  infections are asymptomatic (Kowal, 1985).
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3.2. PERSKTENCE/SURVIVABILITY IN MEDIA
       Because sludge that is covered with soil hi a landfill does not dry rapidly and sludge at
the working face of the landfill is left uncovered only a few hours a day, paniculate suspensions
(dust) will not be a major exposure pathway from landfilled sludge.  Persistence and transport
by air  are not addressed hi this review or by the SLDGFLL model because the groundwater
pathway presents the worst-case for human exposure to enteric pathogens.
3.2.1.  Persistence/Survivability  in Soil.  The type of organism, the degree of predation by
other microorganisms, the amount of sunlight and the physical composition of the soil, including
moisture content, pH and temperature, affect the persistence or survivability of microorganisms
hi soil (Gerba, 1985; Kowal, 1985; Sorber and Moore, 1987; Moore et al., 1988; Yates and
Yates,  1988; Bitton and Harvey, 1992). Temperature is the best indicator of pathogen survival
time (U.S.  EPA, 1988b).  Moist, cool soils appear to enhance survival tune; soils with greater
water-holding  capacity, such as those with organic matter (i.e., sludge), may be conducive to
pathogen survival.
       Bacteria.   Sorber and Moore (1987), U.S. EPA  (1988a,b; 1991a), Yates and Yates
(1988) and Bitton and Harvey (1992) discuss bacterial persistence hi soil.
       There is a wide range of recorded survival times for bacteria in soil.  At soil depths of
0-30 cm in sludge-amended soil, Sorber and Moore (1987) observed that tune to achieve a
90% reduction hi numbers (Tgo) for Salmonella is  3 - 61 days.  They suggested  that a 90%
reduction hi Salmonella will occur within 3 weeks of sludge application. Feachem et al. (1983)
concluded that Salmonella may survive longer hi the environment than Shigellae at temperatures
> 30C.   According to Gerba and Bitton (1984), 2-3  months should reduce the levels of
pathogenic  bacteria hi the soil to  non-harmful levels.  Survival tunes are  species and strain
specific.  For example, the reported Tgo for Salmonella typhimurium ranges from 3.2 - 29.4 days
and for Salmonella adelaide ranges from 7 - 46.5 days (U.S. EPA, 1991a).
       Sorber and Moore (1987) noted that the exceptionally long survival tunes ( > 6 months)
recorded hi the literature for Salmonella are from experiments where high levels (106 - 1010/L)
of laboratory-grown organisms were added to soil. They reported detectable levels of indigenous
Salmonella have been found 3-5  months after application.  Yanko et al. (1978) reported that
                                         3-15

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Salmonella were found 4-5 months after digested anaerobic sludge was disked into soil.  After
injecting both untreated and anaerobically digested sludges 15 cm into soil, no viable Salmonella
were detected at any time after injection of anaerobically  digested sludge.  Salmonella were
detected in the plot receiving untreated sludge up to 3 months after injection (Carroll and Ross,
1983; Sekla and Stackiw, 1983).
       Donnelly and Scarpino (1984) found at least one species of pathogenic bacteria survived
for two years under simulated landfill conditions; in general, the density of microorganisms
recovered in the leachate was 1-2 orders of magnitude lower than the density originally added
in the sludge.
       According  to Yates and Yates (1988),  soil moisture may be  the dominant factor for
survival of enteric bacteria hi soil; enteric bacteria are extremely sensitive to drying and survive
longer in moist soil than in dry soil.  In sandy soil, desiccation may contribute to rapid die-off,
according to Watson (1980). However, Chao et al. (1988) reported that although drying reduced
the number of introduced Yersinia enterocolitica in the soil, that organism tolerated desiccation
better than either Escherichia coli or Staphylococcus aureus.  The presence of organic matter
in the soil improves the water-holding capacity of the soil, which enhances bacterial survival,
and, at the same time, provides conditions conducive to regrowth (Temple et al., 1980).
       Most of the literature reports on bacterial survival hi soil or in sludge that is applied to
land, not on survival in sludge landfills. Because soil moisture is an important determinant of
survival, Crane and Moore (1984) concluded that minimizhig moisture loss by burying sludge
in landfills will enhance survival of bacteria.
       Sorber and Moore (1987) found that temperature was the only physical  or meteorologic
parameter related to microorganism survival hi their review of the transport  and survival of
pathogens in sludge-amended soil.  Colder temperatures appear to favor longer survival (Jones
et al., 1983).  Some bacteria have survived freezing or subfreezing  temperatures (Yates and
Yates, 1988). Because low temperatures prolong survival,  temperature may be a consideration
in the siting of monofills.
        The presence and activity of other microorganisms, such as  other bacteria or protozoa,
in the soil may decrease survival of pathogenic  bacteria hi soil, possibly because of competition
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for nutrients (Yates and Yates, 1988). The pH of the soil may affect such factors as availability
of nutrients, activity of antimicrobial agents and the ability of enteric bacteria .to survive. Each
strain appears to have an optimum pH range for survival.  Generally, acid levels (3 - 4) are
detrimental to survival and, for some enteric bacteria, inactivation is enhanced at highly alkaline
pH values (  9.5) (Yates and Yates, 1988).   The physical and chemical conditions found in
monofills are diverse and site-specific, based on soil type, and can  change with inputs to the
landfill (U.S. EPA, 1988b).
      These abiotic factors also interact to influence bacterial persistence hi the soil. Zibilske
and Weaver (1978) reported that Salmonella typhimurium died rapidly (within 3 days) in dry clay
soil or at high temperatures (39C), but moisture  hi the soil mitigated the effects of the high
temperature,  and lower temperatures mitigated the effects of dry conditions.  Guy and Visser
(1979) reported that the combination of high clay content and low soil pH reduces survival time
of E.  coll.
      U.S. EPA (1993) cites inactivation rates for bacteria (Shigella, Salmonella and E.  coli)
in moist soil ranging from 0.016 - 6.39 logio/day.  The default rate used by the model is 0.016
Iog10/day, the most conservative value of the range.
      Viruses.  Detailed information on the persistence  of viruses in soil may be found in
Sorber and Moore (1987), U.S. EPA (1988a,b;  1991c) and Bitton and Harvey  (1992).
      U.S. EPA (1991c) cites inactivation rates ranging from 0.0017 - 3.69 log^/day for
viruses. Under field conditions,  Cogger et al. (1988) report an inactivation/retention rate for
viruses hi soil and groundwater of 0.89 Iog10/day; Powelson et al. (1993) give a field composite
of retention by soil and inactivation by soil and groundwater of 10.4 Iog10/day.  These were used
by the model for inactivation rates in soil and water.
       The most important single factor hi virus persistence and inactivation hi the environment
is temperature (Yates and Yates, 1988).  Temperatures of 60C or above inactivate most enteric
viruses (Morris and Darlow,  1971; Gerba and Bitton,  1984); however, some  types, such as
HAV, have been shown to withstand higher temperatures (U.S. EPA, 1988a).  Sobsey et al.
(1986) found that at 25C, poliovirus exhibited 90  - 99%  inactivation in  12  weeks, but
inactivation of poliovirus and echovirus at this temperature was 99.9 - 99.99% after 12 weeks.
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According to Yates and Yates (1988), cations of some salts affect the inactivation of some
viruses in water and may affect viral adsorption and thermal stability in soil.
       Soil drying is a major detrimental factor to viruses in sludge-soil mixtures (Bitton et al.,
1981).  The  synergistic  influence of temperature and soil desiccation are cited by Bitton et al.
(1987) as a primary control of viral persistence in the soil. In a field study, Hurst et al. (1980b),
found that periodic drying followed by aeration enhanced viral inactivation.  Hurst et al. (1980a)
report that virus survival and drying do not correlate linearly hi soil, but survival decreases as
soil moisture increases up to the saturation point; survival then increases with additional moisture
past the saturation point.
       Hurst (1988) concluded that microbial antagonism is a major factor in viral persistence
in soil,  reporting that aerobic microorganisms exert a significant effect on viral  inactivation hi
sandy soil.   Mechanisms suggested for microbial  antagonism include the effects of metabolic
products and interference with adsorption.
       Enteric viruses are more tolerant of acid than alkaline conditions,  and most are more
stable at pH 7, close to neutral (Rao and Melnick, 1987). Although the effects of pH on viruses
has not been extensively studied, Yates and Yates (1988) suggest that pH indirectly effects viral
survival by controlling adsorption onto soil particles in addition to.other more direct effects.
       Lefler and Kott (1974) found that poliovirus persisted longer in soil watered with organic
matter than in distilled water, but other experiments (Moore et al., 1981; Bitton et al., 1976)
have not confirmed  this relationship of persistence with organic matter.   Organic matter may
interfere with viral adsorption (Moore et al.,1981; Bitton et al., 1976) and may act as an eluting
agent, desorbing viruses from  soil (Gerba, 1984a).  Aggregates or clusters of viruses may
protect them against other environmental factors (Sobsey and Shields,  1987).
       The nature of the sorbent determines whether virus  survival is enhanced  or reduced by
adsorption (Yates and Yates, 1988).  Adsorption to certain metal particles and organic muck
result in inactivation (Moore et al., 1982), but adsorption to silica and  iron oxide does  not
(Murray and Laband, 1979).  Hurst et al. (1980a) and Gerba (1985) report increasing viral
survival with greater adsorption by soil particles. These observations suggest that the nature of
the soil beneath a monofill will influence the adsorption and inactivation of viruses.
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       Hurst et al. (1980a) report that virus survival is influenced by the following conditions:
temperature,  soil moisture content, presence of aerobic microorganisms and degree of virus
adsorption to soil.  In addition they reported that one soil characteristic that was significantly
correlated to survival was soil saturation pH. However,  soil characteristics,  such as soil texture
(clay,  loam or sandy),  cation exchange capacity  (CEC), mineral  and organic content and
presence of van der Waals forces (weak attractive forces between electrically neutral molecules)
and hydrophobic interaction affect survival indirectly by affecting adsorption of viruses to soil
particles.
       In laboratory experiments using sludge-amended desert soils, Straub et al. (1992) report
that soil texture and soil temperature are the  main factors controlling viral  inactivation.  As
temperature increased from 15 to 40 C, inactivation rate increased significantly for poliovirus
type 1 and bacteriophage MS2. Clay loam soils offered more protection to viruses than sandy
loam  soils.
       Goyal and Gerba (1979) studied a number of viruses including human^enteroviruses and
found strain and type differences hi adsorption to soil. Susceptibility to inactivation in soil may
vary with the type and strain also.  Sobsey et al.  (1986) report that the inactivation rates of
poliovirus, reovirus,  echovirus and HAV in several types of soil material at 25C  differed.
HAV, the virus most resistant to thermal inactivation in treatment processes  (Siegel, 1982), was
shown to survive longer than other viruses hi sewage effluents and soils (Hazard and Sobsey,
1985).  Straub et al.  (1992)  noted that although inactivation of poliovirus  and MS2 increased
with increasing temperature over a range of temperatures, a significant increase hi inactivation
rate for bacteriophage PRD-1 occurred  only at the  high end of the range, 40C.
       If the average temperature hi monofills is ~29  C as reported by Suflita et al. (1992)
for the Fresh Kills Landfill,  some viruses will survive  for extended periods  hi the monofill.
       Progress being  made hi the research  on many of the  viruses that have been poorly
understood confirms mat factors which affect them and the interactions  of those factors  are
complex. More research is needed hi this area. For example, many sludge viruses are difficult
to detect and isolate from environmental media (Gerba,  1984b). Rao et al. (1986a) suggest that
the extraction, concentration and enumeration techniques for  some enteroviruses hi soil are
marginally efficient, and techniques are totally inadequate for detecting HAV, Norwalk virus and
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human rotavirus.  Hurst et al. (1991) report an evaluation by a number of laboratories of several
methods for the recovery of enteroviruses from sludge and soil. Scares et al. (1992) found that
recovery of enteroviruses was poor using these methods; they suggest that large volumes of
eluent may be necessary to improve recovery of viruses hi sludge/soil systems.
       Because viruses differ markedly hi their responses to adverse conditions, there is a
possibility that some of the enteric viruses may survive for a long period hi the moist, cool soil
beneath a landfill.  Therefore,  their potential to reach the groundwater will depend on then-
movement through the soil.
       U.S. EPA (1993) cites inactivation rates  for viruses hi moist soil ranging from 0.0017 -
3.69 logio/day. Data from field studies by Powelson et al. (1993) on inactivation and retention
have extended the upper end of the range to 10.4 Iog10/day (see Table 5-3).  The default rate
used by the model is 0.0017, but a value of 0.13 Iog10/day (Straub et al., 1992) was used hi the
model simulations because it was the most conservative of the combined inactivation/retention
rates observed in the field studies.
       Parasites.  The persistence  of parasites hi soil is discussed hi detail hi Kowal (1982,
1985), Storey and Phillips (1985),  Sorber and  Moore (1987) and U.S. EPA (1988a; 1991b).
       The sensitivity of the cysts of the protozoan Entamoeba histofytica to drying hi soil has
been documented, showing survival increases with moisture hi the soil (Kowal, 1985).  In New
Jersey summer weather, cysts survived 18 - 24 hours hi dry soil and 42 - 72 hours hi moist soil
(Rudolfs et al., 1951).   Rudolfs et al. (1951) report an optimum temperature range (28 - 34C)
for survival of E. histofytica in soil, and an optimum soil type, of dark loam with 30-50% sand.
       Helminth ova are more resistant to environmental stresses  than protozoan cysts (Yanko,
1988).  Repeated freeze-thaw conditions hi the  laboratory reduced viability ofAscaris eggs at
4 - 20% soil moisture, with decreasing moisture enhancing reduction (Leftwich et al., 1988).
Burden and Hammett (1979) report little development of Trichuris suis ova to the infective stage
(embryonation) hi whiter; following development  to the infective stage  (62 - 90 weeks), ova
survived for 2 years,  survival rate depending on temperature  when adequate moisture and
oxygen were present. However, some studies have failed to find a correlation between viability
and solar radiation, relative humidity or soil temperatures. For example, Leftwich et al. (1988)
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found no statistical correlation between soil moisture and survival in a Field study with Ascaris
ova.  The interaction of the factors affecting  inactivation may account for the  apparent
discrepancy.
      Because parasites are  sensitive to desiccation and sunlight (Kowal, 1985), subsurface
conditions are expected to be more conducive to survival of helminth eggs than conditions at the
soil surface.  Under cool, moist conditions the eggs and larvae of helminths may survive and
remain infective for years. Storey and Phillips (1985) report that the number of Taenia saginata
eggs surviving 200 days at levels below 12 cm was only slightly lower than the initial number.
      Ascaris may be the most hardy and resistant  of all excreted pathogens (Feachem et al,
1983).  The U.S. EPA (1988b) reports that Ascaris eggs may survive hi soil up to 15 years.
Following application of municipal sludge to land, Jakubowski (1988) found infective Ascaris
eggs survived throughout the  3-year duration of the  study.
       Reporting on the survival of microorganisms  on diapers buried hi a landfill from 1965-
1988, Suflita et al. (1992) found no viable  protozoa or viruses.  The authors conclude that
pathogenic protozoa and viruses do not survive for  long periods of time  at the average
temperature in the landfill, 29.4C.
       Available information  on the survival of protozoan cysts and oocysts and helminth eggs
suggests that they may survive for extensive periods of time hi the moist soil beneath a landfill;
therefore, the risk to human health will be determined hi large part by retention hi soil.
3.2.2.     Persistence/Survivability in  Water.      Many   of   the   factors   affecting
persistence/survivability  of microorganisms  hi soil  also affect their presence  hi water.  The
survival of bacteria, viruses  and parasites hi water is discussed hi U.S. FJA (1991a,b,c).
Available information is limited on survival and inactivation rates hi groundwater.
       Bacteria. U.S. FJPA (1991a) cites inactivation rates for bacteria hi water ranging from
0.0228 - 3.01 logto/day. According to Matthess and Pekdeger (1985), enteric bacteria show very
little growth hi groundwater.  However, survival time for bacteria hi groundwater may be longer
than in soil because conditions of moisture, temperature, pH and absence of sunlight and other
microorganisms may  be favorable.   The biological, physical and chemical conditions of the
groundwater and the  processes that control transport of bacteria (see Section 3.3.2)  are the
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primary  factors influencing bacterial survival in groundwater.   The presence  of competing
microorganisms in the groundwater, and in surface waters, decreases patho^n survival.
       According to Feachem et al. (1983), the survival of Vibrio  cholerae in water and of
Shigella  in the environment at temperatures  >30C would  be  less than that of Salmonella.
Chao et al.  (1988) observed that  Yersinia  enterocolitica survived significantly longer in
groundwater than in river water because of the relative scarcity of predators and toxin producers
in the groundwater.
       Because conditions hi the groundwater will be favorable to the survival of bacteria,  it
is important that conditions in the soil above enhance retention and promote inactivation of
bacteria. U.S. EPA (1993) cites inactivation rates for bacteria in water ranging from 0.0228 -
3.01 logjo/day.  The default inactivation rate for bacteria hi water is 0.0228 Iog10/day.
       Viruses.  As hi soil, there are a number of interacting  factors  affecting viral persistence
hi water,  including temperature, chemicals,  pH,  light,  biologic  factors  (the presence of
microflora and the substances they  produce) and suspended particulate  matter  (Melnick and
Gerba, 1980). Keswicket al. (1982) attribute the longer survival of viruses hi groundwater than
hi surface  water to the  lower temperatures,  protection from sunlight and lack of microbial
antagonism hi groundwater.
       Temperature is the single most important factor hi viral  decay hi groundwater (U.S.
EPA, 1988a).  According to Yates and Yates (1988), lower water temperatures enhance viral
survival  and  infectivity.  Temperature is significantly correlated  with the inactivation of
poliovirus  1, echovirus  1 and MS-2 coliphage hi groundwater samples  (Yates  et al., 1985).
Hurst et  al. (1989b) found that incubation temperature, viral serotype  and water source were the
significant factors affecting  persistence  of three  human enterovirus  serotypes hi water.
Kapuscinski and Mitchell (1980) report that the effects of temperature on virus inactivation may
be indirect, through control of other inactivation mechanisms.
       Yates et al. (1990) found a significant correlation between inactivation of coliphage and
increasing  bacterial numbers, suggesting  that bacteria  may  produce  some  substance that
inactivates viruses.  However, when more  than 30 groundwater samples were studied, Yates
(1990) found no consistent effect of indigenous bacteria on virus inactivation. Similarly, Jansons
et al. (1989) report no association between rate of virus inactivation (echoviruses 6, 11 and 24;
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coxsackievirus  type B5; and poliovirus type 1) and bacterial numbers  in groundwater.   The
effects of other microbial activity on viruses remains to be resolved.
       Optimum pH ranges vary with virus serotype but, in general, the pH levels of natural
waters are not detrimental  to virus survival (Bitton et al.,  1987).   Thermal  inactivation of
bacteriophage increased at 0.15 M concentration of CaCl2 and BaCl2, but concentrations of 0.002
and 0.01 M partially prevented inactivation at 60C (Yates and Yates, 1988). Similarly,   1 M
concentrations  of MgCl2 have  been reported to enhance  the stabilization of poliovirus at
temperatures from 4 - 50C (Yates and Yates,  1988).  Nitrate, ammonia, sulfate, iron, total
dissolved solids (TDS), hardness, pH and turbidity in groundwater samples were not found to
be correlated with inactivation for poliovirus 1, echovirus 1 and MS-2 coliphage (Yates et al.,
1985).
       Aggregated viruses are more resistant to chemicals such as chlorine and bromine  than
those in single-particle  suspensions. Rao et al. (1986a) found that viruses adsorb to solids in
water and that  the association may  be protective, prolonging survival.
       Hurst et al.(1989a)  review  methods to  concentrate and assay viruses hi water. They
conclude that concentration is best  accomplished by direct adsorption of viruses on a filter or
granular solid with subsequent elution.  Optimum quantitation may be achieved by variations of
nucleic acid hybridization, which balances sensitivity, rapidity  and determination of viral
infectivity, according to these researchers. Margolin et al. (1991) report that nucleic acid probes
are 1,000-fold more sensitive than serological tests, and DeLeon  and Gerba (1991) report
enhancements of gene probe sensitivity hi the detection of rotaviruses hi water. Sobsey et al.
(1990) report a simple membrane  filter method for concentrating and enumerating F-specific
coliphages hi water. Benton and Hurst (1990) and Williams and Hurst (1988) discuss variations
and enhancements, respectively, to plaque assays.  Resolution of the many problems associated
with enumeration of viruses will facilitate the development of reliable human risk assessments
for sludge monofills.
       Conditions in the. subsurface are conducive to viral survival for long periods of tune once
they reach the groundwater.   Since they are  small and  may be carried great distances, the
potential for reaching the groundwater well and, therefore, the potential for human infection is
great.
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       U.S. EPA (1993) cites inactivation rates for viruses in water ranging from 0.0039 - 2.383
Iog10/day.  Other values were determined by Cogger et al. (1988) and Powelson et al. (1993)
under field conditions, 0.89 Iog10/day and 10.4 Iog10/day for nonpathogenic virus tracers. The
default value for virus inactivation in water is 0.0039, but a value of 0.13 Iog10/day (Straub et
al.,  1992) was used  in the model simulations because it was the  most conservative of the
combined inactivation/retention rates observed in the field studies.
       Parasites. Protozoan cysts and oocysts may persist in water, but temperature will affect
the likelihood of their survival.  Both surface water (Madore et al.,1987) and filtered, treated
public water (Hayes et al., 1989) have been found to be contaminated with Cryptosporidium
oocysts.   Extremes of temperature can reduce oocyst viability (Tzipori,  1983);   30-minute
exposure to below freezing or above 65 C temperatures was  found to destroy infectivity of
Cryptosporidium oocysts.  When aqueous solutions of Cryptosporidium oocysts were stored at
4C, infectivity began to decline at 2  - 6 months, but viability continued for 6 - 9 months and
infectivity for up to 12 months (Payer and Ungar,  1986).
       Giardia spp. cysts have the potential to survive relatively long periods in water below
20C, with  rapid  inactivation occurring  above  20C  (Jakubowski, 1990).   Freezing is
detrimental to suvival, but low levels of excystation have been reported after storage below
freezing  temperatures (Jakubowski,  1990).   Entamoeba histofytica and Giardia lamblia are
temperature sensitive  hi water, surviving best at 12 - 22C and 4 - 8C, respectively (Mitchell,
1972; Grenfell  et al., 1986; Jakubowski, 1990).
       No data were found on the survival  of helminth eggs in water.
       Because the factors affecting persistence/survivability hi water are poorly understood,
information is limited on inactivation or die-off rates in groundwater.

3.3.  TRANSPORT
       Good management practices require the use of clean soil as sludge cover in landfills and
of drainage ditches and dikes to control runoff from the working face.  Therefore, surface runoff
should not be an exposure pathway of pathogens from monofills. Since the working face will
be too moist for the formation of particulates by wind or activity during the 8 - 12 hours it is
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not covered by soil, transport of participates in air will not be addressed.  The major route for
pathogen transport from the landfill is  by leachate from  the sludge through the soil to the
groundwater.
       Further discussion on the transport of pathogens in soil may be found hi Gerba (1985),
Sorber and Moore (1987), Yates and Yates (1988), U.S. EPA (1988a;  1991a,b,c) and Bitton and
Harvey (1992).
3.3.1. Transport in Soil.  In wastewater, pathogens are loosely bound and may move freely
through the soil, but most of the pathogens in sludge are tightly bound and must be desorbed
before transport hi soil (U.S. EPA, 1988b).  In general, the texture of the soil and the size of
the organism will affect how far a particular pathogen can travel.
                            
       Bacteria. Data on bacterial transport in soil has been compiled by McCoy and Hagedorn
(1979), Moore et al. (1988) and Bitton and Harvey (1992), and representative bacterial transport
rates were generated in U.S. EPA (1991a).  Calculated rates ranged from 0.1-96 m/day.
       Factors affecting bacterial transport in soil include soil physical characteristics:  texture,
particle size distribution,  clay content, organic matter type and content, pH, CEC and pore size
distribution.  Additional factors that affect bacterial transport in soil are soil environmental and
chemical  factors  such as temperature,  moisture content, soil water flux (saturated versus
unsaturated flow), chemical makeup of ions hi the soil solution and their concentrations, bacterial
density and  dimensions  and  nature of the  organic  matter hi the waste effluent  solution
(concentration and size) (Moore et al.,  1988; Crane et al., 1983).
       Gerba and Bitton (1984), as well as Moore et al. (1988), conclude that filtration and
adsorption govern bacterial movement through soil.  Both soil characteristics and bacterial size
and shape affect filtration.   Coarse, gravelly or fissured soil  allows more  bacteria to pass
through, and, conversely, fine grained  soil, such as clay or loam, retards movement. Larger
bacteria will  move through the larger soil pores that allow  greater velocities (Yates and Yates,
1988), but larger cells generally move less in the soil than smaller ones (Alexander et al., 1991).
According to Smith et al.  (1985), the application of organic  matter adds a filtering mat or
clogged zone of bacteria and extracellular materials that should slow bacterial movement.
       Adsorption impedes transport of bacteria into the groundwater, and fine-textured soil,
such  as clay, promotes adsorption  (Hagedorn and McCoy,  1979).  Factors that  enhance
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adsorption, such as the presence of cations, allow greater times for inactivation while delaying
movement. Higher flow velocities and cell concentrations of an inflowing solution decrease the
probability of adsorption and, therefore, promote migration.
       Because of the effects of filtration and adsorption, the depth from the sludge application
site to the groundwater is the major factor influencing contamination of the groundwater.  After
4 years of heavy sludge application, Liu (1982) reports that 90% of the surviving bacteria were
found in the upper 20 cm of soil. Using a methodology that differs from the one currently being
used  in the SLDGFILL model for groundwater transport, Germann et al. (1987) found that
naturally occurring precipitation events will result in minimal transport of microbes deeper than
2 - 3  m in soil; but prolonged or very intense storm events may carry microbes more than 100
m into the vadose zone.  Hagedorn (1984) summarizes that bacterial movement is limited to a
few dozen centimeters when carried by percolating water hi unsaturated soil,  but much longer
distances are possible under saturated flow conditions.
       Viruses. Mack et al. (1972) found that poliovirus traveled at least 90 m underground;
and Keswick and Gerba (1980) find that viruses have penetrated to depths of 67 m and travelled
horizontally as far as 408 m. Noonan and McNabb (1979) record a rate of  -300 m/day for
phages in groundwater; Gerba (1984b) reports that coliphage traveled 1600 m in 16 hours.
       Adsorption and filtration, which enhance the persistence of viruses in soil, inhibit their
movement through soil (Yates and Yates, 1988); adsorption plays the dominant role (Reneau et
al., 1989).  Most viruses  applied to soil are retained hi the upper soil layers  (the first 15 cm)
(Rao  et al., 1986a), and increasing the concentration of viruses hi the applied sewage increases
the numbers of viruses retained at various  levels but does not affect the maximum depth of
penetration (Lance and Gerba, 1980).  Lance and Gerba (1982) suggest that viruses applied in
sludge will be adsorbed to solids and will not be likely to move into the soil. In their laboratory
experiment, Pancorbo et al. (1988) conclude that viruses were immobilized by adsorption when
sludge was applied to soil columns under saturated flow conditions.
       The pH of the soil affects virus adsorption to soil and therefore affects transport.  Bales
et al. (1991) performed experiments with columns of silica beads and found that change in pH
affected phage adsorption/desorption.  Gerba et al. (1981) divided viruses into two groups based
on their adsorption by soil; viruses that are poorly adsorbed were greatly affected by the soil pH,
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as well as by CEC and organic matter, but viruses that are highly adsorbed are not correlated
        j
with these soil characteristics (Gerba et al., 1981).  The high alkaline pH levels that cause viral
desorption from soil would not be expected beneath a landfill because wastewater effluents are
typically acidic (Rao et al.,  1986a).
       Because adsorption is inversely proportional to the permeability of the soil (Wang et al.,
1980), migration will be promoted by coarse-textured soils that do not adsorb well (Yates and
Yates, 1988).  Both viral aggregates and paniculate-associated virus will be strained or filtered
out with finer textured soils With smaller pores, such as clay.  McKay et al. (1993) report that
two bacteriophages travelled 4 m horizonally from a source trench; transport velocity ranged
from 2 - > 5 m/day. They suggest that the phage were too large (20 - 80 nm) or formed bonds
with other particles such that they were, too  large to enter the small pores of the soil matrix,
moving instead  through  the  fractures.  However, the  colloidal phage was attenuated in the
fractures at  a rate of 1 log cycle per meter of travel.
       Virus-specific differences  in  soil adsorption probably result  from physicochemical
differences  in virus  capsid  surfaces (Sobsey and Shields, 1987).  The sludge/virus bond is
partially related to their electrical charges  (U.S. EPA,  1988b).   Based on  their laboratory
experiments of phage adsorption to silica beads, Bales et al. (1991) conclude that, hi general,
hydrophobic effects may be more important to adsorption than electrostatic forces in soil. Bales
and Li (1993) suggest that even small amounts of hydrophobic organic material in a porous
medium can retard virus transport.
       Particle surface area, as well as CEC, the presence or abence of organic matter and
mineral content, affect transport by their influence on adsorption.   In their review of viral
survival and transport, Sobsey and  Shields  (1987) cite reports that  certain types of organic
material may prevent viral adsorption to soil.  For example, humic and  fulvic acids, which are
naturally present hi soil and water, cause desorption as well as preventing adsorption, thereby
increasing viral transport. The soil columns experiments of Powelson et al. (1991) support the
conclusion that natural humic material and sewage sludge organic matter increase unsaturated
flow transport by interfering with viral adsorption.  Sobsey and Shields (1987) note that all
evidence hi  the literature does not support this effect for organic material, and more research
is needed.
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       Unsaturated soil restricts movement of viruses (Lance and Gerba, 1984); unsaturated flow
conditions may enhance adsorption of viruses by holding them in close proximity to soil surfaces
(Gerba and Bitton, 1984). With unsaturated flow conditions, poliovirus did not move below 40
cm in a column of loamy sand, but with saturated flow conditions, the virus traveled at least 160
cm (Lance and Gerba, 1984).  Increasing the application rate of water increases the number of
viruses moving through the soil until a breakthrough point is reached, after which there is no
further increase in movement (Lance and Gerba, 1980).  Lance and Gerba (1980) suggest that
at this pouit the water starts to move only through the large pores; according to Lance and Gerba
(1982), both field and laboratory studies suggest that water flow velocity will be the most
important soil characteristic that affects viral movement.
       Reports of microorganisms traveling 15 m or more may represent movement through the
macropores  under saturated soil  conditions, according to Reneau et al.  (1989).  Controlled
experiments indicate that most viruses are deactivated within 3 m of the source.  This conclusion
is supported by column studies over short distances (deactivation within 40 cm with unsaturated
flow). Under nearly saturated flow conditions (50 ft/day infiltration rate), Gerba et al. (1991)
report that 99% phage reduction occurred after passage through 15 ft (4.6 m). Greater removal
was observed with a slower infiltration rate (3 ft/day).  They conclude that 99% viral reduction
is possible with passage through  15 feet of soil,  and even greater reductions may be possible
with enteric viruses that are not adsorbed to soil as well as phage.
        Parasites. According to the U.S. EPA (1988b), the size of helminth ova (~ 15 - 80 ^m
dia) and of the larger protozoan cysts may inhibit their vertical migration through the soil into
the groundwater; the presence of vertical cracks  or fissures hi the soil  will permit vertical
migration of large parasites with the potential for groundwater contamination.  Burden  and
Hammet (1979) found the majority of helminth (Trichuris suis) ova were located in the 20 - 30-
cm subsurface fraction 30 months after application to a pasture hi southern England, presumably .
because the ova were washed through the cracks and fissures common on the plots.
       Based on transport hi laboratory soil columns, Storey and Phillips (1985) estimated that
the ova of the helminth Ascaris lumbricoides could move 100 cm hi 65 years if the average
annual rainfall were 152 cm (60 in.).  In another study, protozoan (Entamoeba histolytica) cysts
and Ascaris eggs were unable to pass through 61  cm (24 in.) of sand (Metro, 1983a).
                                          3-28

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       The smaller protozoan parasites, such as Giardia and Cryptosporidium, may have the
potential for vertical migration through soil into groundwater. Crytosporidium has been reported
to contaminate water from public water supplies subjected to rapid sand filtration  and other
filtration  methods  (Hayes et  al., 1989;  Casemore,  1991).    Casemore  (1991)  reports  a
contaminated groundwater well; presumably percolation through the soil had occurred.
3.3.2. Transport in Groundwater.
       Bacteria. In porous aquifers (sand and gravel), the velocity of groundwater ranges from
< Im/day to a few m/day; the velocity ranges from 0.3 - 8000 m/day in hard rock aquifers and
is < 26,000 m/day  in karstic aquifers (Matthess and Pekdeger, 1985).
       Adsorption  and filtration limit the movement of bacteria in groundwater, delaying
bacterial movement relative  to the groundwater.   Bacteria are filtered and  prevented from
migrating by the layer of sorptive small particles  and microbial slime at the sediment-water
boundary.  However, Matthess and Pekdeger (1985) suggest that the effects of sedimentation
have  been overestimated  for bacteria.  Because the density of microorganisms is so small
(1 g/cm3), the kinetic energy of the microorganism as it is transported by the groundwater flow
to the surface of a soil grain is not enough to overcome repulsing surface forces. When bacterial
concentrations are high, flocculation and aggregation may limit transport to the aquifer.
       Viruses,^ Because viruses are small,  the filtering action of porous  aquifers is not as
effective for removing viruses  as it is for bacteria (Bitton and Harvey, 1992).  High  cation
concentrations and  loamy aquifers may remove those viruses that adsorb well (Matthess  and
Pekdeger, 1985).   With heavy rainfall, desorption will occur  because  of decreasing  cation
concentrations,  and further transport will result.  Continuous adsorption/desorption will retard
the passage of viruses relative to the rate of groundwater flow and permit tune for inactivation.
       Viruses  have been found at depths of 90 m underground  (Mack et al.,  1972); Keswick
and Gerba (1980) report that viruses have travelled horizontally for distances  as great as 408 m.
Noonan and McNabb (1979) record a rate of movement of viruses hi groundwater of  300
m/day, and Martin  and Thomas (1974) report a velocity of 36-180 m/day.
       Bales and Li (1993)  conclude  that more release of colloids may occur with chemical
perturbations associated with rainfall than with constant chemical conditions for long periods of
tune.  Increasing the ionic concentrations of some salts hi the  soil retards the movement of
                                         3-29

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viruses (Sobsey and Shields, 1987).  Rainfall will decrease the ionic concentration of soil water;
consequently,  viruses  will  desorb  and  migrate,  increasing the potential  for  ground water
contamination (Gerba,  1983).  Periods of drying between rainfall events will reduce desorption
and movement, according to Lance  et al. (1976).
       Further discussion on the transport of microorganisms in groundwater can be found in
Kowal (1982), Gerba (1985), Yates  and Yates (1988), U.S. EPA (1988a;  1989b,c; 1991c) and
Bitton and Harvey (1992).
       Parasites.  No  data were found on the transport  of helminth ova or protozoan cysts in
water.
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  4.  DESCRIPTION OF THE SLDGFELL MODEL AND SITES FOR MODEL RUNS

      A variety of methods have been developed for assessment of risks from municipal sludge.
The SLDGFILL model incorporates features of the unsaturated zone transport model used in
U.S. EPA's risk assessment methodology for chemicals in municipal sludge (U.S. EPA, 1989a)
as well  as features  of an earlier model for sludge pathogen risk  assessment (U.S. EPA,
1989b,c).  Other U.S.EPA-approved models for groundwater transport such  as SESOIL and
AT123D have been used to model subsurface transport of chemicals.  Approaches to modeling
transport of bacteria and viruses in the subsurface are reviewed in Bitton and Harvey (1992).
The SLDGFILL model is intended to be simple, requiring few parameters and running on a
personal computer, while using features found hi larger, more sophisticated transport models as
well as models for exposure and infectivity.  As shown during development of the SLDGFILL
model (U.S. EPA, 1993), the results of model runs agree with laboratory data on pathogen
transport and with results of an independently derived virus transport model (Yates and Ouyang,
1992).

4.1.   OVERVIEW OF THE SLDGFILL MODEL AND METHODOLOGY
      The Pathogen Risk Assessment Model for Municipal Sludge Landfilling (SLDGFILL)
was adapted from the pathogen risk assessment model for land application of municipal sewage
sludge (U.S. EPA, 1989b,c)  to address  the disposal of dewatered sludges (>15% solids) in
sludge-only landfills {monofills) that use  trench, area fill or diked containment methods
(U.S. EPA, 1988a).  SLDGFILL is considerably simpler than the parent model because it deals
only with the groundwater  pathway and only with landfills from which pathogens may be leached
into groundwater.  The other exposure routes are less significant  or better regulated through
good sludge management practices (U.S. EPA, 1988a; 1989a).  The receptor is a person
ingesting water directly from a  drinking-water well near the site.
      SLDGFILL  has retained  the saturated zone  groundwater  transport and infection
algorithms from the parent model. A groundwater transport subroutine for unsaturated soil was
added (U.S. EPA, 1993).  Based on the van Genuchten model (van Genuchten and Alves, 1982),
SLDGFILL combines features of the saturated zone transport subroutine from the parent model
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with U.S. EPA's unsaturated zone transport methodology for chemicals hi sludge (U.S. EPA,
1989a).
       SLDGFILL is a  compartment-vector  model with four compartments:  bulk sludge,
unsaturated soil, saturated soil and groundwater (drinking-water) well.  The model begins with
a trench filled with dewatered municipal sludge as the worst case for the source term.  The
number of organisms in each compartment is  calculated for a column with  a square cross-
section, 1 m (all units are metric) on each side  and the entire depth of the compartment.  The
number of organisms may increase or decrease by transfer from one compartment to another or
may decrease by inactivation or death.  Growth equations are not included because viruses and
human parasites do not reproduce without a suitable host and the growth of pathogenic enteric
bacteria is not favored by conditions hi a municipal sludge monofill. Under certain conditions
(e.g.,  addition of compost or other nutrient mixtures) bacteria may increase by regrowth;
however, such substrates are not considered hi this application, which is specific for monofills.
       Transfers are assumed to be unidirectional, from sludge through soil to the well  from
which the exposed individual drinks. The numbers of pathogens to be transferred are calculated
on a daily basis.  Inactivation or die-off is included hi each transfer step, as well as retardation
and dispersion, which are calculated by the groundwater transport subroutines.
       For simplicity, the model employs  a Poisson distribution calculation to determine the
probability of infection.  The Poisson approximation gives results similar to those of other
proposed dose-response models (U.S. EPA, 1993). In SLDGFILL, the probability of infection
has a component that is proportional to the exposure concentration and a component that is an
exponential function of the minimum dose required to establish an infection (MID). When the
probabilities of infection at various MIDs are graphed as functions of exposure concentration,
a family of sigmoid curves results (Figure 4-1).  At low exposure concentrations, the probability
of infection is exponentially related to the concentration, with an exponential slope proportional
to the MID. In contrast, at high concentrations the probability of infection rises very slowly
with increasing concentration.  As a result, changing the MID from 1 to 5 can cause a decrease
of several orders of magnitude hi probability of infection at low exposure concentrations, while
the same change may result hi less than a two-fold reduction in probability of infection at high
concentrations.
                                           4-2

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                           2      3      4      5      6     7     8
                         PATHOGEN CONCENTRATION IN WATER (No./L)
              .MID-1
. MID-2
.MID-3
.MID-5
.MID7
 9    10


MID=10
                             10             20             30
                       PATHOGEN CONCENTRATION IN WATER (No./L)
                                                 40
                   .MID=10
     . MID=20
     .MID=30 _a_MID=40
               . MID=50
   Figure 4-1. Probability of Infection from a Representative Pathogen, Graphed

as a Function of Exposure Concentration at Various Minimum Infective Doses (MIDs)

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4.2.  TEST SITES FOR MODEL RUNS
       Six sites were chosen to provide a variety of soil types, topography and meteorologic
patterns. The sites were Anderson Co., TN; Chaves Co., NM; Clinton Co., IA; Highlands Co.,
FL; Kern Co.,  CA; and Yakima Co., WA.  Other than Anderson County, TN, for which more
detailed meteorologic data were available to the authors, specific sites were chosen with the goal
of geographic diversity.  Data on soil properties and precipitation were taken from U.S. Soil
Conservation Service soil surveys,  which have been developed for most counties 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.
4.2.1. Site 1: Anderson County, TN.  Values of site-specific variables were chosen to reflect
conditions at an agricultural location  hi the Clinch River Valley of East Tennessee.
       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 hi 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 hi
       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 hi each horizon.  These fragments commonly increase hi
       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 (USDA, 1981a).
       For this analysis, a slope of 10% was used, because it was assumed that sites with slopes
> 10% (6) would not.be used because of the likelihood of excessive runoff.
       Narrative Climatologic Summary. The following climatologic summary for Oak Ridge,
Anderson County, TN, was taken from NOAA (1981):
       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.
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        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 hi various years.  July
        is usually the hottest month but differences between the mean temperatures of the
        summer months of June, July, and August are also relatively small.  The highest
        mean monthly  temperature may occur hi  either of the months June, July,  or
        August and  the highest temperature of the year has occurred  hi 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 [38C] 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
        [12C] with the greatest average range  hi spring and fall and the  smallest hi
        winter.  Summery nights  are seldom  oppressively  hot and humid.   Ix>w 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
        whiter to  a summertime minimum but seasonal differences are small.

-The monthly average temperatures at this location ranged between a low of 2.8  C and a high

 of 24.8 C.  Average annual precipitation, approximately 139 cm, was calculated from data hi
 the soil survey (USDA, 1981a).

 4.2.2.  Site 2: Chaves County, NM.  Values for site-specific variables for Site 2 were chosen
 to represent an agricultural area near Roswell, a city hi southeast New Mexico.
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       Description of Soil.  The soil chosen for the model ran is the Pecos Series, which

comprises fine, mixed, thermic Torrertic Haplustolls.  It is further described as follows (USDA,

1980):

       The Pecos series are deep, moderately well drained,  very slowly permeable soils
       on flood plains.   The soils  formed  hi  calcareous,  saline, stratified, clayey
       alluvium.  Slope is 0 to 1 percent.

       Typically, the surface layer is reddish brown silty clay  loam about 12 niches
       thick. The upper 10 inches of the substratum is reddish brown clay, the next 20
       niches is reddish brown silty  clay and silty clay loam, and the lower part to a
       depth of 60 inches or more is brown loam and  fine sandy loam.  Salinity is
       moderate.  Available water capacity is high.

Pecos soils are of hydrologic group D, characterized by a very slow infiltration rate (high runoff

potential)  when thoroughly wet.  They consist chiefly of clays that have a high shrink-swell

potential,  soils that have a permanent high water table, soils that have a claypan or clay layer

at or near the surface and soils that are shallow over nearly impervious material.  These soils

have a very slow rate of water transmission (USDA, 1980).

       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 hi 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 hi common with the geographic situation,
       discourages a significant part of the heat and humidity that originates hi the south
       hi 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 whiter.  Subzero cold spells are of short duration. Whiter is the season of
       least precipitation (NOAA, 1981).

The monthly average temperatures at this location ranged between a low of 4.2 C and a high

of 26.2 C.  Average annual precipitation, approximately 30 cm, was calculated from data hi

the soil survey (USDA, 1980).
                                          4-6

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4.2.3.  Site 3: Clinton County, IA.  Values for site-specific variables for Site 3 were chosen

to represent an agricultural area in eastern Iowa in a county that borders or. the Mississippi

River.

       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 niches 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 niches to 60 niches.

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

moderate infiltration rates and moderate  rates  of water transmission.
       Narrative Climatologic Summary.  Because a meteorologic report for Clinton County

was not included hi 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 whiter with readings as low as 32 below [-36C], when the ground
       is snow covered, to the hot, dry weather  of the air masses from the desert
       southwest hi 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 ah- masses dominate Dubuque weather, with' the invasions of Gulf air
       rarely occurring  hi the whiter.

       The seasons vary widely from year to year at Dubuque; for example, successive
       invasions of cold ah- from the north may just reach this far one whiter and bring
       a long, cold whiter with snow-covered ground from mid-November until March,
       and many days o'f sub-zero temperatures, while another season the cold air may
       not reach quite this far and 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.
                                          4-7

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The monthly average temperatures at this location ranged between a low of -7.5 C and a high

of 23.2 C. Average annual precipitation, approximately 93 cm, was calculated from the soil

survey (USDA, 1981b).
4.2.4.  Site 4: Highlands County, FL.  Values for site-specific variables for Site 4 were chosen

to represent a sandy soil hi central Florida.  These soils can be productive for agriculture but

can be improved greatly by amendment (USDA, 1989).
       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 hi marine and eolian deposits.  These soils
       are on moderately high ridges hi 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 inches 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 hi 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.
       "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 (NOAA, 1981).
       Orlando, by virtue of its location hi 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 hi the afternoon (sometimes
       to 20 percent  hi the whiter).

       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

                                           4-8

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       above 95 [35C] are rather rare). Too, a breeze is usually present, and this also
       contributes towards general comfort.

The monthly average temperatures at this location ranged between a low of 15.8 C and a high

of 28.0 C. Average annual precipitation, approximately 138 cm, was calculated from the soil

survey (USDA,  1989).

4.2.5. 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 California.

       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, well drained soils on alluvial fan, stream
       flood plains, and stream terraces.  These soils formed hi mixed alluvium derived
       from granitic rock.  Slope ranges from 2 to 9 percent.

       Clay content ranges from 5 to 18 percent hi 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.

       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 whiter.  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
                                          4-9

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       percent of all precipitation falls from October through April, inclusive. Snow in
       the valley is infrequent, with only a trace occurring hi about one year out of
       seven.  Thunderstorms also seldom occur hi the valley (NOAA,  1981).

The monthly average temperatures at this location ranged between a low of 8.5 C and a high

of 28.8 C. Average annual precipitation, approximately 26 cm, was calculated from the soil

survey (USDA,  1981c).

4.2.6. 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 hi south-central Washington along the Yakima River.  This is a region of

fairly low rainfall, but which is successfully farmed by irrigation.
       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 hi 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.
       Narrative Climatologic Summary. Yakima is located hi a small east-west valley
       hi 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  hi  marked
       variations  hi  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 hi
        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


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      the southwestern United States. On these occasions, the cold arctic ah* 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 whiter on
      the average.  On about 21 days during the whiter the temperature will fail to rise
      to the freezing point.  In January and February 1950, there were 4 consecutive
      days colder than-20 [-29C], including -25 [-32C] on February 1.  However,
      over one-half of the winters remain above 0 degrees [F (-18C)] (NOAA, 1981).


The monthly average temperatures at this location ranged between a low of -1.5 C and a high

of 22.3 C.  Average annual precipitation, approximately 20 cm, was calculated from the soil

survey (USDA, 1985).


4.3.  USE OF SOIL AND CLIMATOLOGICAL DATA

      Parameter values derived from the soil and ciimatological data presented above are used

hi the evaluation of pathogen risks at the hypothetical test sites. These parameters are discussed

hi Section 6.3 below.
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                   5. RATIONALE FOR PARAMETER VALUES

       Parameters of the SLDGFILL model are identified by name and by parameter number
in Tables 5-1 through 5-5 at the end of this section (e.g., parameter  1, DSATZN, is denoted
[P(l)]).  During its development, the model was shown to lack sensitivity to several of the
parameters (U.S. EPA, 1993).  In this evaluation of pathogen risks, testing concentrated on the
parameters to which the model was most sensitive. They are DSATZN [P(l)], ANRAIN [P(4>],
EVAP [P(5>], WCSAT [P(6>], PATHDN [P(12)], INACTB [P(13>], INACTW [P(15>], SSPNDB
[P(16>], MID [P(18>], DUNSAT [P(21>], VGW [P(22>] and XWELL [P(24)].

5.1. ASSUMPTIONS
       It is assumed that helminth ova are too large to move through the soil.  Bacteria and
viruses  are  considered to  move freely,  with limitations described by parameters of  the
groundwater transport subroutine.
       The time step used in this model is one day.  The model  is intended to be used for
distances from a few meters to several hundred meters and for times from several days to a few
years, and a description of short-term (e.g., hourly) events is not necessary. In particular, the
daily step is sufficiently precise for the groundwater subroutine, which deals with the relatively
slow transfer of groundwater rather than with rapid events such as  surface runoff (assumed to
be prevented by site management).
       Two key features of microbial fate-and-transfer models are inactivation and transport in
groundwater.  It is assumed in this model that inactivation or death of pathogens hi municipal
sludge, soil  and groundwater follows exponential kinetics with a constant inactivation rate.
Because of the long-term nature of the model runs and the protective effects of the sludge  layer,
it is assumed that effects of temperature on inactivation need not be accounted for other than hi
the overall inactivation rate parameters.
       Composted or incinerated sludges are  not considered in this methodology,  which is
limited to dewatered  sludge.   Although liners may  be  used hi the wide-trench or  area fill
methods, hi order to achieve a more consevative modeling approach, this methodology assumes
no liners. It is assumed that the soil compartments are homogeneous.  Unsaturated soil and the
                                         5-1

-------
aquifer may have different properties, but the model's calculations are based on homogeneity
throughout each compartment.  Transport of pathogens through solution channels, cracks and
cavities in the subsurface may be much more rapid than the rates calculated by the model.
       The groundwater transport algorithm  used in this model is based  on advection and
dispersion  (U.S. EPA,  1989b,c).  Advection  is bulk flow through the soil, and dispersion
describes the diversion of particles from direct flow lines by the presence of a network of pores
and channels.  Specific consideration of filtration/clogging in soil pores or adsorption/desorption
reactions between pathogens and soil particles is not included hi the model.
       The calculated concentration of pathogens reaching a drinking-water aquifer beneath the
site is used to calculate the concentration  in the well.  This assumption is conservative because
in a model using a one-dimensional solution of groundwater transport,  vertical mixing is not
included.
       The receptor at risk is a person who drinks 2 L of water daily from a groundwater well
located downgradient from the monofill.  The probability of infection is calculated in this model
by a simple Poisson relationship, hi which it is assumed that ingestion of less than an infective
dose of the pathogen hi one day never causes infection and ingestion of at least an infective dose
in one day always causes infection.  The effect of each day's  exposure is independent of the
outcome for any other day.   These assumptions ignore additive effects of exposure or the
induction of immunity by subinfective exposures because the U.S. EPA (1992) has suggested
that groundwater protection should be based on the assumption that each day's exposure acts
independently. To model risk to sensitive individuals, low values of infective dose [MID, P(18)]
were used.  The assumption that there is no acquired immunity also allows for protection of
knmunocompromised persons.  Because infection is based on the absolute number of pathogens
ingested, an adult drinking 2  L of water daily is at greater  risk than a child, who would on
average drink less.  Neonatal effects are not considered because the risk of infection to pregnant
women has not been shown to be greater than to other persons.

5.2.  INPUT PARAMETERS
       Parameters describe physical characteristics of the site, nature and amount of sludge and
properties  of the pathogens.   Default values and descriptions of the parameters are described
                                          5-2

-------
AQUIFR
PORWTR
ANRAIN
EVAP
WCSAT
SATCND
SMRSLP
= P(2)
= P(3)
= P(4)
= P(5)
= P(6)
= P(7)
-/ = P(ll
below and summarized hi the accompany ing tables.  The ranges of these values are taken from
literature reviewed for this study (Section 3) and from U.S. EPA (1989b,c; 1991a,b,c; 1993).
In some cases, intermediate values have been added to provide additional detail.
5.2.1.  Site-Specific Parameters.  Site-specific parameters describe physical properties of the
site's underlying soil and the relevant weather conditions.  They are:
       DSATZN     = P(l)       Depth to saturated zone under sludge (m)
                                 Thickness  of aquifer (m)
                                 Volumetric moisture content  of  saturated  soil, or pore
                                 water
                                 Annual rainfall (cm)
                                 Fraction of rainfall lost by evaporation and  surface runoff
                                 Saturated water content of subsurface soil (fraction)
                                 Saturated conductivity rate of subsurface soil (m/s)
                                 Slope of the soil moisture retention curve.
       Depth to the saturated zone varies with topography; typical values for depths were taken
from a sludge monofill process design manual (Walsh, 1978). Although a default value of 20
m is indicated in the model documentation (U.S. EPA, 1993), for these assessments a value of
2 m was used because it is closer to the typical minimum depth to groundwater at several test
sites (Section 6.3). As an assessment of flooding of the monofill, a conservative value of 0 m
was also used.  The ranges of other parameters (Table 5-1) were chosen from  information
reviewed previously (U.S. EPA, 1989b,c; 1993).
       Site-specific values for DSATZN [P(l)], ANRAIN [P(4)], EVAP [P(5)], WCSAT [P(6)]
and SATCND [P(7)] at the six test sites are given in Section 6.3 below. Data for other sites can
be  found  in such reports  as  USDA Soil Conservation Service  soil  surveys and NOAA
climatological data summaries.
5.2.2.  Bulk Sludge Parameters. These parameters describe bulk sludge hi a full trench (i.e.,
one to which no more sludge  is to be added).  A full trench as a starting  point simplifies
calculations while providing a maximum source term of sludge pathogen number hi the monofill.
The loading parameters and rationales for then: choice are:
       DEPTH      = P(8)       Depth of sludge hi trench (m)
                                         5-3

-------
       SOLIDS      = P(9)       Fraction of sludge solids
       BLKDEN     = P(10)      Bulk density of wet sludge (kg/m3)
       PATHDN     = P(12)      Density of pathogens in sludge (number/kg dry weight)
       Typical values for depth of sludge in the monofill trench [DEPTH, P(8)], fraction of
sludge solids [SOLIDS, P(9)] and bulk density of sludge [BLKDEN, P(10)] were taken from the
municipal sludge landfill process design manual (Walsh, 1978).  The density of pathogens in
municipal sludge was taken from Section 3.1.1 and from reviews presented previously (U.S.
EPA, 1993).
       Values used in the model simulations are given in Table 5-2.
5.2.3.   Pathogen-Specific  Data.   Organism-specific  properties  characterize  survival of
pathogens, then: interaction with sludge and soil particles and then: infective dose. They are:
       JNACTB     = P(13)      Inactivation rate in bulk sludge (Iog10/day)
                                 Inactivaition rate hi soil (Iog10/day)
                                 Inactivation rate hi water (Iog10/day)
INACTS
BSTACTW
SSPNDB
= P(14)
= P(15)
= P(16)
       SSPNDS     = P(17)
Sludge-to-water resuspension factor
[(number/g sludge)/(number/cm3 water)=cm3/g]
Soil-to-water resuspension factor
[(number/g sludge)/(number/cm3 water)=cm3/g]
       MID
              = P(18)     Infective dose (pathogens)
       It is known that the survival of municipal sludge pathogens is enhanced by moisture,
organic matter and moderate pH values (Kowal, 1985; U.S. EPA, 1988a).  In this study it is
assumed that the moisture and nutrient content of municipal sludge protect pathogens from
inactivation; this assumption is probably overly conservative, but it provides a reasonable worst-
case assessment of potential risks from pathogen transport. Depth of monofill application should
allow the protective effects of sludge to remain constant; the variable INACTB [P(13)] assumes
the default exponential die-off rate of 0 logs/day.
       Inactivation rates of bacteria and viruses in soil [INACTS, P(14)] and water [INACTW,
P(15)] have been reviewed by U.S. EPA (1993). The ranges of inactivation rates given in Table
                                          5-4

-------
5-3 are taken from that review.  Sources of data were Hurst et al. (1978), Cubbage et al. (1979),
Reddy et al. (1981), Keswick et al. (1982), Bitton et al. (1983), Yates et al. (1.85), Sorber and
Moore (1987), Kutz and Gerba (1988), Moore  et al.  (1988), Hurst (1988) and Hurst et al.
(1989b).  Recent studies of virus transport in soil have shown inactivation or retention by soil
at rates of 0.13 Iog10/day (Straub et al., 1992), 0.89 Iog10/day (Cogger et al., 1988) or  1.35 to
10.4 Iog10/day (Powelson et al., 1993).   These values,  which were  determined  under field
conditions similar to  those of  a municipal sludge landfill, are probably more realistic than
laboratory-derived rates reported hi the model documentation (U.S.  EPA,  1993).  Because
retention is a result of interactions between pathogens and soil particles, retention  rates might
better be  expressed as logio/m rather than Iog10/day or scaled as a function of groundwater
velocity.  However, because the reported variation in retention rates is so great,  the most
conservative, 0.13 Iog10/day, was used without adjustment for groundwater velocity. This value
was substituted for both INACTS [P(14)] and INACTW[P(15)] because they were derived as a
composite of retention by soil and inactivation hi soil and groundwater.
       The ratio of particulate-bound  to  suspended organisms is given by the  parameters
SSPNDB  [P(16)] for sludge and SSPNDS  [P(17)] for soil.  Values hi Table 5-3 (Drewey  and
Eliassen, 1968; Marshall, 1971; Gerba etal., 1975; Surge andEnkiri, 1978; Reddy etal., 1981)
are taken from U.S. EPA (1993). Although the actual distribution between water and particles
is likely to depend on chemical composition of the sludge or soil and of the water  solution, it
is assumed that the distribution is constant through the compartment and that suspended  and
particulate-bound pathogens are hi equilibrium.  These assumptions are not realistic, but are
required for computations; the assumption of homogeneity is probably adequate for  long times,
and the assumption of equilibrium is likely to overestimate the concentration of pathogens hi free
soil water.
       Dose-response  data for municipal sludge pathogens were discussed by U.S. EPA (1993).
Conservative values of MID [P(18)] will be used at the low end of a range of infectious doses
hi the analyses.  The highest reported value of infectious dose for each pathogen type is much
higher than the highest values to be used hi the analysis.  However, the low values hi the ranges
to be used should provide protectively conservative risks for each pathogen type.
       Values used hi the model simulations are  given hi Table 5-3.
                                          5-5

-------
5.2.4.  Groundwater Transport Parameters.  Groundwater transport parameters are used by
the transport subroutine and describe the soil and groundwater through which the pathogens pass
to the well.  They are:
                                Retardation coefficient in the saturated zone (dimensionless)
ROW
RUS
= P(19)
= P(20)
       DUNSAT    = P(21)
       VGW
       DGW

       XWELL
             = P(22)
             = P(23)

             = P(24)
Retardation   coefficient   in   the   unsaturated   zone
(dimensionless)
Hydrodynamic dispersion coefficient hi  the unsaturated
zone (cnWhr)
Velocity of groundwater hi the saturated zone (cm/hr)
Hydrodynamic dispersion coefficient hi the saturated zone
(cm2/hr)
Distance from the sludge trench to the groundwater well
(m)
       Values to be used hi the model simulations are given hi Table 5-4.
5.2.5. Parameters for Evaluation of Test Sites
       In previous modeling studies  of soil  amendment with municipal sludge (U.S. EPA,
1991a,b,c), six representative test sites were described. Properties of soil and climate at these
sites were used to choose values for model parameters, and the outcome was then determined.
Variability hi the results gave some indication of the importance of climate and soil type on
applicability of the soil amendment technology. Selected values for these same sites were used
to test the sensitivity of the SLDGFILL model to site-specific parameters.   The parameters
chosen were DSATZN |P(1)], ANRAIN [P(4>], EVAP [P(5>], WCSAT [P(7>] and SATCND
[P(8)]; other site-specific parameters have little or no effect  on model outcome (U.S.  EPA,
1993).  Parameter values are listed hi Table  5-5.  Default values were used for groundwater
transport parameters for the sites.
       Depth to the saturated zone [DSATZN, P(l)] may vary seasonally and with the amount
of water hi the trenches.  Values given hi Table 5-5 were taken from USDA Soil Conservation
Service soil survey reports  (USDA 1980; 1981a,b,c; 1985; 1989) on minimum depth to the
water table for the soil types listed.  Annual rainfall amounts were calculated from average
                                          5-6

-------
rainfall data given in soil surveys (Section 4.2). In several cases, values were also included
above and below which rainfall could be expected during 20% of the years.  These values were
taken to represent the 80% upper and lower confidence intervals in a normal distribution. From
them, the assumed standard deviation was calculated, and the 95% upper and lower confidence
intervals were calculated.  When these values were not available, the same ratio of upper and
lower values to the mean was used.  EVAP, the fraction of rainfall that evaporates or runs off,
was the default value of 0.5 except in the case of Chaves Co., NM, which was described as
being very dry (assigned a value of 0.7), and Highlands Co., FL (assigned a value of 0.6).
WCSAT was calculated from soil bulk density.  Soil bulk density was usually given in the soil
surveys.   If not,  a value of 0.45 for WCSAT was used.  SATCND was listed in  tables of
physical properties of the soils in the soil surveys.
                                          5-7

-------

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                                      6. RESULTS

        The following sections describe the outcomes  of model runs intended  to show how
 variations in parameters whose values are not accurately known can affect the probability of
 infection by bacteria and viruses in monofilled sludge. Section 6.1  describes the effects for
 bacteria, Section 6.2 describes results for viruses, and Section 6.3 describes results for bacteria
 and viruses using site-specific data characteristic of six different locations chosen to have a wide
 range of climate and soil types.

 6.1.   MODEL RESULTS  FOR BACTERIA
        The transport of bacteria from sludge into the unsaturated zone, through the saturated
, zone, and into wellwater is illustrated in Figure 6-1. In this model run, all parameter values
 were the default values given  hi Tables 5-1 through 5-3.  The results show that the concentration
 of bacteria hi wellwater is much lower than hi the saturated zone beneath the monofill trench.
 Figure 6-1 also  shows  the tunes required  for bacterial pathogens to  traverse the soil
 compartments. The concentrations of bacteria in each transport medium reach a steady state hi
 which entry of bacteria into the medium is balanced by their removal from the medium by
 transport or inactivation.
        The calculated probability of infection to a person drinking wellwater 50 m downgradient
 from the monofill was 6 X10"2 per day when MID = 10, or 3 x 1Q-6 per day  when MID = 20.
 The U.S. EPA has recommended a maximum probability of infection of 1X lOr4 per year as a
 protective limit for pathogen  exposure hi groundwater (U.S. EPA, 1992). This corresponds to
 approximately 2.6xlO"7 per  day (Regli et al.,  1991).  Therefore, the projected probability of
 infection from bacterial pathogens from sludge hi wellwater is approximately 200,000-fold above
 the U.S. EPA benchmark value when the MID is 10 but is only ten-fold above the benchmark
 value when MID ?= 20.
 6.1.1.  Site-Specific Parameters.  Site-specific parameters describe physical properties of the
 site's underlying soil and the relevant weather conditions. Parameter  values and rationales for
 then-choice are given in Section 5.2.1.
        Previous studies (U.S. EPA, 1993) showed that the model outcome  is not sensitive to
                                          6-1

-------
 20
    0
  50           100           150          200
                TIME (days)

.UNSATURATEDZONE  _*_ SATURATED ZONE
                                                                   250
DEFAULT VALUES, 240 DAYS
                  Figure 6-1. Kinetics of Bacterial Transport
                                 6-2

-------
variations in parameters P(2), P(3), P(7) or P(ll).  The effects of variations in the sensitive
parameters, P(l), P(4), P(5) and P(6), on predicted concentrations of bacteria in wellwater and
probabilities of infection are  shown in Table 6-1.  In these model runs only  the indicated
parameter was varied from the default values given in Table 5-1. Table 6-1 also indicates how
the outcome varies with changes hi the parameter values.
       Interpolation of the results  showed that an unsaturated zone of at least 6 m is required
to reduce the probability of infection at the drinking water well below the U.S. EPA benchmark
risk of infection of 10"4 annually, or 2.6 xlO'7 daily (U.S. EPA, 1992).
6.1.2. Bulk Sludge Parameters.  These parameters describe bulk sludge hi a full trench.  A
full trench as a starting point simplifies calculations while providing a maximum source term of
sludge pathogen number hi the monofill. The loading parameters and rationales for their choice
are given hi Section 5.2.2.
       Studies during model development (U.S. EPA, 1993) showed that outcomes of the model
runs are not sensitive to parameters  P(8), P(9) and P(10). The effect of PATHDN [P(12)]  on
the concentration of bacteria hi wellwater and the associated risk of infection are shown hi Table
6-2.   The results showed that the concentration of bacteria hi wellwater is proportional to the
concentration of bacteria in the sludge.
6.1.3. Pathogen-Specific  Data.    Organism-specific  properties characterize  survival  of
pathogens, their interaction with sludge and soil particles and their infective dose.   Parameter
values and the rationales for their choice are given hi Section 5.2.3.  The response of the model
to variations hi INACTS [P(14)]  and SSPNDS  [P(17)] was shown to  be minor (U.S. EPA,
1993).  The results of variation hi  the other pathogen-specific data are shown hi Table 6-2.
Over the range of variation hi SSPNDB |P(16)] that was tested (20 to 2000), there was less than
5% variation  hi the calculated concentration of bacteria hi wellwater.  However,  the other
parameters, PATHDN [P(12)], INACTB [P(13)J, INACTW [P(15)]  and MID [P(18)] had
significant effects on the steady-state concentrations  hi wellwater and on the risk of infection.
Particularly important were inactivation/retention rates hi sludge and hi soil. Figure 6-2 shows
that inactivation of pathogens hi sludge prevents the attainment of steady-state  levels,  as the
number of viable pathogens hi the source trench decreases with tune.
                                          6-3

-------
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   0
50
       .P(13)-0 Log/day

       .P(13)-0.1 Log/day
  100            150
  TIME (days)

.P(13)=0.001 Log/day .

.P(13)=1.0 Log/day
200
                                     P(13)=0.01 Log/day
Figure 6-2. Effect of INACTB [P(13)] on Kinetics of Bacterial Transport to Wellwater

                                     6-6

-------
6.1.4.  Groundwater Transport Parameters.  Groundwater transport parameters are used by
the transport subroutine and describe the soil and groundwater through which tjje pathogens pass
to the well. These parameters and the rationales for their choice are presented in Section 5.2.4.
Of these,  ROW [P(19)], RUS [P(20>] and  DGW [P(23>]  had little influence on  the model
outcome (U.S.  EPA,  1993).  The effects of variations in the sensitive parameters,  DUNSAT
[P(21)], VGW [P(22)] and XWELL [P(24)], are summarized in Table 6-3. The results showed
that concentrations of bacteria in wellwater are proportional to the logarithm of the dispersion
coefficient hi the unsaturated zone.  The  velocity of groundwater was also important to the
model  outcome, primarily because, at low velocities, a longer tune is required for bacteria to
arrive  at the well, and more inactivation occurs to lower the concentration and risk from the
wellwater. Figure 6-3 illustrates how groundwater velocity affects the tune required for bacteria
to arrive at the groundwater well.  In this figure,  the results have been multiplied by different
factors to visualize the kinetics; the changes of scale also illustrate the inactivation and retention
that accompany slower travel through the subsurface.
       Similarly, distance to the well [XWELL, P(24)] is very important because, as the
distance increases,  inactivation and  retention of bacteria by soil increase  and  bacteria are
dispersed, decreasing their concentration.  These effects make the setback distance of the well
from the sludge source a primary factor hi managing risk to groundwater consumers.  Because
the logarithm of the bacterial concentration in wellwater is proportional to XWELL [P(24)]
(Table 6-3), a protective setback distance can be determined graphically. Figure 6-4 shows that
the setback distance required for a daily probability of infection less than 2.6 x 10~7 (using default
parameter values) is 110 m.  When  DSATZN  [P(l)], the distance from the sludge trench to
groundwater, is increased from 2 m to 5m, the setback distance becomes approximately 75 m
(Figure 6-4).    When MID  [P(18)] is 20,  the required setback distance is  reduced to
approximately 60 m (Figure 6-4).  These results indicate the importance of accurate data on
operating  characteristics of the sludge monofill and on infectivity, survivability and transport of
the pathogens.

6.2.   MODEL RESULTS FOR VIRUSES
       Parameters for transport and inactivation of viruses hi the soil and for infection by viruses
                                          6-7

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 P(22)=36cm/hr

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          200
    TIME (days)

P(22)=10.8 cm/hr

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      figure 6-3.  Effect of VGW [P(22)j on Bacterial Transport to Wellwater

                                    6-9

-------
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Figure 6-4.  Graphical Determination of Well Setback Distances for Bacterial Pathogens

                                    6-10

-------
are significantly different from the corresponding parameters for bacteria.  Figure 6-5 presents
the kinetics of viral transport through the unsaturated and saturated zones to a well at 50 m from
the sludge trench.  The steady-state concentration of viruses hi wellwater,  0.005/L, was about
three orders of magnitude lower  than for bacteria  because  the  experimentally  derived
inactivation/retention rate used as the revised default value for viruses [0.13 Iog10/day (Straub
et al., 1992)] is much higher than the default inactivation rate for bacteria hi soil.  The
maximum virus concentration hi the well is reached by day 80 and remains constant for at least
another 500 days.  The associated maximum daily risk of infection by consumption of wellwater
is IxlO'2.
6.2.1. Site-Specific Parameters.  Site-specific parameters describe physical properties of the
site's underlying soil and the relevant weather conditions. The parameters for viruses  are the
same as those for bacteria (Table 5-1). Table 6-4 summarizes the results of varying site-specific
parameters.  The effects of these parameters on model results for viruses are qualitatively similar
to the effects for bacteria (Sect. 6.1.1). Of these parameters, DSATZN [P(l)] was the only one
that could be varied to decrease the daily risk of infection below the benchmark of 2.6 x 10"7 and
is therefore the most important for risk management.
6.2.2. Bulk Sludge Parameters. Bulk sludge parameters describe sludge in the disposal trench.
Except for the density  of viruses  [PATHDN,  P(12)], these parameters are  the same as for
bacteria.  The effects of variations hi PATHDN [P(12)] on concentrations of virus in wellwater
and the associated risk are shown  hi Table 6-5. As with bacteria, the calculated steady-state
concentration of virus hi wellwater is proportional to the density of viruses hi sludge.
6.2.3. Pathogen-Specific Parameters.  Organism-specific parameters characterize survival of
viruses, then: interaction with sludge and soil particles and their infective doses.  All virus-
specific parameters [P(13) through P(18)] are different from bacteria-specific parameters.  The
results of varying pathogen-specific parameters on predicted concentrations of virus hi wellwater
and the associated risks to wellwater consumers are shown hi Table 6-5. Inactivation of viruses
hi bulk sludge, described by the parameter INACTB [P(13)], has qualitatively the same effect
on virus  concentrations as it had on bacteria (Figure 6-2).
       An accurate description of virus inactivation hi soil and water is extremely important to
the consequences of sludge leaching.  Field studies of virus transport (Cogger et al.,  1988;
                                          6-11

-------
0
  0
50
     . UNSATURATED ZONE

     . WELL (results X1000)
100
 TIME (days)
150
200
                       SATURATED ZONE (results X10)
                  Figure 6-5. Kinetics of Virus Transport

                                6-12

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Ijzerman et al., 1993; Straub et al., 1992; Powelson et al., 1993) provided infonnation on the
combined effects of inactivation and retention of viruses in soil.  Their results were incorporated
in the evaluation of risk from viruses by substituting their reported inactivation/retention rates
for both  INACTS  [P(14>]  and  INACTW [P(15)], because the observed results  described
inactivation in both soil and soil pore water. Table 6-5 shows that the high inactivation/retention
rates observed hi the field [0.89  Iog10/day (Cogger et al., 1988) and 1.35 Iog10/day (Powelson
et al., 1993)] result in very low calculated values for risk from wellwater 50 m from the sludge
trench.  These results emphasize the need for reliable field data for inactivation and retention
of viruses during transport hi soil.
6.2.4.  Groundwater Transport Parameters. Groundwater .transport parameters describe the
soil and groundwater through which pathogens migrate hi the saturated zone.  Default values for
viruses are the same as for bacteria.  The effects of varying groundwater transport parameters
are qualitatively the same for viruses as for bacteria (Table 6-6).
       Setback distances for groundwater wells were calculated by varying XWELL [P(24)] and
interpolating from the results to a virus concentration of 2x 10"7/L, the proposed (Regli et al.,
1991) concentration for the allowable daily risk of 2.6 x 10"7 to wellwater consumers (U.S. EPA,
1992).   The calculated setback distances  depended on  a number of parameters,  the most
important being DSATZN  [P(l)]  (Section 6.2.1),  INACTS [P(14)] and INACTW [P(15)].
Setback distances for several combinations of these parameters are given hi Table 6-7. The
results  hi Table 6-7 show that for any given inactivation/retention rate, a combination of facility
design  (distance to groundwater) and well setbacks can be used to manage downgradient risks
from groundwater ingestion.

6.3    RESULTS FOR TEST SITES
       Because most of the site-specific parameters had similar values at the different test sites,
and because many of those parameters had little influence on the outcome of model runs,  the
model  runs yielded similar results for  the test sites.  Figure 6-6 presents a comparison of
bacterial concentrations hi wellwater using the site-specific values listed in Table 5-1,.  The
steady-state concentrations  ranged from approximately 3.0 pathogens/L to approximately 4.2
pathogens/L.  The corresponding probabilities of infection, using MID=10, ranged from 0.05
                                         6-15

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Table 6>7. Well Setback Distances for i
Protection of Wellwater Consumers from Viruses*
DSATZN
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0.0017
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2695
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165
105
12.5
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"Distances calculated to ensure maximum virus concentration in the well is below
2xlO'7/L.
blnactivation/retention rate of nonpathogenic virus tracer.
6-17

-------
0
50
            .Oak Ridge, TN
            , Orlando, FL
200
 100            150
   TIME (days)
_*_ Roswell, NM     x  Dubuque, IA
^_BakersfieId,CA  _a_Yakima,WA
 figure 6-6. Comparison of Bacterial Concentrations in WeUwater at Test Sites
                                 6-18

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to 0.2.  Using the field-derived inactivation/retention rate constant of 0.13 Iog10/day (Straub et
al., 1992), the resulting virus concentrations were approximately 0.4/L, with an associated range
of probabilities of infection of 0.54 to 0.56 per day. These concentrations are higher than those
resulting from the default values for site-specific parameters, most likely because of differences
hi transport rates through the unsaturated soil as described by parameters WCSAT  [P(6)]  and
SATCND [P(7)].  At an inactivation/retention rate of 0.89 Iog10/day (Cogger et al.,  1988), the
range of virus concentrations was 9.7 XlO'19 to 9.9 xlO'19 and  the associated probability of
infection was  
-------

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                                  7.  CONCLUSIONS

7.1    RESULTS OF MODEL CALCULATIONS
       Calculations by the SLDGFILL model indicate that there are risks to people who use
wellwater downgradient from a sludge landfill. Under default conditions, the risk from both
bacterial and viral pathogens hi a well 50 m from the sludge monofill exceeds the U.S. EPA
target of not more than 1X10"4 excess cases of enteric infection from groundwater (U.S.  EPA,
1992).  However, because pathogens are inactivated and are retained by soil during subsurface
transport, the concentration of pathogens in the well can be reduced by  increasing either the
vertical distance from the sludge trench to groundwater [DSATZN, P(l)] or the lateral distance
of the well from the sludge trench [XWELL, P(24)].  By using the model iteratively with site-
specific values for DSATZN, it  is possible to calculate a setback distance for the well that is
protective of human health.
       Using default values for bacterial pathogen density, infective dose and inactivation  rates,
a safe setback distance of 110 m was calculated.  That distance was reduced to 30 m when an
MID of 20 was used hi place of 10. The calculated setback distance for viruses, using the  field-
derived inactivation rate constant of 0.13 Iog10/day (Straub et al., 1992) and other default values,
was 165 m (Table 6-7). In another study (Powelson et al., 1993), the inactivation/retention rate
for bacteriophage hi soil was observed to average 0.89 Iog10/day, which would correspond to a
setback distance of only 12.5 m if pathogenic viruses display the same retention properties. The
disparity between these results emphasizes the importance of accurate field-determined rates of
inactivation and retention of representative pathogenic organisms.
       The  proportionality of pathogen concentration hi the wellwater to PATHDN [P(12)]
(Table 6-2) implies that  for MID  =  10 and XWELL  = 50 m, the  maximum allowable
concentration of bacterial pathogens hi landfilled sludge should be approximately 1 x 104 bacterial
.pathogens/kg. For higher values of MID [P(18)], INACTW [P(16)j or DSATZN [P(l)], higher
concentrations hi sludge would be tolerable.  For example, a previous estimate of MID = 100
(U.S. EPA, 1993), which is probably conservative for most bacterial pathogens,  results in
acceptable risk to groundwater drinkers at approximately 29 pathogens/L, corresponding to a
bacterial pathogen density hi sludge of 4.6xl05/kg.  Similarly,  for viruses, the acceptable
                                          7-1

-------
density in sludge for an annual risk of 1 x 10"4 is calculated to be 4/kg when XWELL=50 m.
Therefore, it is important to know both the pathogen density hi the sludge and.realistic infective
doses of pathogens present hi the sludge to be landfilled.
       Results of varying site-specific parameters (Tables 6-1 and 6-4) showed that except for
DSATZN [P(1)L there was little effect on pathogen transport.  Accordingly, when parameters
for test sites  were used hi comparative model runs, there was little difference from site to site
in risk to groundwater consumers (Section 6.3). However, the depth of unsaturated soil below
the sludge trench was important hi deterrnining the concentration of pathogens reaching  the
groundwater  well. The summary data for the test sites (Table 5-5) indicate that the minimum
distance  from soil surface to groundwater was less  than 2 m hi some cases.  In areas  with a
shallow water table, transport of pathogens from the monofill to downgradient wells poses a
more severe risk to human health than hi areas with a permanently deep water table. Therefore,
prudent risk  management would require that the depth to groundwater should be included hi
determinations of safe setback distances for downgradient wells.
       Data  on survival and transport  of Crytosporidium and Giardia are fragmentary and
inconsistent.  Preliminary modeling results suggest that because of relatively long-term survival,
low infective dose, and small size, Cryptosporidium and possibly Giardia and other protozoa
could potentially be of much greater risk than other parasites typically found hi municipal
sewage sludge.   However, because the  modeling  results  were based more on worst-case
assumptions than on validated data, they can be misleading and are not presented here.  There
is a need for definitive research on transport and inactivation characteristics of Cryptospsoridium
and other protozoa.

7.2    UNCERTAINTIES
       This section presents a discussion  of sources of uncertainty hi the risk assessment for
microbial pathogens hi landfilled municipal sludge.   Because the interactions  among different
pathogens and soil cannot be precisely characterized and the composition of soil cannot be
known throughout the area between the monofill and the groundwater well, uncertainties are
inherent  hi the input data.  In addition, properties of the pathogens, including their survival hi
soil, subsurface transport and infectivity, are both variable and not precisely characterized.
                                          7-2

-------
 Therefore, results of the assessments  presented above may not be precisely correct but are
 nevertheless  sufficiently accurate to indicate what additional research is needed to improve
 pathogen risk assessment for municipal sludge.
 7.2.1. Model Parameters.  The ranges of model parameters shown in Tables 5-1  and 6-1
 through 6-6 indicate the degree of uncertainty in input data.  In most cases, reasonable values
 were chosen that would be conservative enough to be protective of human health.  In particular,
 the value of DSATZN [P(l>] was lower than most of the reported range of distances from sludge
 disposal sites to groundwater, the value used for inactivation of pathogens in sludge [INACTB,
 P(13)=0] was highly conservative, and very conservative values were used for infectivity [MID,
 P(18)]. Each of these parameter values probably underestimates the risk of pathogen infection
 to the person drinking  wellwater.   In  addition,  the distance from sludge source to the well
 [XWELL, P(24)] is a lateral distance from sludge trench to wellhead; the distance to the water
 intake of a well screened at some distance below ground surface  (e.g.,  20 m) would be
 somewhat farther than the distance indicated by XWELL, resulting hi an additional overestimate
 of exposure by the model.
 7.2.2. Environmental Media.  The model is based  on the assumption that environmental media
 (sludge, soil and water) are homogeneous.  Although the assumption  is essential for a simple
 computational model, it is not realistic.  Inhomogeneities hi soil may increase or decrease the
 rate of subsurface transport of pathogens.  The impact of inhomogeneities may best be evaluated
 by running the model with a wide range of parameter values to simulate subsurface transport at
 rates that are both faster and slower than those expected for homogeneous media. This was done
 as part of the sensitivity analyses hi the development of the .model (U.S.  EPA, 1993).
 7.2.3.  Infectivity.   There is a great deal of uncertainty in calculating the risk of infection from
exposure data, because of both variability among individuals hi sensitivity and uncertainty about
the best models to use to calculate the probability of infection. The relation between infection
and disease is also highly variable.   U.S. EPA recognized this uncertainty hi developing its
groundwater rules (U.S. EPA, 1992).   They recommended as a  conservative approach that
infection should be used as  the endpoint for risk assessment and that daily exposures should be
treated as independent rather than additive hi either causing disease or inducing resistance.
                                          7-3

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                                8.  RESEARCH NEEDS

       The key parameters required to predict the  risk from microbial pathogens in sewage
sludge are the density of pathogens in the sludge, the rates of subsurface transport, including
retention and inactivation, and the infectivity of the organisms hi drinking water.  Additional
research is needed to provide a basis for choosing realistic  parameter values.
       The inherent difficulty hi producing a reliable risk assessment model is the minimal
information available to test the most important  parameters and ensure that the model is
representative of real conditions.  One of the difficulties hi clarify ing concentrations, viability
and transport of microbial pathogens hi the environment is the lack of standard, accurate and
rapid detection methods.   Hurst et al.  (1989a) review the methods  for concentrating and
detecting viruses hi water, and Brigmon et al. (1992) briefly discuss methods for detection of
bacteria in environmental samples.  Methods  based on selective media (Watkins et al., 1988),
tracer strains of E.  coli (Finch et al., 1987), membrane filter recovery assays (Santiago-Mercado
and Hazen, 1987)  as well as DNA hybridization (Margolin et al., 1991) are currently under
scrutiny.  Deriving  realistic  representative parameter values for survival and transport will
depend on the development of fast, accurate standard methods hi order to compare research
results with model predictions.
       Recent field studies have added significantly to the understanding of virus inactivation
and retention during subsurface transport (Cogger et al., 1988; Straub et al.,  1992; McKay et
al., 1993; Powelson et al., 1993), and current research into colloidal transport hi the subsurface
is increasing the available data applicable to pathogen transport hi soil and groundwater (Kaplan
et al., 1993).  However, reports of microbial transport are inconsistent.  Powelson et al. (1991)
concluded that sewage sludge-derived organic matter increased the unsaturated-flow transport
of bacteriophage MS-2, a potential model for viral pathogens.  In contrast, other authors have
demonstrated that organic matter inhibits virus transport through the unsaturated zone (Bales and
Li, 1993). Definitive data are needed, both to confirm these recent research results and to
expand the knowledge of inactivation/retention rates and transport processes  hi the subsurface
soil  and groundwater.  Additional field research  is also  needed to provide  information on
transport hi different types of soil.
                                           8-1

-------
       As explained in Sections 2 and 3, the transport processes and inactivation/retention rates
of protozoan cysts and oocysts are poorly understood and inadequately documented to allow for
modeling of these organisms.  Smith (1992), in his review of Cryptosporidium, emphasizes the
need for rapid, standardized techniques for detection of protozoan oocysts.  But information on
protozoan concentrations hi sludge (Jakubowski et al., 1991; Sykora et al., 1991; Crawford and
Vermund, 1988), survival tunes and environmental stability (Barer and Wright, 1990; Casemore,
1991; Jakubowski, 1990) and low infective dose (Karanis et al., 1992; West, 1991; Molbak et
al., 1990; Miller et al.,  1990; Blewett et al., 1993.) indicate a need for information sufficient for
modeling these protozoan parasites.
       In addition to the simple  Poisson model  for probability of infection used  in the
SLDGFILL model, several models for infectivity of viruses and bacteria are available  (Haas,
1983;  Rose and Gerba,  1991; Regli et al.,  1991).   The output of these models should be
validated and the models calibrated for pathogens at low concentrations in drinking water, since
the results of repeated ingestion of low concentrations of pathogens may not be the same as the
results of ingesting single, more  concentrated doses.
       In summary, carefully controlled field and laboratory research is needed in the following
subject areas:
            methods  that are rapid, accurate and standardized for detection of pathogens hi
              sludge and hi environmental media;
            understanding of retention/inactivation of pathogens hi the subsurface and reliable
              inactivation rates,  at least for representative pathogenic microorganisms;
            data on subsurface transport  and retention rates under varying  environmental
              conditions;
            epidemiological or field data for verifying and validating the model; and
            information on infectivity of representative  pathogens in drinking water at low
              concentrations.
                                            8^2

-------
                                              ICES
Alexander, M., RJ. Wagenet, P.C. Baveye, J.T. Gannon, U. Mingelgrin and Y. Tan. 1991.
Movement of Bacteria Through Soil and Aquifer Sand. Robert S. Kerr Environmental Research
Laboratory, Office of Research and Development, U.S. EPA, Ada, OK. EPA/600/2-91/010.

Bales, R.C. and S. Li.  1993.  MS-2 and poliovirus transport in porous media: Hydrophobic
effects and chemical perturbations.  Water Resour. Res. 29(4): 957-963.

Bales, R.C., S.R. Hinkle, T.W. Kroeger and K, Stocking.  1991.  Bacteriophage adsorption
during transport through porous media: Chemical perturbations and reversibility.  Environ. Sci.
Technol. 25: 2088-2095.

Barer, M.R. and A.E.  Wright.  1990.  A review:  Cryptosporidiwn and water. Lett. Appl.
Microbiol. 11: 271-277.

Benton,  W.H. and C.J. Hurst.  1990.  Sequential inoculation as an adjunct in enteric virus
plaque enumeration.  Water Res. 24(7):  905-909.

Berk, R.S. 1983.  An introduction to the world of microorganisms.  Jn: Microbiology: Basic
Principles and Clinical Applications, N.R. Rose and A.L. Barron, Ed.  Macmillan Publishing
Company, NY.  p. 3-10.

Bitton, G. and R.W.  Harvey.  1992.  Transport of pathogens through soils and aquifers.  In:
Environmental Microbiology, Ralph Mitchell, Ed. John Wiley & Sons, NY. p.  103-124.

Bitton, G., N. Masterson and G.E. Gifford.  1976.  Effect of a secondary treated effluent on
the movement of viruses through a cypress dome soil.  J. Environ. Qual. 5(4): 370-375.

Bitton, G., S.R. Farrah, O.C. Pancorbo and J.M. Davidson. 1981.  Fate of viruses following
land  application of sewage  sludge.  I. Survival and transport patterns in  core  studies under
natural conditions.  In:  Viruses and Wastewater Treatment, M. Goddard and M. Butler, Ed.
Pergamon Press, Oxford.  (Cited in Gerba and Bitton,  1984)

Bitton, G., S.R. Farrah, R.H. Ruskin, J. Burner and YJ. Chou.  1983. Survival of pathogenic
and indicator organisms in ground  water. Ground Water 21(4): 405-410.

Bitton, G., J. E. Maruniak and F.W. Zettler.  1987. Virus survival in natural ecosystems.  In:
Survival and Dormancy of Microorganisms,  Y. Henis, Ed. John Wiley and Sons, NY.  p. 301-
332.
                                         9-1

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Black,  M.I., P.V.  Scarpino, CJ. O'Donnell, K.B. Meyer, J.V. Jones and E.S. Kaneshiro.
1982.  Survival rates of parasite eggs in sludge during aerobic and anaerobic digestion. Appl.
Environ. Microbiol. 44(5): 1138-1143.

Blaser, M.J. and L.S. Newman.  1982.  A review of human salmonellosis: I. Infective dose.
Rev. Infect. Dis. 4: 1096-1106.

Blewett, D.A., S.E. Wright, D.P. Casemore, N.E. Booth and C.E. Jones.  1993. Infective dose
size studies on Cryptosporidium parvum using gnotobiotic lambs. Water Sci. Technol. 27(3-4):
61-64.

Brigmon,  R.L.,  S.G. Zam,  G. Bitton and S.R. Farrah.   1992.  Detection  of Salmonella-
enteritidis in environmental  samples by monoclonal antibody-based ELISA.  J.  Immunol.
Methods 152(1): 135-142.

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