In 2014-15, the Illinois State Water Survey conducted their largest synoptic measurement of water levels (i.e., heads) in Cambrian-Ordovician sandstone wells since 1980. The study covered 33 counties in the northern half of Illinois where demands for water are satisfied, in part, by sandstone aquifers. The Wisconsin Geological and Natural History Survey also measured sandstone wells in 10 counties in southern Wisconsin. These observations were used to generate head contours of the sandstone aquifers. These contours provide insight into the direction and magnitude of groundwater flow. They also can be compared with historic measurements, providing insight into the impact of changing groundwater withdrawals through time. In predevelopment conditions, heads in the Cambrian-Ordovician sandstone aquifers were near or above land surface. Due to pumping from the sandstone aquifers, heads have decreased over time; this decrease is referred to as drawdown. In 2014, drawdown in northeastern Illinois was typically over 300 ft and exceeded 800 ft in the Joliet region. Three factors drove this large drawdown. First, demands for water from sandstone aquifers are much greater in northeastern Illinois than in the rest of the study region. Second, the sandstone aquifers are overlain by aquitards, which are low permeable materials that limit vertical infiltration of water. Third, the Sandwich Fault Zone limits water flowing into the sandstone aquifers of northeastern Illinois from the south. Heads near the center of the cone of depression continue to have a decreasing trend. The more severe drawdown in northeastern Illinois has resulted in local areas where heads have fallen below the top of the sandstone, known as desaturation. Desaturation of a sandstone aquifer can create a number of water quality and quantity concerns. The uppermost sandstone, the St. Peter, was observed to be partially desaturated in portions of Will, Kane, and Kendall Counties under non-pumping conditions. Other areas in these counties are at risk of desaturation under pumping conditions or with the installation of additional wells connecting the St. Peter to deeper, more heavily stressed sandstones. Simulations from a groundwater flow model indicate that the risk of desaturation will increase with increased future withdrawals. Despite the relatively small demand for water throughout much of central Illinois, heads have been declining since predevelopment, likely due to the shale overlying the sandstone. This shale serves as an aquitard, minimizing vertical infiltration of groundwater to the sandstone. Sustained drawdown in this region could potentially induce flow from the southern half of the state, where water in the sandstone is highly saline and not suitable as a drinking water supply. Drawdown in northwestern Illinois was also typically small (<100 ft), primarily due to two factors: 1) low demands from the sandstone aquifers and 2) the absence of shale aquitards. The notable exception is in Winnebago County, near Rockford, where demands are historically high and drawdown was on the order of 100-200 ft. While the quantity of water in the aquifer is not a concern in this region, large withdrawals could result in reductions of natural groundwater discharge to surface waters, impacting stream ecosystems under low flow conditions. Drawdown since predevelopment was over 300 ft in southeastern Wisconsin, with the greatest drawdown in Waukesha County of over 400 ft. Recent trends indicate heads in the Waukesha area are recovering, although they are still well below predevelopment levels. This report is also associated with the fact sheets: Changing Groundwater Levels in the Cambrian-Ordovician Sandstone Aquifers of Northern Illinois, 1980-2014, Groundwater Availability in Northeastern Illinois from Deep Sandstone Aquifers, and Sources of Water for Communities in Northeastern Illinois.
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