Mapping the Big Sioux Aquifer from the Air to Plan Sioux Falls' Water

By Joshua F. Valder, Gregory C. Delzer, Janet M. Carter, Bruce D. Smith and David V. Smith. USGS Fact Sheet 2016–3075, September 2016, with the City of Sioux Falls.

Sioux Falls is the fastest-growing community in South Dakota, and it needs a sustainable municipal water supply. The Big Sioux aquifer — glacial outwash sand and gravel, hydraulically connected to the Big Sioux River — supplied about 90 percent of the city's source water in 2015. To plan, the city partnered with the USGS to build a groundwater-flow model, fed by airborne electromagnetic (AEM) data.

The study area and surficial geology near Sioux Falls. Map: USGS.

What the study will deliver

The aim is to map the aquifer's hydrogeologic framework and build a flow model to judge its vulnerability and sustainability, including:

• an inventory of the aquifer's groundwater;
• aquifer properties for the model — saturated thickness, specific yield, porosity;
• the effects of new wells on existing ones and on the river;
• the path of any contaminant plumes;
• maps of streamflow capture areas, which may be more vulnerable.

Why build a groundwater-flow model. Diagram: USGS.

Why a model — and why from the air

Groundwater-flow models let managers project changes (such as new wells) and test scenarios (drought, heavier pumping), and show what data are still needed.

Building one starts with the aquifer's shape: its extent and thickness. That is hard here. Buried channels laid down by repeated glacial advances and retreats are hidden under glacial drift and can't be read from the surface. Earlier test holes and wells are one to several miles apart, leaving gaps to guess.

AEM fills the gaps. It senses how easily electrical currents flow through different deposits (resistivity), and water-bearing sand and gravel have a distinctive signature. From the air it covers large areas densely, crosses rough terrain, and disturbs nothing. Long used for minerals, it is now common in geology and hydrology.

A helicopter tows the "bird," a cylindrical sensor, near Sioux Falls, flying about 200 feet up at about 70 miles per hour. USGS photo by David Smith.

The 2015 survey

• Who: the City of Sioux Falls, the USGS South Dakota Water Science Center and Crustal Geophysics and Geochemistry Science Center, and CGG Canada Services, which flew the helicopter.
• How: the RESOLVE frequency-domain system, chosen for its near-surface resolution and resistance to electromagnetic noise such as power lines.
• How much: about 540 line-miles over about 56 square miles, mainly the flood plain between Dell Rapids and Sioux Falls, plus the Sioux Falls Regional Airport.
• Checks: ground resistivity transects in advance, and test holes and wells from an earlier study to interpret the results. Ground methods alone would have taken too long, with forest and cropland hard to reach.

The helicopter and the RESOLVE sensor. USGS photo.

Power lines complicate data collection and analysis. USGS photo.

From data to model

The USGS processed the data into resistivity–depth sections for 2D maps and 3D views, defining the depth and extent of the aquifer and the Precambrian Sioux Quartzite bedrock beneath. With drillers' logs and aquifer tests, these build the framework; then a flow model can be built, calibrated and used to test development scenarios for future city water projects.

Sources

Based on Construction of a Groundwater-Flow Model for the Big Sioux Aquifer Using Airborne Electromagnetic Methods, Sioux Falls, South Dakota, by Joshua F. Valder, Gregory C. Delzer, Janet M. Carter, Bruce D. Smith and David V. Smith, USGS Fact Sheet 2016–3075, U.S. Geological Survey; a work of the United States government in the public domain. The map, diagram and photographs are taken from the fact sheet's PDF; a flight-line map on Google Earth imagery is not reproduced.