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amec c12DCE, and 1:12DCE. This sample depth at 145 feet bgs appeared to represent the approximate vertical extent of COCs in the vicinity of the well MW-5 cluster. An evaluation of these results and the results of borings down gradient of the MW-5 well cluster presented in the CAP concluded that the maximum vertical extent of CDCs from the site is unlikely to extend much beyond 145 feet bgs (AMEC, 2010a). 6.0 FATE AND TRANSPORT OF CHLORINATED VOLATILE ORGANIC COMPOUNDS IN GROUNDWATER FROM THE SITE The fate and transport from the site of the CVOCs in groundwater were discussed in detail in the CAP and are summarized in the following sections. The groundwater data indicate that CVOC concentrations in groundwater beneath the site are declining, as described in the following section. 6.1 MOVEMENT IN GROUNDWATER Because of the fine-grained nature of the sediments and the relatively shallow horizontal gradient, groundwater beneath the site is estimated to travel very slowly. The groundwater flow velocity for this site is estimated to be on the order of 10 to 30 feet per year, as documented in monitoring reports submitted to the RWQCB since December 2007 (AMEC, 2010a). CVOCs typically migrate in groundwater at much slower rates than the bulk groundwater flow velocity, primarily due to adsorption-desorption reactions with the aquifer material. The effect of these reactions is to retard CVOC movement in groundwater, sometimes significantly. The transport of dissolved PCE in groundwater in a fine-grained, organic-rich aquifer environment such as exists beneath the site and vicinity may be 3 to 5 times slower than the bulk groundwater. Using the groundwater velocity estimates above, PCE in groundwater may be migrating at a rate of 2 to 10 feet per year(AMEC, 2010a). 6.2 DEGRADATION OF CHLORINATED VOLATILE ORGANIC COMPOUNDS IN GROUNDWATER The concentrations of CVOCs in groundwater generally are reduced over time by naturally occurring processes. These processes include physical mechanisms and transport processes, such as dispersion, advection, sorption, and volatilization; chemical mechanisms, such as hydrolysis and elimination; and biological mechanisms, such as aerobic and anaerobic biodegradation (AMEC, 2010a). Data collected thus far indicate that the concentrations of the CVOCs in groundwater beneath the site and off site are being naturally reduced by anaerobic biodegradation. Under anaerobic biodegradation, microbially-mediated processes induce reductive dechlorination and transformation of CVOCs to metabolic byproducts, or daughter products. The AMEC Geomatrix, Inc. I�\13000s\13290\Archive\13290-49.DOC