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�N <br /> am <br /> s <br /> wu <br /> i rz I m <br /> o ❑ � w:Ecom M � r= . <br /> rz a�rmo a 'ion w »• <br /> k E00 a "1000 r' rz n ]00 ( we <br /> ►L. -wu lay +1+ ,,:x, lAO rz lox <br /> z• r u ryl ,.'y <br /> 4rryr�lrY YI•r <br /> f <br /> 9 1m ap � i00 300 0 <br /> •Lp O <br /> aos.a.ra.a, !L0 apo Spp app <br /> Figure 6a_ BTEX plume contours(ppb) on May 7, 1986,at the Figure 6c BTEX plume contours{ppb} on February 8, 1989, at <br /> Rhode!stand service station the Rhode Island service station <br /> IN �N i <br /> apo <br /> AM' <br /> rz <br /> J IV& >ti. .wH 1, u r•Ip <br /> Gn+M+aler F1. <•rwMrNrr itir <br /> 1 <br /> Im X0 400 300 /W <br /> O Ip0 <br /> O�r ie � J00 �p0 70r am <br /> Figure 6b BTEX fume contours �r`R' <br /> p (ppb] on October 5, 1987, at Figure 6d BTEX plume contours(ppb}on April 18, 1990,at <br /> the Rhode Island seance station the Rhode Island service station <br /> and variations in monitoring well concentrations can be observed to decrease in extent and concentration over <br /> attributed to random factors such as water table fluctua- time In order for this behavior to be observed under <br /> lions, sampling variability, and analytical uncertainty, normal circumstances, the rate of natural attenuation <br /> then the plume is at steady state in order for a plume must be greater than the rate of contaminant addition <br /> to reach steady state, the rate of natural attenuation to ground water Typically, this behavior is observed <br /> must equal the rate of contaminant addition to the aqui- when the residual source of contamination is removed <br /> fer from the source Thus, if a plume ceases to migrate or depleted significantly by either active remediation <br /> and appears relatively constant from one sampling event efforts (free product recovery, soil venting, excavation, <br /> to the next, then natural attenuation is occurring If a etc) or natural source attenuation (flushing, biodegra- <br /> plume has reached steady state,then easily applied ana- dation,etc) Thus,if the source concentrations at a site <br /> lYtical models (Domenico 1987) for ground water flow decrease significantly over time it is probable that the <br /> and transport can be compared to plume centerline con- plume will expand to a maximum extent and then the <br /> Itntrations to quantify the rate of natural attenuation leading edge will recede if natural attenuation is occur- <br /> Recedtng Plume ring Data from a service station in Rhode Island from <br /> behavior O , <br /> In some cases, contaminant plumes have been the!adingto 19�edge of theillustratehis plume reced dver fromi 215rfeet <br /> SPRING 1994 GWMR ■ 167 <br />