Passive Soil Vapor Extraction - GSI Environmental Inc.

SRNL-STI-2009-00571Rev. 18.2.2 Scenario 2: PSVE May Not be Cost-Effective at Site Where New Wells areRequiredSite Information: Preliminary pilot-scale testing ($10,000) demonstrated that a portion ofthis federal site was not adequately covered by the 10 wells that make up the existingASVE system. To meet PSVE technology requirements and performance objectives, 5additional wells will be installed that target the same depth intervals as the existing wells.These wells are to be screened in lithologically-isolated intervals that will increase theeffectiveness of PSVE, but because these intervals are relatively deep (40—60 ft bgsand 80—100 ft bgs) and must advance through several difficult intervals, the quote fromthe subcontractor for drilling these wells is $50,000 (including all mobilization andoversight costs). The primary capital costs are associated with installation of Baroball TMvalves at each wellhead ($500), and it is estimated that the entire PSVE system can beinstalled in a single day (labor costs of $1,000). The monitoring schedule for the PSVEsystem is identical to that for the previous ASVE system, and the primary O&M savingsoccur as a result of eliminating power requirements for the ASVE blowers ($2,000annually), all components of the GAC treatment system ($3,000 annually), and theassociated maintenance of these systems ($1,000 annually).Table 8.3: Cost Evaluation for Site-Specific Scenario 2.Key Cost ComponentsPilot test cost $10,000Drilling cost for installing new PSVE wellsEquipment costs for Baroball TM valvesLabor cost for installationTOTAL COST $61,500Key Savings Components (relative to ASVE)*Power SavingsTreatment SavingsOther O&M savingsTOTAL SAVINGS (excluding costs)TOTAL SAVINGS (including costs)*Discount rate = 3%$50,000 ($5,000 per well)$500 ($50 per well)$1,000 ($1,000 for 1-day installation period)$2,000 per year$3,000 per year$1,000 per year$6,000 per year$5,000 over 20 year operating period(break even point occurs after 17 years)Results of Cost-Benefit Evaluation: For this type of idealized site, the switch to PSVE willrepresent a net cost savings only after an extended operating period of 15 to 20 years(based on net present value). Consequently, PSVE may only be cost-effective over areasonable project lifecycle if the number of new PSVE wells can be decreased to 3 orless (e.g., focusing only on a source area) or if greenhouse gas emissions from theASVE system become an important cost driver. Alternatively, it may be that the switchfrom ASVE to a PSVE system does not completely eliminate the need for above-groundtreatment of vapors, such that a small GAC treatment unit may have been required forsome or all of the PSVE wells. This is not an unrealistic scenario when new extractionwells are installed it is possible that initially “high levels” of contaminant mass are55 Enhanced Attenuation TechnologiesPassive Soil Vapor Extraction