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4.2.2 In Situ Bioremediation 4.2.2.1 Description r Bioremediation is a process in which microorganisms degrade contaminants in situ. When microorganisms ` are stimulated to grow and use the contaminants as a food and energy source,contaminants are broken down and destroyed. A more detailed description of the bioremediation process is presented in Section 4.2.3.1. Since this is a biological process,a certain range ofpH and temperature along with a sufficient ►. amount of oxygen,nutrients,and moisture are necessary for the microorganisms to act on the organic contaminants ofconcem. Little to no residual treatment is required. The implementation ofthis technology includes using a direct-push drill rig and injecting an oxygen release compound(ORC�)through drive rods into the subsurface treatment area. The ORC®slowly releases oxygen over time,resulting in oxygen diffusing through the groundwater and enhancing biodegradation and destruction of BTEX compounds. i 4.2.2.2 Evaluation r. Based on the site location,geology,hydrogeology,and the groundwater sampling conducted to date,all ofthe necessary conditions are present for the natural biodegradation ofthe contaminants. As was show in Table 3-2,concentrations ofBTEX compounds and TPH-g have decreased dramatically since inception ofthe groundwater monitoring program;however,these concentrations have recently leveled offin the source monitoring wells MW-2 and MW-2A. The limiting factor in natural biodegradation ofcontaminants is the presence of sufficient oxygen to carryout aerobic processes,as oxygen can be rapidly consumed by microbes. In the in situ bioremediation alternative,the amount ofoxygen in the groundwater would be enhanced by injection of the ORC®in the source area contaminated groundwater. Higher dissolved oxygen levels are generated from this technique than those possible with the AS system. Moderate pH levels are also maintained. WESTON therefore recommends that the in situ bioremediation technology be considered further. 4.2.3 Intrinsic Remediation with Monitoring 4.2.3.1 Description r, Intrinsic remediation is a remedial approach that relies on natural attenuation to remediate dissolved contaminants in groundwater. Mechanisms of natural attenuation of dissolved fuel hydrocarbons in groundwater include advection,dispersion,dilution from recharge,sorption,volatilization,hydrolysis,and biodegradation. Ofthese processes,hydrolysis and biodegradation are the onlymechanisms that transform contamniatits into innocuous byproducts. Intrinsic bioremediation occurs when indigenous microorganisms work to bring about a reduction in the total mass of contaminants in the subsurface without the addition of ` nutrients. 11