Evaluating Potential Groundwater Resources on State Lands in El Paso County. Texas Using Airborne Geophysics

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2002

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Abstract

We employed ground-based and high-resolution airborne geophysical methods in combination with an analysis of available water-well data and geologic information to identify potentially favorable groundwater resources in a 372 km^2 area within the Hueco Bolson in eastern El Paso County, Texas. Analysis of water-well data from the survey area demonstrates that:

  • Good quality groundwater (less than 1,000 mg/L total dissolved solids) is present in places.
  • Water-level depths on the floor of the basin range from 90 to 130 m and are strongly correlated with surface elevation.
  • Most groundwater sampled in this area is classified as fresh or slightly saline.
  • Water salinity increases with depth.
  • Reported specific capacities of wells range from 0.5 to 36 gal/min/ft. Most existing wells draw from depths of 110 to 180 m, which is within the exploration range of the airborne time-domain electromagnetic (TDEM) instrument.

Ground-based geophysical studies conducted before the airborne survey demonstrated that:

  • Conditions were favorable for the acquisition of good quality electromagnetic (EM) data.
  • Changes in electrical conductivity with depth are likely to be related to changes in water saturation, host sediment type, and water quality.
  • Increases in conductivity detected at depths below the water table are likely to be caused by increasing groundwater salinity with depth.

An airborne geophysical survey flown by Fugro Airborne Surveys in August 2001 acquired TDEM and passive magnetic field data at 64,773 locations along north–south flight lines spaced at 400-m intervals. Magnetic field data correlated well with mapped fault locations. The Megatem II airborne TDEM system used in this survey achieved exploration depths that exceeded the depth to water over more than 90 percent of the survey area. Exploration depths exceeded 280 m over more than half the survey area. Airborne TDEM data were processed to produce horizontal slices that depict apparent conductivity changes at 10-m depth intervals across the area. Above the zone of water saturation, geologic features such as faults are prominent in the data. At deeper depths below the water table at about 100 m, apparent conductivity values correlate reasonably well with existing groundwater quality data for water wells with accurate locations that are near flight lines.

Largely on the basis of airborne survey results, we identified two areas of low conductivity below the water table that are favorable locations for groundwater exploration on State-owned land. The largest, with an area of 23 km^2, includes a State tract (T&P Railroad, block 78T2, section 18) located southwest of Montana Vista at depths of 110 to 140 m. The other covers about 8 km^2 in the northwest corner of the survey area at depths of 100 to 150 m. Three State tracts (T&P Railroad, block 79T2, sections 15, 16, and 21) are included within this area. Although these areas are favorable sites for groundwater exploration, geophysical and available water-well data suggest that these resources are limited and that local, high-volume production will likely be accompanied by significant water-level decline and salinity increase.

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