vyEPA
United States
Environmental Protection
Agency
Point Source Calculator: A Model for Estimating Chemical
Concentration in Water Bodies
Dirk F. Young and Alie Muneer
Office of Chemical Safety and Pollution Prevention
U.S. Environmental Protection Agency
Washington, D.C. 20460
June 2019
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Table of Contents
1. Abstract 1
2. Introduction 2
3. Conceptual Model 2
4. Model Inputs 5
5. Analysis and Post Processing 8
6. References 11
Appendix 1. The Variable Volume Water Model: Full Documentation 12
Appendix 2. User Guidance for the Point Source Calculator 52
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1. Abstract
The Point Source Calculator (PSC) is a tool designed to estimate acute and chronic
concentrations of chemicals directly applied to water bodies. Waterbodies may include flowing
waters like streams and river segments and more static waters like lakes and ponds. Direct
applications of chemical may be simulated in a flexible manner from simple to complex
repetitive events or as completely unique daily events defined on a daily scale. The PSC is a
graphical user interface which gathers the user's inputs and runs USEPA's Variable Volume
Water Model (VVWM). Required inputs are the same as those for the VVWM, but the PSC
graphical interface facilitates user interaction for the direct-application problem. Post processing
of the PSC is also relevant to the direct-application problem and includes the ability to analyze
concentrations in comparison to target concentrations of concern (CoC), including number of
days above the CoC and number of consecutive days above the CoC.
1
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2. Introduction
The Point Source Calculator (PSC) is a tool for estimating chemical exposure in surface
waters from point source discharge(s). The PSC is a user interface that processes input and
output for the Variable Volume Water Model (VVWM). The VVWM has been used by the
USEPA Office of Pesticide Programs in assessing pesticide aquatic exposure since 2008 and is a
major component of the USEPA Pesticide Water Calculator (PWC) (Young, 2019) and the
Pesticide in Flooded Applications Model (PFAM) (Young, 2012, 2013). Similarly, the Office of
Pollution Prevention and Toxics (OPPT) will now use the VVWM as the exposure calculating
tool for the PSC. Details of the VVWM are given in Appendix 1.
The PSC is like the PWC and PFAM in that it is a user-friendly interface that generates a
VVWM input file, runs the VVWM, and processes the data. The model's name (PSC) better
reflects that releases from effluent pipes of waste-water treatment plants or direct industrial
dischargers (point sources) are the releases of interest. The PSC was designed to meet the
specific needs of OPPT, which is to assess chemicals that flow directly into a water body from
point source discharges and to compare the modeled surface water and sediment chemical
concentrations to target concentrations of concern (CoCs). Thus, the PSC user interface and
input and output requirements are different than for PWC or PFAM.
3. Conceptual Model
The conceptualization of the processes in the PSC is shown in Figure 1. In this
conceptualization, the VVWM is used to represent a segment of a water body which receives a
direct application of a chemical. The chemical immediately mixes with the water column of the
segment. The water column is coupled to a sediment layer, and the chemical can move into the
sediment by a first-order mass transfer process. The fate and transport of the chemical can be
estimated by user-supplied inputs for water column degradation (e.g., metabolism, hydrolysis,
and photolysis), volatilization, benthic degradation (e.g., metabolism and hydrolysis) and
partitioning to suspended sediment and benthic solids (e.g., an organic carbon-normalized
Chemical Input Volatilization
Water Column
Inflow
Washout. Dispersion
Degradation due to:
metabolism, hydrolysis,
photolysis, etc.
Water-Column-to- Benthic
Mass Transfer
Degradation due to:
metabolism,
hydrolysis, etc.
Benthic Region
Figure 1. Depiction of the chemical processes in the PSC.
2
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partitioning coefficient (Koc) or an sorption distribution coefficient (Kd)). A more detailed
process description is given in the VVWM documentation in Appendix l.1
The waterbodies that can be modeled include flowing waters such as streams or rivers or
more static waters such as reservoirs or lakes. As shown in Figure 2, the waterbodies can be
located anywhere within a watershed, including stream segments high in the watershed or large
reservoirs at the watershed exit. In all cases, the waterbody is modeled as a single segment of
interest (comprising a water column and a benthic region), with the segment of relevance being
the one that receives the direct application of the chemical (Figure 3). For a flowing waterbody,
the dimensions are the actual width and depth of the water body, while the length should be
reflective of the dispersivity (length should equal twice dispersivity) of the flowing body. A good
starting value for length of a flowing waterbody may be around 30 meters as estimated from
dispersivity data from U.S. rivers and streams (Fisher et al., 1979). For a static or near-static
water body, the dimensions should be those of the actual water body averages.
1 The WWM is a computer routine used in several applications (e.g., PSC, PWC, PFAM). When used with previous
applications (e.g., PWC), the WWM has accepted input mass through runoff. The WWM, however, is also
capable of accepting point source inputs as required by the PSC. Specifically, the WWM function in the PSC is to
model releases from a point source discharge, usually the effluent pipe of a waste water treatment plant. Although
the WWM manual in Appendix 1 retains some references to the pesticide model for run-off (PRZM) as background
information, PSC is intended to be used for point source discharges and PRZM will not typically be used with PSC,
but PRZM output files could be useful for land-applied chemicals.
3
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o
Waterbody or segment of interest
Figure 2. Conceptualization of some possible wateibody locations for the PSC segments of interest.
Waterbodies could be stream segments high in the watershed or large reservoirs at the watershed exit.
4
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Segment of Relevance
FLOW
Figure 3. Depiction of the relevant segment of a flowing water addressed by the PSC.
4. Model Inputs
As shown in Figure 4, the PSC allows users to input the chemical properties of the
substance as well as mass inputs for the relevant segment, which can be specified by the user in a
variety of ways. Users can specify that chemical mass input occurs according to an on-off
schedule, according to a time series file, or as input from a PRZM5 file (Young and Fry, 2014).
Additionally, users may provide CoCs as shown in Figure 5 as points of comparison with the
estimated concentrations. Descriptions of waterbodies are shown in Figure 6, as depicted by the
PSC Scenario tab. Details of the various inputs can be found in Appendix 2.
5
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2 Point Source Calculator (version 1.02): Calculations fay the WWM
BP
File Scenario Point Source Calculator
5®!™^..! | Toxicity Scenario Results ¦ More Info j
Chemical ID
Sorption Coefficient (ml/g)
Water Column Halflife (days)
Photolysis Halflife (days)
Hydrolysis Halflife (days)
Benthic Halflife (days)
Volatilization
No Volatilization
Koc ®> Kd
@
@
@
°C
'Latitude
BC
eC
Estimate Henry's Const
Use Henry's Const
o
Molecular Weight
Vapor Pressure (torr)
Solubility (mg/L)
Henry's const (atm m3/mol)
Heat of Henry (J/mol)
Reference Temp (°C)
Mass Release Schedule
Use
Specify Mass
n
~
Use a Time Series File
~ Use PRZM5 Output File
Working Directory:
Outfile Family Name:
Offset Days On Days Off Mass (kg/day)
RUN
Figure 4. PSC inputs for chemical properties and mass inputs.
6
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jjj Point Source Calculator (version 1.02): Calculations by the WWM
File Scenario Point Source Calculator
Chemical i Toxicity j Scenario Results More Info
["71 Do Toxicity Analysis
Water Column Concentration of Concern (mq/L)
123
133
34
Acute (1 Day Avg)
2-Day Avg
3-Day Avg
4-Day Avg
7-Day Avg
21-Day Avg
28-Day Avg
60-Day Avg
User D efi n ed: 90 -Day Avg
45
5
90D
12
3
00
Benthic Concentration of Concern Qig/L)
1-Day Benthic Avg
3-Day Benthic Avg
7-Day Benthic Avg
28-Day benthicAvg
60-Day Benthic Avg
User Defined: 34 -Day Avg
Pore Water
(M9/L)
55
G5
50
25
lb
53
Total/Dry Sed
(MO/kg)
1200
710
23
33
3
1
Working Directory: C:\Usere\Dirk\Desktop\ExamplePSC\
Outfile Family Name: example
RUN
Figure 5. PSC Inputs for Toxicity Analyses.
7
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JJf Point Source Calculator (version 1.02): Calculations by the WWM
File Scenario Point Source Calculator
Chemical | Toxicity j[Sc^arioj Results | More Info]
Scenario ID MyScenario
I Get Weather 1 C:\Users\Dirk\Desktop\ExamplePSC\TenYearsofWeathe
Width (m) 10
Depth (m) 10
Length (m) 30
Use Constant Row Rate (mVday) 1.0
© Mo Base Row
Water Column Parameters
DFAC 1-19
Water ColumnSS (mg/L) 30
Chlorophyll (mg/L) 0.005
Water Column foe 0.04
Water Column DOC (mg/L) 5.0
Water Column Biomass (mg/L) 0.4
Bent hie Parameters
Benthic Depth (m) 0.005
Benthic Porosity 0.50
Bulk Density (g/cm3) 1 35
Benthic foe 0.04
Benthic DOC (mg/L) 5.0
Benthic Biomass (g/m2) 0.006
Mass )tfer Coeff. (m/s) 1e-15
Working Directory: C:\Users\Dirk\Desktop\ExarnplePSC\
Outfile Family Name: example
RUN
Figure 6. PSC Scenario Definition Tab.
5. Analysis and Post Processing
The PSC calculates daily water concentrations based on the input information. From
these estimated daily concentrations, the highest acute and chronic values are found and reported
to the user interface. The results are given on an output page as shown in Figure 7. A time series
graph of water column and benthic pore water concentrations is displayed on this tab as well.
Additionally, full detailed output files with additional information such as number of consecutive
days above the CoC can be found in the output files. Finally, there are additional analyses
presented on the last tab as shown in Figure 8. Flere theoretical distributions of how the chemical
tends to distribute in the environment are given, which can indicate which compartments the
chemical will tend to be found. Also, the overall long-term average half-lives are given, which is
useful for identifying the effective dissipation processes. Fuller descriptions are given in
Appendix 2.
8
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[J! Point Source Calculator {version 1,02): Calculations by the WWM
File Scenario Point Source Calculator
Chemical j Toxicity Scenario
Results
More Info
Water Column
Benthic
Total Cone.
tjig/L)
Days > CoC
(Fraction)
Pore Water Days >CoC
(pg/L) (Fraction)
Total Benthic
Days >CoC
(Fraction)
1-Day Avg
663000
1.000
1-day Avg
15.9
0.000
786
0.000
2-Day Avg
662000
1.000
3-day Avg
15.9
0.000
786
0.185
3-Day Avg
661000
0.999
7-day Avg
15.9
0.000
786
0.973
4-Day Avg
659000
0.999
28-day Avg
15.9
0.000
786
0.964
7-Day Avg
656000
0.93S
60-day Avg
15.8
0.493
785
0.984
21-Day Avg
654000
0.995
34 day Avg
15.9
0.000
786
0.991
28-Day Avg
653000
0.993
60-Day Avg
643000
0.984
90 Day Avg
629000
1.000
Copy Graph
800000-
600000-
400000-
•5
200000-
Water Column [Total) Pore Water
500
1000
1500 2000
Days
2500
3000
3500
Working Directory: C:\Usere\Dirk\Desktop\ExamplePSC\
Outfile Family Name: example
RUN
Figure 7. PSC Output page.
9
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Point Source Calculator {version 1.02): Calculations by the WWM
File Scenario Point Source Calculator
Chemical | Toxicity | Scenario | Results [ More
i Info
Chemical Tendency for Distribution
0.99694624
Aqueous Phase Fraction in Water Column
SS-Sorbed Phase Fraction in Water Column
DOC-Phase Fraction in Water Column
Bio-Phase Fraction in Water Column
Aqueous Phase Fraction in Benthic Region
Water Column to Whole System
0.00147149
0.00129631
0.00028595
0.00747117
0.96771717
Effective Dissipation Processes
Conversions
Pone Water (Ug/L.) to Total / Diy Sediment (ligAg) 49.6
Pore Water iig/L) to Total / Wet Sediment Ijjgy'kg) 36.2
I.'
Washout Halflife id ays)
0
Water Metabolism Halflife (days)
61.8
Hydrolysis Halflife (days)
82.7
Photolysis Halflife (days)
0.251 E-i-04
Volatile Halflife (days)
5.511E+D7
Total water Column Halflife (days)
3.489E+01
Benthic Metabolism Halflife (days)
0.275E+04
Benthic Hydrolysis Halflife (days)
O.IIOE-^05
Total Benthic Halflife (days)
0.220E+04
Working Directory: C:\Users\Dirk\Desktop\ExamplePSC\
Outfile Family Name: example
RUN
Figure 8. Additional analyses regarding distribution tendency and relative degradation
processes.
10
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6. References
Fisher, H.B, List, E.J, Koh, R.C.Y., Imberger, J., and Brooks, N.H. (1979) Mixing in Inland and
Coastal Waters. Academic Press, Ney York, NY. pp. 126-127.
Young, D.F. 2014. The Variable Volume Water Model, EPA-734-F-14-003, United States EPA
Washington DC.
Young, D.F. and Fry, M.M. 2014. PRZM5 A Model for Predicting Pesticide in Runoff, Erosion,
andLeachate: User Manual. EPA-734-F-14-002. United States EPA, Washington DC.
Young, D.F. 2013. Pesticides in Flooded Applications Model (PFAM): Conceptualization,
Development, Evaluation, and User Guide, United States Environmental Protection Agency,
Washington DC, EPA-734-R-13-001.
Young, D.F., 2012. Development and Evaluation of a Regulatory Model for Pesticides in
Flooded Applications. Environmental Modeling & Assessment 17(5), 515-525.
Young, D.F. 2019. U.S. Environmental Protection Agency Model for Estimating Pesticides in
Surface Water, in Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment,
Mitigation, and Management. American Chemical Society, Washington DC. pp 435-450.
11
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Appendix 1. The Variable Volume Water Model: Full
Documentation
Appendix 1, 12
VVWM
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A EPA
United States
Environmental Protection
Agency
The Variable Volume Water Model
USEPA/OPP 734F14003
June 26, 2014
Dirk F. Young
Environmental Fate and Effects Division
Office of Pesticide Programs
U.S. Environmental Protection Agency
Washington, D.C. 20460
Appendix 1, 13
VVWM
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Contents
1 Introduction 1
2 The Varying Volume Water Body Model 1
2.1 Conceptualization and Mathematics 1
2.2 Solute Holding Capacity Ratio (0) 5
2.3 Effective Water Column Dissipation (Tl) 6
2.3.1 Hydrologic Washout v vi y 6
2.3.2 Metabolism (|j.bio_l) 6
2.3.3 Hydrolysis (jihydr l) 7
2.3.4 Photolysis (jiphoto) 7
2.3.5 Volatilization (^volatilization) 8
2.4 Effective Benthic Region Dissipation (T2) 11
2.4.1 Benthic Hydrolysis (jihydr_2) 11
2.4.2 Benthic Metabolism (jibio_2) 11
2.5 Mass Transfer Coefficient (Q) 12
2.6 Daily Piecewise Calculations 14
2.6.1 Volume Calculations 14
2.6.2 Initial Conditions 14
2.7 Analytical Solution 15
3 The USEPA Standard Water Bodies 16
3.1 Farm Pond 19
3.2 Index Reservoir 19
3.3 Custom Water Body 20
4 VVWM Evaluations 20
4.1 Solute Holding Capacity Ratio Sensitivity 20
4.2 Washout and Overflow Sensitivity 22
4.3 Photolysis Sensitivity 24
4.4 Volatilization 26
5 Testing and Comparison of VVWM Solution with EXAMS 28
6 Computer Program Implementation 29
6.1 Executable and the Command Line 29
6.2 Input Files 30
Appendix 1, 14
VVWM
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6.2.1 General Input File 30
6.2.2 ZTS Input File 32
6.2.3 Meteorological File 33
6.3 Output Files 34
6.3.1 Regulatory Summary Output File 34
6.3.2 Daily Values Output File 34
7 Summary 34
8 References 36
Appendix 1, 15
VVWM
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1 Introduction
USEPA's Office of Pesticide Programs (OPP) uses computer models to estimate
pesticide exposure in surface waters resulting from pesticide applications to agricultural fields.
These models are used to simulate pesticide applications to agricultural fields, the subsequent
fate and transport in surface waters, and ultimately, estimated environmental concentrations
(EECs) that are both protective and scientifically defensible. Using historical meteorological
data from the region specified in the risk assessment, PRZM (Carsel et. al, 1997) calculates daily
runoff and spray drift fluxes from "standard" fields over a simulation period (typically 30 years).
These standard fields are parameterized to represent particular crops and regions of the United
States (e.g., corn grown in Ohio). Another model EXAMS (Burns, 1985) simulates standard
water bodies, that receive pesticides from the standard fields. Because EXAMS is difficult to
implement in a user-friendly environment, OPP has created a new program, the Variable Volume
Water Body Model (VVWM). VVWM behaves much like EXAMS, simulating the USEPA
standard water bodies (i.e., farm pond and index reservoir) but with greater efficiency and
flexibility for incorporation into a user interface. The VVWM also allows for variations in water
body volume on a daily basis due to runoff, precipitation, and evaporation. Temperature, wind
speeds, and pesticide dissipation processes are also allowed to vary daily.
2 The Varying Volume Water Body Model
2.1 Conceptualization and Mathematics
The VVWM is conceptualized in Figure 1 and consists of two regions: a water column
and a benthic region. Each individual region is completely mixed and at equilibrium with all
phases in that region, with equilibrium described by a linear isotherm. The two regions are
coupled by a turbulent-mixing, first-order mass-transfer process. As Figure 1 also shows, the
pond volume may vary by inputs of precipitation and runoff and by outputs of evaporation and
overflow.
Appendix 1, 16
VVWM, pg. 1
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Direct pesticide
application to littoral
region by runoff,
erosion and spraydrift evaporation Preciptation
volatilization
Washout During Overflow
Littoral region
littoral region degradation
due to metabolism, —
hydrolysis, and photolysis
Varying Littoral
Region Depth
littoral/benthic mass transfer
Direct pesticide
application to benthic
region by erosion
solids
Overflow depth
benthic region
benthic region
degradation due
to metabolism
and hydrolysis
Figure 1. Graphic of the standard water body showing inputs, outputs, and transformation
processes.
The mathematics are solved by daily piecewise analytic solutions. The temporal
resolution is one day because daily inputs are readily acquired (i.e., runoff, rainfall, and
evaporation data are 24-hour totals), and regulatory needs seldom require finer resolution. The
water body volumes and flow rates are also daily values, consistent with the input data
resolution. For the analytic solution, water body properties are held constant each day, but may
vary from day to day.
All individual dissipation processes (e.g., metabolism, hydrolysis, and volatilization) are
represented as first-order in concentration, as described later. On any given day, solute mass in
the water body is described by two differential equations, namely a mass balance on the water
column:
dSsed I dSMo 1 dSDoc 1 dcx
msed 1 ~7~ + Mbro 1 -T-mDOC 1 ~^ + V1 =
at at - at at
~QC 1 ~QCsedSsed ~QCbioSbio ~QCdOCSDOC ~ aip\ "^2)
~~ V\!J-photoC\ ~ V\Mbio_a\C\ ~ V\!*hydr_\C\ ~ V\MvolC\ (1)
^se d^b io_sed 1 ^ sed ^biota f^bio_biota\^biota
~mDOcflbio DOC\SDOC
and a mass balance on the benthic region:
Appendix 1, 17
VVWM, pg. 2
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Vllibio-a2C2 V2^hydr2C2 + C1(C\ C2)
sedf^bio-sed2^ sed2 ^biota ^bio-biota2^biota2 ^ ^
'mDOcfibio-DOC2SDOC2 ~^Ssed2
Where
B = burial rate of sediment, [kg/s]
ci = aqueous concentration in water column, [kg/ m3]
C2 = aqueous concentration in benthic region, [kg/ m3]
Csed = concentration of suspended sediment in water column = mSed i/vi [kg/m3]
Cdoc = concentration of DOC in water column = mDoc/vi, [kg/m3]
Cbio = concentration of biota in water column = mbio/vi, [kg/m3]
msed1 = mass of suspended sediment in water column, [kg]
mDoc 1 = mass of DOC in water column, [kg]
mbio 1 = mass of suspended biota in water column, [kg]
mSed 2 = mass of suspended sediment in water column, [kg]
mDoc 2 = mass of DOC in benthic region, [kg]
mbio 2 = mass of biota in benthic region, [kg]
Ssed i = sorbed concentration on suspended sediment in water column, [kg/ kg]
sdoc i = sorbed concentration on suspended DOC in water column, [kg/ kg]
Sbio i = sorbed concentration on suspended biota in water column, [kg/ kg]
Ssed2 = sorbed pesticide concentration on benthic sediment, [kg/ kg]
sdoc 2 = sorbed pesticide concentration on benthic DOC, [kg/ kg]
Sbio 2 = sorbed pesticide concentration on benthic biota, [kg/ kg]
vi = volume of water in region 1 on the specific day, [m3]
V2 = volume of water in region 2, [m3]
Q = volumetric flow rate of water out of water column, [m3/s]
a = 1st order water column-to-benthic mass transfer coefficient, [m3/s]
Hhydr = 1st order hydrolysis rate coefficient, [s"1]
Uphoto =lst order photolysis rate coefficient, [s"1]
Hvoi = effective 1st order volatilization rate coefficient, [s"1]
Ubio ai=lst order aqueous-phase metabolic degradation rate coefficient in water column, [s"1]
Ubio sedi = 1st order sediment-sorbed metabolic degradation rate coefficient in water column, [s"1]
Ubio bioi = 1st order biota-sorbed metabolic degradation rate coefficient in water column, [s"1]
Ubio doci = 1st order DOC-sorbed metabolic degradation rate coefficient in water column, [s"1]
Ubio a2 =lst order aqueous-phase metabolic degradation rate coefficient in benthic region, [s"1]
Ubio sed2 = 1st order sediment-sorbed metabolic degradation rate coefficient in benthic region, [s"1]
Ubio bio2 = 1st order biota-sorbed metabolic degradation rate coefficient in benthic region, [s"1]
Ubio doc2 = 1st order DOC-sorbed metabolic degradation rate coefficient in benthic region, [s"1]
The following assumptions are made: (1) suspended matter in the water column has
negligible volume, (2) hydrolysis, photolysis, and volatilization act only on dissolved species, (3)
within a single region (water column or benthic), the rate coefficient for biological metabolism is
the same for both dissolved and sorbed forms of pesticide (e.g., |j,bio i = |J,bio ai = |~lbio_sedl
|~lbio DOC1 |~lbio biotal, cUld |^tbio_2 |~lbio_a2 |~lbio_sed2 |~lbio DOC2 |~lbio_biota2), (4) the hydrolysis T3,tC
sed 2 bio 2 DOC2
— + — + mnnr9 —+ v9 —-
dt 102 dt ' " - dt 2 dt
Appendix 1, 18
VVWM, pg. 3
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coefficient in the benthic region is the same as that in the water column, (5) linear isotherm
equilibrium exists within each region among all sorbed species. With these assumptions, we can
rewrite equations (1) and (2) in a simpler form as follows:
^ = -riCl-Q0(Cl-c2) (3)
dt
^- = -r2c2+0(Cl-c2) (4)
where
dt
ri - — + tphoto + M'hydr + ^vol Kl + ^bio (5)
V1
t-, r BKd2
r2 - Kl^hydr + llUo_2 + 7^ (6)
2
Q = i r (7)
lmsed_2^sed_2 + mbio_2Kbio_2 + mDOC_2KDOC_2 + V2 )
^ (msed_2Ksed_2 + mbio_2K-bio_2 + mDOC_2KDOC_2 V2) s0s
(y--r — r (o;
lmsedjKsed_l + mbiojK-bioJ + mDOCjK DOC 1 + V1 /
where fwi and fw2 are the fractions of solute in the aqueous phase within the water column and
benthic regions, respectively, as defined by the following equations:
V
^Wl _ 7 T? ~T? ~T? ^
Vmsed 1 sed 1 + mbio l^bio 1 + mDOC l^DOC 1 + V1 )
fw2=-
rn
2^sed 2 + mbio 2^bio 2 + mDOC 2^DOC 2 V2 >
(10)
and where Ksedi, Kbioi, Kdoc 1 are the linear isotherm partitioning coefficients for suspended
sediments, biota, and DOC in the water column, and Ksed 2, Kbio 2, Kdoc 2 are the linear isotherm
partitioning coefficients for sediments, biota, and DOC in the benthic region (all with units of
m3/kg).
The term, fwi, varies daily depending on the volume of the water body (vi) as described
below in Section 2.6 Daily Piecewise Calculations. We assume that the mass of sediment, biota,
and DOC remain constant. However, this assumption has very little impact on the model output
Appendix 1, 19
VVWM, pg. 4
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since partitioning to these species is insignificant, except when given extremely high partitioning
coefficients.
Given a set of initial conditions, equations (3) and (4) completely describe the standard
water bodies. It is clear that there are only four parameters that influence the concentration—r 1,
r 2, Q, and 0. H is the effective overall degradation rate in the water column, [s"1]. His the
effective overall degradation rate in the benthic region, [s"1]. Q is a mass transfer coefficient
describing transfer between the benthic and water column, [s"1]. 0 is the ratio of solute holding
capacity in the benthic region to that in the water column, [unitless]. The sections that follow
describe the details of the components of these equations with respect to the standard water
bodies.
2.2 Solute Holding Capacity Ratio (0)
The solute holding capacity ratio (0) is the ratio of solute holding capacity in the benthic
region to the solute capacity in the water column, as defined by equation (8). The individual
partitioning coefficients (Kdsed, Kd biota, and Kd doc) used in equation (8) are generally not
directly measured for a pesticide assessment. To account for these unknown coefficients, the
standard water bodies use various estimation means that relate the various partitioning
coefficients to the organic carbon partitioning coefficient (Koc), which is usually known in a
pesticide assessment process.
For the sediment, the partitioning coefficient is directly proportional to Koc, with the
constant of proportionality being the amount of organic carbon in the sediment, which is a set to
standard values for the standard water bodies (see Table 1). The fraction of organic carbon (foc)
is assumed to be the same in the benthic and water column. The sediment partitioning
coefficients can thus be determined from the following equation:
Ka.„dJ =Kd„, 2 =fotKo,(0 001^) (11)
where Koc = organic carbon partitioning coefficient, [mL/g]
foc = fraction of organic carbon in sediment [unitless]
Note that the units of the coefficients in equations (1) to (10) are all given in s.i. form, which is
maintained throughout this document. However, for some fundamental parameters such as Koc,
which is usually presented in units of mL/g, common units and conversion factors are used.
The partitioning coefficients for DOC are determined from the default empirical
relationships described in the EXAMS documentation (Burns, 2000). The VVWM incorporates
the notion of Burns (2000) that benthic DOC has higher partitioning characteristics than water
column DOC for standard water bodies:
^,=0.2114^(0.001^) (12)
OOOl^f) (13)
The partitioning coefficients for biota are also determined from default empirical
relations described in the EXAMS documentation:
Appendix 1, 20
VVWM, pg. 5
-------
K«. ,=Kllo s=0.436r
0.35
1 I 3 \
(oooi^f) (14)
By inserting equations (11) through (14) into equation (8) and substituting specific values
from Table 1 into equation (8), the solute holding capacity (0) can be written as a function of
solely Koc, as presented in Figure 2 for both the standard pond and reservoir.
2.3 Effective Water Column Dissipation (ri)
The overall dissipation rate in the water column (Ti), as defined in equation (5) is the
sum of contributions from hydrologic washout and degradation by mechanisms of biological
metabolism, photolysis, and hydrolysis. The specific methods and assumptions that are used in
the VVWM to determine these individual first-order dissipation processes are described below.
2.3.1 Hydrologic Washout
vviy
The first term in equation (5), Q/vi, represents the effective first-order dissipation rate
due to flow moving pesticide out of the water body. Flow out of the water body only occurs if
meteorological conditions produce enough water inflow to cause the water body to overflow (see
Section 2.6 Daily Piecewise Calculations). The washout term acts on all forms of pesticide
(aqueous dissolved and sorbed to suspended matter), as is apparent from equation (1) and the
definitions for Xsed, Xbio, and Xdoc. This means that the settling of suspended solids is not
explicitly considered in the VVWM, and pesticides in both dissolved and suspended sorbed
forms can flow out of the reservoir.
Flow is obtained from an input file or entered as a constant baseflow. The input file
provides a daily flow and is typically generated by the PRZM model as a zts file (see section
6.22) Baseflow will work is additive to any flow from the zts file.
2.3.2 Metabolism (Hbio_i)
In the registration process of pesticides, an estimate of the aqueous degradation rate under
aerobic conditions is supplied by the registrant. Such estimates are derived from laboratory tests
following standard EPA-approved protocols, which are typically conducted in aqueous/sediment
systems at 20 to 25° C. These tests generally do not differentiate between degradation occurring
on the dissolved and sorbed forms of the pesticide; an overall degradation rate is generally all
that is available. Therefore, the VVWM treats the sorbed-phase and aqueous-phase degradation
rates as the same, which makes both equal to the overall rate.
As temperature varies in a water body, the USEPA has established a standard for
temperature adjustments of the aerobic metabolism rate when regulating pesticides as follows:
T-t„
Ubio 1 = H25 x 2V 10 ' (15)
where |LX25 = laboratory measured aerobic metabolism rate at 25°C, [s"1]
T = temperature of modeled water body, [°C]
Tref = temperature at which laboratory study was conducted, [°C]
Appendix 1,21
VVWM, pg. 6
-------
This temperature adjustment doubles the metabolism rate for every 10°C rise in temperature, and
halves the rate for every 10°C decrease. Air temperature is taken from the meteorological data
that corresponds to the crop/location scenario being simulated. The VVWM uses the previous
30-day average temperature and adjusts the temperature daily. (Note: EXAMS made
temperature adjustments on a monthly calendar basis, which required tracking of the Gregorian
calendar).
2.3.3 Hydrolysis (jihydr i)
The hydrolysis rate is directly obtained from experimental measurements, as supplied by
pesticide registrant data submissions. In the VVWM, the effective hydrolysis rate is the
experimentally-determined overall hydrolysis rate from tests conducted at the pH of interest. In
a typical USEPA assessment, the pH is 7 (Note: Because pH is not included explicitly in the
VVWM, the appropriate input is the overall hydrolysis rate, not the specific neutral-, base-, or
acid-catalyzed hydrolysis rate coefficients, as in EXAMS).
Unlike the metabolism rate, temperature adjustments of the hydrolysis rate are not made
by the VVWM. Temperature-dependent hydrolysis characterizations are not generally made for
the registration process, and the USEPA has not adopted a standard adjustment for temperature
effects on hydrolysis. Therefore, the hydrolysis rate is as follows:
^hydr 1 — overall, pH 0^)
where ^overall, pH = laboratory-measured overall hydrolysis rate at pH of interest, [s"1].
The VVWM uses the assumption that hydrolysis acts only on dissolved species.
Therefore, the effective hydrolysis rate is reduced by the fraction of total pesticide that is present
in dissolved aqueous form (fwi), as defined in equation (9) and implemented in equation (5).
2.3.4 Photolysis ((iphoto)
Photolysis rates are derived from standard laboratory tests following USEPA-approved
protocols. These tests are designed to estimate the photodegradation rate for near-surface
conditions at a specific latitude and under clear-sky conditions. The VVWM adopts the methods
given by EXAMS (Burns 1997, 2000) to account for latitude adjustments, light attenuation, and
cloud cover:
^photolysis — ^lat ^cloud ^atten ^measured
(17)
where fiat = latitude adjustment factor, [unitless]
fcioud = cloudiness adjustment factor, [unitless]
fatten = attenuation factor to absorption, [unitless]
^measured = measured near-surface photolysis rate coefficient at reference latitude and clear
atmospheric conditions [sec"1]
Although cloudiness does not affect the current standard water bodies (fcioud is set to a
standard value of 1), fcioud is included here for the purposes of formality and because it may be
considered in future versions.
Appendix 1, 22
VVWM, pg. 7
-------
The latitude of the standard water body varies, depending on the desired location in the
U.S. where the pesticide assessment is being made. The effect that latitude has on incident light
is accounted for by the latitude adjustment factor (fiat), which the VVWM adopts from EXAMS
(Burns, 2000). Full details of the reasoning behind fiat can be found in the EXAMS
documentation, and only the resulting equation is given here:
_ 191700 + 87050cos(0.0349 x Lsim) Qg)
~ 191700+ 87050cos(0.0349 xLref)
where Lref = reference latitude at which the measured photolysis rate was determined, [degrees]
LSim = latitude of the simulated scenario, [degrees]
The light attenuation factor (fatten) described by Burns (2000) has also been adopted; the
full details are available in the EXAMS documentation:
f =
atten
l-cxp[-(D[icXd|)a]
(DfJ(di)a
(19)
where Dfac = EXAMS-defined distribution factor default value = 1.19, [unitless]
di = depth of water column, [m]
a = total absorption coefficient, [m"1]
The absorption coefficient (a) is calculated from EXAMS default conditions—that is,
from the spectral absorption coefficient assuming that the wave length of maximum absorption
occurs at 300 nm:
a = 0.141 +101[CCHL ] + 6.25[Cdoc ] + 0.34[CSed ] (20)
where Cdoc, Csed have been previously defined under equation (1), and Cchl is the chlorophyll
concentration [mg/L],
Temperature effects are not considered in the above equations, except when the water
temperature is 0°C or below. Photolysis is inhibited, as in EXAMS. Temperature effects are
not considered since the USEPA generally does not receive temperature dependent data for the
registration process and has not adopted a standard temperature adjustment for photolysis.
2.3.5 Volatilization (^volatilization)
The VVWM uses a two-film model for volatilization calculations and all of the default
volatilization assumptions as described in the EXAMS documentation (Burns, 2000). The
concentration of a pesticide in the atmosphere is assumed to be negligible, and thus volatilization
becomes a first-order dissipation process. The overall volatilization rate coefficient is expressed
as follows:
Ak ,
Mw = — (21)
Vi
Appendix 1, 23
VVWM, pg. 8
-------
where A = surface area of water column, [m2]
kvoi = volatilization exchange coefficient, [m/s]
k = k
and the volatilization exchange coefficient comprises liquid-phase and gas-phase resistances:
1 1 1
ir=ir+Mr (22)
Kvol Kw \RT / a
where kw = liquid-phase resistance [m/s]
ka = gas-phase resistance, [m/s]
H = Henry's law constant (m3atm/mol)
R = the universal gas constant (8.206 x 10"5 m3atm/mol/K)
T= temperature (K)
The VVWM uses the EXAMS methods of referencing the liquid exchange resistance of
pesticides to the liquid resistance of oxygen, and uses molecular weight as the sole surrogate for
molecular diffusivity variations among compounds. Further details can be found in the EXAMS
documentation (Burns, 2000), but the resulting relationship is as follows:
(23)
where ko2 = oxygen exchange constant at 20°C, [m/s]
MW = molecular weight of pesticide, [g/mol]
The oxygen exchange constant is determined from the empirical relationship of Banks
(1975). Adjustments are also made for temperatures other than 20°C. Note that although
EXAMS uses a reference temperature of 20°C for the Banks (1975) relationship, it is not clear
from Banks (1975) what the actual reference temperature should be. Schwarzenbach et al.
(1992) used a 10°C reference for the same relationship. Until further clarified, a 20°C reference
temperature is used. For wind velocities (vwind) less than 5.5 m/s, ko2 is calculated as:
k02 =(4.19x10-'V^;)(l.024(T-2°!)
(24)
and for wind velocities greater than or equal to 5.5 m/s, ko2 is:
ko2=3.2xl0-'(u10)2(l.024
-------
Ui _ M^/Zp)
u2 log(z2/z0)
where zo is the boundary roughness height, which is assumed to be 1 mm for the standard water
bodies. Given a wind speed (measured at 10 m) from the meteorological file, the equivalent
wind speed at 0.1 m is:
log(0.1/0.00l)
U°' " log(l0/0.00l) U'°" 10 <27)
In the VVWM, wind speed varies on a daily basis, unlike in EXAMS where the average monthly
wind speed varies on a monthly basis.
The gas-phase resistance is referred to as water vapor resistance, and an empirical
relationship based on a linear regression of laboratory-derived data from Liss (1973) relates the
water vapor exchange velocity to wind speed:
ka H20 = 0.00005 + 0.0032u01
where ka,H2o = the water vapor exchange velocity (m/s)
uo.i = wind speed velocity measured at 0.1 m above the surface (m/s)
The exchange rate of a pesticide is then related to the exchange rate of water by:
k -k D"
K- a,H20
a,H->0
(28)
where a (not to be confused with the alpha in equation 1 and 2) is a value that depends on the
conceptual model believed to describe volatilization and ranges from 0.5 for the surface renewal
model to 1.0 for the stagnant film model (Cusler,1984 ; Schwarzenbach et al., 1993). The
VVWM uses a value of 1.0 for a; thus, implying a stagnant film model. However, some
laboratory data suggest that a may be better represented with a value of 0.67 (Mackay and Yuen,
1983). The diffusion coefficient of the pesticide is related to the diffusion coefficient of water by
the common approximate relationship (e.g., Schwarzenbach et al., 1993):
D.
D.
18
MW
0.5
(29)
Substituting (29) into (28) gives:
k = k
18
MW
(30)
Appendix 1, 25
VVWM, pg. 10
-------
The resulting relationship is:
k = fo.00005 + 0.0032un, 1/-^-
a L v MW
18
(31)
MW
The Henry's Law constant is generally not available from pesticide registration
submissions, so it is approximated in the VVWM from vapor pressure and solubility. The
Henry's Law constant also is not adjusted for temperature, as this information is not supplied in
the pesticide registration, and OPP has not adopted a standard temperature adjustment factor.
The resulting relationship is:
(vp/760)
(Sol/MW) ( }
where vp = vapor pressure [torr]
Sol = solubility [mg/L]
2.4 Effective Benthic Region Dissipation (ri)
The overall benthic degradation in the VVWM, as defined in equation (6), is only
affected by biodegradation and hydrolysis. As with the water column, OPP assumes that
biodegradation in the benthic region affects all forms of pesticide (both dissolved and sorbed
forms) and that hydrolysis affects only aqueous dissolved forms (see equation 6 and definition of
fw2).
2.4.1 Benthic Hydrolysis (|J,hydr_2)
In the current standard water bodies, the pH of the entire system (benthic and water
column) are held at a constant pH of 7, although a subsequent paper will suggest using scenario-
specific pH values. Benthic hydrolysis is assumed to occur at the same rate as hydrolysis in the
water column; the previous discussion of hydrolysis in the water column applies to the benthic
region:
^hydr 2 = ^hydr 1 (33)
2.4.2 Benthic Metabolism (|0.bio_2)
In the VVWM, benthic metabolism is assumed to occur under anaerobic conditions.
Therefore, anaerobic metabolism rates are derived from laboratory tests following standard EPA-
approved protocols. These studies are typically conducted in aqueous/sediment systems at 20 -
25°C. As with water column metabolism, OPP assumes that sorbed-phase degradation occurs at
the same rate as aqueous-phase degradation, and temperature effects on metabolism are handled
in the same way. Thus, the effective rate is the following:
Appendix 1, 26
VVWM, pg. 11
-------
M-bio 2 M* measured X ^
(34)
where ^measured = laboratory measured anaerobic metabolism rate at Tref
T = temperature of modeled water body [°C]
Tref = temperature at which anaerobic laboratory study was conducted [°C],
2.5 Mass Transfer Coefficient (Q)
The mass transfer coefficient (Q) defined in equation (7) is an overall coefficient that
includes all means of pesticide exchange between the water column and benthic regions. This
includes exchange through the aqueous phase as well as by mixing of sediments between the two
compartments. The physical process of this combined mixing is assumed to be completely
described by a first-order mass transfer coefficient (a). The parameter a is referenced to the
aqueous phase, but implicitly includes exchange due to mixing of sediments as well as aqueous
exchange. In compartment modeling, it is unnecessary to explicitly model the individual
exchange mechanisms (as EXAMS does) since all phases of pesticide within a compartment are
at equilibrium. Therefore, the concentration of a pesticide in any given form (aqueous or sorbed)
dictates the concentration of the other forms of the pesticide.
In the VVWM, the a term is based upon parameters and assumptions given in the
EXAMS documentation. Although not explicitly presented as such, EXAMS uses a boundary
layer model to exchange pesticide mass between the water column and benthic regions. EXAMS
defines the parameter DSP, which represents a Fickian-type dispersion coefficient in the benthic
sediment. This dispersion coefficient acts on the total concentration within the benthic region,
implying that sediment-sorbed pesticide moves through the benthic region at the same rate as
dissolved-phase pesticide (e.g., via bioturbation). The rate of mass change in the benthic region
is approximated under steady state conditions across a boundary layer of constant thickness:
where M2 = total pesticide mass in benthic region
A = area of benthic/water column interface, [m2]
D = effective overall dispersion coefficient in benthic media (includes both sorbed and
dissolved phases), [m2/s]; DSP in EXAMS
Ax = thickness of boundary layer, [m]
91 = total partition coefficient for total concentrations, [unitless]
Cti = total concentration in water column, [kg/m3]
Ct2 = total concentration in benthic region, [kg/m3]
The total concentrations in the water column and benthic regions are calculated as follows:
^ = A-5-(9iCT1-CT2)
dt Ax
(35)
(36)
Appendix 1, 27
VVWM, pg. 12
-------
c _C2[v2+X(m2Kd2)j (3?)
^T2
where ci and vi are the aqueous-phase concentration and the aqueous volume, as previously
defined under equation (1); Z(miKdi) and X(m2Kd2) are short-hand notation for the sum of all
solid masses and the respective Kds presented under equation (1) for the water column and
benthic regions, respectively; Vti and Vt2 are the total volumes of the water column and benthic
region, respectively, which include both the water and the solids volumes. The total pesticide
mass in the benthic region is expressed as follows:
M2=c2(v2+Xm2Kd2) (38)
The total partitioning coefficient is defined as the ratio of Ct2 to Cti when the system is at
equilibrium:
C
9? = —— (when benthic region is at equilibrium with water column) (39)
T1
By substituting in the definitions of Cti and Ct2 from equations (36) and (37) and recognizing
that at equilibrium ci = C2, the total partitioning coefficient becomes:
^ (40)
lVl+ZmiKdl) VT2
Substituting equations (36) to (40) into equation (35) yields the following:
dM2 _ AD(v2+Xm2Kd2)
dt Ax VT2
(ci-c2) (41)
Comparing equation (41) with equation (2), we can see that:
a = AD(v,+Im,Kj (42)
and that Q is:
Ax VT2
„ AD
Ci = (43)
VT2Ax
where D = overall water column -to-benthic dispersion coefficient (m2/s)
Ax = boundary layer thickness (m)
A = area of water body (m2)
Appendix 1, 28
VVWM, pg. 13
-------
D in the above equation is set to a constant (Table 1) for the USEPA standard pond. The
value of D was originally chosen to be on the order of Fickian-type dispersion coefficients in
sediments, as observed in field studies reported in the EXAMS documentation. Although
equation (42) implies a mechanistic meaning to a, it is difficult to adequately transform Fickian-
type dispersion coefficients into first-order mass transfer coefficients for finite volume
compartments, and it is equally difficult to define a boundary layer thickness, especially when
there is sediment and aqueous mixing. EXAMS suggests that the boundary layer thickness be
equal to the distance between the center of the water column and the center of the benthic region,
but the actual boundary layer thickness is difficult to estimate and likely is more related to
benthic animal life than water column depth.
Attempting to model the benthic mass transfer parameter as a function of water column
depth would be speculative, so the VVWM currently maintains a constant thickness.
2.6 Daily Piecewise Calculations
Because we retain an analytical solution, the VVWM is solved in a daily piecewise
fashion, in which the volume of the water column changes at the beginning of the day and
remains constant for the duration of that day. Mass is conserved in the water column by
recalculating a new beginning day concentration with any volume change.
2.6.1 Volume Calculations
The volume of the water column aqueous phase is calculated from daily runoff,
precipitation, and evaporation for any day as follows:
Vj = v0 + R+ P — E — S for 0 < VI < Vmax (44)
where vo = the aqueous volume of the previous day (m3)
R = daily runoff into the water body (m3)
P = daily direct precipitation on water body (m3)
E = daily evaporation of runoff (m3)
S = daily seepage = 0 (neglected) (m3)
Daily runoff is taken from the PRZM model output. Daily precipitation and evaporation
are taken from the meteorological file. Seepage at this time is not considered, as in EXAMS. If
the newly calculated volume (vi) is greater than vmax, then the volume for the day is set to Vmax,
and the excess water is used in the calculation of washout. The minimum water volume is zero,
but it is set to an actual minimum to prevent numerical difficulties associated with calculations
involving infinity and zero. There also may be some practical physical lower boundary
appropriate for the minimum volume, such as those associated with soil water holding capacity,
water tables, and refilling practices of pond owners. These factors need to be explored further.
2.6.2 Initial Conditions
Initial concentrations are determined by the pesticide mass inputs from PRZM and spray
drift. PRZM gives daily outputs for pesticide mass associated with aqueous-phase runoff and
erosion solids. All of the pesticide in aqueous-phase runoff and half of the pesticide associated
Appendix 1, 29
VVWM, pg. 14
-------
with the erosion solids are delivered to the water column, and the remaining half of solids-
associated pesticide is delivered to the benthic region. Pesticide may also be delivered to water
bodies by spray drift, which is delivered solely to the water column. In addition, pesticides may
also exist in the water bodies from previous inputs. For the VVWM, there is an instantaneous
volume change at the beginning of the day due to hydrologic conditions (Section 2.6.1 Volume
Calculations); thus the concentration in the water column is adjusted accordingly. The initial
concentrations, upon addition of new pesticide mass, are then expressed as follows:
G
/>
wl
10
+MdJ+j^C,
J wl, prior
""10, prior
(45)
Of) - — (XdMeroS1on) + Qo,pnor
V,
(46)
2
where Mmnoff = mass of pesticide entering water body via runoff (kg)
Merosion = mass of pesticide entering water body via erosion (kg)
Mdrift = mass of pesticide entering water body via spray drift (kg)
C10,prior aqueous concentration in water column before new mass additions (kg/m3)
C20,prior = aqueous concentration in benthic region before new mass additions (kg/m3)
VI, prior the water column volume from the previous day (m3)
fwl,prior fwl from the previous day
Xd = fractional initial distribution (between water column and benthic region) of the
pesticide associated with eroded solids as it enters the water body
2.7 Analytical Solution
Equations (3) and (4) along with the initial conditions represent the two equations
describing the standard water bodies. These equations are in the form of the following:
^ = Ac1+Bc2 (47)
dt
^ = ECl+Fc2 (48)
dt
where
A = -r!-Q@
B = Q0
E = Q
f = -r2 - q
Equations (47) and (48) have the solution:
Appendix 1, 30
VVWM, pg. 15
-------
(49)
c = Xj + Yj
2 1 B 1 B
(50)
where
^ =
X*, =
A + F + A + F)2 - 4(F A - BE)
2
A + F - 7( A + F)2 - 4(F A - BE)
=
^2 ^
I^-IO 20
B
A2 — /lj
Yi =
c -1 ^ — I c
v on I ^ l"^--11
B
A2 — y^i
Average concentrations can be determined over any interval in which all parameters
remain constant. In the VVWM, parameters change on a daily basis, so the average water
column concentration is expressed as follows:
C, „„ =
X,
-e^t2 +¦ Yl
l,t2
X
i eMi
Y,
x,ti
ri (t2 - ti) r2(t2 -tj rx (t2 - tx) r2(t2 -tx)
(51)
where Ci,avg = average water column concentration of time from ti to t2 [kg/m3]
ti = beginning of the time interval considered [s"1], (zero for our case of daily estimates)
t2= end of the time interval considered [s"1], (86,400 seconds for our case of daily
estimates)
3 The USEPA Standard Water Bodies
All parameters in the above equations, except for the pesticide-specific parameters, have
standard values set by the USEPA for the standard farm pond and index reservoir scenarios
(Table 1). Many of these values have no documentation and simply have evolved over many
years of repeated, unquestioned use. Table 2 shows how the parameters in the VVWM simplify
and replace previous EXAMS parameters and expressions, and Table 3 lists the original EXAMS
standard parameters. The VVWM also gives the option to define a custom-sized water body.
Appendix 1, 31
VVWM, pg. 16
-------
Table 1. Standard Parameter Values
For the VVWM.
Parameter
Units
Farm
Pond
Values
Index
Reservoir
Values
Notes
VI
3
m
20,000
144,000
water column volume
V2
m3
249.8
1,314
aqueous benthic volume(a)
A
m2
10,000
52,555
surface area, calculated (vi/di)
di
m
2.0
2.74
water column depth
di
m
0.05
0.05
benthic depth
Illsed 1
kg
600
4,320
based on suspended solids
concentration of 30 mg/L (see Csed i)
mbio i
kg
8.0
57.60
based on biota concentration of 0.4
mg/L
mDoc i
kg
100
720
based on DOC concentration of 5 mg/L
foe
0.04
0.04
fraction of organic carbon (water
column and benthic)
Illsed 2
kg
6.752 x 105
3.552 x 106
(b)
mbio 2
kg
0.0600
0.3156
(c)
mDoc i
kg
1.249
6.570
(d)
PH
7
7
CcHL
mg/L
0.005
0.005
chlorophyll concentration
Cdoc
mg/L
5
5
DOC concentration
Csed 1
mg/L
30
30
suspended solids concentration
Cbio
mg/L
0.4
0.4
biomass concentration
D
m2/s
8.33 x 10"9
8.33 x 10"9
sediment dispersion coefficient
Ax
m
1.02
1.39
benthic/water column boundary layer
thickness
Vt2
500
2,630
total volume of benthic region (di x A)
W calculated from: VOL2*BULKD*(1.-100./PCTWA)
C5) calculated from: (BULKD)(VOL2)(100000)/PCTWA (see Table 2)
(c) calculated from: BNMAS*AREA*.001(see Table 2)
(d:i calculated from: DOC*v2/1000
Table 2. VVWM Equivalents of EXAMS Parameters.
VVWM
Parameters
Expressed in Terms of EXAMS Parameters
mi
rkgi
(SUSED)(VOLi) (lO"3)
m2
[kg]
f BULKD VoLiio'mHk3kg]
^PCTWA/100J 2\ m3 X gj
VI
|m3l
VOLi
V2
[m3]
(vol2)(bulkd{i- 100 1 *
v A \ pctwaJ
Q
[m3/s]
STFLO (3600 s/hr)
MAI
[s-1]
(KBACWi)(BACPL)/(3600s/hr)
Appendix 1, 32
VVWM, pg. 17
-------
|asi
[s-1]
(KBACW2)(BACPL)/(3600s/hr)
[IA2
[s-1]
(KBACSjXbNBACJ
100gl
( 1 hrl
(PCTWA ^
I 100 J
V g J
^3600 s J
|as2
[s-1]
(kbacs2Xbnbac2)
f PCTWA A
{ 100 J
fio-100^
I g J
( 1 h0
^3600 s J
[s-1]
(area)(dsp)
(CHARLXV0L2)
Kdi
m3/kg
(KOC)(FROC)( 10"3 m3/L)
Kd2
m3/kg
(KOC)(FROC)( 10"3 m3/L)
* Assumes that the density of water is 1,000 kg/m3
Appendix 1, 33
VVWM, pg. 18
-------
Table 3. EXAMS Standard Parameters.
EXAMS Parameter
EXAMS
EXAMS Value for
Value for
Standard Drinking
Standard
Water Reservoir
Pond
PRBEN
—
0.5
0.5
PCTWA
—
137
137
BULKD
g/mL
1.85
1.85
FROC
—
0.04
0.04
CHARL
m
1.05
DSP
m2/hr
3.00 x 10"5
3.00 x 10"5
AREA
2
m
10000
52600
VOLi
3
m
20,000
144,000
VOL2
3
m
500
2,630
DEPTHi
m
2
2.74
SUSED
mg/mL
30
0.005
CHL
mg/L
0.005
0.005
DOC1
mg/L
5.0 mg/L
5.0 mg/L
DOC2
mg/L
5.0 mg/L
5.0 mg/L
LAT
34
39.1
BNMAS
g/m2
0.006
0.006
BNBACi
—
—
—
BNBAC2
cfu/lOOg
37
37
BACPLi
cfu/mL
1
1
BACPL2
—
—
DFAC
—
1.19
1.19
WIND
m/s
metfile
metfile
STFLO
m3/hr
0
Average daily
rainfall (from 36
years of data)
TCEL
°C
monthly avg
monthly avg
3.1 Farm Pond
The standard farm pond, representing a highly vulnerable exposure scenario, is a pond
located at the edge of a pesticide-treated field. The pond dimensions (1 ha area by 2 m depth),
originally based on a Georgian farm pond size, are in accordance with USD A guidance for pond
construction for an appropriately-sized pond fed by a 10-ha watershed—that is, approximately 2
acres of drainage per acre-ft of storage in central Georgia (USDA, 1982). In the farm pond,
where inflow is assumed to exactly balance evaporative losses (leaching is not modeled). Table 1
gives some of the standard parameters for the pond.
Appendix 1, 34
VVWM, pg. 19
-------
3.2 Index Reservoir
The index reservoir represents a natural or artificial lake fed by perennial and ephemeral
streams, varying in flow due to precipitation, evaporation, and runoff from the surrounding
watershed and groundwater discharge. The reservoir is a potential drinking water source that
may be affected by pesticide runoff, spray drift, and leaching to groundwater. The reservoir is a
fixed volume water body with outflow equated to runoff that enters the reservoir. Table 1 gives
some of the standard parameters for the index reservoir.
3.3 Custom Water Body
A custom water body also can be defined in the VVWM with specific dimensions,
including the field area [m2], water body area [m2], initial depth [m], maximum depth [m], and
hydraulic length [m]. The custom water body can be of varying volume, or of constant volume
with (or without) flow through. This third option allows for greater flexibility in evaluating
pesticide fate and transport in a non-standard receiving water body.
4 VVWM Evaluations
4.1 Solute Holding Capacity Ratio Sensitivity
As Figure 2 shows, the standard index reservoir has a lower solute holding capacity ratio
than the standard pond, and this is due to the greater water column depth of the reservoir. The
point where 0 is equal to 1 represents the Koc for which the solute capacity in the benthic region
is equal to that in the water column. For the pond, equal capacities occur at Koc of 730 mL/g,
and for the reservoir, the equal capacities occur at 1,000 mL/g. Of course, the water column and
benthic regions are not at equilibrium, so the actual distribution of solute will not be evenly split
between benthic and water column at these Koc values. These values and Figure 2, however,
give some physical insight into how the standard water bodies can potentially distribute solute.
It is also of interest to examine the relative significance of the individual media within
each region with regard to the distribution of solute among them. Figure 3 shows the relative
capacities of the individual media (aqueous and sorbed to biota, DOC, and suspended sediment)
within the water column as a function of Koc. Up to a Koc value of-10,000 mL/g, only the water
phase is significant. Up to Koc values of 100,000, biota partitioning is not significant, and at a
Koc value of 100,000, the combined capacities of all sorbed species amounts to less than 20
percent of the total water column capacity. It can also be seen that, for the standard water
bodies, DOC and suspended sediments have nearly equal capacities for solute.
Figure 4 shows the relative capacities for the benthic region. For the benthic region of
the standard water bodies, DOC and biota partitioning are not significant at any Koc value; the
relative fractions for DOC and biota are on the order of 10"7 to 10"5, which cannot be seen in the
Koc range shown (Figure 4). At a Koc of about 9 mL/g, solute is evenly distributed between the
pore-water-dissolved fraction and the sediment-sorbed fraction. At Koc values above 1,000 mL/g,
the vast majority of solute in the benthic region is sorbed to sediment.
Appendix 1,35
VVWM, pg. 20
-------
1000
100
10
0.1
0.01
Pond
Reservoir
10
100 1000
Koc
10000 100000
Figure 2. Solute holding capacity as a function of KoC for the USEPA standard water
bodies.
1.2
o
TO
Q.
TO
o
U)
c
0.8
c
o
5)
0)
0) 0£
- 2
o
c
_3
O
w
TO
o
I-
o
c
o
_ 0.6
0.4
"C 0.2
<3
Capacity of Water
Capacity of DOC
Capacity of Suspended Sediment
Capacity of Biota
10 100 1000 10000 100000
Koc (ml/g)
Figure 3. Relative solute holding capacity of individual components in water column.
Appendix 1, 36
VVWM, pg. 21
-------
2 '5> 0.6
Capacity of Water
Capacity of DOC
Capacity of Sediment
Capacity of Biota
10
100 1000
Koc (ml/g)
10000
100000
Figure 4. Relative solute holding capacity of individual components in benthic region.
4.2 Washout and Overflow Sensitivity
Figures 5 and 6 show how the VVWM overflow modification affects pesticide
dissipation in the standard pond and standard reservoir, respectively. The effective dissipation
half-life due to washout of a pesticide is shown for a range of typical annual average runoff flow
rates as determined from OPP's standard scenarios. This figure only gives an idea of the
potential long-term effect of the VVWM washout addition. Short-term effects will be quite
variable since washout is calculated on a daily basis, and during overflow events, the effective
half-life may differ greatly from long-term averages.
Appendix 1, 37
VVWM, pg. 22
-------
1000
900
800
>
re
¦c
700
600
^ 500
x
® 400
300
o
£
m 200
100
0
0.0005 0.001 0.0015 0.002 0.0025 0.003
Flow Rate (m3/s)
Figure 5. Effective half-life of pesticide due to washout in the standard pond as currently
parameterized (1 hA area, 2 m deep). Range of flow rates are for the current standard field
size (10 hA).
450
400
« 350
300
250
200
150
100
0
0.01
0.02
0.03
0.04
0.05
Flow Rate (m3/s)
Figure 6. Effective half-life of pesticide due to washout in the standard reservoir as
currently parameterized (5.26 hA, 2 m deep). Range of flow rates are for the current
standard field size (10 hA).
Appendix 1,38
VVWM, pg. 23
-------
4.3 Photolysis Sensitivity
With the above considerations, the effective photolysis rate in the standard water bodies
only depends on the laboratory-measured photolysis rate, the latitude of the water body, and the
reference latitude of the measured photolysis rate. The effective photolysis rate can be written in
terms of these parameters. For the farm pond, the effective rate is calculated from the following
equation:
flatfatten
1913 + 868.8cos(0.0349 x Lsim)
191700 + 87050cos(0.0349 x Lref)
l-cxp[-(DrJ(d|)a]
(Dfjdja
(52)
Values for the standard water bodies are given in Table 1. Given the values for standard water
bodies in Table l(a = 42.096 m"1); fatten = 0.009981 for the farm pond; fatten = 0.007286 for the
reservoir; and fiat =s 0.804 for 34°.
From equation (52) for a standard farm pond at latitude of 34° and with a reference
laboratory latitude of 0°, the effective aqueous-phase photolysis rate is 124 times lower than the
measured laboratory rate. For the standard reservoir at the same latitude, the rate is 170 times
less than the laboratory determined value. As with hydrolysis, photolysis is assumed to act upon
only dissolved forms of pesticide; therefore, the overall effective hydrolysis rate is further
reduced by the factor fw in equation (5).
A plot of the inverse of equation (52) shows its effect on the half-life as given in Figure
7. This figure shows that depth is nearly proportional to the increase in half-life at the scale
shown. A closer look at depth in Figure 8 shows that the direct proportional relationship begins
at about 0.02 m, indicating that the photolysis has fully attenuated by this depth. Further
increases in half-life are simply due to the greater amount of volume in the water column.
Appendix 1, 39
VVWM, pg. 24
-------
0 0.5 1 1.5 2 2.5 3
Depth (m)
Figure 7. The effect of depth on the effective half-life due to photolysis, showing the almost
proportional linear relationship of half-life with depth.
6
5
4
3
Ll_
2
1
0
CD
X
0
0.02
0.04
0.06
0.08
0.1
Depth (m)
Figure 8. Smaller scale depth figure, showing that reductions in photolysis half-life become
proportional (linear) with depth after about 0.02 m.
Appendix 1, 40
VVWM, pg. 25
-------
4.4 Volatilization
The effect that wind speed has on effective half-life is given in Figure 9 for the standard
pond. The figure shows that wind speed variations will have an increasingly dramatic effect as
Henry's law coefficient is reduced. The use of daily wind speeds in the VVWM thus has
significant short-term implications (acute concentrations) for low Henry's law compounds.
Volatilization as calculated by the VVWM is relatively insensitive to changes in
temperature because OPP has not adopted a temperature adjustment standard for the Henry's
Law coefficient and volatilization data (as a function of temperature) required for registration.
Thus, OPP currently assumes that the Henry's Law coefficient is constant regardless of
temperature.
100 -
90 -
80 -
70 -
£ 60 -
re
2,
50 "
30 -
20 -
10 -
0 -
0 20 40 60 80 100 120 140 160
Henry's Law Constant (m3 atm/mol x 10s)
Figure 9. Effect of Henry's Law Constant and wind speed (measured at 6m) on effective
volatilization half-life of aqueous phase. MW= 100, Temp = 25 °C.
wind speed = 1 m/s
wind speed = 2 m/s
wind speed = 10 m/s
Appendix 1,41
VVWM, pg. 26
-------
300
Temp = 30 C
Temp =20 C
Temp = 5 C
250
200
150
¦g
Li
ti-
cs
X
100
0
20
40
60
80
100
120
140
160
Henry's Law Constant (m3 atm/mol x 106)
Figure 10. Effect of Henry's Law Constant and temperature on effective volatilization half-
life of aqueous phase. The lack of temperature sensitivity is a result of not considering the
effect of temperature on Henry's Law Constant. Wind speed = 1 m/s, MW=100.
Appendix 1, 42
VVWM, pg. 27
-------
1
- WWM
° EXAMS
0.8
0.6
0.4
0.2
0
0
20
40
60
80
100
days
Figure 11. Comparison of the volatilization mechanisms of the WWM and EXAMS for
conditions: solubility = 100 mg/L, MW=100, vapor pressure = 0.1 torr, Koc = 1 mL/g, wind
speed = 1 m/s, temperature = 25° C, and an input mass of 0.02 kg to the water column. A
constant volume condition was used for the VVWM.
5 Testing and Comparison of WWM Solution with EXAMS
Individual processes of the VVWM analytical solution were tested by comparing the
output with that of EXAMS. For these tests, a constant volume condition was imposed on the
VVWM, so that only the processes common to both EXAMS and the VVWM were tested.
Individual processes were tested by either zeroing out all other dissipation or making them
insignificant, and using a single initial aqueous-phase input. The results from a test of the
volatilization routine are shown in Figure 11. Here the analytical solution for volatilization in the
VVWM is captured and correctly formulated. Other processes such as hydrolysis, photolysis,
metabolism, and benthic mass transfer were tested in a similar manner, and all tested equally
well. Combined processes with multiple inputs, including spray drift, erosion, and runoff, as
read from PRZM output files, were also tested. An example is given in Figure 12, which shows
excellent agreement with EXAMS, and further verifies the proper formulation of the processes
within the VVWM.
Appendix 1, 43
VVWM, pg. 28
-------
6
- VVWM
° EXAMS
5
4
3
2
1
0
0
100
200
300
days
Figure 12. Comparison of VVWM with EXAMS for the following conditions: MW = 100,
solubility = 100 mg/L, vapor pressure = 0.01 torr, aerobic half-life = 10 days, anaerobic
half-life = 100 days, KoC = 100 mL/g, wind speed = 1 m/s, temperature = 25 °C, and
arbitrarily selected PRZM input fluxes. A constant volume condition was used for the
VVWM.
6 Computer Program Implementation
6.1 Executable and the Command Line
Running the VVWM requires the executable and three input files: a general input file, a
"ZTS" file, and a meteorological file. The executable is run from a command line with the
following command:
fortranvvwm.exe "inputfilename "
where for tranvvwm.exe is the name of the executable, and inputfilename is a command line
argument that specifies the path and name of the General Input File. For example,
C:\> fortranvvwm.exe "C:\My Documents\Test\MyFirstInputFile.txt"
Appendix 1, 44
VVWM, pg. 29
-------
In this case, the fortranvvwm.exe file is located on the C: directory and the input file is named
MyFirstInputFile.txt and located in the C:\My Documents\Test\ directory. Note: Quotation
marks around the command line argument are necessary if there are any blank spaces in the
argument.
6.2 Input Files
6.2.1 General Input File
The input file is a text file with the structure given in Table 4. For lines that hold
multiple inputs, the data is separated by a comma or space. The first line specifies where
additional input will be read and where the output will be delivered.
Table 4. General Input File Format.
Line
Fortran Variable Name
Type
Description
1
output filename
character(256)
Full path and name of main output file (less suffix).
This establishes the base name and location of the
output files.
This also specifies the name of the *.zts file that will
be read for the mass and water flow. This input file
must be named outputfilename.zts where
outputfilename is the string defined by the variable
outputfilename.
2
UNUSED
3
nchem
integer
1 = parent only, 2 = parent and degradate, 3= parent,
degradate 1, degradate 2 (sequential)
4
iskoc
logical
Establishes whether the sorption coefficient is Koc or
Ka; True = Koc, False = Kd
5
kocall(i)
real
Sorption coefficient (mL/g); the number of values
should match nchem
6
aer_aq_all(i)
real
Water column degradation half-life (days); the number
of values should match nchem
7
temp_ref_aer_all(i)
real
Reference temperature for water column degradation;
the number of values should match nchem
8
anae_aq_all(i)
real
Benthic degradation half-life (days); the number of
values should match nchem
9
temp_ref_anae_all(i)
real
Reference temperature for benthic degradation; the
number of values should match nchem
10
photoall(i)
real
Photolysis half-life (days); the number of values
should match nchem
11
RFLATall(i)
real
Reference latitude for photolysis; the number of
values should match nchem
12
hydroall(i)
real
Hydrolysis half-life (days); the number of values
should match nchem
13
UNUSED
14
UNUSED
15
UNUSED
16
MWT(i)
real
Molecular Weight; the number of values should match
nchem
17
VAPRall(i)
real
Vapor Pressure (torr); the number of values should
match nchem
Appendix 1, 45
VVWM, pg. 30
-------
18
SOLall(i)
real
Solubility (mg/L); the number of values should match
nchem
19
xAerobic(i)
real
Molar Conversion Factor for water column
degradation; the number of values should match
(nchem-1): parent to degradate 1, degradate 1 to
degradate 2
20
xBenthic(i)
Real
Molar Conversion Factor for benthic degradation; the
number of values should match (nchem-1): parent to
degradate 1, degradate 1 to degradate 2
21
xPhoto(i)
Real
Molar Conversion Factor for photolysis; the number
of values should match (nchem-1): parent to degradate
1, degradate 1 to degradate 2
22
xHydro(i)
real
Molar Conversion Factor for hydrolysis; the number
of values should match (nchem-1): parent to degradate
1, degradate 1 to degradate 2
23
UNUSED
24
UNUSED
25
UNUSED
26
UNUSED
27
UNUSED
28
QT
real
Q10 factor by which degradation increases for every
10 °C rise in temperature.
29
scenarioid
Character(50)
Text to describe the field scenario. Used for naming
output files.
30
metfilename
Character(256)
Full path and file name of the meteorological file.
31
UNUSED
32
UNUSED
33
UNUSED
34
burialflag
logical
If set to .TRUE, this will activate pesticide removal by
sediment burial.
35
UNUSED
36
UNUSED
37
UNUSED
38
UNUSED
39
Doverdx
real
Mass transfer coefficient (m/s) as defined by D/Ax in
Eqn.46
40
PRBEN
real
Xd in equation 40 and 41
41
benthic depth
real
Depth of benthic region (m)
42
porosity
real
Porosity of benthic region (--)
43
bulkdensity
real
Bulk density of benthic region (g/mL). Mass of solids
per total volume.
44
FROC2
real
Fraction of organic carbon on sediment in benthic
region.
45
DOC2
real
Concentration of dissolved organic carbon in benthic
region (mg/L)
46
BNMAS
real
Areal concentration of biosolids in benthic region
(g/m2)
47
DFAC
real
Photolysis parameter defined in eqn. 23
48
SUSED
real
Suspended solids concentration in water column
(mg/L)
49
CHL
real
Chlorophyll concentration in water column (mg/L)
50
FROC1
real
Fraction of organic carbon on suspended sediment in
water column.
Appendix 1, 46
VVWM, pg. 31
-------
51
DOC1
real
Concentration of dissolved organic carbon in water
column (mg/L)
52
PLMAS
real
Concentration of biosolids in water column (mg/L)
53
UNUSED
54
UNUSED
55
UNUSED
56
napp
integer
Number of spray drift events that will be used to apply
pesticide mass to pond
57
appdate_sim_ref(i)
integer
Dates of spray drift events reference to days of the
simulation (first day of simulation =1)
58
simtypeflag
integer
Flag to identify the type of water body: 1= User
defined parameters; 2=USEPA Pond; 3=USEPA
Reservoir; 4 = Reservoir with f
59
afield
real
Area of adjacent runoff producing field. This is used
to convert area-normalized pesticide mass in the mass-
input file to actual mass (m2).
60
area
real
Area of water body (m2).
61
depthO
real
Depth at which the input concentrations of physical
parameters (e.g., suspended solids, CHL., etc) were
measured.
62
depthmax
real
Maximum depth that water can rise before overflow
(m).
63
spray(i)
real
Mass of pesticide (kg) delivered from spray drift
corresponding to dates of appdate sim ref(i)
64
flowaveraging
integer
Number of days that are used to average the influent
water flow. If = 0, then the flow rate to be used in the
program is the average flow rate of the entire
simulation.
65
baseflow
real
Provided an additional constant flow through the
waterbody m3/s
66
Cropped fraction
real
Holds the Fraction of Cropped Area. Of the
watershed. Only used so that it is recorded in the
output. Program does not use these values for
calculations
6.2.2 ZTS Input File
The ZTS file contains daily mass inputs, water flows, and sediment deliveries. The ZTS file is
automatically created by the PRZM model or it may be manually created. It must be named as:
inputfilename. zts
where inputfilename is the same as that used above for the Input File and likewise specifies the
full path and name of the file. The ZTS file has a format as shown in Table 5. Each line (except
the first three) represents the daily values for each input variable. Data on each line may be
separated by a space or comma. The number of data lines in the file must correspond to the
number of days in the meteorological file.
Appendix 1, 47
VVWM, pg. 32
-------
Table 5. ZTS File Format.
Line #
Data
1
not read
2
not read
3
not read
4
X, X, X, 0, B, MRp, MEp, MR1, ME1, MR2, ME2
N
X, X, X, Q, B, MRp, MEp, MR1, ME1, MR2, ME2
Where
N refers to the last line in the ZTS file. It corresponds to the number of records in the
meteorological file.
X is dummy data that is not used, but must be in place. In a PRZM-generated ZTS file
these are the year, month, and day values.
Q is the daily water per field area that flows into the water body (cm/ha/day). This is
used for calculating washout and volume changes of the water body, if these options
are chosen.
B is the daily solids per field area that enters the water body (tonnes/ha/day) and is used
for burial if that option is chosen.
MRp is mass of pesticide per field area entering water body by runoff (g/ha/day)
MEp is mass of pesticide per field area entering water body by erosion (g/ha/day)
If degradate 1 is being simulated (nchem >1), then the following would be entered:
MR1 is mass of degradate 1 per field area entering water body by runoff (g/ha/day)
ME1 is mass of degradate 1 per field area entering water body by erosion (g/ha/day)
If degradate 2 is being simulated (nchem =2), then the following would be entered:
MR2 is mass of degradate 2 per field area entering water body by runoff (g/ha/day)
ME2 is mass of degradate 2 per field area entering water body by erosion (g/ha/day)
6.2.3 Meteorological File
The meteorological file is specified in line 30 of the input file. This file has the same formatting
as that required by PRZM. The fortran formatting for each line is:
IX, 312, 4F10.0
With the input variable of: MM, MD, MY, PRECIP, PEVP, TEMP, WIND
where
MM = meteorological month
MD = meteorological day
MY = meteorological year
PRECIP = precipitation (cm/day)
Appendix 1, 48
VVWM, pg. 33
-------
PEVP = pan evaporation data (cm/day)
TEMP = temperature (°C)
WIND = wind speed (cm/sec)
Example Partial Meteorological File:
010161
0 .
.00
0 .
.30
9.
5
501.
. 6
240.
, 3
010261
0 .
.10
0 .
.21
6.
3
368 .
. 0
244 .
, 3
010361
0 .
.00
0 .
.28
3.
5
488 .
. 3
303.
, 0
The meteorological file determines the simulation time. The simulation will start at the first date
and end with the last date in this file. Dates must be continuous in the file. The file does not
have to start or end on any particular calendar date; the program accepts partial years.
6.3 Output Files
6.3.1 Regulatory Summary Output File
A summary file that contains USEPA regulatory values for concentration is produced for
each chemical simulated and is named:
outputfilename_scenario_ID_waterbodytext_Parent-Degradate.txt
where
outputfilename - as specified in Line 1 of input file.
scenario ID - as specified in Line 29 of input file.
waterbodytext - Depending on the water body simulated, this will be "Custom", "Pond",
or "Reservoir" if simtypeflag (Input Line 57) = 1, 2, or 3, respectively
Parent-Degradate - This will be "Parent", "Degradatel", or "Degradate2" and indicates
which of the products are contained in the file.
6.3.2 Daily Values Output File
An output file that contains the daily values for water body depth, water column
concentration, and benthic pore water concentration is created with the name:
outputfilename_scenario_ID_waterbodytext_Parent-Degradate_daily.txt
7 Summary
Many of the individual processes and components of the USEPA VVWM (e.g.,
metabolism, photolysis, volatilization) are consistent with EXAMS. The VVWM differs from
EXAMS in ways that are intended to improve upon modeling methods. This includes improving
the characterization of temporal variability, hydrologic balances, and the efficiency and speed at
which computations are made. These differences are summarized below:
Appendix 1, 49
VVWM, pg. 34
-------
1. The VVWM changes parameter values on a daily basis (e.g., temperature, wind, flow),
corresponding to the daily input data from the meteorological file and from PRZM.
EXAMS changes parameters on a monthly basis, using calendar month averages for
values.
2. The VVWM can implement daily changes in temperature, which are based on the
preceding 30-day average air temperature, thereby simulating the temperature lag of
water bodies with air temperature. EXAMS can only make changes on a monthly basis,
and temperatures used in the standard water bodies do not lag air temperatures, but
instead are current calendar month averages.
3. The VVWM considers variations in the water body volume due to hydrologic inputs;
EXAMS does not.
4. The VVWM is solved analytically and is specifically designed to solve the standard two-
region OPP water body scenarios.
Appendix 1, 50
VVWM, pg. 35
-------
8 References
Banks, R. B., 1975. Some Features of Wind Action on Shallow Lakes. Journal of the
Environmental Engineering Division., ASCE. 101(EE5), 813-827.
Burns, L.A., Cline, D.M., and Lassiter, R.P., 1982. Exposure Analysis Modeling System
(EXAMS): User Manual and System Documentation. EPA-600/3-82-023, U.S. EPA.
Burns, L.A., 1997. Exposure Analysis Modeling System (EXAMS II) Users Guide to Version
2.97.5. EPA/600R-97/047, U.S. EPA.
Burns, L.A., 2000. Exposure Analysis Modeling System (EXAMS): User Manual and System
Documentation. EPA/600/R100/081, U.S. EPA.
Carsel, R., J. Imhoff, P. Hummel, J. Cheplick, and A. Donigan, 1997. PRZM 3.1 Users Manual,
National Exposure Research Lab, Office of Research and Development, U.S. Environmental
Protection Agency, Athens, Georgia.
Burns, L.A., 1985. Models for predicting the fate of synthetic chemicals in aquatic ecosystems,
in: Validation and Predictability of Laboratory Methods for Assessing the Fate and Effects of
Contaminants in Aquatic Ecosystems, ASTM STP 865, T.P. Boyle, Ed., American Society of
Testing Materials, Philadelphia, pp 176-190.
Cusler, E.L., 1984. Diffusion: Mass Transfer in Fluid Systems, Cambridge University Press, New
York
Liss, P.S., 1973. Processes of Gas Exchange Across an Air-Water Interface. Deep Sea Research,
20(3), 221-238.
Schwarzenbach, R.P., Gschwend, P.M., and Dieter, D.M., 1993. Environmental Organic
Chemistry, John Wiley & Sons, New York.
Appendix 1, 51
VVWM, pg. 36
-------
Appendix 2. User Guidance for the Point Source Calculator
Appendix 2, 52
-------
Point Source Calculator
User Guidance
(Revised July 31, 2018)
Contact Information:
Dirk F. Young
U.S. Environmental Protection Agency
Washington, DC
young. dirk@epa. gov
Purpose: Point Source Calculator for calculating chemical concentrations due to direct chemical
inputs to the water body.
Menu Items
File manipulations are performed on the menu bar. The first menu item is File, with submenus
Retrieve All and Save All. Retrieve All will open a file browser and allow a user to upload a
previously created input file into the interface. The input files are text files that can be created
either with the PSC interface or with a text editor. The Save All command will open a file
browser and allow the user to save the inputs from the PSC interface into a text file.
The naming of output files is determined by the name of the file saved or retrieved. The name
and directory of the output files are always presented at the bottom of the GUI.
Users must use either Retrieve All or Save All before running a simulation. If not, an error
message will appear instructing the user to do so. This is necessary because the use of Save or
Retrieve establishes the location where output files will be created.
Chemical Tab
The chemical properties tab allows users to enter of chemical properties. The definitions are
summarized here:
Chemical Properties Section
Chemical ID can be used to name the chemical that is being studied. The content of this box is
not used in the program nor is it used for file naming.
Sorption Coefficient either as Koc or Kd, both in mL/g. Koc is the organic-carbon-normalized
sorption coefficient; Kd is the adsorption-desorption coefficient. Sorption coefficients are the
same in all compartments.
Water Column Half Life is the half-life (days) of a chemical in the water column. This
parameter is applied to all phases of the chemical in the water column (unlike hydrolysis or
photolysis inputs). If there is no degradation, leave this parameter blank.
Appendix 2, 53
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Temperature Associated with the Water Column Value is the temperature (°C) at which the
water column degradation study was conducted. During a simulation, the degradation rate is
adjusted by temperature, with this temperature input being the reference.
Photolysis Half Life is the near-surface aquatic half-life (days) of the chemical due to photolysis.
If there is no degradation, leave this parameter blank.
Photolysis Reference Latitude is the latitude that the photolysis value is intended to simulate.
Hydrolysis Half Life is the half-life (days) of the chemical due to hydrolysis at the simulated pH.
A half-life of zero is interpreted to mean that the compound does not degrade by this process.
Benthic Half Life is the half-life (days) of the chemical in the benthic compartment. This
parameter acts on all phases of the pesticide/chemical substance in the benthic compartment. If
there is no degradation, leave this parameter blank.
Temperature Associated with the Benthic Compartment Value is the temperature (°C) at which
the benthic metabolism study was conducted.
Volatilization Section
No Volatilization - Checking this option will exclude volatilization.
Estimate Henry's Constant - Checking this option will cause the program to calculate Henry's
Constant from molecular weight, vapor pressure, and solubility.
Use Henry's Constant - Checking this option will cause the program to use the input value for
the Henry's Law constant.
Molecular Weight is the molecular weight of the chemical (g/mole). This parameter only affects
the volatilization rate.
Vapor Pressure is the vapor pressure (torr) of the compound at a representative temperature to
be simulated. This parameter only affects the volatilization rate and only if "Estimate Henry's
Constant" is selected.
Solubility is the solubility (mg/L) of the pesticide/chemical substance at a representative
temperature to be simulated. Solubility is used only in the volatilization routine; it does not cap
concentrations in this program. This parameter only affects the volatilization rate.
Heat of Henry is the enthalpy of phase change from aqueous solution to air solution
(Joules/mole). This enthalpy can be approximated from the enthalpy of vaporization
(Schwarzenbach et al., 1993), which can be obtained from EPA's Estimation Program Interface
(EPI Suite™) among other sources. Enthalpy for pesticides/chemical substance obtained in a
literature review ranged from 20,000 to 100,000 Joules/mole (average 59,000 Joules/mole).
Some example enthalpies for pesticides/chemical substance are shown below:
Appendix 2, 54
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Metalochlor 84,000 Joule/mole, Feigenbrugel et al. 2004
Diazonon 98,000 Joule/mole, Feigenbrugel et al. 2004
Alachlor 76,000 Joule/mole, Gautier et al., 2003
Dichlorvos 95,000 Joule/mole, Gautier et al., 2003
Mirex 91,000 Joule/mole, Yin and Hassett, 1986
Lindane 43,000 Joule/mole, Staudinger et al. (2001)
EPTC 37,000 Joule/mole, Staudinger et al. (2001)
Molinate 58,000 Joule/mole, Staudinger et al. (2001)
Chlorpyrifos 17,000 Joule/mole, Staudinger et al. (2001)
Enthalpies can also be estimated by EPI Suite™. Open the software, then select the
HENRYWIN subprogram on the left of the EPI Suite™ screen. On the top menu of the
HENRYWIN window item, select the ShowOptions, then select Show Temperature Variation
with Results. Enter the chemical name of interest and then push the Calculate button. EPI
Suite™ will give the temperature variation results in the form of an equation: HLC (atm-
m3/mole) = exp(A-(B/T)) {T in K}. The enthalpy of solvation in Joules/mole is equal to
8.314*B. Example of enthalpies estimated from EPI Suite™ are shown below:
Pendamethalin 62,000 Joules/mole
Carbaryl 58,000 Joules/mole
Carbofuran 54,000 Joules/mole
Molinate 54,000 Joules/mole
Endosulfan 37,000 Joules/mole
Reference Temperature for Henry's constant is the temperature at which the vapor pressure,
solubility, and Henry's Law constant apply or were measured at (°C).
Henry's Constant (atm-m3/mole): Allows Henry's Law Constant to be entered directly when it
is available. If Henry's Law Constant is not available, it can be calculated automatically by
checking the appropriate radio button.
Mass Release Schedule
There are 3 ways to input mass into the system: by specifying a repeating schedule, by reading
an input time series file that specifies the daily mass, or by reading a PRZM5 standard output
file.
Specify Mass
Choosing this option allows the user to specify a repeating schedule that runs through the entire
simulation. Up to three mass input schedules can be superimposed upon each other.
Offset - the number of days after the start of the simulation before the mass input pattern begins.
Days On - the number of consecutive days that mass is input into the system for this schedule.
Appendix 2, 55
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Days Off- the number of consecutive days that mass is not input into the system for this
schedule.
Mass (kg/day) - the mass input into the system during the Days On of this schedule.
Use a Time Series File
Choosing this option allows the program to read a file that contains daily values for water flow
and mass. The structure is one day of data per line with the data separated by whitespace (blanks
or tabs with the amount of whitespace being inconsequential). The first three columns are
dummies (program does not use them), but a user may wish to reserve these for day, month, year
for their own records. The next two columns are water flow (m3) and chemical mass (kg) that
occur for that day. A typical file may look like the following:
dummy dummy dummy water (m3) mass (kg)
The length of the file does not have to correspond to the weather file. But the program will
assume that the first day of the time series will correspond to the first day of the weather file, and
it will assume that the values are in chronological order and that there is no missing days or data.
Use PRZM5 Output File
Choosing this option allows the program to read a standard PRZM5 ".ZTS" file which specifies
the daily mass as well as water flow into the system. The required file has the same structure as a
PRZM5 .ZTS file (Young and Fry, 2014). All data are delimited by whitespace and each line
represents one day of data. Data must be in chronological order and must include every day of
the simulation. In the .ZTS file, the order of the data on a line is as follows:
year, month, day, daily flow (cm), daily sediment (tonnes), massl (g/cm2), mass2 (g/cm2), ...plus
other data
The PSC does not use all data in the .ZTS file. The PSC reads in the .ZTS file as follows:
dummy, dummy, dummy, daily flow (cm), dummy, mass entering (g/cm2), dummy
where dummy is a place holder (the number should be in the file, but the PSC does not use it).
Only the 4th (daily flow) and the 6th (massl) are used by the PSC. The remaining data serve
only as place holder and can be replaced with a zero, so a typical file with 5 days of data may
look like this:
1 1 2014 123.9
1 2 2014 144 1.8
1.7
A s Q 119.90
One April 14 1.23e2
1.9e4
0.01
0 0 0 0.12 0 1.34 0
0 0 0 0.17 0 1.34 0
0 0 0 0.13 0 0 0
Appendix 2, 56
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0 0 0 0.00 0 0 0
0 0 0 0.17 0 100.6 0
Watershed Area:
The time series input file is structured as a PRZM5 output file as if the inputs were normalized to
an area as represented by a watershed. Thus, total flow and total mass delivered into the PSC is
the input file values times the watershed area using appropriate unit conversions.
Toxicity Tab
This Tab allows the user to enter Concentrations of Concern (CoCs) that correspond to several
time averaging schemes. The program will use these values for analysis if the user chooses to
check the Do Toxicity Analysis box at the top of the page. The program will calculate the
number of days that the concentrations are above the CoCs and how many consecutive days that
the concentrations remain above the CoCs.
Scenario Tab
Scenario ID is text that will be used in the output file naming. It is helpful if it is indicative of
the scenario characteristics.
The Get Weather button allows specification of a weather file. The weather file should be
organized without a header and into the following white-space-delimited columns:
date, precipitation (cm), pan evaporation (cm), average temp (°C), wind speed (cm/s)
The date should be presented as a number consisting of the two-digit numerical values for month
day year and compiled together for example December 15, 1992 should be written as 121592.
January 3, 1991 should be written as 010391 (or 10391). The program will read the entire date
value in as a single integer and parse the value. Because the file is recognized as being white-
space delimited the date should not contain any internal spaces. For example, February 7, 1992
which is 020792 can be written as 20792 but not as 2 792. Daily metrological files for the United
States that will work for PSC are available from the US EPA at:
https://www.epa.gov/ceam/tools-data-exposiire-assessment. The files at that address contain
additional columns of information that have no effect on PSC.
Width of Mixing Cell [m] is the width of receiving water body.
Depth of Mixing Cell [m] is the depth of receiving water body.
Length of Mixing Cell [m] is the width of receiving water body. Note that for a flowing water
body such as a stream or river, this length value should correspond roughly to twice the
dispersivity (2D/v, where D is the dispersion coefficient and v is the velocity of the stream or
river) characteristic of the flowing water body. A good starting value may be around 30 meters
as estimated from the median of data in Fisher et al. (1979).
Use Constant Flow Rate [m3/sec] specifies the base flow through the receiving water body.
No Base Flow—there will be no constant flow through the system.
Appendix 2, 57
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DFAC[-] is a parameter defined as is in EPA's exposure analysis modeling system (EXAMS). It
represents the ratio of vertical path lengths to depth. Default value is set to 1.19 as suggested by
EXAMS documentation.
Water Column SS [mg/L] is the suspended mass in the water column. Default value is set to that
used by the USEPA/OPP standard farm pond.
Water Column Biomass [mg/L] is the biomass in water column which impacts photolysis and
has a very minor impact on sorption.
Chlorophyl [mg/L] represents the chlorophyll concentration in the water column. Default value
is set to that used by the EPA standard farm pond. This parameter only affects the photolysis
rate.
Water Column Foc is the fraction of organic carbon associated with suspended sediment. Default
values are set to those used by the EPA standard farm pond.
Water Column DOC [mg/L] represents the dissolved organic carbon concentration in the water
column.
Benthic Depth [m] is the depth of the benthic compartment. This is another difficult to estimate
parameter; however, literature and EPA's own calibrations suggest about 0.05 m.
Benthic Porosity is the porosity of the benthic compartment: [pore space volume per total
volume]. Default value is set to that used by the USEPA/OPP standard farm pond.
Bulk Density [g/cm3] is the rationally defined bulk density: [mass of sediment per total volume
of sediment]. Default value is set to that used by the USEPA/OPP standard farm pond.
Benthic Foc is the fraction of organic carbon associated with benthic sediment. Default value is
set to that used by the USEPA/OPP standard farm pond.
Benthic DOC [mg/L] represents the dissolved organic carbon concentration in the water column.
Default value is set to that used by the USEPA/OPP standard farm pond.
Benthic Biomass [g/m2] biomass per square meter in the benthic zone. This parameter has little
influence on results; it is a holdout from early model development. Default value is set to that
used by the USEPA/OPP standard farm pond.
QT[-] (not user accessible in PSC) is the Q10 value for metabolism. Fixed in the PSC to a value
of 2.
Mass Transfer Coefficient [m/s] represents the mass transfer coefficient between the water
column and the benthic zone. It accounts for all means of mass transport and is referenced to the
Appendix 2, 58
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surrogate driving force of aqueous concentration differences. It is a difficult parameter to
measure. Literature and EPA's own calibrations suggests a starting estimate of 10"8 m/s.
Results Tab
This page provides a graph of daily aqueous concentrations and some analyses with the
Concentration of Concern (CoC). The first columns of results (Total Cone, or Pore Water) gives
the maximum value of the chemical over the averaging periods specified on the Toxicity page.
The Days > CoC column present a fraction which is the ratio of the total number of days
exceeding the CoC to the total number of days in the simulation.
Additional analyses and summaries are provided in the main output file, including the maximum
number of sequential days above the CoC. The main output file will have a name that starts with
the Outfile Family Name (see bottom of the Point Source Calculator GUI) and will be appended
with the scenario ID and "Parent.txt". So, for a chemical in which the user save the inputs as
ChemA and used a scenario VirginiaMountains, the output file will have the name
ChemA_VirginiaMountains_Parent.txt. This file will contain all the results from the simulation,
some of which do not appear in the interface so a more detailed analysis is available if desired.
More Info Tab
This page provides additional useful information characterizing the chemicals behavior. The way
the chemical tends to distribute itself is presented as well as a comparison of the different
mechanisms of dissipation.
Output Files
Two output files are generated. They can both be located in the Working Directory as specified
at the bottom of the interface. All files associated with a particular run will have the Output file
Family Name (also specified at the bottom of the interface) in the file name. One output file will
contain all the post processed output that is in the Results tab as well as some additional
analyses. The other file will contain daily concentrations in both the water column and the
benthic region.
Appendix 2, 59
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Citations
Feigenbrugel,V. Calve,S.L. Mirabel,P., Louis, F. (2004). Henry's law constant measurements for
phenol, o-, m-, and p-cresol as a function of temperature, Atmospheric Environment, 38(33),
5577-5588
Fisher, H.B, List, E.J, Koh, R.C.Y., Imberger, J., and Brooks, N.H. (1979) Mixing in Inland and
Coastal Waters. Academic Press, Ney York, NY. pp. 126-127.
Gautier, C., Stephane Le Calve, Philippe Mirabel (2003). Henry's law constants measurements of
alachlor and dichlorvos between 283 and 298 K, Atmospheric Environment, 37(17) 2347-2353.
Schwarzenbach, R.P., Gschwend, P.M., and Imboden, D.M. 1993. Environmental Organic
Chemistry, John Wiley & Sons, New York.
Staudinger, J. and Roberts, P.V., 2001. A Critical Compilation of Henry's Law Constant
Temperature Dependence for Organic Compounds in Dilute Aqueous Solutions. Chemosphere,
44(4), 561-576.
Yin, C.; Hassett, J. P. (1986) Gas-partitioning approach for laboratory and field studies of mirex
fugacity in water. Environ. Sci. Technol. 1986, 12, 1213-1217.
Young, D.F. and Fry, M.M. 2014. PRZM5 A Model for Predicting Pesticide in Runoff, Erosion,
and Leachate: User Manual. EPA-734-F-14-002. United States EPA, Washington DC.
Appendix 2, 60
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