ASSESSING CHANGES IN THE DAKOTA AQUIFER POTENTIOMETRIC SURFACE IN NORTHWEST IOWA BETWEEN 2008 AND 2023

In northwest Iowa, demands on the Dakota aquifer have increased due to the declining quality of surface water, lack of other productive aquifers in the area, and increases in pumping for municipal, rural, and private water supply, ethanol production, and confined animal feeding operations. The last study focused on the Dakota aquifer in Iowa was completed in 2008 by the Iowa Geological Survey. The objective of this current study was to collect water levels from existing wells across the Dakota aquifer extent to update the regional scale potentiometric surface for the unit. In Iowa, the Dakota aquifer exists primarily across a 16-county area in the northwest corner of the state. It is composed primarily of sandstone and conglomerate units of the Nishnabotna Member of the Dakota Formation and the sandstone-rich portions of the overlying Woodbury Member. To identify wells suitable for water level measurements across the study area we used the GeoSam database maintained by the Iowa Geologic Survey. There are 1,302 Dakota aquifer wells in Iowa. However, the precise hydrostratigraphic characterization for many of these wells is uncertain because of a lack of well construction data, stratigraphic picks, or both. For example, the stratigraphy defined for many of the Dakota aquifer wells was simply described as ‘Cretaceous’ which means the well could be open to the Nishnabotna Member, the Woodbury Member, and/or the overlying upper Cretaceous units. Consequently, we focused on a set of 52 wells that were deliberately drilled for a Dakota aquifer study conducted in the early 1980s. Water levels were collected from all measurable wells and used to generate a potentiometric surface representative of 2023 conditions. A preliminary comparison to the 2008 potentiometric surface shows spatial variability in the changes in magnitude and inferred regional groundwater flow directions likely related to areas where more or less pumping has occurred. Currently, continuous water level data collected from three USGS Dakota aquifer monitoring wells are being used to assess the rate of change between the 2008 and 2023 potentiometric surfaces. These analyses help to quantify the impacts of increasing use on the Dakota aquifer and will be valuable to help determine new Dakota aquifer drilling and monitoring well locations for future phases of the study.