County Geologic Atlases and Related Hydrogeologic Research

PROJECT OVERVIEW
The Minnesota County Geologic Atlas program is an ongoing effort begun in 1982 that is being conducted jointly by the University of Minnesota's Minnesota Geological Survey and the Minnesota Department of Natural Resources (DNR). The program collects information on the geology of Minnesota to create maps and reports depicting the characteristics and pollution sensitivity of Minnesota's ground-water resources. County Geologic Atlases are used in planning and environmental protection efforts at all levels of government and by businesses to ensure sound planning, management, and protection of land and water resources. The Minnesota Geological Survey will use this appropriation to:

• Initiate geologic atlases for Sherburne and Morrison counties;
• Continue work on county geologic atlases already in progress;
• Make collected data available in a digital format;
• Investigate the hydrologic properties of the St. Lawrence Formation in southeastern Minnesota;
• Evaluate methods for investigating groundwater flow pathways in urban areas, using Rochester, MN as the test area.

OVERALL PROJECT OUTCOME AND RESULTS
The Minnesota Geological Survey maps sediment and rock because these materials control where water can enter the subsurface (recharge), where and how much water can reside in the ground (aquifers), where the water re-emerges (discharge), and at what rates this movement occurs. This information is essential to managing the quality of our water and the quantity that can be sustainably pumped. This project completed geologic atlases for Sherburne and Morrison counties, and contributed to atlas work in Anoka, Wright, Hennepin, Hubbard, Becker, Wadena, St. Louis, and Lake counties. Information about the geology is gleaned from the records of domestic wells, and from drilling conducted for this project. In Sherburne County we used 14,450 wells and 5 cores and in Morrison County we used 6,400 wells and 21 cores, and soil borings and geophysical surveys. From the data we created maps of the geology immediately beneath the soil; the aquifers within the glacial sediment; and the shape, elevation, and rock types of the bedrock surface. These maps and data support monitoring, wellhead protection, water appropriation, clean-ups, and supply management.

The deep bedrock aquifers in southeastern Minnesota are in most places not yet significantly impacted by pollution and presumed to be protected by low permeability overlying geologic layers, called aquitards. Even though aquitards are an important control on recharge and contaminant transport, their hydrologic characteristics are poorly understood compared to aquifers. This subproject investigated the St. Lawrence Formation through existing data, new data on fracturing, and by constructing an instrumented borehole to test the water-bearing characteristics. We learned that the St. Lawrence acts to retard vertical water flow where it is buried by more than 50 feet of overlying rock, but fails to do so in more shallow settings. Parts of the formation convey water horizontally in either setting.

A third subproject traced ground water movement in the Rochester area by examining the chemistry of the water. We learned that flow patterns are changing, apparently in response to high capacity pumping.

PROJECT RESULTS USE AND DISSEMINATION
County geologic atlases are distributed in print and digital formats. The digital format allows us to include all the data that support the maps and the ability to change the maps or create new ones. The products are available from the MGS web site (http://www.mngs.umn.edu/index.html). We also conduct post-project workshops in the map area to familiarize users with the products and their applications. The products are also distributed to libraries. Products of the Morrison County Geologic Atlas have been applied to finding new municipal water supplies in Little Falls and Motley. We expect both these atlases will be applied to understanding the widespread distribution of nitrate in ground water in this part of Minnesota. Additional funding from DNR has allowed us to continue to collect data from the instrumented borehole constructed for the St. Lawrence subproject. This additional data will be combined with what we have in a formal MGS Report of Investigations. The Rochester study is likely to improve computer simulations of water flow and influence decisions about the distribution and pumping rates of the wells that supply the city.