USGS Groundwater Information: Branch of Geophysics
Geophysical investigations have become a fundamental part of USGS Virginia Water Science Center (VA WSC) studies in the Shenandoah Valley to enhance understanding of ground-water/surface-water interactions and karst terrain.
"It's a folded and faulted area, which happens to have quite a bit of carbonate rock, so you have karst topography and karst flow systems. Geophysics is another tool in our tool box to try to understand it," explained David Nelms, Ground-Water Specialist in the VA WSC.
Surface-water/ground-water interactions are an important area of study for the VA WSC. The northern Shenandoah Valley area is experiencing economic and population growth, resulting in increased water supply needs. Much of the domestic water supply comes from ground water, and public utilities are located on the Shenandoah River and its tributaries. During the 1998-2002 drought, some streams, wells, and springs in the area went dry. This brought increased public attention to the fact that the local ground-water and surface-water systems are connected, and raised awareness of the importance of a better understanding of these complex flow systems. An improved understanding of surface-water/ground-water interactions will also aid scientists researching ecological issues in the Valley, such as several large fish kills that have occurred in recent years.
The VA WSC participated in an Office of Ground Water, Branch of Geophysics (OGW BG) technology demonstration and evaluation project exploring the use of fiber-optic distributed temperature sensing (FO-DTS) technology to identify areas of ground-water discharge into the Shenandoah River. As part of the study, funded by the Ground-Water Resources Program (GWRP), OGW BG staff worked with the VA WSC to install two 1.3-kilometer stretches of FO cable in the Shenandoah River (Fig. 1) and to collect over a million temperature readings over a few days (Fig. 3). The FO-DTS data are being interpreted in conjunction with recent continuous-resistivity profiling on the river, multi-method borehole geophysical surveys, and geological mapping to try to better characterize the underlying geologic units and structure and how they affect ground-water and surface-water in the area.
In the Shenandoah Valley, the USGS also uses surface-geophysical methods to better understand the local and regional karst landscape. Karst aquifers can have the benefit of providing large water supplies. However, they can also pose a wide array of geologic and hydrologic challenges and hazards that affect people's use of the land, including ground subsidence, sinkholes, ground-water contamination, and (or) unpredictable water supplies. Questions have been raised about the impact of construction blasting on the flow of springs and wells. Sinkholes in particular are a common problem in the Valley, especially in association with wells and stormwater runoff. The results can sometimes be disastrous; in one instance, a home was destroyed when a sinkhole developed after a new well was installed.
Geophysical methods can be used to rapidly and non-invasively image the subsurface to help identify structures that pose potential hazards, such as voids, and to better characterize ground-water flow paths in an area. With assistance from OGW BG and funding from USGS Eastern Region, the VA WSC is exploring the application of near-surface geophysical methods to better understand the complex karst terrains. Methods that have been tested so far include direct-current (DC) electrical resistivity, multi-channel analysis of surface waves (MASW) using a seismic land streamer for rapid data collection, and multifrequency electromagnetic induction surveys. The VA WSC also is working with the USGS Geology Discipline Crustal Imaging & Characterization Team under a USGS Venture Capital Fund grant to determine if Love-wave analysis and related seismic methods can be useful in the area (Fig. 2).
Clarke County, which is one of the counties being studied in the Shenandoah Valley, currently requires some geophysical investigations in advance of development and construction projects. According to Nelms, any new methods demonstrated to be effective through the WSC work have the potential to be adopted by the County. "These are relatively new methods," says Nelms. "If you can get something more definitive so that people know what's going on below them, it's helpful."
To support technology transfer and get the word out about the results of the VA WSC work, Nelms is developing a Clarke County geophysics section for the VA WSC web site. Interested USGS personnel and cooperators will be able to learn about the geophysical investigations being conducted by the Science Center and to see samples of these data and results.
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