HR: 14:40h
AN: H13L-05    [Abstracts]
TI: Arsenic Release Mechanism in an Intensively Irrigated Agricultural Region of the Alluvial Aquifer, Eastern Arkansas, USA
AU: * Kim, B
EM: bxk03@uark.edu
AF: ENDY Ph.D program, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701, United States
AU: Steele, K F
EM: ksteele@uark.edu
AF: Department of Geosciences, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701, United States
AU: Davis, R K
EM: ralphd@uark.edu
AF: Department of Geosciences, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701, United States
AU: Sharif, M U
EM: mushari@uark.edu
AF: ENDY Ph.D program, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701, United States
AU: Kresse, T M
EM: tkresse@usgs.gov
AF: Water Science Center, US Geological Survey, 401 Hardin Rd., Little Rock, AR 72211, United States
AU: Fazio, J A
EM: fazio@adeq.state.ar.us
AF: Water Division, Arkansas Department of Environmental Quality, 8001 National Drive, Little Rock, AR 72219, United States
AB: Twenty one of 118 wells from the alluvial aquifer in the Bayou Bartholomew watershed in eastern Arkansas exceed the maximum contaminant level (MCL) of 10 ìg/L for As. This watershed is one of the most intensive agricultural ground-water irrigation regions in the U.S.A. It was hypothesized that the intensive ground-water irrigation caused significant water level fluctuation that could affect the geochemical evolution and mobilization of As in the alluvial aquifer. In order to test the hypothesis and conceptualize the As mobilization mechanism, laboratory column experiments and field work were conducted. Ground-water level and quality was monitored at 3 monitoring sites (a shallow and a deep well at each site) and 21 irrigation wells. Disaggregated sediments from the borehole of 3 monitoring wells were packed in 6 in (D) * 2 ft (L) acrylic columns proportional to the field sediment profile in terms of lithology and thickness. Field collected ground water was passed through a pre- treatment column also packed with sediments collected in the field in order to a reducing environment similar to field conditions. The regenerated water was used as input for three separate columns: 1) a column exposed to air representing oxic water-level fluctuation, 2) a column isolated from air representing anoxic water-level fluctuation, and 3) a column isolated from air with continuous flow. ORP, pH, conductivity, DO, and temperature were measured in situ in the columns, and water was collected periodically for chemical analyses. Ground water was collected from the monitoring and irrigation wells during the recharge season (April 2007) and growing season (July 2007), and analyzed for major and trace ions. The ground-water quality was generally similar with laboratory column experiments (e.g. As: <5 – 88 ìg/L and 10.3 – 354 ìg/L, Fe: 0.016 – 38 mg/L and 0.029 – 50.5 mg/L for field and laboratory, respectively). Statistical and graphical analyses, and geochemical modeling with PHREEQC indicated that surface complexation of arsenic onto the iron oxyhydroxides and ion exchange were the main geochemical processes causing As mobilization and transport in the two columns representing water-level fluctuation, whereas mineral dissolution and ion exchange were the main geochemical processes operating in the continuous flow column. A small amount of competitive sorption and reductive dissolution of iron oxyhydroxides were observed in all columns. These distinct conditions in the laboratory (oxic and anoxic fluctuation, and continuous flow) are aggregated in the natural environment. Based on the laboratory column experiments and ground-water chemistry from the field samples, the following mechanisms were determined for arsenic mobilization: 1) In an oxidizing environment (during the growing season when the water level is lowered and/or fluctuated by irrigation) arsenic was sorbed onto the iron oxyhydroxides in the sediment, 2) when a reducing environment was developed, arsenic was released into the ground water by the reductive dissolution and the common ion effect, resulting in the increase of the arsenic concentrations during the spring season, and 3) infiltration of phosphate, carbon, and nitrogen increased the common ion effect (competitive sorption and interference with sorption) and the development of reducing condition during the recharge season.
DE: 1831 Groundwater quality
DE: 1834 Human impacts
DE: 1842 Irrigation
DE: 1847 Modeling
DE: 1880 Water management (6334)
SC: Hydrology [H]
MN: 2007 Fall Meeting