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The impact of emergency pumpage at the Decatur wellfields on the Mahomet aquifer: model review and recommendations

 Volume/Number:  2001  
 Issuing Agency:  Illinois State Water Survey 0038
 Description:  The City of Decatur operates a series of ten groundwater wells in DeWitt and Piatt Counties that serve as an emergency water supply in times of low surface water levels in Lake Decatur. The City of Decatur contracted with Layne-Geosciences, Inc. (LGI) to develop a computer model of the groundwater system to simulate the effects of pumpage on the Mahomet Aquifer and surrounding wells. The LGI model was completed in April 1999. In response to lowering lake levels, Decatur began pumping their wells in November 1999 for 84 days at daily rates from 3 million gallons a day (mgd) to 16 mgd. The Illinois State Water Survey (ISWS) reviewed and tested the LGI model against the known drawdown encountered during the 84 days of operation. The LGI model was found to be only marginally successful in reproducing the measured water levels. The largest error occurred in the Piatt County area where the model significantly overpredicted the drawdown. These errors were the result of several factors, including errors in the aquifer thickness map, calibration to data only within 5 miles of the wellfield, errors in the location of pumping wells, the use of general head boundaries throughout the model, and, most importantly, the absence of a hydraulic connection between the Mahomet Aquifer, the Glasford Aquifer, and the Sangamon River near Allerton Park. Additional data available in the ISWS well records, and new data provided by Decatur through Guillou and Associates, Inc., indicate a connection between the aquifer system and the Sangamon River. Adding this connection represents a change in the conceptual model of the flow system not included in the LGI model. When this connection was added, a much closer match between observed and calculated water levels was obtained. Future work should focus on developing a more complete understanding of the connections between the aquifer system and the Sangamon River. Those efforts should include a pump test of the Cisco wellfield with complete monitoring of the river and aquifers. Monitoring of water levels at selected locations should continue and expand. The groundwater flow model should be re-calibrated using the new data and the improved understanding of the flow system. The results of these activities can provide an improved assessment of the potential of the Decatur wellfield for future use. 
 Agency ID:  CR-2001-11 
 ISL ID:  000000000839   Original UID: 999999994322 
  Documents:  20060930185946_ISWSCR2001-11.pdf