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Influence Of Groundwater Discharge On Arsenic Contamination In Sediments
Citation:
FORD, R. G., K. G. SCHECKEL, B. K. LIEN, T. LUXTON, P. J. CLARK, S. D. ACREE, R. R. ROSS, B. BRANDON, AND G. LOMBARDO. Influence Of Groundwater Discharge On Arsenic Contamination In Sediments. Presented at Battelle Fifth International Conference On Remediation Of Contaminated Sediments, Jacksonville, FL, February 02 - 05, 2009.
Impact/Purpose:
To conduct a field investigation to evaluate the impact of a discharging arsenic plume on sediment contaminant characteristics at a site adjacent to a landfill in northeastern Massachusetts.
Description:
A field investigation was conducted to evaluate the impact of a discharging arsenic plume on sediment contaminant characteristics at a site adjacent to a landfill in northeastern Massachusetts. Site characterization included assessment of the hydrologic and chemical samples collected throughout the groundwater/surface water (GW/SW) transition zone. The magnitude and spatial distribution of contaminated groundwater discharge was determined through estimation of water flux through the upgradient aquifer, direct measurements of discharge through sediments at discrete locations using a bidirectional advective flux meter, and mapping of shallow sediment temperature as an indirect indicator of groundwater discharge. Chemical characteristics of groundwater along specific transport pathways were determined for samples acquired from discrete upgradient intervals within the aquifer and shallow portions of the aquifer underlying a cove with elevated levels of arsenic contamination in sediments. Sediment cores were retrieved from transects across the impacted cove and characterized for element chemistry and arsenic solid phase speciation. Depth-discrete samples of surface water and suspended solids were collected throughout the cove in areas with suspected high and low discharge of contaminated groundwater. Based on estimates of the groundwater flux throughout the upgradient aquifer and measured arsenic concentrations, the greatest flux of arsenic discharged along the western shoreline of the cove. Contaminated groundwater discharge was characterized as reducing with relatively uniform elevated concentrations of ferrous iron. The predominant form of arsenic in the plume was arsenite. Sulfate concentrations varied in groundwater around the perimeter of the cove with the lowest concentrations occurring within the region of highest arsenic discharge. The distribution groundwater discharge was heterogenenous throughout the cove. There was general correspondence between shallow sediment temperature, chemical signatures of groundwater discharge, and elevated concentrations of arsenic in sediments. Solid phase speciation of arsenic in sediments demonstrated a predominant association of As(V)/As(III) with iron oxyhydroxides and As(III) with iron sulfides. The spatial distribution of these various species depended on location with the cove relative to plume discharge. Elevated concentrations of arsenic and ferrous iron were observed in deep surface water overlying cove sediments. Concentrations of these dissolved constituents decreased dramatically at shallower depths due to oxidation-precipitation of ferrous iron and sequestration of arsenic. Assessment of relationships between surface water chemistry and sediment geochemistry relative to the spatial distribution of plume discharge within the cove indicated that elevated concentrations of ferrous iron and arsenic in deep surface water was derived from a combination of direct groundwater discharge and dissolution of As-bearing iron oxyhydroxides that settle out from the water column. [This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.]