Sorption of trace metals to an aluminum precipitate in a stream receiving acid rock-drainage; Snake River, Summit County, Colorado

The quality of water in streams that are contaminated by acid drainage from mines and from the weathering of mineralized rocks improves as the water flows downstream. The purpose of this study was to investigate the geochemical processes that occur in one such stream and to determine the fate of the trace metals that are removed from the water. The stream chosen for this purpose was the Snake River, Summit County, Colorado, which is affected by natural acid rock-drainage (ARD) containing SO 4, Al, Fe, and various trace elements such as Zn, Cu, Pb, Ni, and others. Most of the Fe in the Snake River is removed from solution by the oxidation of Fe 2+ to Fe 3+ and the subsequent precipitation of Fe-oxyhydroxides that form a massive ferricrete deposit near the springs that feed the river. Further downstream, the Snake River (pH=3.0) mixes with water from Deer Creek (pH= 7.0) thereby increasing its pH to 6.3 and causing SO 4-rich precipitates of Al-oxyhydroxide to form. The precipitates and associated organic C complexes sorb trace metals from the water and thus have high concentrations of certain elements, including Zn (540–11,400 ppm), Cu (34–221 ppm), Pb (90–340 ppm), and Ni (11–197 ppm). The concentrations of these elements in the precipitates that coat the streambed rise steeply in the zone of mixing and then decline downstream. The trace element concentrations of the water in the mixing zone at the confluence with Deer Creek decrease by 75% or more and are up to 3 orders of magnitude lower than those of the precipitates. Sorption curves for Zn, Cu, Pb, Ni, and SO 4 were derived by stepwise neutralization of a sample of Snake River water (collected above the confluence with Deer Creek) and indicate that the trace metals are sorbed preferentially with increasing pH in the general order Pb, Cu, Zn, and Ni. Sulfate is removed between pH 4 and 5 to form an Al-hydroxysulfate and/or by sorption to microcrystalline gibbsite. The sorption data determined from the neutralization experiment were used to account for the downstream decrease of trace-metal concentrations in the precipitates. The results of this study demonstrate that the partitioning of trace metals in the Snake River is not only a function of pH, but also depends on the progressive removal of trace metals as the water of the Snake River flows through its confluence with Deer Creek. The chemical composition of the water also determines what compounds precipitate with increasing pH.