HR: 11:50h
AN: H22B-06    [Abstracts]
TI: Lithogenic vs Biogenic Stream Water Chemistry: Following the Solute Flush
AU: * Bain, D J
EM: djbain@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. / M.S. 420, Menlo Park, CA 94025 United States
AU: Anderson, S
EM: suzanne.anderson@colorado.edu
AF: University of Colorado at Boulder, 1560 30th Street, Campus Box 450, Boulder, CO 80309 United States
AU: Bullen, T
EM: tdbullen@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. / M.S. 420, Menlo Park, CA 94025 United States
AU: Fitzpatrick, J
EM: jfitzpat@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. / M.S. 420, Menlo Park, CA 94025 United States
AU: Schulz, M
EM: mschulz@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. / M.S. 420, Menlo Park, CA 94025 United States
AU: Vivit, D
EM: dvvivit@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. / M.S. 420, Menlo Park, CA 94025 United States
AU: White, A
EM: afwhite@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. / M.S. 420, Menlo Park, CA 94025 United States
AB: Mediterranean hydrologic systems are driven by strong intra-annual variation in precipitation. Summer drought is followed by a pronounced solute flush at the beginning of the wet season. Solutes accumulate near the soil surface during dry periods via evapoconcentration and dry deposition. In a set of nested watersheds draining the Santa Cruz, California marine terraces, a differentiation between shallow soil water with biogenic solutes (enriched via evapotranspiration and biologic nutrient cycling) and deep soil water with lithogenic solutes (imprinted by chemical weathering) allows interpretation of flow pathways to the stream. The shallow soil waters are more concentrated than deep soil waters at the beginning of the wet season. Stream chemistry is a mixture of lithogenic deep soil water and biogenic shallow soil water; we expect the lithogenic component to increase downstream. However, the composition of the water along a downstream transect shows no clear shift to lithogenic compositions downstream, especially in the early wet season. The lithogenic water influence may be minimal as most flow paths are lateral across a thick argillic horizon and rarely encounter lithogenic zones. However, the continued influence of biogenic solutes seems to result from movement of evapoconcentrated water through the flow system. Simultaneous collection of groundwater and soil water along the transect shows the slow movement of flushed solutes through the system. Soil waters generally are most concentrated during the first precipitation events and become increasingly diluted throughout the remainder of the wet season. Immediately following the flushing of shallow soils, a concentration peak is observed in perched groundwater near the drainage divide. A similar peak appears in perched groundwater sequentially downstream, finally appearing in groundwater near the base of the catchment after several months. This pattern of flushed solutes in perched groundwaters may result from the movement of flushed solutes along progressively longer flowpaths or hyporheic exchange between surface water and solute rich ground waters along the transect. In both cases, the flushed-solute enriched waters moving through the groundwater system contribute to the continued biogenic signature of surface waters.
DE: 0483 Riparian systems (0744, 1856)
DE: 1065 Major and trace element geochemistry
DE: 1804 Catchment
DE: 1806 Chemistry of fresh water
DE: 1830 Groundwater/surface water interaction
SC: Hydrology [H]
MN: Fall Meeting 2005