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EM 1110-1-4001 3 Jun 02 b Mass Removed Given the often highly uncertain quantity of material released or mass present in the subsurface prior to remediation,the comparison of the mass removed to the mass initially present is a poor criteria for shutdown At some sites,such as a Superfund site in Arizona,more mass was removed in the pilot test than was estimated during the remedial investigation to be present at the entire site,and mass removal was still high at the end of the test c Target soil concentrations As discussed above,many states'target cleanup levels,especially for petroleum hydrocarbons,ultimately limit the residual concentrations of contaminants in the soil Since soil sampling is both costly and potentially disruptive,the site operator will want to be quite certain that the soil samples will show that the cleanup levels have been attained before they are collected For this reason,the shutdown sampling is typically conducted in stages,whereby the attainment of one criterion will trigger the next level of testing,and so on,until achievement of cleanup levels is confirmed For example,the first criterion might be the attainment of a target vapor concentration in monitoring points,based on a correlation between extracted vapor and soil concentrations If this target were met,the system might be shut down for a number of days,after which the in-situ soil gas concentrations and composition would be analyzed If the soil gas results following shut down met target levels,only then would actual soil samples be collected Finally,the results of the soil analyses would be compared with the actual cleanup levels for residual soils At this point,the system might be shut down,but often the equipment will remain in place for some period of time in the event that future confirmatory samples show that concentrations have risen above cleanup levels again, in which case system operation would be resumed The use of soil sampling for confirmation of cleanup and system shutdown must consider carefully the heterogeneous distribution of soil concentrations at a site and the uncertainties associated with sampling soils for VOCs Soil sampling to confirm cleanup requires the use of statistically based sampling strategies to quantify the certainty of achieving goals(USEPA, 1989e) The current SW-846 method 5035 is strongly recommended for sampling soils for VOCs d Extracted Vapor Concentrations and Composition (1) In many cases,the SVE system is operated until the concentrations in the extracted vapors either drop to non-detectable levels or to some asymptotic (but low)level There are some caveats to this method, however First,although the decrease of concentrations in the extracted vapor is an indication of the effectiveness of the system,it is certainly not conclusive evidence that the concentrations in the soil have decreased proportionally Johnson,Kemblowski,and Colthart(1990b)list other potential reasons for decreases in vapor concentrations • Water table upwellmg • Soil drying • Diffusion constraints • Short-circuiting Use of the concentrations in the influent or from an individual extraction well for shutdown decisions,in the absence of other data, is also prone to errors due to the large component of relatively clean flow to many extraction wells that often enters the subsurface near the well This large component of the flow travels through the zones near the well that are thoroughly flushed after some operation compared to soils at greater distances from the vent It is quite easy to design an SVE system that reaches low influent • concentrations while still leaving a significant quantity of mass in the soils,especially near stagnation 9-6