GEOLOGY AND GROUND-WATER RESOURCES OF WEBSTER COUNTY, IOWA
IowaDepartment of Natural Resources, Geological Survey Bureau
Water-Supply Bulletin No. 4, 1955, 257 p.
Prepared cooperatively by the United States Geological Survey and the Iowa Geological Survey
Webster County, comprising an area of 718 square miles just northwest of the center of Iowa, had a population of 44,241 in 1950, with 25,115 in Fort Dodge, the principal city. Some 94.4 percent of the county is in farm land; corn is the principal crop and is used in the raising of hogs and cattle, an important occupation in this part of the country. Mineral products include gypsum, clay, coal, sand, gravel, and limestone.
The mean annual precipitation at Fort Dodge is 31.21 inches, of which more than 3 inches normally occurs during each of the months May, June, July, August, and September. The average number of growing days is 150. The warmest month generally is July; during December, January, and February the average temperature normally is below freezing.
The upland area, comprising over 80 percent of the county, is mostly a gently undulating, slightly eroded glacial-drift plain. Morainal hills of low relief occur in the extreme southern and northern parts of the county. The Des Moines River flows through the county from north to south and, together with its tributaries, drains the entire county except the southwestern part, which is tributary to the Raccoon River. The Des Moines River has cut a deep, narrow valley about 90 feet below the upland in the northern part of the county and about 220 feet below the upland in the southern part. The tributary streams commonly have shallow valleys more than a few miles back from the Des Moines River.
Glacial deposits of Pleistocene age, ranging in thickness from 50 feet in the north to 175 feet in the south, mantle the indurated rocks over all the upland area, but indurated rocks ranging in age from Mississippian to Cretaceous are exposed in places along the valleys of the Des Moines River and its tributaries.
Rock cuttings obtained from many wells in Webster and surrounding counties give control on the subsurface geology. Red serpentinized basalt, presumably of pre-Cambrian age, was encountered in a well at a depth of 2,290 feet, or 1,310 feet below sea level, at Fort Dodge. The stratigraphic sequence includes rocks of late Cambrian, Ordovician, Devonian, Mississippian, Pennsylvanian, Permian, and Cretaceous age. Rocks of Silurian, Triassic, and Jurassic age are not known to occur in the county.
In the northwestern part of the county, a thick section of shale and sandstone of Cretaceous age has been downfaulted in respect to adjacent limestone and sandstones of Paleozoic age. These strata rest on strata of undetermined age and appear to surround a core of igneous rock. Only the easternmost part of the structural basin lies in Webster County, the remainder being in Calhoun, Pocahontas, and Humboldt counties. The abrupt lateral change in lithology, the contorted and brecciated condition of the strata, the circular outline, and the igneous core suggest a volcanic structure. Faulting has occurred also in the vicinity of Fort Dodge and has created a graben, about one-third of a mile wide at Fort Dodge, which trends in a direction somewhat north of east.
Many shallow wells obtain small quantities of hard water containing considerable iron from sands and gravels in the Pleistocene drift sheets. Two large sand-and-gravel-filled buried channels occur near Duncombe and Gowrie and give promise of yielding much water.
Cretaceous strata generally yield little water to wells in Webster County, even where thick because of downfaulting along the western margin of the county. The Fort Dodge formation of Permian age likewise contains little water, and the shales in the formation probably prevent recharge to underlying limestones from waterbearing beds in the drift.
Sandstones of Pennsylvanian age generally yield little water to wells, but where they occur as channel fills, particularly in the central part of the county, they yield moderate supplies.
Mississippian rocks form aquifers which supply many farm wells, particularly in the northern part of the county, and yield small to moderate supplies of hard water which may have an objectionably high fluoride content. Devonian strata yield little water to wells in Webster County, and unusually hard water can be expected because of the gypsum content in the Cedar Valley and Wapsipinicon limestones.
The St. Peter sandstone and the upper part of Prairie du Chien formation, both of Ordovician age, are relatively good aquifers and yields of 50 to 200 gallons a minute with moderate drawdowns can be expected from them.
The most consistently high yielding zone of aquifers is formed by the lower part of the Prairie du Chien, the Jordan sandstone, and the St. Lawrence formation. The transmissibility of these beds is between 50,000 and 110,000 gallons a day per foot at Fort Dodge. The water from these aquifers is hard and in places contains objectionable concentrations of iron. The strata below the St. Lawrence formation are not promising as a source of water in Webster County, and the available water is likely to have a high chloride content.
The towns having municipal water-supply systems are Badger, Callendar, Dayton, Duncombe, Gowrie, Harcourt, Lehigh, and Fort Dodge. Yields from wells at Fort Dodge are abnormally high, probably because of the fractured condition of the strata; four wells finish in the Mississippian rocks, one in the Devonian rocks, and one in the Jordan sandstone. Water levels in the well field have declined from a reported elevation of 62 feet above land surface in 1911 to approximately the land surface in 1951. The pumpage from the field had increased to about 3.6 million by 1950. Water levels may be expected to decline between 1 and 2 feet a year if pumpage continues to increase at the same rate as during the past 15 years; but if pumpage is stabilized at 3.6 million gallons a day, the water levels may decline no more than 10 feet within the next 30 years. These postulated declines in water level will be accelerated by drilling any additional private wells that will draw water from the same water-bearing beds.