Title
Arsenic Geochemistry in Warm Springs Ponds: New Field and Experimental Results
Abstract
Silver Bow Creek flows into the Warm Springs Ponds Operable Unit (WSPOU), where various containment cells are used to precipitate copper and other metals (e.g., Cd, Mn, Pb, Zn). Lime is added seasonally to increase the pH and assist in removal of metals from the water column. Although the WSPOU is effective at removing copper and other cationic trace metals, concentrations of dissolved arsenic exiting the facility are elevated (> 20 mg/L) during low-flow periods each summer and fall.
A series of laboratory mesocosm experiments with pH adjustment have been completed using upstream Silver Bow Creek water and Pond 3 water, as well as shallow sediment from Pond 3. In addition, new field data have been collected from the south end of Pond 3 using PVC piezometers and sediment pore-water diffusion samplers (peepers). Results of the mesocosm experiments (open system, with continuous stirring) show no increase in dissolved As concentration with increase in pH up to pH’s > 11. Therefore, we believe that the release of As into the ponds in low-flow periods is not due to changes in pH alone. Instead, we hypothesize that arsenic release is linked to microbial reduction of ferric oxide minerals in the organic-rich sediment. Dissolved sulfate is also microbially reduced in the sediment to H2S, which also influences the geochemistry of As and Fe. Upwards diffusion of dissolved As from the sediment pore-water into the pond water is the most likely explanation for the increase in As concentration of the WSPOU in low-flow periods.
Start Date
24-4-2015 12:00 PM
End Date
24-4-2015 2:00 PM
Document Type
Poster
Arsenic Geochemistry in Warm Springs Ponds: New Field and Experimental Results
Silver Bow Creek flows into the Warm Springs Ponds Operable Unit (WSPOU), where various containment cells are used to precipitate copper and other metals (e.g., Cd, Mn, Pb, Zn). Lime is added seasonally to increase the pH and assist in removal of metals from the water column. Although the WSPOU is effective at removing copper and other cationic trace metals, concentrations of dissolved arsenic exiting the facility are elevated (> 20 mg/L) during low-flow periods each summer and fall.
A series of laboratory mesocosm experiments with pH adjustment have been completed using upstream Silver Bow Creek water and Pond 3 water, as well as shallow sediment from Pond 3. In addition, new field data have been collected from the south end of Pond 3 using PVC piezometers and sediment pore-water diffusion samplers (peepers). Results of the mesocosm experiments (open system, with continuous stirring) show no increase in dissolved As concentration with increase in pH up to pH’s > 11. Therefore, we believe that the release of As into the ponds in low-flow periods is not due to changes in pH alone. Instead, we hypothesize that arsenic release is linked to microbial reduction of ferric oxide minerals in the organic-rich sediment. Dissolved sulfate is also microbially reduced in the sediment to H2S, which also influences the geochemistry of As and Fe. Upwards diffusion of dissolved As from the sediment pore-water into the pond water is the most likely explanation for the increase in As concentration of the WSPOU in low-flow periods.