Aquifer Characterization and Drought Assessment Ocheyedan River Alluvial Aquifer

The Iowa Geological Survey completed a hydrogeologic evaluation of the water resources in the Ocheyedan River aquifer in Osceola, O’Brien, Clay and Dickinson counties in Iowa. The work was funded under the Water Plan Program of the Iowa Department of Natural Resources. The primary objective of this study was to evaluate the aquifer for future water supply development. Future work will include a calibrated groundwater flow model of the Osceola County Rural Water District (OCRWD) northern wellfield, which will be used to predict future well interference, available drawdown, optimal maximum pumping rates, and quantifying induced (river) recharge. Based on the groundwater elevation data, surface water flows toward OCRWD and Iowa Lakes Regional Water (ILRW) wellfields. Without this induced recharge, high capacity production wells and irrigation wells would not be able to sustain their pumping rates during prolonged droughts. Groundwater recharge sources are precipitation, induced recharge from surface water, and seepage from glacial drift and terraces along the valley wall. A groundwater study conducted in 2011 calibrated an average annual groundwater recharge of approximately 8 inches within the alluvium near the ILRW wellfield in Clay County (Gannon, 2011). At an average groundwater recharge rate of 8 inches per year, approximately 13 billion gallons per year (bgy) of precipitation would recharge the aquifer. If we classify a severe drought as half the annual precipitation (Gannon, 2006), and we assume this corresponds to an estimated recharge of 4 inches per year (half the average recharge), approximately 6.5 bgy of precipitation recharge enters the aquifer during a severe drought. Much of this recharge is removed from the aquifer through river baseflow and evapotranspiration. The actual rate of groundwater available for use would need to be quantified using hydrographs, analytical methods, and numerical modeling. Total current water usage for the study area, not including private wells, is estimated at 3.1 billion gallons per year (8.6 million gallons per day), with a peak usage of 16.33 million gallons per day. Well interference likely occurs between the irrigation wells and the OCRWD and ILRW wells during peak summer-time usage. The application of a calibrated groundwater flow model will help evaluate the magnitude and significance of this well interference. The distribution of potential well yield was estimated by converting the transmissivity value to specific capacity, and multiplying this by one-half the saturated sand and gravel thickness. Potential well yields greater than 400 gallons per minute (gpm) are found near Spencer, ILRW, and OCRWD. There appears to be areas between the City of Everly and south of OCRWD, and near the upper Little Sioux River that have potential well yields less than 100 gpm.