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multiple sets of data give a high degree of confidence Mw-, to the attenuation rate When interpreting the results of this approach, it is important to understand that if 0 Well " weu the mass of contaminant remains constant over time <MEi)this could indicate that the inputs fromsources are MWiMw s ' balanced by attenuation mechanisms, or that no source is active and no actual degradation is occurring Mw-a While the mass balance approach can provide precise ' quantitative evaluation of natural attenuation, the OA"rovmmeConcentratwnCentou" approach is often impractical due to the extensive moni- • MonHonnp well toring data and intensive data analyses required An • Source Area appropriate approach for most sites from a cost-effec- Figs 3 Recommended ground water monitonng well network ' tiveness perspective would be to utilize more easily along the primary plume flow path for demonstrating natural applied indicators such as plume characteristics and dis- attenuation solved oxygen in conjunction with a confirmatory moni- IV ' toring program Evaluation of plume characteristics over time may not allow precise quantification of rates ' of attenuation but it will reliably determine if contami- 10 ' i nant migration is limited or prevented by natural phe- ' romera Dissolved oxygen indicators are not used to ' quantify natural attenuation but to determine if aerobic U 10= a ficant attenuation mechanism o biodegradation is a signi ' The indicators described below have been derived from Cl detailed studies of natural attenuation and are sup- 10` ported by data from multiple sites Grounawneer Flog. Decreasing Primary Flow Path Concentrations In order to demonstrate natural attenuation in this source 1000 `xx' manner it is important to place a sufficient number of l'oa'm" Distance from Source (ft) ground water monitoring points as close along the pr1- Figure 4 Benzene concentrations along the plume flow path at mary plume flow path as possible (Figures) To identify a New York service station. I the primary flow path, monitoring points such as .%4W-7 and MW-8 must be placed to define the transverse extent los j of the plume If natural attenuation is occurringat a �ee Ips � � is -� sept i9v3 spill or release site the contaminant concentrations will �—Ats Imo, decrease with distance from the source along the flow path of The plume Figure 4 clearly demonstrates this behavior for a service station in New York where Baso- line was released The beniene concentrations along the ; ! primary flow path decreased from 2600 ppb at the U 1o' • �! source to 2 7 ppb at a distance 1425 feet downgradient The decrease in concentrations with distance from the to' • source directly indicates that some degree of natural attenuation is occurring If no natural attenuation (I a 10ND •+ID No no biodegradation no source decay no volatilization 0 Soo 1000 1500 or insignificant dispersion) was occurring, then concen- Distance from Source trations would remain relativeiv constant to the leading Figure 5 Toluene concentrations along the plume centerline at edge where a sharp front would be observed In reality three monitoring events at a New York service station some degree of natural attenuation will always be Notes:Source concentirations were not measured on"2 and observed due to physical processes such as dispersion 9/92 due to the presence of free product P and volatilization Evaluation of the centerline concen- Not ail wells were sampled at each monitoring event - trations over time will determine if the plume is migrat- trations should be monitored routinely for use as Ind- mg or not Toluene centerline concentrations from the icators of natural attenuation Vew York service station are shown in Figure 5 over a six-month period In addition to decreasing with dist- Steady-State Plume ante from the source the toluene plume retreated over In addition to centerline concentrations, historical time as the nondetect point for toluene receded toward monitoring data for the entire plume can be examined the source in successive monitoring events The receding to determine if the contaminant plume is at steady state, toluene plume probably resulted from decreasing source receding, or continuing to migrate If the extent of the concentration over time Thus,plume centerline concen- plume has not changed over a significant period of time, 166 a SPRING 1994 GWMR