A Helicopter Time Domain Survey for Hydrogeologic Mapping, Northwest Paradox Valley, Colorado and Utah
Bruce Smith, Jared Abraham, Kenneth Watts, Andrew Nicholas, George Breit, Benjamin Bloss, and Paul Bedrosian
The Paradox Valley, on the Colorado - Utah border, is one of several collapse features created by dissolution of Pennsylvanian salt in the core of uplifts in the salt anticline region of the Colorado Plateau. The area is also crossed by the Uravan mineral belt which contains significant vanadium-uranium deposits in Jurassic sandstones. The U.S. Geological Survey (USGS) contracted an airborne time domain helicopter electromagnetic (AEM) survey consisting of about 1800 line kilometers with 200 meter line spacing that covered the northwest part of the Paradox Valley. Preliminary maps and observations from the geophysical survey are presented. Historically, salinity of the Dolores River increased substantially as it crossed the valley due to the in-flow of a very high total dissolved solid groundwater (TDS about 256,000 milligrams per liter) derived from dissolution of the underlying salt diapir. In 1996 the U.S. Bureau of Reclamation began operation of a salinity control project, the Paradox Valley Unit, to reduce the salt load to the Dolores River by intercepting part of the salt plume before it enters the river and injecting it into a limestone unit at a depth of about 4.2 to 4.9 kilometers. Part of the objective of the AEM survey is to provide information to define locations where underlying salt is being dissolved and on the depth to the freshwater-brine interface. Preliminary results clearly map the high conductivity groundwater plume in greater detail than has been known from the limited drill holes. Results from the geophysical survey can help constrain a three-dimensional numerical model of groundwater flow and brine discharge to the Dolores River. In addition to the groundwater applications, the AEM data will be evaluated to characterize the electrical properties of the geologic setting (sandstone channels) for sandstone-hosted ore deposits. Interpretation of the AEM data will be important to land use managers in better understanding groundwater quality and resources as well as the geologic framework of the Paradox Valley and Colorado River Basin.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012