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OFFICE OF

CHEMICAL SAFETY AND

POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

PC Code: 053001

DP Barcodes: 404861

MEMORANDUM July 17, 2013

SUBJECT: Metaldehyde: Tier II Drinking Water Exposure Assessment for the Existing 

Watercress Use, the Existing Special Use for Invasive Mollusk Eradication, 

and the Proposed Wetland Taro Use (Reg. Nos. 6836-107 (technical) and

71096-13).

TO: Barbara Madden, Risk Manager

Laura Nollen, Risk Manager Reviewer

Risk Integration, Minor Use, and Emergency Response Branch

Registration Division

FROM: James Hetrick, Ph.D., Senior Science Advisor

 Melanie Biscoe, Physical Scientist

Environmental Fate and Effects Division

THROUGH: Dana Spatz, Branch Chief

Environmental Risk Branch 3

Environmental Fate and Effects Division (7507P)

Executive Summary & Background

Metaldehyde (2,4,6,8-tetramethyl-1,3,5,7-tetraoxacyclooctane) is a molluscicide used to control 

slugs and snails. Interregional Research Project #4 (IR-4) requested new or expanded uses of 

metaldehyde for the following crops/crop groups:

 Grass (grown for seed)

 Leaf petioles subgroup 4B

 Mint

 Taro (wetland)

 Corn (field)

 Soybean (regional registration)

 Caneberry subgroup 13-07A

 Bushberry subgroup 1307-B

 Berry, low growing, subgroup 13-07G
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When considering this request, the Environmental Fate and Effects Division (EFED) determined 

that Wetland Taro would have the highest annual use rate of all the proposed new or expanded 

uses, as well as the highest annual rates for all existing uses. Although EFED estimated 

preliminary drinking water exposures for Wetland Taro, EFED determined that Wetland Taro is 

not appropriate for inclusion in a national drinking water assessment because its use is likely

limited to the Hawaiian island of Kauai. EFED also determined that Wetland Taro is not 

appropriate for inclusion in a regional drinking water assessment, as is applied to submerged

fields that release to surface water systems. Drinking water estimates for such an aquatic 

application would not be reflective of actual drinking water concentrations, because ground 

water is the source of drinking water in Hawaii’s Taro-producing areas and the majority of the 

Hawaiian Islands generally (based on the Safe Drinking Water Information System (SDWIS), 

accessed at http://www.epa.gov/enviro/facts/sdwis/search.html).

When reviewing the metaldehyde labels, EFED determined that the existing Watercress use had 

a higher application rate than the remaining proposed uses, as well as a higher application rate 

than the other agricultural uses on the metaldehyde label. With regard to non-agricultural uses, 

EFED determined that the Special Use for Exotic Mollusk Eradication had the highest 

application rate. EFED has therefore prepared a drinking water assessment for the existing 

watercress use and the existing “Special Use” for exotic mollusk eradication. The “Special Use” 

use pattern and application rate are based on information in IR-4’s amended label submission for 

“OR-CAL Slug-Fest 4.0” (EPA Reg. No. 71096-13). The watercress use pattern and application 

rate used in this assessment are based on the label for “Slug-Fest All Weather Formula” (EPA 

Reg. No. 71096-4) approved on September 11, 2012, as watercress is not included on the label 

for “OR-CAL Slug-Fest 4.0” (EPA Reg. No. 71096-13).

The recommended Estimated Drinking Water Concentrations (EDWCs) for use in human health 

risk assessment are based on the exotic mollusk eradication use rate of 2 lbs a.i./acre applied in 6 

broadcast (ground) applications for a total maximum annual application of 12 lb a.i./acre. 

Although there is no specific re-application interval stated in the “Special Use” box on the 

metaldehyde label, EFED assumed there would be a minimum 14-day interval based on label 

directions for applications to grass (seed crop), forage and hay. EFED also assumed that 

applications would occur for six consecutive years because this is maximum number of siteyears for use according to APHIS. The computer models PRZM/EXAMS (surface water) and 

PRZM-GW (ground water) were used to develop the EDWCs for use in the human health risk 

assessment. 

The highest estimated 1-in-10-year annual peak (acute), annual average (chronic) and 30-year 

annual average (cancer) concentrations of metaldehyde in surface water sources of drinking 

water are 205, 136, and 83 μg/L, respectively. For ground water sources of drinking water, the 

estimated peak and post-breakthrough average concentrations were 1,740 and 635 μg/L, 

respectively (Table 1).

EFED recommends using the maximum concentrations from the ground water simulation for 

human health risk assessment (i.e., 1,740 and 635 μg/L). EFED recognizes that, due to 

substantial uncertainty in the metaldehyde hydrolysis half-life data, there is uncertainty in the

ground water simulation results. Refinement options in this regard are limited, as an additional 
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hydrolysis study is not likely to provide sufficient data to narrow the metaldehyde hydrolysis 

half-life, which may range from to 10 to 242 years. 

Table 1. Maximum Tier II estimated drinking water concentrations (EDWCs) in parts per 

billion (μg/L) resulting from applications of metaldehyde at an annual application rate of 

12 lb a.i./acre.

Drinking water source 

(model)

1-in-10 year 30-year simulation

Acute Chronic Acute Chronic

Surface water 

(PRZM-EXAMS) 205 136 - 83

Ground water 

(PRZM-GW) - - 1740 635

Data Uncertainties

The EPA Office of Pesticide Programs published the metaldehyde Reregistration Eligibility

Decision (RED) on July 27, 2006 and issued the associated Data Call-In on September 3, 2008. 

No additional environmental fate data were required for metaldehyde as part of the reregistration 

process, and EFED considers the environmental fate database for metaldehyde to be fulfilled.

There is substantial uncertainty in the metaldehyde hydrolysis half-life data, as calculated halflives range from 10 to 240 years (MRID 41114701). Because the longest hydrolysis studies are 

generally carried out over the course of one year, an additional hydrolysis study is not likely to 

provide sufficient data to narrow the metaldehyde hydrolysis half-life. This uncertainty has 

important implications for ground water simulation results, as PRZM-GW is highly sensitive to 

hydrolysis half-life inputs. These implications are discussed in more detail in the Drinking 

Water Modeling section.

Previous Drinking Water Assessments

The residue of concern in drinking water for human health risk assessment is metaldehyde 

parent. The most recent drinking water assessment was conducted for the 2006 metaldehyde 

RED. For surface water, EDWCs were generated using the Tier II PRZM-EXAMS model. The 

maximum 1-in-10 year peak EDWC was 110 μg/L (Florida cole crops scenario), the maximum 

1-in-10-year annual average EDWC was 55.8 μg/L (California leafy vegetable scenario), and the 

maximum overall average EDWC was 31.9 μg/L (California leafy vegetable scenario). For 

ground water, EDWCs were generated from a Tier I SCI-GROW analysis. The maximum 

EDWC for ground water was 62.5μg/L (based on ornamental use rates). 

It should be noted that the DWA for the RED included an EDWC for the watercress use. The 

watercress EDWC in the RED DWA was 1,500 μg/L, and was calculated by diluting the 

application rate in the volume of a 1-acre paddy with a water depth of 6 inches. This EDWC did 

not consider degradation, sorption, or distance from drinking water intake, and was therefore not 

used in the dietary assessment for the RED.
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Monitoring data for metaldehyde are limited. Metaldehyde was only an analyte in the 2010 and 

2011 USDA Pesticide Data Program (PDP). There were no detections of metaldehyde in surface 

water.

Current Assessment

The current assessment:

 Estimates drinking water concentrations expected from ground spray applications of 

metaldehyde on turf for the purposes of exotic mollusk eradication

 Estimates drinking water concentrations expected from application to watercress fields

 Estimates drinking water concentrations consistent with recent EFED policy changes 

concerning

Ground water modeling - EFED is implementing the use of PRZM-GW for Tier 1 and Tier 2 

modeling for residues in ground water1, while SCI-GROW model is being used as a Tier 1 

screen. Therefore both PRZM-GW and SCI-GROW estimates are reported in this assessment. 

PRZM-GW allows for the estimation of pesticide concentrations with multiple years of 

applications and the consideration of different scenarios. The SCI-GROW model represents a 

generic vulnerable site where one or multiple application(s) may be made within a single year.

The ground water EDWCs for metaldehyde are higher with PRZM-GW. The increase is due in 

part to the consideration of multiple years of application. The simulated average concentration 

and post-breakthrough averages from PRZM-GW are greater than the value estimated by SCIGROW because they represent multiple years of metaldehyde use.

 

1 Brady, Donald. 2012. Memorandum: Approval of PRZM-GW for Use in Drinking Water Exposure Assessment.
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Uses Assessed

The uses evaluated in this assessment are described in the following paragraphs, as well as in 

Table 22.

”Special Use” for Exotic Mollusk Eradication

The existing use sites for exotic mollusk eradication include the following:

 Fallow land

 Barrier strips

 Uncultivated non-agricultural areas

 Recreational areas

 Non-food or non-feed brush

 Weed or dense vegetative areas

 Railroad, pipeline, highway, power and telephone rights of way and roadsides

 Guardrails and fences

 Lumberyards

 Storage areas

 Seaports

 Industrial facility sites, including yards and walkways around industrial buildings, 

parking areas, parks, golf courses, other public areas

 Airports and similar industrial non-crop areas

This use is limited to State and/or Federal invasive mollusk eradication operations. Only ground 

(i.e., broadcast) applications are allowed for this use. The maximum single application rate for 

all exotic mollusk eradication uses is 2 lb a.i/acre, and the maximum annual application rate is 12

lb a.i./acre (i.e., 6 applications of 2 lb a.i/acre per year). Although the label does not specify a 

minimum reapplication interval, EFED assumes that a 14-day reapplication interval is used. 

This assumption is based on the 14-day minimum re-application interval for grass (seed crop), 

forage and hay specified on the label instructions for “OR-CAL Slug-Fest 4.0” (EPA Reg. No. 

71096-13) submitted by IR-4. With regard to surface water EDWCs, this assessment calculates 

EDWCs for application to turf, as the turf scenario is expected to be representative of (or 

sufficiently conservative with respect to) the use sites listed above. Use of a Percent Cropped

Area (PCA) adjustment when calculating turf EDWCs is not appropriate given the presence of 

non-agricultural and non-turf pesticide uses on the metaldehyde label2.

Watercress

Only ground (i.e., broadcast) applications are allowed for the watercress use (Table 3). The 

maximum single application rate for watercress is 2 lb a.i/acre, and the maximum annual 

application rate is 8 lb a.i./acre (i.e., 4 applications of 2 lb a.i/acre per year). The “Slug-Fest All 

Weather Formula” (EPA Reg. No. 71096-4) label states that no more than two applications may

 

2 Brady, Donald. 2012. Memorandum: Guidance on Development and Use of Percent Cropped Area and Percent 

Turf Area Adjustment Factors in Drinking Water Exposure Assessments: 2012 Update. 

http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html.
Page 6 of 28

be made per harvested crop, and that the second application can be made no closer than three 

days prior to harvest. The label also states that:

 The average depth of water in the watercress beds must be between 1 to 15 inches

 Metaldehyde applications are to be made to the watercress canopy

 The watercress field must be drained for a minimum period of 24 hours prior to 

metaldehyde application and must remain drained for 24 hours following the application

 When the flow-through irrigation cycle is resumed, the effluent from the first hour from 

metaldehyde-treated beds must be held or recirculated on-site for five days or longer

Although the label does not specify a minimum reapplication interval, EFED assumed a 20-day

reapplication interval based on growth and harvest patterns of watercress in Florida, a major 

watercress production state. 

Table 2. Metaldehyde Uses and Rates Assessed 

Use

Single 

Application 

Rate 

lb a.i./acre 

Re-application 

Interval 

Maximum 

Annual Rate lb 

a.i./acre 

Application 

Method

Percent Crop 

Area (PCA) 

Adjustment 

Factor*

Existing

“Special Use” for 

Invasive Mollusk 

Eradication

2 14 days 12 Ground n/a

Watercress 2 20 days 8 Ground n/a

*Only applicable for surface water modeling using PRZM-EXAMS.

Environmental Fate and Transport Characterization

Based on submitted laboratory data, metaldehyde is expected to be moderately persistent and 

mobile in the environment (Table 4). The major routes of dissipation for metaldehyde are

through runoff, leaching and aerobic degradation. 

Based on data submitted for one soil type, metaldehyde degrades under aerobic conditions with a 

half-life of approximately two months, eventually breaking down to acetaldehyde, which readily

mineralizes to carbon dioxide. Metaldehyde appears to be persistent in anaerobic environments, 

with a half-life of over 200 days reported for a soil maintained under anaerobic conditions. A 

Henry’s Law constant of 2.43x10-8atm-m

3

/mol and vapor pressure of 1.1x10-5torr suggest that 

volatility is not a major route of transport in the environment.
Page 7 of 28

Metaldehyde is not susceptible to either hydrolysis or photolysis under typical environmental 

conditions. Therefore, hydrolysis and photolysis are not expected to be major routes of 

dissipation in the environment. 

Metaldehyde is very mobile in soil, with adsorption coefficients (KF values) ranging from 0.10 to 

0.44 in four soils, and corresponding Koc values between 14.6 and 55.7. These relatively low 

values indicate potential for leaching and runoff in the open environment. 

Table 3. Physical/chemical properties and environmental fate source data for metaldehyde

Drinking Water Modeling

Property Value Source and/or Comments

Chemical Name

2,4,6,8-tetramethyl1,3,5,7-

tetraoxacyclooctane 

Molecular Weight 176.2 g/mole Epi Suite

Solubility in Water

200 mg/L (17°C) Product Chemistry, supported by 

Epi Suite results at 20°C

45,360 mg/L (25°C) Epi Suite

Vapor Pressure (25°C) 0.000011 torr Product Chemistry

Hydrolysis Half-life (25°C)

pH 5: 3,649-88,455

pH 7: 4,308-66,092 MRID 41114701

pH 9: stable 

Aqueous Photolysis Half-life (pH 7) stable MRID 41337401

Soil Photolysis Half-life stable MRID 41507701

Aerobic Soil Metabolism Half-life 67.2 days half-life for one soil 

MRID 41546001

Anaerobic Soil Metabolism Half-life 222 days half-life for one soil 

MRID 41507701

Organic Carbon-Normalized Soil 

Partition Coefficient (KFoc)

36.25 mL/g MRIDs 41228001, 41675101 

Average KFoc (41.2, 55.7, 14.6, 33.5)

Soil Adsorption Coefficient (KF) 0.234 mL/g

MRIDs 41228001, 41675101; 

Average KF(0.103, 0.223, 0.175, 

0.436) 

Henry’s Law Constant 2.43x10-8atm-m

3

/mol

Calculated from vapor pressure, 

solubility, and molecular weight 

per input parameter guidance
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Invasive Mollusk Eradication Use

Models and Scenarios Used

Detailed descriptions, documentation, and links for running the models discussed in the 

paragraphs below may found at http://www.epa.gov/oppefed1/models/water/index.htm. 

Surface Water Modeling

The estimated drinking water concentrations (EDWCs) for the invasive mollusk eradication use 

of metaldehyde were generated using PRZM3 (version 3.12.2 compiled 05/11/05, Carsel, 1997)

and EXAMS (version 2.98.04.06 compiled 04/24/05, Burns, 2002). PRZM simulates pesticide 

fate and transport as a result of leaching, direct spray drift, runoff and erosion from an 

agricultural field and EXAMS estimates environmental fate and transport of pesticides in surface 

water body. The EDWCs assessment for surface water uses single or multiple sites which 

typically represent a high-end exposure scenario from pesticide use on a particular crop or noncrop use site. PRZM and EXAMS were linked by the program (PE5, version 01 compiled 

07/23/07). The linked PRZM and EXAMS models simulate the impact of daily weather on the 

treated agricultural field over a period of thirty years. During this time, pesticide is washed-off of 

the field into the water-body by twenty to forty rainfall/runoff events per year. Each new addition 

of pesticide to the water-body adds to the pesticide which has arrived earlier either through 

previous runoff events or through spray-drift and begins degrading on the day it reaches the 

water. Longer-term, multiple-day average concentration values are calculated based on the peak 

day value and subsequent values considering degradation processes. 

Within the linked PRZM and EXAMS models, the “index reservoir” is used for drinking water 

assessments. The index reservoir is used in a manner similar to the “standard pond” used in 

ecological exposure modeling, except that flow rates have been modified to reflect local weather 

conditions. The index reservoir is approximately 82 m wide and 640 m long, with an area of 5.3 

ha. The area of the entire watershed is 172.8 ha. Weather and agricultural practices are 

simulated for 30 years so that the 1-in-10-year exceedance probability at the site can be 

estimated. The simulation is generated using 30 years of meteorological data, encompassing the 

years from 1961 to 1990.

In this assessment, EFED’s Standard and California Red-Legged Frog (RLF) scenarios were 

used within the linked PRZM/EXAMS. For turf, the FL and PA Standard Turf scenarios were 

used, as well as the CA Red-Legged Frog (RLF) Turf scenario. The metadata for each scenario 

identifies the growing season for the crop of concern in that scenario. In this assessment, 

EDWCs were calculated for each month of the growing season identified in the scenario 

metadata. For turf, EDWCs were calculated for the 15th day of each month of the growing 

season. 

The Percent Crop Area (PCA) is a generic watershed-based adjustment factor that is applied to 

pesticide concentrations estimated for the surface water component of the drinking water 

exposure assessment using PRZM/EXAMS with the index reservoir. When appropriate, the 
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EDWCs generated by the linked PRZM/EXAMS models are multiplied by the maximum percent 

of crop area (PCA) in any watershed (expressed as a decimal) generated for the crop or crops of 

interest. Guidance for using PCAs and a thorough discussion of this method and comparisons of 

monitoring and modeling results for selected pesticide/crop/site combinations is located at

http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html. In this assessment, PCA 

adjustment factors were not used because the label includes numerous agricultural and nonagricultural use sites for metaldehyde, and the co-occurrence of metaldehyde uses within a 

watershed cannot be ruled out. 

Ground Water Modeling

Tier I and II groundwater EDWCs for metaldehyde resulting from its exotic mollusk eradication 

use were derived with PRZM-GW (Pesticide Root Zone Model for Groundwater, version 1.0, 

August 31, 2012), using the GW-GUI (Graphical User Interface, version 1.0, August 31, 2012). 

PRZM-GW is a one-dimensional, finite-difference model that estimates the concentrations of 

pesticides in groundwater. It accounts for pesticide fate in the crop root zone by simulating 

pesticide transport and degradation through the soil profile after a pesticide is applied to an 

agricultural field. PRZM-GW permits the assessment of multiple years of pesticide application 

(up to 100 years) on a single site. Six standard scenarios, each representing a different region 

known to be vulnerable to groundwater contaminations, are available for use with PRZM-GW 

for risk assessment purposes. In the PRZM-GW simulations, each of these standard scenarios 

was used. PRZM-GW output values represent pesticide concentrations in a vulnerable 

groundwater supply that is located directly beneath a rural agricultural field. Note that, unlike in 

surface water modeling, PCAs are not used with PRZM-GW per EFED guidance. 

For comparison purposes, EDWCs were also calculated using the Screening Concentration in 

Ground Water model (SCI-GROW v2.3, compiled 08/08/03). SCI-GROW was developed by

fitting a linear model to ground water concentrations with the Relative Index of Leaching 

Potential (RILP) as the independent variable. Ground water concentrations were taken from 90-

day average high concentrations from Prospective Ground Water studies. The RILP is a function 

of aerobic soil metabolism and the soil-water partition coefficient. There is no PCA adjustment 

for SCI-GROW. The output of SCI-GROW represents the concentrations of metaldehyde 

residues that might be expected in shallow unconfined aquifers under sandy soils.

Modeling Input Parameters

The model input parameters for PRZM-EXAMS and PRZM-GW are summarized in Table 4. 

Model input values were prepared following EFED input guidance3,4. 

In contrast to surface water EDWCs calculated using PRZM-EXAMS, PRZM-GW EDWCs are 

highly impacted by hydrolysis half-life inputs. This is because hydrolysis is generally the only 

 

3

Parameters are selected as per Guidance for Selecting Input Parameters in Modeling the Environmental Fate and 

Transport of Pesticides; Version 2.1, October 22, 2009.

4 Baris, R.; Barrett, M.; Bohaty, R.; Echeverria, M.; Kennedy, I.; Malis, G.; Wolf, J.; Young, D. PRZM-GW Input 

Parameter Guidance; Health Canada, U.S. Environmental Protection Agency, October 12, 2012.
Page 10 of 28

transformation pathway considered at soil depths greater than one meter. Even if a chemical is 

considered stable to hydrolysis, PRZM-GW input parameter guidance recommends using a 

hydrolysis half-life calculated from the guideline hydrolysis study (OCSPP 835.2120) for better 

estimates of groundwater concentrations (this is supported by EFED’s comparison of PRZMGW results to groundwater monitoring data). In the case of metaldehyde, the calculated 

hydrolytic half-lives range from 10 to 242 years based on data from a 32-day hydrolysis study. 

This wide range of half-lives cannot be refined, as an additional hydrolysis study is not likely to 

provide sufficient data to narrow the metaldehyde hydrolysis half-life, 

Because of the imprecision in the hydrolytic half-lives calculated, as well as the uncertainty 

associated with extrapolating half-lives so far beyond the 32-day study duration, a hydrolytic 

half-life calculated for a pH of 7 using log-linear regression analysis was used as the PRZM-GW 

hydrolysis input parameter. This half-life is 4,716 days, as indicated in Table 4. For comparison 

purposes, upper-bound PRZM-GW EDWCs resulting from the assumption that no hydrolytic 

degradation of metaldehyde occurs are also reported in the Modeling Results section. SCIGROW input parameters include the maximum single application rate , the maximum number of 

applications, and the aerobic soil metabolism half-life as well as the Koc (36.25 ml/g).

Table 4. Input values used for modeling the metaldehyde mollusk eradication use on turf 

with PRZM-EXAMS and PRZM-GW.

Model Input Parameter Input Value and Unit

Used as a 

PRZM-GW 

Input?

Source/Comments

Maximum Application 

Rate Turf: 2.24 √

kg/ha Labeled Use, Per 

2006 RED Addendum

Maximum Number of 

Applications

6 √

Maximum yearly 

application rate stated on 

label (equivalent to 13.44 

kg/ha)

Method of Application Surface applied, not 

incorporated (In PRZMEXAMS, CAM = 1, IPSCND 

= 1 )

v

Label

Application Efficiency 0.99 (ground spray) Input parameter guidance

Spray Drift Fraction 0.064 (ground spray) Input parameter guidance

Type of Application Ground PRZM/EXAMS manual

Date of Application

15th day of each month.

√ (30 days 

after postemergence 

date in 

PRZM-GW 

scenario)
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Model Input Parameter Input Value and Unit

Used as a 

PRZM-GW 

Input?

Source/Comments

Minimum Interval 

between Applications

Turf: 0-14 days, depending 

on label interpretation; 

used 14 days in model

√

Label

Soil Partition Coefficient 

KF 0.234 ml/g √

MRID 41228001; Average 

KF (0.103, 0.223, 0.175, 

0.436) 

Henry’s Law Constant 2.43 x 10-8

Hydrolysis

stable √ (4716 

days)

MRID 41114701; although 

metaldehyde is stable, a 

specific half-life is used for 

PRZM-GW, as the model is 

sensitive to hydrolysis 

half-life values.

Aerobic Soil Metabolism 

Half-life 201 days √

MRID 41546001 (67-day 

half-life times 3 for single 

value)

Aerobic Aquatic 

Metabolism Half-life 

(25oC)

402 days

MRID 41546001 (201-day 

aerobic soil half-life times 

2)

Anaerobic Aquatic 

Metabolism Half-life 666 days

MRID 41507701 (222-day 

anaerobic soil half-life 

times 3)

Aqueous Photolysis Halflife

stable

MRID 41337401

Vapor pressure (25oC)

0.000011 torr

Product Chemistry

Solubility in water (25oC)

45,360 mg/L

EpiSuite

Molecular Wt.

176.2 g/mole

Epi Suite
Page 12 of 28

Modeling Results

Surface Water

For each PRZM-EXAMS scenario, EDWCs were calculated for each month of the growing 

season described in the scenario metadata. Table 5 reports the maximum EDWCs calculated for 

each PRZM-EXAMS (i.e., surface water) scenario. The maximum 1-in-10 year acute EDWCs in 

surface water ranged from 115 to 205 μg/L. The maximum 1-in-10 year annual surface water 

EDWCs ranged from 69 to 136 μg/L. The maximum 30-year average surface water EDWCs 

ranged from 47 to 83 μg/L. The California RLF Turf scenario resulted in the maximum 

EDWCs. The PRZM-EXAMS output data for the surface water EDWCs for the California RLF 

Turf scenario are attached in Appendix 2. 

Table 5. Maximum (Per Scenario) Tier II Surface Water EDWCs for Existing Invasive 

Mollusk Eradication (6 applications of 2 lb a.i./acre metaldehyde applied to turf each year 

with a reapplication interval of 14 days).

1. Use 

Assumptions

2. PRZMEXAMS 

Scenario

3. First 

Application 

Date

4. 1-in-10 year 

EDWC

5. 30-

Year 

Average

Peak Annual Overall

month-day μg/L

Ground

Application

CA RLF Turf

15-01 193 136 83

15-10 205 130 77

FL Turf

15-08 111 62 47

15-09 115 60 43

15-10 109 69 42

PA Turf 15-08 190 119 63

Ground Water

For each PRZM-GW scenario, EDWCs were calculated for each year in a 30-year simulation 

where metaldehyde was applied for the first 6 years only at the maximum annual rate of 12 lb 

a.i./acre per year. In each of the 6 years of application, it was assumed that there were 6 

applications of 2 lb a.i./acre. The first application in each year was assumed to be 30 days after 

the plant emergence date described in the scenario metadata. Each of the five subsequent 

applications is assumed to be 14 days apart based on the minimum re-application interval for 
Page 13 of 28

grass (seed crop), forage and hay specified on the label instructions for “OR-CAL Slug-Fest 4.0” 

(EPA Reg. No. 71096-13) submitted by IR-4. 

There is substantial uncertainty in the PRZM-GW modeling assumptions. First, the assumption 

that metaldehyde is only used for 6 consecutive years at the application site is based on 

information on the maximum number of application site-years submitted by the United States 

Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) as part 

of its comments on the 2006 Reregistration Eligibility Decision5. There are no restrictions on the 

metaldehyde label concerning how many years metaldehyde may be used at a particular site

targeted for invasive mollusk eradication. Second, selection of the within-year application 

window influences the EDWCs. For example, selecting an application window that starts at 

plant emergence versus one that starts 60 days after plant emergence can vary peak groundwater 

EDWCs by approximately ±50 μg/L. Third, there is uncertainty in calculating EDWCs using of 

a 14-day re-application interval, as a re-application interval not specified in the “Special Use” 

box describing the directions for invasive mollusk eradication. 

Table 4 reports the EDWCs calculated for the 30-year simulation for each PRZM-GW (i.e., 

ground water) scenario. The peak EDWCs in the 30-year simulation ranged from 731 to 1,740

μg/L. The simulation average EDWCs ranged from 181 to 557 μg/L, and the post-breakthrough 

average EDWCs ranged from 205 to 635 μg/L. The Wisconsin Corn scenario resulted in the 

maximum EDWCs for the invasive mollusk eradication use. Figure 1 shows the modeled 

increase and decline of metaldehyde concentrations in groundwater from this use over 30 years 

for the Wisconsin Corn scenario. It should be noted that the DelMarVa Corn and Florida Citrus 

scenarios resulted in EDWCs similar to those calculated for the Wisconsin Corn scenario.

For comparison purposes, EDWCs for the mollusk eradication use were also calculated using 

SCI-GROW, which resulted in an acute and chronic EDWC of 149 μg/L. The SCI-GROW 

output data are available in Appendix 1. 

 

5

Seeley, Kenneth R. October 4, 2006. USDA APHIS Comments on Metaldehyde RED. May be found at 

www.regulations.gov in docket ID number EPA-HQ-OPP-2005-0231.
Page 14 of 28

Table 6. Tier I and II estimated drinking water concentrations (EDWCs) in parts per billion (μg/L) 

resulting from 6 consecutive years of metaldehyde use in an invasive mollusk eradication program 

where 6 applications of 2 lb a.i./acre are applied each year with a reapplication interval of 14 days.

Application 

Method PCA Scenario

30-Year Simulation*

Peak Simulation 

Average

PostBreakthrough 

Average

μg/L

SCI-GROW

Ground n/a n/a 149

PRZM-GW

Ground n/a

FL Citrus 1720 365 377

DELMARVA Corn 1730 291 296

FL Potato 1180 220 231

GA Peanuts 731 181 205

NC Cotton 1260 290 322

WI Corn 1740 557 635

*Note that while this is a 30-year simulation, the EDWCs are a result of only 6 years of consecutive use.

Figure 1. Estimated Groundwater Concentrations of Metaldehyde from 6 years of use in a 30-Year 

Simulation using PRZM-GW Wisconsin Corn Scenario (ppb = μg/L)*

*Maximum Single Application rate 2 lb a.i./acre; Maximum Annual 

Application rate 12 lb a.i./acre; Minimum Re-Application Interval 14 

days; Initial application modeled is 30 days post-emergence, based on 

emergence date in the scenario
Page 15 of 28

As discussed in the Modeling Input Parameters section, there is additional uncertainty in the 

PRZM-GW EDWCs presented in for the invasive mollusk eradication use due to the uncertainty

in the hydrolysis half-life data. For comparison purposes, Table 7. includes the PRZM-GW 

results using a hydrolysis half-life of 4,719 as well as the PRZM-GW results using the 

assumption that no hydrolytic degradation of metaldehyde occurs. 

Table 7. PRZM-GW 30-year simulation outputs resulting from 6 consecutive years of 

metaldehyde use in an invasive mollusk eradication program where 6 applications of 2 lb 

a.i./acre are applied each year with a reapplication interval of 14 days.

FL Citrus DELMARVA 

Corn FL Potato GA Peanuts

NC 

Cotton WI Corn

Hydrolysis Half-Life of 4,716 Days

Highest Daily Value 

(μg/L) 1720 1730 1180 731 1260 1740

Simulation Average 

Value (μg/L) 365 291 220 181 290 557

Average 

Breakthrough Time 

(days)

239 217 247 993 1055 874

Throughputs 46 50 44 11 10 13

Post Breakthrough 

Average (μg/L) 377 296 231 205 322 635

No Degradation as a Result of Hydrolysis

Highest Daily Value 

(μg/L) 1780 1770 1220 802 1380 1820

Simulation Average 

Value (μg/L) 380 297 228 200 318 584

Average 

Breakthrough Time 

(days)

239 217 247 993 1055 874

Throughputs 46 50 44 11 10 13

Post Breakthrough 

Average (μg/L) 392 302 239 226 353 666
Page 16 of 28

Watercress Use

Models and Scenarios Used

Detailed descriptions, documentation, and links for running the models discussed in the 

paragraphs below may found at http://www.epa.gov/oppefed1/models/water/index.htm. 

Surface Water Modeling

The Tier I EDWCs for the watercress use of metaldehyde were generated using the Tier 1 Rice 

Model (version 1.0, May 8, 2007). The Tier I Rice Model is used to estimate aquatic exposures 

for applications directly to water. The model estimates exposure in a water body holding a 10 cm 

water depth. When a pesticide is applied to a water body, the model assumes that it will 

instantaneously partition between a water phase and a sediment phase based on the partition 

coefficient of the chemical. The formula of the Tier I Rice Model v1.0 is as follows:

Cw = mai

 0.00105 + 0.00013Kd

and, if appropriate:

Kd = 0.01Koc

where:

Cw = water concentration (μg/L)

mai = mass applied per unit area (kg/ha)

Kd = water-sediment partitioning coefficient (L/kg)

Koc = organic carbon partitioning coefficient (L/kg)

The Tier I Rice Model was calibrated to generate estimates that are similar to or greater than 

dissolved concentrations measured within flooded paddies and in discharged paddy water. The 

calibration involved determination of the sediment interaction depth by calibrating the model to 

exceed residues measured in paddy water in the vast majority of dissipation studies. The model 

does not account for pesticide degradation, volatilization, dilution, or other dissipation processes.

The model was not evaluated or calibrated for concentrations measured in sediment. 

Based on the results of the Tier I modeling (see the Modeling Results section below), refined 

Tier II EDWCs for watercress were generated using the Pesticide Flooded Application Model 

(PFAM) (version 0.70). PFAM was developed as a flooded agriculture model for use in 

pesticide regulatory work. This model has been developed under USEPA Guidance6, and 

external independent evaluations have been positive7. 

 

6

USEPA 2009. Guidance on the Development, Evaluation, and Application of Environmental Models. EPA/100/K09/003 March 2009. US Environmental Protection Agency, Washington, DC.

7

Luo, Y. 2011. Report 263: Review and Evaluation of Pesticide Modeling Approaches in Rice

Paddies. California Department of Pesticide Regulation, Environmental Monitoring Branch. June

29, 2011. http://www.cdpr.ca.gov/docs/emon!surfwtr/protocols/study263protocol.pdf
Page 17 of 28

PFAM includes both hydrological processes and chemical processes. The water body depth may 

change due to precipitation, refill, drainage, evaporation, and weir-height changes. The model 

consists of two regions: a water column and a benthic region. Each individual region is 

completely mixed and at equilibrium with all phases within the individual region, and 

equilibrium within each region follows a linear isotherm8. The two regions are coupled by a firstorder mass transfer process. Equations for chemical processes (i.e., hydrolysis, bacterial 

metabolism, photolysis, and sorption) within each region are similar to the EXAMS model 

described earlier in this assessment. The model accounts for plant growth, metabolism, 

hydrolysis, photolysis, and metabolism in water, sediment, and soil (when no water is present), 

sorption, and volatilization. Water, sediment, and pesticide may flow out of the field. Changes in 

water body conditions (temperature, water levels, wind speed, etc.) and the resulting changes in 

degradation rates occur on a daily time step. Pesticide application and flooding sequences are 

mapped onto the time series in 1-year cycles for the length of the simulation. For instance, a 

typical exposure assessment assumes the pesticide is used every year for 30 years (the length of 

most EFED meteorological files). Output is delivered as released mass of pesticide as well as a 

daily time series of concentrations.

PFAM estimates exposure in the paddy effluent, which is eventually mixed with surface water 

which potentially flows to a drinking water intake. Mixing of paddy water with other surface 

water will result in dilution of the paddy water and reductions in the drinking water 

concentrations; this is especially true for estimated chronic concentrations. In order to account 

for this, PFAM was set up to flow (mass and volume) into the index reservoir, the standard 

drinking water body used by EFED to estimate surface water source drinking water 

concentrations using the varying volume pond model9. This portion of the model is still under 

development and evaluation, so the derived estimates using PFAM with the index reservoir are 

considered provisional. 

Ground Water Modeling

Ground water modeling was not conducted for watercress because PRZM-GW or SCI-GROW 

models are not appropriate to represent saturated flow condition with a constant head pressure 

from watercress bogs or rice paddies. 

Modeling Input Parameters

The model input parameters for the Tier I Rice Model were the average Kd (0.234 L/kg, MRID 

41228001) and the maximum single application rate (mai =4.65 kg/ha). The model input 

parameters for PFAM are summarized in Table 8 and 9. Model input values were prepared 

following EFED input parameter guidance10. The PFAM output file is shown in Appendix 3.

 

8

USEPA. 2004. Exposure Analysis Modeling System (EXAMS): User Manual and System Documentation.

EPA/600/R-00/81-023. May 2004. Ecosystems Research Division. United States Environmental Protection Agency.

9

FIFRA SAP. 2004. Refined (Level II) Terrestrial and Aquatic Models for Probabilistic and Ecological Assessment 

of Pesticides. March 30-April2, 2004. http://www.epa.gov/scipoly/sap/meetings/2004/033004 mtg.htm

10 USEPA 2013. Dirk Young, Office of Pesticide Programs Environmental Fate and Effects Division. Pesticides in 

Flooded Applications Model (PFAM) User Guidance Version 1.00 (Jan 23, 2013). Washington, DC.
Page 18 of 28

Table 8. PFAM Input Parameters for Metaldehyde Use on Watercress

6. Input Parameter 7. Value 8. Source and Comments

Chemical Property and Fate Inputs

Water Column Half-life (days) at 

25°C 402 MRID 41546001 (201 day aerobic soil 

half-life times 2)

Benthic Compartment Half-Life 

(days) at 25°C 666 MRID 41507701 (222 day half-life times 

3 for single value)

Unflooded Soil Half-life (days) at 

25°C 201 MRID 41546001 (67 day half-life times 3 

for single value)

Aqueous Near Surface Half-life 

(days) at 40° Latitude Stable MRID 41337401

Hydrolysis Half-life (days) Stable MRID 41114701 

Organic Carbon Partition 

Coefficient (mL/goc) (Koc)

23.4

Calculated Koc from Kd based on Input 

Parameter Guidance; Average Kd of 

0.234 from four soils (0.103, 0.223,0.175, 

0.436) MRID 41228001

Molecular Weight (g/mol) 176.2 Product Chemistry

Vapor Pressure (torr) at 25°C 0.00002 Product Chemistry

Solubility (mg/L) at 25°C 45,360 EPISuite

Heat of Henry (J/mol) 37,413 Estimated using HENR YWIN program in 

EPISuite 

Henry Reference Temperature (°C) 25 Product Chemistry/EPISuite

Application Inputs

Number of Applications 2 Label (EPA Reg. No. 71096-4)

Applied Mass (kg/ha) 2.352 Label (EPA Reg. No. 71096-4)

Application Dates 21-Oct and 10-Nov Based on information from MRID 

45718601 and Smith 200711.

Slow Release No

Label indicates that the product is slowrelease. Because the release rate is not 

specified, it is assumed that the a.i. is 

immediately released upon application.

Location Inputs

Meteorological files w12834.dvf

Meteorological data available in PRZMEXAMS 5 installation. Station 

corresponds to Daytona Beach, FL.

Crop Inputs

Zero Height Reference 1-Oct Based on information from IFAS 199912.

 

11 Smith, Erika N. 2007. Watercress (Nasturtium officinale) Production Utilizing Brook Trout (Salvelinus 

fontinalis) Flow-through Aquaculture Effluent. Thesis submitted to the Davis College of Agriculture, Forestry, and 

Consumer Sciences at West Virginia University. Morgantown, West Virginia.

12 IFAS, 1999. University of Florida Institute of Flood and Agricultural Sciences (IFAS) Extension document FPS590 (Originally published 1999, revised in 2007, reviewed in 2011).
Page 19 of 28

6. Input Parameter 7. Value 8. Source and Comments

Days from Zero Height to Full 

Height 19 Estimated based on information from Smith 200713.

Days from Zero Height to Removal 60 Estimated based on information from 

Smith 200713.

Maximum Fractional Areal 

Coverage 1 Conservative assumption

Physical Inputs

Reference depth (m) 0.1524 Corresponds to a water depth of 6 

inches.

Watershed Inputs

Area of Application (m2) 362,880

21% of the watershed drainage acreage 

(1,728,000 m2), based on the PCA for rice 

as a conservative assumption.

Area of Surrounding Watershed

(m2)

1,365,120

Difference in watershed drainage 

acreage (1,728,000 m2) and area of 

application

Curve Number of Surrounding 

Watershed 83

Curve number for a contoured pasture 

with Fair hydrologic conditions and a Soil 

Group of D

 

13 Smith, Erika N. 2007. Watercress (Nasturtium officinale) Production Utilizing Brook Trout (Salvelinus 

fontinalis) Flow-through Aquaculture Effluent. Thesis submitted to the Davis College of Agriculture, Forestry, and 

Consumer Sciences at West Virginia University. Morgantown, West Virginia.
Page 20 of 28

Table 9. Planting, Flooding, Drainage, Application, and Harvest Event Inputs for Florida 

Watercress PFAM Modeling (shaded events are flood events used in the PFAM “Floods” Tab)

Event Day Date

Water 

Height 

(m)

Fill 

Level 

(m)

Wier 

Level 

(m)

Min 

Level

Turn 

Over 

(m) 

(per 

day)

Plant 

Seedlings 0 1-Oct 0 0 0 0 0

First Flood 1 2-Oct 0.1524 0.1524 0.1524 0.1524 14.2

Drain 19 20-Oct 0 0 0 0 0

Application 20 21-Oct 0 0 0 0 0

Flood with 

Retention 21 22-Oct 0.1524 0.1524 0.1524 0.1524 0

Flood with 

Turnover 26 27-Oct 0.1524 0.1524 0.1524 0.1524 14.2

Drain 39 9-Nov 0 0 0 0 0

Application 40 10-Nov 0 0 0 0 0

Flood with 

Retention 41 11-Nov 0.1524 0.1524 0.1524 0.1524 0

Flood with 

Turnover 46 16-Nov 0.1524 0.1524 0.1524 0.1524 14.2

Harvest 1 60 30-Nov 0.1524 0.1524 0.1524 0.1524 14.2

Modeling Results

EFED conducted Tier 1 and PFAM model simulations for metaldehyde use on watercress (Table 

10). 

Table 10. Maximum Tier I (Single Application) and Tier II estimated drinking water 

concentrations (EDWCs) in parts per billion (μg/L) 

Scenario Peak Chronic

μg/L

Tier 1 4,303

Tier 1 

PCA correction1 904

PFAM11,120 665

1- Rice PCA was used as a surrogate for watercress.

2-

EFED estimated acute and chronic EDWCs for metaldehyde using the refined Tier I Rice Model and 

Pesticide Flooded Application Model (PFAM) without the index reservoir. These estimated 

concentrations result in conservative estimates of drinking water concentrations and reflect possible 

concentrations that may occur when a drinking water intake is near the location where paddy water is 

released, and mixing with uncontaminated water is minimal. The Tier I Rice Model output has been 

shown to generate concentrations that are similar to measured concentrations in paddy waters, and is 

conservative when compared with surface water concentrations downstream of paddy discharges.

The percent crop area for rice (PCA=0.21) was used as surrogate PCA for watercress.
Page 21 of 28

Monitoring Data

Surface water data from one state and three nationwide pesticide monitoring programs were 

reviewed to provide additional context concerning the potential presence of metaldehyde in 

ambient water resources. These water monitoring programs do not target sample collection to 

peak use locations or application times for particular pesticides, nor do they target sample 

collection to high precipitation events (i.e., high-runoff events). Additionally, the sampling 

frequency in these monitoring programs was not designed to capture peak concentrations. 

Therefore, the monitoring data reported by these programs may not represent the highest 

concentrations in ambient surface water, and are considered qualitative information for the 

purposes of this ecological risk assessment.

USGS-EPA Pilot Reservoir Monitoring Program – Metaldehyde is not an analyte in this

program.

USGS National Water-Quality Assessment (NAWQA) Program Data – Metaldehyde is not 

analyte in the USGS NAWQA monitoring program. NAWQA program data are available at 

http://infotrek.er.usgs.gov/nawqa_queries/index.jsp.

California Surface Water Monitoring Program Data - Metaldehyde is not analyte in the

California surface water monitoring program. California Surface Water Monitoring Program 

data are available at http://www.cdpr.ca.gov/docs/emon/surfwtr/surfdata.htm.

Pesticide Data Program (PDP) Data - The PDP drinking water monitoring project provides nontargeted pesticide occurrence data in raw, finished (treated/tap), and ground water in the United 

States. In 2010 and 2011 PDP surface water samples were tested for metaldehyde and there 

were no detections. PDP data are available at http://www.ams.usda.gov/AMSv1.0/pdp
Page 22 of 28

Appendix 1. SCI-GROW Outputs for Invasive Mollusk Eradication Use

 SCIGROW

 VERSION 2.3

 ENVIRONMENTAL FATE AND EFFECTS DIVISION

 OFFICE OF PESTICIDE PROGRAMS

 U.S. ENVIRONMENTAL PROTECTION AGENCY

 SCREENING MODEL

 FOR AQUATIC PESTICIDE EXPOSURE

SciGrow version 2.3

chemical:Metaldehyde

time is 5/21/2013 13:17:48

------------------------------------------------------------------------

 Application Number of Total Use Koc Soil Aerobic

 rate (lb/acre) applications (lb/acre/yr) (ml/g) metabolism (days)

------------------------------------------------------------------------

 2.000 6.0 12.000 3.62E+01 201.0

------------------------------------------------------------------------

groundwater screening cond (ppb) = 1.49E+02 

************************************************************************
Page 23 of 28

Appendix 2. PRZM-EXAMS Output for Invasive Mollusk Eradication Use – California 

RLF Turf Scenario

Initial Application on January 15

stored as FINALCAdwTurf14_15-01.out

Chemical: Metaldehyde

PRZM environment: CATurfRLF.txt modified Tueday, 20 February 2007 at 12:03:48

EXAMS environment: ir298.exv modified Thuday, 29 August 2002 at 15:34:12

Metfile: w23234.dvf modified Wedday, 3 July 2002 at 09:04:22

Water segment concentrations (ppb)

Year Peak 96 hr 21 Day 60 Day 90 Day Yearly

1961 48.05 47.79 46.7 44.72 43.28 30.17

1962 89.69 89.23 87.28 85.22 83.59 62.4

1963 172 171 168 159 153 110

1964 98.54 98.03 95.9 94.33 92.98 73.46

1965 68.99 68.64 67.55 65.35 63.94 50.49

1966 109 108 106 105 103 76.8

1967 194 193 189 182 177 137

1968 221 220 217 209 201 154

1969 225 224 221 217 212 161

1970 125 124 122 120 118 92.95

1971 135 134 131 127 123 89.59

1972 80.45 80.03 78.28 76.35 74.95 59.36

1973 154 153 151 149 144 109

1974 109 109 107 102 99.8 78.46

1975 168 167 165 159 154 117

1976 140 140 138 136 132 98.12

1977 113 112 110 106 102 77.82

1978 94.5 94.01 91.95 90.28 88.71 67.85

1979 186 185 182 176 173 125

1980 141 140 138 136 132 100

1981 108 108 106 104 101 78.96

1982 95.15 94.81 93.06 89.68 87.54 67.16

1983 96.14 95.63 93.88 91.02 88.02 66.61

1984 66.74 66.51 65.14 62.95 61.55 47.97

1985 90.58 90.11 88.07 83.52 80.2 58.05

1986 88.64 88.16 86.16 83.38 80.95 61.88

1987 121 121 119 117 115 83.04

1988 79.94 79.51 77.8 75.84 74.43 58.38

1989 61.94 61.61 60.2 57.83 56.1 43.64

1990 78.9 78.48 76.71 74.73 73.16 53.66

Sorted results

Prob. Peak 96 hr 21 Day 60 Day 90 Day Yearly

0.032258064516129 225 224 221 217 212 161

0.0645161290322581221 220 217 209 201 154

0.0967741935483871194 193 189 182 177 137

0.129032258064516 186 185 182 176 173 125

0.161290322580645 172 171 168 159 154 117

0.193548387096774 168 167 165 159 153 110

0.225806451612903 154 153 151 149 144 109

0.258064516129032 141 140 138 136 132 100

0.290322580645161 140 140 138 136 132 98.12

0.32258064516129 135 134 131 127 123 92.95

0.354838709677419 125 124 122 120 118 89.59

0.387096774193548 121 121 119 117 115 83.04

0.419354838709677 113 112 110 106 103 78.96

0.451612903225806 109 109 107 105 102 78.46

0.483870967741936 109 108 106 104 101 77.82

0.516129032258065 108 108 106 102 99.8 76.8

0.548387096774194 98.54 98.03 95.9 94.33 92.98 73.46

0.580645161290323 96.14 95.63 93.88 91.02 88.71 67.85

0.612903225806452 95.15 94.81 93.06 90.28 88.02 67.16

0.645161290322581 94.5 94.01 91.95 89.68 87.54 66.61
Page 24 of 28

0.67741935483871 90.58 90.11 88.07 85.22 83.59 62.4

0.709677419354839 89.69 89.23 87.28 83.52 80.95 61.88

0.741935483870968 88.64 88.16 86.16 83.38 80.2 59.36

0.774193548387097 80.45 80.03 78.28 76.35 74.95 58.38

0.806451612903226 79.94 79.51 77.8 75.84 74.43 58.05

0.838709677419355 78.9 78.48 76.71 74.73 73.16 53.66

0.870967741935484 68.99 68.64 67.55 65.35 63.94 50.49

0.903225806451613 66.74 66.51 65.14 62.95 61.55 47.97

0.935483870967742 61.94 61.61 60.2 57.83 56.1 43.64

0.967741935483871 48.05 47.79 46.7 44.72 43.28 30.17

0.1 193.2 192.2 188.3 181.4 176.6 135.8

Average of yearly averages: 82.994

Inputs generated by pe5.pl - Novemeber 2006

Data used for this run:

Output File: FINALCAdwTurf14_15-01

Metfile: w23234.dvf

PRZM scenario: CATurfRLF.txt

EXAMS environment file: ir298.exv

Chemical Name: Metaldehyde

Description Variable Name Value Units Comments

Molecular weight mwt 176.2 g/mol

Henry's Law Const. henry 2.43e-8 atm-m^3/mol

Vapor Pressure vapr .000011 torr

Solubility sol 45360 mg/L

Kd Kd .234 mg/L

Koc Koc mg/L

Photolysis half-life kdp days Half-life

Aerobic Aquatic Metabolism kbacw 402 days Halfife

Anaerobic Aquatic Metabolism kbacs 666 days Halfife

Aerobic Soil Metabolism asm 201 days Halfife

Hydrolysis:pH 7 0 days Half-life

Method: CAM 1 integer See PRZM manual

Incorporation Depth: DEPI cm

Application Rate: TAPP 2.24 kg/ha

Application Efficiency:APPEFF 0.99 fraction

Spray Drift DRFT 0.064 fraction of application rate applied to pond

Application Date Date 15-01 dd/mm or dd/mmm or dd-mm or dd-mmm

Interval 1 interval 14 days Set to 0 or delete line for single app.

app. rate 1apprate 2.24 kg/ha

Interval 2 interval 14 days Set to 0 or delete line for single app.

app. rate 2apprate 2.24 kg/ha

Interval 3 interval 14 days Set to 0 or delete line for single app.

app. rate 3apprate 2.24 kg/ha

Interval 4 interval 14 days Set to 0 or delete line for single app.

app. rate 4apprate 2.24 kg/ha

Interval 5 interval 14 days Set to 0 or delete line for single app.

app. rate 5apprate 2.24 kg/ha

Record 17: FILTRA

IPSCND 1

UPTKF

Record 18: PLVKRT

PLDKRT

FEXTRC 0.5

Flag for Index Res. Run IR Reservoir

Flag for runoff calc. RUNOFF total none, monthly or total(average of entire run)

Initial Application on October 15

stored as FINALCAdwTurf14_15-10.out

Chemical: Metaldehyde

PRZM environment: CATurfRLF.txt modified Tueday, 20 February 2007 at 12:03:48

EXAMS environment: ir298.exv modified Thuday, 29 August 2002 at 15:34:12

Metfile: w23234.dvf modified Wedday, 3 July 2002 at 09:04:22
Page 25 of 28

Water segment concentrations (ppb)

Year Peak 96 hr 21 Day 60 Day 90 Day Yearly

1961 50.42 50.17 47.45 31.68 22.38 5.518

1962 55.79 55.5 51.46 48.2 46.77 37.66

1963 60.71 60.41 57.8 51.46 49.76 40.84

1964 63.69 63.36 60.66 55.76 53.81 44.22

1965 168 167 104 72.02 58.92 48.28

1966 166 165 162 154 149 115

1967 170 170 166 158 153 116

1968 115 115 102 94.27 90.96 72.98

1969 119 118 114 111 103 86.17

1970 121 121 118 107 103 82.15

1971 120 119 117 111 107 80.9

1972 122 121 118 102 82.48 65.31

1973 239 238 201 143 117 97.58

1974 236 235 230 219 211 155

1975 150 150 147 140 135 101

1976 85.91 85.49 83.75 79.9 77.06 58.98

1977 75.88 75.48 72.86 65.99 63.6 51.34

1978 74.31 74.03 72.67 69.17 66.59 51.09

1979 150 149 89.63 61.23 53.95 45.15

1980 147 146 143 136 131 99.13

1981 148 141 119 103 99.51 81.49

1982 159 158 150 137 132 106

1983 193 192 152 145 139 106

1984 206 205 202 178 169 137

1985 199 198 194 185 178 132

1986 116 116 113 108 104 77.64

1987 91.58 91.12 89.2 84.96 81.81 62.06

1988 91.91 91.44 67.14 59.73 57.52 45.45

1989 90.03 89.58 87.69 83.58 80.53 63.09

1990 79.81 79.54 77.91 75.57 73.07 55.97

Sorted results

Prob. Peak 96 hr 21 Day 60 Day 90 Day Yearly

0.032258064516129 239 238 230 219 211 155

0.0645161290322581236 235 202 185 178 137

0.0967741935483871206 205 201 178 169 132

0.129032258064516 199 198 194 158 153 116

0.161290322580645 193 192 166 154 149 115

0.193548387096774 170 170 162 145 139 106

0.225806451612903 168 167 152 143 135 106

0.258064516129032 166 165 150 140 132 101

0.290322580645161 159 158 147 137 131 99.13

0.32258064516129 150 150 143 136 117 97.58

0.354838709677419 150 149 119 111 107 86.17

0.387096774193548 148 146 118 111 104 82.15

0.419354838709677 147 141 118 108 103 81.49

0.451612903225806 122 121 117 107 103 80.9

0.483870967741936 121 121 114 103 99.51 77.64

0.516129032258065 120 119 113 102 90.96 72.98

0.548387096774194 119 118 104 94.27 82.48 65.31

0.580645161290323 116 116 102 84.96 81.81 63.09

0.612903225806452 115 115 89.63 83.58 80.53 62.06

0.645161290322581 91.91 91.44 89.2 79.9 77.06 58.98

0.67741935483871 91.58 91.12 87.69 75.57 73.07 55.97

0.709677419354839 90.03 89.58 83.75 72.02 66.59 51.34

0.741935483870968 85.91 85.49 77.91 69.17 63.6 51.09

0.774193548387097 79.81 79.54 72.86 65.99 58.92 48.28

0.806451612903226 75.88 75.48 72.67 61.23 57.52 45.45

0.838709677419355 74.31 74.03 67.14 59.73 53.95 45.15

0.870967741935484 63.69 63.36 60.66 55.76 53.81 44.22

0.903225806451613 60.71 60.41 57.8 51.46 49.76 40.84

0.935483870967742 55.79 55.5 51.46 48.2 46.77 37.66

0.967741935483871 50.42 50.17 47.45 31.68 22.38 5.518
Page 26 of 28

0.1 205.3 204.3 200.3 176 167.4 130.4

Average of yearly averages: 77.3666

Inputs generated by pe5.pl - Novemeber 2006

Data used for this run:

Output File: FINALCAdwTurf14_15-10

Metfile: w23234.dvf

PRZM scenario: CATurfRLF.txt

EXAMS environment file: ir298.exv

Chemical Name: Metaldehyde

Description Variable Name Value Units Comments

Molecular weight mwt 176.2 g/mol

Henry's Law Const. henry 2.43e-8 atm-m^3/mol

Vapor Pressure vapr .000011 torr

Solubility sol 45360 mg/L

Kd Kd .234 mg/L

Koc Koc mg/L

Photolysis half-life kdp days Half-life

Aerobic Aquatic Metabolism kbacw 402 days Halfife

Anaerobic Aquatic Metabolism kbacs 666 days Halfife

Aerobic Soil Metabolism asm 201 days Halfife

Hydrolysis:pH 7 0 days Half-life

Method: CAM 1 integer See PRZM manual

Incorporation Depth: DEPI cm

Application Rate: TAPP 2.24 kg/ha

Application Efficiency:APPEFF 0.99 fraction

Spray Drift DRFT 0.064 fraction of application rate applied to pond

Application Date Date 15-10 dd/mm or dd/mmm or dd-mm or dd-mmm

Interval 1 interval 14 days Set to 0 or delete line for single app.

app. rate 1apprate 2.24 kg/ha

Interval 2 interval 14 days Set to 0 or delete line for single app.

app. rate 2apprate 2.24 kg/ha

Interval 3 interval 14 days Set to 0 or delete line for single app.

app. rate 3apprate 2.24 kg/ha

Interval 4 interval 14 days Set to 0 or delete line for single app.

app. rate 4apprate 2.24 kg/ha

Interval 5 interval 14 days Set to 0 or delete line for single app.

app. rate 5apprate 2.24 kg/ha

Record 17: FILTRA

IPSCND 1

UPTKF

Record 18: PLVKRT

PLDKRT

FEXTRC 0.5

Flag for Index Res. Run IR Reservoir

Flag for runoff calc. RUNOFF total none, monthly or total(average of entire run)
Page 27 of 28

Appendix 3. PFAM Output for Watercress Use 

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**************** Inputs *******************

Chemical # 1

 402.000000000000 

Water Column System Halflife(except photo & hydrolysis = 

 666.000000000000 Benthic Compartment Halflife (except hydrolysis) = 

 1000000.00000000 Photolysis Halflife = 

 1000000.00000000 Hydrolysis Halflife = 

 176.200000000000 Molecular Wieght = 

 2.000000000000000E-005 Vapor Pressure = 

 45360.0000000000 Solubility = 

 23.4000000000000 Koc = 

 25.0000000000000 Aerobic Reference Temper = 

 25.0000000000000 Anaerobic Reference Temper = 

 40.0000000000000 Reference Latitude = 

 37413.0000000000 Enthalpy of Henry = 

 25.0000000000000 Henry Reference Temperature 

****************** End of Run ************************

******************************************************

Variable Volume Water Model, Version 0.0

*******************************************

Performed on: 7/17/2013 at 7:58

Standard Reservoir, Area = 52555 m2, Depth = 2.73 m 

Chemical # 1

Peak 1-in-10 = 1.12E+03 ppb

Chronic 1-in-10 = 6.65E+02 ppb

4-day avg 1-in-10 = 1.12E+03 ppb

21-day avg 1-in-10 = 1.11E+03 ppb

60-day avg 1-in-10 = 1.07E+03 ppb

90-day avg 1-in-10 = 1.07E+03 ppb

YEAR Peak 4-day 21-day 60-day 90-day Yearly Avg

 1 9.00E+02 8.99E+02 8.99E+02 8.94E+02 8.89E+02 5.92E+02

 2 9.29E+02 9.26E+02 9.18E+02 8.87E+02 8.71E+02 5.75E+02

 3 6.51E+02 6.50E+02 6.45E+02 6.30E+02 6.20E+02 4.02E+02

 4 8.49E+02 8.47E+02 8.45E+02 8.29E+02 8.15E+02 5.85E+02

 5 1.13E+03 1.12E+03 1.12E+03 1.09E+03 1.06E+03 6.05E+02

 6 9.00E+02 8.99E+02 8.99E+02 8.95E+02 8.93E+02 6.31E+02

 7 1.08E+03 1.08E+03 1.08E+03 1.07E+03 1.07E+03 6.56E+02

 8 8.66E+02 8.65E+02 8.65E+02 8.62E+02 8.60E+02 5.58E+02

 9 6.11E+02 6.07E+02 6.03E+02 5.90E+02 5.86E+02 4.79E+02

10 1.15E+03 1.14E+03 1.12E+03 1.06E+03 1.00E+03 6.57E+02

11 8.99E+02 8.96E+02 8.93E+02 8.78E+02 8.59E+02 5.19E+02

12 6.02E+02 6.02E+02 6.01E+02 5.90E+02 5.83E+02 4.51E+02

13 1.05E+03 1.05E+03 1.05E+03 1.04E+03 1.02E+03 7.06E+02

14 9.67E+02 9.66E+02 9.66E+02 9.56E+02 9.40E+02 6.69E+02

15 1.12E+03 1.12E+03 1.12E+03 1.11E+03 1.08E+03 5.83E+02

16 6.62E+02 6.57E+02 6.54E+02 6.02E+02 5.85E+02 4.68E+02

17 8.85E+02 8.83E+02 8.78E+02 8.47E+02 8.16E+02 5.19E+02

18 6.98E+02 6.94E+02 6.87E+02 6.27E+02 5.92E+02 3.49E+02

19 7.28E+02 7.27E+02 7.26E+02 7.14E+02 7.09E+02 5.06E+02

20 1.04E+03 1.04E+03 1.03E+03 9.86E+02 9.45E+02 6.57E+02

21 8.52E+02 8.48E+02 8.44E+02 8.14E+02 8.06E+02 4.72E+02

22 8.82E+02 8.78E+02 8.68E+02 8.26E+02 7.91E+02 4.22E+02

23 5.34E+02 5.27E+02 5.09E+02 4.66E+02 4.48E+02 2.94E+02

24 8.87E+02 8.85E+02 8.81E+02 8.68E+02 8.51E+02 5.87E+02

25 8.81E+02 8.71E+02 8.38E+02 7.50E+02 6.87E+02 5.04E+02

26 8.38E+02 8.32E+02 8.18E+02 7.79E+02 7.36E+02 4.85E+02

27 9.37E+02 9.31E+02 9.15E+02 8.72E+02 8.54E+02 6.66E+02

28 7.35E+02 7.26E+02 6.95E+02 6.39E+02 6.17E+02 4.23E+02
Page 28 of 28

29 8.32E+02 8.29E+02 8.21E+02 7.82E+02 7.47E+02 5.40E+02

30 9.05E+02 8.95E+02 8.34E+02 6.32E+02 5.22E+02 5.78E+02

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Effective compartment halflives averaged over simulation duration:

washout halflife (days) = 117.9621 

water col metab halflife (days) = 502.1136 

hydrolysis halflife (days) = 1000042. 

photolysis halflife (days) = 1.3676309E+08

volatile halflife (days) = 2446864. 

total water col halflife (days) = 95.50826 

zero burial

benthic metab halflife (days) = 831.8600 

benthic hydrolysis halflife (days) = 3527164. 

total benthic halflife (days) = 831.6638 

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