Resolving Biogeochemical Processes Controlling Metal Ion Fate and Transport within Soils.
Scott Fendorf, Matthew Ginder-Vogel, Matthew Polizzotto, Benjamin Kocar, and Katharine Tufano. Dept. of Geological and Environmental Sciences, Stanford University, Building 320, Room 118, Stanford, CA 94305-2115
Soils are inherently complex systems, exhibiting extremely heterogeneity in their physical, chemical, and biological states. As a consequence, resolving processes controlling the fate and transport of ions/chemical is a challenging task. In order to meet this challenge and gain a comprehension of processes governing elemental cycling, we need to conduct multidisciplinary studies that capture chemical states derived from biogeochemical conditions resulting within specific physical environments of soils. Advances in spectroscopic and microscopic techniques now afford an unparalleled opportunity to obtain information not only on specific elements but on the microbial communities at a sub-micron scale. Thus, we are now able to obtain information at the scales of heterogeneity residing within soils. Of course, this opportunity is not easily realized and comes with the challenge of gaining information that is statistically relevant to field-scale processes. Here we illustrate the utility of (micro)spectroscopic and microscopic techniques in deciphering conditions and phases controlling, or stimulating, the migration of three elements that, independently, represent severe threats to environmental quality: arsenic, chromium, and uranium. The present case in Asia exemplifies the devastating impact these toxins can have on human health. At present, in Bangladesh alone more than 57 million people are being subjected to arsenic concentrations exceeding drinking water standards of 10 ƒÝg/L set by the World Health Organization with catastrophic results. Nearly 2 million people have developed severe arsenicosis, 125,000 skin cancer, and up to 7,000 cases of internal cancer are attributable to arsenic exposure from drinking water. Thus, it is critical that we have a detailed understanding of the biogeochemical processes impacting their dissolved concentrations, and availability for biological uptake in general, along with their propensity to migrate within surface and subsurface waters. X-ray micospectroscopy has provided essential data on the solid-phases controlling dissolved arsenic levels, and, in fact, provided key information on redox fluctuations that serve to provide a continual source of arsenic to the aqueous phase. Similar to arsenic, the migration of chromium and uranium within soils is largely governed by redox reactions. In contrast to arsenic, however, chromium and uranium are typically more mobile under aerobic than anaerobic conditions. Within structured media such as soils, redox conditions are not uniform and rather vary depending on the proximity to advective flow paths. Thus, chromium and uranium oxidation state can vary appreciably at the sub-aggregate scale. We provide evidence from microspectroscopic examination that uranium reduction resides at the junction of advective and diffusive domains while chromium reduction transpires throughout the soil matrix.