Hydrogeologic Sensitivity of the Water Table Aquifer

Minnesota Department of Health (MDH) Source Water Protection initially developed county-scale nitrate risk maps in 2007 for nine southeastern Minnesota counties to assist with state and local water quality planning efforts and support a nitrate occurrence study (Southeast Minnesota Water Resources Board, 2009). Hydrogeologic sensitivity mapping was done as part of the nitrate risk mapping effort. Hydrogeologic sensitivity of the water table aquifer was generated using guidelines provided in Minnesota DNR (1991). Areas were ranked as low, moderate, high, or very high hydrogeologic sensitivity based on permeability of near-surface bedrock or unconsolidated geologic materials, estimated depth to water, and land slope. LOW (1 point) was assigned to areas 1) covered by geologic materials primarily composed of clay or shale close to the surface, or 2) where land slopes were greater than 12% (regardless of underlying geologic materials).MODERATE (2 points) was assigned to areas not already assigned LOW, and underlain by modified clay till (clay plus a significant sand or gravel fraction). MODERATE was also assigned where land slopes were at least 6% and at most 12% (regardless of the rank of underlying geologic materials, except that material already ranked LOW remained LOW).HIGH (3 points) was assigned to areas not already assigned LOW or MODERATE, and underlain by unconsolidated sands and gravels. HIGH was also assigned to areas where limestone, dolomite, or sandstone were close to the surface and covered by loess. VERY HIGH (4 points) was assigned to areas not already assigned LOW, MODERATE or HIGH, where loess was absent, and shallow bedrock was limestone, dolomite, or sandstone.