GIS-BASED QUATERNARY GEOLOGY MAPPING OF ST. CROIX COUNTY, WISCONSIN

St. Croix County is situated along the St. Croix River in west-central Wisconsin and is located just 30 miles east of Minneapolis-St. Paul, Minnesota. According to U.S. Census Bureau statistics, the St. Croix County population has grown by 12.7% since 2000 and by more than 70% since 1970, making it the fastest growing county in Wisconsin. State and county officials anticipate that the population in western St. Croix County will increase an additional 55 % by the year 2020, placing further demands on county natural resources.

The rural non-farm population, which increased by 13,000 residents between 1970 and 1990, places significant stress on the environment as the number of wells, septic systems, and roads increase. In response to the population growth of St. Croix County, the U.S.G.S. sponsored a STATEMAP 1:100,000 GIS-based mapping project to establish the nature and extent of the Quaternary glacial deposits that cover the county. Several till units and associated meltwater deposits are found in St. Croix County including the sandy, Superior-derived Copper Falls and River Falls Formations as well as the Keewatin-derived Pierce Formation, which consists of two clay-rich lacustrine members and two clayey basal till members. The significant clay content of the Pierce Formation units presents serious limitations to foundation construction and septic system installation, especially in the eastern half of the county.

Mapping the distribution of glacial deposits in St. Croix County also provides a construction materials inventory that can be used to identify the most accessible sources of sand and gravel. Understanding the nature and distribution of the Quaternary units is especially important in eastern St. Croix County where dissolution of the underlying carbonate bedrock is indicated by the numerous karst features. The karst features, in conjunction with a thin, permeable, sediment cover, make local aquifers susceptible to surface contamination. One half of the watersheds in St. Croix County currently have a significant percentage of wells that exceed the state’s Groundwater Preventive Action Limit for nitrates of 2 ppm. The data collected and the map generated by this project will be available to St. Croix County officials to aid their zoning and development planning efforts.