The title of the report presented to the Johnson County Board of Supervisors during its work session on Wednesday was very technical: “Simulation of Groundwater Flow in the Silurian Aquifer, Eastern Iowa (2020–2045).” That was appropriate given the complex nature of its subject matter. But the report’s ultimate message about the groundwater supply much of the county relies on was summed up in four words by the U.S. Geological Survey’s Jude Thomas.
“It’s a finite resource,” she told the supervisors.
Thomas is the section chief for Groundwater and Geophysics in the USGS Central Midwest Water Science Center. The research for the aquifer study began in 2022, and follows a 2006 version of the study that modeled aquifer activity from 2006 to 2022. Thomas was at the work session on Wednesday to provide the supervisors with an overview of the report’s findings.
“Continued population growth has increased water use in model area by over 25% since the USGS 2006 Silurian model,” the new report found. (The text of Thomas’s presentation can be found starting on page 5 of the Board of Supervisors May 14 agenda package. The USGS will publish the final version of the report later this year.)
The Silurian aquifer is one of several bedrock aquifers underlying Iowa. It extends throughout most of the state, except for its northwest corner. It’s the most important source of groundwater for much of Johnson County.
“Protecting [the Silurian aquifer] requires thoughtful, strategic decision-making across communities,” Director of Johnson County Planning, Development and Sustainability Josh Busard said in a statement before the work session. “This study gives us a clear picture of where we stand today, and more importantly, it provides a forecasting tool that helps us plan responsibly for the future.”
The report examined an area of the aquifer spanning over 1,500 square miles, centered on Johnson County but also touching on parts of Linn, Benton, Iowa, Washington, Louisa, Muscatine and Cedar counties.
The aquifer’s name refers to the Silurian period, the geological period in which the rock formation formed, beginning 443.1 million years ago. The Silurian was the shortest period of the Paleozoic era, lasting only about 23.5 million years. It immediately preceded the Devonian period, which is better known in Iowa thanks to the Devonian Fossil Gorge in Coralville.
Despite Coralville’s connection to the Devonian, the city’s wells tap into the older Silurian aquifer for its water. North Liberty, Tiffin and Solon rely on the Silurian for their cities’ water, and so do some of the unincorporated areas of Johnson County. Iowa City has a few wells that tap into the Silurian, but they are little used. The city’s water supply comes from alluvial wells adjacent to the Iowa River.
Alluvial wells are shallow wells in loose geological formations near rivers and streams that draw their water from those sources. As long as the flow of a river or stream remains fairly constant, the adjacent wells are a reliable water source that recharges relatively quickly. Bedrock aquifers, like the Silurian, are different.
“The water they drink out of the Silurian, typically, the majority of it would be hundreds to thousands of years old,” Thomas explained to Little Village after her appearance before the board. “It’s a bedrock aquifer; it’s just not going to recharge in a time-step that’s like an alluvial aquifer.”
The report found a low recharge rate throughout most of the area. In the relatively few locations where the bedrock is nearer the surface, the report projects a recharge rate of 11.85 centimeters of water per year. For the vast majority of the area, the projected recharge rate for the Silurian was 0.23 centimeters per year.
The report takes into consideration the changing climate over the next 20 years, including the potential for recurring droughts. Along with droughts, like the four-year-long statewide drought that ended in May last year, the changing climate also has an increasing number of high-volume rain events that have lead to flash flooding in recent years. But the water released in those rain events runs off quickly into streams, rivers and storm drains, especially if it falls on soil that has been dry for a prolonged period. Little of that water filters down through the dirt and layers of rock to reach the storage area of the Silurian aquifer.
During the board’s discussion of the report, Supervisor V Fixmer-Oraiz noted that since “we’re not able to recharge [the Silurian], then we’re really looking at how we manage the drawdown.”
The report looked at drawdown — the amount of water removed from the aquifer for use — and projected the amount of water stored in the Silurian. The drawdown projections were based on current usage and discussions with city staff members in the area regarding their estimates for the growth of water usage over the next 20 years.
The report found that storage in the aquifer is “relatively minor” and is projected to continue to decrease in the years leading up to 2045.
“In the Iowa City area , where Iowa City isn’t necessarily using the Silurian, but others are — like Tiffin, like Coralville — we saw drawdowns of 13 to 43 meters,” Thomas told the supervisors. “And similarly in the Solon area, we saw drawdowns ranging from four to 13 meters.”
Rod Sullivan, the longest-serving member of the board, noted the direct role of the supervisors in preserving the Silurian, is limited, because its mostly cities drawing water from it. Cooperation and coordination between the county and the cities will be needed. But the report itself is evidence that cooperation is already occurring. Twelve cities joined the county to help cover the cost of the USGS research and preparation of the report.
The condition of Iowa’s aquifers has been getting more attention in recent years, especially as the recent multi-year drought put increasing strain on groundwater resources throughout the rural parts of the state. The increasing number of data centers in the state has also focused new attention on water resources, because the centers use massive amounts of water for cooling purposes.
Iowa currently has 30 data centers, 20 of which are in the greater Des Moines area, and more are planned. Two major data centers are scheduled to be built in southwest Cedar Rapids, in an industrial park near the Eastern Iowa Airport. Google is preparing to construct a second major data center in Council Bluffs.
Concerns about what the drought, data centers and other developments are doing to Iowa’s aquifers got the Iowa Legislature to allocate $250,000 last year to fund a study of the state’s aquifers by the Iowa Geological Survey.
When the funding was approved last May, Rep. Norlin Mommsen of Dewitt, the Republican chair of the House Agriculture and Natural Resources Appropriation Committee, said it was becoming “more critical we know what [water] we’ve got and what depth it is.”
Mommsen cited concerns that data center water usage might prove too heavy a drain on resources communities use for drinking water.
“There are a lot of unknowns out there,” he said.
