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4.0 REMEDIAL ACTION As detailed in section 2 3, authorization to proceed to remediation was granted by PHS/EHD in 1997, and Clearwater Group submitted a Corrective Action Plan and cost estimate The UST Cleanup Fund pre-approved Upgradient Environmental's cost estimate to use Oxygen Releasing Compound (ORC) in early 1998, and site remediation began in April 1998 The remedial procedures are described in section 2 3 1 4.1 Dissolved Oxygen Concentrations The purpose of infecting ORC into the soil was to increase the concentration of dissolved oxygen in the groundwater In the presence of oxygen, soil microbes can degrade fuel hydrocarbons by the process of respiration In this process, the hydrocarbons are oxidized (they donate electrons) and oxygen is reduced (it accepts electrons) Frequently, then, oxygen is the limiting factor in the process of natural biodegredation of the contaminants Increasing the concentration of oxygen should therefore lead to natural attenuation of hydrocarbons in the groundwater Concentrations in excess of 3 mg/l (ppm) are generally considered indicative of aerobic conditions suitable for intrinsic bioremediation, whereas concentrations less than this signify anaerobic conditions The concentration of dissolved oxygen was measured on seven occasions in 1998 for comparison to changes in hydrocarbon concentrations The first measurement took place at the time of ORC injection (April) to establish a pre-remediation baseline Concentrations were then measured for three consecutive months (May-July), and then again in October- December Each time, a battery-powered submersible pump was used to purge the standing water from the monitor wells, and a portable sensor was used to measure the concentration in percent DO and in milligrams per liter (mg/1) at intervals during purging Neither the vapor extraction wells nor the sparging wells were purged because of limited water or slow recharge rates The sensor was calibrated against a 0% DO solution and against ambient air (100%) Experience indicated that concentrations measured in percent and mg/l did not correlate well, and it is uncertain which is the better measure of true concentrations In general, however, dissolved oxygen concentrations measured by either method declined as the standing water was removed, indicating that oxygenated water in the well bore was being removed and replaced with less-oxygenated water from the surrounding soil Final readings at the conclusion of purging are tabulated in Tables 5 and 6 and graphed in Figure 22 Unfortunately, no sustained increase in oxygen concentrations is evident in the data from any of the wells In all cases, concentrations were less than 3 mg/l at the end of purging, implying that truly aerobic conditions were probably never attained in the aquifer 4 2 Hydrocarbon Concentrations The extent of benzene in the groundwater was discussed in section 3 2, and changes in the benzene concentration over time were discussed there Here, the discussion will focus on changes in TPH-g concentration with time Because these changes have varied in both 7