PRELIMINARY HYDROGEOCHEMISTRY OF A SANDSTONE KARST IN PINE COUNTY, MN

A sandstone karst is located in east central Minnesota. This karst is developed in Mesoproterozoic Hinckley Sandstone and has surface expression in the overlying Wisconsinan glacial deposits. The Hinckley Sandstone is in fault contact with Keweenawan basalt from the 1.1 Ga Midcontinent Rift. The heterogeneous permeability structure of the Hinckley Sandstone drives this karst system. This permeability structure of the sandstone is reflected by its diverse geochemistry as an aquifer, and the development of surficial karst features.

The Hinckley Sandstone contains several different waters, which exist in close proximity with limited mixing. These waters have a variety of origins. The waters are dilute (average total dissolved solids = 188) and range from Ca/Mg/HCO3 waters to waters where Na is the most abundant cation to waters where Cl is the most abundant anion. Cl/Br ratios less than seawater indicate the presence of old, residual chloride components. Cl/Br ratios of >10,000 in other samples indicate anthropogenic components. Some of the Na-rich waters in the Hinckley Sandstone appear to be associated with groundwater flow paths through adjacent Keweenawan basalt. These waters have boron concentrations near the 600ppm drinking water standard, while over 80% of sampled basalt wells exceed this standard.

Preliminary carbon dating of groundwater may point to a significant recharge event about 4,000 – 6,000 years before present.