Environmental Health - Toxic Substances
Visualizing Contamination Pathways in the Subsurface
U.S. Geological Survey (USGS) scientists have developed or adapted several non-invasive methods to identify potential pathways through which contaminants could potentially migrate beyond what would normally be expected at groundwater contamination sites. In some cases, these same geologic features, such as faults, fractures, and gravel beds, can also act as barriers to contaminant migration. Identifying such subsurface features is a key component of characterizing groundwater contamination sites, designing groundwater cleanup operations, and assessing the performance of subsurface remediation systems. Current technologies for detecting potential subsurface contaminant pathways rely primarily on direct measurements from wells. Such measurements are invasive, expensive, time consuming, and provide only limited information on the location of contaminants and how contaminants are moving through time. The following are two examples of the application of non-invasive surface geophysical methods to locate subsurface geological features at groundwater contamination sites.
Locating a Subsurface Fault Zone
At the Naval Air Warfare Center (NAWC) in New Jersey, USGS scientists tested the ability of a non-invasive geophysical method known as the multichannel analysis of surface waves (MASW) seismic method to locate a fault zone that has a major influence on the movement of groundwater and contaminants at the site. Most seismic methods measure the reflection or refraction (bending) of compressive waves of energy (seismic waves) by geologic features such as faults or changes in rock type. A small seismic source, such as an explosive charge or dropping a heavy weight on the ground, is used to create micro “earthquakes,” which cause measurable seismic waves to radiate into the subsurface. In this application, the dropped weight was accelerated by a rubber band, and a series of small ground motion detectors measured the resulting seismic waves. The MASW method analyzes the behavior of seismic waves travelling along the surface of the earth (similar to the waves from a stone dropped in water). At the NAWC site, the MASW method effectively measured the location of the fault as well as the orientation of gently dipping sedimentary bedrock and the thickness of the overlying regolith (a layer of fragmented and weathered rock material that overlies bedrock) at the site.
Detecting Gravel Layers in the Unsaturated Zone
At the Amargosa Desert Research Site (ADRS) in Nevada, USGS scientists tested the ability of non-invasive multielectrode resistivity surveys to map the underground extent of gravel layers in a thick and dry unsaturated zone. Mapping the gravel layers was key to determining the potential movement of gaseous volatile organic compounds and radioactive tritium migrating from a closed low-level radioactive waste-disposal facility near the ADRS. Data from the resistivity surveys was used to create two-dimensional images of subsurface resistance to the flow of electricity. Since the gravel layers have less residual moisture than the surrounding finer grained sediments, they showed up as layers with higher electrical resistivity. The results of this study showed that multielectrode resistivity surveys can image subsurface features, such as the gravel layers, in arid environments to a depth of about 10 meters. This rapid, inexpensive, non-invasive method can be applied at other sites to help characterize the hydrologic and geologic framework of contaminated sites in arid environments.
Ivanov, J., Miller, R.D., Lacombe, P., Johnson, C.D., and Lane, J.W., 2006, Delineating a shallow fault zone and dipping bedrock strata using multichannal analysis of surface waves with a land streamer: Geophysics, v. 71, no. 5, p. A39-A42, doi:10.1190/1.2227521.
Lucius, J.E., Abraham, J.D., and Burton, B.L., 2008, Resistivity profiling for mapping gravel layers that may control contaminant migration at the Amargosa Desert Research Site, Nevada: U.S. Geological Survey Scientific Investigations Report 2008-5091, 30 p. (On-line only.)