Nitrate Risk to the Water Table Aquifer

Nitrate-nitrogen (nitrate) risk ranking maps have been constructed for the water table aquifer beneath eleven southeastern Minnesota counties, the seven-county Twin City Metropolitan Area, and twelve additional counties throughout central and south-central Minnesota. Additional counties will be mapped as base data becomes available. Nitrate loading to the subsurface is estimated by reclassification of land use data (ranging from 1 point for relatively low to 5 points for relatively high estimated nitrate loading). Hydrogeologic sensitivity for the water table aquifer is ranked from low (1 point) to very high (4 points) based upon the permeability of geologic materials (surficial geology, soil parent material type and bedrock type), land slope, and the relationship between depth to water and depth to bedrock. Adding the estimated nitrate loading raster to the hydrogeologic sensitivity raster simulates nitrate release to the water table, and produces the nitrate risk map for the water aquifer. The final point score can fall within nitrate risk ranges classed on the final map as low (up to 4 points), moderate (5 to 6 points), or high (7 to 9 points). The results are checked by comparison to available nitrate chemical data collected from wells within the county being mapped.