HR: 1340h
AN: B23B-1046    [Abstracts]
TI: Trace Metal Cycling Within Riparian Wetland and Hyporhiec Regions Of A Northern Temperate Stream Catchment
AU: * Shafer, M M
EM: mmshafer@facstaff.wisc.edu
AF: University of Wisconsin-Madison, 660 North Park Street, Madison, WI 53706 United States
AU: Kerr, S
EM: sckerr@wisc.edu
AF: University of Wisconsin-Madison, 660 North Park Street, Madison, WI 53706 United States
AU: Overdier, J
EM: overdier@facstaff.wisc.edu
AF: University of Wisconsin-Madison, 660 North Park Street, Madison, WI 53706 United States
AB: Our study addresses trace element cycles and metal-binding ligand production and transport within riparian wetland and hyporheic zones of stream catchments. The research is being carried out at extensively instrumented stream catchments in the Northern Temperate Lakes Region of north-central Wisconsin - a system that is also under investigation by the NSF-funded Long Term Ecosystem Research (LTER) program and the USGS Water Energy and Biogeochemical Budgets (WEBB) program. Within the study system we have identified flow paths of contrasting redox, in addition to zones of positive and negative ground water recharge. Detailed temporal data sets of water and ligand chemistry data are being obtained from dense networks of nested piezometers and hyporheic samplers placed in these contrasting regions/flow paths. In addition to major element chemistry, data on over 25 trace metals are acquired using high-resolution ICP-MS. Variations in groundwater flow direction and velocity are key factors driving chemistry in the hyporheic zone. Discharging hyporheic systems are characterized by sub-surface constituent peaks, reflecting, in part, an increased residence time due to the interaction of upwelling groundwater and entrained stream water. Sub-surface peaks are typically absent form recharging systems. Sulfate reduction and iron redox cycling are associated with active cycling of Cu, Zn, and several other trace metals within the hyporheic zone. DOM cycling also appears to influence metal chemistry in the hyporheic, both indirectly through OM oxidation and coupled sulfate and nitrate reduction, and apparently directly via complexation (e.g. with cadmium). Sulfate supply, primarily sourced from surface water, is limiting sulfide production in the hyporheic during certain periods of the year. Stream chemistry variations, particularly in the recharging system, are important drivers of hyporheic major and trace element cycling, as in addition to sulfate, levels of DOM and several trace elements are substantially greater in surface water than groundwater. The importance of sulfide and iron in regulating ground water fluxes of certain trace metals to the stream is also evident when oxic and anoxic flow paths are compared. Along a predominantly oxic flow path from regional ground water to stream riparian porewaters, concentrations of both Mn and Zn drop markedly (10-100 fold). Arsenic is also lost to riparian and wetland sediments. Levels of Cu along anoxic flowpaths (0.06 nM Cu, 50-700 nM sulfide, 30-40,000 nM Fe) are strikingly reduced from that observed in oxic zones (4-6 nM, Cu, <20 nM sulfide, 100 nM Fe). Studies are in progress to characterize the oxidation state of Cu in the contrasting micro-environments. Large seasonal variations in Pb levels are observed, with particularly elevated concentrations seen in spring, coinciding with greater DOC fluxes.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0461 Metals
DE: 0497 Wetlands (1890)
DE: 1804 Catchment
DE: 1806 Chemistry of fresh water
SC: Biogeosciences [B]
MN: Fall Meeting 2005