The 2007 Iowa General Assembly, recognizing the increased demand for water to support the growth of industries and municipalities, approved funding for the first year of a multiyear evaluation and modeling of Iowa’s major aquifers by the Iowa Department of Natural Resources. The task of conducting this evaluation and modeling was assigned to the Iowa Geological and Water Survey (IGWS). The first aquifer to be studied was the Lower Dakota aquifer in a sixteen county area of northwest Iowa.
The geology of the northwest Iowa study area is characterized by a sequence of unconsolidated upper Cenozoic (less than about 5 million years) deposits, overlying Mesozoic Cretaceous (about 100 million years) sandstones, shales, and limestones, Paleozoic (520 million to 280 million years) sandstones, shales, and limestones, and Precambrian (2.8 billion to 1.1 billion years) quartzites, granites, and related rocks. The major sources of groundwater in this area are the sandstones in the Cretaceous Dakota Formation.
Most of the water-producing sandstones in the Dakota Formation are found in the basal Nishnabotna Member, although scattered sandstones in the overlying Woodbury Member also yield signifi cant amounts of groundwater. For this study, IGWS geologists defi ned the Lower Dakota aquifer as the water-bearing sandstone and conglomerate units of the Nishnabotna Member and the directly adjacent, and hydrologically contiguous sandstone bodies in the overlying Woodbury Member. An intensive one-year study of the geology and hydrogeology was undertaken to provide a more quantitative assessment of the aquifer, and to construct a groundwater fl ow model that can be used as a planning tool for future water resource development.
The hydraulic properties of the Lower Dakota aquifer vary considerably in both the lateral and vertical direction, and were obtained from aquifer pump test analyses. Based on aquifer test results, the hydraulic conductivity ranges from 22 to 81 feet per day, with an arithmetic mean of 47 feet per day. Transmissivity values range from 2,700 to 12,000 feet squared per day, and are controlled primarily by the aquifer thickness. The storage coeffi cient of the Lower Dakota aquifer ranges from 1.8 x 10-5 to 2 x 10-3. The arithmetic mean storage coeffi cient is 3.3 x 10-4.
Recharge to most of the Lower Dakota aquifer is through relatively thick confi ning beds that include Cenozoic (Pleistocene) glacial till and upper Cretaceous shale units. Due to the relatively thick confi ning units, the rate of recharge in the lower Dakota is very small. Calibrated recharge rates varied from 0.15 inches per year to 0.05 inches per year over most vii of the study area. A calibrated recharge rate of 3 inches per year was used in the Sioux City area due to thin or absent confining units.
A numerical groundwater flow model of the Lower Dakota aquifer was developed using four hydrogeologic layers. The model was created using Visual MODFLOW version 4.2. Hydrologic processes include net recharge, hydraulic conductivity, specifi c storage, fl ow through boundaries, no flow boundaries, well discharge, river boundary, and groundwater upwelling.
The calibrated model provides a good correlation for both steady-state and transient conditions. Root mean square errors of 14.8 and 9.4 feet were calculated for the steady-state and transient fl ow models. Simulated water level changes are most sensitive to recharge in the steady-state model, and pumping rates in the transient model.
The Lower Dakota aquifer has tremendous development capacity. The current summer time usage is approximately 31.6 million gallons per day, and winter usage is approximately 20.2 million gallons per day. This is well below the development potential for the aquifer. The actual volume of groundwater available for development depends on location. Based on water balance and predictive model simulations, both the Storm Lake and Cherokee areas are at or near the sustainability threshold.