ARCHIVED REPORTS_2011 REVISED FEASABILITY STUDY

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amec�9 be treated. An area 120 acres in size and 15 feet in saturated thickness, with an average porosity of 0.3, has a pore volume of 540 acre-feet, or approximately 176 million gallons. Because this area is so large, a full-scale bioaugmentation program would likely involve a bio- curtain configuration intended to curtail the movement of VOC-affected groundwater rather than treatment of the entire pore volume. Although the bio-curtain is expected to be more cost- effective than an area-wide bioaugmentation program, major limitations to effectiveness and cost are identified later in this document. The proposed bio-curtain alignment is shown in Figures B.2-1. The water table is approximately 25 feet below ground surface in the WWTP area. For cost estimating purposes, the bio-curtain length is assumed to be 4,500 feet and the target treatment interval is assumed to be from approximately 120 to 135 feet bgs to the base of the C1-zone of Intermediate Aquifer. The total thickness of the treatment area is assumed to be 15 feet. The assumed treatment area for this feasibility analysis is based on groundwater samples from the existing monitoring well network which are too far apart to base a final design. Full scale implementation of this bio-curtain would require a significant amount of additional subsurface characterization along the proposed alignment to ensure adequate distribution of microbes across zones of groundwater with high VOC concentrations. 3.2 EFFECTIVENESS Bioaugmentation could be effective if microbes could be distributed evenly across the entire bio-curtain alignment at the target concentration. However, effective delivery may not be achieved because of filtration and attachment of injected bacteria to aquifer sediments, as discussed in earlier sections of this technical assessment. Attachment and filtration of injected bacteria would likely cause clogging of pore-spaces near the injection location resulting in additional delivery limitations. Although partial treatment could result from this scenario, it is possible that VOC-affected groundwater could pass through large sections of the intended bio- curtain alignment without treatment because of the uneven distribution of microbes, where decreased permeability would coincide with areas of higher microbial concentration. Therefore, AMEC concludes that the effectiveness of a full-scale bio-curtain is highly uncertain. 3.3 IMPLEMENTABILITY Of the above potential side-effects associated with bioaugmentation described in Section 2.2, the two most undesirables are the increase in total dissolved solids (TDS; from increased dissolved metals, alkalinity and organic carbon) and the increased mobility of arsenic in the aquifer system. These are undesirable because both TDS and arsenic concentrations have already exceeded groundwater quality objectives in the past and continue to be major groundwater quality concerns for stakeholders in the Ripon area. The addition of organic substrates during bioaugmentation could exacerbate these existing groundwater quality AMEC Geomatrix, Inc. \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTFS Assessment_Apx B_01 2711\Attachment B.2\Attach B-2.doc 132-8