DIRECT DETERMINATION OF ARSENIC AND IRON SPECIATION IN SEDIMENTS AND GROUNDWATER USING X-RAY ABSORPTION SPECTROSCOPY: A TIDAL MARSH CASE STUDY

The mobility of arsenic in groundwater is tied directly to coupled redox reactions, often involving iron, that determine attenuation mechanisms such as sorption or precipitation. Adjacent to the site of a former pesticide manufacturing facility in East Palo Alto (CA), soil and groundwater arsenic concentrations exceed 1000 mg/kg and 100 mg/L, respectively. Although groundwater velocities are ~100 ft/yr towards San Francisco Bay to the east, monitoring of arsenic concentrations in wells over the last twelve years has shown arsenic levels down gradient of the plume boundary to be less than 0.010 mg/L. This indicates that the plume boundary has not moved appreciably during this time and suggests that natural processes in the tidal marsh-mixing zone are effectively limiting the subsurface migration of arsenic. The speciation of arsenic and iron as a function of depth was determined in situ using X-ray absorption spectroscopy on fresh, wet sediment samples taken over a 12-ft. interval (9.5 feet of core recovered). Arsenic oxidation state (As(III) or As(V)) in sediments is related directly to lithologic horizons and depth to groundwater. Surficial As(V) is reduced to As(III) ~0.5 ft. below the surface in a clay and sand zone. At ~5-6 ft. depth, a zone of mixed As(III)/As(V) is found, below which all arsenic is oxidized to As(V). This oxidized zone coincides with groundwater level in a sand layer and with relatively high total arsenic concentrations. We hypothesize that arsenic is attenuated in the sand layer by oxidation to As(V) and sorption to mineral surfaces. Dissolved iron (Fe2+) is found in concentrations in excess of 1 mg/L, indicating iron reduction. Dissolved arsenic down gradient of the core is present as As(III). Thus, arsenic attenuation depends on the rate and extent of arsenic oxidation, probably coupled to iron and facilitated by microbial activity, in the upper shallow groundwater zone.