2 research outputs found
Changing Groundwater Levels in the Sandstone Aquifers of Northern Illinois and Southern Wisconsin: Impacts on Available Water Supply
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.published or submitted for publicationis peer reviewedOpe
Preliminary statewide projections of county-level water demands in Illinois: Draft project completion report
Illinois State Water Survey and Southern Illinois University CarbondaleOpe