Hydrogeology of the Rock River Region

There are three primary sources of groundwater utilized in the Rock River Region: major sand and gravel aquifers, major shallow bedrock units, and the deeper Cambrian-Ordovician Sandstone Aquifers. The web maps below show withdrawal data from public supply wells in the year 2013 along with some context about the geology.

One of the more valuable tools to assess groundwater conditions is a transmissivity map. Transmissivity is a term that defines how much groundwater can potentially flow through an aquifer; the higher the transmissivity, the more productive an aquifer will be. The most transmissive aquifer is the sand and gravel, but it is also spatially variable, as apparent in the map below. The most spatially extensive area of high transmissivity occurs in Lee, Whiteside, Bureau, and Henry Counties, comprising the Green River Lowlands. Outside of this area, the primary high transmissive areas occur in the comparatively narrow bedrock valleys that run through Boone, Winnebago, and Ogle Counties and along the Mississippi River in Carroll and Whiteside Counties.

Material at or near bedrock surface has often developed secondary porosity, particularly in carbonate units. Geologic units in the map below are often used as shallow bedrock aquifers where they are less than 500 feet from land surface. The orange dots represent shallow bedrock wells that were active at some point between 1980 and 2013 . The heaviest use of the shallow bedrock occurs around Rock Island.

For the Cambrian-Ordovician Sandstone Aquifer, transmissivity is expected to be relatively uniform throughout northern Illinois. Wells open to a single sandstone aquifer generally have a transmissivity of 5,000 square feet per day; those open to multiple can approach a transmissivity of 10,000 square feet per day. Regardless, this is much smaller than transmissivity in the shallow aquifers where they are present. However, the deep sandstone has been an important source of water for northern Illinois for over a century, largely due to the hundreds of feet of available head above the aquifer that allows for comparatively large drawdowns. The primary issue with the deep sandstone is that withdrawals are often unsustainable due to limited recharge.

In some areas of the Rock River region, the sandstone is at bedrock or land surface; here an increase in secondary porosity is likely to increase transmissivity of the sandstone (orange area in the plot below). Recharge to the sandstone is also much greater in these orange shaded areas. However, withdrawals in these areas are more likely to capture flow that would otherwise discharge to streams, creating potential ecological impacts.

Where present, the sand and gravel aquifers are generally used to meet water supply due to their comparatively high transmissivity, although they are also more at-risk to contamination. Shallow bedrock aquifers are also used if available, most prevalently around Rock Island where permeable Silurian-Dolomite is either at land surface or overlain by Pennsylvanian-Mississippian-Upper Devonian units. When these shallow aquifers are not available, can't provide needed quantities of water, or are contaminated, the Cambrian-Ordovician Sandstone Aquifer is commonly used.

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.