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r: In general, with no remediation systems in-place on-site, the extent of a subsurface hydrocarbon = ) plume could be increasing, steady (essentially constant), or shrinking with time, depending on the relative magnitude of the hydrocarbon loading rate and the degradation rate. In the f' subsurface, hydrocarbons are dissolved and added to the plume as fresh ground-water flows through hydrocarbon-impacted soils where benzene, toluene, ethylbenzene, and xylenes (BTEX) leach from the soil into ground water. The mechanisms which reduce the concentration at a monitoring point or reduce the mass within the plume include dispersion, adsorption/retardation, biodegradation, and volatilization. ==` The extent and migration of subsurface hydrocarbon plumes are governed by the relative magnitude of the hydrocarbon loading rate to ground water and the relative magnitude of the attenuation rates. Initially, a plume will expand because the biodegradation rate is less than the loading rate. This is evident shortly after a release has occurred, before the biodegradation rate adjusts to the new loading rate. For a plume to remain steady (essentially the same size), mass loading is balanced by mass ..i removal (intrinsic biodegradation = mass loading). This steady-state condition controls the =1� migration of the plume and is attributed to the effects of intrinsic biodegradation. The effects of dispersion and adsorption will affect the concentration gradient, but will not remove hydrocarbon mass from the plume. VERIFICATION OF CLEANUP Site closure will ultimately depend upon the SJCPHS and the CVRWQCB. Hydrocarbon vapor concentrations from bioventing wells will be plotted versus time. When this curve approaches Fj an asymptotic slope, confirmation soil borings will be sampled to confirm the progress and effectiveness of in-situ bioremediation efforts. Soil samples will be collected from the +" confirmation borings at 5-foot depth intervals and analyzed for TPH-g using California DoHS approved EPA Method 8015 modified for gasoline, and for BTEX using EPA Method 8020. The soil will be considered remediated when laboratory analyses reveal that TPH-g levels have been reduced to 10 mg/Kg or less, and benzene levels have been reduced to 0.1 mg/Kg or less =- in accordance with the guidelines set forth in the State of California LUFT Manual. Ground-water remediation might be expected to be considered completed when BTEX E=' concentrations fall below the State Maximum Contaminant Levels (MCLS) of 1, 150, 700 , and 1750 /Agl f, respectively. W.W. Intivin, Inc. r Project No. 33016.07 December 1995 10 J