Geology and ground-water resources of Linn County, Iowa

Linn County, in east-central Iowa, covers about 713 square miles and lies in the Western Young Drift section of the Central Lowlands physiographic province. The normal annual rainfall in the county is about 88 inches and the annual mean temperature is about 48°F. The population in 1960 was 136,899, of which 75 percent was urban.

Ground water is a vital natural resource in Linn County-all municipal, farm-domestic, livestock, and most industrial supplies are obtained from this source. An estimated 24 mgd (million gallons per day) of ground water was used in the county in 1964.

The principal aquifers are alluvium, buried channel deposits, Silurian-Devonian limestones and dolomites, and the Jordan Sandstone. All yield fair-to-good quality water, although the water is hard and locally contains high concentrations of iron. All are capable of yielding as much as 500 gallons or more per minute to wells.

Alluvium has been developed for water supplies only at Cedar Rapids, where withdrawals in 1964 averaged 12 mgd for municipal supplies and 2.7 mgd for industrial supplies. The alluvial aquifer yields up to 2,000 gpm (gallons per minute) to wells in the Cedar Rapids well field. Similar yields from alluvium may be available along several reaches of the Cedar and Wapsipinicon Rivers. Smaller quantities are available from the alluvium of Prairie Creek. Alluvial aquifers are readily recharged by precipitation and induced infiltration.

Buried channel deposits occur in preglacial or interglacial valleys that were carved into the bedrock. These old valleys, whose trends roughly parallel the present Cedar River and Prairie Creek, contain water-bearing alluvial deposits that are covered by glacial drift. Data from a few wells tapping these deposits indicate that yields of up to 500 gpm of good-quality water are available from this source. The most favorable areas for development of water supplies are those areas where the channel deposits underlie and receive recharge from the alluvium of the Cedar River and Prairie Creek.

The Silurian-Devonian aquifer's county-wide occurrence, near-surface position, and ability to yield as much as several hundred gallons per minute of good-quality water makes it the most widely used aquifer in Linn County. During 1960-64, withdrawals averaged about 1.5 mgd for domestic-livestock use, about 0.6 mgd for small community use, and about 4 mgd for industrial-commercial use. Withdrawals are concentrated in the Cedar Rapids area, where 65 percent of the withdrawals from the aquifer occur. This concentrated pumpage has caused a progressive lowering of the aquifer's piezometric surface in downtown Cedar Rapids. During the past 70 years, water levels in wells in this locality have declined about 105 feet in the center of the cone of depression and about 26 feet about one mile from the cone's center. Water level in the center of the cone presently is declining at an average rate of 1 foot per year. Because the rate of decline in the same area was determined to have been 2 to 3 feet per year during the 1940's and 50's, the cone is believed to be stabilizing or pumpage is being reduced. The aquifer probably could withstand an additional 150 to 200 feet of piezometric lowering in the Cedar Rapids area, but individual wells would be adversely affected.

The Jordan aquifer, which underlies the entire county, is considered to be the most isotropic and homogeneous aquifer in Linn County. Yields of 1,000 gpm or more of fair-to-good quality water from this source are believed to be available anywhere in the county. The aquifer is not yet developed extensively; an average of about 2.4 mgd was pumped during 1964 for industrial and municipal use in the Cedar Rapids-Marion area.

The shallow bedrock and glacial drift aquifers yield only small quantities of good quality water. Their widespread extent and shallow depth, however, make them suitable for the development of small supplies for domestic and livestock use.