UNOXIDIZED GLACIAL TILL CONTROLS AQUIFER VULNERABILITY IN THE U.S. MIDWEST

Multiple Pleistocene-age glacial advances across the northern United States and Canda left behind fine-textured deposits of loess, till and other diamictons that serve as regional aquitards preventing vertical migration of contaminants to underlying aquifers. The stratigraphic sequence of glacial deposits often consists of an upper oxidized, fractured and weathered till overlying an unoxidized and unleached (UU) till. In Linn County, Iowa, the hydrogeology of a thick pre-Illinoian section investigated with nested wells and chemical tracers showed that oxidized portion of the till profile was a zone of recent groundwater circulation. The UU till portion of the section provides groundwater protection in excess of 100s to 1000s of years. High nitrate concentrations in private drinking water wells cluster in regions with little or no UU till cover. New mapping in Blackhawk County, Iowa utilizing spatial coverages of infiltration recharge, stratigraphy, contaminant sources and capture zones confirms the significance of UU till in assessing aquifer vulnerability. Overall, the extent and thickness of UU till controls vertical groundwater flow and potential contaminant transport the glaciated U.S Midwest.