Monroe County relies heavily on its aquifers and streams for drinking water, irrigation, and other uses; however, increased water use, high concentrations of certain constituents in ground water, and droughts may limit the availability of water resources. Although the most densely populated parts of the county use water from the Great Lakes, large amounts of ground water are withdrawn for quarry dewatering, domestic supply, and irrigation.
Unconsolidated deposits and bedrock of Silurian and Devonian age underlie Monroe County. The unconsolidated deposits are mostly clayey and less than 50 feet thick. Usable amounts of ground water generally are obtained from thin, discontinuous surficial sand deposits or, in the northwestern part of the county, from deep glaciofluvial deposits. In most of the county, however, ground water in unconsolidated deposits is highly susceptible to effects of droughts and to contamination.
The bedrock is mostly carbonate rock, and usable quantities of ground water can be obtained from fractures and other secondary openings throughout the county. Transmissivities of the Silurian-Devonian aquifer range from 10 to 6,600 feet squared per day. Aquifer tests and historical information indicate that the Silurian-Devonian aquifer is confined throughout most of the county. The major recharge area for the Silurian-Devonian aquifer in Monroe County is in the southwest, and ground-water flow is mostly southeastward toward Lake Erie. In the northeastern and southeastern parts of the county, the potentiometric surface of the Silurian-Devonian aquifers has been lowered by pumpage to below the elevation of Lake Erie.
Streams and artificial drains in Monroe County are tributary to Lake Erie. Most streams are perennial because of sustained discharge from the sand aquifer and the Silurian-Devonian aquifer; however, the lower reaches of River Raisin and Plum Creek lost water to the Silurian-Devonian aquifer in July 1990.
The quality of ground water and of stream water at low flow is suitable for most domestic uses, irrigation, and recreation. In ground water, dissolved solids and hydrogen sulfide are present at concentrations objectionable to some users. Indicators of ground-water contamination from agricultural activities—pesticides and nitrates—were not present at detectable concentrations or were below U.S. Environmental Protection Agency (USEPA) limits. In stream water, some treatment to remove bacterial may be necessary in summer months; nitrate concentrations, however, were found to be below USEPA limits.
Tritium concentrations indicative of recent recharge to the Silurian-Devonian aquifer are present in the southwest-to-northeast-trending band from Whiteford to Berlin Townships. Generally, where glacial deposits are thicker than 30 feet, recharge takes more than 40 years. Carbon isotope data indicate that some of the ground water in the Silurian-Devonian aquifer is more than 14,000 years old.
Mild droughts are common in Michigan, but long severe droughts, such as those during 1930-37 and 1960-67, are infrequent. The most recent drought, during 1988, was severe but short. Ground-water levels declined throughout the county; the largest declines were probably in the southwest. Shallow bedrock wells completed in only the upper part of the Silurian-Devonian aquifer and near large uses of ground water were especially susceptible to the effects of drought. Deep bedrock wells continued to produce water through the drought of 1988.
During droughts, streamflow is reduced because of low ground-water levels and high consumptive uses of surface water. In 1988, annual discharge on the River Raisin was near normal, but monthly averages were below normal from March through August. The quality of surface water during droughts is similar to that during normal low-flow conditions.