The Rio Grande is well-studied as a regionally important water source, but the small, poorly characterized springs that surface within the Rio Grande rift are also a vital resource. Several of these springs have water chemistries that suggest a mixing of larger volume meteoric recharge with small volume, deeply-sourced fluids. It has been hypothesized that deep-seated faults within the rift provide conduits for the ascent of deeply-derived fluids, while others have proposed that upwelling sedimentary basin brines represent a significant salinity input to the modern river. This study provided the first hydrochemical data on a comprehensive suite of springs and wells in the Sevilleta National Wildlife Refuge, and tested and refined existing models for water quality in the rift using hydrochemistry, microbial characterization, and geochemical modeling along a series of transects.
The purpose of this study was to examine the geochemical inputs which characterized a small number of springs and wells within a hydrologic system exhibiting minimal anthropogenic influence on the Sevilleta National Wildlife Refuge (SNWR). Because such analyses had not been performed previously in the study area, there was no existing information on what sources wildlife in the SNWR used for drinking or what geochemical compounds were percolating through the subsurface as groundwater moved toward the Rio Grand.
Sampling Design: The sampling region encompassed the Sevilleta National Wildlife Refuge. Major springs were sampled once a quarter. Wells were sampled once in the summer 2008 field season.
Experimental Design: All water samples (including replicates) were analyzed for major ions. Each sample site (no replicates) was analyzed for stable isotopes of oxygen and hydrogen and trace elements. A select number of samples was analyzed for stable isotopes of carbon and 87Sr/86Sr ratios.
Field Methods: In the field, water samples were collected in at least two 125mL pre-cleaned or sterile high density polyethylene bottles. Prior to collection, each bottle was pre-contaminated three times with the sample water and emptied downstream from the locality. To minimize degassing, all unfiltered, unacidified samples were collected with zero headspace, either by submerging the bottle and capping under water or filling the bottle to overflowing and then capping.
Samples for inductively coupled plasma (ICP) analysis were filtered through a 0.45µm membrane attached to the sampling syringe and acidified with 16N HNO3. pH, conductivity, and temperature were measured in the field with an Oakton pH/CON 300 Series pH/conductivity/TDS/ºC meter.
Well water samples were collected in three 125mL and two 30mL high-density polyethylene (HDPE) bottles. Wells were purged of up to three well volumes of water to ensure groundwater, and not borewater, was sampled. Well purge times were calculated from the USGS Techniques of Water Resources Investigation (TWRI) Book 9, chapter 4 A.
Prior to collection, each bottle was pre-contaminated three times with groundwater and emptied outside of the well. Because of well design, some samples were collected from wells in an acid-washed 500mL HDPE bottle attached to an extension to reach the pour point, then used to fill other sample bottles. In these cases, the 500mL bottle was also pre-contaminated three times with well water. To minimize degassing, unfiltered, unacidified samples were collected with zero headspace by filling the bottle to overflowing and then capping. The same preservation methods used for surface samples were followed for well samples.
Samples collected for DNA extraction and microbial community analysis were filtered through 0.2 micrometer sterile discs. Water was filtered until the filter became clogged, generally after filtering between 60 and 200 ml, depending on the suspended load of the spring water. The filter was subsequently preserved in a sucrose lysis buffer (SLB) (20 mM EDTA, 200 mM NaCl, 0.75 M sucrose, 50 mM Tris-HCl, pH 9.0) and refrigerated at -80ºC until DNA extraction. Waters collected for delta 13C analysis were unfiltered and preserved in the lab with HgCl2 following the methods of Torres et al., 2005.
Laboratory Procedures: In the laboratory, samples were refrigerated at 4°C in the dark until analysis. All waters were analyzed for major cations and metals on a Perkin Elemer ICP-OES and X series ICP-MS, respectively. Anions were measured on a Dionex DX-500 Ion Chromatograph and alkalinity was determined by End Point Titration. Stable isotopes of hydrogen, oxygen, and carbon were analyzed for select samples. Delta 18O and delta 13C were determined with a Finnigan MAT Delta Plus Mass Spectrometer, while delta D was analyzed with a Finnigan MAT 252 Mass Spectrometer. 87Sr/86Sr ratios were determined with a Neptune Multicollector ICP-MS.
QA/QC: Water sample charge balances were checked for analytical accuracy. All concentrations were measured after calibrating instruments with at least 3 standards.