HYDROGEOLOGY AND STRATIGRAPHY OF THE DAKOTA FORMATION IN NORTHWEST IOWA
J.A. Munter, G.A. Ludvigson, and B.J. Bunker
Iowa Department of Natural Resources, Geological Survey Bureau,
Water Supply Bulletin 13, 1983, 55 p.
A hydrogeologic investigation of the Dakota aquifer in northwest Iowa utilizing outcrop analysis and over 30,000 feet of research drilling has resulted in the recognition of two members of the Dakota Formation. The lowermost Nishnabotna Member consists predominantly of a medium- to coarse-grained quartz arenite sandstone with interbedded layers of shale, siltstone and lignite. This unit is 200 to 300 feet thick in many of the western and north-central parts of the Dakota aquifer. The Nishnabotna Member is overlain by the Woodbury Member, which is commonly 100 to 150 feet in thickness and consists of shale interbedded with very fine- to fine-grained sandstone, siltstone and lignite. Individual sandstone beds have limited lateral continuity, but provide low yields to domestic wells in the area. Post-Cretaceous erosion has removed portions of the Dakota Formation in most of the study area.
The Dakota aquifer is confined by Cretaceous-age shales and limestones in the western part of study area, and by fine-textured glacial drift in nearly all of the study area. The thickness of the drift is commonly 200 to 400 feet.
The Dakota aquifer in Iowa is recharged by downward percolation through the confining units throughout the study area, and is also recharged by lateral groundwater inflow from Minnesota. Natural discharge occurs to the lower reaches of most of the major rivers in the area, to several Paleozoic aquifers, and to South Dakota beneath a 30-mile segment of the Big Sioux River.
Comparison of recently collected water-level data with historic records on a regional scale suggests that water levels in the aquifer are neither rising nor lowering at a detectable rate. Several local areas appear to have experienced long-term water-level declines, but the extent and magnitude of these declines are not well known.
Controlled pumping test data from the Dakota aquifer yield an average hydraulic conductivity of 48 feet/day for sandstones in the Nishnabotna Member of the Dakota Formation. Some of the pumping test data appear to have been influenced by heterogeneity in the aquifer. It is suggested that shale or mudstone lenses in the aquifer tend to retard the response of observation wells that are located at relatively large distances from production wells, resulting in low estimates of transmissivity, even where thickness and overall textural properties of the aquifer appear to be relatively uniform.
Water quality in the Dakota aquifer varies from calcium-bicarbonate type to calcium-sulfate type water. The low-sulfate type water (less than 250 mg/l) typically occurs in areas that are inferred to have relatively high recharge rate, mostly in the southwest part of the study area. The high-sulfate type water (greater than 1000 mg/l sulfate) occurs in areas with thick confining units such as in the north, northeast and central parts of the study area.