Geochemical and microbial reactions affecting the long-term performance ofin situ 'iron barriers'

The in situ application of granular iron (Fe-0) has become popular for the destruction of halogenated organic compounds and for the immobilization of specific metals in groundwater. However, a knowledge gap exists concerning the long-term performance of the Fe-0-barriers. The corrosion of Fe-0 may p roduce mineral precipitates that alter the system's hydraulic integrity. Fo r example, data from existing barriers show varying trends in pH, alkalinit y, mineral precipitation, and microbial activity. Although the chemical beh aviors are site-specific, this paper discusses the concepts involved in dev eloping a generic approach for predicting the trend of aqueous and surface speciation, and the resulting effects on Fe-0 treatment systems. Observatio ns from existing Fe barriers are summarized, and the chemical and microbial processes that influence chemical speciation, both in water and on surface s, are reviewed. A conceptual geochemical model is presented, which illustr ates the factors that must be considered in developing a quantitative model that can be used to design monitoring plans for timely detection of cloggi ng in Fe-0 reactive barriers. In order to develop quantitative predictive m odels, field and laboratory research should: (1) assess the extent and rate s of media deterioration by analyzing coupled chemical and microbial reacti ons; and (2) identify the controlling mechanisms for hydraulic alteration w ithin and around Fe-0 barriers. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.