HR: 09:30h
AN: H51E-06 [Abstracts]
TI: Denitrification in the Sediments of a Nitrate-rich River Draining an Agricultural Watershed
AU: * Ruehl, C
EM: cruehl@es.ucsc.edi
AF: Earth Sciences Dept.
UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Fisher, A
EM: afisher@es.ucsc.edu
AF: Earth Sciences Dept.
UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Wheat, G
EM: wheat@mbari.org
AF: Global Undersea Research Unit, P.O. Box 475, Moss Landing, CA 95039
AU: Hatch, C
EM: chatch@es.ucsc.edu
AF: Earth Sciences Dept.
UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Wankel, S
EM: sdwankel@usgs.gov
AF: USGS - Menlo Park, 345 Middlefield Road, Menlo Park, CA 94025
AU: Kendall, C
EM: ckendall@usgs.gov
AF: USGS - Menlo Park, 345 Middlefield Road, Menlo Park, CA 94025
AU: Kim, S
EM: skim@es.ucsc.edu
AF: Earth Sciences Dept.
UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Los Huertos, M
EM: marcos@ucsc.edu
AF: Environmental Studies Dept.
UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AU: Shennan, C
EM: cshennan@ucsc.edu
AF: Environmental Studies Dept.
UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064
AB:
The interface between surface water and ground water in stream systems has been shown to exert strong control over the
quality and quantity of waters at the surface and recharging to aquifers. We are investigating the removal of nitrate in a
river draining the Pajaro Valley, a coastal, agriculturally-rich watershed that features elevated nitrate levels and
groundwater pumping in excess of recharge (overdrafting). During summer base flow (discharge $\sim$0.3 m$^{3}$/sec), when no
significant precipitation has fallen in the basin for 2 to 3 months, the Pajaro River consistently loses 0.1 to 0.2
m$^{3}$/sec of its discharge to the underlying alluvial aquifer between Chittenden Road and Murphy's Crossing Rd, a 10 km
stretch east of Watsonville, CA. Nitrate concentrations during base flow, which are typically in excess of the EPA limit for
drinking water (10 mg NO$_{3}$-N/L), decrease by $\sim$30% along this stretch. Molar DIN:TDP ratios during summer baseflow
are typically 300-400, suggesting strong phosphorus limitation (relative to nitrogen) to assimilative uptake in this system.
We observed zones of nitrate depletion (i.e. nitrate concentrations lower than, but other solutes equal to, stream
concentrations) at various points in the subsurface of the stream during low-flow conditions. Nitrate in these zones was
isotopically heavier ($\epsilon$=-21$\permil$ and -10$\permil$ for $^{15}$N and $^{18}$O, respectively), suggesting that
denitrification (and/or DNRA) is the mechanism of nitrate removal. As nitrate concentrations were reduced downstream,
residual nitrate was not fractionated as strongly as in the subsurface ($\epsilon$=-9$\permil$), which could be caused by the
incomplete return of denitrified (and thus heavier) nitrate to the main channel after seeping into the streambed.
$\delta$$^{18}$O values in surface water increased a greater amount over the stretch later in the water year
($\epsilon$=-16$\permil$ in October, compared to -4$\permil$ in July), suggesting nitrification might act as a nitrate source
to this reach at this time. Thus, true denitrification rates may be greater than estimates based on observed nitrate removal
in this reach.
DE: 4805 Biogeochemical cycles (1615)
DE: 1803 Anthropogenic effects
DE: 1806 Chemistry of fresh water
DE: 1836 Hydrologic budget (1655)
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting