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Table 3 interpolation and integration of monitoring w,-il data If the total mass decreases over time and the approach to Evaluate Additional Site Assessment Data Used is applied properly, then this technique is a direct and to Evaluate Natural Attenuation quantitative method of demonstrating natural t Site Assessment Data Application attenuation However, the cost associated with the extensive monitoring network required for this Date of contaminant release Estimate expected extent approach and the relative complexity of the analyses of plume migration precludes its application at most sites other than Historical concentrations along Evaluate status of plume research sites In addition, proper application of the the primary flow path from the (i a steady state, mass balance approach requires understanding factors source to the leading edge decreasing migrating) such as variable sources and water table fluctuations Background DO levels Determine if sufficient The study conducted by Chiang et al (1989)demon- upgradient of the source and D O is present for strated that natural aerobic biodegradation of BTX was plume aerobic biodegradation the major mechanism responsible for a reduction in (' 1-2 mg/1.) soluble benzene mass at a Michigan gas plant facility D O levels inside and outside Identify inverse The study included mass balance and nonlinear least the contaminant plume correlation indicative of squares analvses to determine benzene attenuation aerobic biodegradation rates consideration of potential attenuation mecha- Alkahnity hardness pH and Evaluate geochemical nisms other than biodegradation, statistical correlation soluble Fe inside and outside indicators of natural of D O and BTX levels solute transport modeling and the contaminant plume attentuation microcosm studies to show BTX biodegradation The Locations of nearest ground Identifv areas of natural application of the mass balance approach is described water recharge areas (c g ground water aeration in this section while other aspects are used a5 examples lakes streams canals retention to following sections ponds catch basins and Figure 2 shows the decrease in total benzene mass &ones) in ground water from November 1984 to Mav 1986 The plume sampled in No-ember 1984 contained an approxi- around water concentrations will provide useful data mate total benzene mass of 9 83 hg while the plume for evaluating natural attenuation The manner in which sampled in January 1985, October 1985 January 1986 these data can be used to indicate and demonstrate and Mav 1986 contained successively lower benzene natural attenuation is described in the remainder of this masses of 5 66 kg, 2 27 kg, 2 10 kg and 0 99 kg respec- section tively During the sampling period of November 1984 Plume Characteristics to May 1986 there was no pumping of ground water so the disappearance of ben/ene was attr,buted to natural The most definitive indicators of natural attenuation attenuation mechanisms A material balance analysts (I e,plume characteristics such as decreasing total mass was conducted to determine the first-order rate constant of contaminant, receding plume, steady-state plume, and the net influx rate from the source using four sets and dissolved oxygen concentrations) are based on of data from the site taken between November 1984 actual concentrations obtained during periodic monitor- and January 1986 The first-order attenuation rate con- ing events With the appropriate monitoring data, the Stant was determined to be 0 95 percent/day and the mass of contaminant present and the extent, rate of influx rate was 0 0194 kg/dav The large number of moni- migration, and distrioution of contaminant concentra- toring wells and rigorous material balance analvses with tions can all be used to indicate that natural attenuation IS occurring Several indicators and methods of demon- 10 strating natural attenuation based on the plume charac- teristics are described in the following paragraphs. In all these approaches,data from monitoring points with 8 consistent sampling methods and appropriate screened intervals are used As indicated by Robbins (1989) and 6 Chiang et al (1992x), apparent attenuation could be observed if data from inconsistent sampling methods or 56 improver screened intervals are used 4 Mass Balance Approach 2 The mass balance approach,described in more detail 2 Z ZF11 91 by Chiang et al (1989) and summarized below, consti- 1111 ,0 9 1 tutes a rigorous demonstration of natural attenuation 0 In this approach,an extensive monitoring well network Nov-84 Jan-85 Oct-85 Jan-86 May-86 Is Installed to define the complete vertical and horizontal Figure 2 Total soluble benzene mass to ground water at the extent of the soluble plume The total mass of a contami- Michigan gas plant from November 1984 to May 1986 nant In the soluble plume is monitored over time by SPRING 1994 GWMR ■ 165