Risk to Groundwater Supplies in the Rock River Region

The Illinois State Water Survey considers both sustainable water usage and risk when considering groundwater resources.  Sustainable supply  is discussed in a separate story map, while this story focusses on risk in the Rock River region. It is important to note that this analysis is intended to identify areas that might require further investigation, not to make a definitive statement of water supply for that municipality. There are many factors (such as alternative sources and local recharge to the aquifer) that should be investigated on a local scale.

As a method of screening potential at-risk systems, the ISWS compares water use needs to transmissivity for those communities that use groundwater to meet some or all of their supply. A risk metric ranging from 1 to 10 is assigned for each community, with a higher number meaning greater risk. For example, a community pumping 100 gallons per minute per square mile (gpm/mi^2) from an aquifer with an average transmissivity of 10 gallons per minute per foot (gpm/ft) would have a risk metric of 10. In contrast, a community pumping the same amount from an aquifer with an average transmissivity ten times greater (100 gpm/ft) would have a risk metric of 1. A number of other important factors, such as proximity to streams or availability of recharge to the aquifer, are not currently considered in this analysis; those would be considered in a local analysis.

The left-hand side of the slider map below shows the risk ranking for communities that use the shallow aquifer (sand and gravel or shallow bedrock), either without or in conjunction with the deeper Cambrian-Ordovician Sandstone Aquifer. The right hand side shows the transmissivity of the shallow aquifer in square feet per day.

Perhaps the most important observation is that Rockford has a lower risk ranking (2) than many other communities in the Rock River Region; this is primarily because the demands are spread over a fairly large area, including a bedrock valley filled with coarse-grained, high transmissive material. Due to Rockford’s large size and the variety of geologic units that wells are open to, this story is more nuanced, as discussed in  this story map . Rockford also relies on sandstone wells in addition to shallow sand and gravel wells.

The communities with a higher risk ranking in the Rock River region are generally smaller, such as Scales Mound which has a risk ranking of 10. The reason for the high risk is that the transmissivity map does not indicate any major or minor sand and gravel aquifers, nor any highly permeable bedrock aquifers. The relatively small demands are offset by the small area and transmissivity. In these cases, a more detailed local analysis is often appropriate to understand the local geology.

While the above analysis is based on total transmissivity, it is important to consider any additional risk from deep demands, which are often  unsustainable . The map below shows areas where current (red) and future (orange) risk to the St. Peter sandstone exist. The current risk area, based on an analysis of the 2014 synoptic measurement of water levels in northern Illinois and Wisconsin and a recent model simulation, indicates areas where the deep St. Peter Sandstone is either partially desaturated or has lost at least 50% of the predevelopment available head.

A very common, and understandable, critique of this analysis is that many communities fall into the "Current Risk" zone and do not have immediate water supply issues. Indeed, this is true, but it is important to not conflate "risk" with "certainty" in this case.

So what does risk to the uppermost sandstone (the St. Peter) actually mean? The St. Peter is prone to caving when it becomes partially or completed saturated, which is a strong possibility in the red and orange areas. The solution to this issue has generally been to drill deeper into the Ironton-Galesville sandstone aquifer, which is  highly unsustainable  and can accelerate declines in water levels. As a result, this cascading impact can, sometimes after a few years or decades, result in depletion of groundwater resources, as is currently happening in Northeastern Illinois.

A very important observation is that, due to relatively constant or decreasing demands from the sandstone aquifer in the Rock River Region, risk generally does not increase between current day (areas shaded red) and 2050 model simulations (areas shaded orange ). That does bode well for the water supply over the coming decades, because the very slow rate due to unsustainable withdrawals does not appear to manifest in additional risk. However, the deep aquifer is highly responsive to changes in demands, so any increase in sandstone demands has the potential to increase risk. This would need to be analyzed on a case-by-case basis.

Another risk to consider, particularly in the Winnebago County region, is that declines in the sandstone have induced a gradient from areas with known contamination. Well construction, leaving wells open to multiple aquifers, can create shortcuts for this contamination to potentially enter the groundwater supply. It is recommended that the region continue to work with state and federal EPA to monitor this possible situation.

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.