HR: 1340h
AN: H13G-1673    [Abstracts]
TI: Seasonal Variation in Arsenic Speciation in a Shallow Aquifer
AU: * Illera, V
EM: vramon@ucmerced.edu
AF: University of California, Merced, P.O. Box 2039, Merced, CA 95344, United States
AU: O'Day, P A
EM: poday@ucmerced.edu
AF: University of California, Merced, P.O. Box 2039, Merced, CA 95344, United States
AU: Root, R A
EM: rroot@ucmerced.edu
AF: University of California, Merced, P.O. Box 2039, Merced, CA 95344, United States
AU: Rivera, N
EM: nrivera@ucmerced.edu
AF: University of California, Merced, P.O. Box 2039, Merced, CA 95344, United States
AU: Rafferty, M T
EM: mrafferty@sspa.com
AF: S.S. Papadopulos & Associates, Inc., 116 New Montgomery Street, Suite 900, San Francisco, CA 94105-3629, United States
AU: Vlassopoulos, D
EM: dimitri@sspa.com
AF: S.S. Papadopulos & Associates, Icn., 815 SW Second Ave., Suite 510, Portland, OR 97204- 3026, United States
AB: Seasonal variation in arsenic speciation and concentration in sediments in a shallow aquifer were studied with respect to changes in water table elevation and rainfall. Sediment cores were collected at different times from 2004 to 2007 at a marsh site adjacent to San Francisco Bay (in East Palo Alto, CA), which experiences a strong wet winter/dry summer seasonality. The site is a former pesticide manufacturing plant that has undergone remediation and surface capping. Mobilization of post-remediation residual arsenic in sediments is inhibited by natural subsurface attenuation. This study examines the dynamics of seasonal changes on groundwater level and subsurface redox conditions, and its potential impact on arsenic mobilization. Cores (3-cm diameter) were collected from depths of 2.7-3.5 m, intersecting the range of seasonal elevation change in the water table. Groundwater level from a nearby well was monitored with continuous data logging. Sediment samples were analyzed for total and extractable element concentrations and characterized by arsenic and iron synchrotron X-ray absorption spectroscopy (XANES and EXAFS). Three distinct redox zones are recognized spectroscopically within the shallow aquifer: a reduced zone in unsaturated sediments (~0.5-1.5 m depth) where arsenic is present as As-sulfide phases (orpiment or realgar); a transition zone between reduced and oxidized zones at the depth of water table (~1.5 m ±0.5 m) with mixed arsenic oxidation states; and an oxidized zone permeated with oxic groundwaters (~1.5-3 m depth) where only As(V) is present. Sediment samples from the reduced zone had arsenic concentrations from 50 to 150 mg kg^-1. Arsenic concentrations decreased to a minimum in the transition zone to 20 mg kg^-1, and reached a maximum in the oxidized zone (around 200 mg kg^-1). Arsenic XANES spectra showed a progressive change from mostly arsenic sulfides in the upper reduced sediments (component sum ~100%\) to a mixture of orpiment and As(V) deeper in the reduced zone (80%\ orpiment, 20%\ As(V)). In the transition zone, the As(III)/(V) ratio varied with depth, in general from 90%\ As(III)/10%\ As(V) at 1.30 m to 30%\ As(III)/60%\ As(V) at 2.00 m. In samples from between 2.50-2.70 m, As(V) was the main species, although the spectroscopic data indicated a small amount of orpiment and As(III). Iron XANES spectra indicated the formation of pyrite in some samples with arsenic sulfides. Comparisons of annual data showed that the limits of the transition zone were shifted vertically due to regional precipitation and elevation changes in the water table. During the dry season when water table elevation was at a minimum, sorbed As(III) was the dominant species in the partially saturated transition zone. In the winter, water table rise causes oxidative dissolution of sulfides in the flooded reduced zone, with persistent sulfide minerals at shallower depths that are not always saturated annually.
DE: 1836 Hydrological cycles and budgets (1218, 1655)
SC: Hydrology [H]
MN: 2007 Fall Meeting