HYDROCHEMISTRY OF THE VALLE TOLEDO, VALLES CALDERA, NEW MEXICO

Initial groundwater resource investigations were conducted in the Valle Toledo and other areas near Los Alamos, New Mexico during the late 1940’s and early 1950’s. The investigations included drilling of numerous test holes and installation of several observation and pumping wells. Test holes within the Valle Toledo penetrated an artesian sand and gravel aquifer 122 m thick. Results of the water resource investigations determined that pumping and development of the aquifer would deplete flow to the San Antonio River and potentially obstruct surface-water rights; hence, groundwater resources in the Valle Toledo were not developed. This investigation was conducted to evaluate hydrochemical characteristics and groundwater residence times. Groundwater samples were collected and analyzed for major ions, trace elements, low-level tritium, radiocarbon in dissolved inorganic carbon (DIC), and stable isotopes of hydrogen, oxygen, and carbon in DIC. Groundwater is characterized by a sodium-calcium-bicarbonate composition with total dissolved solids (TDS) ranging from 100 to 130 mg/L. Concentrations of nitrate as nitrogen range from 0.02 to 0.04 mM (0.3 to 0.5 mg/L) and sulfate range from 0.01 to 0.03 mM (1.4 to 2.8 mg/L). Water-rock interactions are not extensive based on the TDS content and chemical composition of the groundwater samples. Compositions of δ²H and δ¹⁸O range from -92.6 to -88.1 ‰ and from -12.9 to -12.2 ‰, respectively, suggesting that snowmelt and rain provide local recharge to the aquifer system. Activities of tritium were less than detection (0.2 tritium units, 0.6 pCi/L), with no modern component of water present. This suggests that recharge to the artesian aquifer is greater than 60 years. Unadjusted radiocarbon activities as fraction modern carbon in dissolved inorganic carbon range from 0.82 to 0.74, corresponding to estimated ages ranging from 1,521 to 2,400 years. The San Antonio River is the principal drainage from the Valle Toledo and is designated as a High Quality Coldwater Aquatic Life water source. Discharge of groundwater from the Valle Toledo supports headwater baseflow to the San Antonio River. Groundwater discharging to the San Antonio River is not susceptible to present-day contamination, as reflected by the average age exceeding 1,500 years.