HR: 0800h
AN: H41F-0344    [Abstracts]
TI: Diel changes in stable carbon isotope ratios and trace element concentrations in the Clark Fork River, MT.
AU: * Parker, S
EM: sparker@mtech.edu
AF: Dept. of Chemistry and Geochemistry, Montana Tech 1300 W. Park St., Butte, MT 59701 United States
AU: Gammons, C
EM: cgammons@mtech.edu
AF: Dept. of Geological Engineering, Montana Tech 1300 W. Park St., Butte, MT 59701 United States
AU: DeGrandpre, M
EM: Michael.DeGrandpre@umontana.edu
AF: Dept. of Chemistry, University of Montana, Missoula, MT 59701 United States
AB: A diel (24-hr) water sampling was conducted on the Clark Fork River near Deer Lodge, Montana from 31-July to 1-August of 2003. The Clark Fork River is located in southwestern Montana and its upper reaches have been heavily affected by historic mining and smelting activities in the Butte and Anaconda areas. River floodplains and sediment beds contain significant quantities of metals (iron, aluminum, copper, zinc, lead, cadmium.) and arsenic. Two sites about 1.2 km apart, were sampled with a mean transit time for the water of 2.5 hours between the sites. The river in the study reach is characterized by oxic conditions, moderate alkalinity, moderate biological productivity and a pH range of about 8.0 to 8.5 during the summer low water months. During the mid-summer months, water in the Clark Fork River and its tributary streams is diverted for irrigation. Some of this water returns to the main stem in a chemically modified form as surface or groundwater return flow. This greatly complicates the hydrogeology and nutrient balance of the watershed. The two samplings sites used in this study make it possible to analyze the changes in chemical and physical properties of the water as it travels between the sites. Results reported here show that there is a large degree of temporal and spatial variability across the study area. In situ instruments were deployed and hourly water samples were collected for analysis from both sites. Diel concentration cycles are documented for dissolved forms of manganese, zinc and nitrate. Particulate forms of zinc, aluminum, copper, iron and manganese are shown to undergo twenty-four hour concentration changes. Total suspended solids (TSS) also show a diel change with the mass of TSS increasing at night. Dissolved arsenic is shown to undergo a diurnal concentration cycle at both sampling sites that was out of phase by the average 2.5 hour transit time between the two sites. This arsenic fluctuation may be a pulse of As that is being advected down river from a site above the sampling area. One component of this project was to demonstrate the presence of a diel stable carbon isotope ($\delta$$^{13}$C) cycle mediated by the use and production of dissolved carbon dioxide. Aquatic plants use carbon dioxide during photosynthesis and there is a carbon isotope fractionation associated with the removal of CO$_{2}$ from the water column. This work demonstrates the presence of a diel cycle in the stable carbon isotope ratio $\delta$$^{13}$C at both sites. The magnitude of the carbon isotope cycle is significantly different at the two sites and this is correlated with the rates of photosynthesis and respiration. The difference in productivity at the two sites is associated with the difference in nutrient levels and the nitrate to phosphate ratio.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 0330 Geochemical cycles
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting