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EPA/600/6-90/002 A
November 1989
PATHOGEN RISK ASSESSMENT FOR
LAND APPLICATION OF MUNICIPAL SLUDGE
VOLUME I: METHODOLOGY AND COMPUTER MODEL
U.S. Environmental Projection Agency
Begion 5, Library (5PL-16)
230 S. Dearborn Street, Room 1670
Chicago, IL 60604
Environmental Criteria and Assessment Office
Office of Health and Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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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.
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PREFACE
Section 405 of the Clean Water Act requires the U.S. Environmental Protection
Agency to develop and issue regulations that identify: (1) uses for sludge including
disposal; (2) specific factors (including costs) to be taken into account in determining
the measures and practices applicable for each use or disposal; and (3) concentrations of
pollutants that interfere with each use or disposal. To comply with this mandate, the
U.S. EPA has embarked on a program to develop four major technical regulations: land
application, including distribution and marketing; landfilling; incineration and surface
disposal. The development of these technical regulations requires a consideration of
pathogens as well as chemical constituents of sludge. Public concern related to the
reuse and disposal of municipal sludge often focuses on the issue of pathogenic
organisms. The purpose of this report is to describe a proposed methodology and
associated computer model designed to assess the potential risks to human health posed
by pathogens in municipal sewage sludge applied to land as fertilizer or soil conditioner.
Volume I: Methodology and Computer Model describes the conceptual framework of
the risk assessment methodology and the structural organization, including assumptions
and components, of the computer model. Volume II: User's Manual contains background
information to provide the user with an understanding of the actual functioning of the
model. This information includes descriptions of operating variables and their default
values, explanations of the various subroutines, and the mathematical basis for process
and transfer functions.
The first draft of this document was prepared by Science Applications International
Corporation under a U.S. EPA Interagency Agreement with the Department of Energy.
Portions of the document were also developed by the University of Cincinnati under a
Cooperative Agreement with the U.S. EPA.
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DOCUMENT DEVELOPMENT
Cynthia Sonich-Mullin, Project Officer
Norman E. Kowal
Randall J.F. Bruins
Larry Fradkin
Environmental Criteria and Assessment Office
U.S. Environmental Protection Agency
Cincinnati, Ohio
Marialice Wilson, Project Manager
Science Applications International Corporation
Oak Ridge, TN
Charles T. Hadden
Analysas Corporation
Oak Ridge, TN
Mary P. Stevenson, Model Revision
Jennifer Webb Chason, Model Revision
Ernie L. Burress, Model Revision
Science Applications International Corporation
Oak Ridge, TN
Milovan S. Beljin, Sensitivity Analysis
Groundwater Research Center
The University of Cincinnati
Cincinnati, OH
Contributors
Rafael Livneh
Steven G. Buchberger
T. Michael Baseheart
Pasquale Scarpino
Scott Clark
The University of Cincinnati
Cincinnati, OH
Charles A. Sorber
University of Pittsburgh
Pittsburgh, PA
Charles P. Gerba
University of Arizona
Tucson, AZ
Richard Ward
The Christ Hospital
Cincinnati, OH
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TABLE OF CONTENTS
Page
1. EXECUTIVE SUMMARY 1-1
2. INTRODUCTION AND DESCRIPTION OF GENERAL APPROACH 2-1
2.1. PURPOSE AND SCOPE 2-1
2.2. DEFINITION AND COMPONENTS OF RISK ASSESSMENT . . 2-2
2.3. RISK ASSESSMENT IN THE METHODOLOGY
DEVELOPMENT PROCESS 2-2
2.3.1. Hazard Identification 2-3
2.3.2. Dose-Response Assessment 2-8
2.3.3. Exposure Assessment 2-10
2.3.4. Risk Characterization 2-13
2.4. POTENTIAL USES OF THE METHODOLOGY IN
DETERMINING RESEARCH NEEDS 2-13
2.5. LIMITATIONS OF THE METHODOLOGY 2-14
3. SLUDGE MANAGEMENT PRACTICES 3-1
3.1. INTRODUCTION 3-1
3.2. MODEL OVERVIEW 3-2
3.3. APPLICATION OF LIQUID SLUDGE FOR
PRODUCTION OF COMMERCIAL CROPS FOR HUMAN
CONSUMPTION (PRACTICE I) 3-14
3.4. APPLICATION OF LIQUID SLUDGE TO GRAZED
PASTURES (PRACTICE II) 3-18
3.5. APPLICATION OF LIQUID SLUDGE FOR
PRODUCTION OF CROPS PROCESSED BEFORE ANIMAL
CONSUMPTION (PRACTICE III) 3-24
3.6. APPLICATION OF DRIED OR COMPOSTED SLUDGE TO
RESIDENTIAL VEGETABLE GARDENS (PRACTICE IV). . . . 3-30
3.7. APPLICATION OF DRIED OR COMPOSTED SLUDGE TO
RESIDENTIAL LAWNS (PRACTICE V) 3-33
4. EXPOSURE PATHWAYS TO HUMAN RECEPTORS 4-1
4.1. EMISSIONS FOLLOWING APPLICATION/INCORPORATION . . 4-2
4.2. PARTICULATES FROM SOIL SURFACE 4-3
4.3. SURFACE RUNOFF 4-4
4.4. DIRECT CONTACT 4-4
4.4.1. Direct Removal 4-4
4.4.2. Crop Surface 4-5
4.4.3. Mowed Grass 4-5
4.5. GROUNDWATER AT AN OFFSITE WELL 4-5
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TABLE OF CONTENTS (continued)
4.6. FUGITIVE EMISSIONS (AEROSOLS) FROM IRRIGATION. . . 4-6
4.7. CROPS/PRODUCTS 4-7
4.7.1. Vegetable Crops 4-7
4.7.2. Pasture Crops 4-8
4.7.3. Processed Animal Feed 4-8
4.7.4. Manure 4-8
4.7.5. Meat 4-9
4.7.6. Milk 4-10
5. MODEL DESCRIPTION AND EXAMPLE CALCULATIONS FOR
EXPOSURE PATHWAYS 5-1
5.1. EMISSIONS FOLLOWING APPLICATION/INCORPORATION . . 5-3
5.2. PARTICULATES FROM SOIL SURFACE 5-6
5.2.1. Onsite Exposures 5-7
5.2.2. Offsite Exposures 5-8
5.2.3. Tilling Operations 5-11
5.3. DIRECT CONTACT 5-11
5.4. WATER 5-12
5.4.1. Surface Runoff 5-12
5.4.2. Groundwater at an Offsite Well 5-13
5.4.3. Fugitive Emissions (Aerosols) from Irrigation 5-15
5.5. CROPS/PRODUCTS 5-16
6. SOURCES OF UNCERTAINTY 6-1
7. SENSITIVITY ANALYSIS 7-1
7.1. INTRODUCTION 7-1
7.2. MODEL TESTING 7-1
7.3. SENSITIVITY ANALYSIS 7-2
7.4. METHODOLOGY 7-3
7.5. RESULTS AND COMMENTS 7-6
8. REFERENCES 8-1
APPENDIX A: Variables and Default Values A-l
APPENDIX B: Operations Guide B-l
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LIST OF TABLES
No. Title Page
2-1 Pathogens of Primary Concern in Sewage Sludges 2-4
3-1 Sludge Management Practices 3-3
3-2 Compartments Included in the Sludge Management Practices 3-10
6-1 Levels of Uncertainty Associated with Pathogen Risk Assessment .... 6-2
7-1 Input Variables - Practice I. Salmonella 7-4
7-2 Sensitivity Analysis Practice I: Number of Pathogens in Compartment 6. 7-7
7-3 Sensitivity Analysis Practice I: Number of Pathogens in Compartment 7 7-9
7-4 Sensitivity Analysis Practice I: Number of Pathogens in Compartment 12 7-11
7-5 Sensitivity Analysis Practice I: Number of Pathogens in Compartment 13 7-13
A-l Position Number, Name, Default Values, and Definition of Input
Variables A-2
A-2 Pathogen-Specific Default Values A-5
A-3 Proposed Initial Value Menu for Pathogen Concentration
Parameter, ASCRS A-7
A-4 Variables and Default Values for Subroutine RISK A-8
A-5 Variables and Default Values for Subroutine RAINS A-11
A-6 Variables and Default Values for Subroutine GRDWTR A-12
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LIST OF FIGURES
No. Titk Page
3-1 Input/Output Diagram for Practice I-Application of Liquid Sludge
for Production of Commercial Crops for Human Consumption .... 3-4
3-2 Input/Output Diagram for Practice II~Application of Liquid Sludge
to Grazed Pastures 3-5
3-3 Input/Output Diagram for Practice III—Application of Liquid Sludge
for Production of Crops Processed before Animal Consumption .... 3-6
3-4 Input/Output Diagram for Practice IV-Application of Dried or
Composted Sludge to Residential Vegetable Gardens 3-7
3-5 Input/Output Diagram for Practice V~Application of Dried or
Composted Sludge to Residential Lawns 3-8
5-1 Effect of Infective Dose (MID) on Probability of Infection 5-4
7-1 Practice I: Number of Pathogens in Compartment 6, Parameter P2 . . . 7-15
7-2 Practice I: Number of Pathogens in Compartment 6, Parameter P45 . . 7-16
7-3 Practice I: Number of Pathogens in Compartment 12, Parameter P8 . . 7-17
7-4 Practice I: Number of Pathogens in Compartment 12, Parameter P9 . . 7-18
7-5 Practice I: Number of Pathogens in Compartment 12, Parameter P37. . 7-19
7-6 Practice I: Onsite Person Infection Probability, Parameter P2 7-20
7-7 Practice I: Groundwater Drinker Infection Probability, Parameter P6, . . 7-21
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1. EXECUTIVE SUMMARY
This document describes a methodology and associated computer model for assessing
the risk to humans of pathogens in treated municipal sewage sludge applied to land.
Land application of sludge in this methodology refers to the distribution of sludge on or
just below the soil surface where it is employed as a fertilizer or soil conditioner for
growing human food-chain and non-food-chain crops. The two categories of land
application addressed in this model are (1) agricultural utilization and (2) distribution
and marketing (D&M), and the source of microbial pathogens is (1) liquid or (2) dried or
composted municipal sewage sludge.
The approach used for the model provides a structure capable of supporting both
stochastic and deterministic mathematical relationships, i.e., it is a dynamic model that
can incorporate site-specific data while allowing process functions to be dependent on
environmental factors, such as temperature and rainfall. The model structure provides a
flexibility that permits addition and/or deletion of sludge management practice
compartments as well as modifications in process and transfer functions. The model is
designed to run on a personal computer with a minimum of 540 KB of free memory.
Currently limited by a lack of data, the model will be able to utilize data gathered in
the future to enhance its predictive accuracy.
The purpose of the model is to determine the probability of infection of the human
receptor from pathogens in the land-applied sludge. The ultimate objective is to use
the model to assist EPA in its regulatory activities, but the immediate uses include (1)
further development of the pathogen model as a research and risk assessment tool and
(2) the application of the methodology in the performance of actual pathogen risk
assessments.
The five municipal sewage sludge management practices addressed by the model are:
application of liquid treated sludge (1) for production of commercial crops for human
consumption, (2) to grazed pastures, and (3) for production of crops processed before
animal consumption; and application of dried or composted sludge (4) to residential
vegetable gardens and (5) to residential lawns.
The computer model represents the compartments and transfers among
compartments of the five management practices. The compartments are the various
locations, states or activities in which sludge or sludge-associated pathogens exist; they
vary to some extent among practices. In each compartment, pathogens either increase,
decrease or remain the same in number with time, as specified by "process functions"
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(growth, die-off or no population changes) and "transfer functions" (movement between
compartments). The population in each compartment, therefore, generally varies with
time and is determined by a combination of initial pathogen input, "transfer functions"
and "process functions." The populations of pathogens in the compartments representing
human exposure locations, together with appropriate intake and infective dose data, are
used to estimate human health risk.
Considering modern disposal practices, almost any pathogenic organism can be
found in municipal sewage. Because of the difficulty of designing a model that could
accurately simulate the survival and environmental movement of more than a few
microbial species, organisms or organism groups were selected to represent the enteric
pathogens most commonly found in sludge. The current version of the model deals with
only three of these selections: Salmonella spp. representing the bacteria; Ascaris
lumbricoides. the parasites (both helminth worms and protozoa); and enteroviruses (a
grouping of several animal viruses), the enteric viruses.
Exposure of an individual to enteric pathogens can lead to (1) no effect, (2) a
subclinical (asymptomatic) infection or (3) a clinical (symptomatic) infection. Although
subclinical infections are not clinically detectable, that individual by either direct or
indirect transmission of the pathogenic organisms may cause disease to develop in
others. In this methodology, infection rather than disease is used to measure risk.
Exposure pathways, i.e., migration routes of pathogens from or within the
application or disposal site to a target organism or receptor, for sludge applied to land
include the following:
• inhalation or ingestion of emissions from application of sludge or tilling
of sludge/soil;
• inhalation or ingestion of windblown or mechanically generated
particulates;
• swimming in a pond fed by surface water runoff;
• direct contact with sludge-contaminated soil or crops (including grass,
vegetables, or forage crops);
• drinking water from an offsite well;
• inhalation and subsequent ingestion of aerosols from irrigation;
• consumption of vegetables grown in sludge-amended soil;
• consumption of meat or milk from cattle grazing on or consuming forage
from sludge-amended fields.
Since the model provides only an approximation of environmental transport mechanisms,
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it does not represent every possible exposure pathway. It does, however, trace the flow
of pathogens through the major routes leading to possible human exposure.
The dose required to cause infection is based on the virulence or infectivity of the
pathogenic organism and on the susceptibility of the exposed population or individual
receptor. The "minimum infective dose" or MID is typically the dose required to infect
50% of the population. The uncertainty in measuring infectious doses greatly weakens
the power of any quantitative risk assessment. The model is designed, therefore, so
that the user can supply a best estimate of infectious dose for the particular pathogen
and practice being modeled.
Risk assessments ordinarily proceed from source to receptor. That is, the source,
or sludge disposal/reuse practice, is first characterized, and contaminant movement away
from the source is then modeled to estimate the degree of exposure to the human
receptor. Health effects are then predicted based on the estimated exposure and dose-
response relationships. This computer model sums the hourly exposures of a human
receptor to pathogens in each exposure compartment and computes the daily (24-hour)
probability of the human receptor receiving an exposure exceeding an infective dose
(e.g., for Salmonella, the default MID=10).
Many factors contribute to the uncertainties associated with the present risk
assessment model. Chief among these is the lack of quantitative data describing the
processes involved. Even when available, data are highly variable with regard to (1) the
initial concentrations of microbial pathogens in wastewater and sludge; (2) processes of
microbial transport and inactivation; (3) dose-response relationships; and (4) exposure
levels and receptor susceptibility.
A sensitivity analysis was performed, but because of the large number of input
parameters and the uncertainty related to the values of parameters, it should be viewed
as preliminary. However, the analysis does indicate that the model is very sensitive to
the inactivation rate of microorganisms in soil, as well as to the parameters used to
calculate the fractions of pathogens transferred from surface soil to subsurface soil,
from subsurface soil to groundwater and from surface soil to surface runoff water.
Accordingly, these parameters should be selected with great care, especially as they are
all likely to be site-specific. Because the data available to support choices of the
values are limited, research efforts should be directed to these areas in order to
increase the accuracy of the model.
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2. INTRODUCTION AND DESCRIPTION OF GENERAL APPROACH
2.1. PURPOSE AND SCOPE
This report is one of a series that presents methodologies for assessing the
potential risks to humans or other organisms that may result from management practices
for the disposal or reuse of municipal sewage sludge. The practices addressed by this
series include land application, distribution and marketing programs (D&M), landfilling,
incineration, ocean disposal and surface disposal. In particular, these reports discuss
methods used to evaluate potential health and environmental risks from toxic chemicals
or pathogenic organisms that may be present in sludge. This document considers only
the land application of sludge, including its distribution and marketing, and assesses
potential pathogen-induced health risks associated with this practice.
For the immediate future, the risk assessment methodology and accompanying model
may serve to (1) contribute to the continued development of the risk assessment process
for pathogens and (2) provide a working version of the model for performance of actual
pathogen risk assessments. The risk assessment procedures presented in this report
constitute one approach to evaluating technology-based sludge management options.
Ultimately, these procedures may be used by the U.S. EPA Environmental Criteria and
Assessment Office to help develop technical criteria for microbial pathogens in sludge,
the initial effort being a prototype criteria document for bacteria. The methodology
and model may also be useful in developing guidance for the selection of sludge
management options by local authorities. These uses are not the focus of this
document, however, and will not be discussed. Neither does this methodology address
potential risks associated with the treatment, handling or storage of sludge,
transportation to the point of reuse or disposal, or accidental release.
This study is based on the "Sewage Sludge Pathogen Transport Model Project" (U.S.
EPA, 1980), undertaken to assess the risk from pathogens associated with the
reuse/disposal of municipal sludges by the options of land application and D&M. Widely
referred to as the "Sandia Model," the model and methodology were initially developed
by the BDM Corporation in cooperation with Sandia National Laboratories and the U.S.
Department of Energy, and they have been modified by the University of Cincinnati in
cooperation with the U.S. EPA. Most recently, development has been continued by
Science Applications International Corporation, Oak Ridge, TN, under contracts and an
interagency agreement involving Analysas Corporation, the U.S. EPA and the U.S.
Department of Energy.
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2.2. DEFINITION AND COMPONENTS OF RISK ASSESSMENT
The 1983 National Academy of Sciences report (NRC, 1983) defines risk assessment
as "the characterization of the potential adverse health effects of human exposures to
environmental hazards." By contrast, risk management is defined as "the process of
evaluating alternative regulatory actions and selecting among them." This selection is
made through consideration of costs, available technology and other nonrisk factors.
The National Academy of Sciences organized the risk assessment process into four
components:
(1) hazard identification - "the process of determining whether exposure to an
agent can cause an increase in the incidence of a health condition...."
(2) dose-response assessment - "the process of characterizing the relation
between the dose of an agent...and the incidence of an adverse health
effect in exposed populations and estimating the incidence of the effect as
a function of human exposure to the agent."
(3) exposure assessment - "the process of measuring or estimating the intensity,
frequency, and duration of human exposures to an agent...or of estimating
hypothetical exposures that might arise...."
(4) risk characterization - "the process of estimating the incidence of a health
effect...by combining the exposure and dose-response assessments." (NRC,
1983)
The U.S. EPA has broadened the definitions of hazard identification and dose-response
assessment to include the nature and severity of the toxic effect in addition to the
incidence (U.S. EPA, 1989a). This outline of the risk assessment process provides the
framework for the description of the pathogen risk assessment methodology and model
in this report.
2.3. RISK ASSESSMENT IN THE METHODOLOGY DEVELOPMENT PROCESS
The process of developing a risk assessment methodology begins by defining the
management practice. Land application refers to the distribution of sludge on or just
below the soil surface where it is employed (1) as a fertilizer or soil conditioner for
growing human food-chain and non-food-chain crops, (2) in land reclamation or (3) to
utilize the land as a sludge treatment system. Five categories of land application are
recognized: agricultural utilization, forest land utilization, drastically disturbed land
utilization, dedicated land disposal, and distribution and marketing (D&M). The last of
these refers to the giveaway or sale of bulk or bagged sludge or sludge products to the
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public, commercial growers or local governments for use as fertilizers or soil
conditioners for food- and non-food-chain vegetation. Usually in this category, the
sludge has undergone some dewatering treatment to reduce the volume of sludge before
distribution. In addition, humus material or nutrient additives may have been blended
with the sludge to increase its fertilizer or soil conditioning value and to give it a more
acceptable texture. The sludge may also have been composted to reduce its biochemical
oxygen demand, odor and pathogen load.
Risk assessment for pathogens in municipal sludges requires the following input
data:
• Survival capability, numbers (level or concentrations) and types of pathogens
present in the sludge, along with a consideration of their virulence or infective
dose (minimal infective dose or MID);
• The sludge reuse/disposal option used and the conditions of sludge application
(quantities, frequencies, application method, etc.); and
• The fate of the pathogens in the environment, including the magnitude, duration
and routes of exposure from the applied sludge to human receptors.
Considering modern disposal practices, almost any pathogenic organism can be found
in municipal sewage. The following section characterizes the pathogens of concern in
sewage sludges.
23.1. Hazard Identification. Hazard identification normally consists of identifying the
critical effect, which is the adverse effect occurring at the lowest dose. In the case of
pathogen risk assessment, the hazard identified is any adverse consequence to human
health resulting from exposure to pathogenic microorganisms. The microbial composition
of sewage wastes is variable and heterogeneous. Therefore, a quantitative hazard
identification for microbial pathogens may be difficult to achieve because it involves an
aggregate of hazard assessments for each of a generally unknown number of species of
pathogenic organisms, often present in poorly characterized concentrations.
The presence of pathogens in municipal sludge is well documented. Several reviews
include surveys of pathogens present in sludge at different stages of treatment, and
most comment on the relative pathogenesis of these populations (WHO, 1981; Kowal,
1982, 1985; U.S. EPA, 1986). Table 2-1 presents the pathogens most commonly found in
sewage sludge and the diseases caused by them.
For purposes of discussion, sewage-borne pathogens are generally divided into four
or five major groups: bacteria, viruses, protozoa, helminths and, sometimes, fungi.
Fungi are generally not significant pathogens in sewage except in relation to composting
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TABLE 2-1
Pathogens of Primary Concern in Sewage Sludges
Type
Organism
Effect
Bacteria Campylobacter jejuni
Escherichia cpli
(pathogenic strains)
Leptospira spp.
Salmonella spp.
Shigella spp.
Vibrio cholerae
Yersinia enterocolitica
Viruses Adenovirus
Astrovirus
Calicivirus
Coronavirus
Enteroviruses
Coxsackievirus A
Coxsackievirus B
Echovirus
Hepatitis A virus
Numbered enteroviruses
Poliovirus
Epidemic non-A non-B
hepatitis
Norwalk viruses
Pararotavirus
Parvovirus
Reovirus
Rotavirus
Snow Mountain Agent
Protozoans Balanridium coli
Cryptosporidium sp.
Entamoeba histolytica
Giardia lamblia
Toxoplasma gondii
gastroenteritis
gastroenteritis
Weil's disease
gastroenteritis, enteric fever
gastroenteritis
cholera
gastroenteritis
respiratory disease, eye
infections, gastroenteritis
gastroenteritis
gastroenteritis
respiratory infections
gastroenteritis, meningitis,
meningitis, herpangina, fever,
respiratory disease
myocarditis, congenital heart
anomalies, pleurodynia, respiratory
disease, fever, rash, meningitis
meningitis, diarrhea, rash, fever,
respiratory disease
infectious hepatitis
conjunctivitis
meningitis, paralysis, fever
hepatitis
gastroenteritis
gastroenteritis
aplastic crisis, erythema,
hydrops fetalis
not clearly established
diarrhea, vomiting
gastroenteritis
balantidiasis
gastroenteritis
amebic dysentery
giardiasis
toxoplasmosis
fetal death,
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TABLE 2-1 (cont.)
Type
Organism
Effect
Helminths
Fungi
Ancvlpstoma duodenale
Ascaris lumbricoides
Hymenolepis nana
Necator americanus
Strongyloides stercoralis
Taenia sp.
Toxocara spp.
Trichuris sp.
Aspergillus fumigatus
Candida albicans
Cryptococcus neoformans
Epidermophyton spp.
and Trichophyton spp.
Trichosporon spp,
Phialophora spp.
hookworm
ascariasis
taeniasis
hookworm
abdominal pain, nausea, diarrhea
taeniasis
visceral larva migrans
whipworm
aspergillosis or respiratory
infections
candidiasis
subacute chronic meningitis
ringworm and athlete's foot
infection of hair follicles
deep tissue infections
* Source: U.S. EPA, 1985a; Gerba, 1983; Thurn, 1988; Hurst, 1989.
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of sluage, as discussed below. Most pathogenic microorganisms found in sewage cause
gastroenteric disease of some form, although secondary effects of the organisms may
also be important.
23.1.1. Pathogenic Bacteria — Escherichia coli and Campylobacter are significant
causative agents of waterborne gastroenteritis throughout the world. Mortality is
generally low except in persons with low natural resistance to infection.
Leptospira is found in the urine of infected animals. It can cause a variety of
usually mild symptoms. Its spread is usually limited to contact with infected urine.
Salmonella is a major causative agent of food poisoning and typhoid fever. It can
contaminate vegetable, meat and dairy products produced on sludge-amended land. The
organism is readily transferred from contaminated to uncontaminated foodstuffs by direct
contact or via the hands of persons preparing the food. Spread of sludge-borne
Salmonella can effectively be curbed by good sanitary practices during food preparation
(WHO, 1981). The relative importance of sludge as a source of Salmonella infection may
be small compared to other sources, such as highly mechanized food preparation methods
in which insufficient care is given to sanitation. A report by the World Health
Organization points out that the worldwide incidence of disease caused by Salmonella
has risen (WHO, 1981), as has the incidence in the United States (U.S. PHS, 1985), while
agricultural use of untreated sewage and sludge has fallen.
Shigella causes bacillary dysentery, a disease characterized by severe discomfort but
usually low mortality rates. The disease is spread very easily by the fecal-oral route
under conditions of poor sanitation. Therefore, a single case of sludge-borne infection
may give rise to many cases of disease in the population.
Vibrio cholerae is the causative agent of cholera, which has a high mortality rate if
not treated, but a low mortality rate if appropriate supportive measures are taken.
Yersinia enterocolitica causes a gastroenteritis with low mortality rates. The
disease appears only sporadically in the United States.
23.1.2. Viruses — Adenoviruses are infectious by inhalation or by ingestion.
Ingested adenovirus causes a mild gastroenteritis, but aerosol infections can cause
serious respiratory disease or blindness.
Enteroviruses cause relatively mild gastrointestinal symptoms, but they may also
invade the circulatory system and attack the nervous system or other major organs.
Poliomyelitis is a well-known, but now rare, complication of infection with poliovirus,
while viral meningitis is not uncommon. Hepatitis A virus attacks the liver, but
permanent damage is rare.
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Diseases caused by the other viruses listed in Table 2-1 are less significant, most
only causing mild gastroenteritis. As techniques for detecting and isolating viruses
become more sophisticated, more viruses are found in sewage and sludge. The medical
significance of these recently-found viruses is still being determined.
2.3.1.3. Protozoans -- The protozoan pathogens cause a variety of symptoms by
colonizing the gastrointestinal tract. Protozoan diseases may be debilitating but are
rarely fatal in developed countries. Protozoa are present in sewage and sludge as cysts,
dormant structures resistant to adverse environmental conditions.
2.3.1.4. Helminths -- The pathogenic helminths include a variety of worms, some
of which are only incidental parasites of humans. Among them are pinworms,
roundworms, whipworms, and a variety of tapeworms. The larval stages of helminths
often migrate through the body before maturing in the gut and can cause serious tissue
and organ damage. Adult forms primarily cause malnutrition and anemia while residing
in the gut. Helminths are present in sewage and sludge as ova.
23.1.5. Fungi -- Fungi are predominantly opportunistic pathogens in sewage.
Infection with fungi associated with sludge is generally by aerosol or direct contact
routes. Fungal infections are uncommon in healthy individuals, so that infections are
often associated with low resistance or compromised immunity and are likely to be
persistent.
As a part of EPA's efforts to evaluate the feasibility of pathogen risk assessment
for sludge, more comprehensive data on pathogenic organisms are discussed in Pathogen
Risk Assessment Feasibility Study (U.S. EPA, 1985a) and Development of a Qualitative
Pathogen Risk Assessment Methodology for Municipal Sludge Landfilling (U.S. EPA,
1986).
Although performing risk assessments for all, or at least most, of the pathogens
present in sewage sludge would be ideal, designing a model that could accurately
simulate the survival and environmental movement of more than a few specific organisms
would be difficult. Therefore, organisms or organism groups were selected to represent
the enteric pathogens most commonly found in sludge. The following criteria have been
used to select these representative pathogens:
1. The pathogen is known to be present in municipal sludge.
2. The pathogen is known to cause human disease.
3. More data are available for the representative pathogen than for others in
the same microbial group.
4. Its survivability is typical of other members of the group.
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5. Minimum infective doses (MIDs) are known.
6. The pathogen survives outside the human host.
7. The infective routes—ingestion, inhalation, or skin contact—are known.
The pathogenic organisms initially selected by the EPA as representative of
pathogens present in municipal sludges were:
1. Salmonella spp., as an example of pathogenic enteric bacteria;
2. Enteroviruses, as an example of enteric viruses;
3. Entamoeba histolytica and Giardia lamblia. as examples of parasitic protozoans;
4. Ascaris lumbricoides and A. lumbricoides var. suum. as examples of
helminths; and
5. Aspergillus fumigatus. as a representative of pathogenic fungi.
The current version of the model deals with only three of the representative
pathogens—Salmonella spp. representing the bacteria; Ascaris lumbricoides. the parasites
(both helminth worms and protozoa); and enteroviruses (a grouping of several animal
viruses), the enteric viruses. No representative for the fungi was selected because the
available information concerning this group is inadequate to support a quantitative
assessment. Moreover, the scientific literature provides little documentation of fungal
disease as a consequence of exposure to wastewater or sludge.
The selection and use in risk determination of representative pathogens does not
preclude the performance of risk assessments with other pathogens of particular
importance, such as infectious hepatitis virus.
23.2. Dose-Response Assessment. Consequences of exposure to a pathogen are
variable. The organism may (1) not penetrate host defenses; (2) initiate a transitory
colonization which is self-limiting or eradicated by host defenses such as inflammation
or the immune response; (3) establish a long-term infection without overt symptoms; (4)
cause mild or acute disease; or (5) kill the host. These responses depend, to some
extent, on properties of both the pathogen and the host and, usually, on the number of
infecting microbial cells present. Thus, there is no clearly defined exposure that will
always lead to infection, even with only a single microbial species or strain. An
understanding of the dose-response relationship for each pathogen is important in
estimating the risk associated with the presence of the pathogen in sludge.
The dose-response assessment characterizes the relation between exposure to
pathogens and occurrence of adverse health effects. This relationship is based on the
number of viable organisms ingested and the dose of the organism required to cause
infection in a susceptible host. Exposure levels calculated by the model and assumed or
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experimentally derived values for infectious dose are used to assess the risk of
infection.
Exposure of an individual to enteric pathogens can lead to (1) no effect, (2) a
subclinical (asymptomatic) infection or (3) a clinical (symptomatic) infection. Although
subclinical infections produce no symptoms in the infected host, that individual, by
either direct or indirect transmission of the pathogenic organisms, may cause disease to
develop in others. In this methodology, infection rather than disease will be used to
measure risk.
This methodology assumes that exposure to enteric pathogens will not result in in-
fection unless the organisms are actually swallowed. Most exposures, therefore, should
result from consumption of contaminated foods or liquids. Risks due to inhalation of
enteric pathogens will be considered only because the organisms can be subsequently
swallowed. Disease could result through routes of exposure other than the alimentary
tract, and initial propagation of enteric organisms could occur at a site different from
the intestine of the infected individual. Such mechanisms of infection and propagation
are uncommon for enteric pathogens, however, and will not be considered for purposes
of risk assessment.
The dose required to cause infection is based on the virulence or infectivity of the
pathogenic organism and on the susceptibility of the exposed population or individual
receptor. Virulence, or degree of pathogenicity of an organism, is a somewhat
quantitative reflection of the ability of the organism to establish infection and may
depend on the organism's ability to overcome host defenses. The virulence of a given
organism can vary, depending on its recent history. In addition, the medium in which
the dose is administered can also affect the observed response. Similarly, resistance
mechanisms (including barriers to infection, inflammatory responses, and specific immune
responses) vary among individual hosts. Humans vary in their susceptibility to
pathogens, depending on route of exposure, age of the exposed individual, quality of
normal bodily defense systems, existing microbial populations in the host, and other
poorly defined properties. The differences in susceptibility of the host population are
usually reflected in the consequences of infection. Thus, experimental exposure of an
immunologically experienced population may elicit a secondary immune response but little
infection, whereas a previously unexposed population may exhibit a higher incidence of
infection and disease.
The "minimum infective dose" or MID is typically the dose required to infect 50% of
the population. Estimates of the number of microorganisms required to produce
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infection can be made either by exposing volunteers to known doses of the pathogen or
by inferring from epidemiological data the probable levels of exposure that are
associated with observed frequencies of infection or disease.
When virus concentrations are determined by infecting an indicator cell line, the
information gained is a ratio of infective activities. The most quantitative
determination, the plaque assay, yields a concentration of infective virus units; but the
dose producing the plaque formation response must also be known because, in some
instances, more than one infectious virus particle may be required to initiate plaque
formation. When infection can only be measured by cytopathic effect, it may be
necessary to calculate a Tissue Culture Infective Dose (usually TCID5Q, i.e., dose
required to infect 50% of the cultures). In this case, also, the dose-response
relationship of the indicator cell line to infection must be known for an accurate
calculation of the dose-response effect in humans.
The reported infectious doses may vary widely for a given pathogen. For example,
the reported MID for Salmonella varies from 10^ to lO** organisms (Blaser and Newman,
1982; Kowal, 1982, 1985) and for poliovirus from 1 TCIDso to around 4 x 105 TCID50
(Kowal, 1982). The uncertainty in measuring infectious doses greatly weakens the power
of any quantitative risk assessment. The model is designed, therefore, so that the user
can supply a best estimate of infectious dose for the particular pathogen and practice
being modeled. The following MIDs are used as default values in the model:
Salmonella. MID=10; Ascaris lumbricoides. MID=1; and enteroviruses, MID=1.
2.3J. Exposure Assessment The exposure assessment step begins with the
identification of pathways of potential exposure, that is, migration routes of pathogens
within or from the application or disposal site to a target organism or receptor.
Pathogens leached into surface water can be transported as runoff into rivers, lakes or
oceans. They may also become associated with particulate materials in the runoff water
and be deposited as sediments along the route of transport. Contaminated soil may
become airborne as a result of wind erosion or mechanical disturbance and, thus, may be
inhaled or deposited on crop surfaces. Contaminated irrigation water or liquid sludge
may form aerosols, which can be carried offsite by the wind.
An important factor in the significance of these exposure pathways is the survival
time of pathogens, most of which are poorly adapted to survive in soil. Microorganisms
are inactivated in soil at rates that vary with the type of organism, the degree of
predation by other microorganisms, the amount of sunlight, and the physical and
chemical composition of the soil, including moisture content, pH and temperature (Gerba
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et al., 1975; Kowal, 1985). Bacteria and viruses survive well at neutral or slightly
alkaline pH, whereas acid soils reduce survival times. Moist, cold soils contribute to
increased survival time of bacteria, viruses, protozoan cysts and helminth ova; and,
therefore, soils with a higher percentage of organic matter that have a greater water-
holding capacity may be more conducive to pathogen survival. Virus survival time in
soil decreases with dessication and higher temperatures, and protozoan cysts are also
extremely sensitive to drying. Likewise, helminth eggs and larvae are susceptible to
die-off when exposed to desiccation and sunlight, but in cool, moist soil they may
remain infective for several years (Kowal, 1985). Soil protozoa prey on bacteria;
competition and antagonism from endemic soil microorganisms decrease bacteria survival
time, although soil microorganisms seem to have less of an effect on virus degradation
(Kowal, 1985).
Following surface application, the transport of pathogens through unsaturated soil is
influenced by the porosity, ionic composition and pH of the soil. Bacteria are readily
retained by filtration in soil. Most bacteria do not migrate more than about 50 cm in
soil, although extensive migration can be observed in gravelly or fissured soil (U.S. EPA,
1986; Kowal, 1985). Viruses are not as readily filtered from the percolating water
because of their small size and can be found in groundwater after application of
effluent to sandy soil (Goyal et al., 1984; Wellings et al., 1975). However, they do
adsorb to soil organic matter and clay particles (Wang et al., 1985; U.S. EPA, 1986).
The extent of adsorption of bacteria and viruses depends on the ionic strength of the
soil pore water and the ionic composition of the soil. Adsorbed microorganisms can be
washed free by water of low ionic strength, so that a heavy rain may result in an
increase in the number of virus particles and bacteria being released into groundwater.
Experimental studies have shown, however, that if virus suspensions in soil columns are
allowed to drain dry, live viruses are not readily eluted by addition of water (Lance &
Gerba, 1980). Therefore, it is likely that inactivation of virus particles occurs upon
drying, despite the protective properties of soil. Protozoan cysts and helminth ova are
large enough that they exhibit very little migration through soil.
Bacteria and viruses have been observed to move readily through saturated soils
(Lance and Gerba, 1984), indicating that rapid transport of pathogens in an aquifer is
probable. As a result, care must be taken in choosing sites for land disposal of sludge
and wastewater so as to avoid infiltration of pathogens into any aquifer.
In this pathogen risk assessment methodology, humans are the receptors of concern.
The available risk assessment models for microbial pathogens are variable and limited in
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their treatment of exposure pathways. Such pathways to humans for land application of
sludge and wastewater can include th« following:
• Ingestion of soil or sludge;
• Inhalation of aerosols;
• Leaching of pathogens from land application sites to surface water and
groundwater, and subsequent ingestion;
• Transfer of pathogens to vegetation or food crops and subsequent ingestion;
• Transfer from soil, water or vegetation to animals by contact or ingestion and
subsequently, transfer to humans.
Since the model provides only an approximation of environmental transport mechanisms,
it does not represent every possible exposure pathway. It does, however, trace the flow
of pathogens through the major routes leading to possible human exposure.
The following human receptors are the exposed individuals whose probability of
infection by microbial pathogens is calculated by this model:
• Onsite person who is exposed by direct contact with soil, vegetables, or forage
or by inhalation and subsequent ingestion of aerosols (particulates or liquid);
• Offsite person who is exposed to particulate or liquid aerosols carried by wind;
• Food consumer who eats vegetable crops, meat or milk produced on sludge-
amended soil;
• Groundwater drinker who consumes water from a well near the sludge
application site;
• Pond swimmer who ingests a small amount of water while swimming in the pond
that receives the surface runoff from the application site.
In assessing human exposure, it would be preferable to define the full spectrum of
potential levels of exposure and the number of individuals at each level, thus
quantifying the exposure distribution profile for a given exposure pathway. Such a task
exceeds the scope of the present effort; however, by varying the values of the
parameters that determine exposure, the user may gain an appreciation for the range of
potential risks faced by exposed individuals. A list of default values defining reasonable
worst-case assumptions is provided for use in testing the model. However, the
compounding of worst-case assumptions can lead to improbable results. Therefore, the
key to effective use of this methodology will be a careful and systematic examination of
the effects of varying each of the input parameters, using estimates of central tendency
and upper-limit and lower-limit values to gain an appreciation for the variability of the
result. An approach to this is provided in Chapter 7, Sensitivity Analysis.
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Exposure, for purposes of this methodology, will be determined for a conservatively
defined human receptor. 1 The definition of the human receptor will vary with each
human exposure pathway. Chapter 4 enumerates the exposure pathways and defines the
human receptor in qualitative terms; e.g., for the home garden scenario, the human
receptor is a person producing much of his or her own crops on sludge-amended soil.
The human receptor is not defined quantitatively in Chapter 4, but relevant information
that allows the user to do so (such as available data on the ranges of crop consumption
rates) is provided in Volume II: User's Manual.
2.3.4. Risk Characterization. Risk characterization consists of combining the exposure
and dose-response assessment procedures to estimate the incidence of a health effect.
Risk assessment analysis ordinarily proceeds from source to receptor. That is, the
source, or sludge disposal/reuse practice, is first characterized, and contaminant
movement away from the source is then modeled to estimate the degree of exposure to
the human receptor. Human health effects are then predicted based on the estimated
exposure and dose-response relationships. This computer model sums the exposures of a
human receptor to pathogens every hour and computes the daily probability of the
human receptor receiving an exposure exceeding an infective dose (for Salmonella, the
MID=10). Each exposure compartment adds a current value every hour to the
accumulated exposure, resulting in a total daily (24-hour) exposure that is used in the
calculation of risk to the described human receptor for the exposure compartments in
that practice.
2.4. POTENTIAL USES OF THE METHODOLOGY IN DETERMINING
RESEARCH NEEDS
One of the values of the pathogen risk assessment methodology and computer model
described herein is its ability to identify areas in which additional research is needed.
For example, a major hurdle in any risk assessment is estimating exposure by a variety
of routes or pathways to a population that varies according to activity patterns. The
use of a conservatively defined human receptor is based, at least in part, on the
1 The definition of the human receptor does not include workers exposed in the
production, treatment, handling or transportation of sludge. This methodology is
geared toward protection of the general public. It is assumed that workers can
be required to use special procedures or equipment to minimize their exposure to
sludge-borne contaminants. Agricultural workers, however, might best be
considered members of the general public since the use of sludge may not be
integral to their occupation.
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difficulty in estimating exposure of a population to a changing level, or dose, of
pathogens. Information on infectious dose for most pathogens is limited, and
distribution of pathogens in soil or groundwater is poorly understood. This model
assumes random distribution of pathogens in environmental media, a commonly-made
assumption, but probably frequently violated. Further research on pathogen exposure
pathways and infectious dose levels would facilitate the predictive accuracy of this
model and its successors.
Another obvious data gap, illustrated by this methodology and model development, is
the degree of survival and transport of pathogens in the environment. Information on
the fate of pathogens in groundwater and subsurface soil is extremely limited. The
concentration and survival rates of pathogens leaching through soil into groundwater are
unavailable for viruses, protozoa, and helminths, while bacterial concentration data are
few (U.S. EPA, 1986). More data are needed concerning the transport of pathogens
through the unsaturated zone, especially with respect to rainfall effects.
In conjunction with the results of the sensitivity analysis, this model should be
laboratory- and field-validated, thereby revealing other research needs.
2.5. LIMITATIONS OF THE METHODOLOGY
Limitations of the calculation methods for each pathway are given in Chapter 5,
along with examples of calculation methods. However, certain limitations common to
any model, including the calculation methods, are stated in this section. In several
cases, simplifying assumptions have been made to prevent the model from becoming too
cumbersome for practical application. If the user were required to input all possible
variables, the time required to collect the information and to enter it prior to a model
run would be prohibitive. As a result, the flexibility of the model has been restricted
to some extent.
The predictive value of the model depends on reliable input parameters and on the
accuracy with which initial pathogen concentrations are determined. Municipal sludges
are highly variable mixtures of residuals and by-products of the wastewater treatment
process, and the distribution of microbial types in sludge will depend, to some extent,
on the effects of sewage constituents on competition among and between microbes.
Also, variations in sewage may result in varying efficacy of treatment, so that the
concentration of a particular pathogen cannot be precisely predicted. Variability in
weather or farming practices is likely to result in differing rates of growth or die-off
in soil, air and water. Exponential growth and die-off rates are assumed to apply until
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the end of the practice, even though under certain circumstances linear growth or die-
off rates may be more appropriate; consequently, the modeled rates may not be
completely realistic.
Times of transfer from compartment (see Section 3.2 for a description of transfers
and compartments) to compartment are, in some cases, arbitrary, rather than being
functions of processes in the source compartment, and default times may not be
realistic. Timing of irrigation and rainfall are specified by the user, but with very
limited options. Although an irrigation schedule may be established by the farmer, good
agricultural practice will allow the schedule to vary according to the effects of
temperature, wind and rainfall on immediate soil conditions. This degree of flexibility
has not been built into the model.
Algorithms for generation of particulate clouds due to tilling or wind are
necessarily oversimplified. For the purposes of this model, too many factors are
required to describe accurately the types of farm equipment used, the conditions of soil
moisture, ground cover, variations in wind speed and direction, size and location of
fields, and location of the human receptor. The model is based on approximations or
stipulated default values of these parameters. Similarly, for offsite exposure to airborne
pathogens, variations in wind direction and speed are not considered. In the default
condition, concentrations are calculated only for a line directly downwind from the
source. Concentrations at a specified distance from the center of the plume can be
calculated, but the entire plume is not described in a single run of the model.
The algorithm used for movement of pathogens by surface runoff and sediment
transport has been shown to be adequate for describing these processes for water and
soil, but it has not been validated for microbial transport. It is, at best, only an
approximation of reality, and it is accurate only to the extent that each of many
variables is a good description of actual conditions at the study site. The model
assumes that surface runoff is collected in an onsite pond. No allowance is made for
variations in terrain or soil type in the area being modeled.
Transport of pathogens through the aquifer to an offsite well is modeled by an
adaptation of an algorithm for transport of chemicals. The model is limited primarily by
uncertainties in the appropriate values to be used for dispersion coefficient and
retardation factor, but it is also constrained to a receptor directly downflow from the
source in the center of the contaminant plume. It assumes instantaneous mixing of
pathogens with the groundwater upon their input into the compartment, and it is
intended to describe only transport in a straight line from a point source to the
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receptor well. It also assumes homogeneity of the aquifer medium, with no cracks or
solution channels. These assumptions are almost certainly not entirely correct for any
site. In addition, the model makes no distinction between confined and unconfined
aquifers; instead, it calculates transport strictly on the basis of distance. For example,
a well drilled to a depth of 75 m into a confined aquifer 100 m from the source will be
treated the same as a shallow well in an unconfined aquifer at a distance of 125 m from
the source, even though a confined aquifer may have no contact with overlying
groundwater.
Simplifying assumptions have been made in the descriptions of farming, gardening
and home use practices, and processing of crops for consumption or sale. The default
irrigation option is spray irrigation because the model assumes that generation of
aerosols by spray irrigators will provide a worst-case situation.
The model also assumes that the probability of infection is adequately described by
the Poisson distribution; however, there is no precise exposure value below which
infection will not occur and above which it will always occur. Exposures are treated as
acute exposures accumulated over one day, with no consideration for effects of chronic
exposures. In addition, the methodology compartmentalizes risks according to separate
exposure pathways.
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3. SLUDGE MANAGEMENT PRACTICES
3.1. INTRODUCTION
Sludge is a mixture of solids and liquid resulting from some process that causes or
allows settling of particulate material from suspension. Sludges can be generated by a
number of industrial processes, and many of them are classified as hazardous wastes
(Sittig, 1979). Chemical sludges are generally too toxic to support the growth of micro-
organisms. Sewage and its associated sludge, in contrast, are rich in organic nutrients,
are low in toxic chemicals and have a reasonable pH range. Consequently, they readily
support microbial growth. Indeed, because bacterial decomposition of organic materials
constitutes a major part of sewage treatment, it is essential that sewage support
bacterial growth. This discussion, therefore, is limited to sludge generated by treatment
of municipal or household sewage.
Sludge is a byproduct of sewage treatment. Domestic and municipal wastes include
solids, and during the treatment process, dissolved organic and inorganic materials are
converted to solids. A Federal requirement for secondary treatment of all sewage
(Clean Water Act, P.L. 92-500 and amendments) has ensured that the quality of
wastewater discharged from treatment facilities is high, but, as a result, the amount of
sludge • generated has increased. The per capita production of sewage was estimated
over a decade ago to be around 100 gal/day (400 L/day) for a residential population and
more than 300 gal/day (1200 L/day) in a more highly industrial area (James, 1976).
Total municipal sludge production was estimated in 1982 at more than 6.8 million metric
tons dry weight (DW), with an anticipated two-fold increase by the year 2000. A survey
of 6.5% of the U.S. treatment plants in 1982 revealed that land application and
distribution and marketing (D&M) practices accounted for 2.87 million tons DW, or 42%
of total sludge production (U.S. EPA, 1983b). The use of landfills for sludge disposal is
limited both by the availability of vacant land and by objections of the public to having
sludge disposal sites located near their homes. Therefore, other disposal alternatives
are necessary. U.S. EPA guidelines encourage municipalities to consider land application
of sludge whenever feasible.
Because municipal sewage contains human sanitary waste, microorganisms that
colonize humans will be present in sewage. Among these microorganisms will be some
that cause disease. Therefore, assessing the efficacy of treatment processes in reducing
the concentrations of pathogenic microorganisms and, subsequently, assessing the risk to
human health posed by those pathogens are essential if sludge disposal might lead to
3-1
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subsequent human contact.
There are few quantitative methodologies available at this time for assessing risk to
human populations from sludge disposal practices. Responses of different pathogens to
treatment conditions, interactions of pathogen types with environmental conditions and
responses of human populations to pathogens over different exposure routes are complex.
Information in these areas is incomplete and often inconsistent, leading to a great deal
of uncertainty about quantitation.
Extensive background information relevant to the conceptual risk assessment
framework for land application of sewage sludge has been provided by the U.S. EPA
(1986). This key study addresses the pathogens associated with sewage sludge, as well
as exposure pathways and the potential risks to humans from each of the pathways.
Most of that information will, therefore, not be repeated here. The following discussion
briefly defines the practices included within the land application and D&M management
options.
3.2. MODEL OVERVIEW
A total of five sludge management practices, listed in Table 3-1 and illustrated in
Figures 3-1, 3-2, 3-3, 3-4 and 3-5, are included in the present model. Two of the
practices use heat-dried or composted sludge for residential purposes, and three use
liquid sludge for a group of commercial farming operations. Since each of these two
types of sludge represents a wide range of sludge treatment possibilities, the extent of
treatment or conditioning prior to land application must be approximated for each case
(i.e., the pathogen concentration in the applied sludge must be specified). The computer
model represents the compartments and transfers among compartments of the five
management practices. The compartments are the various locations, states or activities
in which sludge or sludge-associated pathogens exist; they vary to some extent among
practices. Compartments that represent sources of human exposure are designated with
an asterisk in the flow diagrams for each practice. In each compartment, pathogens
either increase, decrease or remain the same in number with
time, as specified by "process functions" (growth, die-off or no population changes) and
"transfer functions" (movement between compartments). Process functions are
designated by RHOx, and the transfer functions are designated as TRxy where x is the
compartment from which pathogens are being transferred and y is the compartment to
which they are being transferred. For example, a transfer from Compartment 1 to
Compartment 2 would be TR12. The population in each compartment, therefore,
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TABLE 3-1
Sludge Management Practices and Descriptions
Practice Description"
I Application of Liquid Sludge for Production of Commercial Crops for Human
Consumption
II Application of Liquid Sludge to Grazed Pastures
III Application of Liquid Sludge for Production of Crops Processed before Animal
Consumption
IV Application of Dried or Composted Sludge to Residential Vegetable Gardens
V Application of Dried or Composted Sludge to Residential Lawns
Two types of sludge are used in this model - liquid and dried/composted. The extent
of treatment or conditioning prior to application is variable and must be determined for
each case.
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Subsurfaca
Soil
Groundwater
Offstte
Well
Application
Incorporation
Soil
Surface
Crop
Surface
Harvesting
Commercial
Crop
Irrigation
Water
Application/Tilling
Emissions
External
Source
FIGURE 3-1
Input/Output Diagram for Practice I - Application of Liquid Sludge
for Production of Commercial Crops for Human Consumption
3-4
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Application/Tilling
Emissions
External
Source
FIGURE 3-2
Input/Output Diagram for Practice II - Application of Liquid
Sludge to Grazed Pastures
3-5
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Application/Tilling
Emissions
External
Source
FIGURE 3-3
Input/Output Diagram for Practice III - Application of Liquid Sludge
for Production of Crops Processed before Animal Consumption
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Subsurface
Soil
Application
Application/Tilling
Emissions
FIGURE 3-4
Input/Output Diagram for Practice IV - Application of Dried
or Composted Sludge to Residential Vegetable Gardens
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Application
Incorporation
Soil
Surfaca
14
Crop
Surface
Application/Tilling
Emissions
Direct
Contact
11
Soil Surfaca
Water
FIGURE 3-5
Input/Output Diagram for Practice V - Application of Dried
or Composted Sludge to Residential Lawns
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generally varies with time and is determined by a combination of initial pathogen input,
"transfer functions" and "process functions." The populations of pathogens in the
compartments representing human exposure locations, together with appropriate intake
and infective dose data, are used to estimate human health risk.
Considering modern sanitary disposal practices, any pathogenic organism can be
found in municipal sewage. It would be difficult to design a model that could
accurately simulate the survival and environmental movement of more than a few
specific organisms. For this reason, organisms or organism groups were selected to
represent the enteric pathogens most commonly found in sludge (see 2.3.1).
The approach used to develop the sewage sludge pathogen transport model provides
a structure capable of supporting both stochastic and deterministic mathematical
relationships, i.e., it is a dynamic model that can incorporate site-specific data while
allowing process functions to be dependent on environmental factors, such as
temperature and rainfall. The model structure provides a flexibility that permits
addition or deletion of sludge management practice compartments as well as
modifications in process and transfer functions. The model is designed to run on a
personal computer with a minimum of 540 KB of free memory.
The modeling effort itself can serve as a tool for identifying areas in which data
are currently nonexistent or incomplete and for which further research is needed.
Informed judgment estimates were incorporated into the model's treatment of these less-
studied areas. As new information becomes available, these estimates can be removed
from the model and replaced with supported data. The model can be progressively
modified, thus constantly enhancing its predictive accuracy.
Although each practice listed in Table 3-1 is different, all five practices share
common characteristics. All compartments that appear in one or more of the five
sludge management practices are listed in Table 3-2. The first 14 compartments, most
of which are common to all practices, are described below.
APPLICATION (1) represents the application of sludge to a field (default size 10
ha) or to a yard or garden of specified size. The starting time for the simulation (T=0)
is when application of sludge begins. Liquid sludge may be applied by spread-flow
techniques, by spray, or by subsurface injection. The application rate and pathogen
concentrations are variables to be entered by the user of the model. Default values are
different for each practice and are given in Appendix A, Table A-l. The position
number of each input variable listed in Table A-l is designated in the text by brackets,
e.g., APRATE p\1)]. APMETH [P(6)], CATTLE [P(78)], CROP [P(66)] and IRMETH
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TABLE 3-2
Compartments Included in the Sludge Management Practices
Compartment
Name and Number
Application
Incorporation
Application/Tilling
Emissions
Soil Surface
Particulates
Surface Runoff
Direct Contact
Subsurface Soil
Groundwater
Irrigation Water
Soil Surface Water
Offsite Well
Aerosols
Crop Surface
Harvesting
(Commercial) Crop
Animal Consumption
Meat
Manure
Milk
Hide
Udder
Liquid Sludge
Management Practices
I II III
1
2
3*
4
5*
6*
7*
8
9
10
11
12*
13*
14
15
16*
1
2
3*
4
5*
6*
7*
8
9
10
11
12*
13*
14
17
18*
19
20*
21
22
1
2
3*
4
5*
6*
7*
8
9
10
11
12*
13*
14
15
17
18*
19
20*
21
22
Dried/Composted Sludge
Management Practices
IV V
1 1
3* 3*
4 4
5* 5*
7* 7*
8 8
11 11
14 14
15
16*
Indicates exposure compartment
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[P(17)] are used as flags only; TCULT [P(67)], TRAIN [P(13)] and TSLOTR [P(85)] are
used as either flags or variables, depending on the values assigned to them.
During spread-flow and spray application, sludge will be spread thinly on the soil,
where it will be subject to drying, heating and solar radiation, thus losing the
protective benefits provided by bulk sludge. The model assumes, therefore, that
inactivation will occur at a rate characteristic of the organism in soil at 5°C above the
ambient temperature (Brady, 1974; USDA, 1975) and that liquid sludge is absorbed by the
upper 5 cm of soil surface during this time. The default time for transfer from
APPLICATION (1) to INCORPORATION (2) is 24 hours, which allows a field treated with
liquid sludge to dry sufficiently to plow or cultivate. If the injection option is chosen,
the liquid sludge goes directly from APPLICATION (1) to SUBSURFACE SOIL (8).
During spray application of liquid sludge or application of dry composted sludge,
droplets or loose particulates may become airborne. Liquid aerosols are modeled by a
Gaussian-plume air dispersion model that calculates the downwind concentration of
airborne particulates. Dry particulate emissions are calculated using models for
generation of dust by tilling or mechanical disturbance of soil. Both are represented as
transfers from APPLICATION (1) to APPLICATION/TILLING EMISSIONS (3*).
INCORPORATION (2) involves the mixing, by plowing or cultivation, of the sludge
and sludge-associated pathogens evenly throughout the upper 15 cm of soil. Process
functions associated with this compartment are the same as for the relevant pathogen
type in soil. Particulate emissions generated by cultivation are represented by a
transfer from INCORPORATION (2) to APPLICATION/TILLING EMISSIONS (3*)
beginning at hour 24, extending for enough time to cultivate the field (at a rate of 5
ha/hour) or till the garden or lawn (at a rate of 0.005 ha/hour). At the end of this
time, all remaining pathogens are transferred to SOIL SURFACE (4).
APPLICATIONyTILLING EMISSIONS (3*) is an exposure compartment that receives
the dust, or suspended particulates, generated by application or by the cultivating or
tilling of dried sludge or sludge-soil mixture. It also receives aerosols generated by
spray application of liquid sludge. All process functions associated with this
compartment are incorporated in the aerosol subroutines (Section 4.6). Onsite emissions
are assumed to settle back to the soil surface at the end of the generation period.
Exposure in this compartment is by inhalation; but, as in all inhalation exposures, model
simplification limits the exposure to the pathogens assumed to be swallowed after the
inhaled dust or aerosol spray is trapped in the upper respiratory tract, swept back to
the mouth by ciliary action and swallowed.
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SOIL SURFACE (4) describes the processes occurring in the upper 15 cm (Practices
I, IV and V) or upper 5 cm (Practices II and III) of the soil layer. Microbes are
inactivated at rates characteristic for moist soil at 5°C above the chosen ambient
temperature. Transfers from SOIL SURFACE occur by wind to WIND-GENERATED
PARTICULATES (5*) at a time chosen by the user (TWIND [P(23)]), by surface runoff
and sediment transport after rainfall events to SURFACE RUNOFF (6*), by a person
walking through the field or contacting soiled implements or clothing or by other casual
contact to DIRECT CONTACT (7*), by leaching after irrigation or rainfall to
SUBSURFACE SOIL (8), by resuspension during irrigation or rainfall to SOIL SURFACE
WATER (11), or at harvest to CROP SURFACE (14).
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind. Process functions are the same as for the organism in air-dried soil
at the ambient temperature. The exposed individual is standing in the field (onsite) or
at a user-specified distance downwind from the field (offsite) during a windstorm. The
wind-generated exposure is calculated from user-specified values for duration (DWIND
[P(24)]) and severity (WINDSP [P(25)] of the windstorm (default values 6 hours at 18
m/sec (40 mph)).
SURFACE RUNOFF (6*) describes an onsite pond containing pathogens transferred
from SOIL SURFACE (4) by surface runoff and sediment transport after rainfall.
Inactivation rates in this compartment are characteristic of microbes in water and are
much lower than rates for soil. Water is removed from the pond by infiltration and
recharge of the groundwater aquifer, but the model assumes that no microbes are
transferred by this process. The human receptor is an individual who incidentally
ingests 0.1 L of contaminated water while swimming in the pond. This compartment is
also an exposure compartment for cattle drinking 20 L of water daily (Practice II).
DIRECT CONTACT (7*) is the exposure compartment for a worker or a child less
than 5 years old who plays in or walks through the field, yard or garden, incidentally
ingesting 0.1 g of soil or vegetation. This human receptor represents the worst-case
example of an individual contacting contaminated soil or soiled clothing or implements.
No process functions are associated with this compartment because it is strictly an
exposure compartment, and no transfers are made from this compartment because the
number of pathogens in it is negligible.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 5 (II and III) or 15 (I, IV and V) cm depth and the water table.
It also serves as the incorporation site for subsurface injection of liquid sludge.
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Process functions in SUBSURFACE SOIL are the same as for moist soil at ambient
temperature. The transfer from SOIL SURFACE (4) occurs after each rain or irrigation
event as a result of leaching from the soil surface. The time of transfer is calculated
by dividing the depth of rainfall or irrigation by the infiltration rate. Transfer to
GROUNDWATER is arbitrarily set at one hour later. At present, the relation between
unsaturated water flow and subsurface transport has not been well-established. Thus,
this model lacks a satisfactory subroutine to describe pathogen transport from the
subsurface soil to groundwater. Instead, user-specified constants are used to describe
the fraction of pathogens transferred from SOIL SURFACE (4) to SUBSURFACE SOIL
(default SUBSOL [P(44)]= 0.0005 for bacteria, 0.001 for enteroviruses and 0 for helminth
ova and protozoan cysts) and from SUBSURFACE SOIL (8) to GROUNDWATER (9)
(default FRGRND [P(53)]= 0.001 for bacteria and enteroviruses and 0 for helminth ova
and protozoan cysts).
GROUNDWATER (9) describes the flow of pathogens in the saturated zone. Process
functions are the same as for other water compartments. Transfers occur to
IRRIGATION WATER (10) if the groundwater is used for irrigation or to OFFSITE WELL
(12*) if used as drinking water. The number of pathogens transferred to IRRIGATION
WATER (10) is based on the concentration of pathogens in the groundwater compartment
and the total depth of irrigation. The transfer to OFFSITE WELL (12*) is described by
a modification of the subsurface solute transfer model of van Genuchten and Alves
(1982). Because microbes in suspension are passively transported by bulk water flow and
interact with soil particles by adsorption and desorption, they behave similarly enough
to dissolved chemicals that existing solute transport models can be used to describe
their fate in the saturated zone (Gerba, 1988).
IRRIGATION WATER (10) describes the transfers for pathogen-contaminated water
used for irrigation. No processes are associated with this compartment because it is
intended as a transition compartment. Irrigation of the field, lawn or garden takes
place a user-specified number of times each week (NIRRIG [P(19)]). This irrigation
water may come from either an onsite well fed by GROUNDWATER (9) or from an
outside source of treated, liquid sludge. The default conditions vary by practice. In
either case, AEROSOLS (13*) are generated unless a non-spray option is chosen. Spray
irrigation is the default since other methods would not be expected to cause a
significant exposure to workers or offsite persons. In addition to aerosol emissions,
irrigation transfers pathogens to CROP SURFACE (14) and to SOIL SURFACE WATER
(11).
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SOIL SURFACE WATER (11) represents any irrigation water or rainfall in contact
with the ground prior to infiltration. This compartment describes the temporary
suspension of pathogens in such a water layer and their subsequent transfer to CROP
SURFACE (14) or to SOIL SURFACE (4). Process functions are the same as for other
water compartments.
OFFSITE WELL (12*) is the exposure site for a human receptor drinking 2 L/day of
contaminated water whose pathogens have been transported through groundwater.
Process functions are the same as for groundwater. The groundwater transport
subroutine supplies the concentration of pathogens in the well at a user-specified
distance from the source. No transfers out of the compartment are specified because it
is an exposure compartment only.
AEROSOLS (13*) describes fugitive emissions from spray irrigation, which occurs at
a default rate of 0.5 cm/hour for 5 hours. The source of irrigation water producing
AEROSOLS can be an onsite well (i.e., GROUNDWATER (9)) or liquid sludge. Process
functions are described in Chapter 5, as is the Gaussian-plume model used to calculate
concentrations of airborne microbes downwind. The human receptor is an onsite worker
or a person offsite who is exposed during the time of irrigation.
CROP SURFACE (14) describes contamination of vegetable or forage crops by
transfer of user-specified amounts to or from SOIL SURFACE (4), from IRRIGATION
WATER (10), or to or from SOIL SURFACE WATER (11). Process functions are not well
characterized, but default values were chosen to be the same as in the soil surface.
The following summaries describe each of the five modeled practices.
3.3. APPLICATION OF LIQUID SLUDGE FOR PRODUCTION OF COMMERCIAL
CROPS FOR HUMAN CONSUMPTION (PRACTICE I)
Liquid sludge may be applied as fertilizer/soil conditioner for the production of
agricultural crops for human consumption or for animal forage or prepared feed. Both
existing (40 CFR 257.3-6) and proposed (U.S. EPA, 1989b) regulations prohibit direct
application of sewage sludge to crop surfaces. Therefore, this model practice is
designed for a single application of liquid sludge, which is incorporated into the soil
before the crop is planted. Regulations also require various waiting periods before the
planting of crops that will be consumed uncooked by humans. These restrictions,
however, are optional in the model and can be tested.
The following description summarizes the practice-specific compartments, processes
and transfers.
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APPLICATION (1) represents the application of liquid sludge to a field (size given
by AREA [P(7)j, default 10 ha). The pathogen concentration (ASCRS [P(l)], Table A-2)
and application rate (APRATE [P(2)], default 10,000 kg/ha, or 10 T/ha) can be entered
by the user of the model. Spray application is not expected to be the method of choice
for applying sludge in this practice, because installing sprayers for a single use would
not be practical. The user, however, may choose this option by specifying APMETH
[P(6)j = -1. Spread-flow application is the default method of application (APMETH = 1).
The default application rate (APRATE [P(2)]) is 10 T/ha, which the model treats as a
single application. Subsurface injection can be chosen by specifying APMETH = 0. In
this case, no sludge pathogens are introduced to the soil surface except by irrigation.
During APPLICATION, inactivation of pathogens will occur at a rate characteristic
of the organism in soil at the expected soil surface temperature. This temperature is
taken to be 5°C above ambient air temperature (TEMP [P(8)]) to allow for surface
warming (Brady, 1974; USDA, 1975). It is assumed that surface-applied liquid sludge is
absorbed by the upper 5 cm of soil surface during this time. The default time for
transfer from APPLICATION (1) to INCORPORATION (2) is 24 hours, which allows a
field treated with liquid sludge to dry sufficiently to plow or cultivate. Alternatively, if
the injection option is chosen, transfer to SUBSURFACE SOIL (8) occurs at 10 hours.
Aerosol emissions are modeled as the transfer of liquid sludge from APPLICATION
(1) to APPLICATION/TILLING EMISSIONS (3*) only if spray application is specified.
INCORPORATION (2) involves the mixing, by plowing or cultivation, of the sludge
and sludge-associated pathogens evenly throughout the upper 15 cm of soil. Particulate
emissions generated by cultivation are represented by a transfer from INCORPORATION
to APPLICATIONATILLING EMISSIONS (3*) beginning at hour 24, and extending for
enough time to cultivate the field (at a rate of 5 ha/hour). At the end of this time, all
remaining pathogens are transferred to SOIL SURFACE (4). During incorporation,
pathogens will be inactivated at a rate characteristic of the organism in moist soil at
the expected soil surface temperature. This temperature is taken to be 5°C above
ambient air temperature (TEMP [P(8)]) to allow for surface warming (Brady, 1974; USDA,
1975). INCORPORATION (2) dilutes the pathogens by mixing the sludge with soil; the
concentration of pathogens becomes (ASCRS pathogens/kg * APRATE kg/ha)/(2* 106
kg/ha).
APPLICATIONmLLING EMISSIONS (3*) is an exposure compartment that receives
the dust, or suspended particulates, generated by tilling the dried sludge or sludge-soil
mixture. Tilling occurs at INCORPORATION (2) and at times specified by TCULT
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[P(67)J. Additional tilling does not occur if TCULT = -2, but can be called biweekly
(TCULT = 0) or at a single time given by TCULT > 0. The concentration of airborne
particulates in this compartment is calculated using an equation for dust emissions
during cultivation (U.S. EPA, 1985b). The source strength and exposure equations are
described in Chapter 5 and in Volume II: User's Manual. This compartment is strictly
an exposure compartment, and no die-off of pathogens is assumed. Exposure
calculations are made by Subroutine RISK.
This compartment also receives pathogens from aerosols generated by spray
application of liquid sludge if APMETH [P(6)] = -1. The source strength of the aerosol
is calculated from the rate of application and concentration of pathogens in the liquid
sludge (Section 4.6).
SOIL SURFACE (4) describes the processes occurring in the upper 15 cm of the soil
layer. No practice-specific differences from the general description above are expected.
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind at a user-supplied time TWIND [P(23)] (default 60 hours). Die-off
rates are those expected for pathogens in air-dried soil at the ambient temperature.
The exposed individual is standing in the field or at a user-specified distance (default
200 m) downwind from the field during a windstorm. The wind-generated exposure is
calculated from user-specified values for duration (DWIND [P(24)]) and intensity
(WINDSP [P(25)]) of the windstorm (default values 6 hours at 18 m/sec (40 mph))
assuming, as a worst case, that there is no plant cover on the soil surface (COVER
[P(30)] = 0). Exposure calculations are made by Subroutine RISK.
SURFACE RUNOFF (6*) describes an onsite pond containing pathogens transferred
from SOIL SURFACE (4) by surface runoff and sediment transport after rainfall. The
human receptor is an individual who incidentally ingests 0.1 L of contaminated water
while swimming in the pond. Exposure calculations are made by Subroutine RISK.
DIRECT CONTACT (7*) is the exposure compartment for a worker or for a child
less than 5 years old who plays in or walks through the field, incidentally ingesting 0.1
g of soil. This human receptor represents the worst-case example of an individual
contacting contaminated soil or soiled clothing or implements. Exposure calculations are
made by Subroutine RISK.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 15 cm depth and the water table. It also serves as the
incorporation site for subsurface injection of liquid sludge (APMETH [P(6)j = 0).
Pathogens are transferred from SOIL SURFACE by leaching after a rain or irrigation
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event. The fraction of pathogens transferred is a user-supplied value (SUBSOL [P(44)],
default pathogen-specific (Table A-2)). Die-off rates are characteristic of pathogens in
soil at the ambient temperature.
GROUNDWATER (9) describes the flow of pathogens in the saturated zone.
Process functions are the same as for other water compartments. The volume of water
represented by GROUNDWATER (9) is calculated from the average thickness in meters
of the aquifer (variable AQUIFR [P(9)], default 10), the average porosity of the aquifer
(POROS [P(10)j, default 0.3) and the surface area. The model assumes that pathogens
are uniformly distributed throughout the aquifer. Transfers occur to IRRIGATION
WATER (10) if the water is needed for irrigation (which is the default condition).
Transfers to OFFSITE WELL (12*) for use as drinking water are described by Subroutine
GRDWTR, the modified solute transport model.
IRRIGATION WATER (10) describes the transfers of pathogen-contaminated water
used for irrigation. No processes are associated with this compartment because it is
intended as a transition compartment. The default source of IRRIGATION WATER (10)
is GROUNDWATER (9) from an onsite well, with no contribution from other sources
(DILIRR [P(18)] = 0). An additional source of contaminated irrigation water can be
modeled by specifying its relative contribution (0 < DILIRR j< 1). The concentration of
pathogens in this source is given by the variable COUNT [P(22)]. Transfer to AEROSOL
(13*) is possible if spray irrigation is used; this is the default option because spread-
flow irrigation would not be expected to cause a significant exposure to workers or
offsite persons. Irrigation transfers pathogens to SOIL SURFACE WATER (11) and to
CROP SURFACE (14) after sufficient time has passed to allow a crop surface to form
(TCROP [P(68)J, default 720 hours). SOIL SURFACE WATER (11) is the compartment
representing water in contact with the ground prior to infiltration. Transfers occur to
SOIL SURFACE (4) and to CROP SURFACE (14). Die-off rates are assumed to be the
same as for pathogens in water.
OFFSITE WELL (12*) is the exposure site for a human receptor drinking 2 L/day of
contaminated water whose pathogens have been transported through groundwater. The
groundwater transport subroutine (Subroutine GRDWTR) supplies the concentration of
pathogens in the well at a user-specified distance from the source (default value 50 m).
Exposure calculations are made by Subroutine RISK.
IRRIGATION AEROSOLS (13*) describes fugitive emissions from spray irrigation,
done at a rate of IRRATE [P(20)] cm/hour (default 0.5) for 5 hours, NIRRIG [P(19)]
times per week (default=2). Process functions are described in Chapter 5, as is the
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Gaussian-plume model used to calculate concentrations of airborne microbes downwind.
The human receptor is an onsite worker or a person offsite who is exposed during the
time of irrigation. Exposure calculations are made by Subroutine RISK.
CROP SURFACE (14) describes contamination of vegetable crops by transfer of
pathogens from SOIL SURFACE (4), IRRIGATION WATER (10) or SOIL SURFACE
WATER (11). No transfers to CROP SURFACE occur before TCROP [P(68)]f the time at
which the plants have emerged (default 720 hours). Vegetables can be grown
aboveground, on-ground or below-ground. These are represented by tomatoes, zucchini
and carrots, respectively. Pathogen concentrations are determined as number/crop unit
for each sludge management practice. Process functions are assumed to be influenced
by drying, thermal inactivation and solar radiation and are thus most characteristic of
pathogens in surface soil (5°C above ambient temperature).
HARVESTING (15) occurs at THARV [P(69)] (default 1800 hours). At this time, all
pathogens remaining on CROP SURFACE (14) are transferred to HARVESTING (15),
which represents a single harvest of all of the crop. The same process functions apply
as in CROP SURFACE (14). The crop is held for 24 hours before being processed. The
number of pathogens is then transferred to COMMERCIAL CROPS (16*).
COMMERCIAL CROPS (16*) is the compartment in which further processing takes
place. The number of pathogens/crop unit following processing is calculated in this
compartment and is the figure used in the vegetable-exposure risk calculations. A
24-hour pathogen exposure is computed by Subroutine VEG.
Before being consumed, vegetables normally are processed in some way. Included in
the program is a series of user-selectable processing steps. The user has the option of
choosing any or all processing steps, listed in Table A-4, and of specifying some
conditions within processing steps. In the default condition, the human receptor
consumes minimally prepared vegetables (washed, but not peeled or cooked) at a rate of
81 g tomatoes, 80 g zucchini or 43 g carrots per eating occasion (Pao et al., 1982).
3.4. APPLICATION OF LIQUID SLUDGE TO GRAZED PASTURES (PRACTICE II)
In this practice, liquid sludge is applied as fertilizer, soil conditioner and irrigation
water for the production of forage crops for pasture. This model practice is designed
for repeated applications of liquid sludge on a field with a standing forage crop used
for pasture. Spray irrigation is the method of choice for this practice since it is
effective for delivering large amounts of sludge to a large area. In this way, the
pasture is also used as a final treatment and disposal system for the treated sludge. An
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irrigation rate of 0.5 cm/hour for 5 hours for a total depth of 2.5 cm twice weekly is
used as the default condition for operation of the model, but the rate (IRRATE [P(20)]),
the total weekly depth (DEPTH [P(21)]) and the number of times per week (NIRRIG
[P(19)]) can be changed by the user. A sludge solids concentration of 5% is assumed.
The model assumes that each hectare of pasture supports 12 head of cattle,
although both area and herd size may be varied. This may be a higher density than is
the common practice for fields that receive no irrigation, but with adequate irrigation,
sufficient forage is expected to be produced. Current and proposed regulations require
various waiting periods before animals can be grazed. These requirements can be tested
by the model.
APPLICATION (1) represents the application of liquid sludge to a pasture (size given
by AREA [P(7)J, default 10 ha). The pathogen concentration (ASCRS [P(l)], Table A-2)
and application rate (APRATE [P(2)], default 12.5 T/ha) can be entered by the user of
the model. Spray application is expected to be the method of choice for applying
sludge in this practice (APMETH [P(6)] = -1), in which case the initial application is
assumed to be the same as for one day's irrigation (2.5 cm depth = 250 m^/ha, or 12.5
T/ha at 5% sludge solids). However, the user may choose spread-flow application
(APMETH = 1) or subsurface injection (APMETH = 0).
During APPLICATION (1), inactivation of pathogens will occur at a rate
characteristic of the organism in soil at the expected soil surface temperature. This
temperature is taken to be 5°C above ambient air temperature (TEMP [P(8)]) to allow
for surface warming (Brady, 1974; USDA, 1975). It is assumed that surface-applied
liquid sludge is absorbed by the upper 5 cm of soil surface during this time. The
assumed time for transfer from APPLICATION (1) to SOIL SURFACE (4) is 15 hours.
INCORPORATION (2) is omitted in this practice because incorporation by cultivation
is not reasonable when there is a standing crop.
APPLICATION/TILLING EMISSIONS (3*) is an exposure compartment used to
calculate exposures to aerosol emissions generated by application of liquid sludge if the
spray option (APMETH [P(6)] = -1) is used. The source strength of the aerosol is
calculated from the rate of application and concentration of pathogens in the liquid
sludge (Section 4.6). Tilling of pasture crops is not expected to occur, so no dust
emissions are calculated. Exposure calculations are made by Subroutine RISK.
SOIL SURFACE (4) describes the processes occurring in the upper soil layer. On
the assumption that SOIL SURFACE will not be as deep in this practice as in practices
with an incorporation step, a value of 5 cm, corresponding to 6.V*10-> kg/ha, has been
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assigned to its depth. Therefore, the dilution of pathogen-laden sludge by soil is less
than in Practices I, IV and V. The pathogen load is assumed to be uniformly
distributed throughout this layer. Process functions are those given for pathogens in
soil.
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind at a user-supplied time TWIND [P(23)j (default 60 hours). Die-off
rates are those expected for pathogens in air-dried soil at the ambient temperature.
The exposed individual is standing in the field or at a user-specified distance downwind
from the field during a windstorm. The wind-generated exposure is calculated from
user-specified values for duration (DWIND [P(24)}) and severity (WINDSP [P(25)]) of the
windstorm (default values 6 hours at 18 m/sec (40 mph)). The fraction of soil surface
with plant cover in this practice is COVER [P(30)] = 0.9. Exposure calculations are
made by Subroutine RISK.
SURFACE RUNOFF (6*) describes an onsite pond containing pathogens transferred
from SOIL SURFACE (4) by surface runoff and sediment transport after rainfall. The
human receptor incidentally ingests 0.1 L of contaminated water while swimming in the
pond. This compartment is also an exposure compartment for cattle drinking 20 L of
water daily. Exposure calculations are made by Subroutine RISK.
DIRECT CONTACT (7*) is the exposure compartment for a worker or for a child
less than 5 years old who plays in or walks through the field, incidentally ingesting 0.1
g of soil and 0.1 g of vegetation. This human receptor represents the worst-case
example of an individual contacting contaminated soil or soiled clothing or implements.
Exposure calculations are made by Subroutine RISK.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 5 cm depth and the water table. It also serves as the
incorporation site for subsurface injection of liquid sludge (APMETH [P(6)] = 0).
GROUND WATER (9) describes the flow of pathogens in the saturated zone. Process
functions are the same as for other water compartments. The volume of water
represented by GROUNDWATER is calculated from the average thickness in meters of
the aquifer (variable AQUIFR [P(9)J, default 10), the average porosity of the aquifer
(POROS [P(10), default 0.3) and the surface area. The distribution of pathogens
throughout the aquifer is assumed to be uniform. Transfers occur to IRRIGATION
WATER (10) if the water is needed for irrigation (which is the default condition).
Transfers to OFFSITE WELL (12*) for use as drinking water are described by Subroutine
GRDWTR, the modified solute transport model.
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IRRIGATION WATER (10) describes the transfers of pathogen-contaminated water
used for irrigation. No processes are associated with this compartment because it is
intended as a transition compartment. IRRIGATION WATER comes by default from a
source of liquid sludge (COUNT [P(22)] = 12.5 T/ha * 5*104 pathogens/kg = 6.25*108
pathogens/kg and DILIRR = 1); the user may specify irrigation from an onsite well fed
by GROUND WATER (DILIRR [P(18)] = 0). Transfer to AEROSOL (13*) occurs if spray
irrigation is used; this is the default option because spread-flow irrigation would not be
expected to cause a significant exposure to workers or offsite persons. Irrigation
transfers 10% of the pathogens to CROP SURFACE (14) and 90% to SOIL SURFACE
WATER (11).
SOIL SURFACE WATER (11) is the compartment serving as a source and recipient
of pathogens for SOIL SURFACE (4) and for CROP SURFACE (14). It describes the
suspension of crop- and soil-associated pathogens, as well as those transferred by
irrigation, in the layer of water resulting from irrigation or rainfall. It also describes
their transfer back to either SOIL SURFACE (4) or CROP SURFACE (14). The
residence time for pathogens in this compartment is determined by the depth of water
and by the infiltration rate. The process functions are those associated with ou.er
water compartments.
OFFSITE WELL (12*) is the exposure site for a human receptor drinking 2 L/day of
contaminated water whose pathogens have been transported through groundwater. The
groundwater transport subroutine (Subroutine GRDWTR) supplies the concentration
of pathogens in the well at a user-specified distance from the source (default value 50
m). Exposure calculations are made by Subroutine RISK.
IRRIGATION AEROSOLS (13*) describes fugitive emissions from spray irrigation,
done at a rate of IRRATE [P(20)] cm/hour (default 0.5) for 5 hours, NIRRIG [P(19)]
times per week. The default source of irrigation water producing IRRIGATION
AEROSOLS (13*) is liquid sludge, although it may be GROUNDWATER (9). Process
functions are described in Chapter 5, as is the Gaussian-plume model used to calculate
concentrations of airborne microbes downwind. The human receptor is an onsite worker
or a person offsite who is exposed during the time of irrigation. Exposure calculations
are made by Subroutine RISK.
CROP SURFACE (14) describes contamination of the forage crop by transfer of
pathogens from SOIL SURFACE (4), IRRIGATION WATER (10) or SOIL SURFACE
WATER (11). The model assumes that 10% of the solids in liquid sludge applied as
IRRIGATION WATER are retained by the plant surfaces and 90% reach SOIL SURFACE
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(ASLSUR [P(41)] = 0.9). Process functions are assumed to be -influenced by drying,
thermal inactivation and solar radiation, and are thus characteristic of pathogens in
surface soil (5°C above ambient temperature).
HARVESTING (15) is not applicable in this practice.
COMMERCIAL CROP (16*) is not applicable in this practice.
ANIMAL CONSUMPTION (17) describes the ingestion of CROP SURFACE (14) by
cattle grazing in the pasture. Cattle consume a specified amount of forage (default
FORAG [P(81)] = 7 kg dry wt) daily, as well as an amount of pathogen-contaminated
soil (default SCNSMP [P(83)] = 1.1 kg). The fraction of CROP SURFACE (14) pathogens
consumed per cow is given by the ratio of FORAG [P(81)j to the forage yield per
hectare, default value 1.6 kg/m2 (Whittaker, 1975).
The model assumes that because of the high acidity of the cow's rumen and the
effects of competition by rumen bacteria, infection of the cow by bacterial pathogens
will occur only if the number of bacteria ingested by each cow is greater than
l*lofyday. Human enteroviruses and parasites from domestic sewage are assumed not to
be infective for cattle. Transfers from ANIMAL CONSUMPTION (17) are to MEAT
(18*), MANURE (19) and MILK (20*).
MEAT (18*) is the compartment describing transfer of pathogens from ANIMAL
CONSUMPTION (17) to meat. The human receptor is assumed to consume 0.256 kg of
meat daily (U.S. FDA, 1978). The model allows for inactivation of pathogens in meat by
cooking, assuming reasonable cooking times and temperatures.
Pathogens can be transferred to MEAT (18*) by systemic infection with bacterial
pathogens. The default value for this transfer (DTCTMT [P(59)]) is zero since no data
could be found quantifying the contamination of meat from a systemic Salmonella
infection, and neither Ascaris nor poliovirus should transfer from the gut of the cattle.
The user can assign a value to DTCTMT if contamination of MEAT by a systemic
infection is assumed. The transfer would occur daily but only for the Salmonella
pathogen type.
If the beef cattle option is chosen (variable CATTLE [P(78)j = -1), the cattle
grazing the pasture will be slaughtered at day TSLOTR [P(85)]. At slaughter, each
animal becomes 270 kg of MEAT. When cattle are butchered, the MEAT is often
contaminated by enteric bacteria present in the gut of the cattle. This transfer is
modeled as being from MANURE (19) to HIDE (21) and then MEAT (18*). HTM [P(64)],
the fraction of pathogens in HIDE transferred to MEAT at time of slaughter (TSLOTR
[P(85)]), applies to all three pathogen types. There are no transfers out of this
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compartment, but the pathogen population is used in meat exposure risk calculations.
After slaughter, no risks are assumed to occur from non-meat portions of the carcass.
MANURE (19) describes the source of contamination of MEAT (18*) and MILK (20*)
with pathogens excreted by an infected cow. Enteroviruses and parasites do not enter
this compartment because they are assumed not to infect cattle. Pathogenic bacteria
appear in the compartment if the number in ANIMAL CONSUMPTION (17) exceeds
l*10°/day/cow. Cattle are assumed to produce 4 kg/day DW of manure, and a cow
infected with an enteric pathogen will excrete 1*10^ bacteria/g. Pathogens from
MANURE are transferred to SOIL SURFACE (4), HIDE (21) and UDDER (22).
MILK (20*) is the compartment describing production and consumption of milk from
cattle pastured on the sludge-amended field when the dairy cattle option is chosen
(CATTLE [P(78)j = +1). This is the default condition. In this practice, the default size
of the herd is 12 head/ha, and a yield of 15 L of milk/cow/day is assumed. Milking
occurs twice daily and, in commercial practices, the milk is immediately chilled and held
at 1°C until pasteurization. However, as described in Chapter 4, the default condition
is for consumption of raw milk because commercial production of milk poses an
extremely small hazard of exposure to pathogens.
In the model, pathogens can be transferred to MILK (20*) directly from the
compartment ANIMAL CONSUMPTION (17). This transfer will simulate the possible
effects of a systemic Salmonella infection. The user must supply a value for DTCTMK
[P(60)j, the variable which specifies the fraction of the pathogens transferred from the
ANIMAL CONSUMPTION compartment. The default value for this variable is zero
because transfer of pathogens from the blood of even a septicemic cow to milk is
unlikely. Neither Ascaris nor enteroviruses are known to infect cattle.
In the model, pathogens resulting from using contaminated utensils and from
careless handling are combined as a transfer, which occurs at each milking, from the
manure-contaminated UDDER (22) compartment. All three pathogens can enter MILK
(20*) by this route. There are no transfers out of this compartment, but the pathogen
population is used in the milk-exposure risk calculation.
The default condition will model the consumption of raw milk which has been stored
for 24 hours. This condition will give a worst-case probability of infection. Exposure
calculations are made by Subroutine RISK, which assumes that the human receptor
consumes 2 kg milk/day, roughly three times the national average milk consumption
(U.S. FDA, 1978).
HIDE (21) describes the route of transfer of pathogenic enteric bacteria from
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MANURE to MEAT. A fraction of pathogens from MANURE (TMTH [P(62)], default
0.001) is transferred hourly to HIDE and has an hourly die-off rate of 0.04. At time
TSLOTR [P(85)], a fraction of the pathogens in HIDE (default HTM = 0.1) is transferred
to MEAT (18*).
UDDER (22) provides for an indirect transfer of pathogens from manure to MILK.
A fraction of pathogens from MANURE (TMTU [P(63)]5 default 0.001) is transferred
hourly to UDDER, from which the pathogens have an hourly removal rate of 0.04.
Therefore, the number of pathogens in UDDER is zero unless infection of the cow has
occurred. At each milking, a fraction (UTM [P(65)], default 0.05) of the manure on the
udder is assumed to fall into the milk. These pathogens provide the exposure associated
with raw milk in Subroutine RISK.
3.5. APPLICATION OF LIQUID SLUDGE FOR PRODUCTION OF CROPS
PROCESSED BEFORE ANIMAL CONSUMPTION (PRACTICE III)
In this practice, liquid sludge is applied as fertilizer, soil conditioner and irrigation
water for the production of forage crops to be processed and stored for animal feed.
This model practice is designed for repeated applications of liquid sludge on a field with
a standing forage crop. The model assumes that spray irrigation will be used for the
application of liquid sludge in this practice because this method is effective for
delivering large amounts of sludge to a large area. In this way, the field is also used
as a final treatment and disposal system for the treated sludge. An irrigation rate of
0.5 cm/hour for 5 hours for a total depth of 2.5 cm twice weekly is used as the default
condition for operation of the model, but the rate (IRRATE [P(20)]), the total weekly
depth (DEPTH [P(21)]) and the number of irrigations per week (NIRRIG [P(19)]) can be
changed by the user. A sludge solids concentration of 5% is assumed.
APPLICATION (1) represents the application of liquid sludge to a field (size given
by AREA [P(7)], default 10 ha). The pathogen concentration (ASCRS [P(l)], default
pathogen-specific (Table A-2)) and application rate (APRATE [P(2)], default 12.5 T/ha)
are variables that can be entered by the user of the model. Spray application is
expected to be the method of choice for applying sludge in this practice (APMETH
[P(6)] = -1), in which case the initial application is assumed to be the same as for one
day's irrigation (2.5 cm depth = 250 m^/ha, or 12.5 T/ha at 5% sludge solids). The user,
however, may choose spread-flow application (APMETH = 1) or subsurface injection
(APMETH = 0).
During APPLICATION (1), inactivation of pathogens will occur at a rate
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characteristic of the organism in soil at the expected soil surface temperature. This
temperature is taken to be 5°C above ambient air temperature (TEMP [P(8)]) to allow
for surface warming (Brady, 1974; USDA, 1975). It is assumed that surface-applied
liquid sludge is absorbed by the upper 5 cm of soil surface during this time. The
default time for transfer from APPLICATION (1) to SOIL SURFACE (4) is 1 hour.
INCORPORATION (2) is omitted in this practice because incorporation by cultivation
is not reasonable when there is a standing crop.
APPLICATIONATILLING EMISSIONS (3*) is an exposure compartment used to
calculate exposures to suspended particulates generated by application of liquid sludge if
the spray option (APMETH [P(6)] = -1) is used. Aerosol emissions are modeled as the
transfer of liquid sludge from APPLICATION to APPLICATION/TILLING EMISSIONS
(3*) only if spray application is specified. The source strength of the aerosol is
calculated from the rate of application and concentration of pathogens in the liquid
sludge (Section 4.6). Tilling of field crops is not expected to occur, so no tilling
emissions are calculated. Exposure calculations are made by Subroutine RISK.
SOIL SURFACE (4) describes the processes occurring in the upper soil layer. It is
assumed that SOIL SURFACE will not be as deep in this practice as in Practices I, IV
and V because there is no incorporation step, and a value of 5 cm, corresponding to
6.7*10* kg/ha, has been assigned to its depth. Therefore, the dilution of pathogen-laden
sludge by soil is less than in Practices I, IV and V. The pathogen load is assumed to
be uniformly distributed throughout this layer. Process functions are those given for
pathogens in soil.
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind at a user-supplied time TWIND [P(23)] (default 60 hours). Die-off
rates are those expected for pathogens in air-dried soil at the ambient temperature.
The exposed individual is standing in the field or at a user-specified distance downwind
from the field during a windstorm. The wind-generated exposure is calculated from
user-specified values for duration (DWIND [P(24)]) and severity (WINDSP [P(25)]) of the
windstorm (default values 6 hours at 18 m/sec (40 mph)). The fraction of soil surface
with plant cover in this practice is COVER [P(30)] = 0.9. Exposure calculations are
made by Subroutine RISK.
SURFACE RUNOFF (6*) describes an onsite pond containing pathogens transferred
from SOIL SURFACE (4) by surface runoff and sediment transport after rainfall. The
human receptor incidentally ingests 0.1 L of contaminated water while swimming in the
pond. Exposure calculations are made by Subroutine RISK.
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DIRECT CONTACT (7*) is the exposure compartment for a worker or for a child
younger than 5 years old who plays in or walks through the field, incidentally ingesting
0.1 g of soil and 0.1 g of crop surface. This human receptor represents the worst-case
example of an individual contacting contaminated soil, soiled clothing or implements.
Exposure calculations are made by Subroutine RISK.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 5 cm depth and the water table. It also serves as the
incorporation site for subsurface injection of liquid sludge (APMETH [P(6)]= 0).
GROUND WATER (9) describes the flow of pathogens in the saturated zone. Process
functions are the same as for other water compartments. The volume of water
represented by GROUNDWATER is calculated from the average thickness in meters of
the aquifer (variable AQUIFR [P(9)], default 10), the average porosity of the aquifer
([P(10), default 0.3) and the surface area. Pathogens are assumed to be distributed
uniformly throughout the aquifer. Transfers occur to IRRIGATION WATER (10) if the
water is needed for irrigation (which is the default condition). Transfers to OFFSITE
WELL (12*) for use as drinking water are described by Subroutine GRDWTR, the
modified solute transport model.
IRRIGATION WATER (10) describes the transfers of pathogen-contaminated water
used for irrigation. No processes are associated with this compartment because it is
intended as a transition compartment. IRRIGATION WATER comes by default from a
source of treated liquid sludge (COUNT [P(22)] = 6.25*108 pathogens/kg and DILIRR
[P(18)] = 1); the user may specify irrigation from an onsite well fed by GROUNDWATER
(DILIRR = 0). Transfer to AEROSOL (13*) occurs if spray irrigation is used; this is the
default option because spread-flow irrigation would not be expected to cause a
significant exposure to workers or offsite persons. Irrigation transfers 10% of the
pathogens to CROP SURFACE (14) and 90% to SOIL SURFACE WATER (11) (ASLSUR
[P(41>] = 0.9).
SOIL SURFACE WATER (11) is the compartment serving as a source of pathogens
for SOIL SURFACE (4) and for CROP SURFACE (14). It describes the suspension of
crop- and soil-associated pathogens, as well as those transferred by irrigation, in the
layer of water resulting from irrigation or rainfall. It also describes their transfer back
to either SOIL SURFACE (4) or CROP SURFACE (14). The residence time for
pathogens in this compartment is determined by the depth of water and by the
infiltration rate. The process functions are those associated with other water
compartments.
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OFFSITE WELL (12*) is the exposure site for a human receptor drinking 2 L/day of
contaminated water whose pathogens have been transported through groundwater. The
groundwater transport subroutine (Subroutine GRDWTR) supplies the concentration of
pathogens in the well at a user-specified distance from the source (default value 50 m).
Exposure calculations are made by Subroutine RISK.
IRRIGATION AEROSOLS (13*) describes fugitive emissions from spray irrigation,
done at a rate of IRRATE [P(20)] cm/hour (default 0.5) for 5 hours, NIRRIG [P(19)]
times per week. The default source of irrigation water producing IRRIGATION
AEROSOLS (13*) is treated liquid sludge, although it may be GROUNDWATER. Process
functions are described in Chapter 5, as is the Gaussian-plume model used to calculate
concentrations of airborne microbes downwind. The human receptor is an onsite worker
or a person offsite who is exposed during the time of irrigation. Exposure calculations
are made by Subroutine RISK.
CROP SURFACE (14) describes contamination of the forage crop by transfer of
pathogens from SOIL SURFACE (4), IRRIGATION WATER (10) or SOIL SURFACE
WATER (11). It is assumed that 10% of the solids in liquid sludge applied as
IRRIGATION WATER are retained by the plant surfaces and 90% reach SOIL SURFACE
(ASLSUR [P(41)] = 0.9). Process functions are assumed to be influenced by drying,
thermal inactivation and solar radiation and are thus characteristic of pathogens in
surface soil (5°C above ambient temperature).
HARVESTING, PROCESSING AND STORAGE (15) describes the processes and
transfers associated with cutting, processing and storing the forage crop. At time
THARV ([P(69)J, default 720 hours), all pathogens remaining in CROP SURFACE (14) are
transferred to this compartment. The crop is baled or rolled and removed from the
field for storage for a period of time specified by STORAG [P(80)J. After the storage
period, transfers to ANIMAL CONSUMPTION (17) begin; the number of pathogens
transferred each day is proportional to the amount of crop fed to the cattle, as
described in ANIMAL CONSUMPTION. Process functions are characteristic of pathogens
in dry soil at the ambient temperature.
COMMERCIAL CROP (16*) is not applicable in this practice.
ANIMAL CONSUMPTION (17) describes the ingestion of the stored crop by cattle.
Cattle consume a specified amount of forage (default FORAG [P(81)j = 7 kg dry wt)
daily, as well as an amount of pathogen-contaminated soil (default SCNSMP [P(83)] = 1.1
kg). The variable ALFALF [P(82)J indicates the percentage of total forage that is the
harvested crop. The fraction of HARVESTING, PROCESSING AND STORAGE (15)
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pathogens consumed per cow is given by the ratio of contaminated FORAG to the forage
yield per hectare, default value 1.6 kg/m^ (Whittaker, 1975). The feeding of
contaminated forage to the cattle ends after a specified number of days, FATTEN
[P(84)], or when the total yield has been consumed.
The model assumes that because of the high acidity of the cow's rumen and the
effects of competition by rumen bacteria, infection of the cow by bacterial pathogens
will occur only if the number of bacteria ingested by each cow is greater than
l*10^/day. Human enteroviruses and parasites from domestic sewage are assumed not to
be infective for cattle. Transfers from ANIMAL CONSUMPTION (17) are to MEAT
(18*), MANURE (19) and MILK (20*).
MEAT (18*) is the compartment describing transfer of pathogens from cattle to
meat. The human receptor is assumed to consume 0.256 kg of meat daily (U.S. FDA,
1978). The model allows for inactivation of pathogens in meat by cooking, assuming
reasonable cooking times and temperatures.
Pathogens can be transferred to MEAT (18*) by systemic infection with bacterial
pathogens. The default value for this transfer (DTCTMT [P(59)]) is zero since no data
could be found quantifying the contamination of meat from a systemic Salmonella
infection, and neither Ascaris nor poliovirus should transfer from the gut of the cattle.
The user can assign a value to DTCTMT [P(59)j if contamination of MEAT by a systemic
infection is desired. The transfer would occur daily but only for the Salmonella
pathogen type.
If the beef cattle option is chosen (CATTLE [P(78)J = -1), the cattle fed the stored
forage will be slaughtered at day TSLOTR [P(85)J. At slaughter, each animal becomes
270 kg of MEAT (18*). When cattle are butchered, the MEAT (18*) is often
contaminated by enteric bacteria present in the gut of the cattle. This transfer is
modeled as being from MANURE (19) to HIDE (21) and then MEAT (18*). HTM [P(64)],
the fraction of HIDE (21) pathogens transferred to MEAT at time of slaughter (TSLOTR
[P(85)]), applies to all three pathogen types. There are no transfers out of this
compartment, but the pathogen population is used in meat exposure risk calculations.
After slaughter, no risks are assumed to occur from non-meat portions of the carcass.
MANURE (19) describes the source of contamination of MEAT (18*) and MILK (20*)
with pathogens excreted by an infected cow. Enteroviruses and parasites do not enter
this compartment because they are assumed not to infect cattle. Pathogenic bacteria
appear in the compartment if the number in ANIMAL CONSUMPTION (17) exceeds
l*10°/day/cow. The model assumes that cattle produce 4 kg (dry wt) of manure per day
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and that a cow infected with an enteric pathogen will excrete 1*1CK bacteria/g.
Pathogens from MANURE (19) are transferred to HIDE (21) and UDDER (22).
MILK (20*) is the compartment describing production and consumption of milk from
cattle fed stored forage crops from the sludge-amended field when the dairy cattle
option is chosen (CATTLE [P(78)] = +1). This is the default condition. In this practice,
the size of the herd is 12 head, and a yield of 15 L of milk/day is assumed. Milking
occurs twice daily, and, in commercial practices, the milk is immediately chilled and
held at 1°C until pasteurization. However, as described in Chapter 4, the default
condition is for consumption of raw milk because commercial production of milk poses
an extremely small hazard of exposure to pathogens.
In the model, pathogens can be transferred to MILK (20*) directly from the
compartment ANIMAL CONSUMPTION (17). This transfer will simulate the possible
effects of a systemic Salmonella infection. The user must supply a value for DTCTMK
[P(60)j, the variable that specifies the fraction of the pathogens transferred from the
ANIMAL CONSUMPTION (17) compartment. The default value for this variable is zero
because transfer of pathogens from the blood of even a septicemic cow to milk is
unlikely. Neither Ascaris nor enteroviruses are known to infect cattle.
In the model, pathogens resulting from using contaminated utensils and from
careless handling are combined as a transfer, which occurs at each milking, from the
manure-contaminated UDDER (22) compartment. All three pathogens can enter MILK
(20*) by this route. There are no transfers out of this compartment, but the pathogen
population is used in milk-exposure risk calculation.
The default condition will model the consumption of raw milk which has been stored
for 24 hours. This condition will give a worst-case probability of infection. Exposure
calculations are made by Subroutine MILK, which assumes that the human receptor
consumes 2 kg milk/day, roughly three times the national average milk consumption
(U.S. FDA, 1978).
HIDE (21) describes the route of transfer of pathogenic enteric bacteria from
MANURE (19) to MEAT (18*). A fraction of pathogens from MANURE (19) (TMTH
[P(62)], default 0.001) is transferred hourly to HIDE. Pathogens in HIDE (21) have an
hourly removal rate of 0.04. At time TSLOTR [P(85)], a fraction of the pathogens in
HIDE (21) (default HTM [P(64)] = 0.1) is transferred to MEAT (18*).
UDDER (22) provides for an indirect transfer of pathogens from manure to MILK.
A fraction of pathogens from MANURE (19) (TMTU [P(63)J, default 0.001) is transferred
hourly to UDDER. Therefore, the number of pathogens in UDDER is zero unless
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infection of the cow has occurred. Pathogens in UDDER (22) have an hourly removal
rate of 0.04. At each milking, a fraction (UTM [P(65)], default 0.05) of the manure on
the udder is assumed to fall into the milk. These pathogens provide the exposure
associated with raw milk in MILK (20*).
3.6. APPLICATION OF DRIED OR COMPOSTED SLUDGE TO RESIDENTIAL
VEGETABLE GARDENS (PRACTICE IV)
Dried or composted treated sludge may be made available to the public as a bulk or
bagged product to be sold or given away. It may be used as fertilizer or soil
conditioner in the production of domestic garden crops for human consumption.
Although some studies have shown that composting is highly effective in removing
pathogens from sludge (Wiley and Westerberg, 1969), other studies have shown that
bacterial pathogens may grow in dried or composted sludge to concentrations of 1x10"
organisms/kg dry weight (U.S. EPA, 1988). Exposure of individuals to materials used in
home gardening would be expected to be more frequent than exposure in a commercial
agricultural setting. Therefore, this practice would be expected to pose a greater risk
of infection. This model practice is designed to describe the application of dried or
composted treated sludge that is incorporated into the soil before the crops are planted.
The user may specify the proportions of aboveground, on-ground and below-ground crops
in the garden.
APPLICATION (1) represents the application of dried or composted sludge to a
garden (size given by AREA [P(7)], default 0.015 ha). The pathogen concentration
(ASCRS [P(l)j, Table A-2) and application rate (APRATE [P(2)], default 25 T/ha) are
variables that can be entered by the user of the model. The default method of
application is spreading by hand (APMETH [P(6)J = 1).
Because the processed sludge is assumed to be stable, there is no die-off during
APPLICATION (1). The entire contents of APPLICATION are transferred to
INCORPORATION (2). The default time for transfer from APPLICATION (1) to
INCORPORATION (2) is 24 hours.
INCORPORATION (2) involves the mixing, by plowing or cultivation, of the sludge
and sludge-associated pathogens evenly throughout the upper 15 cm of soil. Particulate
emissions generated by cultivation are represented by a transfer from INCORPORATION
(2) to APPLICATION/TILLING EMISSIONS (3*) beginning at hour 24, and extending for
enough time to cultivate the field (at a rate of 0.005 ha/hour). At the end of this
time, all remaining pathogens are transferred to SOIL SURFACE (4). During
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incorporation, pathogens will be inactivated at a rate characteristic of the organism in
moist soil at the expected soil surface temperature. This temperature is taken to be
5°C above ambient air temperature (TEMP [P(8)]) to allow for surface warming (Brady,
1974; USDA, 1975). INCORPORATION (2) dilutes the pathogens by mixing the sludge
with soil; the concentration of pathogens becomes (ASCRS pathogens/kg * APRATE
kg/ha)/(2*106 kg/ha).
APPLICATIONATILLING EMISSIONS (3*) is an exposure compartment that receives
the dust, or suspended particulates, generated by spreading the composted sludge or by
tilling the dried sludge or sludge-soil mixture. Particulates released during
APPLICATION (1) are assumed to be undiluted dried sludge, at a concentration equal to
that at compost production sites (0.015 g/m^ (Clark et al., 1983)). Tilling occurs at
INCORPORATION (2) and at times specified by TCULT [P(67)]. Additional tilling does
not occur if TCULT = -2, but can occur biweekly (TCULT = 0) or at a single time given
by TCULT > 0. Particulates released during INCORPORATION (2) are assumed to be
undiluted dried sludge whereas, during later cultivations, they comprise a soil-sludge
mixture. The concentration of airborne particulates in this compartment is calculated
using an equation for dust emissions during cultivation (U.S. EPA, 1985b). The source
strength and exposure equations are described in Chapter 5 and in Volume II: User's
Manual. This compartment is strictly an exposure compartment, and no die-off of
pathogens is assumed other than the die-off rate incorporated in the aerosol
subroutines. Exposure calculations are made by Subroutine RISK.
SOIL SURFACE (4) describes the processes occurring in the upper 15 cm of the soil
layer. There are no practice-specific differences from the general description above.
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind at a user-supplied time TWIND [P(23)] (default 60 hours). Die-off
rates are those expected for pathogens in air-dried soil at the ambient temperature.
The exposed individual is standing in the garden or at a user-specified distance (default
200 m) downwind from the garden during a windstorm. The wind-generated exposure is
calculated from user-specified values for duration (DWIND [P(24)]) and severity (WINDSP
[P(25)]) of the windstorm (default values 6 hours at 18 m/sec (40 mph)) assuming, as a
worst case, that the fraction of soil surface with plant cover is COVER [P(30)] = 0.
Exposure calculations are made by Subroutine RISK.
SURFACE RUNOFF (6*) is not applicable in this practice because runoff is assumed
to return to the domestic sewage treatment system without presenting a risk of
infection to the user of the sludge.
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DIRECT CONTACT (7*) is the exposure compartment for the garden worker or for a
child younger than 5 years old who plays in or walks through the garden site,
incidentally ingesting 0.1 g of soil and 0.1 g crop surface. This human receptor
represents the worst-case example of an individual contacting contaminated soil or
soiled clothing or implements. Before all pathogens have been transferred to SOIL
SURFACE (4), exposure is at the pathogen concentration found in undiluted sludge
whereas, after the transfer, the concentration is that calculated for the soil-sludge
mixture. Exposure calculations are made by Subroutine RISK.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 15 cm depth and the water table. Pathogens are transferred
from SOIL SURFACE (4) by leaching after rain or irrigation. The fraction of pathogens
transferred is a user-supplied value (SUBSOL [P(44)j, default pathogen-specific (Table A-
2)). Die-off rates are characteristic of pathogens in soil at the ambient temperature.
GROUND WATER (9) is not applicable in this practice because no provision is made
for subsequent exposure to pathogens carried by groundwater. It is assumed that
GROUNDWATER is not used for either irrigation or drinking water.
IRRIGATION WATER (10) is not applicable in this practice because irrigation water
is assumed to come from an uncontaminated domestic water supply.
SOIL SURFACE WATER (11) is the compartment serving as a source of pathogens
for SOIL SURFACE (4) and for CROP SURFACE (14). It describes the suspension of
crop- and soil-associated pathogens, as well as those transferred by irrigation, in the
layer of water resulting from irrigation or rainfall. It also describes their transfer back
to either SOIL SURFACE or CROP SURFACE. The residence time for pathogens in this
compartment is determined by the depth of water and by the infiltration rate. The
process functions are those associated with other water compartments.
OFFSITE WELL (12*) is not applicable in this practice.
IRRIGATION AEROSOLS (13*) is not applicable in this practice.
CROP SURFACE (14) describes contamination of vegetable crops by transfer of
pathogens from SOIL SURFACE (4) or SOIL SURFACE WATER (11). No transfers to
CROP SURFACE occur before TCROP [P(68)], the time at which the plants have
emerged (default 720 hours). Vegetables can be grown aboveground, on-ground or
below-ground. These are represented by tomatoes, zucchini and carrots, respectively.
Process functions are assumed to be influenced by drying, thermal inactivation and solar
radiation and are thus characteristic of pathogens in surface soil (5°C above ambient
temperature).
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HARVESTING (15) occurs at THARV [P(69)] (default 1680 hours). At this time, all
pathogens remaining in CROP SURFACE (14) are transferred to HARVESTING (15). The
same process functions apply as in CROP SURFACE (14). The crop is held for 24 hours
before being processed. The number of pathogens is then transferred to CROP (16*).
CROP (16*) is the compartment in which further processing takes place. The
number of pathogens/crop unit is calculated in this compartment and is the figure used
by the vegetable-exposure risk calculation.
A 24-hour pathogen exposure is computed by Subroutine VEG. Pathogen
concentrations are determined as number/crop unit.
Before being consumed, vegetables normally are processed in some way. Included in
the program is a series of user-selectable processing steps. The user has the option of
choosing any or all processing steps and of specifying some conditions within processing
steps. In the default condition, the human receptor consumes minimally prepared
vegetables (washed, but not peeled or cooked) at a rate of 81 g tomatoes, 80 g zucchini
or 43 g carrots per eating occasion (Pao et al., 1982).
3.7. APPLICATION OF DRIED OR COMPOSTED SLUDGE TO RESIDENTIAL
LAWNS (PRACTICE V)
Dried or composted treated sludge may be available to the public as a bulk or
bagged product for use as fertilizer or soil conditioner in the preparation of seed beds
for domestic lawns. Although some studies have shown that composting is highly
effective in removing pathogens from sludge (Wiley and Westerberg, 1969), other studies
have shown that bacterial pathogens may grow in dried or composted sludge to
concentrations of 1*10^ organisms/kg dry weight (U.S. EPA, 1988). Individuals engaged
in preparing a seed bed for a lawn are likely to come into contact with the soil and
any additives used to improve the seed bed. If the soil or the additives contain
pathogens, this practice would be expected to pose a risk of infection. This model
practice is designed to describe the application of dried or composted treated sludge
that is incorporated into the soil before the lawn is seeded.
APPLICATION (1) represents the application of dried or composted sludge to a lawn
(size given by AREA [P(7)], default 0.05 ha). The pathogen concentration (ASCRS
[P(l)], Table A-2) and application rate (APRATE [P(2)], default 25 T/ha) are variables
that can be entered by the user of the model. The default method of application is
spreading by hand (APMETH [P(6)] = 1).
Because the processed sludge is assumed to be stable, there is no die-off during
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APPLICATION (1). The entire contents of APPLICATION are transferred to
INCORPORATION (2). The default time for transfer from APPLICATION to
INCORPORATION is 24 hours.
INCORPORATION (2) involves the mixing, by plowing or cultivation, of the sludge
and sludge-associated pathogens evenly throughout the upper 15 cm of soil. Particulate
emissions generated by cultivation are represented by a transfer from INCORPORATION
to APPLICATIONATLLING EMISSIONS (3*) beginning at hour 24 and extending for
enough time to cultivate the field. At the end of this time, all remaining pathogens are
transferred to SOIL SURFACE (4). During incorporation, pathogens will be inactivated
at a rate characteristic of the organism in moist soil at the expected soil surface
temperature. This temperature is taken to be 5°C above ambient air temperature (TEMP
[P(8)]) to allow for surface warming (Brady, 1974; USDA, 1975). INCORPORATION (2)
dilutes the pathogens by mixing the sludge with soil; the concentration of pathogens
becomes (ASCRS pathogens/kg * APRATE kg/ha)/(2*106 kg/ha).
APPLICATIONmLLING EMISSIONS (3*) is an exposure compartment that receives
the dust, or suspended particulates, generated by tilling the dried sludge or sludge-soil
mixture. Particulates released during APPLICATION (1) are assumed to be undiluted
dried sludge at a concentration equal to that at compost production sites (0.015 g/m3
(Clark et al., 1983)). Tilling occurs at INCORPORATION (2) and at times specified by
TCULT [P(67)j. Additional tilling does not occur if TCULT = -2, but can occur
biweekly (TCULT = 0) or at a single time given by TCULT > 0. Particulates released
during INCORPORATION (2) are assumed to be undiluted dried sludge whereas, during
later cultivations, they comprise a soil-sludge mixture. The concentration of airborne
particulates in this compartment is calculated using an equation for dust emissions
during cultivation (U.S. EPA, 1985b). The source strength and exposure equations are
described in Chapter 5 and in Volume II: User's Manual. The compartment is strictly an
exposure compartment, and no die-off of pathogens is assumed other than die-off rates
incorporated in the aerosol subroutines. Exposure calculations are made by Subroutine
RISK.
SOIL SURFACE (4) describes the processes occurring in the upper 15 cm of the soil
layer. There are no practice-specific differences from the general description above.
WIND-GENERATED PARTICULATES (5*) describes the airborne particulates
generated by wind at a user-supplied time TWIND [P(23)] (default 60 hours). Die-off
rates are those expected for pathogens in air-dried soil at the ambient temperature.
The exposed individual is standing on the lawn or at a user-specified distance (default
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200 m) downwind from the lawn during a windstorm. The wind-generated exposure is
calculated from user-specified values for duration (DWIND [P(24)]) and severity (WINDSP
[P(25)]) of the windstorm (default values 6 hours at 18 m/sec (40 mph)) assuming, as a
worst case, that the fraction of soil surface with plant cover is COVER [P(30)] = 0.
Exposure calculations are made by Subroutine RISK.
SURFACE RUNOFF (6*) is not applicable in this practice, because runoff is assumed
to return to the domestic sewage treatment system without presenting a risk of
infection to the user of the sludge.
DIRECT CONTACT (7*) is the exposure compartment for the lawn worker or for a
child younger than 5 years old who plays in or walks through the lawn site, incidentally
ingesting soil or crop surface. This human receptor represents the worst-case example
of an individual contacting contaminated soil, soiled clothing or implements. Before all
pathogens have been transferred to SOIL SURFACE (4), exposure is at the pathogen
concentration found in undiluted sludge whereas, after the transfer, the concentration is
that calculated for the soil-sludge mixture.
After 840 hours, the time assumed necessary for the lawn to require mowing, the
lawn is mowed weekly, and a fraction, 0.1, (DRECTC, Variable 5 of the Subroutine RISK
variables) of the pathogens associated with CROP SURFACE (14) is transferred to
DIRECT CONTACT. It is assumed that the person mowing the lawn is exposed by
inhalation and subsequent ingestion to 0.1 g of CROP SURFACE (14) at each mowing.
Exposure calculations are made by Subroutine RISK.
SUBSURFACE SOIL (8) describes the processes and transfers for pathogens in the
subsurface soil between 15 cm depth and the water table. Pathogens are transferred
from SOIL SURFACE by leaching after rain or irrigation. The fraction of pathogens
transferred is a user-supplied value (SUBSOL [P(44)j, default pathogen-specific (Table A-
2). Die-off rates are characteristic of pathogens in soil at the ambient temperature.
GROUND WATER (9) is not applicable in this practice because no provision is made
for subsequent exposure to pathogens carried by groundwater. The model assumes that
GROUND WATER is not used for either irrigation or drinking water in this practice.
IRRIGATION WATER (10) is not applicable in this practice because irrigation water
is assumed to come from a non-contaminated domestic water supply.
SOIL SURFACE WATER (11) is the compartment serving as a source of pathogens
for SOIL SURFACE (4) and for CROP SURFACE (14). It describes the suspension of
crop- and soil-associated pathogens, as well as those transferred by irrigation, in the
layer of water resulting from irrigation or rainfall. It also describes their transfer back
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to either SOIL SURFACE (4) or CROP SURFACE (14). The residence time for
pathogens in this compartment is determined by the depth of water and by the
infiltration rate. The process functions are those associated with other water
compartments.
OFFSITE WELL (12*) is not applicable in this practice.
IRRIGATION AEROSOLS (13*) is not applicable in this practice.
CROP SURFACE (14) describes the transfer of pathogens from SOIL SURFACE (4)
and SOIL SURFACE WATER (11) to the mowed or cut grass. The model assumes that,
as the grass emerges (default TCROP [P(68)J = 240 hours), it takes with it a fraction
(SSTCS [P(57)], default 1*105) of the SOIL SURFACE (4) pathogens. Thereafter, the
same transfer occurs daily. There is also a daily transfer back to SOIL SURFACE
(CSTSS [P(56)], default 0.1) and transfers to DIRECT CONTACT (7*) (PSTMG [P(55)J,
default 0.5). Process functions are assumed to be influenced by drying, thermal
inactivation and solar radiation and are thus characteristic of pathogens in surface soil
(5°C above ambient temperature).
HARVESTING (15) is not applicable in this practice. Exposure to CROP SURFACE
(14) as a result of mowing is modeled by routines incorporated into CROP SURFACE
(14) and DIRECT CONTACT (7*).
CROP (16*) is not applicable in this practice.
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4. EXPOSURE PATHWAYS TO HUMAN RECEPTORS
Sludge applied to land surfaces can give rise to human infection through the
following pathways:
• Ingestion of soil or sludge;
• Inhalation of aerosols;
• Leaching of pathogens from land application sites to surface water and
groundwater, and subsequent ingestion;
• Transfer of pathogens to vegetation or food crops, and subsequent ingestion;
and
• Transfer from soil, water or vegetation to animals by contact or ingestion and
subsequently, transfer to humans.
Human ingestion of sludge, contaminated soil, contaminated crop surface,
groundwater, surface water or foodstuffs is considered to be the major source of risk
for infection associated with land disposal of sludge. Contact with and ingestion of
contaminated water and sediments through swimming activities are also considered as a
risk to the human receptor. The significant exposure compartments in each sludge
management practice, indicated with an asterisk in Table 3-2, are pathways for which
exposure calculations are performed by this model.
Although it is possible that many individuals may be exposed to some extent to
sludge-borne contaminants, it is common practice to model a hypothetical individual
whose activities lead to a maximal reasonable exposure to the hazard. The risk of
adverse consequences to this individual is determined, and, if the calculated risk is
acceptable, then the risk to other persons should be negligible. This model sums the
hourly exposure of a human receptor to pathogens in each exposure compartment and
computes the daily (24-hour) probability of the individual receiving an exposure
exceeding an infective dose. The model assumes no cumulative effects beyond one day
and no interaction among pathogens or between the pathogen of interest and
components of the matrix in which it occurs.
The degree of exposure is determined by the number of sludge pathogens ingested
during the relevant time period. This number is determined by the concentration of
pathogens in the consumed materials (determined in each exposure compartment;
Appendix A, Table A-3 lists some proposed initial values for the pathogen concentration
parameter, ASCRS) multiplied by the volume or mass of material consumed. Although
the quantity of potentially contaminated food, water or air consumed can be extremely
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variable, some typical values are assumed. Much of the material in this chapter
describing the assumptions and calculations used for estimating human exposures is
taken from the earlier version of the model (U.S. EPA, 1980). Several user-supplied
parameters are available to change the conditions of human exposure to the pathogens.
During the input phase of running the computer model, the user has the opportunity to
change these values from the default values. These infection risk parameters, together
with their default values and definitions, are listed in Appendix A, Table A-4.
4.1. EMISSIONS FOLLOWING APPLICATION/INCORPORATION
Particulates, defined here as airborne dust or droplets, are generated by many of
the activities modeled in the sludge management practices, such as irrigation, tilling of
the soil or wind erosion. This material and the pathogens associated with it might be
inhaled by people living and working near sludge application sites. Because of the
decay of viable pathogens after incorporation into soil, the maximal exposure to
application aerosols would be expected to occur at the time of application when liquid
sludge is sprayed on the field or dried sludge is tilled into the soil. While it is more
likely that viruses or bacteria will be found in aerosols than the heavier ova and cysts
of parasites, each pathogen is treated in the same manner. Ova and cysts may settle
out from aerosols more rapidly than viruses and bacteria, but they can be resuspended
by winds.
Aerosols are generated during the dumping and spreading of sludge. No data are
available on the generation and transport of aerosols emanating from the dumping
process. However, other data indicate that microorganisms may be transported
substantial distances by winds acting on aerosols from waves on large bodies of water
and that pathogens may be concentrated by evaporation during aerosol formation (U.S.
EPA, 1986). In the case of spray application of liquid sludge, a calculation is done for
onsite exposure to aerosols, but the model assumes that workers will not intentionally
be exposed to the direct spray. A separate calculation is done in which the human
receptor is assumed to be an offsite person at some distance from the aerosol source.
The distance can be specified by the user, or the default value of 200 m can be used.
APPLICATIONyTILLING EMISSIONS (3*) are generated by tilling the sludge into
the soil or by cultivating crops. The dust generated during cultivation is assumed to
settle rapidly and thus would not constitute an offsite exposure. In this case, the
human receptor is assumed to be the operator of a tractor or garden tiller in the
application area. The amount of dust generated by cultivation is calculated during
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INCORPORATION (2) and at TCULT [P(67)], beginning at hour 1 (for worst case) for
dried/composted sludge and after 24 hours (to allow the field to dry) following
application of liquid sludge. The calculation computes the transfer from
INCORPORATION (2) or SOIL SURFACE (4) to APPLICATION/TILLING EMISSIONS
(3*). For all practices, the calculation is done once for each hour of tilling. In
Practices I through III, tilling time is AREA/5, where AREA is the area of the field in
hectares, assuming that one tractor operator will till 40 ha in an 8-hour day. For
Practices IV and V, the tilling rate is assumed to be 0.005 ha/hour, and the time of
tilling is AREA/0.005 hour.
In Practices I through III, the dust cloud is assumed to contain particles at a
concentration calculated by equations for generation of dust by mechanical disturbance
of dry soil (U.S. EPA, 1983a). For Practices IV and V, the concentration of particulates
is assumed to be 15 mg/nA Although dust concentrations during application of dried
sludge are not available, the value of 15 mg/m^ is similar to concentrations detected at
compost preparation sites (Clark et al., 1983). This material is assumed to form a
short-lived cloud, exposing the operator of tilling equipment but not escaping from the
site. The volume of the cloud is defined by the area of application and a height (HT
[P(33)]) set to a default value of 1.6 m. The human receptor is assumed to be located
in the center of the particulate cloud, although in reality the cloud would immediately
drift downwind from the point of generation.
4.2. PARTICULATES FROM SOIL SURFACE
WIND-GENERATED PARTICULATES (5) are released from the soil surface when
soil particles are loosened and entrained by the wind, a process known as wind erosion.
For wind erosion to occur, the surface must be dry and free of cover that can absorb
the wind stress necessary to suspend particles. In Practices II and III, 90% ground
cover is taken as the default value, whereas in Practices I, IV and V, a period during
which there is no ground cover is assumed to provide the worst case for wind erosion.
No particulates are generated if the windstorm coincides with a rainfall or irrigation
event. Pathogens on soil particles are assumed to die off at a temperature-dependent
rate characteristic of air-dried soil.
Wind erosion during a windstorm of 18 m/s (approximately 40 mph) and 6 hours
duration is assumed to result in an offsite exposure to an individual at a default
distance of 200 m from a point source representing one hectare. This individual is
assumed to inhale airborne particulates at a rate of 20 m-^/day for the duration of the
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windstorm, and the pathogens are assumed to be swallowed after inhalation.
4.3. SURFACE RUNOFF
SURFACE RUNOFF (6) occurs whenever the amount of water applied to the soil
exceeds the retention capacity of the soil. The retention capacity varies with soil type,
cover characteristics, land use practices and hydrologic condition. The source of water
for runoff is limited to rainfall, because irrigation is expected to be controlled so that
runoff is minimal. As a result of surface runoff, some of the surface soil is washed
from the site. The extent of sediment transport depends on the intensity of rainfall,
the inherent credibility of the soil and the contour and use of the land. This model
includes a subroutine to calculate the amounts of runoff and sediment transport
associated with each rainfall event. A detailed description of this subroutine is given in
Volume II: User's Manual.
Some soil surface microbes are expected to be suspended in runoff water and will
be removed from the site in association with sediment particles. The model assumes
that sediment transported from the field is representative of the soil surface layer, in
which pathogen distribution is homogeneous. An onsite pond whose volume is 100 m^/ha
of watershed receives surface runoff and sediment. This pond is not expected to be
used as a water source for spray irrigation, because irrigation at the rate of 5 cm/day
twice weekly requires 1000 m^/ha of water each week.
The pond is considered to be a potential exposure compartment, when the human
receptor incidentally ingests water while swimming. Subroutine SWIMER computes the
daily pathogen exposure for the swimmer. The model assumes ingestion of 100 mL of
pond water (Pipes, 1978) on any given day (or 0.004167 L/hour for 24 hours). Because
the use of a runoff pond for drinking water is unlikely and because extensive dilution
of the runoff would occur in other receiving bodies, the pond swimmer is assumed to
represent a worst-case exposure.
4.4. DIRECT CONTACT
4.4.1. Direct Removal. DIRECT CONTACT (7*) occurs when surface soil is transported
offsite either intentionally or casually by people. The amount of soil removed has a
negligible effect on the pathogen content of the SOIL SURFACE (4) compartment, but
small amounts of highly contaminated soil may represent a significant exposure to
sensitive human receptors. Surface soil, defined as the upper 15 cm (or 5 cm,
depending on practice) of soil at the application site, is assumed to be in the vadose
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(unsaturated) zone. Exposure due to inadvertent ingestion of surface soil is determined
by the pathogen concentration in this material (number/g) multiplied by the mass in-
gested (a user-supplied parameter, DRECTS (Variable 6 in Subroutine RISK), default
0.1 g/day). Separate direct exposures to SOIL SURFACE (4), SUBSURFACE SOIL (8)
and MANURE (19) (onsite and offsite) are not included. Typical daily consumption of
dust and dirt has been estimated as approximately 0.02 g for adults (U.S. EPA, 1984),
but it seems reasonable to assume that a field worker may ingest larger quantities.
Calculated rates of soil ingestion among children vary, with typical mean values near 0.1
g/day (Binder et al., 1986). These ingestion estimates depend on measurements of soil
metals, and do not include corrections for quantities of these metals in foods, resulting
in overestimates by a factor of 2-6 (Calabrese, 1988). The 95th percentile value
reported by Binder et al. (1986) was 0.4-0.6 g/day before correction for food. For this
model, the default value for all soil ingestion compartments taken together is 0.1 g
soil/day for Practice I and other agricultural practices (a value which is 5 times the
estimated value for adults (U.S. EPA, 1984)). For Practice I and other agricultural
practices, the human receptor is assumed to be a child younger than 5 years old or a
field worker who casually ingests contaminated soil from the field. In contrast, for
D&M practices the human receptor is a child or worker who ingests unincorporated
sludge or compost.
4.4.2. Crop Surface. Plants growing in sludge-amended soil may be contaminated by
sludge-borne pathogens. Incidental contact with and handling of leaves, stems or fruits
may result in ingestion and consequent infection. In this model, the amount of CROP
SURFACE (14) is given by the variable HAY [P(73)] (default 1.6 kg/m2). Each day the
human receptor casually ingests the crop surface pathogens contained in an amount of
CROP SURFACE given by DRECTC (Variable 5 in Subroutine RISK), default 0.1 g.
4.4.3. Mowed Grass. Grass fertilized by composted sludge in D&M practices may be
contaminated with pathogens in the sludge. Direct skin contact with contaminated grass
is not considered to be a significant exposure hazard, but removal of grass from the
site after it is cut represents a potential for incidental contact and subsequent
ingestion. The exposure calculation assumes that the daily ingestion of cut grass
(DRECTC, Variable 5 in Subroutine RISK) is, as a worst case, equivalent to the
incidental ingestion of soil (0.1 g).
4.5. GROUNDWATER AT AN OFFSITE WELL
GROUNDWATER (9) represents the aqueous component of the saturated zone.
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Microbes are assumed to be leached to GROUNDWATER from SOIL SURFACE (4) via
infiltration of rainwater and irrigation water through SUBSURFACE SOIL (8). However,
quantitative models to describe this leaching have not been developed; as an
approximation, a fraction of the pathogens in SOIL SURFACE (4) is designated as
transferred to SUBSURFACE SOIL (8). This fraction, variable SUBSOL [P(44)], is a
judgment estimate, and the user is encouraged to refine it as better information
becomes available.
Groundwater may be transported from the field to household wells, OFFSITE WELL
(12*), for domestic use. Transport of pathogens from SOIL SURFACE (4) to an
OFFSITE WELL (12*) requires movement of the organisms through the vadose
SUBSURFACE SOIL (8), then through saturated soil to the well. The rates of
inactivation and decay of pathogens are assumed to be different in the saturated zone
from those characteristic of the vadose zone. In most cases, microbes survive much
longer in groundwater than in soil. Microbes reaching the groundwater are assumed to
be uniformly mixed in an aquifer whose thickness (AQUIFR [P(9)j and porosity (POROS
[P(10)] can be specified by the user. The transport subroutine is called after a rainfall
or irrigation event that allows for leaching of microbes into the groundwater. It then
computes the average daily pathogen concentration at an offsite well at a user-specified
distance from the field (default 50 m). The human receptor is an individual drinking 2
L of untreated water daily from the offsite well.
4.6. FUGITIVE EMISSIONS (AEROSOLS) FROM IRRIGATION
The source of water for irrigation may be either an onsite well receiving water
from GROUNDWATER (9) or a source of treated liquid sludge (if spray application is
chosen for Practices II and III). The volume and frequency of irrigation required for a
given agricultural practice depend on a number of variables including crop, soil type and
weather conditions. Irrigation water can be applied as a spray or through ditches or
pipes that are on or in the ground. In this model, however, spray irrigation is assumed
to provide a worst-case exposure to pathogens. IRRIGATION AEROSOLS (13*) are
generated during spraying operations and by wind activity on waters receiving sludge or
sludge-contaminated runoff water. Exposure calculations assume a delivery rate of 0.5
cm/hour for spray irrigation, with a default total amount of 10 cm (5 cm twice weekly).
The rate of irrigation is assumed to be controlled so that no surface runoff occurs.
If the source of irrigation water is contaminated with sludge pathogens, spray
irrigation would be expected to cause contamination of aboveground crop surfaces. On
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the other hand, infiltration via ditches would not contaminate aboveground crops and
would not add significantly to contamination of below-ground crops. The variable
DRECTC (Variable 5 in Subroutine RISK), which describes incidental ingestion of
CROP SURFACE (14), is used when CROP SURFACE is contaminated by irrigation.
Spray irrigation must be considered a significant aerosol source for offsite exposure
of an individual who breathes the airborne pathogens downwind from the field during
the lifetime of the irrigation-generated aerosol. This exposure is calculated in
IRRIGATION AEROSOLS (13*) by a Gaussian-plume model which is called during
irrigation. The calculated exposure to airborne pathogens includes all particle sizes.
However, particles less than 2 microns in diameter are likely to be inhaled, whereas
larger particles are more likely to be swept out of the respiratory tract and swallowed
(Adams and Spendlove, 1970; Fuchs, 1964; Sorber and Guter, 1975). Most sludge-borne
pathogens would be expected to have different infectivities upon inhalation as compared
to ingestion, thereby making predictions of probability of infection difficult. Inhalation
is assumed to be equivalent to ingestion as a route of infection.
4.7. CROPS/PRODUCTS
4.7.1. Vegetable Crops. Edible crops represent the major product of Practices I and
IV, which end with the harvest of vegetable crops. The number of pathogens/crop unit
is calculated in the final compartment of each of these sludge management practices and
is the value used by the vegetable-exposure risk calculation. Crops produced in sludge-
amended fields may be contaminated by pathogens in soil on the CROP SURFACE (14).
The amount of soil associated with each crop unit may be specified by the user.
Transport of human pathogens into crop tissue is assumed not to be significant.
A full 24-hour pathogen exposure rather than a 1-hour exposure is computed by
Subroutine VEG because this subroutine is called only once. Vegetables can be grown
above ground, on ground or below ground. These are represented by tomatoes, zucchini
and carrots, respectively. Pathogen concentrations are determined as number/crop unit.
A crop unit is the average weight of each individual vegetable (tomato = 0.23 kg,
zucchini = 0.23 kg, carrot = 0.10 kg). Daily consumption is also averaged as 81 g, 80 g
and 23 g for these vegetables, respectively (Pao et al., 1982).
Practice I specifies that sludge is incorporated before the crop is planted.
Therefore, the minimum time from APPLICATION (1) to HARVEST (15) is the published
time required for maturation of the crop in question. This value is a variable that can
be supplied by the user (TCROP [P(68)]).
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Before being consumed, vegetables are normally processed in some way. Included in
the program is a series of user-selectable processing steps that are outlined below. The
user has the option of choosing any or all processing steps and of specifying some
conditions within processing steps. The parameters that control processing are listed in
Table A-4.
Processing steps are modeled to account for removal or inactivation of pathogens by
washing, blanching, etc., but for a worst-case analysis, the model assumes that no
processing of the crop occurs other than washing. Thus, the human receptor in
Practice I or IV is an individual who eats one standard serving of the minimally
prepared crop (washed, but not peeled or cooked). Vegetables are usually washed before
being commercially processed or before being cooked or eaten raw in the home. Both
types of washing are designed to remove visible dirt and will also remove 90% of the
pathogens clinging to fresh vegetables (Hersom and Hulland, 1964). Selecting the
washing step by setting IWASH=1 leads to a 90% reduction in pathogens associated with
the vegetables being processed.
4.7.2. Pasture Crops. Contaminated pasture crops may serve as a source of infection
of cattle being grazed on the pasture. Infection of cattle is modeled if the daily
amount of ANIMAL CONSUMPTION (17) is > 108 Salmonella. Ascaris and enterovirus are
assumed not to infect cattle. If infected cattle remain in the same field, the pathogen
load in accumulated manure might add significantly to the pathogen population. The
model assumes that there is a user-defined distribution (TMTSS [P(61)]) of manure
between SOIL SURFACE (4) and CROP SURFACE (14). Default values are set at 70%
and 30%, i.e., 70% of pathogens from the manure of infected cattle reach SOIL
SURFACE (4) and 30% remain on CROP SURFACE (14). This compartment does not
represent a direct exposure for a human receptor.
4.73. Processed Animal Feed. This compartment represents an exposure to cattle by
ingestion of processed feed contaminated with soil and sludge. As discussed in the
description of Practices II and III, cattle must ingest at least 108 bacteria to initiate an
enteric infection. Following infection, each cow excretes 4 kg DW of manure daily at
an arbitrary concentration of 10^ organisms/kg for 4 days. This contaminated MANURE
(19) can contaminate HIDE (21) and UDDER (22), but it does not contaminate CROP
SURFACE (14) because, in this case, crops have already been harvested and processed.
This compartment does not represent a direct exposure to a human receptor.
4.7.4. Manure. This compartment is intended to describe the transfer of pathogens
from infected cattle to HIDE (21) and UDDER (22), as well as the additional pathogen
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load to surface soil resulting from cattle-raising activities on sludge-amended pasture
land. Because of the acidity of the cattle rumen, pathogens in contaminated soil
ingested with forage are unlikely to survive to initiate an enteric infection. The model
assumes that an infective dose of 10^ bacteria or enteroviruses per day is required to
establish an infection. After infection, the pathogen load associated with MANURE (19)
is calculated, assuming production of 4 kg/day DW of manure and a pathogen
concentration of 10^ organisms/g for 4 days. Of these, the fraction 0.001 is assumed to
remain on the animal's hide and the fraction 0.001 on the udder.
4.7.5. Meat. Pathogen exposure through meat consumption occurs when the beef
cattle option is selected in Practice II or Practice III. This exposure risk calculation is
performed on the pathogen population transferred to MEAT (18*) at the time of
slaughter. The human receptor is assumed to consume 0.256 kg/day of meat (U.S. FDA,
1978).
When a cow is slaughtered, the carcass is washed with a solution of hot water and
chlorine, covered with a shroud and cooled to 0°C (Fields, 1979). The effects of this
treatment on meat-associated pathogens are not included in either sludge management
practice process functions or in the exposure risk calculations. Treatment after
slaughter is designed to reduce surface contamination, which is generally the only
pathogen contamination associated with beef production. The model does not attempt to
estimate the area of carcass contaminated and the area consumed by the eater but
treats pathogens associated with meat as if they are spread evenly throughout the
product rather than concentrated on the outside and cut surfaces.
Although contaminated meat, poultry and eggs are the major source of human
salmonellosis, most of the contamination is attributable to endemic levels of Salmonella
in the flocks and herds. Contamination of meat by ingested sludge-borne enteroviruses
and bacteria is not expected to be a significant exposure route for humans, assuming
that only healthy animals are slaughtered and that good sanitary practices are carried
out during meat processing. Meat containing encysted reproductive forms of tapeworms
and other helminths is a significant route of infestation of humans. However, with the
exception of the beef tapeworm, Taenia saginata (Kowal, 1985), the transfer of parasites
from domestic sewage to cattle is not likely, because cattle are not susceptible to the
parasites found with significant frequency in domestic sewage.
The model assumes that if an animal is infected with Salmonella or an enterovirus,
a small fraction (0.001) of the pathogens from MANURE (19) are transferred daily to
HIDE (21). In turn, a fraction of HIDE pathogens is transferred to MEAT (18) at the
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time of slaughter. The default value for this fraction is 0.1 times the number of
pathogens in HIDE, distributed over 270 kg of meat per animal.
4.7.6. Milk. Pathogen exposure through milk consumption occurs when the dairy cow
option is selected in Practice II or Practice III. The default condition models the
consumption of raw milk which has been stored for 24 hours. This condition gives a
worst-case probability of infection.
According to the model's assumptions, each cow delivers 15 L of milk per day. The
pathogen load transferred to MILK (20*) is distributed evenly throughout the entire
quantity of milk collected from the dairy herd. Twelve cows constitute the herd in
Practices II and III when default values are used. The quantity of milk being
considered is, therefore, 180 L per day. Milk from each milking is kept separate.
Process functions act upon milk-associated pathogens during an assumed 24 hours
between milking and consumption. (This time was chosen to simulate the home
consumption of raw milk. Although commercially produced milk is stored longer before
consumption, it is produced under conditions which preclude the presence and growth of
pathogens.) The user may specify both the duration and temperature of raw milk
storage. Default values are 24 hours for TMIS2 and 4°C for TEMI2 (Variables 24 and
20, respectively, in Subroutine RISK, Table A-4).
Unless the cow is infected with a pathogen and develops a septicemia, presence of
the pathogen within the cow's tissues does not occur. Transfer of Salmonella and other
bacterial pathogens from the blood of a bacteremic cow to milk is highly unlikely and is
not included as part of this model. Good sanitation practices in the milking barn,
effective pasteurization, and compliance with health regulations forbidding sale of milk
containing viable Salmonella are assumed to minimize any risk to the public from
sludge-borne pathogenic contaminants in milk. Therefore, the default value for transfer
of pathogens to MILK (20) from MANURE (19) or contaminated SURFACE SOIL (4) is
zero. However, by specifying a fraction of pathogens (UTM [P(65)]) on the udder
surface (UDDER (22)) to be transferred to MILK (20*) at the time of milking, the user
can simulate the production of contaminated milk. In this case the human receptor
would be assumed to drink the milk without pasteurization.
Pathogen exposure is computed by Subroutine MILK, which uses the concentration
of pathogens in MILK (20*) and a daily consumption figure. The human receptor is
assumed to drink 2 kg milk/day, roughly three times the national average daily milk
consumption of 0.756 kg (U.S. FDA, 1978).
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5. MODEL DESCRIPTION AND EXAMPLE CALCULATIONS
FOR EXPOSURE PATHWAYS
The model uses an approach in which each compartment represents a discrete point
in a treatment or application pathway where pathogen populations are computed as a
function of time. Within each compartment (n), the number of organisms may increase
or decrease, as described by a rate parameter (RHOn), and transfer functions (TRxy)
describe the movement of organisms into or out of each compartment. Transfer
functions are denoted by indicating first the number of the compartment from which the
transfer is made (i.e., the donor compartment) and then the number of the receptor
compartment. Thus a transfer from Compartment 1 to Compartment 2 would be
designated TR12.
The model is designed to calculate the concentration of the chosen organism in each
compartment at a number of 1-hour time increments. Within each compartment, the
population of organisms increases or decreases in number at an exponential rate.
The model includes a set of default values for each rate parameter and initial con-
centration to be used in the calculations, and the user has the option of substituting
other values, corresponding to, for example, different organisms, different treatment
conditions or different values of soil temperature, moisture or pH. It contains,
therefore, a great deal of built-in flexibility in generating comparative data on pathogen
concentrations in various exposure pathways. The endpoint of each release pathway
calculation is the number of organisms present in each compartment of the pathway at
any given time.
Values were taken from the literature for inactivation rates for Salmonella. Ascaris
and enterovirus under a variety of conditions corresponding as closely as possible to the
compartments in question. Thus, exponential inactivation of bacteria and enteroviruses
in soil or groundwater, or on food exposed to freezing or cooking temperatures, is
assumed, whereas bacterial growth is expected in food at moderate temperatures.
SLOPE and NTRCPT variables ([P(37)] - [P(40)]) were determined for a least-mean-
squares line fitted to a log transform of inactivation rates under different conditions
(double log transform for viruses). For a more detailed discussion, see Volume II:
User's Manual.
Transfer functions characterizing movement from compartment to compartment may
be linear with time or exponentially or proportionally related to the concentration of
organisms in the source and destination compartments. Calculations for each compart-
5-1
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ment of the model option chosen yield the concentration of the selected pathogen in
each compartment. To simplify calculations of discrete, presumably instantaneous events
superimposed on continuous functions for growth, inactivation and transfer of pathogens,
the exponential functions are converted to linear hourly approximations. Thus an
inactivation rate of 1.2 logs per day means that the number of organisms remaining
after one day will be N = NO*10-L2, but after one hour, N = No*10-°-05 = 0.89*NO.
The hourly inactivation factor is summed with all transfer factors for the compartment
and multiplied by the population of the compartment. This calculation is made once for
each hour.
The concentration of pathogens in environmental media (pathogens per kilogram of
material) is calculated by dividing the number of pathogens in the selected compartment,
N(i), by the weight or volume of material in that compartment. The weight or volume
of material in each compartment for which a residue-exposure risk calculation is made is
stored in the program. In some compartments the amount of material is controlled by a
variable. For example, the user can supply the quantity of water in the groundwater
aquifer by specifying the value of AQUIFR [P(9)J and POROS [P(10)]. In other
compartments the weight is constant.
Soil is assumed to have an average density of 1.33 g/cmr. The weight of soil per
hectare in a SOIL SURFACE (4) compartment in Sludge Management Practices I, IV and
V is 2*1()6 ^ (i na * 15 cm (jeep _ ^33 g/cm3). jn practices II and III, the weight of
soil per hectare is 0.667* 10^ kg (1 ha * 5 cm * 1.33 g/cnp). The concentration of
pathogens is calculated by dividing the contents of the relevant compartment by the
weight of soil in the compartment.
Exposure estimates, calculated as exposure amount x concentration of pathogen, are
made for each exposure compartment. For compartments characterized by a given
dose/day, the dose is converted to dose/hour, and the exposure is summed over 24
hours. For example, to simulate ingestion of soil at the daily geometric mean pathogen
survival, the daily exposure (default 0.1 g from soil and 0.1 g from crop surface in
Practices II and III) is divided by 24, and the exposure estimate is done once for each
hour for 24 hours. This is equivalent to using the geometric mean pathogen
concentration in the exposure calculation. For exposures with a defined lifetime (one or
more hours, or the modeled lifetime of an aerosol, or an operation carried out as part
of the use practice), the hourly exposure is summed over the relevant time period.
The risk of infection is determined using MID values from published literature to
calculate the probability that the dose to the human receptor will reach or exceed that
5-2
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value. Since the pathogens are assumed to be distributed randomly in the exposure
medium, these probabilities are described by a Poisson distribution. Thus a daily
ingestion of 0.1 g of soil containing an average of 1 pathogen per gram yields a risk
estimate of 0.095 if the MID is 1, but 2*10'16 if the MID is 10. Figure 5-1 shows the
calculated probability of infection as a function of average dose for several values of
MID.
5.1. EMISSIONS FOLLOWING APPLICATION/INCORPORATION
Application of liquid sludge by spread-flow or injection practices is assumed not to
result in the generation of offsite emissions. Spray application of liquid sludge is
modeled as generating aerosols that may be transported offsite by a wind whose speed
can be chosen by the user (default value is 4 m/s). For the spray option, an
application rate of 0.5 cm/hour is assumed, as in the case of irrigation. Details of the
method of calculation and sample results are given below in the discussion of
particulates.
Paniculate emissions resulting from spreading and tilling liquid sludge in Practice I
are assumed to be similar to emissions for other agricultural tilling (using a disc, land
plane or sweep plow), as described by U.S. EPA (1983a). This calculation is performed
beginning at 24 hours to compute the transfer from INCORPORATION (2) to
APPLICATION/INCORPORATION EMISSIONS (3*). In Practice I, the calculation is done
once for each hour up to AREA/5, where AREA is the area of the field in hectares,
assuming that one tractor operator will till 40 ha in an 8-hour day. In Practices IV
and V, the length of time is AREA/0.005. The quantity of dust generated by tilling is
empirically described as:
E = 6.04 * k * s°-6 (5-1)
where:
E = emission factor (kg/ha)
k = particle size multiplier (dimensionless)
s = fractional silt content of soil.
5-3
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EFFECT OF INFECTIVE DOSE (MID)
ON PROBABILITY OF INFECTION
z
o
o
Ui
o
>
J
OQ
CO
O
DC
Q.
MID
MID = 5
MID =10
MID = 50
EXPOSURE (NO. OF ORGANISMS)
FIGURE 5-1
-------
The particle size multiplier (k) varies with particle size range as follows:
Size range <30 jam <15 /mi <10 ptm <5 /im <2.5 jam
k value 0.33 0.25 0.21 0.15 0.10
If the silt content of the soil is 40% and the size of particulates inhaled and ingested is
in the <30 /Am range, the calculated amount of particulates generated per hectare by
tilling would be 6.04 * 0.33 * 0.4°-6 = 1.15 kg/ha. The value of 40% silt corresponds to
a loam or clay loam soil, and the size class chosen yields the highest estimated k
factor, making this a conservative assumption.
For the model calculation, the operator of the tilling equipment is assumed to
remain continuously in the particulate cloud generated by tilling. Therefore, the
operator's exposure is equivalent to an 8-hour exposure to a particulate cloud whose
concentration is given by the emission factor divided by the cloud volume per hectare.
If, as a worst case, the operator is exposed at a height of 2 m and the cloud does not
extend upward beyond 2 m, then the concentration of particulates in the cloud is 1150 g
/(2m* 104 m^) = 0.058 g/m3. Tne average respiration rate for adult males is 20
m3/day. Allowing for a marginal increase in rate because of exertion, the operator
might inhale 1 m3/hour, or 8 m3/working day. This value corresponds to 0.46 g
particulates/day.
Tilling of the soil to incorporate liquid sludge occurs after the soil has dried
appreciably. The model, therefore, arbitrarily assumes that the microbes will have mixed
with the upper layer of soil, so that they are distributed in the upper 5% (0.75 cm) of
the soil surface layer. Assuming a soil density of 1.33 g/cc, the associated soil layer
would represent 1*108 g, and the concentration of pathogens in the particulate cloud
would be given by (ASCRS * APRATE)/1*108 g/ha. For liquid sludge, the suggested
values of ASCRS [P(l)] = 5*104 pathogens/kg and APRATE [P(2)j = 1*104 kg/ha yield a
concentration of 5 pathogens/g soil and an exposure of ~ 2.3 pathogens/working day. At
an MID of 10, this exposure would yield a probability of infection of 1.4* 10"4.
The worst case exposure in Practices IV and V would occur if the particulate cloud
were composed entirely of suspended particles of dried or composted sludge. The
concentration of dust generated by tilling composted sludge is assumed to be 15 mg/m3
(Clark et al., 1983). At a concentration of 1*103 bacterial pathogens/g, the exposure
would be 8 m3/day * 0.015 g/m3 * 1*103 pathogens/g = 120 pathogens/day. This
exposure would lead to a probability of infection of 1. To achieve a probability of
infection <0.001/day, the bacterial pathogen exposure must be <3/day. Therefore, the
calculation implies that, for tilling dry sludge, the prudent operator should wear a mask
5-5
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capable of filtering out 98% of the respirable particles.
The exposure calculation assumes that the dry sludge has the same particle size
distribution as soil, which is probably not valid. Because of residual moisture, sludge is
less likely to form a particulate cloud when it is tilled. In addition, tilling would result
in a dilution of sludge with soil in the particulate cloud, and this dilution would reduce
the exposure correspondingly. Clearly, more information is needed on the size and
composition of airborne particulates generated by tilling newly-applied sludge.
5.2. PARTICULATES FROM SOIL SURFACE
The PARTICULATES (5) compartment represents a cloud of airborne particles
generated from SOIL SURFACE (4), at a concentration determined by models for wind
erosion. The time of initiation, strength and duration of the wind generating the
particulate cloud can be specified by the user; default values are 18 m/sec (40 mph) for
6 hours, beginning at 60 hours.
The concentration of wind-generated dust in the air will depend on the soil
composition, moisture, and wind speed. Exposure to windblown dust occurs only under
dry conditions. There is a threshold wind speed below which wind erosion does not
occur. That speed varies with soil type, terrain and amount of cover vegetation, and,
because of large variations in wind speed close to the ground, is typically corrected to
a height of 7 m. In this example, the corrected threshold wind speed is assumed to be
7.5 m/sec. The following estimates are based on the document Rapid Assessment of
Exposure to Particulate Emissions from Surface Contamination Sites (U.S. EPA, 1985b),
which summarizes methodologies to estimate concentrations of airborne pollutants.
Assuming unlimited erosion potential (no ground cover), the emission factor for particles
<10 /u,m in diameter (E^o) is given by the following equation (Gillette, 1981):
EIO = 0.036 (1-V) ([u]/ut)3 F(x) (5-2)
where:
emission factor (average annual emission rate per unit area, in
/^
g/m-/hour)
V = fraction of contaminated surface with cover vegetation (0 for bare
soil)
[u] = mean wind speed (m/sec)
Ut = threshold value of wind speed in m/sec, corrected for height of 7 m
5-6
-------
x = 0.886 ut/[u]
F(x) = an empirical function tending to 1.91 as x becomes less than 0.5,
and toward 0 as x becomes greater than 3. In practice, the
calculation will be of little interest if [u] is not greater than ut, so
x will always be less than 0.886, and F(x) will be sufficiently close
to 1.91 that use of that value will cause only a slight overestimate
of
For a wind speed of 40 mph (18 m/sec) and no vegetation, EIQ is calculated by this
equation to be 0.95 g/m^/hour, or 2.64 g/ha/sec.
5.2.1. Onsite Exposures. For onsite exposure, the concentration may be estimated by
the following equation (Gifford and Hanna, 1973),
P = Cqa/[u] (5-3)
or by the similar equation (Pasquill, 1974),
P = qax/[u]h' (5-4)
where:
qa = area emission rate
[u] = mean wind speed over emission area
C = constant dependent on characteristics of the pollutant and source
x = distance from upwind edge of source to receptor
h1 = height of the box containing the cloud.
If C = x/az (Gifford & Hanna 1973),
C = 1/0.012 = 83.3, and
P = 2.64g/104 m2/sec * 83.3/18 m/sec
= 12.2*10-4 g/m3 (1.2 mg/m3),
or from Pasquill (1974),
P = (2.64 g/104 m2/sec) * 400 m/(18 m/sec * 5m)
= 11.7* lO"4 g/m3 (1.2 mg/m3).
These equations show satisfactory agreement at the distance chosen, which
represents the human receptor at the downwind edge of a 10-ha field (250m * 400m).
The former calculation, which implies an equilibrium condition at long distances but
5-7
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which may give an overestimate for short distances, is used in the model.
The highest concentration expected in SOIL SURFACE (4) is 12.5 pathogens/g,
which would occur if composted sludge were incorporated at a rate of 25 T/ha and a
concentration of 1*10^ pathogens/kg. Exposure to a particulate suspension of this soil
at 1 mg/m^ would lead to no significant probability of infection, even at MID=1.
5.2.2. Offsite Exposures. For offsite emissions, a Gaussian-plume model is used.
Particulates are assumed to be evenly distributed throughout the defined volume of an
aerosol cloud, and the pathogens associated with airborne material are assumed to be
uniformly distributed among the particles. In this model the wind erosion equation can
be applied, or the user may specify a weight of sludge-contaminated soil/ha that is
transferred to an airborne state. The model then calculates the number of pathogens
transferred with that weight of material by using the adjusted concentration of
pathogens in surface soil (for Practice I default conditions, 5*104 pathogens/kg sludge *
1*104 kg sludge/ha / 2*109 g soil/ha = 0.25 pathogens/g soil). This portion of the total
pathogen population is transferred from SOIL SURFACE (4) to PARTICULATES (5).
While it is more likely that viruses or bacteria will be found in wind-borne particulates
than the heavier ova and cysts of parasites, each model pathogen is treated in the same
manner.
The particulate-exposure risk calculation assumes that particulate-associated
pathogens are disseminated downwind after generation and that generation is an
instantaneous event. The downwind movement leads to a reduction in the concentration
of airborne pathogens as a result of dispersion and inactivation. The reduction in
pathogen concentration due to dispersion is estimated using a modification of Pasquill's
diffusion equation, which describes release from a ground-level source with no plume
rise (U.S. EPA, 1985b).
\^ O O ^ O
T -(y2/2<7v2) -(z2/2az2)
Cx.y.z = Trc7yaz u e y e (5-5)
where:
Qc,y,z = number of pathogens per cubic meter of air at a downwind location
described by the coordinates x,y,z after release from a source at
ground level
Of = rate of release of pathogens from the source
°y,z ~ standard deviation of the pathogen concentration in a lateral (y) and
vertical (z) direction
5-8
-------
u = mean wind speed (m/sec).
In the risk calculations, as a worst case, the exposed individual is assumed to be
standing directly downwind (default y=Q). The default value for height of exposure is
1.6 m. The user may specify the distance x in meters between the source and the
exposed individual. The dispersion parameters (0y,z) m tne model were determined for
neutral atmospheric conditions.
The reduction in pathogens due to inactivation is affected by a number of factors
including the age of the cloud, temperature, relative humidity, and solar radiation
(Camann et al., 1978; Dimmick and Akers, 1969; Gregory, 1973; Lighthart and Frisch,
1976). The following equation was used to estimate inactivation of pathogens dispersed
as shown in Equation 5-5.
I = ed(x/u) (5-6)
where:
I = inactivation of the pathogen population
e = base of natural logarithm
d = aerosol inactivation rate constant for a particular pathogen.
The inactivation rate constants used in the model are 50th percentile values for the
aerosol inactivation rates of Salmonella and enteroviruses in water suspensions (Camann
et al., 1978) and an estimated value for Ascaris. For particulate clouds association of
microbes with the particles is assumed to be protective, and a decay rate equal to that
in air-dried soil is used. The symbols x. and u, are as described for Equation 5-5.
The combined effects of dispersion and inactivation are in Equation 5-7, which
predicts the final pathogen concentration.
_ -(Y2/2<7y2) -(Z2/2<7Z2) d(x/u)
rz u e e e (5-7)
where:
Qc,y,z — number of pathogens/m^ of air at a downwind location described by
the coordinates x,y,z after release from a source at ground level
= calculated rate of release of pathogens from the source compartment
at a given time ((mass/sec) * (pathogens/mass))
5-9
-------
x = user-specified distance in meters from the source
y = lateral distance of the receptor in meters from a line directly
downwind of the source
z = receptor height in meters
cry = 0.0295x, the standard deviation of particulate concentration
perpendicular to the wind direction for a specified distance x in
meters (Turner, 1969)
<7Z = 0.012x, the standard deviation of particulate concentration vertically
(Turner, 1969)
d = inactivation rate constant. Values for soil particles are the same as
for bulk soil. Values for water suspensions (Camann et al., 1978):
Salmonella -0.028/sec
Ascaris -0.01/sec
enterovirus -0.002/sec
u = user-specified wind speed (m/sec). The default value for WINDSP
[P(25)]= 18 m/sec for windstorm and BREEZE [P(32)] = 4 m/sec for
irrigation.
Using the calculated windstorm-generated source strength of 2.64 g/ha/sec and 0.25
pathogens/g soil, a source of 10 ha would give a concentration of 4.8* 10"^ pathogens/m^
at a distance of 200 m downwind. For aerosol and sludge intake the average inhalation
rate is assumed to be 20 m^/day or 0.83 m^/hour. The pathogen exposures from offsite
particulates are assumed not to exceed 16 hours/day, but for ease of computation, the
inhalation rate is converted to 0.62 irP/hour, and exposures in the compartment are
accumulated hourly into a 24-hour pathogen exposure. A rough estimation of risk of
infection can be made from these results. The distribution of pathogens on particles of
different size classes is not well described. The equation used above models emissions
based on a calculated source strength of particle <10 ^m in diameter. This size class
may represent approximately 20% of the total particulates, whereas pathogens are likely
to be associated with larger particles as well. Therefore, the emissions might be higher
by a factor of perhaps 4, or about 2*10'2 pathogens/m^ at 200 m from the 10-ha
source. A person breathing the dust-laden air for 16 hours/day at a rate of 20 m3/day
would inhale 13.3 m^ of air, or 0.27 pathogens/day. The probability of infection is
approximately 1.85*10^4 at this dose for organisms with MID as low as 4, and
vanishingly small for higher MID values. The likelihood that any individual would
remain exposed for 16 hours to a windstorm of this magnitude is small, so this estimate
5-10
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demonstrates an exaggerated worst case. In addition, any reductions in pathogen
concentration by inactivation before the windstorm would reduce the exposure, and the
presence of vegetation on the source field would reduce the concentration of suspended
soil.
5.2.3. Tilling Operations. Additional tilling, at a time and for a duration specified by
the user, is modeled by the same calculation as for the transfer from INCORPORATION
(2) to APPLICATION/INCORPORATION EMISSIONS (3*), except, in this case, the
calculation is performed for the transfer from SOIL SURFACE (4) to
APPLICATION/INCORPORATION EMISSIONS (3*). The calculation is done once for
each hour of the duration specified by the user. After complete mixing of the sludge
into surface soil, at an application rate of 1*10^ kg liquid sludge/ha and a concentration
of 5*10^ bacterial pathogens/kg, the average bacterial pathogen exposure for 8 hours of
tilling would be approximately 0.46 g/day * 0.25 pathogen/g = 0.115 pathogens/day. This
exposure would result in a calculated probability of infection of 1*10~1" for MID=10 or
0.12 for MID = 1.
The distribution of dust in the paniculate cloud would not, in all probability, be
uniform. The tractor operator would be more likely to receive a lower dose than
indicated by this calculation, whereas a person working at ground level in the field
during the tilling operation would receive a higher dose. However, the ground-level
worker would be unlikely to be exposed to the particulate cloud at full concentration
for the entire work day, further reducing the probability of infection. Figure 5-1 shows
that to achieve a probability of infection of 10"•'/day (with MID=10), the average
exposure would have to be increased to approximately 3 bacterial pathogens/day, a
factor of more than 25. Any dilution of the particulates by soil would reduce the
pathogen concentration correspondingly, increasing the exposure required for infection.
53. DIRECT CONTACT
In Practice I, sludge is assumed to be tilled into the surface soil immediately after
application. If the soil has a bulk density of 1.33 g/cm^, the dry mass of the surface
soil layer is 2*106 kg/ha (Naylor and Loehr, 1982; Donahue et al, 1983). As a result of
this mixing, the initial concentration of pathogens in the surface soil is given by the
following equation:
CP = CS * AR/MS (5-8)
where:
5-11
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CP = concentration of pathogens in soil (number/g)
CS = concentration of pathogens in sludge (number/kg)
AR = application rate (kg/ha)
MS = mass of soil = 2*106 kg/ha * 103 g/kg = 2*109 g/ha.
By this equation, application of liquid sludge with 5*10^ pathogens/kg at a rate of 1
kg/ha would result in a soil concentration of 0.1 pathogen/g soil.
The default amount of ingested soil and residue is 0.1 g/day. Assuming an initial
concentration of 0.1 pathogen/g soil, the daily exposure is 0.01 pathogen. For MID=10,
the probability of infection would be much less than 10" 16.
5.4. WATER
5.4.1. Surface Runoff. Surface runoff occurs when the amount of water received by
the field exceeds the retention capacity of the soil. The amount of runoff is calculated
by a subroutine that is called once for each rainfall or irrigation event. This
subroutine uses the Modified Universal Soil Loss Equation (Williams, 1975), which
calculates the amount of soil erosion as a function of soil condition and use and as a
function of the peak discharge, calculated from a constructed excess rainfall hyetograph.
Inputs to the subroutine include the time, duration, and amount of rainfall. Rainfall is
modeled as a single event whose intensity increases linearly with time to a peak at a
user-specified storm-advancement coefficient (default value 0.4) and subsequently falls
linearly to the end of the rain. The composite transport resulting from excess rainfall
serves as the transfer function from SOIL SURFACE (4) to SURFACE RUNOFF (6).
Irrigation is assumed to be controlled so that it does not cause surface runoff.
The majority of soil microorganisms are associated with soil particles. A fraction
of these will be suspended by soil surface water and will be transported in suspension
by surface runoff. Studies have shown that the fraction of microorganisms suspended
by excess water is dependent on properties of the soil as well as on type of
microorganism (Burge and Enkiri, 1978; Drewey and Eliassen, 1968; Gerba el al., 1975;
Marshall, 1971; Reddy et al., 1981). Data on adsorption of viruses and bacteria to soil
particles can be fitted empirically to an equation describing a linear chemical adsorption
isotherm:
PARTIC = K * SUSP
where:
5-12
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PARTIC = concentration of soil-bound organisms, number/gram
K = retention coefficient
SUSP = concentration of suspended organisms, number/ml solution.
Reported values of K range from 260 to 1900 for bacteria (Matson et al., 1978) and from
4.6 to 161 for viral particles (Burge and Enkiri, 1978).
Because the total number of organisms in SURFACE RUNOFF (6*) is the sum of
bound and suspended organisms (TOTAL = PARTIC + SUSP), the number of suspended
organisms is
SUSP= TOTAL/(1 + K) or
SUSP/TOTAL = 1/(1 + K).
The constant 1/(1 + K) is variable SUSPND [P(45)]. Default values for SUSPND vary
with pathogen type and also with land use practice; it is assumed that suspension of
organisms in soil surface water is less when there is a grass cover (Practices II, III,
and V) than when there is a substantial fraction of bare ground (Practices I and IV).
Default values of SUSPND [P(45)] are given in Table A-2.
The human receptor for surface runoff is a swimmer who incidentally ingests 0.1 L
of pond water during a single swim. Although some of the microbes transported into
SURFACE RUNOFF (6*) will be associated with sediment and are likely to be deposited
at the bottom of the pond, swimming by the human receptor is likely to resuspend them
and they can be ingested. The model assumes that all pathogens transported to the
runoff pond remain uniformly distributed.
Assuming a pond volume of 100 irP per hectare, an infective concentration (3
Salmonella per 100 mL) represents 3*10^ organisms per hectare, or about 0.6% of the
initial pathogen load when liquid sludge is applied at 10 T/ha. Test runs of the surface
runoff subroutine show that a 2-hour rainfall beginning at 120 hours would have to
total nearly 4.65 cm to yield a total of even 0.15% of the pathogen load as surface
transport. Of that amount, slightly over 50% would be suspended organisms. This
amount of rainfall is well above the 5-year maximum 2-hour rainfall reported for any
agricultural region of the continental United States, so a release of that amount would
be an infrequent occurrence.
5.4.2. Groundwater at an Offsite Well. The groundwater subroutine considers
transport in saturated soil. Computer models for solute transport in groundwater have
5-13
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been applied to the transport of microbes in soil (Haridas, 1983; van Genuchten, 1978,
1986). These models must include terms for the death or inactivation of microbes,
filtration, retardation, adsorption and advection during transport. Clogging of soil pores
by filtration and the subsequent declogging by sloughing off of biofilm can be modeled
(Haridas, 1983), but time-averaged values for retardation are adequate and are more
readily estimated. The mathematical description used in this model is a one-dimensional
solution (van Genuchten and Alves, 1982) of the advection-dispersion equation for
solutes (for a description, see Volume II: User's Manual). Input parameters include
retardation coefficients for saturated soil, as well as distance from source to output
(upper boundary of groundwater or location in field to drinking water well), bulk flow
velocity and hydrodynamic dispersion. Default values for these variables are given in
Table A-6. The value of the retardation coefficient R is chosen to represent poor
adsorption to soil particles during saturated flow.
Accurate values for retardation coefficient R are not known; values may be very
high for extreme adsorption, filtering or clogging or as low as 0.5 for selective flow of
pathogens through large pores and channels (Keswick et al., 1982; Bradford, 1987;
Matthess et al., 1988). For the saturated flow option, a default value of 1 (no
retardation) is used.
Both surface soil and subsurface soil immobilize microorganisms to some extent. As
input into the groundwater transport subroutine, the surface soil is represented as
retaining all but 0.05% of bacteria and 0.1% of virus particles. The default fraction for
transport of bacterial and viral pathogens through the unsaturated zone is 0.001.
Transport through the saturated zone is modeled for a default distance of 50 m from
the edge of the field, with the source of saturated flow taken as a point source
containing all of the contribution from 1 ha.
A probability of infection of 10~3 requires an accumulated daily exposure of 3
pathogens when MID=10. At the modeled water consumption rate of 2 L/day, this would
require a concentration of 1.5 pathogens/L. Using the default values for thickness of
the aquifer (AQUIFR [P(9)]) and porosity (POROS [P(10)], the volume of groundwater
per hectare is 3*10^ m^ or 3*10' L. The total number of pathogens applied in liquid
sludge is assumed to be 5*10° per hectare, so the potential maximum groundwater
contamination, if all pathogens were transported immediately, would be 5*10^/3*10^ = 17
pathogens/L. A maximum of 0.1% of soil surface pathogens is estimated to be
transported to subsurface soil, and 0.1% of subsurface soil pathogens are likely to be
transported to groundwater (Sorber and Moore, 1987). Therefore, the maximum likely
5-14
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exposure to sludge pathogens in drinking water should not exceed 0.2/day.
5.43. Fugitive Emissions (Aerosols) from Irrigation. Direct onsite exposure to
contaminated irrigation water is assumed to be negligible. However, aerosols are
generated by spray irrigation. Sample calculations show that use of GROUND WATER (9)
as a source of irrigation water is not likely to lead to significant pathogen
concentrations in the aerosol. As a worst case, the use of treated liquid sludge for
irrigation is modeled in Practices II and III. The default rate of irrigation is 0.5
cm/hour, or 50 m3/ha/hour. At 5% sludge solids, this rate is equivalent to 2.5* 103
kg/ha/hour, or 695 g/ha/sec. The efficiency of aerosol formation during spraying has
been estimated as about 0.1% or less (Sorber et al., 1984), giving a value of 0.695
g/ha/sec or about 7*10"^ g/m^/sec for the rate of total emissions. If the concentration
of pathogens in liquid sludge is 5*104 pathogens/kg, the area emission rate Qa is
Qa = (7*10'5 g/m2/sec) * 50 pathogens/g
= 3.5*10'3 pathogens/irWsec
and the concentration of airborne pathogens P is
P = 83.3 Qa/[u]
= (83.3 * 3.5* 10'3 pathogens/m2/sec)/ (4 m/sec)
= 7.3*10-2 pathogens/m3.
This concentration would lead to a 5-hour exposure of ~0.4 pathogens for a moderately
active worker in the field (probability of infection approximately 8*10'12).
Fugitive emissions from irrigation spray nozzles are modeled by use of equation 5-7
above. The human receptor for this exposure is assumed to be a person working
outdoors 200 m downwind from the closest irrigation sprayer. Inputs to the subroutine
include the time at which irrigation begins, the duration and amount of irrigation, wind
speed and the concentration of pathogens in the irrigation water. Inactivation rates are
taken to be the 50th percentile values for the aerosol inactivation rates of Salmonella
and enteroviruses (Camann et al., 1978) and an estimated value for Ascaris. They are as
follows:
d = Salmonella -0.028/sec
Ascaris -0.0 I/sec
enterovirus -0.002/sec
Offsite fugitive emissions from a 10-ha source under irrigation would be calculated
according to equation 5-7 above, using values of 4 m/sec for u and the inactivation
5-15
-------
constants given above. According to this calculation, the concentration of bacterial
pathogens at 200 m offsite would be
QZOO = 0.39 pathogens/m^.
A 5-hour exposure to this concentration would lead to inhalation/ingestion of about 2
bacterial pathogens for a probability of infection of 3.8*10'5 for MID = 10.
The above discussions for particulates indicate that the probability of adverse
consequences from exposure to aerosols would be minimal. In addition, studies on the
consequences of spraying treated and untreated wastewater have indicated that no
health risks could be associated with the use of treated wastewater (Camann et al.,
1980; Fannin et al., 1980; Johnson et al., 1980; Northrop et al., 1980; Shuval and Fattal,
1980). Field studies on the application of wastewater at Kibbutz Tzora, Israel (Teltsch
and Katzenelson, 1978) showed that when raw wastewater containing 0-60 Salmonella per
100 mL was sprayed, the average airborne concentration of Salmonella at 40 m was
0.014/nA At an inhalation rate of 1 m^/hour, a 12-hour exposure to this concentration
of organisms would yield an inhaled dose of about 0.17 organisms.
5.5. CROPS/PRODUCTS
If contaminated water is used for irrigation, the crop can potentially be
contaminated. If an aboveground crop (tomato) of 10 cm diameter retains all of the
pathogens in a 0.2-mm layer on the surface of the fruit, the equivalent volume of
irrigation water is approximately 0.01 L. A probability of 10"^ for infection with the
pathogen at MID=10 would require a concentration of 300 pathogens/L. Using the
calculation given above (Section 5.4.1) that the surface runoff pond concentration could
be 30 pathogens/L after a 2-hour rainfall of 4.65 cm or (in Section 5.4.2) that the
potential maximum groundwater contamination would be 17 pathogens/L, the probability
of contamination of crop units to an infectious level by irrigation water would be
negligible.
5-16
-------
6. SOURCES OF UNCERTAINTY
Many factors contribute to the uncertainties associated with the present risk
assessment model, chief among these being the lack of quantitative data. Even when
available, data are highly variable with regard to (1) the initial concentrations of
microbial pathogens in infectious waste, wastewater and sludge; (2) processes of
microbial transport and inactivation; and (3) dose-response relationships. Estimates of
levels of uncertainty of some other contributing factors are given in Table 6-1.
The most obvious factor contributing to uncertainty of the model is imprecise
exposure data. Most of the source materials involve a number of poorly characterized
taxa of organisms distributed heterogeneously through a nonhomogeneous medium.
Variations in pathogen numbers in the applied sludges due to seasonal and yearly
differences can influence exposure levels. The initial concentrations of pathogens in
treated sludge are usually not known precisely, and estimates of changes in these
concentrations within and between compartments of each sludge management practice
are rarely supported. Such estimates are subsequently used to calculate the pathogen
concentration at different points of human exposure. Consequently, the resulting
estimated concentrations may be incorrect by orders of magnitude. As an example,
concentrations of pathogens in groundwater depend on (1) the number of organisms that
enter the soil; (2) their transport into the groundwater, a factor dependent upon soil
type and the particular pathogen, and the distance through soil to groundwater; and (3)
survivability of the pathogen in the particular soil and groundwater environment. Since
these factors are only approximately known at best, predicting exposure levels due to
groundwater contamination is extremely difficult.
Similarly, uncertainties in the prediction of pathogen transport by aerosols center
around estimates of percent aerosolization, biological decay and variations in wind
direction and strength. The greatest uncertainty in this exposure assessment resides in
the estimates of biological decay. Responses of pathogens to treatment conditions and
interactions of pathogen types with environmental conditions differ. Further, responses
of receptor human populations to pathogens introduced by different exposure routes are
complex.
Another factor that produces error in human exposure determination is the volume
of contaminated sample consumed. If exposure is through contaminated food, the
quantity of sludge or soil on the food and the quantity of food consumed are subject to
large variations. If exposure occurs through consumption of water while swimming,
6-1
-------
TABLE 6-1
Levels of Uncertainty Associated with Pathogen Risk Assessment
Category
Contribution to Uncertainty
1. Determination of immune status
2. Assay technique
3. Sensitivity of host
4. Virulence of virus
5. Use of upper 95% confidence limit
6. Route of exposure
7. Choice of dose-response model
8. Synergism/antagonism
9. Dietary considerations
10. Distribution of subjects among
doses and number used
One order of magnitude
One order of magnitude
Several orders of magnitude
Several orders of magnitude
Up to one order of magnitude
One order of magnitude
Several orders of magnitude
Many orders of magnitude
Uncertain
1-2 orders of magnitude
Source: Gerba, 1984
6-2
-------
variations in the volume of water consumed by different individuals is likely. Even the
quantity of drinking water consumed by different persons can vary greatly.
The dose required to cause infection is another major source of potential error.
Based on information generated with human respiratory and enteric viruses, a virus
capable of infecting a specific tissue culture can also infect humans. Thus, a single
infectious unit (I.U.) of virus, as detected in tissue culture, is considered a minimum
infective dose for purposes of this report. If the tissue culture assay is insensitive, the
actual infectious dose could be considerably less than 1 I.U. On the other hand, the
value may be much greater than 1 I.U., as has been determined for echovirus-12 in
infected volunteers (Schiff et al., 1984). Similar sources of error are associated with
assessing risk in the case of other enteric pathogens. In general, the uncertainties in
measuring infectious dose greatly weaken the power of this type of quantitative risk
assessment.
Any consideration of factors that introduce uncertainties to the model must include
the very concept of using representative organisms. Though widely accepted, the
assumption that the species selected accurately represents the potential pathogens
present in the source material is not necessarily reliable. Other sources of error, such
as the immune state and resulting susceptibility of the individual to infection by a
particular pathogen, and the route of entry of the pathogens, will alter the probability
of infection.
6-3
-------
7. SENSITIVITY ANALYSIS
7.1. INTRODUCTION
The goals and objectives for model application determine the nature of the model.
Two categories of models can be distinguished: research models and management models.
The research model should provide indicators for future directions in investigations.
The possibility for gaining an understanding of response of the system to input
parameters is of primary importance. Management models have specified applications,
e.g., long-term planning, evaluation of a specific design, etc. The RISK model is a
research-oriented model at this stage, and the main objectives of model testing and
sensitivity analysis are to better comprehend the system, especially its response to
varying the input parameters, and to narrow the future research directions.
7.2. MODEL TESTING
The main attributes of software quality include reliability, usability, efficiency,
transportability and maintainability. The listed quality factors are related to testing,
sensitivity analysis, validation and verification issues. Program testing (i.e., running the
code with representative sample data sets and comparing the actual results with the
expected results) has been the fundamental technique for determining coding errors.
Because testing is a difficult and time-consuming procedure, increased emphasis should
be placed on ensuring quality throughout the entire process of code development, rather
than trying to add it after the process is complete. Although all errors in the software
are costly, the later in the life cycle that errors are discovered, the more costly the
error. The objective of the verification process is twofold: to check the accuracy of
the computational algorithms used to solve the governing equation, and to assure that
the computer code is fully operational. Verification is also used to evaluate the
sensitivity of the code to various parameter values. Sample problems should be selected
so that the main program and all subroutines are tested. Model validation is often
defined as the comparison of model results with actual measurements from the field or
laboratory experiments. The objective is to determine how well the model describes the
actual system behavior. Validation of environmental models is quite difficult to perform
because of the lack of field data and questionable accuracy of the data. Collected field
data are often published in some earlier studies for a different purpose, often without
any quality assurance and quality control, and thus they are subject to inaccuracies,
interpretive bias, loss of information during transmission, and so forth. The modeling
7-1
-------
procedure, before and after any field experiment has been used to test the validity of
the model, can be considered to reflect a priori and a posteriori knowledge of the
behavior of a system (Beck, 1983).
Sensitivity analysis can yield insight into the nature of the model. A priori
sensitivity analysis establishes the relative magnitudes of changes in the simulated model
output responses to changes in the model parameter values. A posteriori sensitivity
analysis examines the distribution of model responses that are possible, given the
distributions of estimated parameter values.
7.3. SENSITIVITY ANALYSIS
Even without experimental field data having been collected for model evaluation,
certain important questions about the suitability of the model can be posed. The
answers to these questions — questions of a priori sensitivity analysis — may lead to a
restructuring of the model at the conceptualization stage (Beck, 1983). Sensitivity
analysis is part of a feedback loop during both the a priori and a posteriori phases of
the modeling procedure.
The importance of sensitivity analysis has been well known in control engineering
since the 1950s (Tomovic, 1962). Relatively recently has it been applied to water
quality and ecological systems, for example by Gardner et al. (1981), Haas (1983), Jaffe
and Parker (1978), McCuen (1973), and Rose and Swartzman (1981). In groundwater
literature, the question of sensitivity analysis has been discussed by Gilham and
Farvolden (1974) and by Sykes et al. (1983, 1985).
The main objective of the sensitivity analysis is to understand the relative
sensitivity of the model predictions (output) to changes in the values of the model input
parameters fi. The sensitivity coefficient Sjj can be defined as
s;j =
A/3j//3j
where:
AQ = the change in the i-th state variable of the model in response to a
change A/3j in the value of the j-th input parameter;
C j = the nominal value for the i-th predicted state variable response
(e.g., a number of pathogens in a given compartment);
/3j = the nominal value for the j-th input parameter.
7-2
-------
In general, AQ and A/3j are introduced as small changes in the neighborhoods of Q
and &. The formula given above enables the researcher to investigate whether a
certain percentage change in a parameter has no real significance (Sjj=0), whether a
parameter is a dominant parameter, or whether a small change in the value of the input
parameter causes instability in the model output.
If the output response of the model is found to be insensitive to changes in the
value of any parameter, that parameter is characterized as not identifiable. This means
that it is not possible to estimate the influence of that parameter unless the model
relationships are specified in some other form. On the other hand, if a parameter has a
strong influence on a particular output variable, then the quality of that output will
depend strongly on the ability to make accurate estimates of the value of the input
parameter.
7.4. METHODOLOGY
Before sensitivity analysis begins, one must know the nominal values for the input
parameters. A "complete" analysis requires the estimation of frequency distributions for
the set of parameters that will be varied. As a minimum, enough information must be
available to estimate the limits of variability. The nominal, minimum, and maximum
values of the input model parameters are given in Table 7-1. The sensitivity analysis
was performed for Practice I only (Application of Liquid Sludge for Production of
Commercial Crops for Human Consumption) and for the pathogen Salmonella.
Once the nominal values for the parameters were selected, an input data set in
which the values were varied separately was created. The "reference run" is for a
period of 60 days, and includes 5 rain events that occur at 120, 240, 480, 600, and 840
hours of the total simulation time. All 16 compartments of the model had no pathogens
at the beginning of the simulation. The results were printed every 24 hours. The value
of parameter ASCRS (P[lj) was 1E10 for all runs. The relatively large value of the
parameter was chosen only for purpose of the sensitivity analysis and should not be
considered representative of actual sludge applications.
Only one input parameter was changed at a time, for each run, taking the
maximum or the minimum value of the parameter. After each run, the output file
PATHOUT was examined. The file contains information on number of pathogens in each
of 16 compartments and the infection probability for the specified PRINT SAMPLING
RATE (every 24 hours in this case). It was decided that of the 16 compartments, only
compartments that represent sources of human exposure be monitored. The monitored
7-3
-------
TABLE 7-1
INPUT VARIABLES - PRACTICE I. SALMONELLA
# NAME
Nominal
Minimum
Maximum
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
ASCRS*
APRATE
ASCIN
TREG
UPLIM
APMETH
AREA
TEMP
AQUIFR
POROS
FILTR8
MID
TRAIN
RDEPTH
TK
TIRRG
IRMETH
DILJRR
NIRRIG
IRRATE
DEPTH
COUNT
TWIND
DWIND
WINDSP
EPSMLT
ESILT
EHT
SCRIT
COVER
AEREFF
BREEZE
HT
ANDAY
TMAX
TMIN
SLOPES*
NTRCPS*
SLOPEP*
NTRCPP*
ASLSUR
FSSUR
FRRAIN
SUBSOL*
SUSPND*
1.0E+10
1.00E+4
.
**
1.00E+9
+ 1
10
20
10
0.3
2
10
168
5
0
0
0
2
0.5
2.5
60
6
18
0.33
0.4
2
7.5
0
0.001
4
1.6
.
.
.
0.0206
2.113
0.00449
1.435
0.9
1
5.00E-4
5.00E-3
1.0E+10
2.00E+3
.
**
1.00E+9
0
1
0
2
.1
1
1
-2
2.5
0
0
0
1
0.1
0.5
30
3
4
0.1
0.2
0.5
3.75
0
0.0001
1
1
.
.
.
0.0412
2.113
0.0089
1.435
1.0
1
5.00E-6
5.00E-5
1.0E+10
7.00E+4
»
**
1.00E+9
-1
100
43
SO
.5
4
10,000
10
0
1
1
4
1
10
120
12
22.4
0.33
0.8
3
15.0
0.1
0.01
22.4
2
.
.
.
0.0103
2.113
0.00225
1.435
.
1
5.00E-2
5.00E-1
Comments
Unsupported Value
calculated
crop specific
upper limit for pathogen population
see subroutine RAINS
time since rain
time since irrigation
irrigation rate
calculated
user specified
user specified
unsupported estimate
see subroutine RAINS
7-4
-------
TABLE 7-1 (Continued)
INPUT VARIABLES - PRACTICE I. SALMONELLA
NAME
Nominal
Minimum
Maximum
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
FCROP1
FCROP2
FCROP3
FCROP4
FCROP5
FCROP6
FCROP7
FRGRND*
SSWTCS*
PSTMG
CSTSS
SSTCS
CSTSSW
DTCTMT
DTCTMK
TMTSS
TMTH
TMTU
HTM
UTM
CROP
TCULT
TCROP
THARV
YIELD1
YIELD2
YIELDS
HAY
PLNT1
PLNT2
PLNT3
PPG
CATTLE
COWS
STORAG
FORAG
ALFALF
SCNSMP
FATTEN
TSLOTR
1.00E-8
0.06
1.00E-3
1.00E-4
0.012
0
2.00E-4
0.001
-
-
-
.
0.75
-
.
-
-
-
-
.
1
-2
720
1848
2.50E+7
2.50E+7
1.00E+6
.
.
.
.
-
.
-
-
-
.
-
.
-
1.00E-10
0.0006
1.00E-5
1.00E-6
0.00012
0
2.00E-6
0.0001
-
.
.
.
0.1
-
.
-
.
-
.
,-
0
-2
360
924
2.50E+6
2.50E+6
1.00E+5
.
.
.
.
.
.
. .
.
-
.
-
-
.
1.00E-6
6
1.00E-1
1.00E-2
1.2
0
2.00E-2
0.01
-
-
.
.
1.0
-
.
-
-
.
-
.
-1
0
1440
3696
2.50E+8
2.50E+8
1.00E+7
.
.
.
.
.
,.
.
.
.
.
.
.
.
Comments
unsupported estimate
Practice I and IV only; unsupported
Practice I and IV only; unsupported
unsupported estimate
unsupported estimate
unsupported estimate
Practice 11,111, and V only
Practice V only
Practice II, III, and V only
Practice II and III only
unsupported estimate
Practice II and III only
Practice II only
Practice II and III only
Practice II and III only
Practice II and III only
Practice IV only
Practice IV only
Practice IV only
PPG = N(16)/YIELD for Practice
Practice II and III only
Practice II and III only
Practice III only
Practice II and III only
Practice III only
Practice II and III only
Practice Ml only
Practice II and III only
* pathogen specific, Table A-2 User's Manual
** practice and crop-specific
7-5
-------
compartments are 3, 5, 6, 7, 12, 13, and 16. The infection probability was monitored
for each of the five exposure categories: Onsite Person, Offsite Person, Food Consumer,
Groundwater Drinker and Pond Swimmer. The number of pathogens in a compartment or
the infection probability versus time was extracted from the PATHOUT file. Thus, in
addition to four "normal" output files, twelve data files were generated for each run.
Even for the limited number of input parameters being tested, a large number of output
files have been generated (~1000 files). Each of the output data files was sorted to
find the maximum. Those values were entered in a spreadsheet, and the sensitivity
coefficients were calculated for each compartment (Tables 7-2 through 7-5). In
addition, graphs were generated for visual examination of the results (a selected number
of these, Figures 7-1 through 7-7, are included).
7.5. RESULTS AND COMMENTS
The results of the sensitivity analysis are given in Tables 7-2 through 7-5. The
tables do not include Compartments 3 (Application/Tilling Emissions), 5 (Particulates),
and 16 (Commercial Crop) because they did not contain any pathogens for any
combination of the input parameters.
The number of pathogens in Compartment 6 (Surface Runoff) (Table 7-2) was most
sensitive to SLOPES (P[37]), a process function parameter for inactivation of organisms
in soil; AREA (P[7j), area of the field; APRATE (P[2])} application rate; APMETH (P[6]),
application method; NIRRIG (P[19j), number of irrigations per week; and SUSPND
(P[45]), a parameter for fraction of organisms transferred from soil surface to soil
surface water.
The number of pathogens in Compartment 7 (Direct Contact) (Table 7-3) was
sensitive to SLOPES (P[37j), a process function parameter for inactivation of organisms
in soil; AREA (P[7]), area of the field; APMETH (P[6]), application method; and APRATE
(P[2]), application rate.
The number of pathogens in Compartment 12 (Offsite Well) (Table 7-4) was most
sensitive to SLOPES (P[37J), a process function parameter for inactivation of organisms
in soil; IRRATE (P[20j), irrigation rate; APRATE (P[2]), application rate; AREA (P[7]),
area of the field; SUBSOL (P[44]), a parameter for fraction of organisms transferred
from soil surface to subsurface soil; and FRGRND (P[53]), a parameter for fraction of
organisms transferred from subsurface soil to groundwater. AQUIFR (P[9]), aquifer
thickness and POROS (P[10]), porosity of the aquifer, did not have any effect on the
number of pathogens in the compartment; these parameters are applied in the risk
7-6
-------
TABLE 7-2
SENSITIVITY ANALYSIS PRACTICE I*
NUMBER OF PATHOGENS IN CCMPARTMENT 6
NOM
l.OOE+04
l.OOE+00
l.OOE+01
2.00E+01
l.OOE+01
3.00E-01
2.00E+00
l.OOE-fOl
5. OOE+00
2.00E+00
5.00E-01
2.50E+00
6.00E+01
6.00E+00
1.80E+01
INPUT
MIN
2.00E+03
O.OOE+00
l.OOE+00
O.OOE+00
2.00E+00
l.OOE-01
l.OOE+00
l.OOE+00
2.50E+00
l.OOE+00
l.OOE-01
5.00E-01
3.00E+01
3 . OOE+00
4.00E+00
MAX
7.00E+04
-l.OOE+00
l.OOE+02
4.30E+01
5.00E+01
5.00E-01
4. OOE+00
l.OOE+04
l.OOE+01
4. OOE+00
l.OOE+00
l.OOE+01
1.20E+02
1.20E+01
2.24E+01
NCM
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
OUTPUT
MIN
1.411E+08
O.OOOE+00
O.OOOE+00
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.092E+08
7.059E+08
7.052E+08
7.055E+08
7.055E+08
7.055E+08
MAX
4.939E+09
7.055E+08
7.059E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.015E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
SENSITIVITY
Si (MIN)
1.0000
1.0000
1.1111
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0105
0.0007
0.0005
0.0000
0.0000
0.0000
Si (MAX)
1.0001
0.0000
0.0001
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0057
o.oooc
o.oooc
o.oooc
o.oooc
o.oooc
2
6
• •! II
7
8
9
10
11
12
!•
14
19
20
21
23
24
25
* Results derived from this sensitivity analysis should not be
considered representative of actual sludge applications.
7-7
-------
TABLE 7-2 (Continued)
SENSITIVITY ANALYSIS PRACTICE I*
NUMBER OF PATHOGENS IN COMPARTMENT 6
p
27
28
29
31
32
37
39
44
45
46
53
58
68
69
70
NCM
4.00E-01
2.00E+00
7.50E+00
l.OOE-03
4.00E+00
2.06E-02
4.49E-03
5.00E-04
5.00E-03
l.OOE-08
l.OOE-03
7.50E-01
7.20E+02
1.85E+03
2.50E+07
INPUT
MTN
2.00E-01
5.00E-01
3.75E+00
l.OOE-04
l.OOE+00
4.12E-02
8.90E-03
5.00E-06
5.00E-05
l.OOE-10
l.OOE-04
l.OOE-01
3.60E-f02
9.24E+02
2.50E+06
MAX
8.00E-01
3.00E+00
1.50E+01
l.OOE-02
2.24E+01
1.03E-02
2.25E-03
5.00E-02
5.00E-01
l.OOE-06
l.OOE-02
l.OOE+00
1.44E+03
3.70E+03
2.50E+08
NCM
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
OUTPUT
MTN
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
5.856E+06
7.055E+08
7.059E+08
7.112E+08
7.055E4-08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E4-08
MAX
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
3 . 012E+09
7.055E+08
6.690E+08
2.431E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
7.055E+08
SENSITIVITY
Si(MIN)
0.0000
0.0000
0.0000
0.0000
0.0000
0.9917
0.0000
0.0006
0.0082
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
Si (MAX)
0.0000
0.0000
0.0000
0.0000
0.0000
6.5386
0.0000
0.0005
0.0066
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
* Results derived from this sensitivity analysis should not be
considered representative of actual sludge applications.
7-8
-------
TABLE 7-3
SENSITIVITY ANALYSIS PRACTICE I*
NUMBER OF PATHOGENS IN OCMPAKIMENT 7
NCM
l.OOE+04
l.OOE+00
l.OOE+01
2.00E-H)!
l.OOE+01
3.00E-01
2.00E+00
l.OOE+01
5.00E+00
2.00E+00
5.00E-01
2.50E+00
6.00E+01
6.00E+00
1.80E+01
INPUT
M3N
2.00E+03
O.OOE+00
l.OOE+00
O.OOE+00
2.00E+00
l.OOE-01
l.OOE+00
l.OOE+00
2.50E+00
l.OOE+00
l.OOE-01
5.00E-01
3.00E+01
3.00E+00
4.00E+00
MAX
7.00E+04
-l.OOE+00
l.OOE+02
4.30E+01
5.00E+01
5.00E-01
4.00E+00
l.OOE+04
l.OOE+01
4.00E+00
l.OOE+00
l.OOE+01
1.20E+02
1.20E+01
2.24E+01
NCM
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
OUTPUT
MIN
2.464E+01
O.OOOE+00
O.OOOE+00
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.233E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
MAX
8.625E+02
1.232E+02
1.233E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.226E+02
1.233E+02
1.233E+02
1.232E+02
1.232E+02
1.232E+02
SENSITIVITY
Si(Min)
1.0000
1.0000
1.1111
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0010
0.0000
0.0000
0.0000
0.0000
Si (MA)
1.00(
0.00(
O.OOf
0.001
O.OOi
O.OOi
0.00'
0.00
0.00
0.00
0.00
0.00
o.oc
o.oc
o.oc
2
•••<•••••
6
7
8
9
•••MB
10
11
12
14
<•»••»••
19
•••••MB
20
•••••w
21
•••••Ml
23
••»••»•
24
25
* Results derived frcro this sensitivity analysis should not be
considered representative of actual sludge applications.
7-9
-------
TABIE 7-3 (Continued)
SENSITIVITY ANALYSIS PRACTICE I*
NUMBER OF PATHOGENS IN OCMPARIMENT 7
p
27
28
29
31
32
37
39
44
45
46
53
58
68
69
70
NCM
4.00E-01
2.00E+00
7.50E+00
l.OOE-03
4.00E+00
2.06E-02
4.49E-03
5.00E-04
5.00E-03
l.OOE-08
l.OOE-03
7.50E-01
7.20E+02
1.85E+03
2.50E+07
INPUT
MIN
2.00E-01
5.00E-01
3.75E+00
l.OOE-04
l.OOE+00
4.12E-02
8.90E-03
5.00E-06
5.00E-05
l.OOE-10
l.OOE-04
l.OOE-01
3.60E+02
9.24E+02
2.50E+06
MAX
8.00E-01
3.00E4-00
1.50E+01
l.OOE-02
2.24E+01
1.03E-02
2.25E-03
5.00E-02
5.00E-01
l.OOE-06
l.OOE-02
l.OOE+00
1.44E+03
3.70E+03
2.50E+08
NCM
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
OUTPUT
MIN
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
2.265E+01
1.232E+02
1.233E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
MAX
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
2.072E4-02
1.232E+02
1.168E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
1.232E+02
SENSITIVITY
Si (MIN)
0.0000
0.0000
0.0000
0.0000
0.0000
0.8162
0.0000
0.0008
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
Si (MAX)
0.0000
0.0000
0.0000
0.0000
0.0000
1.3636
0.0000
0.0005
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
* Results derived fron this sensitivity analysis should not be
considered representative of actual sludge applications.
7-10
-------
TKBLE 7-4
SENSITIVITY ANALYSIS PRACTICE I*
NUMBER OF PATHOGENS IN CXMPARTMENT 12
p
2
6
7
8
9
10
11
12
14
19
20
21
23
24
25
NCM
l.OOE+04
l.OOE+00
l.OOE+01
O.OOE+00
l.OOE+01
3.00E-01
2.00E+00
l.OOE+01
5.00E+00
2.00E+00
5.00E-01
2.50E+00
6.00E+01
6.00E+00
1.80E+01
INPUT
MIN
2.00E+03
O.OOE+00
l.OOE+00
O.OOE+00
2.00E+00
l.OOE-01
l.OOE+00
l.OOE+00
2.50E+00
O.OOE+00
l.OOE-01
5.00E-01
3.00E+01
3.00E+00
4.00E+00
MAX
5.00E+07
-l.OOE+00
l.OOE+02
4.30E+01
5.00E+01
5.00E-01
4.00E+00
l.OOE+04
l.OOE+01
4.00E+00
l.OOE+00
l.OOE+01
1.20E+02
1.20E+01
2.24E+01
NCM
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.913E+04
4.913E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
OUTPUT
MIN
9.691E+03
1.129E+08
O.OOOE+00
4.846E+04
4.913E+04
4.913E+04
4.817E+04
4.846E+04
4.846E+04
4.853E+04
O.OOOE+00
7.751E+04
4.846E+04
4.846E+04
4.846E+04
MAX
3.392E+05
4.846E+04
3.459E+02
4.846E+04
4.913E+04
4.913E+04
4.862E+04
4.846E+04
4.846E+04
9.018E+04
4.482E+03
2 . 171E+04
4.846E+04
4.846E+04
4.846E+04
SENSITIVITY
Si(Min)
1.0000
2328.76
1.1111
ERR
0.0000
0.0000
0.0120
0.0000
0.0000
0.0014
1.2500
0.7493
0.0000
0.0000
0.0000
Si (MAX
0.001
O.OOC
0.11C
El
O.OOC
0.00(
o.oo:
0.001
O.OOi
0.861
0.90
0.18
0.00
0.00
0.00
* Results derived from this sensitivity analysis should not be
considered representative of actual sludge applications.
7-11
-------
TABLE 7-4 (Continued)
SENSITIVITY ANALYSIS PRACTICE I*
NUMBER OF PATHOGENS IN COMPARTMENT 12
27
28
29
31
32
37
39
44
45
46
53
58
68
69
70
NOM
4.00E-01
2.00E+00
7.50E+00
l.OOE-03
4.00E+00
2.06E-02
4.49E-03
5.00E-04
5.00E-03
l.OOE-08
l.OOE-03
7.50E-01
7.20E+02
1.85E+03
2.50E+07
INPUT
MIN
2.00E-01
5.00E-01
3.75E+00
l.OOE-04
l.OOE+00
4.12E-02
8.90E-03
5.00E-06
5.00E-05
l.OOE-10
l.OOE-04
l.OOE-01
3 . 60E+02
9.24E+02
2.50E+06
MAX
8.00E-01
3.00E+00
1.50E+01
l.OOE-02
2.24E+01
1.03E-02
2.25E-03
5.00E-02
5.00E-01
l.OOE-06
l.OOE-02
l.OOE+00
1.44E+03
3.70E+03
2.50E+08
NOM
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
OUTPUT
MIN
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
1.228E+04
4.846E+04
4.846E+02
4.846E+04
4.846E+04
4.846E+03
4.846E+04
4.846E+04
4.846E+04
4.846E+04
MAX
4.846E+04
4.846E+04
4.846E+04
4.846E+04
4.846E+04
7.484E+04
4.846E+04
4.844E+06
4.813E+04
4.846E+04
4.845E+05
4.846E+04
4.846E+04
4.846E+04
4.846E+04
SENSITIVITY
Si(Min)
0.0000
0.0000
0.0000
0.0000
0.0000
0.7466
0.0000
1.0000
0.0000
0.0000
1.0000
0.0000
0.0000
0.0000
0.0000
Si (MAX)
0.0000
0.0000
0.0000
0.0000
0.0000
1.0887
0.0000
0.9996
0.0001
0.0000
0.9998
0.0000
0.0000
0.0000
0.0000
* Results derived from this sensitivity analysis should not be
considered representative of actual sludge applications.
7-12
-------
TABLE 7-5
SENSITIVITY ANALYSIS PRACTICE*
NUMBER OF PATHOGENS IN COMPARTMENT 13
NCM
l.OOE+04
l.OOE+00
l.OOE+01
2.00E+01
l.OOE+01
3.00E-01
2.00E+00
l.OOE+01
5.00E+00
2.00E+00
5.00E-01
2.50E+00
6.00E+01
6. OOE+00
1.80E+01
INPUT
MIN
2.00E+03
O.OOE+00
l.OOE+00
O.OOE+00
2.00E+00
l.OOE-01
l.OOE+00
l.OOE+00
2.50E+00
l.OOE+00
l.OOE-01
5.00E-01
3.00E+01
3.00E+00
4 . OOE+00
MAX
7.00E+04
-l.OOE+00
l.OOE+02
4.30E+01
5.00E+01
5.00E-01
4. OOE+00
l.OOE+04
l.OOE+01
4. OOE+00
l.OOE+00
l.OOE+01
1.20E+02
1.20E+01
2.24E+01
NCM
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
OUTPUT
MIN
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
MAX
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
1.732E+01
O.OOOE+00
6.033E+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
SENSITIVITY
Si (MIN)
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
Si (MAX)
Era
ERE
ERJ
ERJ
ERI
Era
EH
ER]
Era
ER]
Era
ER1
ER-
ER'
ER
2
6
7
8
9
10
11
12
14
19
^Mn_i
20
21
23
24
25
* Results derived from this sensitivity analysis should not be
considered representative of actual sludge applications.
7-13
-------
TABLE 7-5 (Ctantinued)
SENSITIVITY ANALYSIS PRACTICE*
NUMBER OF PATHOGENS IN COMPARTMENT 13
NCM
4.00E-01
2.00E+00
7.50E+00
l.OOE-03
4.00E+00
2.06E-02
4.49E-03
5.00E-04
5.00E-03
l.OOE-08
l.OOE-03
7.50E-01
7.20E+02
1.85E4-03
2.50E+07
INPUT
MIN
2.00E-01
5.00E-01
3.75E+00
l.OOE-04
l.OOE+00
4.12E-02
8.90E-03
5.00E-06
5.00E-05
l.OOE-10
l.OOE-04
l.OOE-01
3.60E+02
9.24E+02
2.50E+06
MAX
8.00E-01
3.00E+00
1.50E+01
l.OOE-02
2.24E+01
1.03E-02
2.25E-03
5.00E-02
5.00E-01
l.OOE-06
l.OOE-02
l.OOEH-00
1.44E+03
3.70E+03
2.50E+08
NCM
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
cunur
MIN
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
MAX
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
SENSlTlVriY
Si (MIN)
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
Si (MAX)
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
ERR
.* ^
considered representative of actual sludge applications.
7-14
-------
5.0E+009 -i
4.0E+009 :
3.0E+009 -
o
z:
2.0E+009 -
1.0E+009 -
O.OE+000
Practice I
Number of Pathogens in Compartment 6
Parameter P2
10
20
30 40
Time (days)
50
60
FIGURE 7-1
-------
ON
8.0E+008 n
6.0E+008 -
4.0E+008 -
2.0E+008 -
O.OE+000 i 1111
Practice I
Number of Pathogens in Compartment 6
Parameter P45
10 20
30
Time (days)
40
' i •
50
60
FIGURE 7-2
-------
5.0E+004
4.0E+004 -
3.0E+004 -
2.0E+004 -
1.0E+004
O.OE+000
Practice I
Number of Pathogens in Compartment 12
Parameter P8
Max
Nominal
A
Win
i i i i i iii i\ i i i i i i i i i \ i i n i i i i r j r i i i n i 11 \ i i i i i i i i i | rr i r i i r
10
20 30
Time (days)
40 50 60
FIGURE 7-3
-------
00
5.0E+004 -i
4.0E+004 -
3.0E+004 -
o
z
2.0E+004 -
1.0E+004 -
O.OE+000
Practice I
Number of Pathogens in Compartment 12
Parameter P9
0 10 20 30
40 50
Time (days)
60
80 90
FIGURE 7-4
-------
8.0E+004 -i
6.0E+004 -
4.0E+004 -
2.0E+004 -
O.OE+000
Practice 1
Number of Pathogens in Compartment 12
Parameter P37
Max
TT
Nominal
i i i rr> T | i i M i i i i i | i i i i i i i i i | i i 111 i i i i i i i i i i rfrT i i i i i i i i i
0 10 20 30 40 50 60
Time (days)
FIGURE 7-5
-------
PRACTICE I
ONSITE PERSON INFECTION PROBABILITY
PARAMETER P2
JD
O
.Q
O
L-
CL
10
10
10
10
10
10
-12
-14
]
']
i
*j
i
'i
']
i
Jj
i
i
i
i
i
-18.
0.00
Max
10.00
20.00
Time (days)
30.00
40.00
FIGURE 7-6
-------
-j
N>
PRACTICE I
GROUNDWATER DRINKER INFECTION PROBABILITY
PARAMETER P6
Max
Nom
20 30 40
Time (days)
50
FIGURE 7-7
-------
calculation to calculate the concentration of pathogens.
Compartment 13 (Aerosols) (Table 7-5) was essentially empty for all runs except for
the maximum values of NIRRIG (P[19]), number of irrigations per week, and DEPTH
(P[21]), depth of irrigation water. Because the nominal value of the output variable was
0, the sensitivity coefficient could not be calculated for these parameters.
Most of the exposure compartments were shown to be sensitive to the same input
variables. The sensitivity to SLOPES (P[37]) is more extreme when Compartment 12
(Offsite Well) is examined because SLOPES is an exponential parameter and the time
between application of sludge and entry of pathogens into groundwater is relatively
long. In contrast, the sensitivity coefficients of all output compartments for parameter
P[2] (APRATE) reflect the proportional response of total number of pathogens applied to
the amount of sludge applied. The sensitivity coefficients of Compartment 12 for
SUBSOL (P[44]) and FRGRND (P[53]) reflect the role of these parameters in determining
the number of pathogens entering groundwater from surface soil and subsurface soil.
The response to variable P[6] (APMETH) is characteristic of the role of this parameter
as a flag indicating the method of application of sludge; the value of P[6] labeled MIN
in this analysis indicates subsurface application of sludge, which makes pathogens
unavailable for surface runoff to Compartment 6 and makes more pathogens available for
incorporation into groundwater and thus to Compartment 12.
Because of the large number of input parameters and the uncertainty related to the
values of parameters, this sensitivity analysis should be viewed as preliminary.
However, the analysis does indicate that the model is very sensitive to the inactivation
rate of microorganisms in soil, as well as to the parameters used to calculate the
fractions of pathogens transferred from surface soil to subsurface soil, from subsurface
soil to groundwater and from surface soil to surface runoff water. Accordingly, these
parameters should be selected with great care, especially as they are all likely to be
site-specific. Because the data available to support choices of the values are limited,
research efforts should be directed to these areas in order to increase the accuracy of
the model.
7-22
-------
8. REFERENCES
Adams, A.P. and J.C. Spendlove. 1970. Coliform aerosols emitted by sewage treatment
plants. Science 169: 1218-1220.
Beck, M.B. 1983. Sensitivity analysis, calibration, and validation. Jn: Mathematical
Modeling of Water Quality: Streams, Lakes, and Reservoirs, ed. G.T. Orlob, International
Series on Applied System Analysis, John Wiley & Sons, New York, NY. p. 425-467.
Binder, S., D. Sokal and D. Maughan. 1986. Estimating soil ingestion: The use of
tracer elements in estimating the amount of soil ingested by young children. Arch.
Environ. Health 41: 341-345.
Blaser, MJ. and L.S. Newman. 1982. A review of human salmonellosis: I. Infective
dose. Rev. Inf. Dis. 4: 1096-1106.
Bradford, J. 1987. Transport of MS-2 virus through saturated soil columns. M.S.
Thesis, University of Arizona, Tucson, AZ.
Brady, N.C. 1974. The Nature and Properties of Soil. Macmillan Publishing Co., Inc.,
New York. p. 266-276.
Burge, W.D. and N.K. Enkiri. 1978. Virus adsorption by five soils. J. Environ. Qual. 7:
73-76.
Calabrese, EJ. 1988. Improving the risk assessment process. Jn: The 1988 Washington
Conference on Risk Assessment, September 22-23, 1988, Alexandria, VA. Sponsored by
the Center for Energy and Environmental Management.
Camann, D.E., H.J. Harding and D.E. Johnson. 1980. Wastewater aerosol and school
attendance monitoring at an advanced wastewater treatment facility: Durham Plant,
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8-4
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8-7
-------
APPENDIX A. VARIABLES AND DEFAULT VALUES
A-l
-------
TABLE A-l
Position Number, Name, Default Values, and Definition of Input Variables
>
Position
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Name
ASCRS
APRATE
ASCIN
TREG
UPLIM
APMETH
AREA
TEMP
AQUIFR
POROS
FILTR8
MID
TRAIN
RDEPTH
TK
TIRRG
IRMETH
DIL1RR
NIRRIG
1RRATE
DEPTH
COUNT
TW1ND
DWIND
WINDSP
EPSMLT
ESILT
EHT
SCR1T
I
#
1 x 104
0
**
1 x 109
+ 1
10
—
10
0.3
2
*
-2
5
—
—
0
0
2
0.5
2.5
0
60
6
18
0.33
0.4
2
7.5
II
#
1.25 x 104
0
**
1 x 109
-1
10
—
10
0.3
2
#
-2
5
—
-
0
1
2
0.5
2.5
0
60
6
18
0.33
0.4
2
7.5
Default Value
III
#
1.25 x 104
0
**
IxlO9
-1
10
—
10
0.3
2
*
-2
5
—
-
0
1
2
0.5
2.5
0
60
6
18
0.33
0.4
2
7.5
IV
#
2.5 x 104
0
**
1 x 109
+ 1
0.015
--
10
0.3
2
*
-2
5
-
-
0
0
2
0.5
2.5
60
6
18
0.33
0.4
2
7.5
V
#
2.5 x 104
0
**
1 x 109
+ 1
0.05
--
10
0.3
2
*
-2
5
~
--
0
0
2
0.5
2.5
60
6
18
0.33
0.4
2
7.5
Definition
Pathogen density (Pathogens/kg)
Application rate (kg/ha)
Pathogen concentration (Pathogens/ha)
Waiting period (months) required by U.S. EPA Pathogen
Reduction Regulations
Upper limit for pathogen concentration
Application method
Area of field or garden (ha)
Air temperature (°C)
Aquifer thickness (m)
Aquifer porosity
Infiltration rate (cm/hr)
Minimum infective dose (number of pathogens)
Time of rainfall (hr)
Rainfall depth (cm)
Time since last rain began
Time since last irrigation began
Irrigation method
Fraction of irrigation water that is contaminated
Number of irrigations per week
Irrigation rate: (cm/hr)
Depth of irrigation water (cm)
Pathogen concentration in irrigation sludge (pathogens/kg)
Time of windstorm (hr)
Duration of windstorm (hrs)
Wind speed during windstorm (m/sec)
Particle size multiplier for tilling emissions
Fractional silt content of soil
Height of box model (m)
Critical windspeed (m/sec)
-------
TABLE A-l (Continued)
Position Number, Name, Default Values, and Definition of Input Variables
Default Value
Position
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
Name
COVER
AEREFF
BREEZE
HT
ANDAY
TMAX
TMIN
SLOPES*
NTRCPS*
SLOPEP*
NTRCPP*
ASLSUR
FSSUR
FRRAIN
SUBSOL*
SUSPND*
FCROP1
FCROP2
FCROP3
FCROP4
FCROP5
FCROP6
FCROP7
FRGRND*
SSWTCS*
PSTMG
CSTSS
SSTCS
CSTSSW
DTCTMT
I
0
0.001
4
1.6
—
#
#
*
*
*
*
0.9
1
--
*
*
1 x 10'8
0.006
1 x 10"3
1 x 10'4
0.012
0
2 x 10'4
*
0.75
II
0.9
0.001
4
1.6
—
#
#
*
*
*
*
0.9
1
—
*
*
*
*
0.01
1 x 10'5
0.5
0
III
0.9
0.001
4
1.6
--
#
#
*
*
*
*
0.9
1
—
*
*
*
*
0.01
1 x 10'5
0.5
0
IV
0
0.001
4
1.6
—
#
#
*
*
*
*
0.9
..
*
*
1 x 10'8
0.006
1 x 10'3
1 x 10'4
0.012
0
2 x 10'4
0.75
V
0.9
0.001
4
1.6
~
#
#
*
*
*
*
0.9
._
*
*
*
0.5
0.01
0.5
Definition
Percent of ground surface covered by vegetation
Efficiency of aerosol formation
Normal wind speed (m/sec)
Height of receptor downwind (m)
Day in annual temperature cycle, set by user
Maximum monthly average temperature (calculated)
Minimum monthly average temperature (calculated)
Slope of inactivation vs temperature curve, moist soil
Intercept of inactivation vs temperature curve, moist soil
Slope of inactivation vs temperature curve, dry soil
Intercept of inactivation vs temperature curve, dry soil
Transfer fraction: Application to soil surface
Transfer fraction: Application to subsurface soil
Transfer fraction: Soil surface to surface runoff
Transfer fraction: Soil surface to subsurface soil
Transfer fraction: Soil surface to soil surface water
Transfer fraction: Soil surface to crop 1
Transfer fraction: Soil surface to crop 0
JT
Transfer fraction: Soil surface to crop -1
«
Transfer fraction: Crop 1 to soil surface
Transfer fraction: Crop 0 to soil surface
Transfer fraction: Crop -1 to soil surface
Transfer fraction: Subsurface soil to crop -1
Transfer fraction: Subsurface soil to groundwater
Transfer fraction: Soil surface water to crop surface
r
Transfer fraction: Grass removed during mowing
Transfer fraction: Crop surface to soil surface
Transfer fraction: Soil surface to crop surface
Transfer fraction: Crop surface to soil surface water
Transfer fraction: Animal consumption to meat
-------
TABLE A-l (Continued)
Position Number, Name, Default Values, and Definition of Input Variables
Position
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
Name
DTCTMK
TMTSS
TMTH
TMTU
HTM
UTM
CROP
TCULT
TCROP
THARV
YIELD 1
YIELD2
YIELD3
HAY
PLNT1
PLNT2
PLNT3
PPG
CATTLE
COWS
STORAG
FORAG
ALFALF
SCNSMP
FATTEN
TSLOTR
I II
0
0.7
0.001
0.001
0.1
0.05
1
-2
720 0
1800
^7
2.5 x 107
1 x 106
1.6
~
1
12
7
1.1
-2
Default Value
III IV V
0
0.001
0.001
0.1
0.05
-2
0 720 240
720 1680
2.5 x 107
2.5 x 107
1 x 106
1.6
0.4
0.3
0.3
—
1
12
720
7
30
1.1
720
-2
Definition
Transfer fraction: Animal consumption to milk
Transfer fraction: Manure to soil surface
Transfer fraction: Manure to hide
Transfer fraction: Manure to udder
Transfer fraction: Hide to meat
Transfer fraction: Udder to milk
Type of crop
Cultivation time (hr)
Time crop surface is present (hr)
Harvest time (hrs)
Yield of tomatoes (g/ha)
Yield of zucchini (g/ha)
Yield of carrots (g/ha)
Grass present in field, or hay yield (kg/m^)
Fraction of home garden in above-ground crop
Fraction of home garden in on-ground crop
Fraction of home garden in below-ground crop
Pathogen concentration on crop (paqthogens/g)
Type of cattle
Herd size
Length of forage storage (days)
Forage consumed per cow per day (kg)
Percent of feed which is harvested crop
Soil consumed per cow per day (kg)
Number of hours cattle to be fed forage
Time of slaughter (day)
#No default values, must be supplied by user
*Pathogen-specific, see Table A-2
**Practice- and crop-specific
—Calculated internally by the program
-------
TABLE A-2
Pathogen-Specific Default Values
Default Value
Position Name Pathogen I
12 MID
37 SLOPES
38 NTRCPS
39 SLOPEP
Salmonella 10
Ascaris 1
Enterovirus 1
Salmonella 0.0206
Ascaris
Enterovirus 0.00145
Salmonella 2.113
Ascaris
Enterovirus 2.957
Salmonella 0.00449
Ascarjs
II
10
1
1
0.0206
0.00145
2.113
2.957
0.00449
III
10
1
1
0.0206
0.00145
2.113
2.957
0.00449
IV
10
1
1
0.0206
0.00145
2.113
2.957
0.00449
V
10
1
1
0.0206
0.00145
2.113
2.957
0.00449
Definition
Minimum Infective Dose (Number of pathogens ingested)
Process function parameter used to calculate die-off of pathogens
in soil as a function of temperature
Process function parameter used to calculate die-off of pathogens
in soil as a function of temperature
Process function parameter used to calculate die-off of pathogens
in particulates as a function of temperature
Enterovirus
40 NTRCPP Salmonella 1.435 1.435 1.435 1.435
Ascaris
Enterovirus
44 SUBSOL Salmonella 5xlO'4 5xlO'4 5xlO'4 5xlO'4
Ascaris 0000
Enterovirus 0.001 0.001 0.001 0.001
45 SUSPND Salmonella 0.005 0.001 0.001 0.005
Ascaris 0.01 0 0 0.01
Enterovirus 0.01 0.001 0.001 0.01
1.435
5xlO'4
0
0.001
0.001
0
0.001
Process function parameter used to calculate die-off of pathogens
in particulates as a function of temperature
Transfer fraction: Soil surface to subsurface soil
Transfer fraction: Soil surface to soil surface water
-------
TABLE A-2 (Continued)
Pathogen-Specific Default Values
Default Value
Position Name
53 FRGRND
54 SSWTCS
Pathogen I
Salmonella 0.001
Ascaris 0
Entcrovirus 0.001
Salmonella
Ascaris
Enlcrovirus
II
0.001
0
0.001
0.1
0.05
0.1
III
0.001
0
0.001
0.1
0.05
0.1
IV V Definition
Transfer fraction: Subsurface soil to groundwater
0.1 Transfer fraction: Soil surface water to crop surface
0.05
0.1
-------
TABLE A-3
Proposed Initial Value Menu for
Pathogen Concentration Parameter, ASCRS
Estimated
Material
Raw Liquid Municipal Sludge
Liquid Digested Sludge
(Anaerobic)
Liquid Digested Sludge
(Aerobic)
Dried Digested Sludge
Composted Sludge
Sludge Amended Soil
Mean Oreanisms/ks (dry wf) of
Salmonella
5 x
5 x
5 x
1 x
1 x
2 x
105
104
104
103
106**
103
Ascaris
5 x 103
5 x 103
5 x 103
5 x 102
1 x 10°
1 x 10°
Material*
Viruses
5 x
1 x
1 x
1 x
5 x
3 x
105
105
105
104
IQl
103
*Adapted from Sorber and Moore, 1987
**Assumes regrowth as described by Yanco, 1988
A-7
-------
TABLE A-4
Variables and Default Values for Subroutine RISK
Position Name
2 COOKA
3 COOKP
4 COOKS
5 DRECTC
6 DRECTS
7 IBLAN
8 ICAN
9 ICANG
10 ICOOK
11 IFREE
12 IFREG
13 IPAST
14 ISTRH
Default
Value
l.E-5
l.E-5
l.E-30
0.1
0.1
0
0
0
0
0
0
0
1
Definition
Survival of Ascaris during cooking
Survival of enterovirus during cooking
Survival of Salmonella during cooking
Ingestion rate of crop surface from direct
contact in g/day
Ingestion rate soil from direct contact in
g soil/day
Flag for blanching of vegetables
Flags canning sequence. If ICAN=1, the
vegetable-exposure risk calculation will
include the effects of storage of raw
vegetable, washing, blanching, canning,
storage of cans and cooking.
Flag for vegetable canning alone
Flag for cooking of vegetables and meat
Flags freezing sequence. If IFREE=1, the
vegetable-exposure risk calculation will
include the effects of storage of raw
vegetable, washing, blanching freezing,
frozen storage and cooking.
Flag for vegetable freezing alone
Flag for pasteurization of milk
Flag for storage of vegetables before
processing. When ISTRH=1, the effects
of storage on pathogen population are
included in the vegetable-exposure risk
calculation. When ISTRH=0, these effects
are not included.
A-8
-------
TABLE A-4 (Continued)
Variables and Default Values for Subroutine RISK
Position
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Name
ISTRP
IWASH
PASTE
PASTA
PASTP
TEMI2
TEMI4
TEMP2
TEMP4
TMIS2
TMIS4
TMP2M
TMP7F
TMP7N
TSTM2
Default
Value
0
1
l.E-9
0.001
0.001
4
4
7
0
24
24
4
-4
20
720
Definition
Flag for storage of processed vegetables
Flag for washing of vegetables
Survival of Salmonella after pasteurization
Survival of Ascaris after pasteurization
Survival of enterovirus after
pasteurization
Temperature (°C) of milk storage before
pasteurization
Temperature (°C) of milk storage after
pasteurization
Temperature (°C) of storage of vegetables
before processing
Temperature (°C) of storage of frozen
meat
Time (hours) of milk storage before
pasteurization
Time (hours) of milk storage after
pasteurization
Temperature (°C) of storage of meat after
slaughter
Temperature (°C) of storage of frozen
foods
Temperature (°C) of storage of canned
foods
Duration in hours of meat storage
30
TSTR2
168
(unfrozen between slaughter and
freezing)
Duration (hours) of vegetable storage
before processing
A-9
-------
TABLE A-4 (Continued)
Variables and Default Values for Subroutine RISK
Position
Name
Default
Value
Definition
31
32
33
34
35
TSTR4
TSTR7
VOLPND
XDIST
YDIST
120
720
1.E2
200
0
Time of storage of frozen meat
Duration of vegetable storage after
processing
Volume (m^) of runoff pond
Distance (in meters) downwind between
particulate source and exposed individual
Lateral distance (in meters) of human
receptor from a line directly downwind of
aerosol source
A-10
-------
TABLE A-5
Variables and Default Values for Subroutine RAINS
Position
2
3
4
5
6
7
8
9
10
11
12
13
14
Name
PDUR
PTOT
BTLAG
CN
AMC
STAD
USLEK
USLEL
USLES
USLEC
USLEP
PI
»
WSOIL
Default
Value
2
5.0
0.5
80
2
0.4
0.4
3.0
0.25
0.5
1.0
SUSPND
1.33
Function
Duration of rainfall (hours)
Total rainfall (cm)
Basin time lag (hr)
Curve number
Antecedent moisture conditions
Storm advancement coefficient
USLE K value (soil credibility
factor)
USLE L value (slope length factor)
USLE S value (slope steepness factor)
USLE C value (cover management factor)
USLE P value (supporting practices)
Pathogen suspension factor
Bulk density of soil (g/cmr)
A-ll
-------
TABLE A-6
Variables and Default Values for Subroutine GRDWTR
Position
2
3
4
5
6
7
8
9
10
11
12
Name
CA
V
D
R
DZERO
DONE
DBND
ALPHA
XI
DX
XM
DT
Default
Value
FRGRND*N(8)
3.6
60
1.0
0
0
0
0.012
50
50
50
1
Function
Initial number of organisms
Velocity of groundwater (cm/hr)
Dispersion coefficient (cm^/hr)
Retardation coefficient
Exponential growth rate
Exponential inactivation rate
Decaying input concentration
Exponential decay of input (per hr)
Starting distance (m) from source
Distance increment (m) in calculation
Maximum distance (m) from source
Time increment (hr) in calculation
A-12
-------
APPENDIX B. OPERATIONS GUIDE
B-l
-------
PATHOGEN RISK ASSESSMENT FOR LAND APPLICATION
OF MUNICIPAL SLUDGE: COMPUTER MODEL OPERATIONS GUIDE
VERSION 3.1, OCT. 1989
** WARNING **
THIS PROGRAM REQUIRES AT LEAST 540K FREE RAM.
** WARNING **
YOUR CONFIG.SYS FILE MUST BE WRITTEN
TO ALLOW AT LEAST 20 OPEN FILES.
The system default configuration may be 8 simultaneously openfiles.
If so, the program will crash, and it will be necessary to
use a text editor or line editor to add the line
FILES=20 to CONFIG.SYS, and then re-boot the computer.
This program does not require a math coprocessor, but it will run significantly faster if
one is present.
In this operations guide, file names will be printed in UNDERLINED CAPITALS.
Prompts written by the program to the monitor screen will be printed in CAPITALS.
The user's responses will be printed in BOLD CAPITALS, and responses for which the
user must choose a variable or file name are enclosed in .
Function keys will be identified by parentheses (ENTER), (CONTROL). In some cases
responses will be identified by equation numbers (flush right).
This program requires input information to specify over 100 variables and usage options.
Some of these data must be entered in response to prompts that will appear on the
screen of your monitor. Others have default values written into the program. You will
be given the opportunity to change the values of these variables during the input phase
of the program. Default values are listed in Appendk A, Tables A-l through A-6 of
Pathogen Risk Assessment for Land Application of Municipal Sludge, bothj/olume I:
ipu ---- ---- — ._- - ..__..
through A-12 of the User's Manual; tables A-l through A-6 are identical in both
<\ppli(
Methodology and Computer Model and Volume II: User's Manual, and in Tables A-7
through A-12 of the User's Manual; tables A-l through A-6 are identical in b
documents. More details about the variables can be found in the User's Manual.
1. GETTING STARTED
Because the program is too large to fit on a low-density floppy disk, it has been
compressed. To install the program, insert the program disk in drive A and type
A:\INSTALL (ENTER). A batch file on the program disk will create a directory named
\RISKMOD on drive C and expand the program as it is copied to that directory. A
procedure for testing the program is described below in Section 4. INPUT FILES. The
output files should match those in the appendix to this guide.
Gather any information you have that is relevant to the specific case you intend to
model. This information will include the concentration of pathogens in the sludge
(organisms/kg dry wt), size of the area to be considered, frequency and depth of
rainfall, etc. You must enter:
B-2
-------
NAME OF OUTPUT FILE
LENGTH OF MODEL RUN
1)
35)
36
ASCRS
TMAX
TMIN
The following P-variables should be changed to reflect the characteristics of the
particular situation you want to model:
Practice-specific values
6 APMETH 68 TCROP
17 IRMETH 69 THARV
18 DILIRR 74 PLNT1
19 NIRRIG 75 PLNT2
20 IRRATE 76 PLNT3
21 DEPTH 78 CATTLE
30 COVER 79 COWS
66 CROP 80 STORAG
67 TCULT 84 FATTEN
85 TSLOTR
Site-specific values
2 APRATE
7 AREA
9 AQUIFR
10 POROS
11 FILTR8
Environmental values
13 TRAIN
23 TWIND
24 DWIND
25 WINDSP
32 BREEZE
Be sure that you are in the directory containing the program or that the computer's
PATH statement includes that directory. It is not necessary to have a printer attached
to the computer doing the model runs or to print the results immediately. However, if
the output file is to be printed later, either from the same computer or from a disk
file, be sure that the output file name (1-1) is different for each model run.
You will be asked to respond with values of input variables at several points at the
beginning of the program. After each response press the (ENTER) or (RETURN) key.
To begin the model run, enter
RISK
1.1. OUTPUT FILE
After printing a title page, the monitor screen will respond
DO YOU WANT TO ENTER VALUES FROM
THE KEYBOARD OR FROM A FILE?
(ENTER "K" OR "F")
If your response is F, you will be prompted:
ENTER THE NAME OF THE INPUT FILE.
B-3
-------
Enter the name of the input file, which contains the parameters required for that
sample run. (A sample input file is included below in Section 4.) The screen output
described in the remainder of Section 1 will scroll past, but you will not need to
respond to the prompts.
If your response is K, you will be prompted:
ENTER A NAME FOR THE OUTPUT FILE.
YOU MAY USE UP TO 8 CHARACTERS.
Enter the filename
(1-1)
You must supply the filename. Otherwise, the program will not continue.
1.2. TIME PARAMETERS
You must specify the time parameters for your model run. There are no default values
for these parameters. For practices that involve a harvested crop, it is likely that the
model run will be longer than the time required to grow the crop to harvesting, but
shorter model runs to determine immediate effects of sludge application are also possible
and appropriate.
You will be prompted on the monitor screen:
*** SLUDGE PATHOGEN MODEL ***
ENTER VALUES FOR THE FOLLOWING
(PRESS RETURN AFTER EACH):
1. END TIME OF PRACTICE IN DAYS
Enter the number of days you want in the model run, e.g.,
20.
2. PRINTING SAMPLE RATE IN HOURS
The number of pathogens in each compartment at the time interval specified by this
response will be printed in a file named PATHOUT. This file is useful if you want to
see the pathogen number in each compartment at the specified time interval.
Otherwise, any number will do, and large numbers (e.g., 24) will make for smaller files
on your hard disk.
1.3. MODEL SLUDGE USE PRACTICE
You must choose which of the sludge application practices you want to model. These
B-4
-------
practices are described in the descriptive document Pathogen Risk Assessment for Land
Application of Municipal Sludge: Volume I: Methodology and Computer Model. There is
no default value for this entry.
You will be prompted on the monitor screen:
PROVIDE PRACTICE NUMBER
YOUR CHOICES ARE:
1 APPLICATION OF LIQUID SLUDGE FOR PRODUCTION
OF COMMERCIAL CROPS FOR HUMAN CONSUMPTION
2 APPLICATION OF LIQUID SLUDGE TO
GRAZED PASTURE
3 APPLICATION OF LIQUID SLUDGE FOR PRODUCTION
OF CROPS PROCESSED FOR ANIMAL CONSUMPTION
4 APPLICATION OF DRIED OR COMPOSTED SLUDGE
TO RESIDENTIAL GARDENS
5 APPLICATION OF DRIED OR COMPOSTED SLUDGE
TO RESIDENTIAL LAWNS
Enter the appropriate number.
1.4. PATHOGEN TYPE AND INITIAL POPULATIONS IN COMPARTMENTS
You must choose the type of pathogen for the model run. Only one pathogen type can
be modeled during each model run. There is no default value for this entry.
You will be prompted on the monitor screen:
PROVIDE PATHOGEN TYPE
YOUR CHOICES ARE:
1 SALMONELLA (BACTERIA)
2 ASCARIS (PARASITE)
3 ENTEROVIRUS (VIRUS)
Enter 1, 2, or 3.
The monitor screen will respond:
B-5
-------
THE VARIABLES LISTED BELOW MAY BE OPTIONALLY (1-2)
CHANGED FROM THEIR DEFAULT VALUES:
POPL(l-22) (INITIAL PATHOGEN POPULATIONS
OF COMPARTMENTS IN PRACTICE.
[SEE MANUAL FOR DESCRIPTIONS.]
DEFAULT=0.0)
1. TYPE THE NUMBER "1" TO ACCEPT THE CURRENT
VALUES AND CONTINUE WITH THE PROGRAM.
2. TYPE THE NUMBER "2" IF YOU WISH TO LOAD A
PATHOGEN POPULATION INTO A COMPARTMENT.
Enter 1 to accept the default condition, which starts the model with no pathogens in
any compartment. To simulate a starting condition in which some compartments have an
initial pathogen load, enter 2. You will be prompted:
PROVIDE POPL SUBSCRIPT IN THE RANGE OF 1-22.
Enter the number of the compartment you want to modify. The monitor screen will
respond:
PROVIDE NEW VALUE OF THIS POPULATION.
Enter the appropriate number. The program will loop back to the prompt at line (1-2),
allowing you to alter each compartment until you respond with 1, indicating that you
are satisfied with the initial states of the compartments.
1.5. INITIAL PATHOGEN CONCENTRATION IN SLUDGE
You must provide the initial concentration of pathogens in the sludge to be applied.
There is no default value for this entry.
You will be prompted on the monitor screen:
PROVIDE SLUDGE PATHOGEN DENSITY (NUMBER OF PATHOGENS
PER KILOGRAM (DRY WEIGHT) OF APPLIED SLUDGE)
ENTER A FORM SUCH AS 100000 OR 1E5
Enter the concentration of pathogens of the kind you specified. The concentration must
be in the units of number per kg dry weight, and in either typed-out or scientific
notation form. Do not use commas or spaces. If you do not know the concentration of
pathogens in the sludge, you can use the typical values taken from Table A-3.
However, the results will be more useful if values descriptive of the specific sludge
batch are entered.
B-6
-------
1.6. PATHOGEN REDUCTION REQUIREMENTS
U.S. EPA Pathogen Reduction Requirements require a waiting period between sludge
application and land use. The waiting period depends on both the class of sludge
treatment and the type of land use. For a description of sludge treatment classes, see
Section 4.3 of the User's Manual. To establish these waiting periods, you will be asked
to enter the time at which sludge application ends:
WHEN DOES SLUDGE APPLICATION CEASE (DAYS)?
(FOR A SINGLE APPLICATION, ENTER "0".)
Enter the appropriate number. If the option of irrigating with liquid sludge is to be
used, enter the day on which the use of sludge as irrigation water will be ended. The
monitor will next respond:
LAND ACCESS RESTRICTIONS VARY WITH THE CLASS
OF SEWAGE TREATMENT. WHAT CLASS APPLIES TO
THIS MODEL RUN? (ENTER A, B OR C).
Enter the appropriate letter.
1.7. ANNUAL TEMPERATURE CYCLE
Because die-off rates are dependent on ambient temperature, the model uses an annual
air temperature cycle in calculations of process functions. The calculated air
temperature during the model run depends on the time of year and the extremes of
monthly average air temperature at the location being modeled. (For details see the
User's Manual, Section 4.4). To obtain the necessary data for these calculations, the
program will prompt you:
WHEN DOES THE PRACTICE BEGIN?
MONTH (1-12):
DAY (1-31):
Enter the number of the month and the day. You will then be prompted:
WHAT IS THE LOCAL AIR TEMPERATURE RANGE?
JANUARY AVERAGE AIR TEMP. (DEG C):
JULY AVERAGE AIR TEMP. (DEG C):
Enter the monthly average temperatures for these months to serve as minimum and
maximum temperatures for the temperature cycle calculation. Average minimum and
maximum temperatures at several locations in the United States can be found in Table
A-9 of Volume II: User's Manual. In this table, temperatures for January and July are
given in °C as well as in °F.
B-7
-------
1.8. MODEL VARIABLES
The majority of the variables used by the program are specified at this step. These
variables provide the operating conditions for most of the model calculations, so they
should be chosen to describe as accurately as possible the conditions you want to model.
You will be prompted on the monitor screen:
THE DEFAULT VALUES FOR MODEL PARAMETERS
DEPEND ON THE PRACTICE AND PATHOGEN CHOSEN.
YOU HAVE CHOSEN PRACTICE 1
AND PATHOGEN 1 SALMONELLA
PRESS RETURN TO CONTINUE
(1-3)
THE CURRENT VALUES ARE:
POSITION VARIABLE CURRENT VALUE
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
PRESS
CHANGED
BY USER
ASCRS
APRATE
ASCIN
TREG
UPLIM
APMETH
AREA
TEMP
AQUIFR
POROS
FILTR8
MID
TRAIN
RDEPTH
TK
TIRRG
IRMETH
DILIRR
NIRRIG
IRRATE
l.OOOOOE+12
10000.
.00000
.00000
l.OOOOOE+09
1.0000
10.000
.00000
10.000
.30000
2.0000
10.000
-2.0000
5.0000
.00000
.00000
.00000
.00000
2.0000
.50000
RETURN TO CONTINUE
B-8
-------
POSITION VARIABLE CURRENT VALUE
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
PRESS
DEPTH
COUNT
TWIND
DWIND
WINDSP
EPSMLT
ESILT
EHT
SCRIT
COVER
AEREFF
BREEZE
HT
ANDAY
TMAX
TMIN
SLOPES
NTRCPS
SLOPEP
NTRCPP
CHANGED
BY USER
2.5000
.00000
60.000
6.0000
18.000
.33000
.40000
2.0000
7.5000
.00000
l.OOOOOE-03
4.0000
1.6000
345.00
30.000
.00000
2.06000E-02
2.1130
4.49000E-03
1.4350
RETURN TO CONTINUE
POSITION VARIABLE CURRENT VALUE
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
PRESS
ASLSUR
FSSUR
FRRAIN
SUBSOL
SUSPND
FCROP1
FCROP2
FCROP3
FCROP4
FCROP5
FCROP6
FCROP7
FRGRND
SSWTCS
PSTMG
CSTSS
SSTCS
CSTSSW
DTCTMT
DTCTMK
CHANGED
BY USER
RETURN TO CONTINUE
.90000
1.0000
.00000
5.00000E-04
5.00000E-03
l.OOOOOE-08
6.00000E-03
l.OOOOOE-03
l.OOOOOE-04
1.20000E-02
.00000
2.00000E-04
l.OOOOOE-03
.00000
.00000
.00000
.00000
.75000
.00000
.00000
B-9
-------
POSITION VARIABLE CURRENT VALUE CHANGED
BY USER
61 TMTSS .00000
62 TMTH .00000
63 TMTU .00000
64 HTM .00000
65 UTM .00000
66 CROP 1.0000
67 TCULT -2.0000
68 TCROP 720.00
69 THARV 1800.0
70 YIELD1 2.50000E+07
71 YIELD2 2.50000E+07
72 YIELDS l.OOOOOE+06
73 HAY .00000
74 PLNT1 .00000
75 PLNT2 .00000
76 PLNT3 .00000
77 PPG .00000
78 CATTLE .00000
79 COWS .00000
80 STORAG .00000
PRESS RETURN TO CONTINUE
POSITION VARIABLE CURRENT VALUE CHANGED
BY USER
81 FORAG .00000
82 ALFALF .00000
83 SCNSMP .00000
84 FATTEN .00000
85 TSLOTR .00000
PRESS RETURN TO CONTINUE
1. TYPE THE NUMBER "1" IF YOU WISH TO ACCEPT THE
CURRENT VALUES AND CONTINUE WITH THE PROGRAM.
2. PROVIDE P SUBSCRIPT IN THE RANGE OF 2-85
IF YOU WISH TO CHANGE A PARAMETER OF THE MODEL.
TYPE "99" IF YOU NEED TO SEE THE LIST AGAIN
Variables are described in the User's Manual. Default values for the variables in each
practice are listed in Tables A-l and A-2. To accept all of the default values, enter 1.
To change any of these values, enter the number of the variable to be changed (given
under the heading Position in the table). Except for variable 13 (RAINS), you will be
prompted:
PROVIDE NEW VALUE OF THIS P.
B-10
-------
Enter the appropriate number. The program will then loop back to (1-3) until you enter
1, indicating that you are satisfied with the values of the model variables. Except for
variable 13, each time the sequence starting at (1-3) is printed, the new value will be
printed at the appropriate location, and an asterisk will appear in the column labeled
CHANGED BY USER.
1.9. RAINFALL
If you entered 13 to include rainfall in the model run, SUBROUTINE RAINS will be
called, and the prompt will be:
GIVE THE NUMBER OF RAIN EVENTS (BETWEEN 1 & 10)
Enter the appropriate number. The monitor will respond:
GIVE START TIME (IN HRS) OF RAIN EVENT NO. 1 (1-4)
Enter the starting time of the first rainfall. Time is measured from the beginning of
the model run (time=0 at the beginning of sludge application in Compartment I), so a
starting time of 246 hours would call for rain on the llth day. The monitor will
respond:
RAIN NO. 1 AT ##.## (HRS) (1-5)
THE PARAMETERS FOR PATHOGEN TRANSPORT BY SURFACE WATER
MAY BE OPTIONALLY CHANGED FROM THEIR DEFAULT VALUES:
NUMBER PARAMETER CURRENT VALUE
2 PDUR 2.00
3 PTOT 5.00
4 BTLAG .50
5 CN 80.00
6 AMC 2.00
7 STAD .40
8 USLEK .40
9 USLEL 3.00
10 USLES .25
11 USLEC .50
12 USLEP 1.00
13 PI .00
14 WSOIL 1.33
PRESS RETURN TO CONTINUE
1. TYPE "1" TO ACCEPT THE CURRENT VALUES
FOR RAIN NO. 1 AND PROCEED TO NEXT RAIN.
2. TYPE THE NUMBER ("2"-"14") CORRESPONDING TO THE
PARAMETER THAT YOU WANT TO CHANGE.
B-ll
-------
Default values for these parameters are listed in Table A-5 and explained in Section 5.3
of Volume II: User's Manual. If you wish to change any of the parameters from their
default values, enter the appropriate variable number. You will be prompted to enter a
new value for the variable. The program will then loop back to (1-5) until you enter 1,
indicating that you are satisfied with the values of the surface runoff and sediment
transport parameters for that rainfall. The monitor will respond:
ARE YOU DONE WITH RAIN NO. 1 ? (Y/N)
If you have made a mistake or want to change a variable enter N, and the program will
loop back to (1-5). If you are satisfied with the values of the variables for that
rainfall, enter Y to proceed. The program will loop back to (1-4) for each additional
rainfall you specified ("RAIN EVENT NO. 2", "RAIN EVENT NO. 3", etc.), and then back
to (1-3) until you enter 1, indicating that you are satisfied with the values of the
variables. The response will be:
CURRENT VALUES ACCEPTED
1.10. OPTIONAL PROCESS FUNCTIONS
Process functions (growth or inactivation rates) are included in the program as defaults.
Default inactivation rates for Salmonella. Ascaris and enterovirus in moist soil and
Salmonella in dry particulates are temperature dependent, whereas inactivation rates of
Salmonella. Ascaris and enterovirus in water and in aerosols are variables whose defaults
are listed in Table A-12 (for more information see Volume II: User's Manual. Section
4.4).
You will see:
OPTIONAL GROWTH OR INACTIVATION RATES MAY BE ENTERED
TO REPLACE THE DEFAULT VALUES (SEE THE USER MANUAL
FOR A DESCRIPTION OF THE DEFAULT CONDITIONS).
1. ACCEPT VALUES
CURRENT VALUE
2. PROC1 - MOIST SOIL -5.000000
3. PROC2 - DRY PARTICULATES -5.000000
4. PROC3 - WATER SUSPENSION -5.000000
5. HCRIT - DROPLET AEROSOL -2.800000E-02
IF YOU WISH TO CHANGE THE VALUE OF A VARIABLE, ENTER
THE NUMBER. TO ACCEPT THE CURRENT VALUES, ENTER "1".
B-12
-------
You may choose growth or inactivation rates that will override the default conditions
for these variables. Except for aerosols, rates should be entered as the logarithm (base
10) of the fractional survival or growth after one hour (e.g., an inactivation rate
resulting in 10% survival after one hour would be entered as -1). For aerosols (HCRIT),
the rate should be entered as the logarithm of fractional survival after 1 minute.
DO NOT ENTER -5. This is the internal flag setting indicating that default conditions
are to be used. You may use -5.0001 or -4.9999 or any other number close to -5. The
monitor will respond:
CURRENT VALUES ACCEPTED.
1.11. INFECTION RISK PARAMETERS
Infection risk parameters are used in the exposure calculations. Most have to do with
food processing and storage, but variables 4 and 5 describe soil and crop ingestion, and
31-34 are physical parameters for pond size and for exposure to particulate and liquid
aerosols.
You will be prompted on the monitor screen:
THE INFECTION RISK PARAMETERS LISTED BELOW MAY BE
OPTIONALLY CHANGED FROM THEIR DEFAULT VALUES.
(1-6)
NUMBER PARAMETER VALUE NUMBER PARAMETER
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
PRESS
COOKA
COOKP
COOKS
DRECTC
DRECTS
IBLAN
ICAN
ICANG
ICOOK
IFREE
IFREG
IP AST
ISTRH
ISTRP
IWASH
PASTE
PASTA
RETURN TO
.000
.000
.000
.100
.100
.000
.000
.000
.000
.000
.000
.000
1.000
.000
1.000
.000
.001
CONTINUE
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
PASTP
TEMI2
TEMI4
TEMP2
TEMP4
TMIS2
TMIS4
TMP2M
TMP7F
TMP7N
TSTM2
TSTR2
TSTR4
TSTR7
VOLPND
XDIST
YDIST
1. TYPE "1" TO ACCEPT THE CURRENT VALUES
AND CONTINUE WITH THE PROGRAM.
VALUE
.001
4.000
4.000
7.000
.000
24.000
24.000
4.000
-4.000
20.000
720.000
168.000
120.000
720.000
100.000
200.000
.000
2. TYPE THE NUMBER ("2" - "35") CORRESPONDING TO
THE PARAMETER THAT YOU WISH TO CHANGE.
B-13
-------
Default values and definitions for these parameters are listed in Table A-4. If you wish
to change any of the parameters from their default values, enter the appropriate
variable number. You will be prompted to enter a new value for the variable. The
program will then loop back to (1-6), printing new values of variables, until you enter
1, indicating that you are satisfied with the values of the risk parameters. The monitor
will then display a summary of the initial conditions chosen for the model run.
1.12. SUBSURFACE TRANSPORT FOLLOWING RAINFALL AND IRRIGATION
You may specify values of the variables used in subsurface transport calculations.
You will be prompted on the monitor screen:
THE PARAMETERS FOR PREDICTING VIRAL AND BACTERIAL (1-7)
TRANSPORT IN GROUNDWATER AFTER LAND APPLICATION OF
SEWAGE SLUDGE MAY BE OPTIONALLY CHANGED FROM THEIR
DEFAULT VALUES.
NUMBER PARAMETER CURRENT VALUE
2 V 3.600
3 D 60.000
4 R 1.000
5 DZERO .000
6 DONE .000
7 DBND .000
8 ALPHA .012
9 XI 50.000
10 DX 50.000
11 XM 50.000
12 DT 1.000
PRESS RETURN TO CONTINUE
1. TYPE "1" TO ACCEPT THE CURRENT VALUES.
2. TYPE THE NUMBER ("2"-" 12") CORRESPONDING TO THE
PARAMETER THAT YOU WANT TO CHANGE.
Definitions and default values for these parameters are listed in Table A-6 and described
in Section 5.4 of Volume II: User's Manual. If you wish to change any of the
parameters from their default values, enter the appropriate variable number. You will
be prompted to enter a new value for the variable. The program will then loop back to
(1-7), printing the new values, until you enter 1, indicating that you are satisfied with
the values of the subsurface transport parameters.
B-14
-------
2. RUNNING
When the computations begin, the following messages will appear on the screen:
At the first iteration of Practices 1, 2 and 3,
RUNNING . . .
PROBABILITY OF INFECTION
DAY ONSTTEOFFSITE EATER DRINKER SWIMMER
of Practice 4,
RUNNING . . .
PROBABILITY OF INFECTION
DAY ONSITE OFFSITE EATER
or of Practice 5,
RUNNING . . .
PROBABILITY OF INFECTION
DAY ONSITE OFFSITE
After each day's simulation, the number of the day and the probability of infection for
the day will be printed on the screen. After every 20 days' simulation the headings will
be printed again. A copy of this output will appear in the file specified at the
beginning of the model run.
Whenever the subsurface transport subroutine is called, there will be a delay in printing
the output data. This first occurs during day 2 under most circumstances. If the
computer does not have a math coprocessor, the delay may be as much as 15 seconds
per day of the model run, so don't be alarmed if nothing seems to be happening.
The model will run until the day specified unless the number of pathogens in each
compartment falls to 0, in which case the program will be terminated to save
computation time. The response on the monitor will be:
RUN TERMINATED BECAUSE ALL COMPARTMENTS = 0
At the end of the model run, the monitor will respond:
. . . RUN COMPLETE.
B-15
-------
3. RECOVERING THE DATA
After the run has been completed, you will see the reminder,
YOU WILL FIND THE INFECTION PROBABILITY OUTPUT FROM
THIS RUN IN THE FILE YOU SPECIFIED.
To view the results, you can use a word processor or text editor, or use the TYPE
command:
TYPE .
The file will then scroll up the monitor screen. You can stop its progress by typing
(CONTROL)S. It will start again if you strike any key.
To print the results, use the print functions of a word processor or text editor, or type
(CONTROL)P to activate the printer echo mode of your computer, and then
TYPE .
The file should then be printed. This file contains the probability of infection for each
day. The contents of other files are summarized below.
File Name
Description
User Provides Contains a summary of input values and the risk of infection
for each 24 hour period. The name of the file is provided by
the user at the second prompt after invoking the model.
EXOUT Output from the groundwater transport routine. Contains the
parameters used by the subroutine and the composite contents
of the output compartment at each specified distance increment
and for each time interval during the model run.
PATHOUT Contains the number of pathogens in each compartment at the
time interval specified by the user.
RAINS Output from the RAINS subroutine.
B-16
-------
4. INPUT FILES
The model can be run by use of an input file in place of interactive keyboard responses
to prompts. An input file can be created by using a word processor or line editor to
enter all of the appropriate responses in sequence. This file is saved under a file name
(INPUT.IN for this example). The input file on the distribution disk contains several
unrealistic values; however, these values were chosen to allow a larger number of
positive results in the short time of the test model run.
keyboard operation blank lines are entered in response to the prompt
TO CONTINUE. With an input file, however, there must be a character
During interactive
PRESS RETURN'
on the line containing that return. In this example, that dummy character is X. Any
character string of up to four characters can be used. The values in this file are:
TEST
20
8
1
1
1
1E12
0
A
4
1
0
30
X
X
X
X
X
X
68
140
X
X
X
X
X
X
69
180
X
X
X
X
X
X
13
1
120
Title)
Length of run)
Print sample rate)
Practice number)
Pathogen type)
Initial compartment populations)
Pathogen concentration)
Day sludge application ends)
Class of sludge treatment)
Month model begins)
Day model begins)
Minimum average temperature)
Maximum average temperature)
Dummy character to continue 1
Dummy character to continue 2
Dummy character to continue 3
Dummy character to continue 4
Dummy character to continue 5
Dummy character to continue 6
P-value for TCROP)
New value for TCROP)
Dummy character to continue 1
Dummy character to continue 2
Dummy character to continue 3
Dummy character to continue 4
Dummy character to continue 5
Dummy character to continue 6
P-value for THARV)
New value for THARV)
Dummy character to continue 1
Dummy character to continue 2
Dummy character to continue 3
Dummy character to continue 4
Dummy character to continue 5
Dummy character to continue 6
P-value for rainfall)
Number of rainfalls)
Time at which rain begins)
B-17
-------
X
1
Y
X
X
X
X
X
X
1
1
X
1
X
X
9
12
X
10
1
X
11
12
X
1
(Dummy character to continue)
(Accept default values)
(Confirm rainfall values)
(Dummy character to continue 1
(Dummy character to continue 2
(Dummy character to continue 3
(Dummy character to continue 4
(Dummy character to continue 5
(Dummy character to continue 6
(Accept optional P-variables)
(Accept die-off rate parameters)
(Dummy character to continue)
(Accept default risk variables)
(Dummy character to continue^
(Dummy character to continue)
(Position of variable XI)
(New value for XI)
(Dummy character to continue)
(Position of variable DX)
(New value for DX)
(Dummy character to continue)
(Position of variable XM)
(New value for XM)
(Dummy character to continue)
(Accept current variables)
The model run is begun by entering
RISK
F
INPUT.IN
The input data will automatically be supplied to the program from the input file.
If a number of successive runs are to be made, they may best be invoked from a batch
file. In this case, if you want to see the PATHOUT and EXOUT files, use the batch
file to rename them before the next run is invoked. For example, a batch file to run
the model three times, using input files and saving the PATHOUT and EXOUT files
might be:
RISK < RISK1.BAT
RENAME PATHOUT PATHOUT1
RENAME EXOUT EXOUT1
RISK < RISK2.BAT
RENAME PATHOUT PATHOUT2
RENAME EXOUT EXOUT2
RISK < RISKS .BAT
RENAME PATHOUT PATHOUT3
RENAME EXOUT EXOUT3
where RISK1.BAT, RISK2.BAT and RISK3.BAT are batch files of the form
B-18
-------
F
INPUT1.IN.
The intermediate batch files (RISK1.BAT, etc.) are small enough not to exceed the size
limit for redirected standard input. The INPUTn.IN files supply the specific information
for each model run as described in the sample input file.
5. SAMPLE OUTPUT
The output file from the test run described above should look like the following:
TEST
PRACTICE STOP TIME= 20 DAYS
PRINT SAMPLING RATE - IPRNT = 8 HOURS
PRACTICE NUMBER = 1
PATHOGEN = 1 SALMONELLA
NUMBER OF COMPARTMENTS THIS PRACTICE = 16
INITIAL POPULATIONS FOR COMPARTMENTS:
COMPARTMENT 1 = O.OOOOE+00
COMPARTMENT 2 = O.OOOOE+00
COMPARTMENT 3 = O.OOOOE+00
COMPARTMENT 4 = O.OOOOE+00
COMPARTMENT 5 = O.OOOOE+00
COMPARTMENT 6 = O.OOOOE+00
COMPARTMENT 7 = O.OOOOE+00
COMPARTMENT 8 = O.OOOOE+00
COMPARTMENT 9 = O.OOOOE+00
COMPARTMENT 10 = O.OOOOE+00
COMPARTMENT 11 = O.OOOOE+00
COMPARTMENT 12 = O.OOOOE+00
COMPARTMENT 13 = O.OOOOE+00
COMPARTMENT 14 = O.OOOOE+00
COMPARTMENT 15 = O.OOOOE+00
COMPARTMENT 16 = O.OOOOE+00
SLUDGE PATHOGEN DENSITY = l.OOOOE+12 NUMBER/KG
RAIN EVENT NO. 1 STARTING AT 120.00 HRS
B-19
-------
THESE RESULTS ARE BASED ON THE FOLLOWING INPUT PARAMETERS
PRACTICE:
PATHOGEN:
1
1 SALMONELLA
THE DEFAULT VALUES AND THOSE CHANGED BY THE USER ARE
LISTED BELOW.
POSITION VARIABLE ACCEPTED VALUE
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
ASCRS
APRATE
ASCIN
TREG
UPLIM
APMETH
AREA
TEMP
AQUIFR
POROS
FILTR8
MID
TRAIN
RDEPTH
TK
TIRRG
IRMETH
DILIRR
NIRRIG
IRRATE
DEPTH
COUNT
TWIND
DWIND
WINDSP
EPSMLT
ESILT
EHT
SCRIT
COVER
AEREFF
BREEZE
HT
ANDAY
TMAX
TMIN
SLOPES
NTRCPS
SLOPEP
NTRCPP
ASLSUR
FSSUR
l.OOOOOE+12
10000.
.00000
.00000
l.OOOOOE+09
1.0000
10.000
.00000
10.000
.30000
2.0000
10.000
-2.0000
5.0000
.00000
.00000
.00000
.00000
2.0000
.50000
2.5000
.00000
60.000
6.0000
18.000
.33000
.40000
2.0000
7.5000
.00000
l.OOOOOE-03
4.0000
1.6000
345.00
30.000
.00000
2.06000E-02
2.1130
4.49000E-03
1.4350
.90000
1.0000
CHANGED
BY USER
B-20
-------
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
FRRAIN
SUBSOL
SUSPND
FCROP1
FCROP2
FCROP3
FCROP4
FCROP5
FCROP6
FCROP7
FRGRND
SSWTCS
PSTMG
CSTSS
SSTCS
CSTSSW
DTCTMT
DTCTMK
TMTSS
TMTH
TMTU
HTM
UTM
CROP
TCULT
TCROP
THARV
YIELD 1
YIELD2
YIELDS
HAY
PLNT1
PLNT2
PLNT3
PPG
CATTLE
COWS
STORAG
FORAG
ALFALF
SCNSMP
FATTEN
TSLOTR
.00000
5.00000E-04
5.00000E-03
l.OOOOOE-08
6.00000E-03
l.OOOOOE-03
l.OOOOOE-04
1.20000E-02
.00000
2.00000E-04
l.OOOOOE-03
.00000
.00000
.00000
.00000
.75000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
1.0000
-2.0000
140.00
180.00
2.50000E-f07
2.50000E+07
l.OOOOOE+06
.00000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
.00000
B-21
-------
PROCESS VARIABLES FOR DIE-OFF OF PATHOGENS
VARIABLE VALUE
PROC1 - MOIST SOIL -5.000000
PROC2 - DRY PARTICULATES -5.000000
PROC3 - WATER SUSPENSION -5.000000
HCRIT - DROPLET AEROSOL
-2.800000E-02
THE INFECTION RISK PARAMETERS USED
IN THIS MODEL RUN ARE:
NUMBER
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
PARAMETER
COOKA
COOKP
COOKS
DRECTC
DRECTS
IBLAN
ICAN
ICANG
ICOOK
IFREE
IFREG
IP AST
ISTRH
ISTRP
IWASH
PASTB
PASTA
VALUE
.000
.000
.000
.100
.100
.000
.000
.000
.000
.000
.000
.000
1.000
.000
1.000
.000
.001
NUMBER PARAMETER
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
PASTP
TEMI2
TEMI4
TEMP2
TEMP4
TMIS2
TMIS4
TMP2M
TMP7F
TMP7N
TSTM2
TSTR2
TSTR4
TSTR7
VOLPND
XDIST
YDIST
VALUE
.001
4.000
4.000
7.000
.000
24.000
24.000
4.000
-4.000
20.000
720.000
168.000
120.000
720.000
100.000
200.000
.000
THE PARAMETERS CHOSEN FOR GROUNDWATER TRANSPORT ARE:
NUMBER PARAMETER
2
3
4
5
6
7
8
9
10
11
12
V
D
R
DZERO
DONE
DBND
ALPHA
XI
DX
XM
DT
VALUE
3.600
60.000
1.000
.000
.000
.000
.012
12.000
1.000
12.000
1.000
B-22
-------
PROBABILITY OF INFECTION
DAY
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
ONSITE
O.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
9.889E-01
3.994E-02
2.885E-06
1.677E-11
2.220E-11
1.110E-16
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
OFFSITE
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
EATER
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
3.363E-04
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
DRINKER
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
2.220E-16
O.OOOE+00
O.OOOE+00
O.OOOE+00
7.183E-14
1.093E-11
1.079E-10
1.305E-10
3.828E-11
6.130E-12
1.208E-12
3.961E-13
SWIMMER
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
O.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
l.OOOE+00
U.S. GOVERNMENT PRINTING OFFICE 1990/748-159/00454
B-23
------- |