HR: 0830h
AN: H51C-01    [Abstracts]
TI: Quantifying Mountain Front Recharge Using Isotopic Tracers
AU: * Wahi, A K
EM: wahi@hwr.arizona.edu
AF: Hydrology and Water Resources and SAHRA, University of Arizona, Tucson, AZ 85721
AU: Ekwurzel, B
AF: Hydrology and Water Resources and SAHRA, University of Arizona, Tucson, AZ 85721
AU: Ekwurzel, B
AF: Geosciences, University of Arizona, Tucson, AZ 85721
AU: Hogan, J F
AF: Hydrology and Water Resources and SAHRA, University of Arizona, Tucson, AZ 85721
AU: Eastoe, C J
AF: Geosciences, University of Arizona, Tucson, AZ 85721
AU: Baillie, M N
AF: Hydrology and Water Resources and SAHRA, University of Arizona, Tucson, AZ 85721
AB: To improve our conceptual and quantitative understanding of mountain-front/mountain-block recharge (MFR) associated with the Huachuca Mountains of the Upper San Pedro River Basin in Arizona, we employed a suite of geochemical measurements including isotopic tracers and noble gases. MFR is frequently the dominant source of recharge to alluvial basins in the semiarid Basin and Range province. It consists of mountain runoff that infiltrates at the mountain front (mountain-front recharge), and percolation through the mountain bedrock that reaches the basin via the movement of deep groundwater (mountain-block recharge). The rate of MFR can be estimated from a water balance, a Darcy's law analysis, or inverse modeling of groundwater processes. Despite the large volume of research on water resources in the basin and the critical importance of MFR to the water budget, the best estimates of MFR obtained using these methods may have errors as large as 100%. We find that geochemical tracers address mechanistic questions regarding recharge seasonality, location, and rates as well as addressing groundwater flowpaths and residence times. The gradient of stable isotopes of hydrogen and oxygen in groundwater with elevation mirrors that of regional precipitation, providing a constraint on the location and seasonality of recharge. Stable isotopic signatures indicate that MFR is dominated by winter precipitation but has 1/3 or more contribution from monsoon precipitation. Detectable tritium and ¹⁴C values greater than 100 pMC for springs, shallow groundwater in mountain canyons, and from wells along the mountain front indicate decade-scale residence times. Away from the mountain front ¹⁴C values rapidly decrease, reaching 12.3±0.2 pMC near the river. This suggests total basin residence times greater than 10,000 years, consistent with past measurements. Ongoing analysis of noble gas concentrations will provide an indication of recharge conditions. The solubility of noble gases in water depends on temperature and pressure; thus, noble gas concentrations provide a means to distinguish water samples recharged at different elevations.
DE: 1040 Isotopic composition/chemistry
DE: 1806 Chemistry of fresh water
DE: 1829 Groundwater hydrology
DE: 1836 Hydrologic budget (1655)
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2005 Joint Assembly