GROUNDWATER CHARACTERISTICS AND EM-34 SURVEY OF THE LOST DRAW-BROWNFIELD BASIN AREA, TERRY COUNTY, TEXAS

The Lost Draw-Brownfield Basin area of Terry County, Texas consistently supports a near-surface water table despite up-gradient water level declines. Irrigation wells neighboring the area have seen deterioration in water quality that may be related to migration of brackish water when pumping demands are high. Specific gravity, conductivity and chloride determinations were carried out on lake water and groundwater from the study area. Strong linear relationships were found between conductivity and specific gravity of groundwater and lake water samples (y=2 x 106 x - 2 x 106, R2=0.9667) and between log transformed conductivity and log chloride concentrations (y=0.7946 x + 1.5128, R2=0.9475). Comparison of current measurements and data from a previous study suggests that the freshwater/saline water interface has moved eastward approximately a mile in the last twenty years. Trends at individual locations are somewhat obscured by uncertainty or inability in duplicating sampling depths from the earlier study. Chloride appears to be increasing at locations toward the perimeter of the basin, and decreasing at wells in the central basin area. These effects follow an interval of extremely shallow groundwater levels in the central basin area and the cessation of industrial and irrigation groundwater withdrawals from the central basin during which irrigation withdrawals along the western margin of the basin have continued. A Geonics EM-34 device was subsequently deployed using the full range of coil configurations to measure the conductivity of earth materials over a 5.5 x 9.0 mi. area, taking readings every half a mile. The EM-34 results closely mirror indicators of groundwater salinity.