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Introduction

Assessment and management of pesticides require far more information than we can afford to directly measure for all the places, times, and pesticides of interest. In addition, many decisions-such as setting monitoring priorities, approving registration of a new pesticide, and determining how much to spend on a management strategy-inherently depend on predicting the potential effects of pesticides on water quality for locations or amounts of use that have never been directly assessed. In these situations, statistical models and other types of models are used for predicting water-quality conditions at unmonitored locations under a range of possible circumstances. The National Water Quality Assessment (NAWQA) Program is developing a series of statistical models, based on monitoring data and watershed characteristics, to enable estimation of pesticide concentrations for streams that have not been monitored. The Watershed Regression for Pesticides models are referred to as WARP models. The first completed WARP model is for atrazine, one of the most heavily used herbicides in the United States (Figure 1).

figure 1
Figure 1. WARP estimates of annual mean concentrations of atrazine in U.S. streams (nonagricultural sources are not accounted for).

A complete description of the development and performance of the atrazine WARP model is provided by Stone and Gilliom (2009). The model statistically relates atrazine concentrations in streams to watershed characteristics that are determined from data sources with national coverage. The model was developed using standard, widely available statistical techniques. Work is currently underway to extend the WARP model approach to other pesticides.

Frequently Asked Questions About WARP

What is Atrazine?

Atrazine is one of the most heavily used herbicides in the United States. Most agricultural use is associated with corn production-about 85 percent of 75 million lbs/yr in 1997. Atrazine has relatively low and poorly quantified nonagricultural use-estimated at less than 1 million lbs/yr (USEPA, 2003a). Nonagricultural uses include conifer forestry, Christmas tree farms, sod, golf courses, and residential lawns (particularly in the South). Atrazine is highly soluble and mobile in water and relatively long-lived, with a soil half-life of 146 days.

Concentrations of atrazine in agricultural streams generally conform to the geographic distribution of corn cultivation, where applications are greatest (Figure 2). Atrazine is also frequently detected in urban streams, but usually at substantially lower concentrations compared with agricultural streams that are located in areas of high atrazine use (some urban streams in parts of the South where atrazine is used on turf grasses have relatively high concentrations). Concentrations in streams draining watersheds with mixed land use tend to resemble those in agricultural streams because many of these streams have watersheds with relatively high proportions of agricultural land.

figure 2
Figure 2. Distribution of atrazine use on crops and 95th percentile atrazine concentrations in agricultural streams.

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How was the Model Developed?

The WARP models for pesticides are developed using linear regression methods to establish quantitative linkages between pesticide concentrations measured at NAWQA sampling sites and a variety of human-related and natural factors that affect pesticides in streams. Such factors include pesticide use, soil characteristics, hydrology, and climate-collectively referred to as explanatory variables. Measured pesticide concentrations, together with the associated values of the explanatory variables for the sampling sites, comprise the model-development data.

The WARP model for estimating atrazine in streams is based on concentrations measured by NAWQA from 1992 to 2001 at 112 stream sites (Figure 3). The single most complete year of data was used to calculate concentration statistics for each site. The atrazine model actually consists of a series of models, each developed for a specific concentration statistic.

figure 3
Figure 3. Stream sites included in model development.

The models are built using the explanatory variables that best correlate with, or explain, the concentration statistics computed from concentrations observed in streams. Although explanatory variables included in the models are significantly correlated with pesticide concentrations, the specific cause-and-effect relations responsible for the observed correlations are not always clear, and inferences regarding causes should be considered as hypotheses.

In developing the models, all potential explanatory variables were required to have values available from existing data sources that include all locations in the conterminous United States, so that national extrapolation would be possible. About 43 possible variables were considered and these were reduced to 6 explanatory variables that were most significant and yielded optimal model formulations. Each model incorporates an uncertainty analysis, which allows assessment of the reliability of the model predictions and also the expression of model predictions as probabilities that concentrations will exceed a specific value, such as a water-quality benchmark, at a particular location.

Atrazine use intensity is the most important factor in the model that explains atrazine concentrations in streams-the more intensive the use of atrazine in a watershed, the higher the atrazine concentration in the stream. Specifically, estimated atrazine use intensity within each watershed explains 68 percent of the variance in annual mean atrazine concentrations. Four additional variables explain another 14 percent of the variability, most of which is accounted for by precipitation during May and June of the sampling year and rainfall erosivity and soil erodibility-factors used in the revised Universal Soil Loss Equation (Renard and others, 1997). For the most of the model development sites, the months of April through June include the highest atrazine application period. High precipitation during May and June, after atrazine application, increases atrazine runoff to streams. Rainfall erosivity and soil erodibility quantify, respectively, the energy of storms in a specific area (averaged over several years), and the susceptibility of soils to erosion by runoff. As these two factors increase, atrazine concentrations also increase, indicating that transport of atrazine is highest in areas of high-energy rain storms and in areas where soils are most susceptible to erosion. Alternatively, soil erodibility may indicate high surface runoff, rather than actual transport of atrazine with soil particles. Overall, the complete model explains a total of 82 percent of the variance in observed annual mean atrazine concentrations.

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What does the Model Predict?

The atrazine models included for use on this web site to create maps and graphs are the models for the annual mean, annual modeled maximum, annual modeled maximum moving averages (21-, 60-, and 90-day durations) and the 5th, 25th, 50th, 75th and 95th percentiles of annual concentration; models for other percentiles are included in Stone and Gilliom (2009). For each of these annual concentration statistics, the models can be used to estimate the value for a particular stream, including confidence bounds on the estimate, or the probability that a particular value will be exceeded, such as a water-quality benchmark. Each of these options for applying the model has advantages for specific purposes.

The modeled maximum concentration is expected to under predict the true annual maximum concentration. The sampling frequencies of the model devlopment sites were not sufficient to characterize the true annual maximum concentrations (Stone and others, 2008).

When used to estimate the value of a concentration statistic for a stream, such as the mean or 95th percentile, the model computes the median estimate of the statistic for all streams with watershed characteristics that are similar to the stream in question. Thus, the computed estimate for a particular stream has an equal chance of being above or below the actual value of the statistic. The confidence that the estimated value is within a certain magnitude of the actual value is indicated by the 95-percent confidence limits, which encompass 95 percent of the actual values associated with the predicted value.

When used to estimate the probability that a particular stream has an atrazine concentration greater than a specific threshold, usually a water-quality benchmark, the model prediction and uncertainty are combined to estimate the probability for the stream.

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How are Predictions Made?

Model estimates can be made for either concentration values or probabilities for most stream reaches included in the USEPA River Reach file (Nolan and others, 2003), which include more than 600,000 miles of streams and more than 60,000 individual stream reaches with watersheds. The term "stream" refers to all River Reach file segments, regardless of drainage basin area. Model development stations spanned over 5 orders of magnitude in terms of watershed area and model predictions are not biased with respect to watershed area. However, model estimates are not made available for streams with watersheds smaller than 75 square kilometers because of the high potential uncertainty in explanatory variables, such as pesticide use, for small watersheds.

In addition to streams, River Reach file segments may actually represent reservoirs. The atrazine regression models were developed with stream (flowing water) data to predict concentration distributions in streams. The application of these regression models to lakes and reservoirs will provide under predicted concentrations for the annual mean and the annual percentiles (Larson and others, 2004). Model estimates are not made available for River Reach file segments that represent reservoirs.

Annual agricultural pesticide use estimates were developed using annual pesticide use data for individual crops for multi-county areas referred to as Crop Reporting Districts (CRDs) (proprietary data, dmrkynetec, Inc., St.Louis, Missouri) and county-level annual harvested acres of individual crops from either the U.S. Department of Agriculture Census of Agriculture or annual survey data for major crops. The CRD-level use estimates for each crop were disaggregated to county-level use estimates by dividing the mass of a pesticide applied to a crop by the acres of that crop in the CRD to yield a rate per harvested acre. This rate was then multiplied by harvested crop acreages in each county to obtain county-level use. The county-level pesticide use estimates for the U.S. were then combined with national land cover data to generate a raster dataset of pesticide use intensity, following the method described in Nakagaki and Wolock (2005).

Estimating atrazine concentrations for the roughly 60,000 stream reaches required that regression predictor variables be calculated for the entire basin draining to the downstream end of each stream reach. For example, the predictor variables for the stream reach at the mouth of the Mississippi River are based on the chemical and physical characteristics of the entire upstream river basin. The process of computing the explanatory variables incorporated several steps. For illustration, the steps below describe the process for calculating atrazine use intensity. The required geographic data include a raster dataset of atrazine use intensity for the conterminous U.S., the River Reach file, and a raster dataset of incremental catchments, which are the local drainage areas of all headwater streams, tributary streams, and stream segments lying between confluences. The processing steps are:

  1. The incremental catchments are intersected with the atrazine use intensity grid to compute atrazine use in each incremental catchment.
  2. The known association between the individual stream reaches and corresponding incremental catchments is used to assign incremental atrazine use values to each stream reach. Similarly, the correspondence between the individual stream reaches and the incremental catchments is used to assign incremental drainage area values to each stream reach.
  3. The topology (spatial connections) of the stream network is used to accumulate, in a downstream direction, all the incremental atrazine use values. This accumulation provides an estimate of total basin atrazine use for each stream reach. Similarly, the incremental drainage areas are accumulated in a downstream direction to estimate the total drainage area for each stream reach.
  4. The total basin atrazine use is divided by the total drainage area to calculate the atrazine use intensity for each stream reach.

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What are Some Examples of WARP Predictions?

Figure 4 shows predicted annual mean concentrations for 2007. Model predictions of atrazine levels in streams across the Nation show the highest annual mean concentrations throughout the high-use areas of the Corn Belt and the Mississippi Valley and Delta regions, and in some areas of Texas, Pennsylvania, and Maryland. Annual means for a few streams in Missouri, Kansas, and Texas are predicted to exceed 3 ug/L, the human-health benchmark for atrazine. This benchmark is the USEPA MCL for drinking water. As a drinking-water standard, the MCL applies to finished drinking water in public water supplies, whereas the predictions are for untreated stream water. Comparisons of model predictions with human-health benchmarks, however, serve as a screening-level assessment of the suitability of potential drinking-water sources.

figure 4
Figure 4. WARP estimates of annual mean concentrations of atrazine in U.S. streams (nonagricultural sources are not accounted for).

Figure 5 shows the probability that streams will exceed the 3 ug/L benchmark. The streams with a greater than 5-percent probability of exceeding the benchmark represent about 6 percent of the Nation's stream miles (36,829 of 649,935 mi). Approximately 546 stream miles (less than 1/10th of 1 percent of the Nation's stream miles) are predicted to have more than a 50-percent probability of exceeding 3 ug/L.

figure 5
Figure 5. WARP estimates of the probability that the annual mean concentration of atrazine in U.S. streams exceeds the atrazine Maximum Contaminant Level (MCL) for drinking water of 3 ug/L (nonagricultural sources are not accounted for).

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How Can WARP be Used to Assess the Potential for Effects of Pesticides on Humans or Aquatic Life?

The potential for pesticide concentrations in stream water to adversely affect human health or aquatic life is evaluated by NAWQA using screening-level assessments (Gilliom and others, 2006) similar in concept to USEPA screening-level assessments (USEPA, 2004d). The NAWQA screening-level assessments compare site-specific estimates of pesticide exposure ( concentration statistics or concentrations) with water-quality benchmarks derived from standards and guidelines established by USEPA, toxicity values from USEPA pesticide risk assessments, and USGS Health-Based Screening Levels (HBSLs). The USEPA standards, guidelines, and toxicity values were developed by USEPA as part of the Federal process for assessing and regulating pesticides.

Screening-level assessments are not a substitute for either risk assessments, which include many more factors (such as additional avenues of exposure), or site-specific studies of effects. Rather, comparisons of measured or estimated concentrations with water-quality benchmarks provide a perspective on the potential for adverse effects, as well as a framework for prioritizing additional investigations that may be warranted. Concentrations that exceed a benchmark do not necessarily indicate that adverse effects are occurring-they indicate that adverse effects may occur and that sites where benchmarks are exceeded may merit further investigation. The characteristics and limitations of screening-level assessments are summarized below.

Characteristics and Limitations of Screening-Level Assessments

Screening-level assessments are a first step toward addressing the question of whether or not pesticides are present at concentrations that may affect human health or aquatic life. They provide a perspective on where effects are most likely to occur and what pesticides may be responsible. Screening-level assessments are primarily intended to identify and prioritize needs for further investigation and have the following characteristics and limitations:

Human-Health Benchmarks for Pesticides in Water

Benchmarks for assessing the potential for pesticides in water to affect human health are USEPA Maximum Contaminant Levels (MCLs) for regulated compounds, and USGS Health-Based Screening Levels (HBSLs) for unregulated compounds.

Maximum Contaminant Level (MCL) - The maximum permissible concentration of a contaminant in water that is delivered to any user of a public water system. This is an enforceable standard issued by USEPA under the Safe Drinking Water Act and established on the basis of health effects and other factors (analytical and treatment technologies, and cost).

Health-Based Screening Level (HBSL) - HBSLs are concentrations or ranges in concentration (for carcinogens) that can be used as benchmarks to which contaminant concentrations in water can be compared to evaluate water-quality data in a human-health context. HBSLs were developed collaboratively by the USGS, U.S. EPA, New Jersey Department of Environmental Protection, and Oregon Health & Science University (Toccalino and others, 2003, 2004). HBSLs are not regulatory standards, are not enforceable, and water systems are not required to monitor for any unregulated compounds for which HBSLs have been developed. HBSL values are developed by using USEPA Office of Water methodologies and USEPA toxicity values, so they generally are comparable to USEPA drinking-water guideline values such as lifetime health advisory levels and risk-specific dose values (Toccalino and others, 2004). Compounds with existing UPEPA USEPA drinking-water guideline values such as lifetime health advisory levels or a risk specific dose are included as HBSLs.

Aquatic-Life Benchmarks for Pesticides in Water

Benchmarks for assessing the potential for pesticides in stream water to adversely affect aquatic life were of two general types: (1) ambient water-quality criteria for the protection of aquatic life (AWQC-AL), which were developed by USEPA's Office of Water (OW), and (2) benchmarks derived from toxicity values obtained from registration and risk-assessment documents developed by USEPA's Office of Pesticide Programs (OPP). Toxicity data from OPP documents were used to supplement OW criteria to expand the coverage of pesticides and to incorporate the most recent toxicity information used by USEPA. The following is summarized from Gilliom and others (2006), which contains complete sources, references, and values.

Ambient Water-Quality Criteria for Aquatic Organisms

USEPA's OW derives both acute and chronic criteria, each of which specifies a threshold concentration for unacceptable potential for effects, an averaging period, and an acceptable frequency of exceedances.

Acute AWQC-AL - The highest concentration of a chemical to which an aquatic community can be exposed briefly without resulting in an unacceptable effect. Except where a locally important species is very sensitive, aquatic organisms should not be unacceptably affected if the 1-hour average concentration does not exceed the acute criterion more than once every 3 years, on average. The intent is to protect 95 percent of a diverse group of organisms.

Chronic AWQC-AL - The highest concentration of a chemical to which an aquatic community can be exposed indefinitely without resulting in an unacceptable effect. Except where a locally important species is very sensitive, aquatic organisms should not be unacceptably affected if the 4-day average concentration does not exceed the chronic criterion more than once every 3 years, on average. The intent is to protect 95 percent of a diverse group of organisms.

Toxicity Values from Risk Assessments

Seven types of aquatic toxicity values were compiled from OPP's registration and risk-assessment documents. The OPP toxicity values are for specific types of organisms. Acute and chronic values were compiled for fish and invertebrates, and acute values for vascular and nonvascular plants. A value for aquatic-community effects was available only for atrazine. The types and amounts of toxicity data available for different pesticides were highly variable. USEPA estimates the toxicity or hazard of a pesticide by selecting the most sensitive endpoints from multiple acute and chronic laboratory and field studies. For many pesticides, USEPA has completed a screening-level ecological risk assessment, which includes acute and chronic assessments for both fish and invertebrates. For some pesticides, acute assessments have also been completed for nontarget aquatic plants. NAWQA derived benchmarks from OPP toxicity values, generally following OPP procedures.

In recent years, USEPA has developed methods for conducting refined risk assessments, in which probabilistic tools and methods are incorporated to predict the magnitude of the expected impact of pesticide use on nontarget organisms, as well as the uncertainty and variability involved in these estimates. The screening-level benchmarks used in NAWQA analysis and summarized below were derived from the toxicity values reported in USEPA registration and risk-assessment documents.

In the few cases where refined assessments were available, these were given preference. In deriving a benchmark for a given type of organism (such as fish) and a given exposure duration (acute or chronic), the lowest of the available toxicity values was selected for each benchmark, unless a preferred toxicity value was specified in a refined risk assessment-in which case that preferred toxicity value was used instead. For two of the benchmarks-acute-fish and acute-invertebrates-the selected toxicity values were multiplied by the USEPA level of concern (LOC) of 0.5, so that the benchmark for NAWQA screening corresponds to the acute risk level defined by USEPA.

Six benchmarks were based directly on toxicity endpoints used in OPP screening-level assessments:

Acute fish - The lowest tested 50-percent lethal concentration (LC50) for acute (typically 96-hour) toxicity tests with freshwater fish, multiplied by the LOC of 0.5.

Acute invertebrate - The lowest tested LC50 or 50-percent effect concentration (EC50) for acute (typically 48 or 96-hour) toxicity tests with freshwater invertebrates, multiplied by the LOC of 0.5.

Acute vascular plant - The lowest tested EC50 for freshwater vascular plants in acute toxicity tests (typically < 10 days).

Acute nonvascular plant - The lowest tested EC50 for freshwater nonvascular plants (algae) in acute toxicity tests (typically < 10 days).

Chronic fish - The lowest no-observed-adverse-effects concentration (NOAEC), or the lowest-observed-adverse-effects concentration (LOAEC) if a NOAEC is not available, for freshwater fish in early lifestage or full life-cycle tests.

Chronic invertebrate - The lowest NOAEC, or LOAEC if a NOAEC is not available, for freshwater invertebrates in life-cycle tests.

One additional benchmark, a benchmark for aquatic-community effects, was derived from the refined risk assessment for atrazine. This endpoint for atrazine incorporates community-level effects on aquatic plants and indirect effects on fish and aquatic invertebrates that could result from disturbance of the plant community.

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What are the Appropriate Uses of the Model and this Web Site?

Potential users of the model and this web site include:

Applications include:

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What are the Limitations of WARP?

The WARP model for atrazine has several limitations and constraints that are important to understand when applying the model and its results:

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Human-Health Benchmarks

Benchmark Description: The MCL is the maximum permissible concentration of atrazine in water that is delivered to any user of a public water system. This is an enforceable standard issued by USEPA under the Safe Drinking Water Act and established on the basis of health effects and other factors (analytical and treatment technologies, and cost). Compliance of public water supplies is generally assessed by comparing annual mean concentrations determined from quarterly monitoring to the MCL. The underlying health-risk assessment is based on lifetime exposure.

WARP Statistic for Comparison: Annual mean concentration. The annual mean concentration is the appropriate concentration statistic to compare to human-health benchmarks that are based on long-term, lifetime exposure.

Interpretation and Limitations: The MCL applies to finished drinking water (post treatment) rather than raw source water and, thus, source water concentrations estimated by WARP for streams that are public water supply sources may tend to over estimate concentrations for public water systems that use treatment processes that reduce atrazine concentrations. In addition, most streams in the U.S. are not presently used as sources of drinking water and, thus, the MCL comparisons for these streams do not indicate the current level of concern, but the degree of potential concern if they should be considered as future sources.

Benchmark Description: The DWLOC is the 90-day moving average concentration in untreated source water that, if exceeded, indicates USEPA concern and may lead to increased monitoring requirements (USEPA, 2003). The DWLOC for atrazine applies to "total chlorotriazines" (TCT), which includes atrazine plus its chlorinated metabolites.

WARP Statistic for Comparison: Modeled maximum 90-day moving-average concentrations.

Interpretation and Limitations: Atrazine concentration, other factors being equal, is an underestimate of TCT, however, because atrazine is only one component of TCT and one or more atrazine metabolites are usually present when atrazine is present. The DWLOC applies to raw source water for public water supply sources. Most streams in the U.S. are not presently used as sources of drinking water. Benchmark comparisons for streams that are not sources of drinking water indicate the degree of potential concern, if they should be considered as future sources.

Aquatic-Life Benchmarks

Benchmarks for assessing the potential for atrazine in stream water to adversely affect aquatic life are of two general types: (1) ambient water-quality criteria for the protection of aquatic life (AWQC-AL), which were developed by USEPA's Office of Water (OW), and (2) benchmarks derived from toxicity values obtained from registration and risk-assessment documents developed by USEPA's Office of Pesticide Programs (OPP).

Ambient Water-Quality Criteria for Aquatic Organisms

USEPA's OW derives both acute and chronic criteria, each of which specifies a threshold concentration for unacceptable potential for effects, an averaging period, and an acceptable frequency of exceedances.

Acute AWQC-AL-The highest concentration of a chemical to which an aquatic community can be exposed briefly without resulting in an unacceptable effect. Except where a locally important species is very sensitive, aquatic organisms should not be unacceptably affected if the 1-hour average concentration does not exceed the acute criterion more than once every 3 years, on average. The intent is to protect 95 percent of a diverse group of organisms.

Chronic AWQC-AL-The highest concentration of a chemical to which an aquatic community can be exposed indefinitely without resulting in an unacceptable effect. Except where a locally important species is very sensitive, aquatic organisms should not be unacceptably affected if the 4-day average concentration does not exceed the chronic criterion more than once every 3 years, on average. The intent is to protect 95 percent of a diverse group of organisms.

Toxicity Values from Risk Assessments

Seven types of aquatic toxicity values were compiled from OPP's registration and risk-assessment documents. The OPP toxicity values are for specific types of organisms. Acute and chronic values were compiled for fish and invertebrates, and acute values for vascular and nonvascular plants. A value for aquatic-community effects was available only for atrazine. NAWQA derived the benchmarks from OPP toxicity values, generally following OPP procedures. For two of the benchmarks-acute-fish and acute-invertebrates-the selected toxicity values were multiplied by the USEPA level of concern (LOC) of 0.5, so that the benchmark for NAWQA screening corresponds to the acute risk level defined by USEPA. Each benchmark is described below.

Acute fish- The lowest tested 50-percent lethal concentration (LC50) for acute (typically 96-hour) toxicity tests with freshwater fish, multiplied by the LOC of 0.5.

Acute invertebrate- The lowest tested LC50 or 50-percent effect concentration (EC50) for acute (typically 48 or 96-hour) toxicity tests with freshwater invertebrates, multiplied by the LOC of 0.5.

Acute vascular plant- The lowest tested EC50 for freshwater vascular plants in acute toxicity tests (typically < 10 days).

Acute nonvascular plant- The lowest tested EC50 for freshwater nonvascular plants (algae) in acute toxicity tests (typically < 10 days).

Chronic fish- The lowest no-observed-adverse-effects concentration (NOAEC), or the lowest-observed-adverse-effects concentration (LOAEC) if a NOAEC is not available, for freshwater fish in early lifestage or full life-cycle tests.

Chronic invertebrate- The lowest NOAEC, or LOAEC if a NOAEC is not available, for freshwater invertebrates in life-cycle tests.

Aquatic community effects- The benchmark for aquatic-community effects was derived from the refined risk assessment for atrazine. This endpoint for atrazine incorporates community-level effects on aquatic plants and indirect effects on fish and aquatic invertebrates that could result from disturbance of the plant community.

1. All Acute Benchmarks:



Benchmark Description: As described above, each acute benchmark for atrazine is based on short-term toxicity tests and pertains to short-term environmental exposure such as indicated by the modeled maximum concentration of atrazine.

WARP Statistic for Comparison: Modeled maximum concentration.

Interpretation and Limitations: The maximum observed atrazine concentrations in the WARP model development data are underestimates of actual maximums because the sampling frequencies were too sparse to yield a high probability of sampling short-lived conditions. Therefore, the WARP-estimated probability that the modeled maximum atrazine concentration exceeds a particular acute benchmark is an underestimate of the actual probability.


Benchmark Description: The chronic fish and aquatic community effects benchmarks for atrazine are compared to the 60-day moving average concentration of atrazine.

WARP Statistic for Comparison: Modeled maximum 60-day moving-average concentrations.

Benchmark Description: The chronic invertebrate benchmark for atrazine is compared to the 21-day moving average concentration of atrazine.

WARP Statistic for Comparison: Modeled maximum 21-day moving-average concentrations.