Abstract

Ground-water samples were collected from 56 locations throughout the Española Basin and analyzed for general chemistry (major ions and trace elements), carbon isotopes (δ¹³C and ¹⁴C activity) in dissolved inorganic carbon, noble gases (He, Ne, Ar, Kr, Xe, and ³He/⁴He ratio), and tritium. Temperature profiles were measured at six locations in the southeastern part of the basin. Temperature profiles suggest that ground water generally becomes warmer with distance from the mountains and that most ground-water flow occurs at depths <250 m below ground surface. The two dominant water types in the basin are Ca/CO₃+HCO₃ and Na/CO₃+HCO₃, followed by mixed-cation/CO₃+HCO₃. Waters generally evolve from Ca/CO₃+HCO₃ to Na/CO₃+HCO₃ with increasing residence time through Ca-Na cation exchange with clay minerals. Basin ground water can be divided into four hydrochemical zones based on chemical and isotopic composition: West, Southeast, Northeast, and Central Deep. Hydrochemical zone boundaries are roughly correlated with contacts between geologic units or lithosome transitions within the Tesuque Formation.

Geochemical mass-transfer modeling was performed using NETPATH and ¹⁴C ages were adjusted accordingly. Isotopic input parameters were varied within reasonable limits to assess uncertainty in the adjusted ¹⁴C ages. For each sample, a preferred adjusted age was selected from multiple possible adjusted ages based primarily on the fit between measured and modeled δ¹³C values. The range of possible age adjustments for most samples is about 6,000 years or less, indicating that the preferred adjusted age for most samples has a total range of uncertainty of <6,000 years. Preferred adjusted ages range from 0 to 35,400 years. First-order trends in the age distribution include older ages generally occurring farther from rivers on the east side of the basin and farther from the mountains, consistent with both mountain-front recharge and recharge on the basin floor in the form of stream-loss and arroyo recharge. Ages also increase with depth in the Southeast zone, the only area where discrete-depth samples could be collected.

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First posted January 29, 2009