Arsenic geochemistry in a geothermally impacted system : the Los Angeles Aqueduct

Abstract

Elevated arsenic concentrations in the Los Angeles Aqueduct (LAA) derive from geothermal inputs in Hot Creek. This study examines arsenic geochemistry in three sections of the LAA system: Hot Creek, Crowley Lake and Haiwee Reservoir. At Hot Creek, the accumulation of arsenic in the sediments is strongly influenced by hot springs and plants. Solid-phase arsenic is present as As(III) throughout the sediment column with an identifiable organic phase in the surface sediment and a sulfide phase in the deep sediment. Sediments carry only a small fraction of the arsenic load in Hot Creek. Concentration profiles obtained with a novel, high resolution pore water sampler reveal a net flux of arsenic out of the sediments. Sediment-water exchange of arsenic in Hot Creek depends on initial arsenic concentrations and the presence of other ions in solution; this latter effect may be partly due to colloid aggregation and peptization. In Crowley Lake, water column profiles obtained during stratified and well-mixed conditions provide no evidence for sediment-water exchange of arsenic. Algal uptake and transformation of arsenic is not sufficient to perturb the distribution of arsenic species in the water column or to accumulate substantial arsenic concentrations in the sediment. Arsenic is associated, and immobilized, with sulfide in the sediments, which are permanently anoxic. Sediment arsenic concentrations are modestly elevated; release of arsenic from the sediments driven by decreased water column arsenic concentrations would be gradual and would not threaten the water quality of the LAA. In Haiwee Reservoir, arsenic is deposited in the sediments as a result of the Los Angeles Department of Water and Power's interim arsenic management plan. This solid-phase arsenic is unstable with respect to reductive dissolution of the iron oxyhydroxide with which it is associated. A strong redox gradient exists below the sediment water interface and solid-phase arsenic speciation evolves rapidly with depth from an As(V) to an As(III) phase. The potential for release of this arsenic to the overlying water poses a threat to the LAA water supply.