Treatment facilities targeted for the production of potable water from a contaminated groundwater source are generally designed with technologies capable of removing contaminants well below drinking water standards and are thus designed conservatively. For example, some water utilities target non-detect levels of TCE in the treated water even though the applicable federal standard in 2012 is 5 μg/L; current analytical methods are routinely capable of measuring concentrations below 1.0 μg/L. In some instances, water utilities will attempt to the remove contaminants that are not currently regulated, but where state standards or compliance levels have been established or are likely in the future. For example, at a groundwater treatment facility located near the Tucson Airport where TCE-contaminated groundwater has been treated for several years using packed tower aeration, operators are installing a UV-AOP treatment process for removal of 1,4-dioxane, which has been recently detected at low levels. Similarly, the groundwater treatment facilities located in the San Fernando Valley in California (Box 5-5) may be upgraded to include treatment for perchlorate and hexavalent chromium.
Challenges for Wellhead Treatment
Wellhead treatment for removal of chemicals typically present at hazardous waste sites is not a new challenge for water utilities. Treatment systems have been in place for more than 30 years in locations where VOCs impact groundwater sources (e.g., New Jersey, California, Arizona). Despite a long history of using specific treatment technologies (such as packed tower aeration or GAC) for removing organic chemicals from groundwater, the presence of multiple organic chemicals in groundwater resources poses a number of treatment challenges. First, the design of water treatment systems must be based on estimates of the number of chemicals of concern present and their likely maximum concentrations. Accurate estimates of these design values are sometimes difficult to come by, given limited site characterization data or limited knowledge of the nature and extent of the contamination present in the aquifer and within the capture zone of the water supply well. Groundwater fate and transport modeling is inherently limited in terms of accurate predictions, but is currently the best tool available to estimate these design parameters.
Each of the individual treatment systems now in use are subject to operational upsets. For example, an air stripping tower may be effective at removing TCE but should a less volatile organic chemical occur, such as 1,2-DCA, the removal efficiency of the system may not be sufficient to meet the treatment objectives. For GAC, the appearance of compounds less strongly adsorbed may result in more rapid breakthrough of the chemical