The Iowa Geological Survey (IGS) completed a drought assessment to evaluate current and future groundwater storage and availability for the City of Sioux Center’s (City) alluvial wellfield. The City had previously hired DGR Engineering to design two low-head dams along the west fork of the Floyd River. A calibrated groundwater flow model was developed by the IGS to provide the City with a quantitative evaluation of the additional groundwater storage potential of the low-head dams. Various other drought strategies to enhance both aquifer storage and induced (river) recharge were also valuated using the calibrated flow model.
The groundwater flow model was used to estimate the percentage of water obtained from induced recharge. Based on model results, the current (no low-head dams) induced recharge on May 13,2015, was 48%, which corresponds to approximately 365 gallons per minute (gpm). This is an estimate based on Wells 5 through 13 pumping continuously, and Wells 1 through 4 pumping periodically. The proposed low-head dams increased induced recharge to 61%, or 464 gpm. Adding a constructed wetland to the east of Wells 9 through 13 (in addition to the proposed lowhead dams) is shown to increase the induced recharge to 69% or 525 gpm.
The proposed low-head dams would raise the river stage approximately 2.5 feet at the proposed southern dam (currently there is a temporary low head dam that raises the river stage 1.5 feet), and approximately 4 feet at the proposed northern dam. Based on the calibrated groundwater flow model, the proposed low head dams would increase groundwater storage by 46 million gallons. Assuming this additional storage would benefit production Wells 4 through 13, and assuming a worst case drought (Floyd River would cease flowing), the additional groundwater storage would allow production Wells 4 through 13 to continue to operate for approximately 58 days with no streamflow in the river. If the former sand and gravel quarries located in the City Park are used for emergency sources of recharge, and pumped into the west branch of the Floyd River, an additional 10.3 million gallons of water may be added to the groundwater storage. This would increase the days of operation of Wells 4 through 13 to 70 days with no streamflow river.
Based on model results, a proposed constructed wetland (in addition to the proposed low-head dams) to the east of Wells 9 through 13 would increase the total groundwater storage to 74 million gallons. This would allow production Wells 4 through 13 to continue to operate under severe drought conditions (no streamflow on the west fork of the Floyd River) for approximately 93 days. Based on the groundwater flow results from the model, production Wells 11 and 13 would obtain most of their recharge from the river and from the constructed wetland. Currently, approximately 50% of the recharge to Wells 11 and 13 comes from land used for growing corn. Adding the two proposed low-head dams and constructed wetland may result in improvements to groundwater quality.