f A Helicopter Time Domain Survey of a Portion of the Paradox Valley, Colorado and Utah

The U.S. Geological Survey (USGS) contracted a time domain helicopter electromagnetic (TDHEM) survey consisting of about 1800 line kilometers with 200 meter line spacing that covered the northwest part of the Paradox Valley (Figure 1). The quality control for the airborne survey incorporated ground based time domain electromagnetic (TEM) measurements made just prior to the survey. The TEM survey consisted of six sites, three of which were used in an initial comparison with the TDHEM along a test flight line. Preliminary maps and observations from the geophysical survey are presented. 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 valley name is derived from the direction of the flow of the Dolores River which crosses perpendicular to the long axis of the valley. Historically, salinity of the Dolores River increased substantially as it crossed the valley from south to north due to the in-flow of a very high total dissolved solid groundwater (TDS about 256,000 milligrams per liter). The brine is derived from dissolution of the underlying salt diapir. Salinity of the Dolores River is a critical issue because it is a tributary of the Colorado River, which is under strict salinity control from several Federal and State laws and compacts. 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. Under normal operation, the Paradox Unit averages about 14 to 14.5 million gallons of brine injected per month. This results in the disposal of about 128 thousand tons of salt per year. Part of the objective of the TDHEM 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.