Explanation of the Assessment System

The maps included on this site have been developed using the Assessment System for Potential Groundwater Contamination from Agricultural Pesticide Use in North Dakota. Dr.Bruce Seelig, North Dakota State University Extension Soil Scientist, developed the assessment system. This system is designed to identify areas with high potential for groundwater contamination from pesticides. The system uses North Dakota Geological Survey information and USDA Natural Resources Conservation Service (NRCS) county soil surveys to help identify areas of potential groundwater contamination. The computerized application of the assessment system was developed by John Nowatzki, Extension Water Quality Specialist, using the Geographic Information System (GIS) software, ArcInfo.

Step-by-Step Process of Determining Groundwater Sensitivity

The system is a step-by-step procedure for determining groundwater sensitivity to pesticides. The first step requires the determination if an aquifer with a useable supply of water exists. All shallow glacial or alluvial aquifers are considered. Maps showing aquifers on this website were developed using the digitized map of glacial drift aquifers in North Dakota available from the North Dakota Spatial Data Clearinghouse.

The second step looks at pesticide use. The maps on this website assume application of the selected pesticide over the entire area of the aquifers.

The third step is to determine the site properties that affect pesticide movement. The soil and geologic materials act as a filter to protect aquifers from contamination. That filtering process is often referred to as pesticide attenuation. Attenuation is lessening the amount of pesticides reaching the groundwater. An estimate of the potential for materials to attenuate or filter out pesticides is presented as the filtration potential in this assessment system. Four factors are considered when assessing the filtration potential of the materials overlaying an aquifer. These factors are: 1) depth to the saturated aquifer combined with the predominant water flow direction; 2) soil and geologic strata permeability; 3) soil organic matter content; and 4) pesticide leaching potential. The attributes of the materials over aquifers are used to determine whether each of the four factors has a high, intermediate or low potential to attenuate the amount of pesticides reaching the groundwater. The NRCS SSURGO data (digitized county soil survey maps) is used to select the soil properties used in the factors of the assessment system.

Explanation of Each Factor

1) Depth to Aquifer/Water Flow Direction

The predominant groundwater flow direction allows for the designation of recharge areas over aquifers. Waterflow through a soil to the groundwater can be categorized as recharge (downward through the soil) and discharge (upward through the soil). Flowthrough is the term used to describe lateral movement of groundwater through the soil.

The presence and depth of calcium carbonate (lime) is used to assess the long-term water flow direction. In recharge areas the predominantly downward movement of water removes calcium carbonate from upper levels of the soil. For this assessment system, soils of recharge areas lack calcium carbonate in the upper 30" of soil. Depth to calcium carbonate was determined from the attribute tables in the county soil survey report. ArcInfo was used to select soil layers with the lower depth less than or equal to 33". This data set was then selected for soil polygons with a high range of calcium carbonate equal to or less than 1 percent. A groundwater recharge area over a shallow aquifer (less than 50' to the saturation zone) is designated a low potential to filtrate pesticides from percolating water. All other combinations of groundwater flow and aquifer depth are designated high filtration potential areas. Irrigated areas, which are automatically assessed as recharge areas) were not considered in the maps on this homepage.

2) Soil and Geologic Material Permeability

Soils with sandy and sandy-skeletal family particle sizes that overlie sand and gravel geologic materials have low potential for filtration. Soils with fine particle size over geologic materials finer than sand and gravel have a high potential for filtration. All other particle sizes have an intermediate potential for filtration. ArcInfo was used to select the soil polygons with the designated family particle sizes.

3) Organic Matter Content

Soil organic matter content of less than 2 percent in the A horizon has a low filtration potential. Soils with more than 2 percent have a high filtration potential. ArcInfo was used to select soil polygons with a high range of organic matter with less than or equal to 2 percent.

4) Pesticide Chemistry

 The tendency for a pesticide to move with water through soils is also influenced by its chemistry. The Hornsby index for pesticide leaching potential is used in this assessment system. The Hornsby index is a combination of the organic carbon adsorption factor (KOC) and the pesticide half-life (T1/2). The index ratio of KOC and T1/2 is multiplied by 10 to give a leaching index potential for each pesticide. A pesticide with an index of 10 or less has a low filtration potential. If the index is 2000 or greater the pesticide has a high filtration potential. Pesticides that do no meet these criteria are considered to have an intermediate filtration potential.

Groundwater Sensitivity Categories

This assessment system has four groundwater sensitivity categories. The categories reflect combinations of the filtration factors described above. The four categories and their definitions are:

The High Sensitivity category is the worst case in which all four filtration factors are low.

The High Intermediate Sensitivity category has a combination of filtration factors in which one to three factors are low.

The Low Intermediate Sensitivity category has a combination of no low filtration factors but not all have high filtration potential.

The Low Sensitivity category is the best case in which all four filtration factors are high.