The villages of Elkton and Pigeon in Huron County, Michigan, depend on wells completed in the Marshall aquifer to provide a reliable source of potable water. In order to protect the quality of water pumped from these wells, these municipalities need to ensure that potentially harmful contaminants do not enter the Marshall aquifer within the contributing areas of these wells. The Well Head Protection Plan for the State of Michigan requires the delineation of a contributing area based on a 10year time-of-travel for public-supply wells. The U.S. Environmental Protection Agency recommends a 40-year time-of-travel as the basis for contributing areas of wells in confined aquifers.

The Marshall aquifer, composed of permeable sandstone, is the principal bedrock aquifer in the Elkton-Pigeon area and in the Michigan Basin. In the Elkton-Pigeon area, the top of the Marshall aquifer is roughly 90 to 150 feet below land surface and the aquifer is about 80 feet thick. Transmissivity ranges from 40 to 1,300 feet squared per day, effective porosity is estimated to be about 10 percent, and the hydraulic gradient ranges from 0.0013 to 0.0025. The recharge area of the aquifer is several miles southeast of Elkton and Pigeon, where the aquifer is a subcrop beneath the permeable unconsolidated sediments of the Port Huron End Moraine.

Water from wells at Elkton and Pigeon has tritium concentrations of less than 0.8 tritium unit, a concentration indicating that (1) the water in the Marshall aquifer at Elkton and Pigeon is more than 40 years old and (2) the aquifer is highly confined. On the basis of a semianalytical model, contributing areas for a the ,10-year time-of-travel for water-supply wells in Elkton and Pigeon are almost entirely within village boundaries. The contributing areas for a 40-year time-of-travel encompass areas within approximately 1 mile of each village and are several miles from the recharge area of the aquifer.

Control of land-use activities within the contributing areas at Elkton and Pigeon will not ensure that water pumped from these supply wells will remain potable. The Marshall aquifer overlies the Coldwater Shale, which contains brine and brackish water that could migrate toward the pumped wells.