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MOBILITY OF ARSENIC CONTAINING IRON OXIDES IN ENVIRONMENTAL SYSTEMS
To assess the stability of "FeOOHAs" residues in existing drinking water distribution systems and drinking water generated wastes.
The Arsenic Rule, which became effective on February 22, 2002, is going to require public treatment facilities to remove arsenic (As) from drinking water supplies if As exceeds the new ten parts per billion (ppb) drinking water maximum contaminant level (MCL). The date by which systems must comply with the new standard is January 23, 2006. Because arsenic is geologically correlated with iron, it is common to find elevated concentrations of iron in waters which exceed the arsenic MCL. Iron co-precipitation is the most common means of reducing the arsenic concentration in finished drinking waters. The by-product of this treatment is "FeOOHAs" (an arsenic containing iron oxide / hydroxide) which is commonly back-washed off treatment filters to settling ponds (the resulting sludge can be spread on farm land), streams (the "FeOOHAs" settles out on the stream bed bottom) and / or disposed to landfill (if it passes Toxicity Characteristic Leaching Procedures [TCLP]). The mobility of these solids (which can contain parts per million quantities of arsenic) and their impact on the surrounding environment is not well documented and is a question which needs to be addressed. In streams, the seasonal redox changes in the water could liberate the arsenic resulting in additional uptake by fish and benthic aquatic life. In land applications, the application of fertilizers could solubilize a portion of the arsenic. Natural weathering of the solid on soils and/or microbial release could contaminate agricultural crops grown on the treated soils. Finally, the stability of landfill disposal could be influenced by microbial action. In all cases, the arsenic originally removed from the water during treatment could become an exposure source depending on the environmental conditions which surround the discharged "FeOOHAs". Thus, one of the goals of this task is to characterize conditions which induce the mobility of the As in "FeOOHAs" solids associated with drinking water. This should provide some guidance to treatment engineers and potentially minimize arsenic exposures induced by the dissolution of the arsenic from "FeOOHAs" solids.
A second issue is stability of "FeOOHAs" which has deposited over a number of years as a scale within the distribution pipes. Compliance with the new drinking water standard may cause some of the older iron co-precipitation plants to tweak their removal strategy (in order to improve arsenic removal) by adding a pre-oxidation step. Pre-oxidation may cause a change in the water's redox potential and this shift in redox potential may cause the "FeOOHAs" deposits in the distribution system to be resolubilized. An improved understanding of this dissolution process would aid in developing an analytical protocol to measure the extent of the release from the existing pipe prior to installing a new treatment system or modifying an old one. An analytical protocol which at least estimates this removal rate would aid in treatment decision making necessary to comply with the implementation of the arsenic rule. If dissolution of arsenic residues is unavoidable, the release estimates may aid in informing the public of potential episodic exposures. Therefore, another aspect of this task is the development of a protocol that can be used to provide an estimate of the upper limit on these exposures.