Sulfide Oxidation in Marine Sedimentary Rocks as a Source of Trace Metals and Sulfate to Urban California Streams

Watersheds in southern Orange County, CA have received regulatory scrutiny for elevated levels of total dissolved solids (TDS) along with cadmium, nickel, and sulfate as threats to in-stream and marine ecology. Multiple source investigations have failed to attribute these chronic contaminants to anthropogenic sources. Patterns of high TDS in the study region's surface waters correlate poorly with landuse and instead appear to follow geologic substrate. Measurements of springs and seeps reveal groundwater pH as low as 4.8, TDS as high as 8700 mg/L, dissolved concentrations of sulfate up to 50 mM, cadmium up to 1.8 uM, selenium up to 2.4 uM, and nickel up to 14.8 uM flowing directly into creeks. We suggest that subsurface oxidation of sulfide in prevalent Neogene marine sedimentary rock formations is the key weathering mechanism behind this phenomenon. Bulk analysis of the Capistrano and other local formations indicates that they are enriched in select trace metals up to two orders of magnitude relative to average crustal abundance, making them a plausible source of contamination to groundwater, and ultimately, surface water. Though carbonate dissolution in these same formations may offset the acidity at some sites, many groundwater samples were substantially undersaturated in calcite and capable of maintaining low pH and high dissolved metals concentration. While sulfide mineral weathering has been invoked as the cause of significant contamination at former mining sites and undeveloped mineralized regions, our findings indicate this same weathering mechanism may have implications for urbanized catchments that contain marine sedimentary units. Sulfide mineral oxidation can result in substantial sulfate loading, acid production and subsequent mobilization of trace metals and other ions from the surrounding rock matrix, leading to high dissolved contaminant levels. To evaluate water and sulfate sources to these high TDS springs, we measured stable isotopes of water and dissolved sulfate at seven locations. δD and δ18O measurements for water in our study area produced values similar to local precipitation and reference streams from undeveloped watershed, suggesting little influence from imported municipal sources that typically have lighter oxygen and hydrogen isotope signatures. Reduced sulfur, like that found in sedimentary sulfide minerals, typically has a much more depleted isotopic signature as compared to sulfur from evaporite or marine sources. Since this signature remains as sulfide minerals undergo oxidation, the isotopic composition of dissolved sulfate in groundwater may offer insight to the importance of various sources. The δ34S of dissolved sulfate in high TDS groundwater sulfate ranged from 0.4‰ to -27.1‰. As expected, the most acidic groundwater sites are associated with the most negative δ34S values. More investigation will be required to assess whether sulfate reduction or mixing with sulfate from an evaporite produces the range in δ34S of dissolved sulfate we observe. In either case, it is clear that oxidation of a sulfide mineral source contributes substantial quantities of sulfate to the study region's springs and seeps.