MICROBIAL CYCLING OF SULFUR AND IRON CONTRIBUTES TO POOR WATER QUALITY IN SPRING CREEK RESERVOIR, DOWNSTREAM OF IRON MOUNTAIN MINE, CALIFORNIA

Spring Creek Debris Dam (SCDD) was constructed in 1963 to capture metal-rich sediments and to regulate runoff from the Iron Mountain mining complex in Shasta County, California. The formation of Spring Creek Reservoir (SCR) by SCDD had the net effect of improving water quality downstream. Since the early 1990s, remediation upstream at the Iron Mountain Mine Superfund site has greatly improved the quality of water flowing into the reservoir. However, when the water level in SCR is below 720 ft above sea level, the reservoir becomes acidic (pH < 5) and rich in dissolved Fe and Cu. Sediments deposited in the reservoir are likely sources of the metals and acidity. To test this hypothesis and determine biogeochemical processes, we collected a series of sediment cores surrounding the 720 ft reservoir level and examined microbial communities and mineralogy. Principal component analysis revealed that microbial communities within the cores formed two distinct groups: those near 720 ft (low-elevation cores), and those at higher elevations. The low-elevation cores were enriched in sulfate-reducing taxa and contained higher concentrations of Fe- and S-bearing minerals than the high-elevation cores. Based on these data, we developed a conceptual model to explain why pH decreases and metal concentrations increase at low reservoir levels. In our model, groundwater flow is affected by SCR water level and the hydraulic head surrounding SCR. At high water levels, oxygenated ground water does not penetrate as far into the reservoir bed sediments as it does at low water levels. When the SCR water level is high, sulfate-reducing bacteria reduce sulfate in anoxic sediments and the resulting hydrogen sulfide reacts with Fe(II) in the pore water (formed by Fe(III)-reducing microbes) to form Fe-sulfide minerals. When the reservoir level is low, the hydraulic head pushes oxygenated ground water into reservoir bed sediments, and surficial sediments are drained of their pore water and exposed to the atmosphere. The resulting oxidation of Fe-sulfide minerals and of residual Fe(II) in pore water generates acidic, metal-laden water in these conditions.