Airborne Electromagnetic Survey of Paradox Valley
Over a seven-day period in October 2011, an aerial geophysical survey was conducted by SkyTEM Surveys in cooperation with the U.S. Geological Survey in an area over the Paradox Valley, Colo., and a small area in San Juan County, Utah. The purpose of the survey was to obtain geophysical measurements to map the subsurface structure of the rocks and the amount of salinity in the area’s groundwater. This survey was conducted using a low-flying helicopter with time-domain electromagnetic instruments suspended below that measure the electrical resistivity of subsurface areas at different depths and provides detailed, high-resolution data for mapping.
Measurements over the Paradox Valley were obtained in a 3 to 4.5-mile wide by 15-mile long area, from southeast of the Dolores River to the northwestern end of the valley, and in a 2.5-mile wide by 4-mile long area around Buckeye Reservoir, immediately northwest of the Paradox Valley.
The lowest resistivity area was located northeast of the town of Bedrock, Colo., near the area in which brine discharges to the Dolores River. Once all the data from the survey have been fully evaluated, it is anticipated to provide a better understanding of the three-dimensional subsurface distribution of various rock types and the quantity of salinity in the fluids they contain. Results from the analysis may also define locations where underlying salt is being dissolved and the depth to the freshwater-brine interface.
Ultimately, the results from this geophysical survey will support a three-dimensional numerical model of groundwater flow and brine discharge to the Dolores River that the USGS is preparing for the Bureau of Reclamation.
Electromagnetic survey methods have been used effectively in many groundwater studies and particularly for mapping water quality. Because the electrical properties of most rocks are primarily dependent upon the amount of water in the rock, the salinity of the water, and the distribution of the water in the rock, electromagnetic methods are well suited for investigating the subsurface distribution of brine in the Paradox Valley.
The Paradox Valley is formed by a collapsed salt dome. Groundwater in the valley comes into contact with the top of the salt formation, where it becomes nearly saturated with sodium chloride. Saline concentrations have been measured in excess of 250,000 milligrams per liter, by far the most concentrated source of salt in the Colorado River Basin. Groundwater then surfaces in the Dolores River. Reclamation studies show that the river picks up more than 205,000 tons of salt annually as it passes through the Paradox Valley.