The National Pesticide Loss Database: A Tool for Management of Large Watersheds

Poster Presentation at the 53rd Annual SWCS Conference

San Diego, California, July 5-9, 1998

Don W. Goss, Texas Agricultural Experiment Station, Temple, Texas

Robert L. Kellogg, Natural Resources Conservation Service, Washington, D.C.

Joaquin Sanabria, Texas Agricultural Experiment Station, Temple, Texas

Susan Wallace, consultant to the Natural Resources Conservation Service, Washington, D.C.

Walt Kniesel (retired)

Summary

• A National Pesticide Loss Database was created for use as a look-up table for estimates of pesticide losses from farm fields in leachate and runoff. Estimates are available for specific pesticides, soil groups, and climatic regions. The database allows for a more precise determination of the potential for pesticide loss than the Soil-Pesticide Interaction Screening Procedure (SPISP).

• Estimates are available for mass loss and annual concentration. Estimates of concentrations can be contrasted to protective thresholds (such as drinking water standards) to provide insight on the level of environmental risk associated with pesticide use on specific soils.

• The database is designed for use in conjunction with watershed models to target priority areas that may be in need of additional conservation practices aimed at reducing environmental risk.

Development of the National Pesticide Loss Database

Overview

Pesticide leaching and runoff losses were estimated using the pesticide fate and transport model GLEAMS 1. GLEAMS is a process model that uses as inputs soil parameters, field characteristics, management practices, pesticide properties, and climate to estimate pesticide leaching and runoff losses.

GLEAMS estimates were generated for 243 pesticides applied to 120 generic soils for 20 years of daily weather from each of 55 climate stations. This resulted in 1,603,800 runs of 20 years each, or 32,076,000 years of estimates. Pesticide runoff was movement beyond the edge of the field, including both pesticides in solution and pesticides adsorbed to soil material and organic matter. Pesticide leaching was movement beyond the bottom of the root-zone. Separate GLEAMS estimates were made for irrigated and nonirrigated conditions. The model estimates irrigation timing and amounts depending on soil moisture. The crop in these simulations was a generic row crop behaving similar to corn, soybeans, cotton, or sorghum, planted in straight rows.

The daily mass of pesticide that was removed by leaching or runoff in solution or runoff with sediment were recorded for each model run, and summed for each year. The daily volume of water that leached below the root-zone or flowed beyond the edge of the field was recorded and summed over the year, and the daily mass of sediment loss was recorded and summed over the year. Final pesticide loss results are reported as 1) the percentage of total mass of pesticide applied, and 2) the annual concentration of pesticide leaving the field, expressed as the percentage of total mass of pesticide applied per million parts of water or sediment.

Soil Parameters

The 120 generic soils were selected on the basis of soil texture and organic matter content. Twelve textures were combined with four organic matter contents for surface horizons. Four textures were used for subsurface horizons, which were not coarser in texture than the surface horizon. Organic matter content of a subsurface horizon was 20 percent of the surface horizon. The thirty horizon combinations times four organic matter contents made up 120 soils, summarized in Table 1. The hydrologic group (HG) was assigned according to subsurface texture. Using tables found in the GLEAMS manual and relations in the USDA Soil Survey Handbook, the other soil parameters required by GLEAMS were estimated. The Curve Number (CN) is also required, and was assigned as shown in Table 1.

Climate Parameters

GLEAMS includes a climate generator that simulates daily rainfall and temperature. Fifty-five climatic stations across the United States were chosen (Map 1). The program generates daily weather using the mean, standard deviation, and skew for monthly precipitation, maximum temperature, minimum temperature, and the mean and standard deviation of monthly solar radiation. Two other monthly values required are: 1) the probability of having a wet day after a wet day, and 2) the probability of having a dry day after a wet day. A twenty year record was simulated with the climate generator from a 40-year frequency distribution to produce a distribution of pesticide loss estimates that would reasonably represent most weather conditions.

Planting and harvest dates were estimated for each of the 55 climate stations based on mean and standard deviation of monthly low temperatures from the climatic record. This calculation was accomplished in a spreadsheet by:

1. Subtracting the monthly minimum temperature form the monthly mean temperature.
2. Calculating the daily rate of change of this value over a two month period.
3. Determining month by month if this value crosses 37 degrees Fahrenheit during the month. This date in the spring is the planting date. This date in the fall is the harvesting date, or date growth stops.

Pesticide Application Parameters

GLEAMS results were simulated for the 243 pesticides in the SCS/ARS/CES pesticide properties database 2. The pesticide database is included in the GLEAMS model pesticide parameter editor, including foliar characteristics constructed using the procedure by Willis and McDowell 3. The Insect Control Guide 4 and the Weed Control Guide 5 were used to define the action of each compound, when applied, how frequently applied, and recommended rates and methods of application.

Pesticide application timing was based on the planting date, harvest date, and purpose. Pesticide application method was based on planting date and purpose. Some herbicides were designated only for pre-plant application and some only for post-plant application. Those herbicides with applications designated as "all methods" were included with the pre-emergent herbicides. Preplant pesticides were simulated with application seven days before planting. Pre-emergent pesticides were simulated with application on the planting date. Post-plant pesticides were simulated with application fourteen days after the planting date. Over the top insecticides, fungicides, and miticides were simulated in 3 repeat applications commencing after one third of the growing season was completed. For example, an insecticide with a recommended repeat application every five days was first applied one-third of the way through the growing season, and then repeated every five days for a total of three applications. Growth regulators were applied after one fourth of the growing season was completed. Defoliants were applied 5 days before harvest date.

Some soil insecticides and nematicides were incorporated in the soil; some surface applied, and some applied over-the-top of foliage. The SCS/ARS/CES pesticide properties database also includes growth regulators and defoliants, both of which are applied on foliage. The insecticides used as foliar applications were applied at label recommended frequency. The Insect Control Guide included recommendations on the frequency of application, i.e. 3-5 days, 5 days, or 7 days. In GLEAMS, insecticides were applied every 3 days for the 3-5 day recommendation, every 5 days for the 5 day recommendation, and every 7 days for the 7 day recommendation.

Model Output

Output from the GLEAMS simulations included annual estimates of:

1. Mass loss in leachate as a percent of the amount applied.
2. Mass loss dissolved in runoff as a percent of the amount applied.
3. Mass loss adsorbed to eroded soil as a percent of the amount applied.
4. Volume of water percolation in centimeters.
5. Volume of water runoff in centimeters.
6. Sediment loss in kilograms.

The mass loss was expressed as a percentage of the amount applied so that loss estimates could be derived for any application rate. This is necessary because, in practice, application rates often vary from the recommended rates. This also provides the flexibility to apply different application rates across the country to reflect differences in use, or to simulate the effects of proposed policies on pesticide loss. Similarly, the annual concentration was expressed as a ratio of concentration to the mass of pesticide applied so that, when multiplied by the application rate in kilograms per hectare, the concentration in PPM would be obtained.

The 20-year distribution of mass loss and concentration estimates were used to derive prediction equations corresponding to any percentile of the distribution. The maps shown here were generated using the 95th percentile, which is the mass loss or concentration that would be expected to be exceeded only five percent of the time.

National Modeling

The National Pesticide Loss Database was used with the National Resources Inventory (NRI) to simulate pesticide loss by watershed for use in identifying potential priority watersheds for implementation of conservation programs.

The NRI was used as a modeling framework and as a source of land use data and soil data. Each NRI sample point was treated as a "representative field" in the simulation model. The simulation was conducted using 13 crops-barley, corn, cotton, oats, peanuts, potatoes, rice, sorghum, soybeans, sugar beets, sunflowers, tobacco, and wheat-which comprise about 170,000 NRI sample points. The statistical weights associated with the NRI sample points are used as a measure of how many acres each "representative field" represents. Land use for the most recent inventory-1992-was used.

Pesticide use data were taken from Gianessi and Anderson, who estimated the average application rate and the percentage of acres treated by state for over 200 pesticides and for 84 crops for the time period 1990-93. Estimates of percent acres treated and application rate were imputed onto NRI sample points by state and crop. Map 2 was created by multiplying the percent acres treated times the acres represented by each point to obtain the acres treated for each pesticide, and then multiplying by the application rate and summing over the pesticides at each NRI sample point to obtain the total pounds of pesticides applied. These results were aggregated over NRI sample points in each 8-digit hydrologic unit in the 48 states.

Estimates of pesticide loss from the National Pesticide Loss Database were imputed onto the 170,000 sample points according to soil type, geographic location, and pesticide. Mass loss and annual concentration were calculated for each pesticide at each sample point. Mass loss estimates were then aggregated over acres treated in each watershed to produce national maps.

Concentrations were compared to water quality thresholds to derive a measure of environmental risk at each NRI sample point. Health Advisories (HAs) and Maximum Contaminant Levels (MCLs) were used for humans for pesticides that have been assigned drinking water standards by EPA. For other pesticides, "safe" thresholds were estimated from EPA Reference Dose values and cancer slope data. Maximum Acceptable Toxicant Concentrations (MATCs) were used as "safe" thresholds for fish, which were calculated using toxicity data published by EPA.

The extent to which the concentration exceeded the threshold was used as a measure of risk for each pesticide. This risk measure was aggregated over the pesticides at each point and then multiplied by the number of acres treated and summed over the points in each watershed to obtain an aggregate risk measure for each watershed--Threshold Exceedence Units (TEUs) per watershed. TEUs are similar in concept to the acre-feet volumetric measure, since they are a multiple of acres times a measure of magnitude at a point. They are used here only to measure relative risk from one watershed to another; the higher the TEU score, the higher the risk.

Mass Loss

The potential loss of pesticides from farm fields in pounds per watershed was estimated, shown in Maps 3, 4, and 5. The greatest potential for leaching loss is concentrated in the southeast and the Mississippi Embayment. The greatest potential for runoff loss--both dissolved and adsorbed--is concentrated in the midwest and the Mississippi Embayment.

Environmental Risk--Leaching

TEUs per watershed for leachate determined using water quality thresholds for humans (drinking water standards) are shown in Map 6, and TEUs for leachate determined using safe thresholds for fish are shown in Map 10. The risk to humans is greater than the risk to fish. Watersheds with the greatest TEUs for both humans and fish are generally in the southeast.

The percent of the land in the watershed (nonfederal rural land) where the annual pesticide concentrations exceeded one or more threshold value, or a multiple of threshold values, is an alternative approach to identifying potential priority watersheds. Maps 7, 8, and 9 show the percent area where leaching concentrations exceed a multiple of 1, 3, and 5 times the water quality threshold for humans, respectively. (A multiple of 1 indicates that the concentration exceeded the threshold for one or more pesticides, and a multiple of 3 indicates that the concentration was 3 times the threshold for one or more pesticides.) As the multiple is increased, the maps focus in on the watersheds most at risk. The TEU maps, however, are a generally superior measure of risk because it is calculated by aggregating over all the pesticides, whereas the percent area maps may only involve a few or a single pesticide. Similar maps are presented for fish in Maps 11, 12, and 13. The distribution of acres by state are presented in Tables 2 and 3.

Environmental Risk--Runoff

TEUs per watershed for runoff determined using water quality thresholds for humans are shown in Map 14, and TEUs for runoff determined using safe thresholds for fish are shown in Map 18. A comparison of these maps to Maps 6 and 10 shows that the risk from runoff is much greater than the risk from leaching. Watersheds with the greatest TEUs for humans are concentrated in the midwest and the Mississippi Embayment. Watersheds with the greatest risk to fish occur scattered throughout the agricultural area.

The percent of the land in the watershed (nonfederal rural land) where the annual pesticide concentrations in runoff exceeded one or more threshold value, or a multiple of threshold values, is shown in Maps 15, 16, and 17 for humans and Maps 19, 20, and 21. for fish. Multiples of 1, 10, and 25 are shown to focus on the watersheds most at risk. Using a multiple of 25 (indicating that the concentration exceeded 25 times the threshold for one or more pesticides), Maps 17 and 21 show that there is some correspondence between priority watersheds for humans and fish (Iowa, Mississippi Embayment), but there are also marked differences (West Texas, Atlantic Coastal Plain). The distribution of acres by state are presented in Tables 4 and 5.

Comparison Of TEUs To Water Sampling In Texas

A recent cooperative study of atrazine loss in Texas provided a unique opportunity to compare the TEU estimates to measured atrazine concentrations (see map 22). This study was with the Texas Natural Resource and Conservation Commission (TNRCC) and the Texas State Soil and Water Conservation Board. Atrazine concentrations were measured by the TNRCC at the intake of water treatment plants. The location of each water intake was used to assign it to a hydrologic unit, but it may not always be the principal hydrologic unit where the water originated. The values represented in map 22 for maximum atrazine levels are the maximum value measured over a two-year period. Values greater than 3 ppb were usually single measurements.

The map of Threshold Exceedence Units for atrazine shows the potential 95th percentile concentration dissolved in runoff (see map 23). Since this shows a 1-in-20 year event, it is not directly comparable to the 2 years of data collected by the TNRCC. In addition, the TEU map will show a score for a watershed even if it represents only a few (or one) NRI sample points, which would likely not be significant at the watershed scale.

Nevertheless, a correlation is apparent. There were only two watersheds where the TNRCC found atrazine above .01 ppb that did not have a TEU score. Several watersheds with atrazine measurements above 2 ppb corresponded with the highest watershed TEU scores in the state.

The comparison of these two maps confirms that the hydrologic units with highest TEU values, and thus the greatest potential for risk to drinking water from runoff, are the areas with the greatest probability of having measurable levels of pesticides in the water.

Looking Ahead

• The authors will expand the database in late 1998 to include estimates specific to close grown crops, in addition to the estimates currently available for row crops.
• When completed, the database will be made available to the public on CD-ROMs.
• In future years, the database will be expanded to include new pesticides that come on the market, and estimates will be updated using the latest standardized estimates of pesticide parameters.
• Work has commenced on a similar national database for nutrient loss and carbon sequestration.

Table 2. Acres (1,000) Where the Potential Leaching Concentration at the Bottom of the Root Zone Exceeds a Multiple of One or More Water Quality Thresholds for Humans.

Table 3. Acres (1,000) Where the Potential Runoff Concentration at the Edge of the Field Exceeds a Multiple of One or More Water Quality Thresholds for Humans.

Table 4. Acres (1,000) Where the Potential Leaching Concentration at the Bottom of the Root Zone Exceeds a Multiple of One or More Water Quality Thresholds for Fish.

Table 5. Acres (1,000) Where the Potential Runoff Concentration at the Edge of the Field Exceeds a Multiple of One or More Water Quality Thresholds for Fish.

List of Maps