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A Practical Approach to Evaluating Natural Attenuation SIA Rio" WATER of Contaminants in Ground Water JUN 2 0 1995 I by PM McAllister and C Y Chiang QUAUTY CCN I ROL BOARD I i I Introduction Although releases of chemicals to the environment can impact Ibsti'act ground water quality natural pro- he extent of natural attenuation is an important cesses such as biodegradation dis- consideration in determining the most appropnate persion, sorption and hydrolysis (among others) often limit the Tcorrective action at sites where ground water qual- migration of contaminants in ground ity has been impacted by releases of petroleum hydrocar- water Data from several field sites, bons ar other chemicals The objective of this study was to laboratory studies and rescarch develop a practical approach that would evaluate natural sites (Barker et a] 1987 Kemblow- attenuation based on easily obtained field data and field ski et a] 1987 Chiang et al I989, t tested indicators of natural attenuation The pnmary ind- Salanitro 1992, Barbaro et al 1992,Caldwell et al 1992) have demon- icators that can be used to evaluate natural attenuation strated that this 'natural attenua- tion tion ' can significantly limit the include plume characteristics and dissolved oxygen levels in ground water Case studies of actual field sites show that migration of contaminants resulting plumes migrate grate more slowly than expected,reach a steady from releases of petroleum hvdro- carbons in ground water Thus, state,and decrease in extent and concentration when natural attenuation can significantly natural attenuation is occurring Background dissolved reduce the potential impact of a oxygen levels greater than 1 to 2 mgfL and an inverse cor- petroleum hydrocarbon release by relation between dissolved oxygen and contaminant levels preventing contaminants from have been identified through laboratory and fiela studies as migrating in ground water to sensi- tive receptors (e g, water supply key indicators of aerobic biodegradation an important atter.- wells rivers and lakes) Figure I uation mecba=rrL Secondary indicators such as geocherrucal illustrates the dramatic differences data,and more intensive methods such as contaminant mass in the extent of ground water plumes < ' balarices,laboratory microcosm studies,and detailed ground for a conservative compound (atten- uated only by dispersion) and a com- --�r water modeling can demonstrate natural attenuation as well. pound which is attenuated by disper- The recommended approach for evaluating natural attenua- sion,sorption and biodegradation as tion is to design site assessment activities so that required calculated from an analytical solution data such as dissolved oxygen levels and historical plume to the three-dimensional transport flow path concentrations are obtained.With the necessary equation As shown in Figure 1, the data,the primary indicators should be applied to evaluate degraded compound approaches asteady state with the plume extending natural attenuation If the initial evaluation suggests that approximately 1000 feet downgradi- < natural attenuation is a viable correcnve action alternative, ent, while the conservative Com- then a mionitormg plan should be implemented to verify the pound migrates at the average extent of natural attenuation ground water velocity, is 4000 feet downgradient after 10 years, and is ['y still migraung Thus, if the nearest ground water receptor was located Pages 161-173 SPRING 1994 GWMR ■ 161 �.nii�