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4 _ _ . i MMM ON r� further states that "inadequate purging and high tur- bidity in the water samples may account for some of the decrease in concentration seen over time in the ground - water samples collected from the wells. " The decrease in hydrocarbcn contaminant concentrations observed in groundwater samples may be due to degradation of the hydrocar - bons by natural. processes. Biodegradation of contaminants is currently a topic of considerable interest in the environmental t engineering literature. Natural biodegradation of petroleum contamination in both the t dissolved and vapor phases by a number of different indigenous . y bacteria, yeasts, .fungi, and algae is well-documented (Gibson and Abdul. 1988) . Complete reduction of hydrocarbons by these �ranisms results "in the creation of carbon dioxide, water, and - new. cell mass (Canter and Knox 1985 ) . ' Aquifers that contain oxygen can support aerobic microorganisms capable of degrading organic compounds including benzene, toluene, and xylenes (Wilson et al. 1986). Anaerobic conditions in: groundwater support different strains of microorganisms which x biodegrade these compounds, and also may be env.ironments.. which are conducive to mineralization of these compounds by methano- genesis, denitrification, or sulfate reduction (Ridgeway. et al. 1989)". _ Th'e,se 'considerations suggest that biodegradation ofthe hydrocar bons_ by, naturally occurring organisms may. have cc�ntributed to the decline of 'contaminant concentrations observed at the site over the 'last tw.o .years. EA is not :aware that turbidity:-in water quality samples can result in a loweredcontaminant'concentration 'in laboratory analyses of groundwater. .' Turbidity 'is almost .unavoidab,le in water samples extracted from groundwater monitoring wells which e35/71050/2-90/tx 9 x 4 {