AVAILABILITY OF GROUNDWATER IN DECATUR COUNTY, IOWA

W. J. Cagle and W.L. Steinhilber


Iowa Department of Natural Resources, Geological Survey Bureau,
Water Atlas No. 2, 1967, 28 p.

ABSTRACT


Decatur County and several other counties in south-central Iowa comprise an area that has been chronically short of good-quality water. Municipalities, industries and rural water users alike have been affected by the water shortage. Municipalities have experienced serious problems in obtaining potable supplies adequate to keep pace with their growth and development; industrial expansion has been hindered and continues to be hindered by the shortage of good-quality water; and rural supplies for domestic and livestock use are difficult to obtain at many places. The increased use of water for all purposes and periodic drought conditions have greatly magnified an already serious problem of water shortage.

Probably most water used for all purposes in the south-central area is groundwater that is obtained from wells and springs, but water from surface sources is used to a substantial degree. The two sources of groundwater in the area are the bedrock and the unconsolidated deposits of glacial drift and alluvium that lie above the bedrock. Sources of surface water include reservoirs, streams, ponds, and cisterns. Reservoirs are the source of supply for some municipalities and industries in the area and ponds and cisterns supplement rural supplies obtained from wells. Unregulated streams are not a source of supply at present.

Some of the sources of water in the area yield insufficient amounts of water, some have erratic and unpredictable yields, some yield highly mineralized water, and some are not completely understood. Collectively, these factors contribute to the shortage problem in the area.

Obviously the shortage of water could be alleviated by developing additional supplies of good-quality water from one or more of the sources in the area. Additional development, however, depends on a better understanding of the entire hydrologic system. In particular, more detailed information is needed on the occurrence, availability and chemical quality of water from the unconsolidated deposits, on the quality of water from the bedrock, and on the streamflow regimen in the area. The need for these facts on water -- facts that could be made readily available to municipalities, industries, and individuals alike -- prompted an investigation of the water resources of south-central Iowa. The investigation, which will encompass both groundwater and surface-water studies, will culminate in a comprehensive report that will appraise the total water resources of the region.

The groundwater study, which will provide the basic geologic and groundwater data necessary for the comprehensive regional report, was started in 1962 by the U.S. Geological Survey in cooperation with the Iowa Geological Survey. This project, which involves an extensive amount of test drilling, is being conducted on a county-by-county basis. In order to aid the water users in the respective counties, the results of the groundwater investigation will be released as a series of county reports pending completion of the comprehensive regional report. This report on Decatur County is the first in the series; other will be forthcoming.

The objective of this report is to present timely groundwater information that will help solve the supply problems of some water users in Decatur County. The information presented encompasses the location, definition, and estimated potential yields of the water-bearing materials in the unconsolidated deposits and the uppermost bedrock; an evaluation of the general occurrence and availability of water from bedrock sources; and a determination of the chemical quality of water from all groundwater sources.

Test drilling has comprised the major part of the groundwater investigation in Decatur County. A total of 121 test holes were drilled through the unconsolidated deposits and into the uppermost part of the underlying bedrock. Drill samples were taken at 5 feet or less in the unconsolidated deposits to more closely define boundaries between geologic units and, most importantly, to define as accurately as possible the positions of the beds of sand and gravel which are the sources of water in the unconsolidated deposits. Cores up to a maximum of 20 feet and drill samples to a maximum of 80 feet were taken in the bedrock to 1) make a definite determination that bedrock had been penetrated and 2) so that a general evaluation of the water-bearing potential and economic value of these rocks could be made. Most of the test holes were logged with an electric logger to confirm the position and thickness of the sands and gravels, to determine the top of bedrock, and to identify bedrock units. In addition to test drilling, supplemental data were collected by an inventory of rural water supplies.