GEOLOGY AND GROUND-WATER RESOURCES OF CLAYTON COUNTY, IOWA
W.L. Steinhilber, O.J. Van Eck, and A.J. Feulne
Iowa Department of Natural Resources, Geological Survey Bureau
Water-Supply Bulletin No. 7, 1961, 142 p.
Prepared cooperatively by the United States Geological Survey and the Iowa Geological Survey
Clayton County includes 784 square miles in northeastern Iowa and in 1960 had a population of 21,962. For the most part, the county is a dissected upland that is drained mainly by the southeastward flowing Turkey River and its principal tributary, the Volga River. The Turkey River empties into the Mississippi River, which flows southward along the eastern border of the county. The climate is humid continental, and the average annual precipitation is 33.01 inches. The economy of the county is based on farming and the raising of livestock. The natural resources of the county include soil, water, rock, sand, and timber.
Glacial deposits of Pleistocene age, ranging in thickness from a featheredge to slightly more than 100 feet, mantle the indurated rocks throughout much of the upland area in the central and western parts of the county. However, indurated rocks of Cambrian, Ordovician, and Silurian age are exposed in hillsides and valley walls throughout the county.
The principal aquifers in the county, in ascending order, are the Galesville sandstone of the Dresbach group, the Jordan and St. Peter sandstones, and the Galena and Hopkinton dolomites. The Galesville sandstone of Late Cambrian age yields large quantities of water to wells in the northeastern part of the county; the hardness of water ranges from 278 to 305 ppm (parts per million) and the dissolved-solids content ranges from 415 to 509 ppm. The Jordan sandstone of Late Cambrian age yields large quantities of water to several municipal wells in the eastern half of the county; the hardness of the water ranges from 238 to 431 ppm and the dissolved-solids content ranges from 252 to 461 ppm. The St. Peter sandstone of Middle Ordovician age yields small to moderate quantities of water to many farm and a few municipal wells in the eastern half of the county; the quality of the water is comparable to the water from the Jordan sandstone, although the hardness is slightly higher. The Galena dolomite of Middle Ordovician age is the principal aquifer for farm wells throughout all except the southwestern corner of the county. Although locally it yields sufficient water for municipal requirements, in most places it yields only enough for domestic requirements. The hardness of the water ranges from 254 to 415 ppm and the dissolved-solids content ranges from 294 to 477 ppm. The Hopkinton dolomite of Middle Silurian age, which underlies only the southwestern part of the county, yields moderate quantities of water to wells. The water from this formation is less mineralized than the water in the other bedrock formations; the hardness ranges from 231 to 333 ppm and the dissolved-solids content ranges from 247 to 376 ppm.
Small quantities of water for domestic use are obtained from the glacial drift in southwestern Clayton County. In most places, however, the supply is not dependable.
Recharge to the aquifers in the county is by subsurface inflow, seepage, and direct precipitation. The Dresbach group and Jordan sandstone are recharged by subsurface inflow. The St. Peter sandstone is recharged mainly by subsurface inflow from the north and south and partly by downward seepage from overlying formations. The Galena dolomite is recharged mainly by infiltration of direct precipitation on the interstream outcrop areas and partly by downward seepage form the Maquoketa shale and partly by some subsurface inflow. The aquifer of Silurian age is recharged by infiltration of direct precipitation and presumably by downward seepage from the Iowan drift.
Discharge from the aquifers is by subsurface outflow, direct and indirect seepage, and springs. The Dresbach group and Jordan sandstone are discharged by subsurface outflow, although the Jordan discharges some water into Bloody Run Creek by seepage. Discharge from the St. Peter sandstone is partly by direct seepage into the streams along the eastern border of the county, partly by indirect seepage up through the Platteville limestone and Decorah shale in the lower reach of Turkey River, and partly by subsurface outflow to the west and southwest. Discharge from the Galena dolomite and from the aquifers of Silurian age is by seepage into streams that dissect the aquifers and by the numerous springs that issue from the aquifers. The amount of water discharged into the Turkey River drainage system by these seeps and springs is estimated to be at least 10 mgd (million gallons per day) during extended periods of drought and several times this amount during periods of average precipitation. The discharge by springs alone is estimated to be about 11 mgd during periods of near-average to average precipitation. Most of these springs discharge into the Turkey River, but some discharge into the smaller streams along the eastern border of the county.
An estimated 3,000 farm wells withdraw approximately 2 mgd of ground water from the aquifers in Clayton County. Most of this water is pumped from the St. Peter sandstone and the Galena and Hopkinton dolomites. Twelve municipalities in the county pump slightly less than 1 mgd of water from the county's ground-water reservoirs. More than half of the water pumped for municipal use is withdrawn from the Jordan sandstone and the underlying St. Lawrence formation. A few small industries withdraw approximately 200,000 gpd (gallons per day) from the aquifers in the county.
Additional water supplies could be developed from the Jordan sandstone, St. Peter sandstone, Galena dolomite, and aquifers of Silurian age without depleting the ground-water reservoir. Wells drilled into the Galena dolomite and aquifers of Silurian age in particular, should fully penetrate the complete aquifer.