Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico
Tuesday, February 27, 2018: 2:20 p.m.
Andrew Robertson
,
New Mexico Water Science Center, U.S. Geological Survey, Albuquerque, NM
Kenneth Carroll
,
Plant & Environmental Sciences Department, New Mexico State University, Las Cruces, NM
Chris Kubicki
,
Plant & Environmental Sciences Department, New Mexico State University, Las Cruces, NM
Roland Purtchert
,
Climate and Environmental Physics, University of Bern, Bern, Switzerland
The Mesilla Basin/Conejos-Médanos aquifer system, extending from southern New Mexico to Chihuahua, Mexico, is a priority transboundary aquifer under the 2006 United States-Mexico Transboundary Aquifer Assessment Act. Declining water levels, deteriorating water quality, and increasing groundwater use by municipal, industrial, and agricultural users raise concerns about long-term aquifer sustainability. Relative contributions of present-day and “paleo” recharge to sustainable fresh groundwater yields has not been determined and evidence suggests that a large source of salinity at the distal end of the Mesilla Basin is saline discharge from deep groundwater flow. The magnitude and distribution of those deep saline flow paths are not determined.
The contribution of deep groundwater to discharge and salinity in the shallow groundwater and surface water systems of the Mesilla Basin will be determined by collecting groundwater samples and analyzing for geochemical and isotopic tracers, including the radioisotopes of argon and krypton. Analytes include major ions, trace elements, the stable isotopes of water, strontium and boron isotopes, uranium isotopes, the carbon isotopes of dissolved inorganic carbon, noble gas concentrations and helium isotope ratios. Dissolved gases are extracted and captured from groundwater wells using membrane contactors in a process known as ultra-trace sampling. Gas samples are analyzed for radioisotope ratios of krypton and argon by ATTA or low-level counting.
Effectiveness of the ultra-trace sampling device and method was evaluated by comparing results of tritium concentrations to the krypton-85 content. Good agreement between the analyses, especially in samples with undetectable tritium, indicates that the ultra-trace procedure is effective and confirms that introduction of atmospheric air has not occurred. The geochemistry data indicate a complex system of geochemical endmembers, and mixing between these endmembers. Ongoing work seeks to better constrain groundwater ages and mixing models through the coupled use of conventional aqueous geochemical and isotopic analysis and the ultra-trace constituents.
Andrew Robertson, New Mexico Water Science Center, U.S. Geological Survey, Albuquerque, NM
Andrew Robertson is a hydrologist and a unit chief for the hydrogeology and geochemistry program area at the New Mexico Water Science Center. Andrew received a M.S. degree in Water Resources from the University of New Mexico. Since joining the USGS in in 2008, Andrew's work has been focused on using geochemical and isotopic tracers to answer questions relating to groundwater hydrology and contaminant fate and transport.
Kenneth Carroll, Plant & Environmental Sciences Department, New Mexico State University, Las Cruces, NM
My research interests include evaluating the fundamental physical, chemical, and biological processes that impact the transport and fate of chemicals in environmental systems. I use a multidisciplinary approach to research by integrating aspects from several scientific fields to investigate processes at a multitude of scales from the molecular scale to the basin scale. I believe a multidisciplinary approach is required for the evaluation of coupled processes that influence hydrobiogeochemical cycles. I also extend this multidisciplinary approach to my teaching.
Chris Kubicki, Plant & Environmental Sciences Department, New Mexico State University, Las Cruces, NM
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Roland Purtchert, Climate and Environmental Physics, University of Bern, Bern, Switzerland
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