Spatial and Temporal Variability in the Aquatic Cycling of Chromium

Abstract

The signal to noise ratio is fundamental to scientific measurement; environmental chemistry is no exception. To accurately quantify human impacts, natural processes leading to spatial and temporal variability of contaminant distribution must be described. This dissertation focuses on natural and anthropogenic sources of chromium, with an emphasis on the San Francisco Bay estuary. The physical and chemical processes causing spatial and temporal variability in chromium distributions are explored. Those processes include weathering, river flow, photochemically-initiated free-radical redox cycling, geologic oxidation, and reductive scavenging.

Chromium is attractive to study because of the contrasting properties of its two most abundant valences. Chromium(VI) is a carcinogen, whereas chromium(III) is a nutrient. Chromium(III) is a partly hydrolyzed cation which readily adsorbs to particles at natural pH. In contrast, chromium(VI) is an oxyanion with very low particle affinity. Therefore, determination of redox speciation is essential to understanding chromium fate and effects. This research established that essentially all chromium inputs to San Francisco Bay are chromium(III).

Seasonal river flow and the mineralogy of Central Valley sediments dominate the chromium geochemical cycle in San Francisco Bay. Central valley sediments are enriched in chromium and nickel relative to the earth's crust, due to the abundance of serpentinite and other ultramafic minerals. Chromium is mobilized from weathered sediments in the alluvial flood plain, and flushed into San Francisco Bay during high flow periods. This brings up to 1000 kg/day  dissolved chromium to the estuary, dwarfing anthropogenic inputs of 30 kg/day.

During low-flow periods, fluvial inputs are comparable to anthropogenic inputs and other geochemical processes, such as in-situ reduction. That observation led to an interest in chromium reduction pathways. This research focused on mechanisms for indirect photochemical reduction by superoxide radical. Superoxide is produced in natural waters by the photooxidation of organic matter. Diffusion-limited reaction rates between copper and superoxide lead to steady-state copper(I) concentrations in sunlit surface waters. We investigated the chromium-copper(I)-superoxide system using continuous radiolysis to generate superoxide radical in synthetic and natural waters. This research established that while there is no substantial direct reaction between superoxide and chromium(VI), nanomolar copper concentrations can effectively catalyze chromium reduction. Copper catalysis is quenched by chelating organic ligands and chloride complexation, and also inhibited above ~ pH 6.5; these observations can be explained by the speciation of the ions involved. Although copper-catalyzed chromium reduction by superoxide can occur in rainwater, iron is a more viable electron carrier in most natural waters.

A naturally occurring oxidative source of chromium(VI) is revealed in the San Benito Mountains. Chromium(VI) concentrations exceed EPA water quality criteria in San Carlos Creek, upstream from the abandoned New Idria mercury mine. This is apparently due to oxidation of chromite inclusions in the serpentinite deposits abundant in that region. San Carlos Creek mixes with acid mine drainage from the New Idria mine, drastically lowering pH and introducing substantial amounts of iron(II). Consequently, dissolved chromium(VI) is rapidly reduced to chromium(III) and quantitatively scavenged into ferric oxide surfaces. This provides a curious example of an anthropogenic impact mitigating a naturally occurring contaminant. More importantly, the data from New Idria illustrate some of the most important processes in the chromium geochemical cycle: mineral weathering, dissolution, oxidation, reduction, and scavenging.

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