HR: 08:00h
AN: H51G-01 INVITED     [Abstracts]
TI: Seasonality of Diel Cycles of Dissolved Trace-Metal Concentrations in a Rocky Mountain Stream
AU: * Nimick, D A
EM: dnimick@usgs.gov
AF: U.S. Geological Survey, 3162 Bozeman Avenue, Helena, MT 59601 United States
AU: Cleasby, T E
EM: tcleasby@usgs.gov
AF: U.S. Geological Survey, 3162 Bozeman Avenue, Helena, MT 59601 United States
AU: McCleskey, R B
EM: rbmccles@usgs.gov
AF: U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303
AB: Substantial diel (24-hour) cycles in dissolved (0.1-$\mu$m filtration) metal concentrations were observed during summer low flow, winter low flow, and snowmelt runoff in Prickly Pear Creek in southwestern Montana. The stream was alkaline (pH of 7.65-9.06), and dissolved metal concentrations were relatively low (1.8-7.1 $\mu$g/L for As, 18-57 $\mu$g/L for Mn, and 12-123 $\mu$g/L for Zn). The metals are derived from abandoned mine lands in the stream's headwaters; As also is derived from geothermal sources. During seven diel sampling episodes, each lasting 34-61.5 hours, concentrations of dissolved Mn and Zn increased from minimum values in the afternoon to maximum values shortly after sunrise. The timing of diel cycles of dissolved As concentrations exhibited the inverse pattern. The magnitude of concentration increases during individual 24-hour periods ranged from 17-152% for Mn and 70-500% for Zn, and correlated positively with the magnitude of diel increases of pH and temperature, indicating that geochemical processes involving reactive inorganic and organic surfaces on and in the streambed probably control these diel metal cycles. Diel increases of As concentrations (17-55%) were proportionally smaller and less variable among the seasonal sampling episodes than for Mn and Zn, and they correlated poorly with diel increases of pH and temperature. Streamflow among the seven sampling episodes ranged from 0.35-3.3 m$^{3}$/s. The timing of minimum and maximum values of diel streamflow cycles was inconsistent among sampling episodes and had little relation to the timing of metal concentration cycles, indicating that hydrological processes are not a primary control of diel metal cycles. Diel cycles of dissolved metal concentrations may occur at any time of year and during various hydrologic conditions in all streams with dissolved metals and neutral to alkaline pH.
DE: 1806 Chemistry of fresh water
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting