USGS Groundwater Information: Branch of Geophysics
In the Nebraska District Office, Hydrologist Wade Kress has been working in the Data Section to integrate geophysics throughout district projects. "Geophysics doesn't solve every problem," says Kress, "but it provides useful information for most." As a result, Nebraska has been using surface, borehole, and water-borne geophysics in an increasing number of projects, including analysis of bridge scour, placement of monitoring wells, and the development of hydrogeologic frameworks for ground-water modeling.
In one project, two-dimensional (2D) resistivity data were collected along a canal beneath Interstate 80 in Ogallala, Nebraska, where a massive localized flood scoured the canal, removing the approaches to the bridges over the canal. Incorporation of multiple 2D surveys into a three-dimensional data set allowed the USGS to identify a definable scour zone throughout the canal and to approximate the volume of sediment removed during the scour event.
In joint projects with cooperators such as the U.S. Environmental Protection Agency, Nebraska USGS personnel have investigated the use of geophysics to help site monitoring wells. The work showed that geophysics aided in locating wells and in better determining how deep to drill and where to screen the wells, thus increasing efficiency and decreasing the costs of the monitoring and remediation efforts. In another investigation, Nebraska is using multiple surface geophysical methods, including 2D direct-current resistivity, continuous seismic-reflection profiling, time-domain electromagnetic soundings, and magnetic resonance sounding, to characterize an alluvial aquifer system. The surface data are being analyzed in conjunction with resistivity, natural gamma, and neutron logs from boreholes at the site to map the depth and extent of paleo-alluvial valleys throughout the aquifer system. The results will be used by the North Platte Natural Resources District to calibrate groundwater models in areas where there is little or no hydrogeologic information.
The Nebraska District has also collaborated with other Districts on investigations involving new applications of geophysics. For example, Kress worked with the Arkansas District and the Arkansas Department of Environmental Quality in an investigation of the Red River Aluminum site at Stamps, Arkansas, to determine the possible extent and depth of saltwater contamination at the Superfund site. The investigation used surface 2D resistivity and borehole geophysical data to map areas containing subsurface low-resistivity anomalies interpreted as possible salt contamination. Kress encourages other Districts who are getting started with geophysics to work with OGW BG, explaining that he has found the Branch's training courses and field project support helpful. Collaborating with the Branch has also enabled Kress to network with other USGS personnel who are using geophysics across the country.
Kress believes it is important to help people understand how the latest geophysical methods and equipment extend beyond traditional methods. "Many of the properties measured provide more information than just a driller's log or core," he explained. "When you are drilling a well, you are getting a pin prick of information. If you can use geophysics to extend the data two or three dimensionally, you can learn more about things you never knew existed and that would be extremely difficult to find without tools like these."
For more information on the Red River Aluminum site study, see WRIR 03-4292 at http://ar.water.usgs.gov/LOCAL_REPORTS/local_reports.html.