Aquifer Characterization and Drought Assessment Floyd River Alluvial Aquifer

Increased demands for groundwater by agriculture, industries, and municipalities have raised concerns about the future availability of groundwater in Iowa. In 2007, the Iowa Legislature began funding a comprehensive Water Resources Management program, which would be implemented by the Iowa Department of Natural Resources. A key aspect of the program is to evaluate and quantify the groundwater resources across the state using computer simulation models. These models help answer questions such as: “How much water can be pumped from an aquifer over 10, 20, or 100 years?” or “Will my well go dry?” A hydrogeologic study was initiated to understand the shallow groundwater resources in the Floyd River alluvial aquifer (Floyd River aquifer) in northwest Iowa. The primary objective of this study was to evaluate the aquifer for future water supply development. A groundwater flow model of the Floyd River aquifer was created for the City of Sheldon using Visual MODFLOW 2011.1. The model can be used to predict future well interference, drawdown, and maximum sustainable pumping rates. Based on available pumping records, an average of 2.21 billion gallons of water are pumped from the Floyd River aquifer each year. Additional water production is available from the aquifer, but limitations exist during extremely dry years. Maximum estimated well yields exceed 200 gpm near Hospers, Le Mars, Merrill, Sioux Center, and Sioux City. The average well yield is slightly less than 100 gpm. The groundwater flow model for the Floyd River aquifer near Sheldon was used to simulate a severe drought. Water level data during the summer of 2012 were used to help calibrate the model. Based on the mass balance calculations in the model, the percentage of water production supplied by the Floyd River (induced recharge) increased from 54 percent during normal rainfall conditions to 68 percent during a severe drought. The increase in induced recharge allows the City of Sheldon public wells to maintain water production during prolonged dry periods. Limitations in water production exist when streamflow along the Floyd River drops below 1.58 cubic feet per second.