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41 downgradient to only 160 feet downgradient of the source as shown in Figures 6a 6b,6c,and 6d The natural z ground water flow at this site is to the west at approxi- mately 0 4 ft/day This receding plume behavior has been observed at �) i the Boca Raton, Florida service station, shown in W3 BMW„ Figures 7a and 7b, where removal of free product was followed by significant decreases of BTEX levels in monitoring wells at the site The predominant direction iof ground water flow is to the canal bordering the site ��«: on the west with a gradient of approximately 0 003 ft/ Graundwaur Flow n ft and an average ground water seepage velocity of u HTEX Ppb i 10 ft/day In May 1989, free product was found in the x s*MPWMOO. &F*InIl, x s.ma+�,=r,,,,, tank pit monitoring wells located at the corners of the S 1989,when Figure 7a BTEX plume ata Randa service 0 ppb m March underground storage tank area Since May 0 10 feet of free product were detected n MW-1, free 1991 Maximum BTEX conceritraeon = 2300 ppb i product has been removed by hand bailing and no free product has been detected in any wells since May 1990 x As a result of the spill,a ground water plume containing a M^" iBTEX had developed at the site From March 1991 to June 1992 as shown in Figures . Taint 7a and 7b, the plume concentrations decreased signifi- 4 ,a so"' ' ft.M „„,, i F cantly at nearly all wells on the site Over this time the maximum BTEX concentration decreased from 2300 ppb to 110 ppb Table 4 shows the monitoring data from four sampling i events since March 1991 Another method of presenting data for a receding plume and Groundwater Flo �W roughly quantifying the rate of natural attenuation is to evaluate individual well concentrations over time The ��n.g Well r x�amdmr ' first-order rate constants determined by a regression of the four data points for each well are given in Table 4 492 Maximum Plume concentratiionda S= 110 ppban rn June It is emphasized here that this is a highly simplified i analysis which lumps together multiple attenuation and Dint previously described sites (Kembfowski et of I987 transport mechanisms Also the rate constant is a p Chiang et al 1989)and in the literature(Salanitro 1992) rate and is not directly equivalent to the X of Chiang Possible factors accounting for the decrease of the soi- 1 et al (1989) a spatially averaged rate constant This method was applied because all monitors wells at this able BTEX plume include biodegradation,flushing due g to natural ground waiter flow voiatifization due to the site showed a decrease in BTEX levels with a first-order shallow depth to ground water and seasonal water table decay constant in the range of 03 to 1 I percent/day fluctuations i except for MW-10 which showed a slight increase in - concentration from 04 to 25 ppb These decay con- Slower Than Expected Migration stants.although not directly comparable,are consistent i' stewith those reported for biodearadation of BTEX at the stopped lmigrat ng toume needoindicate thathavehed natural attenuation ady state or i Table 4 BTEX Concentrations in Monitoring Wells at a Florida Service Station and First-Order Decay Rates Monitoring Well BTEX (ppb)3/41 IS/9I 6192i Decay Coef.I2J92 %/day MW-1 2300 60 MW-2 940 260 l 9 1 1 8 MW-3 700 150 0 53 ` ,'fit W-4 1600 577 2 060 M W-6 9D0 110 40 044 f MW-8 32 11 066 MW-9 2 39 39 12 1 — 077 - 120 43 28 032 0 4 0 -)5 _ +. i 168 ■ SPRING 1994 GWMR '