GROUNDWATER VULNERABILITY PROJECT
A Water Resource Protection Management Tool

Maps
Data Inputs Nitrate-Nitrogen Probability
Brown County Nitrate-Nitrogen Probability
Cottonwood County Nitrate-Nitrogen Probability
Nicollet County Nitrate-Nitrogen Probability

Publications
Final Report: Groundwater Vulnerability Zoning Project, June 2004 (pdf 2,779 k)

Project Overview
The Groundwater Vulnerability Project is a collaborative effort among several state and county agencies to map the probability of nitrate contamination of groundwater in the Minnesota counties of Brown, Nicollet, and Cottonwood.

Why is Nitrate Contamination a Concern?
Nitrate contamination in drinking water is a serious threat to human health. Nitrate molecules in the bloodstream prevent the uptake and transportation of oxygen throughout the body. Nitrates are directly linked to the “blue baby syndrome,” a serious medical condition in infants who drink formula mixed with nitrate contaminated drinking water. Since groundwater is the primary source of drinking water in Brown, Nicollet, and Cottonwood Counties, it is essential to protect this resource.

Mapping Potential Nitrate Groundwater Contamination
Several factors determine the susceptibility of an area to groundwater nitrate contamination, including:

• Geologic formations (the types of rocks that underlie the soil on the surface);
• Geochemical composition (the chemical makeup of the underlying rock); and
• Land use.

Each of these factors affects, to varying degrees, the probability of nitrates occurring in groundwater.

A series of maps were developed and then layered over each other to estimate potentials for nitrate contamination in the groundwater. Maps used included: land use, surface permeability (which is dependent on geologic formations and composition), the percent of clay above the water table, the depth to bedrock, and the water quality and depth to the water table. Then, each category of mapping factor was assigned a different point value. For example, the land use factor was divided into three categories with “Undeveloped” receiving 1 point, “Residential or Commercial” receiving 2 points, and “Agriculture” receiving 3 points. Higher points represent an increased probability for nitrate contamination.

These maps were then layered over each other to determine an area’s point total in nitrate-contamination probability. (NOTE: Not ALL wells drilled in an area of high probability would necessarily have high nitrate concentrations, but the chances would be greater than they would be in an area of low probability.)

Who Provided the Mapping Data?
The nitrate probability maps are based on extensive databases. County Well Indices (from Brown, Nicollet and Cottonwood Counties) provided data on clay percentages, depth to bedrock, and depth to the water table. The Brown-Nicollet Environmental Health Services produced the water quality database by conducting 10,000 nitrate-concentration analyses of nearly 3,000 wells. The land use database is based on aerial photography acquired between 1988 and 1990. The Minnesota Geological Survey provided the landforms database.

How are the Maps Used?
The nitrate probability maps are used by planning and zoning staff to identify areas that have a high likelihood for nitrate groundwater contamination. Sites for permit applications—such as subdivisions or dairy or hog operations—are identified on the probability maps to determine the risk of groundwater contamination. The maps are NOT used to deny permits, but rather to identify vulnerable areas so that appropriate measures can be added to the permit to protect water quality. This process also educates applicants about the conditions that contribute to nitrate contamination.

So far, the maps have been applied to more than 30 permit applications. Future uses of the maps might include targeting areas for wellhead protection programs and identifying areas that might be susceptible to other contaminants. A high-quality drinking water supply is in everyone’s interest, and nitrate probability mapping can help us achieve it.

Partners

• Brown Nicollet Cottonwood Water Quality Board
• Minnesota Department of Health
• Minnesota Pollution Control Agency
• County Agencies: Soil and Water Conservation District, Water Planning, Environmental Health, Environmental Services, Planning and Zoning
• University of Minnesota County Extension Office