Des Moines Water Works is one of the largest water providers in Iowa, supplying the City of Des Moines and surrounding communities from both groundwater and surface water. As the population grows, so does demand for drinking water — and meeting it sustainably requires a thorough understanding of one source in particular: the Des Moines River alluvial aquifer.
That aquifer is hydraulically connected to the Des Moines River and is made of alluvial deposits and glacial outwash sands and gravels. Beginning in 2018, Des Moines Water Works partnered with the U.S. Geological Survey to build a groundwater-flow model of the area, using several geophysical methods to see what lies beneath the river, Beaver Creek and the floodplain.
The study area and its surficial geology near Des Moines, Iowa (USGS).
What the study aims to deliver
The goal is a hydrogeologic framework — a picture of the aquifer's layers and extent — and a groundwater-flow model built on it. The main benefits:
- a model that can be used for water-supply planning;
- estimates of how hypothetical new wells would affect existing production wells and water levels in the Des Moines River;
- an inventory of the aquifer's groundwater resources;
- estimates of aquifer properties to build the model and deepen general understanding.
Why a model is needed
Groundwater-flow models show how much water is available underground and how it moves. They can help predict trends in groundwater levels, explore drought scenarios, and test the effects of pumping more, less or more variably from supply wells — informing decisions about how vulnerable and sustainable a supply is. A steady-state model already existed for the aquifer near Prospect Park in Des Moines, but a model covering more of the aquifer would be more useful.
Building a model starts with describing the aquifer's extent, materials and properties. Here, that is complicated:
- The aquifer includes the current river channel, high and low terrace deposits, and glacial outwash sands and gravels, resting on Pennsylvanian-age bedrock of shale, fossil-rich limestone and very fine- to medium-grained sandstone.
- Its Quaternary deposits mix river and glacial sediments with a complex history: glaciers advanced and retreated, leaving till and outwash, and the river then reworked those deposits as it eroded and built its channel.
- In Polk County those deposits range from 10 to 225 feet thick.
Geophysical data, combined with geologic maps and records from test holes and observation wells, can reveal far more of that pattern than a surface map.
Four ways of looking underground
In 2018 Des Moines Water Works and the USGS used a suite of methods along the Des Moines River, Beaver Creek and the floodplain. All four can estimate the depth to bedrock, so their results can be checked against one another.
| Method | How it works | What was done |
|---|---|---|
| Continuous resistivity profiling (CRP) | a streamer of 11 electrodes, 10 meters apart, towed behind a boat measures the electrical properties of the materials below; wider electrode spacings reach deeper, and the depths and resistivities are worked out by inversion | about 13 miles of surveys on the river and Beaver Creek, in three lines of 8.2, 4.6 and 0.60 miles, run together with CSP |
| Continuous seismic profiling (CSP) | a sound source sends pulses through the water that reflect off the bottom and the layers beneath; with enough penetration it can trace buried layers and the bedrock surface | run at the same time as CRP |
| Electrical resistivity tomography (ERT) | on land, current is injected and voltage measured across many pairs of electrodes; the readings are inverted into resistivity profiles that can be read as layers | lines of 56 electrodes spaced 16 feet apart |
| Horizontal-to-vertical spectral ratio (HVSR) | a single sensor records the vertical and horizontal components of ambient seismic noise; where bedrock contrasts strongly with the sediment above it (more than 2:1 in shear-wave acoustic impedance), the sediment resonates at a frequency — lower for thicker sediment, higher for thinner | about 40 single-point readings, mostly along ERT lines to cross-check |
Collecting geophysical data: (A) on the Des Moines River, (B) preparing a CRP/CSP survey, (C) a deployed HVSR sensor, (D) cables laid out for an ERT survey. Photographs by Emilia L. Bristow, U.S. Geological Survey.
Putting it together
Together, the datasets show how thick the unconsolidated sediments are and how they are distributed, feeding the hydrogeologic framework. From that framework, a groundwater-flow model of the study area can be built — a tool Des Moines Water Works could use to test aquifer development scenarios and manage local groundwater better.
Sources
- U.S. Geological Survey: "Application of geophysical methods to enhance aquifer characterization and groundwater-flow model development, Des Moines, Iowa," USGS Fact Sheet, prepared with Des Moines Water Works; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from pubs.er.usgs.gov
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