Risk of Groundwater Nitrate Contamination
Source: Environmental Protection Agency
Map Description:
This national risk map was designed to present patterns of risk for nitrate contamination at large regional or national scales. It was produced using an overlay analysis in a geographic information system. The scale of the analysis and the data sets used have various limitations that are found in the interpretation section below. Knowing where and what type of risks to ground water exist can alert water-resource managers and private users of the need to protect water supplies. By targeting regions with the highest risk of nitrate contamination, resources can be directed to areas most likely to benefit from pollution-prevention programs and long-term monitoring.

Cautions for this Product:
Areas shown as high-risk on the national map have contamination potential, but do not necessarily depict areas with actual contamination. Sampling and testing are necessary to determine actual nitrate concentration in ground water located at a specific site. Local hydrogeologic conditions may cause variations in local ground-water quality not evident at the national scale. For example, karst aquifers may contain high nitrate concentrations because of the rapid infiltration of water through sinkholes. These small geologic features that are not mapped can be of greater potential risk than are indicated on this national map. This map describes the contamination potential of ground water near the land surface (less than 100 feet). Ground water from deeper wells completed in confined aquifers is less likely to be contaminated by nitrate. Thus, ground water obtained from deep confined aquifers is less likely to contain nitrate, even in areas shown as high-risk on the national map. Factors other than those used to develop the map patterns in this national map can affect nitrate concentration in ground water. These factors include land use, aquifer type (see principal aquifer map), rainfall and irrigation amounts, and the timing of rainfall in relation to fertilizer and manure applications. Caution should be exercised when predicting the risk potential in areas where these factors may override those used to produce this map. This national risk map was designed for interpretation at the regional (multi-county) scale. Spatial inaccuracies could result if the map is used to predict contamination risk at finer spatial scales (e.g., individual watersheds). The map is intended to identify regions that might benefit from more intensive monitoring of nitrate in shallow ground water. Although this spatial data can stand alone, it may be helpful to refer to the printed Ground Water Atlas chapters when using this data and to the references listed in the metadata section of the web sites. Comments regarding the Index of Watershed Indicators (and Surf Your Watershed) and the associated use of this data layer can be directed to USEPA, Office of Ground Water and Drinking Water, Roger Anzzolin (anzzolin.roger@epa.gov). Comments not related to the IWI that are specific regarding this nitrate data can be directed to the USGS Water Resources Division, Office of Ground Water, Tom Nolan (btnolan@usgs.gov).

For Further Information: