PRI researchers gain new insights into how groundwater flows within the Illinois Basin
A conceptual model of water movement in rock layers many feet underground, published by Prairie Research Institute scientists, shows for the first time that water is flowing steadily south through the thick sequence of rock layers that form the Illinois Basin.
The Illinois Basin is a thick stack of about a dozen rock layers in the center of Illinois that nest within one another like bowls. The edges of the rock layers circle one another, like the edges of the stacked bowls. When viewed as a map, the rock layers form concentric rings, with the oldest layer at the outside and the youngest forming the innermost ring.
Long-term data collection by the Illinois State Geological Survey (ISGS), Illinois State Water Survey (ISWS), and other sources allowed a much more detailed understanding of groundwater movement in the Basin in the study led by Sam Panno, ISGS senior geochemist.
The Illinois Basin gives us water, minerals, and energy. Proving the Basin’s water is flowing is good news for the future of drinking water resources in Illinois, said Panno and Walt Kelly, a groundwater geochemist and head of the Groundwater Science Section at ISWS. Water that moves through rock layers tends to have better quality because inherently salty basin waters are being diluted and replaced by fresh rainwater and snow melt.
“This work is helping us understand where good quality groundwater might be located in places we have never looked at before,” Kelly said. But this finding is only good news for very, very long-term water resource planning. The researchers point out that large parts of central Illinois are still drinking glacial melt from the Ice Age. The meltwater from the glacier seeped into the ground, a process known as recharge, and has stayed in storage there for more than 10,000 years. The youngest groundwater used in central Illinois (e.g., from the Mahomet Aquifer) is post Ice Age, but is still thousands of years old.
“If you paved over entire recharge areas, you really wouldn’t notice it in your well during your lifetime,” Panno said.
However, when the amount of groundwater pumped from an aquifer exceeds recharge, that overuse does have a short-term effect. “The rock layers that hold the water are all connected, and overuse and lack of effective planning still can, and are, now stressing important Illinois aquifers, particularly in the northeastern part of the state,” added Kelly.
The conceptual model used for this work was created by combining geologic data from ISGS research with water quality samples archived in ISWS's Groundwater Quality Database.
PRI’s scientists modeled the movement of water through the basin’s rock layers by tracking chloride ions. Because chloride (the Cl element in common salt, NaCl) is highly soluble, does not oxidize, and does not combine or attach with other material, chloride concentrations act as a “natural tracer,” revealing zones of recharge and groundwater flow paths into, through, and out of the Illinois Basin. It is also possible because the Illinois Basin is devoid of major solid halite (salt) deposits present in other geologic basins, Panno said.
The PRI team found that chloride concentrations were progressively lower along the northern edge of the basin. This demonstrates that surface water is entering the basin from the north, west, and northeast, joining remnants of glacial meltwater stored underground; together the recharging surface water and groundwater slowly move through the different formations.
Three major areas of recharge were confirmed:
- In and around Starved Rock State Park, through fractures (faults) in rock layers on both sides of the Illinois River, major recharge of the St. Peter Sandstone aquifer occurred when the Illinois River flooded with glacial meltwater during the last part of the Ice Age, Panno said.
- The Mississippi River hugging the contours of the basin’s western edge provides a recharge area for the basin and also received considerable recharge during the Ice Age.
- The Ohio River provides highly unusual vertical recharge straight down through the Fluorspar Area Fault Complex along the Kentucky-Illinois border.
“These findings recently helped ISWS and ISGS recommend where the City of Bloomington should site its next public drinking water well, based on the chloride concentration in groundwater samples of the St. Peter Sandstone aquifer,” Panno said.
Since colonial times central Kentucky has been prized for saline springs and salt licks, Panno noted. Those areas line up with the southern limits of the Illinois Basin where groundwater from the basin is discharging to the surface and near surface, he said. In addition, saline groundwater is discharging up through geologic structures (faults and folds) within the basin via fractures associated with the structures and are visible as saline springs once used by Native Americans and settlers as a source of salt in the early frontier.
The paper was published in the journal Groundwater and can be found at http://onlinelibrary.wiley.com/doi/10.1111/gwat.12545/epdf.
Sam Panno, firstname.lastname@example.org, 217-244-2456
Walt Kelly, email@example.com, 217-333-3729