Assessment of Groundwater and Quality - Cedar River Project

The City of Cedar Rapids, Iowa, obtains its municipal water supply from shallow (less than 100 feet below land surface) wells completed in the Cedar River alluvium, an alluvial aquifer underlying and adjacent to the Cedar River. Since 1963, six horizontal collector wells and dozens of vertical wells have been installed in the aquifer, which has provided adequate quantities of generally high-quality water.  Population growth and industrial development have steadily increased the demand for water pumped from the alluvial aquifer. The City’s water use has increased since 1980 when it pumped about 8,847 million gallons (Mgal; 24 Mgal per day) compared to 14,107Mgal (38.255 Mgal per day) in 2022. 

Managers of the City of Cedar Rapids Water Division are concerned about meeting the steady increase in demand for municipal water and protecting the water quality in the alluvial aquifer to ensure a safe water supply for their customers. The managers require information to understand the aquifer’s response to varying climate and pumping conditions, to plan for additional withdrawals to meet future demands, to understand water quality conditions and the movement of contaminants into the aquifer from the Cedar River, and to satisfy requirements for source-water protection programs under the Safe Drinking Water Act. 

Objectives: 

Long-term and ongoing objectives of the project include: 

1. Evaluation of the groundwater flow system and quantifying the interaction between the alluvial aquifer, Cedar River, and underlying carbonate bedrock aquifer.
2. Evaluation of the effects of climate and pumping scenarios on the alluvial aquifer and investigation of potential locations for additional water-supply wells.
3. Characterization of water quality in the alluvial aquifer, Cedar River, and underlying carbonate bedrock aquifer through continuous and discrete water quality monitoring.
4. Assessment of the fate and transport of nutrients in the Cedar River Basin and the relation of decadal changes in land use to nutrient concentrations in the river using time of travel, synoptic and Lagrangian water sampling.
5. Compilation and assessment of data required for source-water protection programs under the Safe Drinking Water Act

Current and Future Activities:

Hundreds of water-quality samples have been collected from observation wells, municipal supply wells, the Cedar River, and Morgan Creek. Physical parameters, common ions, carbon, nutrients, and pesticides are currently being monitored on a quarterly basis at the Cedar River and in observation wells in the Cedar Rapids wellfields. Continuous water quality equipment is deployed on the Cedar River at Palo (collecting nitrate, temperature, specific conductance, dissolved oxygen, pH, and turbidity data), Morgan Creek near Covington (collecting nitrate and temperature), and at an alluvial well in the West Wellfield (CRM-104). 

Nitrate and herbicides are primary threats to water quality in the alluvial aquifer. Most nitrate and herbicides detected in the alluvial aquifer likely are transported with induced infiltration from the Cedar River. Nitrate concentrations in the Cedar River typically are greatest (>10.0 milligrams per liter) in the spring and early summer, with some rebounds in the fall. These peaks correspond to periods of fertilizer and manure applications to upstream cropland. Work to increase understanding of flow and transport of nutrients in the Cedar River Basin has included a series of synoptic studies to assess bacteria and nutrient concentrations in the basin at low and high flow and dye tracing studies from Waterloo to Cedar Rapids to better understand the actual time of travel of compounds in the Cedar River at different discharge rates. Nitrate concentrations in the alluvial aquifer tend to be less than nitrate concentrations in the Cedar River, but also typically are greatest in the same time periods. A study of groundwater geochemistry in the Seminole Well Field indicated that carbonate-equilibrium reactions, weathering of aluminosilicate minerals, cation exchange, and oxidation-reduction reactions affect water quality in the alluvial aquifer, and water quality monitoring and analysis are ongoing. 

Numerical groundwater flow models have been constructed in 1996, 2004, and 2021 to assess groundwater availability by simulating groundwater flow in the Cedar River aquifer, quantifying aquifer recharge sources, and evaluating pumping and climate scenarios. The most recent MODFLOW model, published in 2021, was designed to simulate aquifer conditions under drought stress in response to a period of drought between July 2011 and February 2013 in which the City observed water level declines in their wells. The 2021 model demonstrated a decrease in the rate of seepage from the Cedar River to the alluvial aquifer in periods of low river flow.  This model did not include the newest Ranney radial collector well #5 (RCW5) because it was not operational during the stress periods simulated for the 2021 publication (2011-2013 and 2016-2018). A new effort is underway to use the 2021 model to investigate RCW5 interaction with other wells by simulating the wellfield with the addition of RCW5 and using more recent climate data.  A study of water levels and water quality during a 2020 operational test of RCW5, published in 2023 as “One Ranney Well Can Make a Difference: The Impacts of a Radial Collector Well on Groundwater Level and Quality in the Cedar River Alluvial Aquifer”, found water-level declines in nearby wells and shortening of river-to-well travel time during periods of sustained pumping in RCW5.  

An airborne electromagnetic (AEM) survey of the alluvial aquifer and upland areas to its west was completed in 2017 to provide geological data for the 2021 MODFLOW model and for other purposes. The survey obtained electrical resistivity data using a helicopter-borne sensor, providing a wealth of information about the shape and material of the aquifer and surrounding areas. Other geophysical methods have been used in this area, including electrical and seismic surveys at sites in the floodplain, electrical surveys of the river channel, and borehole methods at monitoring well sites. The horizontal-to-vertical spectral ratio (HVSR) passive seismic method works well in Cedar River alluvial material and can provide rapid information on depth to bedrock in areas with limited borehole data. In 2025, HVSR surveying was done in areas of investigation for wellfield expansion to provide more information about the depth of bedrock around test boreholes.