1 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

DATE: 

SUBJECT: 

TO: 

& 

FROM: 

THROUGH: 

June 25, 2013 

Acetochlor Drinking Water Assessment for Additional Food Uses on Peanuts and 

Sugar Beets (DP Barcode D411002; PC Code 121601} 

KaMe Davis, Risk Manager (RM 25} 

Hope Johnson, Risk Manager Reviewer 

Herbicide Branch 

Registration Division (7505P) 

Thurston Morton, Risk Assessor 

Elissa Reaves, Chief 

Risk Assessment Branch IV 

Health Effects Division (7509P~ -~ 

Environmental 

Michael R. Barrett, 

Risk 

Senior 

Branch 

Che

V . 

~ 

Environmental Fate and Effects Division (7507P} 

Mah T. Shamim, Ph.D., Chief 

Environmental Risk Branch V 

Environmental Fate and Effects Division (7507P) 

Estimated Surface and Ground Drinking Water Concentrations (EDWCs) for acetochlor, an 

acetanilide herbicide, were evaluated for the proposed new uses. This drinking water exposure 

assessment incorporates proposed use patterns for peanuts and sugar beets. Acute and chronic 

exposure estimates for surface supply scenarios representing t:lrese new uses remain below the 

maximum EDWCs previously estimated (see DP Barcode D359995, review dated 6/24/2009), 

however, well water exposure models have been updated, and the acute and chronic exposure 
OFFICE OF CHEMICAL SAFETY

AND POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

 

MEMORANDUM

DATE: June 25, 2013

SUBJECT: Acetochlor Drinking Water Assessment for Additional Food Uses on Peanuts and 

Sugar Beets (DP Barcode D411002; PC Code 121601)

TO: Kable Davis, Risk Manager (RM 25)

Hope Johnson, Risk Manager Reviewer

Herbicide Branch 

Registration Division (7505P) 

& Thurston Morton, Risk Assessor

Elissa Reaves, Chief

Risk Assessment Branch IV

Health Effects Division (7509P)

FROM: Michael R. Barrett, Senior Chemist

Environmental Risk Branch V

Environmental Fate and Effects Division (7507P)

THROUGH: Mah T. Shamim, Ph.D., Chief

Environmental Risk Branch V

Environmental Fate and Effects Division (7507P)

Estimated Surface and Ground Drinking Water Concentrations (EDWCs) for acetochlor, an 

acetanilide herbicide, were evaluated for the proposed new uses. This drinking water exposure 

assessment incorporates proposed use patterns for peanuts and sugar beets. Acute and chronic 

exposure estimates for surface supply scenarios representing these new uses remain below the 

maximum EDWCs previously estimated (see DP Barcode D359995, review dated 6/24/2009), 

however, well water exposure models have been updated, and the acute and chronic exposure 
2

estimates from groundwater sources have increased from previous estimates. The current

(partially revised) acute and chronic drinking water exposure estimates are summarized in Table 

1. Although EFED at this time recommends that the Agency rely on these estimates in human 

health risk assessments, a basis for a refined assessment which would likely lower the EDWCs is

presented in the following paragraphs. Specifically, the Table 1 estimates are based on Tier 1 

modeling; analysis of extensive targeted monitoring data for acetochlor indicates that actual 

exposure at drinking water wells may be significantly lower; further characterization of these 

data is provided below.

Table 1. Tier I Estimated Drinking Water Concentrations (EDWCs) for human health drinking 

water risk assessment.

DRINKING WATER 

SOURCE

(MODEL USED) 

HIGH EXPOSURE SCENARIO

(rate modeled)

MAXIMUM ESTIMATED DRINKING 

WATER CONCENTRATION 

(EDWC)c ( ppb); selected values in bold 

italics.

Conservative Values for Updated Drinking Water Exposure Estimate 

Groundwater

(PRZM-GW) FL Astatula sand Scenario – citrus1 Acute 129.0

Lifetime 82.6

Surface Water

PRZM-EXAMS

Ks sorghum

CA sugarbeet “OP” scenario2

Acute 74.9

Lifetime 4.84

Recommended values are based upon the most conservative Tier 1 modeling results; targeted surface and groundwater monitoring studies provide more insights into exposure levels under actual usage conditions and are 

discussed later in this review. 

1 Citrus is the nominal crop upon which the scenario estimated the highest EDWC is based, but results are used for 

all national assessments in a Tier 1 screen. 2 OP = Scenario parameters originally designed to evaluate exposure to organophosphate use areas; this scenario 

may or may not be appropriately conservative for a national evaluation of impacts from uses of non-OP pesticides 

such as acetochlor. 

Maximum monitoring concentrations representative of in targeted groundwater and surface water

monitoring studies are provided in Table 2 for comparison to the modeled EDWCs. In brief, the 

groundwater values in Table 2 are from observation wells expected to generally be more 

vulnerable than domestic drinking water wells (i.e., downgradient and closer to treatment areas 

and with a shallower screening depth only drawing from surficial aquifers). The surface water 

values are directly representative of either pre- or posttreatment water from community drinking 

water supplies.) Additional details are discussed later in this document. For both surface and 

ground-water sources the maximum monitoring values are lower than modeled values, but the 

differences are largest for the groundwater estimates (i.e., the maximum modeled acute and 

chronic concentrations are 174.1X and 1086.8X greater than the maximum values from 7-year 

sites at the 175 SGW monitoring sites; the differences are even larger when compared with the 

PGW well data.) 
3

A conservative adjustment of monitoring maximum values (to reflect expected concentrations of 

acetochlor if used at maximum label rates) based on aggregated usage intensity and, for finished 

water samples, water treatment acetochlor removal efficiency factors is presented in Table 3

(these values are not recommended for use in exposure assessments without further refinements 

which are beyond the scope of the current assessment). Even with these highly conservative 

adjustment factors modeled groundwater exposure acute and chronic maxima are still much 

higher than the monitored values adjusted for a maximum allowed usage rate. On the other hand, 

the preliminarily adjusted surface water monitoring values in Table 3 are very close to modeled 

values1

. Additional possible refinements of these monitoring informed exposure estimates are 

discussed later in this review.

Table 2. Actual maximum measured acute and chronic exposure Values in the ARP surface 

water drinking supply (SDWS) study. 

Daily Annual 7-year

Source (Study) Sample Type Maximum (ppb) Max. site

(ppb)

Max. site

(ppb)

Groundwater (SGW) Observation wells –

adjac. To treatments.

0.741 0.453 0.076

Groundwater (PGW) PGW on-site 0.027 0.008 <0.005

Surface water (SDWS) Finished drinking 

water

18.21 1.43 0.28

Surface water (SDWS) Raw drinking water 7.19 0.59 0.19

Table 3. Provisional exposure estimates based upon adjustments of measured acute and chronic 

exposure values in the ARP surface water drinking supply (SDWS) study (conservatively 

adjusted to reflect maximum label usage by application of acetochlor usage and/or water 

treatment weighting factors). 

Daily Annual 7-year

Source 

(Study)

Sample Type Maximum (ppb) Max. site

(ppb)

Max. site

(ppb)

Groundwater 

(SGW)

Observation wells –

adjac. To treatments.

3.05 1.86 0.31

Groundwater 

(PGW)

PGW on-site NC NC NC

Surface water

(SDWS)

Finished drinking 

water

74.9 33.79 6.62

Surface water

(SDWS)

Raw drinking water 74.9 7.70 2.48

 1 Note, however, that that the first cut adjusted acute value based upon the maxima single surface water measurement was 93.78 to 

430.3 ppb or up to about 6x greater than the maximum modeled value with PRZM/EXAMS. However, for acute exposure estimation in flowing 

waters, the adjusted value is only a truly valid extrapolation from the monitoring data maxima if all of the applied acetochlor in the watershed 

not only is applied at the maximum rate to the entire watershed but is all applied on the same day – a highly unlikely occurrence in a water 

supply watershed of any size. Still, when actual usage factors associated with intense monitoring data are available, additional analysis could 

be conducted to determine whether PRZM/EXAMS might somewhat underestimate peak exposure values for acetochlor in some usage 

scenarios.
4

Daily Annual 7-year

Source 

(Study)

Sample Type Maximum (ppb) Max. site

(ppb)

Max. site

(ppb)

These values are based upon the following adjustment factors:

1. The maximum measured SGW values are conservatively upwardly adjusted based upon the ratio of the 

maximum label rate to an approximate average annual application rate of acetochlor at the SGW sites.

2. PGW adjusted values were not calculated; however it is noted here that all measurements were in 

groundwater 15 to 30 feet directly below fields treated for 1 or 2 years at maximum label rates.

3. The PGW groundwater residue levels for parent acetochlor were below true quantification limits for all 

eight prospective groundwater monitoring study sites (including multiple observation wells per site) even 

though some of these sites were similar to the most vulnerable of the PRZM-GW scenarios. Consequently, 

no attempt was made to adjust the measured values to adjust for a higher use rate like was done with the 

SGW maximum detections.

4. The original SGW maximum values were adjusted upward to represent usage at the maximum rate in the 

entire watershed based upon an assumed usage intensity during the monitoring period. The assumed usage 

intensity is based upon the median acetochlor application rate per county acre during the7-year monitoring 

period from the 12 US counties with the highest planted corn intensities. An additional assumed water

treatment factor was applied to the surface water estimate – based upon assumption of acetochlor removal 

at a rate of 44.8% (the median removal rate for the 20 SDWS sites with the highest acetochlor 

concentrations and paired raw / finish water sampling; see DP Barcodes : 292340 and 292353: ”

Cumulative Drinking Water Exposure Assessment for Chloroacetanilides (Acetochlor and Alachlor”.

Background:

Acetochlor is currently registered on the following crops:

Field corn, production seed corn, silage corn, sweet corn, popcorn, Miscanthus and other nonfood perennial bioenergy crops, soybean, cotton, and grain and forage and grain sorghum. 

The proposed use rates for acetochlor on sugarbeets and peanuts (summarized in Table 4 and 

Table 5) are at or below other registered crops which all have a maximum annual application rate 

of 3.0 lb a.i./Acre or less. The crop production areas for the proposed registrations appear to have 

relatively little congruence with the areas where currently registered crops are grown.

Table 4. Summary of proposed uses of acetochlor. Use information was obtained from the 

current and proposed labels for MON63410 (WARRANT®) [EPA Reg. No. 524-591] herbicide.

Crop(s) 1

Single 

Application 

rate

(lb a.i./A)

Max. # 

applications 

per Season

Minimum 

Interval

(days)

PHI

(days)

Maximum 

Annual2

Rate

(1b a.i./A)

Sugar beets

(aerial or ground

application)

0.9 – 1.5 3 7 NS 3
5

Crop(s) 1

Single 

Application 

rate

(lb a.i./A)

Max. # 

applications 

per Season

Minimum 

Interval

(days)

PHI

(days)

Maximum 

Annual2

Rate

(1b a.i./A)

Peanuts

(aerial or ground 

application)

0.9 – 1.5 3 7 NS 3

1 Application method timing:

Sugar beets = Preplant / At-planting /Preemergence / Postemergence up to 6 leaf stage.

Peanuts = = Preplant / At-planting /Preemergence / Postemergence up to flowering stage.

2 Technically maximum per crop cycle.

Table 5. Proposed uses of acetochlor by soil type.

Soil % O.M.

Max. # 

applications 

per Season

Minimum 

Interval 

(days)

PHI 

(days)

Min. 

Single 

Rate (lb 

ai/A)

Max. 

Single 

Rate (lb 

ai/A)

Max. 

Annual 

Rate (lb 

ai/A)

Fine < 1.5 3 7 NS 0.938 1.425 3.000

Medium < 1.5 3 7 NS 0.938 1.275 3.000

Coarse < 1.5 3 7 NS 0.938 1.200 3.000

Fine >= 1.5 3 7 NS 0.938 1.500 3.000

Medium >= 1.5 3 7 NS 0.938 1.425 3.000

Coarse >= 1.5 3 7 NS 0.938 1.275 3.000

Methods:

A summary of the environmental fate parameters for acetochlor as used in the surface water and 

groundwater supply exposure estimation is provided in Table 6. For this drinking water exposure 

assessment concentrations of acetochlor degradates are not included because of significantly 

lower toxicological concerns for human health, however, EFED notes that in previous reviews 

we have reviewed available monitoring data which shows that degradates such as the acetochlor 

ethanesulfonic acid and acetochlor oxanilic acid have widespread impact on water resources and 

in particular have a significantly higher potential to leach to groundwater than parent acetochlor

(discussed in detail in DP Barcode: D292331 – “Revised Drinking Water Exposure Assessment 

for Acetochlor” ; 2/16/2006). 
6

Table 6. General fate and physical-chemistry data for acetochlor – as used for model 

input.

Parameter Value1 Source

Molecular Weight (g/mole) 269.77 Product chemistry

Solubility (25o C) (mg/l) 233 Product chemistry

Vapor Pressure (25o C) (torr) 2.8E-5 Product chemistry

Hydrolysis Half-life (25o C) (days)

 pH 3

 pH 6

 pH 9

Stable

Stable

Stable

Stable

MRIDs: 00064805, 

41565144, 41613301

Aqueous Photolysis Half-life (days) Stable MRIDs: 00131388, 

41565145

Soil Photolysis Half-life (days)

(not used for model input)

Stable MRIDs: 00131388, 

00160233, 41565146

Aerobic Soil Metabolism Half-life (days) 13.31 MRIDs: 00064805, 

41565147, 41613301, 

41963316, 41963317

Aerobic Aquatic Metabolism Half-life (days)

(EXAMS model input)

26.62 2 x Aerobic Soil 

Metabolism Half-life

Anaerobic Aquatic Metabolism Half-life (days)

(EXAMS model input)

2513 MRIDs: 41338501, 

41565148, 41613301, 

41963318

Organic Carbon Partitioning Coefficient (Koc, 

L/kg or Kd, L/kg)

Koc = 133 4

Koc = 28 5

MRIDs: 00031329, 

00064805, 41338502,

41565149, 41613301, 

41963319

Soil Partitioning Coefficient (kd, L/kg)

(Used as an alternate to Koc input for PRZM-GW 

and SCI-GROW models to represent sandy soil 

sites)

Kd = 0.126

Range is 

0.126 to 

23.2 6

1 Actual model input values are underlined. 

2 Based upon upper confidence interval of mean of four studies (Attebury, Ray, Drummer, and Spinks 

soils). Per EFED Input Parameter Guidance Document. Half-life range was 10 to 20 days.

3 No aerobic aquatic study available. Guidance Document.

4 Based upon average half-life of anaerobic soil metabolism studies (multiplied by 2 per Input Parameter 5 Average value, Range is 12.5 to 290 in ten soils.

6 Equivalent Koc value to the Kd of 0.126 from Lilly coarse sand, representative of the highest risk 

scenarios / sites modeled with PRZM-GW. 7 Spinks (2.4% OM) = 0.30, Sarpy (0.9% OM) = 0.34, Drummer (3.4% OM) = 0.69, Ray (1.2% OM) = 

0.29, Lily field (0.77% OM) = 0.19, Frensham (1.9% OM) = 1.48, East Jubiliee (2.6% OM) = 2.16, Old 

Paddock ((5.4% OM) = 4.37, French A (1.5% OM) = 2.8, and French B (8.0% OM) = 23.2

Groundwater Exposure
7

The newly implemented Pesticide Root Zone Model for Ground Water (PRZM-GW) model was 

used in this assessment at the Tier 1 screening level along with the Screening Concentration In 

GROund Water (SCI-GROW) version 2.3 . 

In previous drinking water assessments, both Tier 1 modeling with the Screening Concentration 

in Ground Water (SCI-GROW) model and a wealth of monitoring data indicated a significantly 

lower acetochlor parent exposure rate from drinking water wells than surface-water based 

drinking water supplies. However, since the previous assessment PRZM-GW has been 

developed. PRZM-GW can also be used at the screening level to evaluate exposure potential 

from wells that might draw from highly vulnerable groundwater sources: PRZM-GW Version 

1.0 may also be used for higher tier site and region-specific exposure estimates for vulnerable 

groundwaters that might be used as drinking water sources. More details on the exposure models 

discussed in this review can be found at:

http://www.epa.gov/oppefed1/models/water/index.htm 

Surface Water Exposure

The Tier II drinking water assessment was performed using PRZM (v3.12.2; May 12, 2005)/

EXAMS (v. 2.98.04.06; April 25, 2005)2 modeling with index reservoir (IR) scenarios and the 

national default percent cropped area (PCA) adjustment factor (i.e., assuming treatment of 91% 

of the acreage in the modeled watershed). PRZM-EXAMS simulations were conducted for 

parent acetochlor. Approved EFED scenarios for evaluation of drinking water exposure from 

both the sugar beet and peanut uses were used for evaluating the proposed new uses. A scenario 

designed for evaluation of organophosphates used on sugar beets in California was also run – the 

results of this one needs to be cautiously applied to a national assessment and there may be some 

potential to refine these results further if the need arises. The input and output files for the 

modeling are provided in Appendix 1.

Exposure Analysis

Groundwater Exposure Analysis Methodology and Results

PRZM-GW 1.0 and SCI-GROW 2.3 were run twice each as a Tier 1 screen for groundwater 

supplies. The two sets of runs differ in particular in the assumptions used for partitioning of 

acetochlor in the soil matrix. Using an average Koc value for test soils as model input to represent 

acetochlor partitioning acute exposure was estimated to be as high as 22.2 ppb (μg/L) and 

chronic (lifetime) exposure was estimated to be as high as 14.9 ppb. However, a close analysis of 

the adsorption data indicated the degree of adsorption was not proportionally related to the initial 

solution concentration for some test soils and. The calculated Koc coefficient for a sand soil with 

 2 Pesticide Root Zone Model /Exposure Analysis Modeling System.
8

relatively low organic carbon content was significantly lower than for the other test soils. The 

most vulnerable leaching scenarios in PRZM-GW are similarly sandy, low organic content soils 

(and such soils are similarly the basis for the SCI-GROW regression model). Both tier 1 models 

were therefore rerun using Kd or Koc inputs that were representative of the sandy soil. 

Use of acetochlor partitioning coefficient model inputs to more specifically represent the 

vulnerable sites modeled resulted in EDWCs that were up to 5 or 6 times larger than the EDWCs 

calculated using the mean Koc value of all test soils (compare Table 8 with Table 7.) The 

highest estimated EDWCs are 129 ppb for acute exposure and 82.6 ppb for lifetime exposure,

values obtained with the scenario that is based upon sandy soils with shallow ground water in 

Florida’s central ridge (where citrus is the predominant agricultural crop.) While these highest

values are recommended for use in this tier 1 assessment of groundwater-sourced exposure, we 

do note that modestly lower exposure levels were estimated with the Delaware Evesboro sand 

soil scenario, a site that clearly represents impacts of acetochlor use in vulnerable groundwater in 

areas with significant production of crops on the current or proposed acetochlor labels (108 ppb 

acute and 55.4 ppb chronic EDWCs). There are currently no scenarios based upon the proposed 

sugar beets use, but the GA Tifton loamy sand scenario was specifically designed to represent a

site with significant groundwater vulnerability in a major peanut production region.

Table 7. PRZM-GW Tier 1 Estimated Drinking Water Concentrations in groundwater resulting 

from the use of acetochlor, assuming average adsorption (Koc = 133.2).

Scenario1

(reference crop 

site)

Acute 

EDWC

(Highest 

Daily Value)

μg/L

Throughputs

(for 30-year 

simulation)

Chronic 

EDWC 

(Post 

Breakthrough 

Average)

2

μg/L

Average 

Simulation 

Breakthrough 

Time

Days

WI Coloma sand

(corn) 18.5 3.66 12.9 2994.5

DE Evesboro sand

(sweet corn / 

cucumber)

12.8 8.34 6.4 1315.1

FL Astatula sand 

(citrus - Central 

Ridge area)

22.2 8.27 14.9 1325.9

FL Pomona sand

(potato) 0.11 3.58 0.02 3058.9

GA Tifton sandy 

loam

(peanut)

1.64 3.46 1.02 3166.7

NC Norfolk loamy 

sand

(cotton)

1.66 5.13 0.91 2135.6

SCI-GROW 0.42 --- 0.42 ----
9

Scenario1

(reference crop 

site)

Acute 

EDWC

(Highest 

Daily Value)

μg/L

Throughputs

(for 30-year 

simulation)

Chronic 

EDWC 

(Post 

Breakthrough 

Average)

2

μg/L

Average 

Simulation 

Breakthrough 

Time

Days

SCI-GROW 0.42 --- 0.42 ----

Throughputs” is a measure of the pesticide mass flux. Specifically, this is the estimated throughput (pore 

volumes/ retardation factor) that occurred for the simulation. A throughput of one is required to expel the 

center of mass of a Dirac pulse input. Near complete breakthrough will require several throughputs.

The post breakthrough average is the temporal average concentration of pesticide in the represented 

aquifer over the simulation period after one throughput has occurred.

1 Textural class given is that of the top 0.2 meters of the soil profile of the simulated scenario, not the 

officially listed class for the soil series

2 These values are recommended for estimation of lifetime exposure.

. 

Table 8. PRZM-GW Tier 1 Estimated Drinking Water Concentrations in groundwater resulting 

from the use of acetochlor assuming the measured adsorption coefficent from a sandy, low 

sorptive capacity test soil (Kd = 0.126 or Koc = 281

).

Scenario2 / 

Model

First 

Application 

Date

Acute

EDWC

(Highest 

Daily 

Value)

μg/L

Throughputs

(for 30-year 

simulation)

Chronic 

EDWC 

(Post 

Breakthrough 

Average)

μg/L3

Average 

Simulation 

Breakthrough 

Time

Days

WI Coloma sand

(corn) May 1 77.2 12.62 46.0 868.5

DE Evesboro sand

(sweet corn / 

cucumber)

April 10 108 50.61 55.4 216.6

FL Astatula sand 

(citrus - Central 

Ridge area)

Mar. 1 129 46.1 82.6 238.2

FL Pomona sand

(potato) Feb. 15 106 44.56 48.7 246.0

GA Tifton sandy 

loam

(peanut)

May 1 30.6 11.04 13.3 992.8

NC Norfolk 

loamy sand

(cotton)

May 15 14.2 10.4 8.9 1053.8

SCI-GROW --- 0.42 --- 0.42 ----
10

Scenario2 / 

Model

First 

Application 

Date

Acute

EDWC

(Highest 

Daily 

Value)

μg/L

Throughputs

(for 30-year 

simulation)

Chronic 

EDWC 

(Post 

Breakthrough 

Average)

μg/L3

Average 

Simulation 

Breakthrough 

Time

Days

See General footnote in Table 7 for an explanation of “throughputs” and “post-breakthrough average”.

1 Kd taken from the tested coarse-textured soil with the lowest adsorption coefficients (Lilly field coarse sand, pH 

= , CEC = 1.9 meq/100g. For SCI-GROW the equivalent Koc value of 28 ml/g for Lilly field coarse sand was used 

as model input.

2 Textural class given is that of the top 0.2 meters of the soil profile of the simulated scenario, not the officially 

listed class for the soil series. 

3 These values are recommended for estimation of lifetime exposure.

Surface Water Exposure Analysis Methodology and Results

Each of the standard PRZM / EXAMS modeling scenarios available for the proposed uses on 

peanuts and sugar beets was used to simulate drinking water concentrations for the proposed uses 

and to supplement the modeling for previously registered uses in earlier reviews.

Table 9. Estimated surface drinking water concentrations (μg/L) of parent acetochlor for 

various durations of exposure using three scenarios to represent oilseed crop uses.

Duration of Exposure

Scenario

# of 

Applications

1st

Application 

Date

Peak Yearly 30-Year

NC Peanut 2 May 16 37.06 6.02 4.30

MN Sugar Beet 2 May 16 38.31 4.34 3.16

CA Sugar Beet 

(OP scenario) 2 Feb. 1 48.67 8.98 4.85

CA Sugar Beet 

(OP1 scenario) 3 Feb. 1 39.62 7.69 4.54

Highest Previously Modeled Surface Water EDWCs (see DP Barcode D359995 and D332524)

Various Various May 5 74.9 4.84 2.13

EDWCs are based upon the Index Reservoir modeled with the specified standard scenarios using PRZM / EXAMS.

Values presented are estimated upper-bound exposure levels (1 in 10 year return frequency except for the 30-year 

values). The default percent crop area value of 91% based on all agricultural land in the most intensely cropped 

watersheds was used (See: http://www.epa.gov/oppefed1/models/water/pca_adjustment_dwa.html ). The highest 
11

values (from all surface water modeling) for each exposure duration are in bold italics. 1 OP = Scenario parameters originally designed to evaluate exposure to organophosphate use areas; this scenario 

may or may not be appropriately conservative for the currently proposed uses of ethalfluralin on sunflower, 

safflower, and other oilseed crops.

Updated Comparison of Monitoring Data with Modeled Concentrations

In this section the most significant available monitoring datasets for acetochlor are reviewed in 

light of the new EDWCs with a greater emphasis on groundwater data than surface water data 

since the newest Tier 1 groundwater exposure model estimates are higher. A second part of this 

section examines how the monitoring results relate more directly to the modeling by 

extrapolating from the high end concentrations observed to estimate equivalent concentrations of 

acetochlor that would occur if acetochlor was used at the same maximum rates that were 

modeled.

Part 1: High-End Monitoring Concentrations

EFED has previously reviewed the available monitoring data for acetochlor in some detail (see, 

e.g., DP Barcode: D332524 - review dated 9/18/2006, DP Barcode: D332524 - review dated 

9/18/2006, DP Barcode: D332524 - review dated 9/18/2006, DP Barcode: D332524 - review 

dated 9/18/2006, ). For this review, three monitoring datasets were further examined for 

providing context to the groundwater-based EDWCs selected for drinking water exposure in 

dietary risk assessments:

1. US Geological Survey National Water Quality Assessment (NAWQA) program

2. The Acetochlor Registration Partnership (ARP) Midwest Corn Production Area Groundwater 

monitoring program (otherwise known as the “State Groundwater monitoring program 

(abbreviated as SGW in this review).

3. The Acetochlor Small-Scale Prospective Groundwater monitoring program (abbreviated as 

PGW in this review).

Additionally, we briefly summarize ARP data that were specifically targeted to surface water 

based drinking water supplies in high acetochlor use areas to provide an overall comparison of 

modeled water / drinking water concentrations and targeted monitoring data. The focus of the 

current assessment is on updated comparisons of monitoring data to groundwater model 

estimates (which are the driver for the current exposure assessment). 

USGS-NAWQA Groundwater Data

The NAWQA study sites have multiple objectives with only a subset of the study sites focused 

on high pesticide use areas (usually associated with agriculture). Nonetheless, the study is large 

in scope and the monitoring site characteristics are well documented and the program has proven 
12

to provide significant insights into the impact of many pesticides on surface and ground water 

sources.

Much of the NAWQA groundwater monitoring data is from drinking water wells3, making these 

data useful for exposure analysis. Some of the other sampling sites were for groundwater sources 

that might be shallower or more closely associated with the surface-applied pesticide than 

drinking water wells in the region. The data obtained for the current review were retrieved 

between May 9 and 13, 2013 from the US Geological Survey NAWQA Data Warehouse at:

http://infotrek.er.usgs.gov/apex/f?p=NAWQA:HOME:0

Overall, acetochlor monitoring was available from 12231 groundwater samples from 

approximately 7325 sites. Acetochlor was detected in eight groundwater samples at a 

concentration of 0.05 ug/L or greater with the maximum detection level being 0.159 ug/L. 

However, only 3 samples with detectable acetochlor were actually from groundwater wells – the 

other 5 samples were from effluent from a single agricultural drainage pipe / tunnel site (these

are typically installed at a depth of only 3 to 4 feet beneath the surface of otherwise poorly 

drained agricultural fields)

4

. Two samples from drinking water wells that had acetochlor parent 

concentrations above 0.05 ug/L with the highest drinking water detection being 0.114 ug/L from 

a public water supply in Marshall County, Tennessee. The highest concentrations detected were 

about 1000 times less than the Tier 1 acute EDWCs and about 500 times the chronic EDWCs 

(compare Table 10 with Table 1 and Table 8). Although we did not determine the specific sort of 

agriculture production practices locally associated with these sites, at least six of the eight 

samples are located in counties with significant corn production (the major acetochlor use site) 

and a history of moderate to heavy use of acetochlor (details are CBI and not discussed here)5. 

Table 10. Details of groundwater detections of acetochlor in the NAWQA monitoring program.

State County Station ID Site Type

Date of 

Sampling

Well 

Depth 

(feet)

DW?

Value

INDIANA HANCOCK 395124085510801 SB-GWD 7/8/2003 NS1 No 0.159

TENNESSEE MARSHALL 353734086423301 GW 10/12/2000 203 Yes 0.114 2

NEBRASKA POLK 411738097264301 GW 11/15/1996 51 No 0.11

INDIANA HANCOCK 395124085510801 SB-GWD 6/14/2003 NS No 0.081

INDIANA HANCOCK 395124085510801 SB-GWD 5/28/2003 NS No 0.0795

 3 DeSimone, L.A., P.A. Hamilton, and R.J. Gilliom. 2004. Quality of Water from Domestic Wells in Principal Aquifers of the United States, 

1991–2004: Overview of Major Findings. USGS Circular 1332. 

AND

Toccalino, P.L. and J.A. Hopple Quality of Water from Public-Supply Wells in the United States, 1993–2007: Overview of Major Findings. USGS 

Circular 1346.

4 See, e.g., http://www.extension.umn.edu/distribution/cropsystems/dc7740.html

AND

Agricultural Drainage Management Coalition. (undated). Drainage Water Management for Midwestern Row Crop Agriculture. Conservation 

Innovation Grant 68-3A75-6-116. Available at: 

ftp://ftp.wcc.nrcs.usda.gov/wntsc/waterMgt/AGWAM/PF_I_ExecSummaryCIG.pdf 5 Source: US Department of Agriculture Census of Agriculture available at: 

http://www.agcensus.usda.gov/Publications/Historical_Publications/index.php AND Confidential business information submitted by the 

Acetochlor Registration Partnership.
13

NEW JERSEY CAMDEN 394538074581601 GW 3/5/1998 90 Yes 0.0604

INDIANA HANCOCK 395124085510801 SB-GWD 6/14/2003 NS No 0.0594

TENNESSEE COFFEE 353341086074501 GW 5/30/2000 43 No 0.0568

INDIANA HANCOCK 395124085510801 SB-GWD 6/14/2003 NS No 0.0522

Data are all detections > 0.05 ug/L of acetochlor from a total of 12232 samples from 7325 sites.

Site Type Definitions:

SB-GWD = An underground pipe or tunnel through which groundwater is artificially diverted to surface water for 

the purpose of reducing erosion or lowering the water table. A drain is typically open to the atmosphere at the lowest 

elevation, in contrast to a well which is open at the highest point.

GW = A hole or shaft constructed in the earth intended to be used to locate, sample, or develop groundwater, oil, 

gas, or some other subsurface material.

1 NS = Not specified. Each of these sites are from agricultural drainage pipes (tile drains) which are presumable 

several feet or less below the ground surface.

2 From a duplicate sample, not confirmed by second analysis.
14

Acetochlor Registration Partnership Groundwater and Surface Water Monitoring

A common concern regarding the use of monitoring data in pesticide regulatory assessments is 

that the data are not sufficiently targeted to areas with a documented use pattern for the pesticide 

in question and/or the contextual information regarding usage and environmental conditions 

associated with the sampling sites is inadequate to properly interpret the monitoring results. 

While deficiencies in these data do still exist, the ARP monitoring studies to support acetochlor 

registration do have most of these desirable attributes enabling an informed comparison with 

EFED’s prediction models.

As noted above, these data have been previously reviewed by EFED in some detail, but 

comparisons are updated here in the context of updated EDWCs based upon revised Tier 1 

groundwater – based EDWC estimates. 

A summary of all of the Acetochlor Registration Partnership surface and groundwater data 

provided in Figure 1 on a chronic exposure basis. Briefly, these data are from the following 

studies (all targeted to acetochlor corn use areas):

PGW Prospective ground water study (monitoring program). Consisted of field-scale eight 

studies with mandated on-field applications of acetochlor during the first 1 or 2 years of 

the study. The eight sites were chosen to provide a geographically diverse representation 

of acetochlor corn use areas and the selected sites included both highly vulnerable and 

moderately vulnerable groundwater.

SDWS Surface drinking water supply (monitoring program). Focused solely on public water 

supplies utilizing surface water supplies in areas of high corn production (and with 

acetochlor usage documented during the course of the study).

SGW State ground water (monitoring program) – Monitoring with 175 observation wells 

located downgradient of corn fields (usually in corn-soybean rotations) with mandated 

acetochlor applications for approximately 3 out of the first 5 years of the 7-year 

monitoring program. No drinking water wells were sampled but the observation wells 

would in most cases be more vulnerable than any actual domestic drinking water wells in 

the study area.

Additional details on these studies can be found in the DP Barcode D292331 review dated 

2/16/2006. A comparative summary of the results of these studies in terms of the cumulative 

distribution of chronic exposure levels by site is provided in Figure 1. Overall, there was a 

much higher concentration at most of the surface water supply (SDWS) monitoring sites in most 

years than at either the PGW or SGW groundwater monitoring sites (i.e., the cumulative 

frequency distribution lines of the SDWS are to the right of the PGW and the SGW lines

meaning higher concentrations are observed more frequently.) The SDWS (surface water) 

concentrations were higher even though the SDWS sites all sampled from drinking water 

supplies whereas the PGW and SGW studies sampled groundwater in high acetochlor usage 

areas that was expected to be more vulnerable to contamination by applied pesticides than 

actual drinking water wells.
15

Surface and groundwater concentrations both tend to be substantially lower than modeled 

concentrations, but the relatively smaller differences in predicted and observed surface water 

concentrations are plausibly largely associated with differences in modeled usage intensity (and 

for acute exposure especially, underestimation bias factors6) and actual watershed usage 

intensity at the monitored sites (previous EFED reviews have discussed in detail acetochlor and 

alachlor usage intensity associated with surface water-based drinking water supply detection 

levels, see: DP Barcodes: 292340 and 292353, “Cumulative Drinking Water Exposure 

Assessment for Chloroacetanilides (Acetochlor and Alachlor)”, 2/7/2006) . 

Figure 1. Cumulative frequency distribution for parent acetochlor chronic exposure, based on the highest timeweighted annual mean at each site. PGW (Prospective Groundwater study) chronic exposures are the maximum 

TWAM concentrations for each cluster at 9-foot depth for lysimeters and shallow monitoring wells for ground 

water.

ARP Midwest Corn Production Area (State Groundwater) Study

A comparison of the EDWCs (which are taken from groundwater modeling results) with the 

ARP SGW results for the 3 highest and the 90th percentile exposure site is provided in Table 11. 

 6 In flowing surface waters, peak concentrations of many pesticides are observed in spikes that may last minutes, hours, or a day and 

the ratio of the actual peak concentration to the maximum measured concentration of a particular pesticide in a particular stream or other 

flowing water system can be large (depending on the water body, application and weather scenario, and the persistence and sorptivity of the 

pesticide. See: ?????

Cumulative Chronic Exposure 

Based on Maximum Acetochlor TWAMs 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 0.003 0.005 0.01 0.05 0.1 0.5 1 2 4 8 > 8

TWAM (ppb)

Cumulative 

Frequency

PGW-Shallow

Ground Water

PGW 9 ft lysimeters

SDWS-Finished

SDWS-Raw

SGW

Maximum Observed TWAMs:

PGW, 9 ft. = 0.178

PGW, gw = 0.008

SDWS, fin. = 1.428

SDWS, raw = 0.591

SGW = 0.520
16

Especially for acute exposure there is some potential for underreporting of the maximum 

exposure level because the sampling typically occurred at each of approximately 175 test sites 

about once a month (about 68 sample collections per site over a 7-year monitoring period) and 

the actual maximum exposure day at a particular site could therefore be larger. However, the 

relatively low flow velocities in the vadose zone and aquifer should make daily fluctuations in 

groundwater concentrations of a contaminant much smaller over a 30-day period than in flowing 

surface waters (this is also consistent with the predicted concentrations over time with the 

PRZM-GW model –see Figure 3 to Figure 14 in Appendix 1.

The modeled drinking water concentrations (from groundwater sources) exceeded the highest 

concentrations measured in the ARP monitoring by the following margins:

Acute EDWC / monitored maximum = 174.1X

Annual EDWC / monitored maximum = 264.5X

Long-term or Lifetime EDWC / monitored maximum = 1086x

Table 11. Comparison of high-end predicted PRZM-GW) and measured (ARP SGW or 

MWCPA study) acute and chronic acetochlor groundwater concentrations.

Source Site Acetochlor

Overall 

Max

Daily Value

Acetochlor

Average Daily Conc. 

for

Max. Year

Acetochlor

Multi-year

Average

EDWC or SGW study exposure duration

24 hr Annual 30- or 7-

year

PRZM-GW Modeling Highest of 6 

scenarios 129 119.8 82.6

ARP targeted 

Monitoring

#1 Site 

(99.5 %ile)

0.741

(MN25)

0.453

(MN25)

0.076

(KS14)

ARP targeted 

Monitoring

#2 Site 

(98.9 %ile)

0.534

(NE16)

0.178

(KS14)

0.057

(MN25)

ARP targeted 

Monitoring

#3 Site 

(98.3 %ile)

0.453

(KS09)

0.144

(KS19)

0.040

(KS17)

ARP targeted 

Monitoring

95th Percentile Site 0.215

(KS19)

0.052

(KS06)

0.013

(KS09)

ARP targeted 

Monitoring

90th Percentile Site 0.055

(IL15)

0.014

(MN24)

0.005

(IA20)

Site ID for the ARP study associated with each reported monitoring concentration is provided in 

parenthesis.

The wide discrepancy between predicted and observed concentrations cannot be explained by 

differences in modeled and actual usage alone. Acetochlor usage was mandated at each site 

during the course of the study and appears to have averaged around 2/3 of the maximum label 

rate and used on average for about 4 of the 7 years monitored or cumulatively roughly about 35 

to 40% of the label maximum rate over the 7-year monitoring period (detailed usage information 

by site is available in ARP documents submitted to the Agency, but has not been compiled to 

allow a more detailed analysis at this time). Another reason to expect lower concentrations in 
17

targeted monitoring program than modeled is the time lag expected between acetochlor 

applications and the entry of residues into the aquifer from field leaching (it is likely that the lag 

phase between application and movement into groundwater would exceed 7 years at some sites, 

which is what is also predicted by the groundwater modeling - See Figures 4 through 13 in 

Appendix 1). Two lines of evidence are available to examine the likelihood that the residues 

observed in the SGW are representative of longer term impacts on groundwater that might occur 

with continued use of acetochlor in future years:

1. Acetochlor degradate concentrations that were measured in the ARP SGW study.

2. Parent alachlor concentrations that were measured in the ARP SGW study.

Acetochlor degradate concentrations in the SGW. the acetochlor degradate data are 

informative in interpreting the results of acetochlor parent monitoring as there were two 

degradates that serve fairly effectively as a tracer of groundwater recharge from the surface 

following application of acetochlor to the surface. Specifically, the acetochlor ethanesulfonic and 

oxanilic acid (abbreviated as AcESA and AcOXA in the rest of this document) are significantly 

more persistent and mobile in soil and groundwater than acetochlor parent and tend to occur in 

aquifers throughout the acetochlor use area with travel times of soil water from the surface of 

agricultural fields to the aquifer are several years or less. Results show that at least one of the 

acetochlor degradates (more commonly AcESA) were consistently detected at concentrations 

above 0.1 ppb for one year or more at about 50% of the SGW sites and above 1.0 ppb for one 

year or more at more than 20% of the sites (by comparison, parent acetochlor was detected above 

0.1 ppb for one year or more at <4% of the sites and above 1.0 ppb or more for one year or more 

at none of the sites (reflected in the indiscernible difference between the frequency distribution 

lines for AcESA and acetochlor total residues . These data indicate that significant movement of 

soil pore water from the treated surface soils into the aquifers sampled by the SGW wells (all 

located in proximity to acetochlor usage sites) had indeed occurred at the majority of sites over 

the 7-year monitoring period. The ARP SGW imply that acetochlor parent residue levels in 

vulnerable ground water are well below 1 ppb at the vast majority of sites where point sourcetype contamination pathways do not occur (as might happen though with a poorly sealed 

wellhead or in areas of well developed karst features including depressions and extended cavities 

underground).
18

Chronic Exposure Cumulative Frequency Distribution

Maximum TWAM for SGW Sites

0%

20%

40%

60%

80%

100%

120%

0 0.003 0.005 0.01 0.05 0.1 0.5 1 2 4 8 > 8 

Concentration (ppb)

Frequency

ESA

OXA

Total residue

Figure 2. Chronic exposure distribution for acetochlor AcESA and AcOXA degradates 

and for total residues and in the SGW program using the maximum time-weighted 

annualized mean for each site. 

Alachlor concentrations in the SGW. Alachlor is an acetanilide herbicide that is similar to 

acetochlor in its molecular structure and physicochemical properties as well as its field usage 

pattern. Since all of the ARP SGW well samples were analyzed for both acetochlor and alachlor 

(and selected degradates of each herbicide), and since alachlor had a multi-year history of usage 

prior to this study (unlike acetochlor) the prevalence of alachlor at the SGW sites, if greater than 

that of acetochlor, might be a rough indicator of the potential impact of acetochlor at such sites 

once it has a longer history of usage. The SGW alachlor acute and chronic measured 

concentrations are presented along with the predicted concentrations from the PRZM-GW model 

are presented in Table 12 and can be compared with the acetochlor values presented in Table 11. 

The exposure levels for alachlor were only substantially higher than acetochlor for the top two 

wells. Since alachlor had been used for multiple years prior to this study and during the study at 

most of the monitored sites these results imply that level of acetochlor impact on the shallow 

aquifers that were sampled for this study has been largely realized at most sites. An important 

caveat is that, with respect to groundwater impacts, it is uncertain that acetochlor will continue to 

behave in the environment similarly to alachlor.
19

Table 12. Comparison of high-end predicted PRZM-GW) and measured (ARP SGW or 

MWCPA study) acute and chronic alachlor groundwater concentrations.

Source Site Alachlor

Overall 

Max

Daily Value

Alachlor

Average Daily Conc. 

for

Max. Year

Alachlor

Multi-year

Average

EDWC or SGW study exposure duration

24 hr Annual 30- or 7-

year

PRZM-GW Modeling Highest of 6 

scenarios 199 NC 159

ARP targeted 

Monitoring

#1 Site 

(99.5 %ile)

15.887

(KS23)

7.247

(KS23)

1.174

(KS23)

ARP targeted 

Monitoring

#2 Site 

(98.9 %ile)

12.835

(NE16)

1.857

(NE16)

0.573

(NE16)

ARP targeted 

Monitoring

#3 Site 

(98.3 %ile)

0.493

(KS06)

0.104

(KS22)

0.044

(WI22)

ARP targeted 

Monitoring

90th Percentile Site 0.061

(IN01)

0.017

(WI27)

0.008

(KS16)

Site ID for the ARP study associated with each reported monitoring concentration is provided in 

parenthesis.

ARP Small-Scale Prospective Groundwater Monitoring Studies

There were eight study sites included to represent major usage areas (on corn alone at the time of 

these studies, but it remains overwhelming the most important usage site for acetochlor) where 

acetochlor was applied at maximum label rates for one or two years and the subsurface fate was 

tracked to determine impacts of acetochlor and its degradates on shallow unconfined aquifers 

beneath each study site. Parent acetochlor was never detected above 0.01 ppb in shallow 

groundwater in any of the wells at any of monitoring sites (generally there were 6 to 12 pairs of 

wells located throughout each of the eight treated fields that were monitored regularly for 

multiple years after application). In contrast, acetochlor ethanesulfonic acid was detected with a 

clear pattern of movement deep into the vadose zone at all eight sites and into shallow 

groundwater at 6 of 8 sites (single well maximum 90-day moving average concentrations of 3.01, 

0.64, 6.07, 7.98, and 8.13 ppb for the DE, IA, IN, MN, PA, and WI sites, respectively.) At the 

NE site the flux rate was too slow for acetochlor and its degradates to leach all of the way to the 

aquifer over the study time period but a downward movement of acetochlor degradates like 

AcESA to the soil pore water at a 9 foot depth or greater had been established (at comparable 

concentrations to the other PGW sites). 

Part 2: Adjustment of Monitored Values to Reflect Maximum Usage Rates

As discussed above there are extensive targeted data (specifically from the Acetochlor 

Registration Partnership SGW, PGW, and SDWS studies in high-corn use areas) reflecting 

acetochlor parent and degradate concentrations in surface and ground-water in vulnerable water 

bodies. Preliminary analysis indicates that a refined drinking water assessment that is informed 
20

by these data is likely to result in lower exposure assessments. Results of the preliminary 

analysis were presented in the exposure summary above (specifically in Table 3) and are 

discussed further below.

Water Treatment Adjustment Factor. For the SDWS study, a key limitation was that for the 

majority of the 175 surface-water based drinking water supplies only finished water was 

analyzed (no raw water samples. In addition water treatment systems were, not unexpectedly, 

variable from site to site and in their efficiency at removing organics such as acetochlor. 

Furthermore, raw and finished water samples, even at those sites where they were available, did 

not necessarily represent the same water (treated water could have entered the treatment system 

days earlier). Due to these factors, a generic adjustment factor appears to be the best possible 

way to account for the possible reduction in acetochlor concentration in the finished water 

samples. The paired raw / finished water samples were examined for 20 water systems with a 

high level of acetochlor and alachlor and a median finished / raw water acetochlor ratio was 

determined to be 0.552. This was used as a general adjustment factor for the SDWS maximum 

observed finished water concentrations. There is, however, a high amount of uncertainty with 

this adjustment factor as the 10th and 90th percentile values were 0.146 and 1.18, respectively. 

Additional details are found in DP Barcodes 292340 and 292353. 

Usage Intensity Adjustment Factor.

Twelve US counties with the highest corn production intensities in the United States were evaluated for 

acetochlor usage intensity (Table 13). Corn is still by far the major use site for acetochlor and was the 

only use site associated with the ARP SDWS monitoring data. The median county-wide acetochlor usage 

rate in the highest corn production counties was 0.23 lb ai / county acre / year and was used to upwardly 

adjust the acetochlor SDWS monitoring data. Since the maximum label rate for acetochlor is 3 lb ai, this 

amounts to an assumption that the watershed wide acetochlor application rate was. 7.7% of the maximum 

rate if acetochlor was applied every year to the entire watershed (for this analysis no percent crop area 

factor was applied.)

Table 13. Acetochlor average annual usage intensity in high corn production counties during the 

ARP monitoring program years (1995 – 2001).

County State Total County Acres

Acetochl

or 

County 

Wide 

Applicati

on Rate 

(Lb Ai/A)

Hamilton Nebraska 347,012 0.171

York Nebraska 362,446 0.235

Stark Illinois 181,493 0.577

Grundy Iowa 318,161 0.437

De Kalb Illinois 401,480 0.444

Kossuth Iowa 626,323 0.217

Fillmore Nebraska 374,206 0.196

Kearney Nebraska 330,911 0.131

Lee Illinois 464,162 0.230

Humboldt Iowa 281,470 0.395
21

Clinton Indiana 261,239 0.187

Iroquois Illinois 709,411 0.355

Median Acetochlor Application Rate: 0.233

Acetochlor application rate calculations are based upon CBI data submitted to the Agency which is not 

included in this review.

A separate usage adjustment factors were used for the ARP SGW data. Since acetochlor was 

applied at label rates (generally between 1.2 and 3 lb ai/A) at most sites in 4 of the 7 monitoring 

years a range (usage was mandated in at least 3 of the first 5 years for all 175 sites included in 

the study) a conservative assumption was used that acetochlor was applied once at 1.275 lb ai/A 

for 4 of the 7 study years; this equates to an assumption that usage over the 7 study years was 

24.3% of the maximum label rate. 

The results of the above adjustments to the maximum observed single sample (daily or acute 

exposure equivalent) and annual or multi-year concentrations (chronic exposure equivalent) are 

presented above in Table 3. 

Additional analysis could be conducted to provide more refined exposure assessments based 

upon the existing monitoring data and refinement of model inputs, including but not limited to:

• Adjustments on an individual site / watershed basis of observed acetochlor concentrations 

for the SDWS.

• Compilation of reported usage data by site for the SGW study and applying more precise 

site specific usage adjustments.

• Analysis of historical percent crop treated data to determine reasonable upper bound PCT 

factors for exposure assessments.

• Refinement of model inputs for PRZM-GW to include a wider array of scenarios 

representing actual domestic drinking water exposure conditions and more precise 

representation of acetochlor behavior in the environment, especially with regard to 

subsurface degradation and hydrolysis. 

Conclusions

Estimated Drinking Water Concentrations of parent acetochlor have been revised upward in this 

review (see Table 1) based upon new Tier 1 modeling for groundwater-based drinking water 

sources. Although the Health Effects Division is not requesting refinements at this time, because 

of the extensive targeted monitoring data available for acetochlor on groundwater and surface 

waters a comparison between modeled and monitoring concentrations was conducted. The 

results of this comparison suggest that actual drinking water concentrations may be 

significantly lower than the predicted concentrations from groundwater sources, even when 

differences between modeled and actual usage rates and intensities are accounted for. 
22

The most significant differences between model and monitoring results were observed for 

groundwater exposure assessment. Analysis of the monitoring data from the targeted SGW and 

PGW studies in particular strongly suggests that actual exposure is much lower (40 to 400X). 

The comparisons are significant because both studies captured residues from wells that sample 

groundwater that should be more vulnerable than the most vulnerable domestic drinking water 

wells: 

• The SGW study is based upon observation wells downgradient and in close proximity to 

acetochlor treated fields with mandated usage at maximum “typical” rates. 

• The PGW studies only mandated usage at maximum label rates for 1 or 2 years at the 

start of the study but monitored groundwater at a depth of 5 to 30 feet at multiple sites 

with highly vulnerable groundwater (including generally sand or loamy sand soils 

throughout the vadose zone at 5 of the 8 sites) directly beneath the treatment area.

Therefore, EFED anticipates that further refinement of the EDWCs with higher tier modeling 

and more site specific analysis of the existing targeted monitoring data would be of value should 

it be needed for future drinking water exposure assessments.
23

APPENDIX 1: Drinking Water Exposure Modeling Inputs and Results

PRZM-GW Acetochlor DWEC Time Series

Figure 3. Estimated acetochlor concentration over time (30-year simulation; WI sand scenario –

benchmark crop = field corn) ; using Koc = 133.2 input.
24

Figure 4. Estimated acetochlor concentration over time (30-year simulation; WI sand scenario –

benchmark crop = field corn) ; using Kd = 0.126 input.

Figure 5. Estimated acetochlor concentration over time (30-year simulation; DE loamy sand 

scenario – benchmark crop = sweet corn). ; using Koc = 133.2 input.
25

Figure 6. Estimated acetochlor concentration over time (30-year simulation; DE loamy sand 

scenario – benchmark crop = sweet corn); using Kd = 0.126 input.

Figure 7. Estimated acetochlor concentration over time (30-year simulation; FL Pomona sand 

scenario – benchmark crop = potatoes); using Koc = 133.2 input.
26

Figure 8. Estimated acetochlor concentration over time (30-year simulation; FL Pomona sand 

scenario – benchmark crop = potatoes); using Kd = 0.126 input.

Figure 9. Estimated acetochlor concentration over time (30-year simulation; FL Central Ridge

Astatula sand scenario – benchmark crop = citrus); Using doc = 133.2 input.
27

Figure 10. Estimated acetochlor concentration over time (30-year simulation; FL Central Ridge

Astatula sand scenario – benchmark crop = citrus); Using Kd = 0.126 input.

Figure 11. Estimated acetochlor concentration over time (30-year simulation; GA Tifton loamy 

sand scenario – benchmark crop = peanut); Using Koc = 133.2 input.
28

Figure 12. Estimated acetochlor concentration over time (30-year simulation; GA Tifton loamy 

sand scenario – benchmark crop = peanut); Using Kd = 0.126 input.

Figure 13. Estimated acetochlor concentration over time (30-year simulation; NC Norfolk

loamy sand scenario – benchmark crop = cotton); Using Koc = 133.2 input.
29

Figure 14. Estimated acetochlor concentration over time (30-year simulation; NC Norfolk

loamy sand scenario – benchmark crop = cotton); Using Kd = 0.126 input.

PRZM-GW EXAMPLE Alachlor DWEC (Surrogate Chemical Analysis) Time Series Output

Highest Daily Value (ppb) = 199.

Simulation Average Value (ppb) = 116.

Average Breakthrough Time (days) = 2830.6

Throughputs = 3.872

Post-breakthrough Average (ppb) = 159.
30

Figure 15. Estimated alachlor concentration over time (30-year simulation; WI sand scenario –

benchmark crop = field corn) ; using Koc = 123, Aerobic half-life = 34 days, Hydrolysis = 

stable, and Annual application of 4.48 kg ai/ha as model input.

Sample PRZM-GW Alachlor (run for Comparison to Acetochlor Modeling 

and Monitoring) Input Processing (*.pgi) File – Wisconsin Corn Scenario 
Alachlor

0

13.3

True

133.2

2

1

01,08,,,,,,,,,

05,05,,,,,,,,,

1.681,1.681,,,,,,,,,

1,1,,,,,,,,,

,,,,,,,,,,

True

1

last

Wisconsin Corn - WI Central Sands

C:\Models\Inputs\Metfiles\w14920.dvf

0.76

0.36

18

90

100

0.25

01

05

21

07

20

10

1

2

0.1

0

4.0

9

1.63,1.63,1.68,1.68,1.59,1.59,1.59,1.

59

0.088,0.088,0.079,0.079,0.068,0.068,0

.068

0.034,0.034,0.025,0.025,0.016,0.016,0

.016

0.46,0.46,0.14,0.09,0.09,0.08,0.07,0.

07

True

95,95,96,96,97,97,97,97

2.3,2.3,2.5,2.5,1.7,1.7,1.7,1.7

5.6

0.2

50

1

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sample PRZM-GWA Acetochlor Input Processing (*.pgi) File – Wisconsin 

Corn Scenario Using Kd Input Value

Acetochlor

0

13.3

False

0.126

2

1

01,08,,,,,,,,,

05,05,,,,,,,,,

1.681,1.681,,,,,,,,,

1,1,,,,,,,,,

,,,,,,,,,,

True

1

last

Wisconsin Corn - WI Central Sands

C:\Models\Inputs\Metfiles\w14920.dv

f

0.76

0.36

18

90

100
32

0.25

01

05

21

07

20

10

1

2

0.1

0

4.0

9

1.63,1.63,1.68,1.68,1.59,1.59,1.59,

1.59

0.088,0.088,0.079,0.079,0.068,0.068

,0.068

0.034,0.034,0.025,0.025,0.016,0.016

,0.016

0.46,0.46,0.14,0.09,0.09,0.08,0.07,

0.07

True

95,95,96,96,97,97,97,97

2.3,2.3,2.5,2.5,1.7,1.7,1.7,1.7

5.6

0.2

50

1

4/18/2013 5:27:22 PM; Acetochlor; Wisconsin Corn - WI Central Sands

User Interface Version: PRZM-GW (Version 1.01)

***Record 3 

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***Record 6

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***Record 8

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***Record 9

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***Record 10

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***Record 11

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 010571 210771 201071 1

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 010586 210786 201086 1
33

 010587 210787 201087 1

 010588 210788 201088 1

 010589 210789 201089 1

 010590 210790 201090 1

***Record 12

Place Holder

***Record 13

 60 1 0 0

***Record 15

Place Holder

***Record 16

 1 561 0 1 4 1.681 1 0

 8 561 0 1 4 1.681 1 0

 1 562 0 1 4 1.681 1 0

 8 562 0 1 4 1.681 1 0

 1 563 0 1 4 1.681 1 0

 8 563 0 1 4 1.681 1 0

 1 564 0 1 4 1.681 1 0

 8 564 0 1 4 1.681 1 0

 1 565 0 1 4 1.681 1 0

 8 565 0 1 4 1.681 1 0

 1 566 0 1 4 1.681 1 0

 8 566 0 1 4 1.681 1 0

 1 567 0 1 4 1.681 1 0

 8 567 0 1 4 1.681 1 0

 1 568 0 1 4 1.681 1 0

 8 568 0 1 4 1.681 1 0

 1 569 0 1 4 1.681 1 0

 8 569 0 1 4 1.681 1 0

 1 570 0 1 4 1.681 1 0

 8 570 0 1 4 1.681 1 0

 1 571 0 1 4 1.681 1 0

 8 571 0 1 4 1.681 1 0

 1 572 0 1 4 1.681 1 0

 8 572 0 1 4 1.681 1 0

 1 573 0 1 4 1.681 1 0

 8 573 0 1 4 1.681 1 0

 1 574 0 1 4 1.681 1 0

 8 574 0 1 4 1.681 1 0

 1 575 0 1 4 1.681 1 0

 8 575 0 1 4 1.681 1 0

 1 576 0 1 4 1.681 1 0

 8 576 0 1 4 1.681 1 0

 1 577 0 1 4 1.681 1 0

 8 577 0 1 4 1.681 1 0

 1 578 0 1 4 1.681 1 0

 8 578 0 1 4 1.681 1 0

 1 579 0 1 4 1.681 1 0

 8 579 0 1 4 1.681 1 0

 1 580 0 1 4 1.681 1 0

 8 580 0 1 4 1.681 1 0

 1 581 0 1 4 1.681 1 0

 8 581 0 1 4 1.681 1 0

 1 582 0 1 4 1.681 1 0

 8 582 0 1 4 1.681 1 0

 1 583 0 1 4 1.681 1 0

 8 583 0 1 4 1.681 1 0
34

 1 584 0 1 4 1.681 1 0

 8 584 0 1 4 1.681 1 0

 1 585 0 1 4 1.681 1 0

 8 585 0 1 4 1.681 1 0

 1 586 0 1 4 1.681 1 0

 8 586 0 1 4 1.681 1 0

 1 587 0 1 4 1.681 1 0

 8 587 0 1 4 1.681 1 0

 1 588 0 1 4 1.681 1 0

 8 588 0 1 4 1.681 1 0

 1 589 0 1 4 1.681 1 0

 8 589 0 1 4 1.681 1 0

 1 590 0 1 4 1.681 1 0

 8 590 0 1 4 1.681 1 0

***Record 17

 0 1 0

***Record 19

Place Holder

***Record 20

 1000 0 0 0 0 0 2 1 1 0

***Record 26

 0 0 0

***Record 27

 3 0.1 0 4.0

***Record 31

 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.97 10.0

***Record 32

 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6

***Record 32A

 2.00 25.0

***Record 33

 8

*** Record 34,36,37 Horizon 1

 1 10 1.63 0.088 0 0 0

 0.0521 0.0521 0

 1 0.088 0.034 0.46 0.126

 5.6 95 2.3 0 0

*** Record 34,36,37 Horizon 2

 2 10 1.63 0.088 0 0 0

 0.0492 0.0492 0

 5 0.088 0.034 0.46 0.126

 5.6 95 2.3 0 0

*** Record 34,36,37 Horizon 3

 3 20 1.68 0.079 0 0 0

 0.0405 0.0405 0

 20 0.079 0.025 0.14 0.126

 5.6 96 2.5 0 0

*** Record 34,36,37 Horizon 4

 4 20 1.68 0.079 0 0 0

 0.029 0.029 0

 20 0.079 0.025 0.09 0.126

 5.6 96 2.5 0 0

*** Record 34,36,37 Horizon 5

 5 20 1.59 0.068 0 0 0

 0.0174 0.0174 0

 20 0.068 0.016 0.09 0.126

 5.6 97 1.7 0 0
35

*** Record 34,36,37 Horizon 6

 6 20 1.59 0.068 0 0 0

 0.00579 0.00579 0

 20 0.068 0.016 0.08 0.126

 5.6 97 1.7 0 0

*** Record 34,36,37 Horizon 7

 7 800 1.59 0.068 0 0 0

 0 0 0

 50 0.068 0.016 0.07 0.126

 5.6 97 1.7 0 0

*** Record 34,36,37 Horizon 8

 8 100 1.59 0.396 0 0 0

 0 0 0

 50 0.396 0.001 0.07 0.126

 5.6 97 1.7 0 0

*** Record 40

 0

*** Record 42

 YEAR 10 YEAR 10 YEAR 10 0

*** Record 45

 3 YEAR

*** Record 46

 DCON1 TAVE 33 34 1.0E3

 INFL TCUM 33 33 0.0161

 IRRG TSER

FLORIDA POTATO SCENARIO

PRZM Input file:

4/25/2013 4:31:08 PM; Acetochlor; FLORIDA - potato - Met File (13889.dvf) 

Jacksonville

User Interface Version: PRZM-GW (Version 1.01)

***Record 3 

 0.77 0 0 25 1 1

***Record 6

 0

***Record 8

 1

***Record 9

 1 0.1 60 40 1 10 10 10 0 100

***Record 10

 30

***Record 11

 150261 010561 010661 1

 150262 010562 010662 1

 150263 010563 010663 1

 150264 010564 010664 1

 150265 010565 010665 1

 150266 010566 010666 1

 150267 010567 010667 1

 150268 010568 010668 1

 150269 010569 010669 1

 150270 010570 010670 1

 150271 010571 010671 1

 150272 010572 010672 1

 150273 010573 010673 1
36

 150274 010574 010674 1

150275 010575 010675 1

 150276 010576 010676 1

 150277 010577 010677 1

 150278 010578 010678 1

 150279 010579 010679 1

 150280 010580 010680 1

 150281 010581 010681 1

 150282 010582 010682 1

 150283 010583 010683 1

 150284 010584 010684 1

 150285 010585 010685 1

 150286 010586 010686 1

 150287 010587 010687 1

 150288 010588 010688 1

 150289 010589 010689 1

 150290 010590 010690 1

***Record 12

Place Holder

***Record 13

 60 1 0 0

***Record 15

Place Holder

***Record 16

 15 261 0 1 4 1.681 1 0

 22 261 0 1 4 1.681 1 0

 15 262 0 1 4 1.681 1 0

 22 262 0 1 4 1.681 1 0

 15 263 0 1 4 1.681 1 0

 22 263 0 1 4 1.681 1 0

 15 264 0 1 4 1.681 1 0

 22 264 0 1 4 1.681 1 0

 15 265 0 1 4 1.681 1 0

 22 265 0 1 4 1.681 1 0

 15 266 0 1 4 1.681 1 0

22 266 0 1 4 1.681 1 0

 15 267 0 1 4 1.681 1 0

 22 267 0 1 4 1.681 1 0

 15 268 0 1 4 1.681 1 0

 22 268 0 1 4 1.681 1 0

 15 269 0 1 4 1.681 1 0

 22 269 0 1 4 1.681 1 0

 15 270 0 1 4 1.681 1 0

 22 270 0 1 4 1.681 1 0

 15 271 0 1 4 1.681 1 0

 22 271 0 1 4 1.681 1 0

 15 272 0 1 4 1.681 1 0

 22 272 0 1 4 1.681 1 0

 15 273 0 1 4 1.681 1 0

 22 273 0 1 4 1.681 1 0

 15 274 0 1 4 1.681 1 0

 22 274 0 1 4 1.681 1 0

 15 275 0 1 4 1.681 1 0

 22 275 0 1 4 1.681 1 0

 15 276 0 1 4 1.681 1 0

 22 276 0 1 4 1.681 1 0

 15 277 0 1 4 1.681 1 0
37

 22 277 0 1 4 1.681 1 0

 15 278 0 1 4 1.681 1 0

 22 278 0 1 4 1.681 1 0

 15 279 0 1 4 1.681 1 0

 22 279 0 1 4 1.681 1 0

 15 280 0 1 4 1.681 1 0

 22 280 0 1 4 1.681 1 0

 15 281 0 1 4 1.681 1 0

 22 281 0 1 4 1.681 1 0

 15 282 0 1 4 1.681 1 0

 22 282 0 1 4 1.681 1 0

 15 283 0 1 4 1.681 1 0

 22 283 0 1 4 1.681 1 0

 15 284 0 1 4 1.681 1 0

 22 284 0 1 4 1.681 1 0

 15 285 0 1 4 1.681 1 0

 22 285 0 1 4 1.681 1 0

 15 286 0 1 4 1.681 1 0

 22 286 0 1 4 1.681 1 0

 15 287 0 1 4 1.681 1 0

 22 287 0 1 4 1.681 1 0

 15 288 0 1 4 1.681 1 0

 22 288 0 1 4 1.681 1 0

 15 289 0 1 4 1.681 1 0

 22 289 0 1 4 1.681 1 0

 15 290 0 1 4 1.681 1 0

 22 290 0 1 4 1.681 1 0

***Record 17

 0 1 0

***Record 19

Place Holder

***Record 20

 400 0 0 0 0 0 2 1 1 0

***Record 26

 0 0 0

***Record 27

 3 0.1 0.65 7

***Record 31

 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.97 10.0

***Record 32

 21 21 21 21 21 21 21 21 21 21 21 21

***Record 32A

 2.00 25.0

***Record 33

 8

*** Record 34,36,37 Horizon 1

 1 10 1.31 0.13 0 0 0

 0.0521 0.0521 0

 1 0.13 0.09 4.19 0.126

 21 93 0.001 0 0

*** Record 34,36,37 Horizon 2

 2 10 1.73 0.13 0 0 0

 0.0492 0.0492 0

 5 0.13 0.09 0.78 0.126

 21 95 0.2 0 0

*** Record 34,36,37 Horizon 3

 3 20 1.78 0.07 0 0 0
38

 0.0405 0.0405 0

 20 0.07 0.021 0.18 0.126

 21 92 0.6 0 0

*** Record 34,36,37 Horizon 4

 4 20 1.65 0.08 0 0 0

 0.029 0.029 0

 20 0.08 0.009 2.03 0.126

 21 92 1.5 0 0

*** Record 34,36,37 Horizon 5

 5 20 1.65 0.11 0 0 0

 0.0174 0.0174 0

 20 0.11 0.051 2.03 0.126

 21 92 1.5 0 0

*** Record 34,36,37 Horizon 6

 6 20 1.75 0.19 0 0 0

 0.00579 0.00579 0

 20 0.19 0.04 1.05 0.126

 21 93 0.3 0 0

*** Record 34,36,37 Horizon 7

 7 200 1.75 0.19 0 0 0

 0 0 0

 50 0.19 0.007 0.19 0.126

 21 94 0.001 0 0

*** Record 34,36,37 Horizon 8

 8 100 1.75 0.3362 0 0 0

 0 0 0

 50 0.3362 0.001 0.16 0.126

 21 79 8.5 0 0

*** Record 40

 0

*** Record 42

 YEAR 10 YEAR 10 YEAR 10 0

*** Record 45

 3 YEAR

*** Record 46

 DCON1 TAVE 21 22 1.0E3

 INFL TCUM 21 21 0.0199

 IRRG TSER

Sample PRZM-GW Input Processing (*.pgi) File – Florida Potato Scenario Using Kd Input Value

Acetochlor

0

13.3

False

0.126

2

1

15,22,,,,,,,,,

02,02,,,,,,,,,

1.681,1.681,,,,,,,,,

1,1,,,,,,,,,

,,,,,,,,,,

True

1

last

FLORIDA - potato - Met File (13889.dvf) 

Jacksonville

C:\Models\Inputs\Metfiles\w13889.dvf

0.77

0

25

60

40

0.1

15

02

01

05

01
39

06

1

2

0.1

0.65

7

3

1.31,1.73,1.78,1.65,1.65,1.75,1.75,1.75

0.13,0.13,0.07,0.08,0.11,0.19,0.19

0.09,0.09,0.021,0.009,0.051,0.04,0.007

4.19,0.78,0.18,2.03,2.03,1.05,0.19,0.16

True

93,95,92,92,92,93,94,79

0.001,0.2,0.6,1.5,1.5,0.3,0.001,8.5

21

0.2

50

1

PRZM-EXAMS Input and Output results for Peanut and Sugar Beet Uses

NORTH CAROLINA PEANUT SCENARIO

stored as Acet_NC-pnt_rsv-air1.out

Chemical: Acetochlor

PRZM environment: NCpeanutSTD.txt modified Tueday, 29 May 2007 at 13:58:46

EXAMS environment: ir298.exv modified Tueday, 26 August 2008 at 06:14:08

Metfile: w13722.dvf modified Wedday, 3 July 2002 at 10:05:50

Water segment concentrations (ppb)

Year Peak 96 hr 21 Day 60 Day 90 Day Yearly

1961 18.14 17.31 14.1 9.785 7.449 2.035

1962 20.1 19 16.68 12.04 9.293 2.622

1963 41.12 39.21 33.13 22.32 17.1 4.784

1964 18.11 17.2 14.45 10.1 7.738 2.25

1965 31.95 30.22 27.51 20.62 16.23 4.618

1966 32.28 30.75 25.29 17.32 13.21 3.74

1967 23.1 21.84 17.41 15.32 12.47 3.655

1968 29.65 28.34 23.22 15.65 11.98 3.374

1969 34.19 32.59 27.64 19.06 14.55 4.125

1970 19.83 18.89 15.32 10.58 8.175 2.377

1971 39.4 37.61 29.75 20.14 15.52 4.349

1972 20 19.1 15.67 12.63 10.12 2.963

1973 32.23 30.79 26.58 17.42 13.35 3.753

1974 46.98 44.85 37.4 24.11 18.44 5.211

1975 23.88 22.85 19.59 13.64 10.47 3.012

1976 27.24 25.93 22.24 15.57 11.84 3.315

1977 38.14 36.26 29.33 19.02 14.38 3.997

1978 18.17 17.32 15.41 11.74 9.063 2.57

1979 21.5 20.48 18.78 13.75 10.65 3.001

1980 32.05 30.37 25.23 17.84 13.79 3.833

1981 31.48 30.06 25.16 16.06 11.97 3.344

1982 23.15 21.92 18.96 12.7 9.623 2.705

1983 40.89 38.99 32.81 21.53 16.3 4.491

1984 46.36 44.16 34.6 22.38 17.3 4.898

1985 23.28 22.14 17.84 12.87 9.901 2.846

1986 33.99 32.33 26.72 17.09 12.68 3.479

1987 19.86 18.84 14.99 10.64 8.102 2.275

1988 26.33 25.09 20.77 14.46 10.88 2.983

1989 28.41 26.79 20.91 15.24 11.87 3.355

1990 42.21 40.14 31.53 20.4 15.53 4.301
40

Sorted results

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

0.032258064516129 46.98 44.85 37.4 24.11 18.44 5.211

0.0645161290322581 46.36 44.16 34.6 22.38 17.3 4.898

0.0967741935483871 42.21 40.14 33.13 22.32 17.1 4.784

0.129032258064516 41.12 39.21 32.81 21.53 16.3 4.618

0.161290322580645 40.89 38.99 31.53 20.62 16.23 4.491

0.193548387096774 39.4 37.61 29.75 20.4 15.53 4.349

0.225806451612903 38.14 36.26 29.33 20.14 15.52 4.301

0.258064516129032 34.19 32.59 27.64 19.06 14.55 4.125

0.290322580645161 33.99 32.33 27.51 19.02 14.38 3.997

0.32258064516129 32.28 30.79 26.72 17.84 13.79 3.833

0.354838709677419 32.23 30.75 26.58 17.42 13.35 3.753

0.387096774193548 32.05 30.37 25.29 17.32 13.21 3.74

0.419354838709677 31.95 30.22 25.23 17.09 12.68 3.655

0.451612903225806 31.48 30.06 25.16 16.06 12.47 3.479

0.483870967741936 29.65 28.34 23.22 15.65 11.98 3.374

0.516129032258065 28.41 26.79 22.24 15.57 11.97 3.355

0.548387096774194 27.24 25.93 20.91 15.32 11.87 3.344

0.580645161290323 26.33 25.09 20.77 15.24 11.84 3.315

0.612903225806452 23.88 22.85 19.59 14.46 10.88 3.012

0.645161290322581 23.28 22.14 18.96 13.75 10.65 3.001

0.67741935483871 23.15 21.92 18.78 13.64 10.47 2.983

0.709677419354839 23.1 21.84 17.84 12.87 10.12 2.963

0.741935483870968 21.5 20.48 17.41 12.7 9.901 2.846

0.774193548387097 20.1 19.1 16.68 12.63 9.623 2.705

0.806451612903226 20 19 15.67 12.04 9.293 2.622

0.838709677419355 19.86 18.89 15.41 11.74 9.063 2.57

0.870967741935484 19.83 18.84 15.32 10.64 8.175 2.377

0.903225806451613 18.17 17.32 14.99 10.58 8.102 2.275

0.935483870967742 18.14 17.31 14.45 10.1 7.738 2.25

0.967741935483871 18.11 17.2 14.1 9.785 7.449 2.035

0.1 42.101 40.047 33.098 22.241 17.02 4.7674

Average of yearly averages: 3.47536666666667

Inputs generated by pe5.pl - Novemeber 2006

Data used for this run:

Output File: Acet_NC-pnt_rsv-air1

Metfile: w13722.dvf

PRZM scenario: NCpeanutSTD.txt

EXAMS environment file: ir298.exv

Chemical Name: Acetochlor

Description Variable Name Value Units Comments

Molecular weight mwt 269.77 g/mol

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

Vapor Pressure vapr 2.8E-5 torr

Solubility sol 233 mg/L

Kd Kd mg/L

Koc Koc 133 mg/L

Photolysis half-life kdp 0 days Half-life

Aerobic Aquatic Metabolism kbacw 26.6 days Halfife

Anaerobic Aquatic Metabolism kbacs 251 days Halfife

Aerobic Soil Metabolism asm 13.3 days Halfife

Hydrolysis: pH 7 0 days Half-life
41

Method: CAM 1 integer See PRZM manual

Incorporation Depth: DEPI 0 cm

Application Rate: TAPP 1.681 kg/ha

Application Efficiency: APPEFF .95 fraction

Spray Drift DRFT .16 fraction of application rate applied to pond

Application Date Date 16-05 dd/mm or dd/mmm or dd-mm or dd-mmm

Interval 1 interval 7 days Set to 0 or delete line for single app.

app. rate 1 apprate 1.681 kg/ha

Record 17: FILTRA

IPSCND

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)

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

MINNESOTA SUGAR BEET SCENARIO

stored as Acet_MN-SugBt_rsv-air1.out

Chemical: Acetochlor

PRZM environment: MNsugarbeetSTD.txt modified Tueday, 29 May 2007 at 

13:57:22

EXAMS environment: ir298.exv modified Tueday, 26 August 2008 at 06:14:08

Metfile: w14914.dvf modified Wedday, 3 July 2002 at 10:05:52

Water segment concentrations (ppb)

Year Peak 96 hr 21 Day 60 Day 90 Day Yearly

1961 18.66 18.13 15.66 11.28 9.066 2.775

1962 63.48 61.55 53.84 39.54 31.92 10.18

1963 31.99 30.95 27.61 19.77 15.86 5.661

1964 25.71 24.63 20.53 16.49 13.93 4.999

1965 19.49 19.09 17.74 13.52 11.07 4.039

1966 20.5 19.94 17.7 13.09 10.47 3.609

1967 19.18 18.68 17.17 13.37 10.95 3.741

1968 19.12 18.59 16.98 13.2 10.79 3.68

1969 23.44 22.86 20.72 16.82 14.36 4.888

1970 24.78 23.88 20.9 16.54 13.45 4.583

1971 40.81 39.62 34.71 24.77 19.91 6.629

1972 19.58 18.92 16.19 11.77 9.506 3.518

1973 20.14 19.55 17.01 12.53 10.16 3.405

1974 21.74 21.16 18.67 13.74 11.26 3.93

1975 25.18 24.52 22.36 17.92 15.4 5.544

1976 19.28 18.69 16.08 11.66 9.214 3.277

1977 27.07 25.96 21.88 17.2 14.32 4.934

1978 24.28 23.33 19.78 16.42 13.94 4.976

1979 19.37 18.86 16.65 13.19 10.94 3.905

1980 18.79 18.1 15.52 11.76 9.424 3.258

1981 49.87 48.35 42.08 30.88 24.57 7.977

1982 19.51 18.89 16.6 12.83 10.5 3.988

1983 19.16 18.64 16.29 12.4 10.22 3.485
42

1984 39.91 38.3 32.59 23.81 19.59 6.536

1985 32.45 31.34 27.48 21.02 17.37 6.34

1986 19.36 18.74 16.31 14.34 12.05 4.548

1987 31.37 30.25 26.1 18.87 14.95 4.993

1988 18.94 18.22 15.19 10.74 8.307 2.844

1989 25.32 24.5 21.32 15.74 12.46 3.969

1990 31.87 30.55 26.1 20.28 16.61 5.506

Sorted results

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

0.032258064516129 63.48 61.55 53.84 39.54 31.92 10.18

0.0645161290322581 49.87 48.35 42.08 30.88 24.57 7.977

0.0967741935483871 40.81 39.62 34.71 24.77 19.91 6.629

0.129032258064516 39.91 38.3 32.59 23.81 19.59 6.536

0.161290322580645 32.45 31.34 27.61 21.02 17.37 6.34

0.193548387096774 31.99 30.95 27.48 20.28 16.61 5.661

0.225806451612903 31.87 30.55 26.1 19.77 15.86 5.544

0.258064516129032 31.37 30.25 26.1 18.87 15.4 5.506

0.290322580645161 27.07 25.96 22.36 17.92 14.95 4.999

0.32258064516129 25.71 24.63 21.88 17.2 14.36 4.993

0.354838709677419 25.32 24.52 21.32 16.82 14.32 4.976

0.387096774193548 25.18 24.5 20.9 16.54 13.94 4.934

0.419354838709677 24.78 23.88 20.72 16.49 13.93 4.888

0.451612903225806 24.28 23.33 20.53 16.42 13.45 4.583

0.483870967741936 23.44 22.86 19.78 15.74 12.46 4.548

0.516129032258065 21.74 21.16 18.67 14.34 12.05 4.039

0.548387096774194 20.5 19.94 17.74 13.74 11.26 3.988

0.580645161290323 20.14 19.55 17.7 13.52 11.07 3.969

0.612903225806452 19.58 19.09 17.17 13.37 10.95 3.93

0.645161290322581 19.51 18.92 17.01 13.2 10.94 3.905

0.67741935483871 19.49 18.89 16.98 13.19 10.79 3.741

0.709677419354839 19.37 18.86 16.65 13.09 10.5 3.68

0.741935483870968 19.36 18.74 16.6 12.83 10.47 3.609

0.774193548387097 19.28 18.69 16.31 12.53 10.22 3.518

0.806451612903226 19.18 18.68 16.29 12.4 10.16 3.485

0.838709677419355 19.16 18.64 16.19 11.77 9.506 3.405

0.870967741935484 19.12 18.59 16.08 11.76 9.424 3.277

0.903225806451613 18.94 18.22 15.66 11.66 9.214 3.258

0.935483870967742 18.79 18.13 15.52 11.28 9.066 2.844

0.967741935483871 18.66 18.1 15.19 10.74 8.307 2.775

0.1 40.72 39.488 34.498 24.674 19.878 6.6197

Average of yearly averages: 4.7239

Inputs generated by pe5.pl - Novemeber 2006

Data used for this run:

Output File: Acet_MN-SugBt_rsv-air1

Metfile: w14914.dvf

PRZM scenario: MNsugarbeetSTD.txt

EXAMS environment file: ir298.exv

Chemical Name: Acetochlor

Description Variable Name Value Units Comments

Molecular weight mwt 269.77 g/mol

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

Vapor Pressure vapr 2.8E-5 torr

Solubility sol 233 mg/L
43

Kd Kd mg/L

Koc Koc 133 mg/L

Photolysis half-life kdp 0 days Half-life

Aerobic Aquatic Metabolism kbacw 26.6 days Halfife

Anaerobic Aquatic Metabolism kbacs 251 days Halfife

Aerobic Soil Metabolism asm 13.3 days Halfife

Hydrolysis: pH 7 0 days Half-life

Method: CAM 1 integer See PRZM manual

Incorporation Depth: DEPI 0 cm

Application Rate: TAPP 1.681 kg/ha

Application Efficiency: APPEFF .95 fraction

Spray Drift DRFT .16 fraction of application rate applied to pond

Application Date Date 16-05 dd/mm or dd/mmm or dd-mm or dd-mmm

Interval 1 interval 7 days Set to 0 or delete line for single app.

app. rate 1 apprate 1.681 kg/ha

Record 17: FILTRA

IPSCND

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)
44

APPENDIX 2: Chemical structure of acetochlor, acetochlor 

degradates and the herbicide alachlor

N

CH3

Cl

O

O

CH3

H3C

Acetochlor

2'-Ethyl-6'-methyl-N-(ethoxymethyl)-2-chloroacetanilide

N

CH3

SO3H

O

O

CH3

H3C

Acetochlor ethanesulfonic acid

2-[(2-ethyl-6-methylphenyl)(ethoxymethyl) amino]-2-oxoethanesulfonic acid
45

N

CH3

O

CH3

H3C

O

HO

O

Acetochlor oxanilic acid

2-[(2-ethyl-6-methylphenyl)(ethoxymethyl) amino]-2-oxoacetic acid]

Alachlor

2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide
46

EXTRA STUFF PROPOSED TO DELETE:

Table 14. Ratios of maximum modeled to conservatively adjusted maximum monitored acute 

and chronic acetochlor concentrations.

Source 

(Study)

Sample Type Modeled / 

Observed

(acute)

Modeled / 

Observed

(multi-yr. 

chronic)

Groundwater 

(SGW)

Observation wells –

adjac. To treatments.

42.3 466.2

Groundwater 

(PGW)

PGW on-site NC NC

Surface water

(SDWS)

Finished drinking 

water

0.17 0.73

Surface water

(SDWS)

Raw drinking water 0.79 1.95

The SGW and PGW groundwater data do not represent actual drinking water sources, but are 

from observation wells that are deliberately installed in proximity to high usage areas (SGW 

study) or directly in such treatment areas (PGW study – but with applications in only 1 or 2 

years.) 

Usage data were available for these studies and a rough adjustment to the expected 

concentrations for maximum annual applications is provided in Table xxxx. ???

Table 15. Ratios of maximum modeled to unadjusted maximum monitored acute and chronic 

acetochlor concentrations.

Source 

(Study)

Sample Type Modeled / 

Observed

(acute)

Modeled / 

Observed

(multi-yr. 

chronic)

Groundwater 

(SGW)

Observation wells –

adjac. To treatments.

174.1 1086.8

Groundwater 

(PGW)

PGW on-site 4777.8 16520.0

Surface water

(SDWS)

Finished drinking 

water

4.11 17.29
47

Source 

(Study)

Sample Type Modeled / 

Observed

(acute)

Modeled / 

Observed

(multi-yr. 

chronic)

Surface water

(SDWS)

Raw drinking water 10.42 25.47

From: February 16, 2006; DP Barcode: D292331; “Revised Drinking Water Exposure 

Assessment for Acetochlor”

Table 16. PGW sites: Selected soil and aquifer characteristics.

PGW 

Study

Location

NRCS Soil 

Series; 

On-site surface 

soil % O.M. & 

pH

Subsoil 

Textures1

Avg. Hydraulic 

Conductivity2

 

(mm/hr)

Aquifer soil 

textures 

determined1

Depth to 

ground 

water 3

(m)

Pore-water velocity

(m/day) 4

Wisconsin Richford 

loamy sand 

OM = 1.6%

pH = 6.4

Loamy 

Sand

Sand

Sandy 

loam

0-1.2 m

1.2-2.4 m

2.4-3.6 m

3.6-4.8 m

>4.8 m

177

358

810

1482

776

Loamy sand

Sandy loam

Sand

7.6-10 1.9 x 10-3

Ohio Genessee silt 

loam 

Fox silt loam

OM = 2.9%

pH = 7.7

Clay 

loam

Loam

Sandy 

loam

0-1.2 m

1.2-2.4 m

2.4-3.6 m

3.6-4.8 m

>4.8 m

293

153

NA

NA

NA

Sandy loam

Loamy sand

0.6-5.2 0.8 x 10-1

Minnesota Estherville 

sandy loam

OM = 3.5%

pH = 6.3

Sandy 

loam

Loamy 

sand

Sand

0-1.2 m 

1.2-2.4 m 

2.4-3.6 m 

3.6-4.8 m 

>4.8 m 

180

331

NA

NA

NA

Sand

Loamy 

sand

Sandy loam

4

.8-6.4

0.4 x 10-1

Nebraska Kenesaw 

silt loam

Coly-Kenesaw 

silt loam

OM = 1.8%

pH = 5.7

Loam

Silt loam

0-1.2 m 

1.2-2.4 m 

2.4-3.6 m

3.6-4.8 m

>4.8 m

75

45

28

18

84

Silt loam

Loam

Sandy loam

7.0-9.7 0.4 x 10-2

Iowa Marshall silty 

clay loam

Minden silty 

clay loam

OM = 3.9%

pH = 5.6

Silty clay 

loam 

Silt loam

0-1.2 m

1.2-2.4 m

2.4-3.6 m

3.6-4.8 m

>4.8 m

207

84

172

87

1.0

Sand

Silt loam

Loam

1.2-8.5 0.9 x 10-1

Indiana Door loam

Lydick loam

OM = 3.0%

Sandy clay 

loam

Sandy 

0-1.2 m

1.2-2.4 m

2.4-3.6 m

64

190

244

Sand 7-9.1 0.6 x 10-1
48

Table 16. PGW sites: Selected soil and aquifer characteristics.

PGW 

Study

Location

NRCS Soil 

Series; 

On-site surface 

soil % O.M. & 

pH

Subsoil 

Textures1

Avg. Hydraulic 

Conductivity2

 

(mm/hr)

Aquifer soil 

textures 

determined1

Depth to 

ground 

water 3

(m)

Pore-water velocity

(m/day) 4

pH = 6.7 loam

Sand

3.6-4.8 m

>4.8 m

742

978

Pennsylvania Clarksburg silt 

loam

Duffield silt 

loam

OM = 2.7%

pH = 6.3

Loam

Sandy 

loam

0-1.2 m

1.2-2.4 m 

2.4-3.6 m 

3.6-4.8 m

>4.8 m 

NA

382

138

95

19

NA

Sandy loam

Loam

1.8–

7.3

0.4 x 10-1

Delaware Sassafras sandy 

loam

OM = 2.9%

pH = 5.8

Sandy 

loam

Loamy 

sand

Sand

0-1.2 m 

1.2-2.4 m 

2.4-3.6 m 

3.6-4.8 m 

>4.8 m 

30

86

30

129

NA

Sand

Sandy loam

Loamy 

sand

3.3-6.1 0.6 x 10-2

1 Soil texture determined by 3-fraction analysis (% sand, silt, and clay) 2

Vertical saturated hydraulic conductivity determined by constant head permeability method

3

Depth to ground water listed is below ground surface, and the minimum and maximum values are of all 

measurements made in the test plot piezometers during the course of the study 4

Average value determined during the course of the study

This table is modified from a more extended version by the ARP found in Newcombe et al. (2005).

Table xx shows some of the highest observed concentrations in surface water drinking water 

supplies and in observation wells in acetochlor use areas. 

Daily Annua

l

7-

year

Maximu

m

95th

%ile

90th

%ile

Max. 

site

95th

%ile

90th

%ile 

site 

–

Max

. 

year

.

Max

.

95th

%ile

90th

%ile

Groundwat

er

Observatio

n wells –

0.741 0.21

5

0.05

5

0.453 0.05

2

0.01

4

0.07

6

0.01

3

0.00

5
49

Daily Annua

l

7-

year

Maximu

m

95th

%ile

90th

%ile

Max. 

site

95th

%ile

90th

%ile 

site 

–

Max

. 

year

.

Max

.

95th

%ile

90th

%ile

adjac. To 

treatments.

PGW onsite0.027 0.008

Surface 

water

Finished 

drinking 

water

18.21 4.16 NC 1.43 0.37 NC NC NC NC

Surface 

water

Raw 

drinking 

water

7.19 7.09 

(1)

3.45 

(1)

0.59 0.04

3 (1)

0.36 

(1)

3.31 2.10 

(1)

1.56 

(1)

3.31

0.741

2.10

0.05

5

0.453 0.01

4

0.07

6

0.00

5

1.81

1.56

Daily values underestimate the highest exposure levels in many surface waters where water is flowing through 

sufficiently rapidly such that the daily peak concentration could be significantly higher than the peak concentration 

of the sampling dates which were typically at about a two-week interval during the acetochlor runoff season 

following the mostly spring applications of acetochlor.

(1) The SDWS study only did selective monitoring of raw water for about 25 to 44 of the 

175 Community Water Supply systems in the monitored program over each year of the 7-

year study. Consequently, the 95th and 90th Percentile values are somewhat arbitrarily is 

taken from the sites with the 2nd and 4th highest exposure level, respectively (unlike the 

finished water samples where the 95th percentile is from the 9th highest site since most of 

the sites were continuously monitored for finished water concentrations over the entire 7 

years of the study. 

Maximum Single 

Concentration (ppb) Maximum TWAM (ppb)

Maximum 95th %tile 

Concentration (ppb)a

CWS Name Value Year CWS Name Value Year CWS Name Value Year

168-PA-IL 7.19 1998 168-PA-IL 0.59 1998 168-PA-IL 3.31 1998

228-SA-IL 7.09 1996 168-PA-IL 0.43 1996 222-HI-IL 2.10 1998

168-PA-IL 5.89 1996 228-SA-IL 0.40 1996 1070-WY-MO 1.81 1997

228-SA-IL 3.45 1998 222-HI-IL 0.36 1998 228-SA-IL 1.56 1996

606-KA-IL 2.88 1998 1070-WY-MO 0.32 1997 259-SP-IL 1.44 1998

1070-WY-MO 2.50 1997 345-RI-IN 0.30 1997 168-PA-IL 1.39 1996

222-HI-IL 2.36 1998 222-HI-IL 0.26 1996 225-CE-IL 1.38 1998
50

345-RI-IN 2.27 1997 259-SP-IL 0.25 1998 228-SA-IL 1.37 1998

259-SP-IL 2.22 1998 606-KA-IL 0.23 1996 603-BL-IL 1.28 1995

225-CE-IL 2.01 1999 603-BL-IL 0.23 1995 557-DM-IA 1.13 2001

Source Site Acetochlor

Overall 

Max

Daily Value

Acetochlor

Average Daily Conc. 

for

Max. Year

Acetochlor

Multi-year

Average

EDWC or SGW study exposure duration

24 hr Annual 30- or 7-

year

PRZM-GW Modeling Highest of 6 

scenarios 129 119.8 82.6

ARP targeted 

Monitoring

#1 Site 

(99.5 %ile)

0.741

(MN25)

0.453

(MN25)

0.076

(KS14)

ARP targeted 

Monitoring

#2 Site 

(98.9 %ile)

0.534

(NE16)

0.178

(KS14)

0.057

(MN25)

ARP targeted 

Monitoring

#3 Site 

(98.3 %ile)

0.453

(KS09)

0.144

(KS19)

0.040

(KS17)

ARP targeted 

Monitoring

90th Percentile Site 0.055

(IL15)

0.014

(MN24)

0.005

(IA20)

Table 17. Summary presentation of chronic exposure to parent acetochlor: Timeweighted annualized mean concentrations (ppb) in surface and ground water from the 

ARP monitoring program (based on maximum TWAM values observed at each site by 

calendar year) along with WARP model predictions for streams and rivers. 

Study N Maximum 95th

Percentile

Median

Surface Water - SDWS 

raw

44 0.591 0.355 0.042

Surface Water - SDWS 

finished

189 1.428 0.347 0.032

Surface Water - WARP 

model (raw)a 470 0.812 0.435 0.042

Ground Water (shallow) -

PGW site averages

8 <0.03 <0.03 <0.03

Ground Water (shallow) -

PGW cluster maximums

8 <0.03 <0.03 <0.03

Ground Water - SGW 182 0.520 0.039 <0.03 a Includes TWAMs calculated by the WARP model. The WARP results are provided for 

comparison to the ARP monitoring results and include WARP results only for states where 

ARP also had surface water monitoring stations. 

N = total number of sites included in the statistics.
51

20060208 Acetanilide DWA:

Table 18. Distribution statistics for time-weighted multi-year concentrations of chloroacetanilides in CWS from the 

SDWS study (finished water samples only).

Statistic Alachlor 

Equivalents

Acetochlor 

TWC

(PPB)

Alachlor TWC

(PPB)

Count (n) 189 189 189

Maximum 0.286 0.282 0.276

99.8 Percentile 0.238 0.263 0.231

99.5 Percentile 0.166 0.235 0.162

99 Percentile 0.149 0.208 0.148

95 Percentile 0.078 0.125 0.074

90 Percentile 0.039 0.079 0.037

75 Percentile 0.017 0.039 0.014

Median 0.009 0.015 0.008

Mean 0.022 0.034 0.020

Table 19. Distribution statistics for time-weighted multi-year concentrations of chloroacetanilides in CWS from the 

SDWS study (raw water samples only).

Statistic Alachlor 

Equivalents

Acetochlor 

TWC

(PPB)

Alachlor TWC

(PPB)

Count (n) 45 45 45

Maximum 0.041 0.190 0.039

99.8 Percentile 0.041 0.184 0.039

99.5 Percentile 0.041 0.175 0.038

99 Percentile 0.041 0.161 0.037

95 Percentile 0.032 0.112 0.029

90 Percentile 0.030 0.103 0.025

75 Percentile 0.021 0.072 0.017

Median 0.013 0.025 0.011

Mean 0.027 0.052 0.025

20060208 Acetanilide DWA:
52

Table 20. Ratio of finished to raw water concentration for ARP SDWS sites with both sampled1

Ratio ID Alachlor 

Equivalents

Acetochlor F/R 

Ratio

Alachlor F/R Ratio

Average 56.0% 49.4% 56.6%

90th 

Percentile 88.7% 118.3% 86.3%

Median 51.0% 55.2% 51.7%

10th 

Percentile 29.6% 14.6% 34.0% 1 based on the distribution of multi-year mean concentrations for the 20 sites with the highest 

mean alachlor equivalent concentrations and with paired sampling of raw and finished water. 

The top ten single-year mean concentrations (out of approximately