HR: 13:45h
AN: H43D-02    [Abstracts]
TI: Using Geochemical Tracers to Quantify Baseflow Inputs to the San Pedro River, Southeast Arizona
AU: * Baillie, M N
EM: baillie@hwr.arizona.edu
AF: Department of Hydrology and Water Resources, University of Arizona, John W. Harshbarger Building P.O. Box 210011, Tucson, AZ 85721
AU: Hogan, J F
EM: jhogan@hwr.arizona.edu
AF: Department of Hydrology and Water Resources, University of Arizona, John W. Harshbarger Building P.O. Box 210011, Tucson, AZ 85721
AU: Ekwurzel, B
AF: Department of Hydrology and Water Resources, University of Arizona, John W. Harshbarger Building P.O. Box 210011, Tucson, AZ 85721
AU: Ekwurzel, B
AF: Department of Geosciences, Universit of Arizona,
AU: Wahi, A K
AF: Department of Hydrology and Water Resources, University of Arizona, John W. Harshbarger Building P.O. Box 210011, Tucson, AZ 85721
AU: Eastoe, C J
AF: Department of Geosciences, Universit of Arizona,
AB: Groundwater managers in the semiarid Southwest U.S. must balance the demands of growing populations against the needs of rare riparian ecosystems. The Upper San Pedro River Basin in Southeastern Arizona provides water for the rapidly growing area of Fort Huachuca, Sierra Vista, and surrounding communities. This basin also supports the riparian area by providing baseflow to the San Pedro River and shallow groundwater to phreatic riparian vegetation. Before water managers can properly plan for sustenance of the riparian area, they require answers about the inputs of baseflow into the river. What is the temporal and spatial variability of recharge and discharge? What are the relative inputs of different sources (i.e. recharge of monsoon runoff and inflow of basin groundwater) into the riparian aquifer? We have used a suite of geochemical tracers to address these questions. Most precipitation in this region falls during the summer monsoons and winter frontal storms. Because these events have different source areas, their respective stable isotopic signatures differ. We use these isotopic compositions as end members in mixing equations in order to determine the dominant season for recharge. Groundwater in the basin has a narrow range of stable isotope compositions, varying from 62 to 72% winter precipitation. The basin isotopic composition is very similar to water in the mountain front and block, indicating that recharge from these areas is the dominant source for basin groundwater. Basin groundwater residence times, determined using the radioactive isotope carbon-14, are on the order of 10,000 years or more, indicating a low recharge rate. In contrast, riparian groundwater has a wide variation of isotopic values, indicating that this water is a mixture of basin groundwater and monsoon runoff, varying from 20 to 90% basin groundwater. The dominance of basin groundwater or monsoon runoff in different areas of the river correlates well with independent classification of river reaches as either gaining or losing, respectively. Tritium age data indicate that some riparian groundwater, especially in losing reaches, has been recharged recently (less than 40 years). Baseflow varies from 40 to 100% monsoon precipitation, and is closer to basin groundwater in gaining reaches. These results help can water managers better understand the water resources of the basin, and increase their ability to effectively manage the riparian area. For example, the demonstrated importance of monsoon recharge to the riparian aquifer indicates that the construction of retention basins to capture monsoon runoff could be one effective strategy to increase recharge into the basin.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1040 Isotopic composition/chemistry
DE: 1800 HYDROLOGY
DE: 1829 Groundwater hydrology
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2005 Joint Assembly