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Published February 1, 2002 | public
Journal Article

Deposition and Fate of Arsenic in Iron- and Arsenic-Enriched Reservoir Sediments

Abstract

Deposition of arsenic to the sediments of Haiwee Reservoir (Olancha, CA) has dramatically increased since March 1996 as a result of an interim strategy for arsenic management in the Los Angeles Aqueduct (LAA) water supply. Ferric chloride and cationic polymer are introduced into the Aqueduct at the Cottonwood treatment plant, 27 km north of the Haiwee Reservoir. This treatment decreases the average arsenic concentration from 25 μg/L above Cottonwood to 8.3 μg/L below Haiwee. Iron- and arsenic-rich flocculated solids are removed by deposition to the reservoir sediments. Analysis of sediments shows a pronounced signature of this deposition with elevated sediment concentrations of iron, arsenic, and manganese relative to a control site. Sediment concentrations of these elements remain elevated throughout the core length sampled (ca. 4% iron and 600 and 200 μg/g of manganese and arsenic, respectively, on a dry weight basis). A pore water profile revealed a strong redox gradient in the sediment. Manganese in the pore waters increased below 5 cm; iron and arsenic increased below 10 cm and were strongly correlated, consistent with reductive dissolution of iron oxyhydroxides and concurrent release of associated arsenic to solution. X-ray absorption near-edge spectroscopy revealed inorganic As(V) present only in the uppermost sediment (0−2.5 cm) in addition to inorganic As(III). In the deeper sediments (to 44 cm), only oxygen-coordinated As(III) was detected. Analysis of the extended X-ray absorption fine structure spectrum indicates that the As(III) at depth remains associated with iron oxyhydroxide. We hypothesize that this phase persists in the recently deposited sediment despite reducing conditions due to slow dissolution kinetics.

Additional Information

© 2002 American Chemical Society. Received for review May 2, 2001. Revised manuscript received August 23, 2001. Accepted September 28, 2001. Publication Date (Web): January 4, 2002. We thank the Los Angeles Department of Water and Power for access to Haiwee Reservoir and Florian Bohn (Caltech) for his work in testing the gel probe sampler. Funding for this work was provided by the U.S. Environmental Protection Agency (826202-01-0) and the National Science Foundation (EAR-9629276). Part of this research was carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

Additional details

Created:
August 19, 2023
Modified:
October 25, 2023