Groundwater Availability Modeling of the West Nishnabotna Aquifer in Iowa

Although Iowa is not facing an immediate water shortage, increased demand for groundwater by agriculture, industries, and municipalities has raised concerns for the future of the resource. To assist decision makers, an intensive one-year investigation was undertaken to provide a more quantitative assessment of the West Nishnabotna alluvial aquifer. The primary objective of this quantitative assessment was to develop a groundwater flow model to provide planning tools for future water resource development. The hydrologic characteristics of the West Nishnabotna aquifer were evaluated to provide input for model development. A total of 10 aquifer pump and recovery tests and 51 specific capacity tests were used to calculate the aquifer parameters. The hydraulic properties of the West Nishnabotna aquifer were shown to vary considerably both laterally and vertically. Hydraulic conductivity was found to range from 123 to 456 feet/day, with an arithmetic mean of 255 feet/day. Based on aquifer pump test data, the storage coefficient of the West Nishnabotna aquifer ranged from 0.012 at Natural Milk Products (Kirkman Farms) near Kirkman to 0.0001 in the Regional Water wellfield north of Avoca. Recharge to most of the West Nishnabotna aquifer ranged from 3.2 to 5.1 inches per year. In addition to the aquifer pump tests, a network of approximately 20 observation wells were used to evaluate groundwater levels. Included in these observation wells were ten wells in which time series data were collected. Hydrologic information was used to construct a numerical groundwater flow model of the West Nishnabotna aquifer, and consisted of three hydrogeologic layers. The model was created using Visual MODFLOW version 2009.1. Hydrologic processes examined in the model include net recharge, hydraulic conductivity, specific storage, flow-through boundaries, no flow boundaries, well discharge, stream boundaries, and river boundaries. The modeling approach involved the following components: 1. Calibration of a steady-state model assuming no groundwater withdrawal using water level data from historic records. 2. Calibration of a transient model using monthly operating reports and water-use data from 2004 through 2010. Simulated water levels were compared to observed time-series water level measurements. 3. The calibrated models were used to estimate induced recharge from the West Nishnabotna River into the alluvial aquifer. The potential impact to the river during droughts and low flow conditions was also evaluated. The results of this assessment suggest a strong interaction between the West Nishnabotna River and the alluvial aquifer adjacent to the river. This interaction provides security for the water users in the West Nishnabotna aquifer even during droughts and dry periods. The rate and amount of withdrawal the West Nishnabotna aquifer can sustain is limited. Depending on the future pumping rates and assuming an average induced recharge of at least 50 percent, the future availability of water could become an issue. Limits on future groundwater pumping are most critical in the Manning area in Carroll County, where current withdrawals from the West Nishnabotna aquifer average over 1.2 million gallons per day (mgd), and induced recharge averages 67 percent. No historical or current streamgage readings are available immediately downstream of Manning and the West Central Iowa Rural Water Association. Therefore, low streamflow values in this area are unknown. This area is near the headwaters of the West Nishnabotna River, and very few tributaries contribute to the overall flow. A streamflow reading of 4 cubic feet per second (cfs) was measured at the United States Geological Survey (USGS) gage near Harlan in 1982, but the gage was decommissioned the following year. Streamflow values in the West Nishnabotna River south of Manning were likely much less than they were at Harlan that year. The relatively high water usage in the Manning area, the high percentage of induced recharge, and the potential for low streamflow values make this region extremely vulnerable to a severe drought. A contingency plan should be made by the City of Manning, Ag Processing, and West Central Iowa Rural Water Association that addresses a moderate to severe drought.