An integrated geological and geophysical study of the fresh and brackish water boundary in the Hueco Bolson, West Texas.

摘要

The Hueco Bolson in West Texas is located in the Rio Grande rift basin and provides water to El Paso County, Texas and Juarez, Mexico. El Paso Water Utilities (EPWU) drilled many wells up to 1000 ft (305) m into this aquifer, which provide 40% of El Paso's water supply. Stratigraphy and sedimentation in active extensional basins have received little study because they are usually buried and this research helped document these processes. The study area within the West Texas region of the Hueco Bolson Aquifer (HBA) lies within a transition zone between the fresh and brackish water sections of the aquifer. The boundary between fresh and brackish water in this study area (as indicated by previous studies) is more vertical and abrupt than typical fresh and brackish water boundaries which are usually gradual and have a lenticular shape.;A total of twenty-six wells drilled by EPWU were analyzed (grain-size and well log analysis) for this study; three wells in the MW series were drilled in 2003, seven wells in the 500 series drilled in 2006 and sixteen wells drilled in 2005. An increase in salinity, measured in total dissolved solids (TDS), is observed from northwest to southeast (from 374 to 7410 TDS) across the study area. The 600 series wells were drilled to provide brackish water to a desalination plant to increase the longevity of the aquifer and to create a trough in the ground water to help prevent incursion of brackish water from the east into the fresh water section of the aquifer.;Five wells (601, 605, 610, 615, and 509A) located in the fresh and brackish water sections of the HBA were chosen for grain-size analysis. Grain-size distribution for wells 601, 605, 610, 615, and 509A were analyzed using sieves for particles larger than 1 mm and a Malvern particle analyzer for particles smaller than 1 mm. The wells penetrate numerous clay and silt beds. These beds can be correlated between wells and help to vertically subdivide the reservoir. Four depositional environments were identified after grain-size analysis: playa lake, playa margin, alluvial fan, and fluvial environments. There are abrupt changes in the energy in the environments from low energy playa lake and play margin deposits to high energy alluvial fan and fluvial deposits.;Well log correlations were created in Kingdom Suites software to correlate from well 601 to 509A. Correlations show the alluvial fan deposits thin heading into the basin, while the playa lake deposits begin to appear at shallower depths out in the basin. There is an increase in the volume of clay (13.9% to 42.9%) from well 601 to 509A (northwest to southeast). Seven faults were identified during well log analysis, two of which have been previously mapped by Collins and Raney (2000).;A gravity survey was conducted to model the basin and identify the faults seen in cross-section. Two of previous faults mapped by Collins and Raney (2000) located in the study area were extended farther south into the urbanized area of the city of El Paso. The five faults identified in this study were constrained and mapped. One of the five faults identified in this study extends about 12.4 miles (21 km) N-S and appears to belong to a series of step-over faults heading into the basin.;There are a series of three faults (one small and two extending about 21 km N-S across the study area) in the transition zone between the fresh water and brackish water sections of the HBA with increasing amounts of clay in the brackish water section. The combination of clay (impeding the flow of water) and barrier faults are creating a natural seal helping to prevent the incursion of brackish water into the fresh water section of the aquifer.