HR: 0830h
AN: H41D-1040    [PDF]
TI: Effects of Benthic Flux on Dissolved-Mercury Distributions in Camp Far West Reservoir, California
AU: * Kuwabara, J S
EM: kuwabara@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Alpers, C N
AF: U.S. Geological Survey, Placer Hall 6000 J Street, Sacramento, CA 95819
AU: Marvin-Dipasquale, M
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Topping, B R
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Carter, J L
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Stewart, A R
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Fend, S V
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Parchaso, F
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Moon, G E
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Krabbenhoft, D P
AF: U.S. Geological Survey, 8505 Research Way, Middleton, WI 53562
AU: Agee, J L
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Kieu, L H
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AB: Benthic flux measurements of dissolved mercury species were made on sediment cores from Camp Far West Reservoir, a reservoir in which elevated mercury levels in sport fish had previously been documented. The reservoir is located downstream of historic hydraulic placer-gold mining and ore processing activities in the Bear River watershed of the northern Sierra Nevada. Field and laboratory studies were conducted in April and November of 2002 (one of the driest years on record for the area) to provide the first direct measurements of the benthic flux of dissolved mercury species between bottom sediment and the water column at three locations within the reservoir. Ancillary data, including nutrient and ligand fluxes, and benthic biological characterizations, were also gathered to provide a water-quality framework with which to interpret the mercury results. The following are the major observations made from interdependent physical, biological, and chemical data. Bottom water total mercury (Hg$_{T}$) concentrations ranged from 2.94 to 18.3 pM. Dissolved Hg$_{T}$ benthic fluxes were generally higher in April than in November 2002 (based on site averages from replicate cores). Hg$_{T}$ fluxes of 36.3 and -28.4 pmoles-m$^{-2}$-h$^{-1}$ were measured in cores from the deep site which was suboxic in November, contrasting with positive fluxes of 306 and 272 pmoles-m$^{-2}$-h$^{-1}$ at that site in April 2002 when the bottom water was oxic. All six measurements of Hg$_{T}$ flux in April 2002 and five of six in November 2002 resulted in positive values (i.e., out of the sediment into the overlying water column). Consistent with benthic fluxes for Hg$_{T}$, dissolved MeHg fluxes were: (a) generally positive in April 2002, (b) negligible at all sites in November 2002, and (c) at least two orders of magnitude lower than total-mercury fluxes, roughly consistent with concentration differences between species. Observed concentration ranges for MeHg in bottom water ranged from less than the detection limit (0.20 pM) at the two shallower, up-gradient sites to 0.48 $\pm$ 0.03 pM (n=3) at the deepest sampling site in November 2002. Dissolved MeHg concentrations in the bottom waters were significantly lower under the low-flow conditions of November 2002 relative to high flow in April 2002, but spatial trends were not temporally consistent. The reservoir was highly phosphate-limited with molar N:P ratios in water column samples ranging from 136 to $>$5,000 (compared to the Redfield N:P molar ratio of 16). Sulfide benthic fluxes were highest (1,180 $\pm$ 50 nmoles-m-2-h-1) at the deepest site in November 2002 following at least 2 weeks of hypolimnetic anoxia (based on prior water-column monitoring), and lowest at the same site in April 2002 (290 $\pm$ 30 nmoles-m-2-h-1) under oxic, high-flow conditions. Sparse benthic macroinvertebrate distributions, mirrored by low benthic-chlorophyll concentrations, were considered to be inconsequential factors mediating benthic flux. Despite their seasonal variability, the magnitude of benthic flux estimates for Hg$_{T}$ were consistently comparable to or greater than riverine sources during a relatively dry year. Diffusive transport of dissolved, bioavailable mercury species between the reservoir bed and water column may therefore be an important process regulating the concentration of mercury species in the reservoir water column.
UR: http://wwwrcamnl.wr.usgs.gov/solute_transport/index.html
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
DE: 1857 Reservoirs (surface)
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2003 Fall Meeting