Desaturation of Sandstone Aquifers in Northeastern Illinois

In 2014-15, the Illinois State Water Survey conducted their largest synoptic measurement of heads in Cambrian-Ordovician sandstone wells since 1980. The study covered the northern half of Illinois. These observations were used to generate head contours of the sandstone aquifers. In 2014, drawdown from predevelopment conditions 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 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 the areas of greatest pumping, even at wells which were cycled off. Simulations from a groundwater flow model indicate that the risk of desaturation will become more severe 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.