DISTRIBUTION AND POLLUTION SENSITIVITY OF QUATERNARY SAND AND GRAVEL AQUIFERS: SOUTHERN PINE COUNTY, MINNESOTA

Buried sand and gravel aquifers are the most important ground-water resource for domestic users in developing southern Pine County north of the Minneapolis-St. Paul metropolitan area. As part of an ongoing county-level mapping program to evaluate surface water pollution sensitivity, the surficial and buried sand and gravel aquifers were mapped in the 240 square mile area of southern Pine County with a combination of previously mapped surficial geologic features, drillers’ well log data, and surface resistivity imaging.

Approximately 1000 drillers’ logs were correlated with a network of 10 stratigraphic cross sections. Sand and gravel units identified on these logs were classified into three layered aquifer systems according to elevation. The accuracy of the shallowest aquifer map was verified at 11 of 12 locations using direct current surface resistivity imaging methods and inversion modeling software. All three aquifer systems exhibited a strong north-south orientation reflecting the possible predominance of north-south oriented tunnel valley deposition that is evident in the partially exposed upper aquifer layer. The southern portion of the upper aquifer has extensive lateral continuity due to superimposed glacial lake deltaic deposition of sand.

Ground-water chemistry data from 23 existing wells combined with the individual aquifer system potentiometric surfaces reveal patterns of local and regional recharge and pollution sensitivity. A ground-water mound and a high tritium value associated with a partially buried tunnel valley show the shallowest buried aquifer is recharged locally. The middle and deepest buried aquifers have tritium and bedrock chemical constituent values (boron and strontium/magnesium ratio) that indicate greater isolation from recent ground-water recharge and, therefore, less pollution sensitivity. Information on the aquifer system geometries, recharge characteristics, and lateral ground-water flowpaths should help local units of governments plan and implement aquifer protection strategies.