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NRMRL-ADA-01310 Chen, J., Lin, Z, and Azadpour-Keeley**, A. "Iron Precipitation and Arsenic Attenuation - Assessment of Arsenic Natural Attenuation of the Subsurface Using a Geochemical Model (PHREEQC)." In: Hydraulic Science: Challenges for the 21st Century, 2001 Annual Meeting and 20th Anniversary of American Institute of Hydrology, Thunderbird Hotel, Bloomington, MN, 10/14-17/01. 2001, 20. 03/29/2001 Laboratory experiments show that amorphous and poorly crystallized ferric iron hydroxides have much greater capacity to attenuate arsenic compared to clays and other aluminosilicate minerals. Studies (e.g., Lin and Qvarfort, 1996) showed that a sudden change in geochemical conditions could cause a rapid precipitation or significant dissolution of iron hydroxides. The precipitation and dissolution of the iron hydroxides affects greatly the mobility of arsenic in the subsurface environments. Ground-water aquifers which occur under many landfills are in reducing conditions, and change to oxidizing conditions away from the source areas. This sequence of redox conditions gives the aquifers significant natural attenuation capacities for arsenic. Geochemical modeling of a Superfund site indicates that high arsenic concentrations could stay in the aquifers under the landfill for a long time. When arsenic contaminated ground water flows away from the reducing environment (i.e., aquifer under the landfill) and enters an oxidizing environment, arsenic concentration decreases quickly. This indicates that adsorption and coprecipitation of arsenic with ferric iron hydroxides plays an important role in controlling arsenic mobility. Based on the simulation results of the natural attenuation capacity of ground waters under reducing and oxidizing conditions, combinations of natural attenuation with other remedial technologies are discussed. The combinations may lead to meeting the cleanup goal within a shorter time frame than the sole use of natural attenuation approach.

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