Sorption of trace metals to an aluminum precipitate in a stream receiving acid rock-drainage; Snake River, Summit County, Colorado
Introduction
When sulfide minerals such as pyrite are oxidized at or near the surface of the earth, ground and surface waters typically become acidic and are enriched in SO4, Fe, Al, and trace elements (Wentz, 1974). Some of the mobilized trace elements associated with acid drainage pose a potential threat to the health of plants, animals, and humans (Adriano, 1986, McBride, 1994). As acidic Fe- and Al-rich streams are neutralized, amorphous to semi-crystalline oxyhydroxides and hydroxysulfates of Fe or Al can form (Stumm and Morgan, 1996). These precipitates have surface sites that sorb cations and anions depending on their charge, which is primarily controlled by the pH of the surrounding solution. The suspended particles and precipitates on the bottom of stream channels are transported downstream during periods of high flow and may be deposited in lakes and reservoirs.
The Snake River in the Dillon watershed, Summit County, Colorado, provides an opportunity to study the distribution of trace elements between water and precipitates that are composed predominantly of an Al-hydroxysulfate. This watershed lies within the Colorado mineral belt, which has been mined for Au, Ag, and various sulfide minerals since the 1860s (Lovering and Goddard, 1950). Although mining was discontinued in the 1930s, some of the streams are still contaminated by acid mine-drainage (AMD) as well as by acid rock-drainage (ARD). The drainage basins of the Snake River and Deer Creek are underlain by granitic gneisses (Idaho Springs Formation and Swandyke Hornblende Gneiss, respectively) of Precambrian age that were intruded by Tertiary quartz monzonite plutons (Lovering, 1935). The bedrock contains disseminated pyrite as well as quartz veins with pyrite and other metallic sulfides (Lovering and Goddard, 1950). Whole-rock chemical analyses indicate that the Idaho Springs Formation is Al- and Si-rich, whereas the Swandyke Hornblende Gneiss has relatively high concentrations of Ca, Mg, and Fe.
Theobald et al. (1963) reported the presence of brown, tan, and black precipitates on the streambed at the confluence of the Snake River and Deer Creek and determined that some of these precipitates contained Fe, Al, Mn, and the trace metals Pb, Cu, and Zn. McKnight and Bencala, 1988, McKnight and Bencala, 1989 subsequently showed that Fe concentrations in the Snake River vary hourly due to photoreduction of Fe3+ to Fe2+. In addition, McKnight et al. (1992a) correlated the concentrations of Fe and organic C in the precipitates with decreasing metal concentrations and suggested that this relationship implies that some metal cations are sorbed onto humic substances in the precipitates.
Numerous laboratory investigations of trace element sorption to Fe-hydroxides have provided a basis for understanding the partitioning of trace elements between solution and sorbent (Ballistrieri and Murray, 1982, Dzombak and Morel, 1990, Stumm, 1992). In addition, field studies addressing various aspects of the transport and fate of metals in waters receiving AMD by Rampe and Runnells, 1989, Broshears et al., 1996, Schemel et al., 2000, and others have provided information about the interaction of multiple variables such as pH, sorbent composition, downstream transport, etc. that control the distribution of trace metals in natural aquatic environments.
The objective of this paper is to test the hypothesis that the partitioning of trace metals in the Snake River at and just below its confluence with Deer Creek is controlled predominantly by an increase of pH and the resulting precipitation of an Al-hydroxysulfate. This study is part of a larger research project of which the objective is to investigate the weathering of mineralized rocks in the drainage basin of the Snake River and to study the geochemical processes that control the concentrations of trace elements in the Dillon Reservoir, which is one of the principal sources of drinking water for the city of Denver.
Section snippets
Water and precipitate samples
Water and samples of tan to white flocculant precipitates were collected from the streambed over a 60 m interval at the confluence of the Snake River and Deer Creek (Fig. 1). Precipitates were collected using a 60-ml polypropylene syringe with attached plastic ™Tygon tubing. The precipitates and associated water were transferred to high-density polyethylene bottles that were rinsed with stream water at each collection site. In addition, the pH was measured twice at every sampling location. The
Precipitate mineralogy and metal sorption at the Snake River/Deer Creek confluence
The pH of the Snake River increases from 3.0 above the confluence to 6.3 just below the confluence with Deer Creek and then stabilizes at 5.2–5.3 further downstream where the water is more completely mixed (Fig. 2a). A flocculent tan to white precipitate occurs on the streambed in the mixing zone and further downstream. Chemical analyses indicate that the precipitates collected from the Snake River streambed are composed predominantly of Al-hydroxysulfate with significant quantities of
Conclusions
The Snake River is contaminated by acid rock-drainage, which results from the oxidation of pyrite and other sulfide minerals within the underlying bedrock of the drainage basin. As the Snake River mixes with Deer Creek, chemically active, Al-hydroxysulfate precipitates form and sorb trace metals as the pH of the water increases.
Experimental neutralization of Snake River water shows that Pb, Cu, Zn, and Ni are sorbed with increasing pH, whereas SO4 is initially removed from the water at low pH
Acknowledgements
This research was supported in part by funds from the Friends of Orton Hall, Department of Geological Sciences, Ohio State University, and by a Graduate Student Alumni Research Award from the Graduate School, Ohio State University. We thank Ken Bencala, Jenny Webster-Brown, and Don Runnels for their helpful reviews that improved this paper. In addition, I (L. Munk) thank my collegues Giehyeon Lee, Nadine Piatak, and Linda Centeno for their input that resulted from numerous discussions about
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2020, Journal of HydrologyCitation Excerpt :Weathering of pyrite precipitates ferric oxyhydroxides and produces SO42− and H+, which rapidly decreases the pH of surface waters and may cause other compounds to dissolve (Åström and Åström, 1997; Munk et al., 2002). Buffering of acidification by CO32– or HCO3– has been shown to occur in highly acidic streams (Munk et al, 2002; Fortner et al., 2011). Upon depletion of sulphides and carbonates, the role of silicate weathering increases in the proglacial zone (Tranter et al., 2005; Anderson et al., 1997; Walsh, 2013).