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EM 1110-1-4001 3 Jun 02 • b Several data trends are commonly encountered in monitoring contaminant concentrations in soil, soil gas,and vent gas Residual soil contamination(paragraph 2-3c(6))decreases with venting time,and distillation effects are apparent from preferential evaporation of more volatile compounds,leaving heavier compounds behind However, tracking residual contamination accurately requires analyzing a large number of samples because soil,being an unmixed medium,is heterogeneous Analysis of residual contamination is usually limited to before venting,to determine starting concentrations,and after the venting operation is complete,to confirm that treatment goals have been met (Analyses of residual contamination in soil samples are actually analyses of the residual plus aqueous plus vapor phase contaminants) c SVE shutdown should not be considered in isolation at sites with underlying contanunated ground water Concentrations of contaminants in ground water should also be monitored to determine the contribution of contaminants from the aqueous phase to the soil gas A site contaminated with up to 55 mg/kg of PCE in soil was subjected to SVE to achieve a cleanup goal of 1 mg/kg After 9 months,the PCE concentration in vent gas was less that 1 percent of its initial value Soil gas concentrations met shutdown criteria,but soil sample analyses showed PCE concentrations of up to 15 mg/kg It was found that groundwater was recontaminating the soil by capillary action and water table fluctuations Bulk fluid movement during a period when the water table rose and fell evidently accelerated the mass transfer process from the saturated zone to the unsaturated zone Solute was apparently transported up into the vadose zone during a water table rise,and then exposed to soil gas in the vadose zone following a drop in the water table and draining of soil near the capillary fringe Under stagnant water table conditions,by contrast,the mass transfer process would tend to be diffusion limited and therefore four orders of magnitude slower than during a period of bulk fluid movement A rough calculation showed that • groundwater could have contributed 270 kg of the more than 325 kg of PCE that were removed by the SVE system(Urban 1992) By performing extensive simulations using the multi-phase flow model,T2VOC, Williams at al (2000)have demonstrated a similar phenomenon at the Twin Cities Army Ammunitions Plant(TCAAP) d Air,being a mixed medium,is more economical than soil for monitoring the progress of SVE/BV operations Vent gas concentrations can provide a gauge of mass removal from the whole soil volume affected by the SVE/BV system,while soil gas monitoring can resolve spatial variation in vapor phase contaminant concentrations Monitoring of vapor phase compounds,including both VOCs and Oz,CO2 and methane in monitoring points will assist greatly in calibrating flow models and improving confidence in the results of the modeling Another parameter which may be measured is tracer gas concentration at monitoring points after injection of the gas into a specified point This permits estimation of flow velocities to assist in calibration of models and estimation of pore volume exchange rates across the site(USEPA, 1996) e BV is at times employed for treatment of soils contaminated with weathered fuels containing relatively heavy petroleum hydrocarbons In such applications,soil concentrations have been observed to decline moderately fast at first, then the degradation rates decline slowly over time Concurrent shutdown testing performed periodically indicated that oxygen uptake rates declined over time, signaling that most of the more biodegradable constituents had been consumed A risk-based approach to viewing such data might argue that if the remaining constituents are so low in solubility and volatility that they are no longer bioavailable under operating conditions that are known to favor biological activity,they may no longer present a risk,provided that direct contact exposure routes can be prevented through appropriate administrative or containment measures (Smith et a] 1995) i 9-4