Using geochemistry and gravity data to pinpoint sources of salinity in the Rio Grande and fault networks of the Mesilla Basin.

摘要

Fresh water resources are scarce in the American Southwest, especially in the semi-arid to arid regions of El Paso, TEXAS. The Rio Grande and Mesilla Basin are both major suppliers of freshwater for the city of El Paso and its cropland along the river. Maintaining a steady source of freshwater is critical in sustaining the growing demands of irrigation and population in the area.;For decades scientists have been trying to understand the increasing salinity content from ~40 mg/L at the headwaters of the Rio Grande in Colorado to ~1500 mg/L in El Paso, TEXAS. The increasing salinity of the Rio Grande has not only limited its use as city drinking water but has also had an effect on its use as irrigation water. In this region, local farmers use a flooding technique to irrigate their crop fields, including pecan, cotton, and alfalfa fields. The use of highly saline water causes a multitude of problems such as increasing soil salinity and reduction of crop productivity. Multiple salinity sources from both natural and anthropogenic activities have been suggested for the Rio Grande. More recently, technological advances have allowed scientists to approach this issue from a multidisciplinary standpoint. This investigation focuses on locating sources of salinity in the Rio Grande and groundwater and assessing their relationships to the fault networks within the Mesilla Basin (Late Cenozoic) using a combination of water geochemistry and geophysical data.;Previous studies suggest multiple sources of salinity contributing to the overall increased salt content within the Rio Grande. This study assesses two main sources: 1) upwelling of saline groundwater through subsurface faults or fractures, and 2) runoff from the agricultural fields. The southern end of the Mesilla Basin in El Paso provides an ideal location to study the natural upwelling of groundwater along the faults within the basin. Here, previous studies using precision gravity data have shown an intricate array of NS and EW trending fault networks throughout the Mesilla Basin in western El Paso. This study combines these historical datasets with new geophysical investigations and geochemical measurements to understand the distribution of the fault systems and infer groundwater flow paths throughout the region.;Modeling the flow system of the Mesilla basin and its relation to the Rio Grande has provided information on the potential salt influxes from the fault networks of the Mesilla Basin. Most of the salinity observed in the groundwater is due to the dissolution of evaporites in the middle and lower Santa Fe units. In addition, we observed a thermal upwelling component that is supplying the Mesilla basin with some of its higher Chloride/Bromide ratios. Surface water runoff has also contributed to the overall salinity by introducing water characterized by higher Calcium/Sodium ratios coming from the Ordovician limestone rocks exposed at the Franklin Mountains' surface. Furthermore, this study has identified potential areas of salinity introduction into the shallow Mesilla Basin and eventually, the Rio Grande through artificial pumping and irrigation.;The addition of two new gravity surveys has updated the UTEP Database and has established a more confident interpretation of fault locations within the Mesilla Basin. In total, five fault locations were mapped. Three of them were updated locations from previous studies and two of them were new interpretations. Horizontal Gradient Magnitude mapping (HGM) has confirmed the structural complexity of the basin which coincides with the range of chemistry seen in the groundwater.