ABSTRACT
Groundwater containment strategies of the 1970s and 1980s and the physical removal methods developed in the 1980s and 1990s (e.g., aquifer sparging) have not always succeeded in achieving satisfactory contaminant removal from source zones or from sorbed-phase mass in downgradient segments of aquifers. The limitations on early remedial strategies drove a transition from containment and physical removal methods, to in situ treatment strategies, which could achieve greater effectiveness through destruction, rather than removal, of contaminants. The desire for contaminant destruction naturally kindled an interest in chemical reaction mechanisms. There is a wide-ranging repertoire of reaction mechanisms that were developed for water supply and wastewater treatment systems, and many of the reactions now used in chemical reactive zones are "borrowed" from those aboveground systems. The major differences for their use in aquifers are the very large volumes that are engaged in reactions and the loss of reaction control that occurs in their migration from aboveground reactor systems to reactions on the loose in the subsurface.
