The geochemistry of arsenic in reservoir sediments, Milltown, Montana

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5762127

Drever, J I ^[1]; Moncure, G K; Jankowski, P A ^[2]

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1. Univ. of Wyoming, Laramie, WY (United States)
2. ENSR Consulting and Engineering, Fort Collins, CO (United States)

Sediments in the Milltown Reservoir contain elevated concentrations of As and other metals. Elevated concentrations of dissolved As are appearing in an alluvial aquifer below and adjacent to the reservoir sediments. The authors investigated geochemical controls on the mobility of As by analyzing pore waters from the sediments (to a depth of 6.5 m) and by selectively leaching the sediments to determine the chemical form of As and other metals. A vertical succession of redox-related changes occur in the pore water: first Fe concentrations increase to around 40 mg/l, then sulfate decreases to near zero, then As concentrations (as As(III)) increase to around 15 mg/l. Arsenic concentrations above 1 mg/l occur only where sulfate concentrations are very low. A significant fraction of the solid-phase As (and other metals) remains adsorbed on Fe oxyhydroxides even under reducing conditions. They interpret the results as follows: arsenic is derived from primary phases such as arsenopyrite, which undergo oxidation either before reaching the reservoir or in the reservoir during drawdown, when the sediments are exposed to air. In an oxidizing environment, As is retained by adsorption of As(V) on Fe oxyhydroxide. When the sediment becomes reducing, associated with a rise in the water level of the reservoir, Fe oxyhydroxide partially dissolves, but sufficient remains as an adsorbent to keep As concentrations low. Possible reasons for the increase in As concentrations following sulfate reduction are: (1) As(V) is reduced to As(III) only below the sulfate/sulfide boundary; the reduced form is desorbed and appears in solution; (2) arsenic released by dissolution of Fe oxyhydroxides and desorption is precipitated as a sulfide. Concentrations build up only when the supply of sulfide is depleted.

• OSTI ID: 5762127
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
540320* -- Environment
Aquatic-- Chemicals Monitoring & Transport-- (1990-)
58 GEOSCIENCES
580000 -- Geosciences
ADSORPTION
AQUIFERS
ARSENIC
CHEMICAL REACTIONS
CHEMISTRY
DESORPTION
DEVELOPED COUNTRIES
DISSOLUTION
ELEMENTS
ENVIRONMENTAL TRANSPORT
GEOCHEMISTRY
IRON COMPOUNDS
MASS TRANSFER
MONTANA
NORTH AMERICA
OXIDATION
REDUCTION
SEDIMENTS
SEMIMETALS
SORPTION
SURFACE WATERS
TRANSITION ELEMENT COMPOUNDS
USA
WATER RESERVOIRS