Untitled DocumentGroundwater and surface water salinization in the El Paso/Juarez Region
Macro Theme Area:
Basin Scale Water Balance [Project ID: B11]
PI:
Barry Hibbs
CO-PI(s):
Chris Eastoe, James Hogan, Fred Phillips
Basin focus:
Rio Grande
Specific area in
basin /
field sites:
Hueco Bolson Aquifer
Summary/Goals: This project is a joint collaborative between SAHRA and CEA-CREST funded by and NSF "Glue Grant". This project is focused on understanding the causes of groundwater and surface water salinization in the region around El Paso, Texas. The Rio Grande and the trans-boundary aquifers of the Las Cruces NM/ El Paso TX/ Ciudad Juarez MX region are the sole water resources for the 2 million people and 178,000 acres of irrigated agriculture of the region. This region, like many in the southwest, is experiencing dramatic population growth and is projected to reach 3.2 million by 2010. Issues of water quantity and water quality are extremely important for this region due to degradation of surface waters in the Rio Grande, overpumping of the local aquifers, and salinization of groundwater resources. The specific research questions are as follows:
1. What are the flowpaths and residence times of water within aquifers of the El Paso region (e.g. Hueco-Tularosa aquifer, Rio Grande - Rio Bravo aquifer, Mesilla Bolson aquifer).
2. How do aquifer flowpaths control the introduction of solutes and ultimately the salinization of groundwaters?
3. What is the interaction between the surface water of the Rio Grande and the associated groundwaters of the Rio Grande aquifer? What role does saline groundwater play in the salinity increases in the Rio Grande?
4. How has climate change affected groundwater recharge in this region? How much of the groundwater in storage today is Pleistocene water?
Activities and outcomes during past year:
Outcomes during the past year (2006) are focused on the publication of results from three areas of research summarized below and limited to additional geochemical analysis of critical samples to support these publications.
Salinization of the Hueco Bolson Aquifer: One approach to identifying the mechanisms of salinization is the combined analysis of hydrogeologic and hydrochemical data to develop a comprehensive conceptual model of a groundwater basin. To demonstrate this, water samples were collected from 33 discrete vertical zone test holes in the Hueco Bolson aquifer. Samples were analyzed for a suite of geochemical tracers and the data evaluated in light of past and present hydrogeology. On the basis of δ2H and δ18O data, we recognized two regional recharge sources, one originating from western mountain-fronts and one through-flow of the adjacent Tularosa aquifer to the north. Chloride concentrations were strongly correlated with lithologic formations and both Cl/Br and 36Cl ratios suggested the primary chloride source is halite dissolution within a specific lithologic unit. In contrast, sulfur isotopes indicated that most sulfate originates from Tularosa basin Permian gypsum sources. These results yielded a more comprehensive conceptual model of the basin, which suggested that well salinization is the result of direct upconing of waters from the Ft. Hancock formation.
Water sources in the Hueco Bolson Aquifer: Stable O and H isotope data for the Hueco Bolson aquifer distinguish four types of water. Two of the types relate to recharge from the Rio Grande, and plot on the present-day Rio Grande evaporation line (RGEL). One corresponds to pre-dam (pre-1916) river water, in contrast with present-day (post-1916) river water in the basin. Pre-dam water is found in the Hueco Bolson aquifer beneath the Rio Grande flood plain and Cd. Juárez, and is mixed with post-dam river water beneath the flood plain. Two other types relate to recharge of local precipitation. Evidence of temporal change of mean d18O and dD of precipitation is present in both types. Recharge from the Franklin and Organ Mountains plots on the Global Meteoric Water Line (GMWL); such water is found along the western side of the Hueco Bolson aquifer, north of the Rio Grande. Water found in the southeastern Hueco Bolson aquifer, north of the Rio Grande plots on an evaporation trend originating on the GMWL near d18O = -8.5‰ and may be related to slow recharge through fine-grained sediment. Pre-dam water should be recognizable by isotope composition, and will provide useful information on groundwater residence times, in many dammed river basins of southwest North America.
GW-SW exchanges along the Rio Grande: The Rio Grande alluvial floodplain aquifer between El Paso/Juarez and Fort Quitman, Texas has been affected by excessive salinity problems. Historically it had been thought that the dilute waters in this location were derived from recharge from arroyos that drained proximal parts of the Hueco Bolson. Instead, our hydrogen and oxygen isotope data and carbon-14 data indicate that these dilute waters were derived from pre-dam infiltration of the Rio Grande. Salinity sources have been investigated in an intensively investigated reach of the floodplain aquifer, extending from 35 to 60 km below El Paso where surface water and groundwater salinity increase markedly. Halide and Cl isotope mixing curves indicate that a saline unit (45 to 80 m bgs) is the primary salinity source, with a chloride age of about 0.75 m.y. to 1.25 m.y. This is consistent with the timing of the final transformation of the local basin (Hueco Bolson) from a topographically closed and undrained basin, to a through-flowing and drained basin. We interpret this unit to be the residual of a paleo-phreatic (salt) playa that was buried beneath the contemporary Rio Grande floodplain after the Rio Grande cut through the El Paso/Juarez narrows ~650 to 750 k.y.
Plans for the upcoming year:
This project has basically ended as of December 2006 as project goals were accomplished and funding has been spent. Remaining issues are focused on publication and dissemination of results, namely through peer review journal articles already submitted with 2-3 more planned and the incorporation of geochemical data into the larger SAHRA database. We also continue to work closely with EPWU to transfer our results as quickly as possible. This has allowed EPWU to modify and update their present groundwater model to improve water resources planning.
Organization Involvement:
El Paso Water Utilities are coordinating well sampling in the Hueco Bolson.
Shared Resources / Joint Activities:
Providing multidepth drilling and sampling of wells throughout the basin. Participating in joint discussion ans analysis of results through yearly meetings
Location: El Paso, TX, USA
URL: N/A
Hawley Geomatters
Organization Involvement:
John Hawley is providing, on a pro bono basis, updated hydrostratigraphy for the Hueco Bolson aquifer
Shared Resources / Joint Activities:
N/A
Location: Albuquerque, NM, USA
URL: N/A
Sacramento Mountains Watershed Restoration Corp.
Organization Involvement:
Providing groundwater and springs samples from the Sacramento mountain area, located north of the Hueco Bolson
