HR: 11:20h
AN: H51G-04    [PDF]
TI: Tracer Tests and Peeper Samplers Used to Quantify In-Stream Nitrate Fluxes and Removal Rates in an Agricultural Watershed
AU: * Ruehl, C
EM: cruehl@es.ucsc.edu
AF: Earth Sciences Dept. University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Fisher, A
EM: afisher@es.ucsc.edu
AF: Earth Sciences Dept. University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Wheat, G
EM: wheat@mbari.org
AF: School of Fisheries and Ocean Sciences University of Alaska, Fairbanks, 245 O'Neill Building, Fairbanks, AK 99775 United States
AU: Los Huertos, M
EM: marcos@cats.ucsc.edu
AF: Environmental Studies Dept. University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Shennan, C
EM: cshennan@cats.ucsc.edu
AF: Environmental Studies Dept. University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Hatch, C
EM: chatch@es.ucsc.edu
AF: Earth Sciences Dept. University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Hatch, C
EM: chatch@es.ucsc.edu
AF: Environmental Studies Dept. University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AB: The interface between surface water and ground water in riparian zones influences the quality and quantity of waters moving between linked reservoirs. Microbial respiration along this interface, for example, can act as a sink for dissolved nitrate, organic carbon, oxygen, and sulfate. We are investigating the removal of nitrate in a river draining the Pajaro Valley, a coastal, agriculturally-developed watershed that features elevated nitrate levels and groundwater pumping in excess of recharge (overdrafting). During summer base flow (discharge $\sim$ 0.3 m$^{3}$/sec), when no significant precipitation has fallen in the valley for 2-3 months, the Pajaro River consistently loses 0.1-0.2 m$^{3}$/sec of its discharge to the underlying alluvial aquifer along a 10-km reach east of Watsonville, CA. At the same time, nitrate concentrations (which are typically in excess of the EPA MCL), decrease by $\sim$30% along this reach. An observed a decrease in nitratre/chloride ratios along this reach suggests that biologic uptake (assimilative or dissimilative), not dilution, is primarily responsible for the observed decrease in nitrate concentrations. Sulfate/chloride ratios also decrease, and this along with pore water profiles obtained from streambed (peeper) samplers, suggests that dissimilitary nitrate reduction (denitrification) is primarily responsible for a significant fraction of the observed nitrate decrease. We performed a series of tracer experiments to quantify discharge, storage parameters, travel time, and nitrate and sulfate removal rates in the river. We used breakthrough curves obtained from subsurface sampling locations in close hydrologic contact with the stream to quantify subsurface nitrate removal rates not limited by diffusion.
DE: 1803 Anthropogenic effects
DE: 1806 Chemistry of fresh water
DE: 1836 Hydrologic budget (1655)
DE: 1871 Surface water quality
DE: 4805 Biogeochemical cycles (1615)
SC: Hydrology [H]
MN: 2003 Fall Meeting