1

The Ecological Significance of Atrazine Effects on Primary Producers 

in Surface Water Streams in the Corn and Sorghum Growing Region 

of the United States (Part II) 

Addendum to Section VI: Additional Analysis of the 40 AEEMP 

Sampling Sites to Identify Additional Areas That May Exceed the LOC 

Submitted to the FIFRA Scientific Advisory Panel 

For Review and Comment 

May 12 – 15, 2009 

Office of Prevention, Pesticides and Toxic Substances 

Office of Pesticide Programs 

Environmental Fate and Effects Division 

Washington, DC 

May 1, 2009 
2

Addendum to Section VI: Additional Analysis of the 40 AEEMP 

Sampling Sites to Identify Additional Areas That May Exceed the LOC 

This addendum to Section VI (Identifying Waters That Exceed Effects-Based Atrazine 

Thresholds Beyond the 40 Sampling Sites) describes additional analyses conducted by 

the US EPA to evaluate watershed characteristics of the 40 Atrazine Ecological 

Exposure Monitoring Program (AEEMP) sites in order to identify additional watersheds 

with characteristics similar to the sites that exceed the atrazine LOC in multiple years. 

A. Further Evaluation of Watershed Factors Driving Atrazine 

Exposures in the AEEMP Study 

US EPA evaluated a number of pesticide use, soil, hydrologic, weather, and 

watershed characteristics for the 40 AEEMP watersheds. The goal of this 

assessment was to identify those factors, singly or in combination, that best 

distinguish the 3 sites that exceeded the atrazine LOC in multiple years (MO-01, 

MO-02, NE-04) from those sites that did not exceed the LOC. For the time-being, the 

Agency grouped the 6 sites that exceeded the LOC in one year only (IN-05, IN-11, 

MO-03, NE-02, NE-05, NE-07) as a separate population. 

In the 2009 SAP Review Document (dated April 15, 2009), the US EPA noted a 

minimum atrazine use intensity of approximately 0.1 lb ai/watershed acre for the 

sites that exceeded the LOC in one or more years (Figure 1); above that level, 

differences in soil-related factors largely characterized the differences between the 

sites that exceeded the LOC and those that did not. The upstream catchment area 

of the nine sites that exceeded atrazine in one or more years generally had a higher 

percentage of runoff prone soils, as indicated by a high or very high USDA runoff 

class or by the presence of hydrologic group C or D soils. The three sites that 

exceeded the LOC in multiple years had a higher fraction of soils with a shallow 

restrictive layer, as indicated by the presence of a subsurface layer with a 

moderately low or low saturated hydraulic conductivity (Ksat) within 50 cm of the 

surface. 

Because the proposed use intensity threshold is based on a limited number of 

samples (as shown in Figure 1, only three sites are near or below the 0.1 lb ai/A 

level), the US EPA looked at an alternative approach that weighted the soil 

parameters by the use intensity. 

US EPA calculated soil parameter-use intensity indices by multiplying the fraction of 

high/very high runoff soils, hydrologic group C or D soils, or soils with a shallow 

depth to a restrictive layer in the upstream catchment area by the atrazine use 

intensity for the catchment. 
3

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Atrazine Use Intensity, lb ai/ac

*

*

*

o

o o

o

o

o

* Sites exceeded LOC in multiple years

o Sites exceeded LOC in 1 year

Figure 1 Atrazine use intensity, lb ai/ catchment acre, for the 40 AEEMP monitoring sites 

The USDA soil runoff classes rank soils according to their potential for loss of water 

due to surface runoff based on a combination of saturated hydraulic conductivity 

(Ksat) of the surface horizon and slope. Soils with either a high or very high runoff 

potential made up 50% or more of the upstream catchments at sites that exceeded 

the LOC in multiple years (indicated by a red asterisk in Figure 2) and more than 

40% at sites that exceeded the LOC at least once (indicated by a red circle in Figure 

2). Except for NE-01, similar high-runoff soils comprised less than 46% of upstream 

catchments for sites that never exceeded the LOC. 

Weighting the fraction of watersheds with high runoff soils by use intensity provides 

a clearer distinction between sites that exceed the LOC and those that do not 

exceed the LOC (Figure 3). The sites that exceed the LOC in multiple years had a 

use intensity-high runoff index greater than 0.20 while sites that did not exceed the 

LOC had an index of less than 0.17. Sites that exceeded the LOC in one year 

overlapped both groups, with a use intensity-high runoff index ranging from 0.05 to 

0.35. 
4

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Fraction of watershed

High Runoff Potential

Very High Runoff Potential

*

*

*

o

o

o

o

o

o * Sites exceeded LOC in 

multiple years

o Sites exceeded LOC in 

1 year

Figure 2 Fraction of AEEMP watersheds containing soils with high or very high runoff 

potential 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Fraction of watershed x Use Intensity

High Runoff Potential

Very High Runoff Potential

*

*

*

o

o

o

o

o

o

* Sites exceeded LOC in 

multiple years

o Sites exceeded LOC in 

1 year

Figure 3 Use intensity-adjusted fraction of AEEMP watersheds containing soils with high 

or very high runoff potential (use intensity – high runoff index) 
5

The hydrologic soil group classification is used in curve number method commonly 

used in water quality models. Higher portions of rainfall are expected to runoff from 

soils in hydrologic group C or D than soils in group A or B. The classifications are 

based on surface hydraulic conductivity and the water storage capacity of the soil. 

While the two MO sites that exceeded the LOC in multiple years had a high fraction 

of hydrologic group D soils in the upstream catchment (shown as blue bars in Figure 

4), NE-04, which also exceeded the LOC in multiple years, had a much lower 

fraction, less than that of 3 other sites (Figure 4). While the upstream catchments of 

all three sites that exceeded the LOC in multiple years had greater than 85% 

hydrologic group C and D soils, two sites that did not exceed the LOC (IN-03 and 

OH-01) also had similarly high fractions of C and D soils. 

When the fraction of the watershed in C or D soils was weighted based on atrazine 

use intensity, the resulting use intensity-group C&D soils index for the sites that 

exceeded the LOC in multiple years was greater than 0.3. (Figure 5). However, two 

sites that did not exceed the LOC (IN-03 and IN-09) and one site that exceeded 

once (IN-05) also had a similarly high use intensity-group C&D soils index. 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Fraction of watershed

HSG C soils

HSG D soils

**

*

o

o

o

o

o

o

* Sites exceeded LOC 

in multiple years

o Sites exceeded LOC 

in 1 year

Figure 4 Fraction of AEEMP watersheds containing hydrologic group C and D soils 
6

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Fraction of watershed x Use Intensity

HSG C soils

HSG D soils *

*

*

o

o

o

o

o

o

* Sites exceeded LOC 

in multiple years

o Sites exceeded LOC 

in 1 year

Figure 5 Use intensity adjusted Fraction of AEEMP watersheds containing hydrologic 

group C and D soils (use intensity-group C&D soil index) 

US EPA defined the depth to drainage restrictive layers based on the depth to which 

subsurface soil layer with Ksat value of <1 μm/s occurs. Greater than 67% of the 

catchments for the three sites that exceeded the LOC in multiple years contained 

soils that had a Ksat value of less than 1 μm/s within 50 cm of the surface while, for 

the sites that did not exceed the LOC, less than 41% of the catchment areas had 

similar soils (Figure 6). Two additional NE sites that exceeded the LOC once (NE-05 

and NE-07) had fractions of soils with shallow, low-Ksat soils similar to the sites that 

exceeded in multiple years. 

However, when the proportion of soils with a shallow drainage-restrictive layer was 

weighted by use intensity, the three sites that exceeded the LOC in multiple years 

were clearly separated from the remaining sites (Figure 7). The resulting use 

intensity-low Ksat index was greater than 0.25 for the sites that exceeded the LOC in 

multiple years and less than 0.17 for the remaining sites (less than 0.15 for the sites 

that did not exceed the LOC). 
7

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Fraction of watershed

Ksat<1 um/s, 25-50cm

Ksat<1 um/s, <25cm

o

o

o

o

o

o

*

* *

* Sites exceeded LOC in multiple years

o Sites exceeded LOC in 1 year

Figure 6 Fraction of AEEMP watersheds with Ksat <1 μm/s within 50 cm of the surface 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

IA-01IA-02

IL-01IL-02IL-03IL-04IL-05IL-06IL-07IL-08IL-09

IN-01IN-02IN-03IN-04IN-05IN-06IN-07IN-08IN-09IN-10IN-11

KY-01KY-02

MN-01

MO-01MO-02MO-03

NE-01NE-02NE-03NE-04NE-05NE-06NE-07

OH-01OH-02OH-03OH-04

TN-01

Site ID

Fraction of watershed

Ksat<1 um/s, 25-50cm

Ksat<1 um/s, <25cm

**

*

o

o

o

o

o

o

* Sites exceeded LOC in multiple years

o Sites exceeded LOC in 1 year

Figure 7 Use intensity-adjusted fraction of AEEMP watersheds with Ksat <1 μm/s within 

50 cm of the surface (use intensity-low ksat index) 
8

Preliminary multivariate approaches have confirmed the importance of both the 

use-weighted Ksat index as well as depth to low Ksat values, High-to-Very High 

runoff potential, and soil type (D soils) factors in combination. Further evaluation 

of these factors with a larger set of data is key to developing more definitive 

models beyond the current working hypotheses. 

B. Update on Approaches for Identifying Watersheds with 

Characteristics Similar to Those That Exceed the Atrazine LOC 

The 2009 SAP Review Document describes the parameters US EPA used to identify 

other watersheds within the atrazine use area that are similar to the sites that 

exceeded the atrazine LOC in multiple years: 

1) Define the extent of potential vulnerable watersheds based on a minimum 

atrazine use intensity (0.1 lb ai/A) and rainfall (average annual precipitation of at 

least 23 inches). 

2) Identify runoff-prone watersheds that have at least 40% soils with a high or very 

high runoff potential or at least 60% hydrologic group C or D soils. 

3) Identify watersheds that have at least 45% soils with a subsurface layer that has 

a Ksat value of <1 μm/s within 50 cm of the surface. 

Figure 8 shows the extent of NHDPlus watersheds in a 10-state region (OH, KY, TN, 

IN, IL, MN, IA, MO, NE, KS) that meet those criteria. The dark green areas meet the 

precipitation, soils, and use ctiteria; the light green areas meet the precipitation and 

soil criteria, but have a lower atrazine use intensity; the gray areas meet the soil 

criteria but have no no corn or sorghum in the watershed. The dark orange 

background shows the extent of vulnerable watersheds identified by WARP that 

were represented by the 40 AEEMP monitoring sites. The light orange shows the 

extent of the next lower tier of vulnerable watersheds. 

Table 1 provides a breakdown of the area percentages of the NHDPlus catchments 

that met either the precipitation and soil criteria alone or the precipitation, soil and 

atrazine use intensity criteria for the ten state area. Roughly 22% of the area in both 

the upper 20th percentile and the 60-80th percentile tiers of vulnerable watersheds 

as identified using WARP (see the background documents for the 2007 and 2009 

SAP meetings for details) consisted of NHDPlus watersheds that meet both the 

precipitation (average annual rainfall >23 inches) and the soil (>60% hydrologic 

group C or D soils and >45% soils with a low Ksat w/in 50 cm of the surface) criteria. 

This fraction may represent the maximum potential extent of areas where atrazine 

concentrations in flowing streams might exceed the LOC. Because of varying use 

intensities over time and expected crop rotations, the atrazine use intensity is not 

expected to be this great for the entire area. 

NHDPlus watersheds that meet the precipitation, soils, and use criteria cover 15% of 

the area in the upper 20th percentile of vulnerable watersheds and half that much in 
9

the next lower (60-80th percentile) tier. Very few areas outside of these tiers meet 

the criteria. 

Figure 8 Map Showing Distribution of Watersheds That May Exceed the Atrazine LOC 

Based on Atrazine Use, Precipitation, and Soil Criteria. 

Table 1 Percentage of area with watersheds with characteristics similar to the AEEMP 

sites that exceed the LOC based on separate precipitation, atrazine use intensity, and 

soil criteria 

Percent of area 

AEEMP sites exceeding 2 or more years (for the 

entire upper 20th %ile WARP watersheds) 12% (0-25%, 95% CI) 

NHDPlus catchments in the 10-state region (see 

Figure 8) that meet: 

Precipitation + 

soil criteria 

Precipitation + 

soil + use 

criteria 

Upper 20th %ile of vulnerable watersheds (WARP) 22.6% 15.3% 

60-80th %ile tier of vulnerable watersheds (WARP) 22.8% 7.6% 

Remaining area 11.6% 0.4% 
10

US EPA used the use intensity-low Ksat index to identify watersheds that are similar 

to the sites that exceeded the atrazine LOC in multiple years. For each NHDPlus 

watershed in the 10-state region, US EPA multiplied the fraction of the watershed 

with soil that have a Ksat of <1 μm/s within 50 cm of the surface by the atrazine use 

intensity and mapped the resulting use intensity-low Ksat index (Figure 9). The 

watersheds that meet the use intensity-low Ksat index criteria cover a similar 

pattern, though a smaller area, than the watersheds that meet the precipitation-use 

intensity-soil criteria (compare Figures 8 and 9 and Tables 1 and 2). However, the 

percent of area in the upper 20th percentile of vulnerable watersheds is in line with 

the population estimate based on the AEEMP results (Table 2). 

Figure 9 Map Showing Distribution of Watersheds That May Exceed the Atrazine LOC 

Based on the Use Intensity-Low Ksat Index 
11

Table 2 Percentage of area with watersheds with characteristics similar to the AEEMP 

sites that exceed the LOC based on the use intensity-low Ksat index 

Percent of area 

AEEMP sites exceeding 2 or more years (for the 

entire upper 20th %ile WARP watersheds) 12% (0-25% 95% CI) 

NHDPlus catchments in the 10-state region (see 

Figure 8) with a: 

Use-ksat 

index >0.15 

Use-ksat index 

>0.20 

Upper 20th %ile of vulnerable watersheds (WARP) 12% 8% 

60-80th %ile tier of vulnerable watersheds (WARP) 3% 1.5% 

Remaining area 2% 0.1% 

 

C. Update on Evaluating Vulnerable Watershed Criteria with 

Monitoring Data Not in the AEEMP 

In the 2009 SAP Review Document (dated April 15, 2009), the US EPA identified a 

number of monitoring data sources it plans to use to evaluate the results of the 

watershed analysis. To provide a comparable data set for analysis, US EPA 

selected those sites that sampled 1st to 5th order streams (based on Strahler’s 

stream order) during the 2004-2008 time frame. That represented comparable water 

bodies and a comparable time frame for both atrazine use and weather. Where the 

sample frequency is sufficient (bi-weekly to monthly during the growing season), 

USEPA evaluated the chemographs using the PATI LOCMEI. US EPA identified 330 

monitoring stations from NAWQA (USGS, 2009), Nebraska Department of 

Environmental Quality (NDEQ) ambient stream and Blue River Basin Water Quality 

Source Assessment programs (NDEQ, 2008), Kansas Department of Health and 

Environment (KDHE, 2008), Iowa Department of Agriculture (IADA, 2009), 

Minnesota Department of Agriculture (MnDA, 2008), and the USDA ARS in MO 

(Lerch et al, 2008) that met the criteria for stream order and time frame (Table 3). 

Table 3 Summary of Monitoring Data Used to Evaluate the AEEMP Watershed Criteria 

Total number of sites 

Source No. 

Sites 

Max Detect 

>10 μg/L 

Max Detect 

>20 μg/L 

Max detect 

<1 μg/L 

Exceed 

LOC 

NAWQA (10 states) 44 9 7 13 6 

NE Dept. of Envir. Qual. 101 33 25 37 26 

KS Dept. of Health & Envir. 121 15 8 58 

IA Dept. of Agric. 54 3 0 8 2 

MN Dept. of Agric. 9 2 0 0 0 

USDA ARS, MO 1 1 1 0 1 

Total 330 63 41 116 354 

The first step US EPA is taking in evaluating the watershed criteria is to overlay the 

locations of the monitoring sites summarized in Table 3 onto the NHDPlus 

watersheds in the 10-state region that encompasses the 40 AEEMP monitoring 

sites. The Agency will determine how effectively the proposed criteria for identifying 

vulnerable watersheds (precipitation-use-soil combination or the use intensity-low 
12

Ksat index) separate sites that either exceed the LOC or have maximum detects 

greater than 10 or 20 μg/L from sites that did not have atrazine detections greater 

than 1 μg/L. 

As a second step, US EPA plans to further evaluate the proposed criteria for 

identifying vulnerable watersheds and refine, if needed, by comparing the watershed 

characteristics of the additional non-AEEMP monitoring sites that exceeded the LOC 

to the AEEMP sites. 

To date, US EPA has completed the first step of evaluation for the precipitation-usesoil criteria combination. Approximately 15% of the 330 monitoring sites occur in 

NHDPlus catchments that meet the precipitation-use-soil combination criteria used 

to identify watersheds similar to those that exceeded the LOC in multiple years 

(Table 4). Over half of the sites in these vulnerable areas had maximum detections 

>10 μg/L, compared to 20% of all sites. Conversely, while more than a third of the 

total monitoring sites did not have atrazine detections >1 μg/L, only 8% of the sites 

in the vulnerable areas never had detections >1 μg/L. 

Nearly half of sites in the vulnerable areas exceed the PATI LOC, compared to 16% 

for all sites (Table 4). While the sites with precipitation-use-soil criteria similar to the 

AEEMP sites that exceed the LOC in multiple years capture more than half (19 out 

of 35) of the total monitoring stations that exceed the LOC, some sites that exceed 

the LOC occur in areas that did not meet the combined precipitation-use-soil criteria 

similar to the AEEMP sites that exceed the LOC in multiple years. 

Table 0-4 Preliminary Evaluation of Sites Not in the AEEMP That Exceeded the PATI 

LOCMEI

Number of Sites 

Monitoring Data 

Set Location Total Sites 

Max 

Detect 

>10 μg/L 

Max 

Detect 

>20 μg/L 

Max 

detect <1 

μg/L 

Exceed 

LOC 

NAWQA 10 All sites 44 9 (20%) 7 (16%) 13 (30%) 6 (14%) 

states Vulnerable area1 7 5 (71%) 4 (57%) 1 (14%) 3 (43%) 

Nebraska DEQ All sites 101 33 (33%) 25 (25%) 37 (37%) 26 (26%) 

Vulnerable area1 24 17 (71%) 12 (50%) 0 (0%) 13 (54%) 

Kansas DHE All sites 121 15 (12%) 8 (7%) 58 (48%) 

Vulnerable area1 8 3 (38%) 0 (0%) 3 (38%) 

Iowa DA All sites 54 3 (6%) 0 (0%) 0 (0%) 2 (4%) 

Vulnerable area1 8 2 (25%) 0 (0%) 0 (0%) 2 (25%) 

Minnesota DA All sites 9 2 (22%) 1 (11%) 0 (0%) 0 (0%) 

Vulnerable area1 0 0 0 0 0 

Missouri USDA All sites 1 1 1 0 1 

ARS Vulnerable area1 1 1 1 0 1 

All sites 330 63 (19%) 41 (12%) 116 (35%) 35 (16%)2 All Sites 

Vulnerable area1 48 26 (54%) 17 (35%) 4 (8%) 19 (48)2

1 Catchments meet the vulnerable watershed criteria of annual average precipitation >23 

inches, atrazine use intensity >0.1 lb ai per catchment acre, and runoff-prone soils (>60 percent 
13

of the upstream catchment area consisted of hydrologic group C or D soils with >45 percent of 

the soils having a subsurface layer with a Ksat of <1 um/s within 50 cm of the surface).. 

2 Total percentages for sites that exceed the LOC are based on 209 total sites and 40 sites in 

the vulnerable watersheds, excluding the KS sites which did not have a sufficient sample 

frequency to evaluate with the PATI LOC. 

The Agency plans additional analyses of all of these monitoring sites, including an 

evaluation of the upstream catchment characteristics of selected monitoring sites. In 

addition, US EPA plans to conduct a similar evaluation for the use intensity-low Ksat 

index method for identifying watersheds with characteristics similar to the AEEMP 

sites that exceeded the LOC in multiple years. 

To date, the watershed analysis has focused on the 10 states that encompass the 

40 sites in the AEEMP. US EPA plans to expand the watershed analysis to 

encompass the extent of the area identified is the upper 20th percentile of vulnerable 

watersheds based on WARP (Figure 10). 

Figure 10 Geographic extent of watersheds in the upper percentiles of vulnerability 

based on WARP.