Characterizing the Hydrogeological Framework for the Cedar River Alluvial Aquifer System near Cedar Rapids, Iowa, using Airborne Electromagnetic Surveys

Executive Summary

In 2012, Iowa experienced a severe drought that critically affected source-water availability in the well fields, most notably to the high-capacity collector wells that supply water to Cedar Rapids, Iowa. As bank storage became depleted, streamflow declined to near record levels which severely impacted riverbed leakage into underlying alluvium. A need to better understand and model the exchange of surface-water and groundwater as well as the impacts on these resources as climate variation occurs in future years. 

The City of Cedar Rapids in collaboration with the U.S. Geological Survey (USGS) is working to refine a previous groundwater flow model to characterize and forecast future model scenarios for the city.  A better understanding of the groundwater resources used for municipal water will help the city plan to meet current and future demands for increased economic and population growth.  In an effort to meet the future water demands, the City of Cedar Rapids maintains a well network of 53 vertical wells and 5 horizontal collector wells, completed in a complex network of unconsolidated Quaternary deposits that make up the Cedar River alluvial aquifer and the underlying variably fractured carbonate bedrock.  Buried channels and variations in the subsurface alluvial deposits, not identifiable from surface geology, often are challenging to model.  As a result, an airborne electromagnetic (AEM) survey has been conducted to map out characterize the subsurface conditions in and around the Cedar Rapids area.  The AEM will provide a subsurface map of changing resistivity’s that can be used in delineating geologic materials and changing aquifer conditions to better estimate aquifer thicknesses and boundary geometry of the aquifer material. This information will be included in a newly refined groundwater-flow model of the Cedar Rapids area that is being developed by the USGS Iowa/Illinois Water Science Center.

Objectives

The primary objectives for the study are to (1) execute an AEM survey in Cedar Rapids, Iowa, by assisting the City of Cedar Rapids with organizing, planning, and guiding the preparation of the flight details; and (2) characterize the geohydrologic framework and aquifer conditions of the Cedar River alluvial aquifer. This study will improve the overall effectiveness of the existing model, specifically, improving the hydrogeologic framework of the model.  This improvement to the model will help the City better plan, develop, and implement strategies to mitigate the effects of prolonged drought on ground-water availability.

More information and highlights from the flight can be found under the “Field Photos” section in the Office of Groundwater highlights (https://water.usgs.gov/ogw/highlights/) and the companion Media Advisory (https://www.usgs.gov/news/media-advisory-helicopter-fly-low-over-cedar-rapids-groundwater-study).