and thus increase the usage rate for GAC, leading to more frequent replacement and increased costs.

More recently, some organic chemicals have been found in groundwater that are not efficiently removed by either air stripping or GAC. For example, 1,4-dioxane has become a very prevalent contaminant due to its widespread use as a solvent stabilizer (Mohr, 2010). For those utilities who have already installed GAC, this chemical will not be removed effectively, and an additional treatment process is needed. The most widely adopted technology for removing 1,4-dioxane is advanced oxidation, using either the UV/hydrogen peroxide or the ozone/hydrogen peroxide technology. AOP is a more recent addition to the suite of wellhead treatment options for organics, and limited operational experience is available. An additional issue with the use of AOP technologies is the formation of oxidation byproducts that can be problematic. For example, the ozone/hydrogen peroxide technology will convert bromine ion to bromate, which has a 10 μg/L MCL. Other byproducts could also be problematic. Finally, the excess hydrogen peroxide must be destroyed prior to transfer to the water distribution system.

The reliability of any wellhead treatment system to meet treatment goals (usually below MCLs) on a consistent basis is uncertain. Water treatment plants are usually monitored at intervals that reflect the relative constant influent water quality. Often, for VOCs and other regulated contaminants, a running average based on quarterly sampling frequency is used to assess performance. Given the potential variability in influent water quality characteristics, greater frequency of sampling may be appropriate until the reliability of the treatment systems can be confirmed.

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The decision of whether to perform wellhead treatment versus conducting more conventional groundwater remediation goes beyond the ability of technologies to remove contaminants at the point of use. Indeed, there are significant economic considerations, as wellhead treatment can be roughly as expensive as source or plume remediation, depending on the contaminants involved and the site hydrogeology. Second, it is not yet clear that wellhead treatment is a reliable, long-term strategy for accomplishing exposure cutoff (and it certainly cannot prevent exposure via vapor intrusion). These tensions are illustrated at the Northrop-Grumman/Navy facility in Bethpage, Long Island, New York (see Appendix B). In this case, wellhead treatment of a groundwater plume used as a primary water supply was deemed necessary because full containment of the groundwater plume was thought to be technically infeasible and not cost effective. Nonetheless, affected water districts have lobbied for plume remediation because the plume



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