Iowa City Project - North Corridor Silurian-Devonian Aquifer Study

Frequently Asked Questions

What was studied?

The purpose of the study was to develop a better understanding of the sources and amounts of ground-water recharge and discharge and to use that understanding to estimate the effects of future development and drought on the sustainability of the Silurian-Devonian aquifer system. Ground-water sustainability is defined as development and use of ground water in a manner that can be maintained for an indefinite time without causing unacceptable environmental, economic, or social consequences. The primary approach used to study the sustainability of the Silurian-Devonian aquifer system was a ground-water flow model constructed for the area.

Why was the study performed?

Ground water is the primary source for public and private water supplies in Johnson County, Iowa. Bedrock of Devonian and Silurian age (referred to here as the “Silurian-Devonian aquifer system”) is the primary source of water for ground-water supplies in the northern part of Johnson County, which is locally known as the North Corridor.  Population growth within municipal and suburban areas of the North Corridor has resulted in increased amounts of water withdrawn from this aquifer system and water-level declines in parts of the county.

Where was the study performed?

The area covered by the study extends approximately 2 miles beyond the Johnson County line on the north, west, and east sides, and corresponds to the boundary between Johnson and Washington counties between townships 78N and 77N on the south.

When was the study performed?

The study of the Silurian-Devonian aquifer system of Johnson County and the North Corridor began in late 2002 and concluded with the publishing of the final report ( ) in the Spring of 2006.

Who were the participants in the study?

The U.S. Geological Survey (USGS) and the Iowa Geological Survey (IGS), which is a part of the Iowa Department of Natural Resources (IDNR), performed this study in cooperation with the IDNR Water Supply Bureau, City of Iowa City, Johnson County Board of Supervisors, City of Coralville, the University of Iowa, City of North Liberty, and the City of Solon.

How much water is currently being pumped from the aquifer?

During 2004, about 5.7 million gallons per day were pumped by wells from the Silurian-Devonian aquifer system. This pumping was about 29% of the water leaving the aquifer system in 2004. Additionally, about 5.1 million gallons per day of water needed to be removed from the aquifer system in the area of a rock quarry in central Johnson County in order to obtain the best model match between simulated water levels and measured water levels.

Where is the water coming from to supply the pumping from the aquifer?

Water entering the aquifer system (recharge) to replace the amount leaving (discharge) came from a combination of precipitation (76%), reservoir water (15%), and ground-water flow from adjacent counties (9%).

How much water might be pumped from the aquifer in the future?

The model was used to predict the response of the aquifer system to two future scenarios: (1) a population growth scenario, and (2) a hypothetical future drought scenario. The population growth scenario was based on estimates of a future population growth of about 15,400 persons in the rural North Corridor by 2025 and projected increases in public water supplies by the various municipalities. The future drought scenario added an additional water demand factor to the projected population increase. These increases resulted in the simulation of an additional withdrawal of 1.5 and 3.2 million gallons per day, respectively, from the aquifer system by wells.

What changes will be seen if and when that water is pumped from the aquifer?

Most of the water needed to supply future pumping will come from reservoir recharge and reduced outflow to adjacent counties. Water levels could decrease by 6 to 10 ft in the North Corridor, primarily to the east of the Iowa River, based on model simulations of projected 2025 population. The hypothetical drought scenario was based on an assumed reduction of 25% in the amount of rainfall entering the aquifer system and an increase of about 25% in the amount of water being pumped by the 2025 population in the eastern two-thirds of the county. This increased stress on the aquifer system resulted in a simulated 5 to 30 ft decline in water levels in some areas.

Will we run out of water?

Sufficient water is available in Johnson County to supply the present and projected future ground-water needs of the North Corridor, although pumping has lowered water levels in some parts of the aquifer system and future pumping will likely further lower water levels in some areas.  Wells that have pumps set close to present or future water levels, however, may go dry, particularly under drought conditions.

What issues will the model not help with?

The model was not designed to provide answers to questions related to individual well operations, but rather to provide a regional analysis of the ground-water flow system in the county and the North Corridor. The study also did not address the quality and safety of the water being supplied for drinking water.

How accurate is the model?

Water levels from a network of about 40 wells in the Silurian-Devonian aquifer system that were routinely monitored between 1995 and 2004 were used to compare to simulated water levels. The better the agreement between the measured water levels and the simulated water levels, the more confidence is gained in the model results. The model constructed for the North Corridor was able to match water levels, on average, within +/- 11 ft of the measured water levels for 1996 conditions and +/-14 ft for 2004 conditions.

Models are not exact replicas of natural systems, and the ground-water model constructed for Johnson County and the North Corridor is no exception. High quality geologic data are available for the area but assumptions were required to provide the pumping estimates for the large, unknown number of private wells in the North Corridor and for discharge from a rock quarry near Coralville. Data for other recharge and discharge sources, needed to verify the model, also were lacking and added to the uncertainty in the model results.

What type of information would improve this or future models?

Improvements to the model and reduction in the uncertainty of the model results can be achieved by obtaining additional or higher quality data, such as pumping by private and industrial wells, water-balance information for the reservoirs, and water levels in specific parts of the aquifer system.

How can the study results be used?

Education of government officials and citizens on the condition of the resource will enable wise and informed decisions to be made. Conservation by water agencies and citizens will help minimize the decline in water levels, particularly during droughts. Monitoring water levels will provide continuing information on the condition of the resource to help water managers and citizens protect the resource.