12.  IN SITU Bioremediation Of Chlorinated Ethene Source Zones

In situ bioremediation (ISB) was not initially considered a feasible technology for treating chlorinated solvent source zones, and it still faces some skepticism. However, experience over the last decade demonstrates that it is a viable technology for treating some source zones, although realistic objectives must be set. It is reasonable to expect 90-99% reductions in groundwater concentrations and mass discharge from a source in many situations, but it is not likely that ISB can achieve complete cleanup of a source zone within a few years. ISB is best applied in stages, optimizing the design and operations over time. Some residual contamination will remain, and modeling suggests that contaminant concentrations may rebound after treatment, although no rebound has been observed so far at field sites, and it is likely that the accumulation of bacterial biomass and reduced minerals during ISB will sustain treatment for several years after active treatment ends. Practitioners considering ISB for a source zone should be aware of several potential difficulties. The electron donor demand can be so great that it is not feasible to supply enough donor and/or pH buffer, or adding large amounts of donor may cause other problems such as methane production, biofouling of wells or clogging of the subsurface. Using ISB to treat source zones requires careful design, monitoring and continuous optimization throughout treatment, and likely will require extended passive management after treatment.