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table is shallower, ground water temperatures are 10 percent of the observed natural attenuation at most higher, and/or rates of aerobic biodegradation are sites The next section discusses several indicators which slower, volatilization may account for a larger fraction can identify sites where natural attenuation may be of the total mass loss occurring The case studies which follow illustrate how - ' these indicators and other methods may be used to dem- Chemical Transformation onstrate natural attenuation In addition to biological degradation and physical ' phenomena abiotic transformations due to naturally Indicators of Natural Attenuation occurring chemical reactions may result in significant In order to demonstrate that natural attenuation is natural attenuation as well While the BTEX com- occurring, a site assessment must be conducted to ' pounds are not expected to be transformed by chemical characterize the hydrogeology and determine the extent processes, several halogenated compounds undergo of contamination at the site hydrolvsis and dehydrohalogenation reactions under ' typical ground water conditions Table 2 { BTEX Conventional Site Assessment Data Used to Evaluate i "^ Degradation of BTEX due to chemical reactions has Natural Attenuation ' not been shown to occur in the laboratory or the field Typical batch (Kemblowski et al 1987)and flow expert- Site Assessment Data Application i menu(Salanitro 1992x)with sterilized soil show no loss of BTX mass from the aqueous phase after the initial Direction and gradient of Estimate expected rate of ' sorption of BTX to the soil Thus,no evidence indicates ground water flow plume migration that chemical transformation of BTEX to ground water Hydraulic conductivity Estimate expected rate of results in Significant natural attenuation plume migration ' Definition of lithology Understand preferential flow Hydrolysis/Dehydrohalogenabon paths,etc. Although the BTEX compounds are not likely to Aquifer thickness Estimate volatilization and degrade chemically other compounds such as 12- model ground wate-flow _ dichiorrpropane (DCP), 1?-dibromo-3-chloropropane Devth to ground water Esumatc volatilization (DBCP) 1 2-dibromoethane (EDB) and 1,2-dichloroe- Range of water table Evaluate potential source ' thane (1 2-DCA) are attenuated by natural hydrolysis fluctuations smearing influence of or dehydrohalogenation reactions(Ellington et at 1987 fluctuanons on ground water Deelcv et at 1991,Barbash and Reinhard 1989) Deeley concentrations, and variation et a] (1991)have described the possible dehydrohaloge- in flow direction i ' nation and hydrolvsis reactions for DBCP and demon- Delineation of contaminant Compare expected extent _. strated that these reactions do occur in laboratory source and soluble plume without natural attenuation to studies The study results indicated that DBCP is actual expected to have a half-life of approximately 6 1 years under tvpical ground water conditions in California (pH = 7 K and 21 1 C) Higher ground water tempera- Data Requirements. tures and/or higher pH will increase the rate of chemical Some of the data needed to evaluate natural attenua- i transformation In addition it was found that hydrolysis tion are collected during a standard site assessment -- reactions leading to dehalogenated products which are These standard data and their utility in evaluating ' preferred environmentally predominate in ground natural attenuation are described in Table 2 Additional I water systcros Half-lives reported for other compounds data that are not always obtained in site assessments, arc 15 to 25 Years for DCP 1 to 6 years for EDB and but can be used to evaluate the extent of natural attenua- 6 to 64 years for l 2-DCA at 25 C tion,are shown in Table 3 Delineating the soluble plume awl In summary aerobic biodegradation is expected to along the primary flow path to the leading edge and ' be the primary mechanism for degradation of BTEX monitoring it over time is important because the data while chemical transformation may be more significant produced allow determination of rates of plume migra- for other compounds such as DBCP Anaerobic and tion and plume size reduction which can be used to hypoxic biodegradation have not been adequately tested estimate rates of attenuation Thus, direct push tech- in the field to determine if they are significant mecha- niques(such as cone penetrometry[Chiang et al 1992b) nisms of natural attenuation Dispersion will reduce con- and hydraulic soil probing),which allow real-time analy- centrations of any compound as contaminated ground sis of ground water samples,can guide the proper place- water migrates away from the source Sorption can sig- ment of ground water monitoring wells along the prim- nificantly retard many contaminants thereby allowing ary flow path of the plume from the source to the leading J more time for degradation and molecular dispersion to edge Historical monitoring data from this type of pri- 1I occur before a ground water receptor is impacted Vola- mary flow path monitoring network acquired over a tilization is expected to account for approximately 5 to period of time long enough to identify actual trends in 164 P SPRING 1994 GWMR _ - 1 4