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natural attenuation is occurring As an example, data 10 from the New York service station mentioned previously are shown in Figure 9, where the toluene,benzene, and e total xylenes plume centerline concentrations are Plotted vs distance from the source of contamination Although toluene has the highest concentration at the 6 source, it rapidly depletes and reaches nondetect levels o=yDs tt aom within 700 feet downgradient The benzene concentra- a tion decreases at a slower rate than toluene, but at a higher rate than the xylenes up to a distance of 1000 feet downgradtent Given the soil-water partition coeffi- 2 cients for B,T,E,and X provided by Verschueren(1983), a p ' the relative rates of migration should follow the order 500 7000 7500 2000 Benzene >toluene,o-xylene> ethylbenzene m-xylene, 8TX'ppb p-xylene Figure 10 Comparison of measured oxygen and BTX concen- trations in ground water for a Michigan gas plant. However the extent of toluene migration is significantly less than total xylenes indicating probable degradation of toluene $ ' Dissolved Oxygen Indicators _IL Since aerobic biodegradation has been shown (Chiang et al I989) to be a primary natural attenuation $- 0 ' mechanism for BTEX, D O levels in ground water can ° •"_ " �.. be used to indicate natural attenuation as a resua of n0 300 X00 KO Iq0 1000 10W l� aerobic biodegradation As part of the site assessment 1}AO 100 D O levels should be measured in monitoring wells Figure 1 ' inside and outside the plume including upgradient of central plroduction faa Total cility in M concentration contours{ppb}at a the source Background Dissolved Oxygen $ Laboratory Studies (Chiang ct al 1989 Kemblowski Ll al 1987 Salanitro 1992) have idcnufic,d a threshold - i ' of I to 2 mglL D O below which rates of biodegradation are slow Above, 2 m L D O rapid disc � p ppe.aranec. of t ~M B rEX in laboratory microcosms is Usually ob-,Lrved Thus if background D O lCVels upgradILni of the. plume. ° - ' �� .00 JW OW 0W i�00 1100 1000 r exceed a thr4snhold of l to 2 mglL theaerobic biode.Qra- 1000 1`_0 dation is a possible mechanism for natural attenuation Figure 11b Dissolved oxygen concentrations .m The natural ground water flow into the contaminant water at a central production fat M facility i Michigan b� in ground ' area will providt- a continuous supply of D O which can be utih/ed for aerobic biodegradation trations has been documented and proven statistically ' meaningful by Chiang el at (1989)Inverse D 0/BTEX Correlation The correlation has been observed at other sites At most sites where, background D O Ic,vcls are including a central production facility in Michigan as ` MOM than 1 to 2 ppm monitoring wells with the highest Shown by the contour maps of BTX and D O in Figures BTEC concentrations have the lowest D O levels and l la and I lb Another example with a slightly different f ' vice versa At the Michigan gas plant studied by Chiang graphical presentation is the Rhode Island service sta- et al (1989) the D O concentrations were measured tion mentioned previously with the BTEX and D O on two occasions February 1987 and July 1987 The data shown in Figure I2 The physical explanation of l February 1987 BTX data plotted vs D O in Figure 10 the, inverse D O /BTX correlation is that D O is show that the D O was significantly higher in areas of depleted in regions of high BTX concentrations by aero- low BTX (< 1 ppm) but decreased at high BTX levels bic biodegradation of BTX Conversely in areas of low (> 1 ppm) The data were anaiyied to show that low BTX concentration the level of contamination is insuffi- levels of D O statistically correlated with high levels of tient to fully deplete the available D O Thus if D O BTX and high levels of D O statistically correlated levels are significantly depleted within the soluble plume with low levels of BTX The statistical analysis indicated relative to outside the plume,it is probable that natural — a low probability that the observed correlation was sim- aerobic biodegradation is occurring Whether or not the ply due to chance and thus the field data provide clear resultant attenuation is significant will depend on the evidence of a real correlation between D O and BTX supply of oxygen to the affected region, the rate of t This inverse correlation between D O and BTX concen- biodegradation and the BTEX concentrations 170 ■ SPRING 1994 GWMR