Arsenic Redistribution between Sediments and Water near a Highly Contaminated Source ALISON R. KEIMOWITZ,* ,†,‡ YAN ZHENG, †,§ STEVEN N. CHILLRUD, † BRIAN MAILLOUX, †, | HUN BOK JUNG, § MARTIN STUTE, †, | AND H. JAMES SIMPSON †,‡ Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, Department of Earth and Environmental Sciences, Columbia University, New York, New York 11367, School of Earth and Environmental Sciences, Queens College, Flushing, New York 11367, and Department of Environmental Science, Barnard College, Columbia University, New York, New York 10027 Mechanisms controlling arsenic partitioning between sediment, groundwater, porewaters, and surface waters were investigated at the Vineland Chemical Company Superfund site in southern New Jersey. Extensive inorganic and organic arsenic contamination at this site (historical total arsenic >10 000 μgL -1 or >130 μM in groundwater) has spread downstream to the Blackwater Branch, Maurice River, and Union Lake. Stream discharge was measured in the Blackwater Branch, and water samples and sediment cores were obtained from both the stream and the lake. Porewaters and sediments were analyzed for arsenic speciation as well as total arsenic, iron, manganese, and sulfur, and they indicate that geochemical processes controlling mobility of arsenic were different in these two locations. Arsenic partitioning in the Blackwater Branch was consistent with arsenic primarily being controlled by sulfur, whereas in Union Lake, the data were consistent with arsenic being controlled largely by iron. Stream discharge and arsenic concentrations indicate that despite large-scale groundwater extraction and treatment, >99% of arsenic transport away from the site results from continued discharge of high arsenic groundwater to the stream, rather than remobilization of arsenic in stream sediments. Changing redox conditions would be expected to change arsenic retention on sediments. In sulfur-controlled stream sediments, more oxic conditions could oxidize arsenic- bearing sulfide minerals, thereby releasing arsenic to porewaters and streamwaters; in iron-controlled lake sediments, more reducing conditions could release arsenic from sediments via reductive dissolution of arsenic- bearing iron oxides. Introduction Arsenic is a toxic metalloid occurring naturally in soils and sediments throughout the world (1). Its mobility is controlled largely by pH and redox changes, and can be present at high concentrations in both natural and contaminated environ- ments under reduced, circumneutral conditions as well as oxidized and reduced alkaline environments such as Mono Lake (2). Arsenic has been widely used in agricultural and manufacturing applications (3) and has thus become a common anthropogenic pollutant. In the subsurface, arsenic mobility is strongly influenced by redox conditions. Release of arsenic from sediments into associated waters has been attributed to reductive dissolution of iron oxyhydroxides to which arsenic had been sorbed (2, 4), although arsenic and iron solubility have been shown to be decoupled in some environments (5-7), indicating that this mechanism is not solely responsible for high arsenic concentrations in reducing groundwaters. Sulfur species have also been shown to influence dissolved arsenic concentra- tions, particularly under strongly reducing conditions (8) where sulfide can complex arsenic and thereby increase arsenic solubility (9, 10). Sulfide can also remove arsenic from solution by precipitation of sulfide mineral phases (11) or sorption to iron sulfide minerals (12). O’Day et al. (13) recently proposed a conceptual model for arsenic mobility in reducing shallow sediments, which characterizes the behavior of arsenic as either sulfur- controlled, as observed in this study in the Blackwater Branch (BWB), or iron-controlled, as observed in Union Lake (UL). This published model (13) indicates that under high-iron conditions, aqueous sulfide is rapidly consumed in the formation of iron sulfide minerals (14, 15), while under low- iron conditions, aqueous sulfide is available to complex aqueous arsenic and to form solid arsenic-sulfide minerals. The control of arsenic mobility by either iron or sulfur is determined by the iron/sulfur ratio. This work expands upon this conceptual model. Under low-iron, sulfur-controlled conditions, removal of arsenic from solution occurs primarily through formation of solid arsenic sulfides (16), with aqueous thioarsenites as potential intermediates (10). Under high-iron, iron-controlled condi- tions, the consumption of sulfide by available iron prevents formation of aqueous thioarsenites (17). Arsenic oxyanions sorb to the iron sulfides that have formed (18), and further pyritization may subsequently occur (19-21) wherein strong As-Fe bonds form (12, 22). The Blackwater Branch and Union Lake (both contaminated by the Vineland Chemical Com- pany) are classified within this system, as are other environ- ments (4, 8). Little is currently known about the geochemical behavior of thioarsenites under natural conditions, although the implied presence of these compounds in both aqueous and sedimentary phases in BWB sediments indicates a need for a better understanding of their behavior. Site Overview The Vineland Chemical Company manufactured arsenical biocides in Vineland, NJ from 1950 to 1994. During this time arsenic salts were stored improperly, thereby introducing large-scale arsenic contamination to soils and groundwater. A groundwater extraction and treatment plant operating since 2000 removes 6000-7500 m 3 day -1 of highly contaminated groundwater and subsequently discharge treated effluent to the BWB, a small freshwater stream that runs along one side of the site (Figure 1); there are no tributaries to the BWB between the Vineland Chemical site and the Maurice River. Since manufacture of arsenicals commenced, the BWB has delivered arsenic downstream into the Maurice River, Union Lake, and Delaware Bay (23). The associated sediments in these surface water bodies are now noteworthy reservoirs of * Corresponding author phone: (854)365-8793; e-mail: ark@ldeo.columbia.edu. † Lamont-Doherty Earth Observatory. ‡ Department of Earth and Environmental Sciences, Columbia University. § School of Earth and Environmental Sciences, Queens College. | Department of Environmental Science, Barnard College. Environ. Sci. Technol. 2005, 39, 8606-8613 8606 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005 10.1021/es050727t CCC: $30.25  2005 American Chemical Society Published on Web 10/12/2005