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' is occurring A plume undergoing natural attenuation
<br /> ,kill migrate more slowly than would be expected based _ 30 "wit,
<br /> en the average ground water velocity and the retarda- nK +•
<br /> _ I
<br /> „on factor of the contaminant Typically for BTEX 7S 1 °jinD d' wa«F....
<br /> ,3nsport in sandy soils with relatively low organic car- 20 \ �„
<br /> ,,on content (---0 I percent), retardation factors range \ ,i
<br /> 1em
<br /> 15 to 3 0 (Verschueren 1983, Kemblowski et al ' is
<br /> 1957) Thus, the BTEX components are expected to +• „Q\ %117 to
<br /> } migrate at one-third to two-thirds of the average ground -
<br /> aater seepage velocity However if significant natural
<br /> degradation is occurring then the leading edge of the e Ma+�� \\_ •
<br /> plume may be migrating at less than one-third to one- - \
<br /> half of the average ground water seepage velocity if
<br /> monitoring data, obtained via consistent sampling Figurega Benzene concentrations(ppb)in ground water at a
<br /> methods demonstrate that the plume of contamination Florida service station—February 1986
<br /> is moving slower than expected given the average veloc-
<br /> ity of ground water flow and the contaminant retarda- ; S.,,e,
<br /> ]a rwu
<br /> tion factor then degradation is probably limiting the r
<br /> =. 07 �, �S ,, ,,,,� ,,,,,s,,,.�, .S,..n.,.10?
<br /> inop,ement of the plume It should be noted that the =7S o.�oNa 4>s
<br /> retardation factor varies with organic carbon content in M'+"r'•` +S i
<br /> different soil types and should be evaluated on a site- � ' s
<br /> specific basis s,S i
<br /> t
<br /> Natural attenuation due to biodegradation of BTX
<br /> was demonstrated at a Vero Beach,Florida service sta- • • • to
<br /> tion by comparing expected distance of plume migration = S S
<br /> n the actual migration (Kemblowski et al 1987) After
<br /> recovering free product for three years, the recovery • , ° • •
<br /> system was shut down and a monitoring program was • +•
<br /> unplcmented over the next 15 months Several addi-
<br /> tional monitoring points were installed at the site and Figure 8b Benzene concentrations(ppb) in ground water at a
<br /> ground water concentrations of BTX were monitored Flonda service station—April 1987
<br /> These 4ata indicated that natural attenuation mecha- id _
<br /> nisms were degrading the soluble plume before it i
<br /> reached the drainage ditch The ground water concen-
<br /> tratiFigures 8a and 8b,respectively g The source
<br /> ons of benzene as of February 1986 and April 1987 R Xylcncs
<br /> are shown in Fi r _
<br /> y ic,
<br /> area is near,MW-18 and equtpotential lines clearly shoo _ �' "�'-- � �
<br /> that all the ground water that seeps through the site �� \ '""Eihylbcazcac
<br /> flows into the ditch From February 1986 to April 1987 z ��
<br /> four sets of water samples collected for chemical analysis
<br /> sbowed that the area]extent of the plume had not signifi N
<br /> cantly increased The benzene and toluene Concentra- N
<br /> tion distributions measured downgradient from the con- to,
<br /> Laminated zone suggested that the soluble plume was N
<br /> being degraded As a quantitative indicator of natural �� --
<br /> attenuaLion,the expected travel distance of benzene was 0 5W 1M10 15DU
<br /> estimated from the time recovery was halted in February Distance from Source (ft)
<br /> 1986 to the April 1987 sampling event using Equation 1 Figure 9 Comparison of B,7 E, and X flow path conceima-
<br /> bons measured in February 1993 at a New York service sta-
<br /> L _ VAT tion
<br /> R (1) ditch which was only 45 feet from the source The actual
<br /> where travel distance of the plume's leading edge was only
<br /> L is the travel distance, feet, V is the average about 10 feet over this time Thus,the benzene distribu-
<br /> ground water seepage velocity, 019 ft/day, At is the tion appeared to have reached steady state with natural
<br /> [rave) time between the two dates, 410 days, and R is attenuation preventing the soluble plume from impact-
<br /> the retardation factor (deternuned by laboratory sore- ing the ditch
<br /> 11011 studies with soils from the site),158 dimensionless
<br /> sing Equation 1 and the above parameters, the Relative Mlgrabon of Multiple Contaminants ,
<br /> exPected travel distance from the source was calculated Analyzing the relative migration rates of benzene,
<br /> 10 be 19 feet, however, the plume had not reached the toluene, and xylene individually can also indicate that
<br /> SPRING 1994 GWMR a 169
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