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Publication Abstract

Meeting East-Central Illinois Water Needs to 2050: Potential Impacts on the Mahomet Aquifer and Surface Reservoirs Roadcap, George, H. Vernon Knapp, H. Allen Wehrmann, David R. Larson, 2011  Illinois State Water Survey, Champaign, IL,  ISWS CR 2011-08    Full Text Available

In this study we examined the impact of current and future water demands on streams and aquifers in east-central Illinois through the use of computer-based models. A numerical groundwater flow model was used to examine the Mahomet Aquifer, the principal groundwater resource in the region. Analytical and deterministic models were used to examine the four large surface water supply reservoirs. Future water supplies and demands out to the year 2050 were evaluated under three scenarios: baseline growth, more resource intensive, and less resource intensive scenarios.

Based on a conceptual model of groundwater flow and recharge in the Mahomet Aquifer system, a numerical groundwater flow model was developed that is composed of three aquifers and three confining layers. Model-predicted drawdowns from predevelopment to 2005 conditions show a large cone of depression in the Champaign region and relatively small drawdowns in limited regions of the confined portion of the aquifer stretching from Clinton through Normal to Morton. In the Havana Lowlands region the model matched field observations that showed no significant drawdowns since the area became heavily irrigated. The predicted increases in drawdown for the three scenarios mimic the historical drawdown with the largest increases in the Champaign cone of depression. The computed mass balance of the model changed with the 262 million gallons per day (mgd) baseline increase in pumpage from 1930 to 2050 largely by inducing an additional 150 mgd of recharge and decreasing baseflow discharge by 99 mgd. A majority of the baseflow reduction, or 82.3 mgd, came from large streams in the unconfined areas at the western end of the aquifer. The remaining 16.7 mgd of baseflow loss came from smaller streams flowing over confined portions of the aquifer with losses in individual watersheds ranging from 7 percent for Salt Creek to 16 percent for the upper Sangamon River above Lake Decatur.

None of the current groundwater users in the Mahomet Aquifer could be considered “at risk” for a future water shortage under the three demand scenarios. The model budget indicates that the available 2050 water supply for the aquifer as a whole is 2.3 times greater than the projected baseline demand. However, hydraulic conditions vary tremendously across the aquifer, and therefore, the aquifer-wide budget does not guarantee that the demand from additional high-capacity wells not in the prescribed demand scenarios will have acceptable impacts to private wells or baseflow in small streams. In this study, the Mahomet Aquifer has been subdivided into areas based on the expected types of impact that the new high capacity wellfield might have.

Surface waters are typically highly replenishable sources, but their availability for water supply can be greatly restricted during periods of extended drought. Knowledge of water availability in streams and reservoirs during times of drought or other periods of low flow is provided in this study through a combination of historical records and both analytical and deterministic models. Statistical estimates of streamflows were developed using streamflow gaging stations and enhanced by the Illinois Streamflow Assessment Model that considers variability in hydrologic records, regional similarity in flow conditions, and external influences such as water withdrawals and wastewater effluents.

Hydrologic changes associated with potential changes in future climate were evaluated using a deterministic watershed simulation model of the Sangamon River basin. Water yields from community reservoirs were analyzed through the use of water budget models that include various hydrologic records for data input.

The surface water supply analysis for east-central Illinois focuses primarily on water yields of community reservoir supply systems: Bloomington, Danville, Decatur, and Springfield. A drought vulnerability classification for these reservoir yields is based on the estimated probability that these community supplies could experience water shortages during an extreme drought as represented by the historical drought of record. Water budget analysis indicates that one of the community systems (Springfield) is an inadequate system, with greater than a 50 percent probability that a shortage would occur during a drought of record condition; two other communities (Bloomington and Decatur) are at-risk systems, with greater than a 10 percent probability of shortage. If each community’s water demand increases as projected by the critical trend (baseline) scenario, both the Bloomington and Decatur systems will be classified as inadequate by 2020 unless supplemental sources are developed by that time. Water budget analyses and the resulting system classifications include considerations for drought response and water conservation measures as outlined in each community’s Drought Action Plan. Drought preparation through water conservation measures could reduce the probability of shortages, but not to the extent where the overall drought vulnerability classification for any of these communities would be changed. Development of supplemental sources is considered essential for reducing each community’s vulnerability to an extreme drought.

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