Groundwater Availability Modeling, Des Moines River Aquifer, Palo Alto and Emmet Counties, Iowa

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 to 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 more fully understand the shallow groundwater resources in the West Fork of the Des Moines River alluvial aquifer (Des Moines River aquifer). The primary objective of this study was to evaluate the potential impact of the new Iowa Lakes Regional Water wellfield near Osgood (Osgood wellfield, Palo Alto County) on the nearby irrigation wells. A computer simulation model of the Des Moines River aquifer was created using Visual MODFLOW version 2010.1. The model predicts future well interference, drawdown, and maximum sustainable pumping rates. The groundwater flow model for the proposed Osgood wellfield involved six new public wells with an annual permitted water use of 539 million gallons per year (mgy). The Osgood wellfield is divided into a north wellfield (proposed Well 6) and south wellfield (proposed wells 1 through 5). A total of 14 irrigation water use permits (24 known irrigation wells) and two existing public water use permits (City of Emmetsburg and City of Graettinger) are located in the model area with permitted water use totaling 1.15 billion gallons per year. Worst-case historical drought conditions based on the 1958 drought were simulated, and the impact of the new Osgood wellfield on the nearby irrigation wells was evaluated. Maximum additional drawdowns in the nearby irrigation wells caused by the pumping of the Osgood wellfield ranged from 4.2 feet to 5.8 feet in the south wellfield, and 0.4 to 0.7 feet in the north wellfield. Based on the mass balance calculations in Visual MODFLOW, the percentage of water production supplied by the Des Moines River and Jack Creek (induced recharge) increased from 10.9 percent during normal rainfall conditions to 58.4 percent during a severe drought. The increase in induced recharge prevents much higher drawdowns in both the irrigation wells and the Osgood wellfield wells. Without the recharge from the Des Moines River and Jack Creek, a severe drought would significantly reduce the water production in the area wells. Based on the model results, adequate water resources are available to meet the current and future water withdrawals in the Des Moines River aquifer. Adjustments in pumping cycles and rotating active and inactive wells may be necessary during a severe drought. The irrigation wells may need to pump during the night when water demand is lower for the Osgood wellfield, or Iowa Lakes Regional Water may want to pump additional water from the north wellfield (Well 6) to reduce the pumping stress on the south wellfield (wells 1 through 5). Cooperation would be necessary for both Iowa Lakes Regional Water and the irrigators. This proactive approach can be a useful planning tool during a severe drought. The model can also be used to evaluate the maximum sustainable withdrawal from the area, and to potentially limit new water use permits and prevent over-allocation of the groundwater resources.