Nitrate-Nitrogen (Nitrate) Risk Background Information
Source Water Protection
Nitrate Risk Map Example
Nitrate Mapping Method
To generate a nitrate risk map, MDH used geologic and soil base data to produce a Geographic Information System (GIS) map of hydrogeologic sensitivity of the water table aquifer. Next, MDH used recent cropland and urban land use data to generate a map of estimated nitrate loading to the subsurface. Finally, summing the hydrogeologic sensitivity and estimated nitrate-loading maps by map addition created a final map of predicted nitrate risk.
Wells with elevated nitrate concentrations are likely to occur in areas ranked as high or moderate risk. However wells in these areas also may provide drinking water with acceptably low nitrate concentrations. Localized problems - such as poor well construction, improper drainage, or onsite wastewater contamination - can also lead to elevated nitrate concentrations but cannot be predicted using the risk maps.
The 2011 revised mapping method minimizes discrepancies resulting from differences in base data. The county maps are designed to be used at the county-scale only (no greater than 1:100,000).