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Diel changes in water chemistry in an arsenic-rich stream and treatment-pond system

January 1, 2007

Arsenic concentrations are elevated in surface waters of the Warm Springs Ponds Operable Unit (WSPOU), located at the head of the upper Clark Fork River Superfund site, Montana, USA. Arsenic is derived from historical deposition of smelter emissions (Mill and Willow Creeks) and historical mining and milling wastes (Silver Bow Creek). Although long-term monitoring has characterized the general seasonal and flow-related trends in As concentrations in these streams and the pond system used to treat Silver Bow Creek water, little is known about solubility controls and sorption processes that influence diel cycles in As concentrations.

Diel (24-h) sampling was conducted in July 2004 and August 2005 at the outlet of the treatment ponds, at two locations along a nearby reconstructed stream channel that diverts tributary water around the ponds, and at Silver Bow Creek 2 km below the ponds. Dissolved As concentration increased up to 51% during the day at most of the stream sites, whereas little or no diel change was displayed at the treatment-pond outlet. The strong cycle in streams is explained by pH- and temperature-dependent sorption of As onto hydrous metal oxides or biofilms on the streambed. Concentrations of dissolved Ca2+ and HCO3 at the stream sites showed a diel temporal pattern opposite to that of As, and geochemical modeling supports the hypothesis that the concentrations of Ca2+ and HCO3 were controlled by precipitation of calcite during the warm afternoon hours when pH rose above 9.0. Nightly increases in dissolved Mn and Fe(II) concentrations were out of phase with concentrations of other divalent cations and are more likely explained by redox phenomena.

Publication Year 2007
Title Diel changes in water chemistry in an arsenic-rich stream and treatment-pond system
DOI 10.1016/j.scitotenv.2007.06.029
Authors C.H. Gammons, T.M. Grant, David A. Nimick, S.R. Parker, M.D. DeGrandpre
Publication Type Article
Publication Subtype Journal Article
Series Title Science of the Total Environment
Index ID 70031472
Record Source USGS Publications Warehouse
USGS Organization Toxic Substances Hydrology Program