| | | | Title: | | | | | Volume/Number: | 1998 | | | | Issuing Agency: | | | | | Description: | In 1993, with funding from the Long Range Water Plan Steering Committee (LRWPSC), the Illinois State Water Survey (ISWS) and the Illinois State Geological Survey (ISGS) began a study of the sand-and-gravel aquifers in southwest McLean and southeast Tazewell Counties to estimate the availability of ground water and determine the hydrogeologic feasibility of possibly developing a regional water supply. The study area includes the confluence of the buried Mahomet and Mackinaw Bedrock Valleys (confluence area) and contains part of one of the largest sand-and-gravel aquifers in Illinois, the Sankoty-Mahomet Sand aquifer. The study had two goals: (1) to determine the quantity of water a well field in the Sankoty-Mahomet Sand aquifer could yield, and (2) to determine the possible impacts to ground-water levels and existing wells that might occur in the Sankoty- Mahomet Sand aquifer and overlying aquifers from the development of a well field pumping 10-15 million gallons of water a day (mgd). Two major tasks were completed to meet the study goals. The first task was a hydrogeologic characterization of the glacially deposited (glacial-drift) aquifers within the confluence area. Results of the hydrogeologic characterization were published in 1995 (Herzog et al., 1995a and b) and a summary of their findings are in the appendices. The second task, and the subject of this report, was the development of a computer-based, mathematical model of the ground-water flow in the glacial deposits (ground-water flow model). The ground-water flow model was used to simulate the effects of a hypothetical well field for various locations within the study area and to provide an estimate of the amount of ground water a regional well field could yield from the Sankoty-Mahomet Sand aquifer within the confluence area. The characterization of the hydrogeology of the glacial-drift aquifer system was simplified to allow the development of a ground-water flow model. The generalized hydrogeology resembled a layer cake with uneven layers, some of which were discontinuous. The layers included relatively impermeable bedrock overlain by three sand-and-gravel aquifer layers that are generally separated by aquitard layers. Due to the complexity of the spatial distribution of the sand-and-gravel deposits above the Sankoty-Mahomet Sand aquifer, these shallower deposits were generalized as two aquifer layers. Although none of the aquifer deposits represented by the shallower aquifer layers are capable of sustaining a 10-15 mgd water supply, the thickness, distribution, and hydraulic properties of these deposits are important for a complete understanding of the hydrology of the model area. In some parts of the area covered by the ground-water flow model, two or more of the aquifer layers are in direct contact, providing a 'window' of hydraulic connection between the aquifer layers. In other parts of the model area, one or more of the aquifer layers are absent. Using the information from the hydrogeologic mapping and water-level data, chloride concentrations, and percent modern carbon data from observation wells, an updated conceptual understanding of the groundwater flow system for the Sankoty-Mahomet Sand aquifer was developed that described the movement of ground water into and out of the model area. Ground water in the Sankoty-Mahomet Sand aquifer generally flows through the Mahomet Bedrock Valley from the southeast, westward to the Illinois River and northward through the Mackinaw Bedrock Valley. The natural ground-water discharge areas for the Sankoty-Mahomet Sand aquifer in the study area are the Mackinaw River in the west-central part of the study area and Sugar Creek in the southwestern part of the study area. In some areas very close to the rivers, ground water is flowing upward from the Sankoty-Mahomet Sand aquifer through the upper aquifers and into the stream beds. There is a slight hydraulic gradient (slope) east of the model area that steepens where the flow enters the study area, even though the aquifer volume increases. This slope increase is caused by a greater amount of recharge entering the aquifer due to hydraulic connections with overlying aquifers. The areas of connection are more numerous in the west and north portions of the model area, as demonstrated by increases in water levels, decreases in chloride concentrations, and increases in modern carbon isotope concentrations in the Sankoty-Mahomet Sand aquifer. Down gradient of these connections, the chloride concentrations remain low, which suggests that the influx of ground water through these connections provides the majority of the recharge in these areas. Water pumped from the Normal west well field south of Danvers, which has wells penetrating into one of these upper aquifer connections, has low chloride values, indicative of water coming from the upper sands. Although the size of the original study area was about 260 square miles, the area to be modeled (model area) was expanded to 1,100 square miles. This expansion was necessary to reduce the effects of the model boundary conditions on simulated water levels in the study area. The simulated water levels are strongly influenced by the boundary conditions, which reduce the accuracy of the simulated water levels near the boundaries. The ground-water flow model was developed using Visual Modflow software. Three aquifer layers sandwiched between four aquitard layers are simulated in the model. Bedrock forms the lowest aquitard; till units form the others. The hydraulic property values of each hydrogeologic unit were assigned to the corresponding layer in the ground-water flow model where the unit was present. When a unit was absent, the layer was assigned the value of an overlying or underlying hydrogeologic layer. The model's boundary conditions control the regional flow into and out of the study area, discharge to and from the streams, infiltration from precipitation, and removal of water by production wells. The model was calibrated to match water levels measured in area wells in 1994 and to match the baseflow gains and losses in the Mackinaw River and Sugar Creek. The mean absolute error of the simulated water levels was 4.99 feet, which was only slightly greater than the errors associated with the potentiometric surface maps, indicating a good match between the model and the characterization of the ground-water flow system. The ground-water flow budget calculated using the model shows that 80 percent of the water coming into the model is from infiltration of precipitation, 11 percent is from the regional Mahomet aquifer in the east, and 8 percent is from river leakage. The budget also shows that 57 percent of the surface and ground water that leaves the model area does so through discharge to the rivers, 33 percent to the regional ground-water flow to the north and to the west, and the remaining 10 percent to existing production wells. (See online pub for remining abstract...) | | | | Date Created: | 9 24 2004 | | | | Agency ID: | COOP-19 | | | | ISL ID: | 000000000797 Original UID: 999999993916 FIRST WORD: Hydrogeology | |
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