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Background Papers

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A Regulator's Perspective on In Situ Bioremediation John M. Shauver Michigan Department of Natural Resources Lansing, Michigan SUMMARY Bioremediation, like any technology applied to clean up a contaminated site, must first be approved by government regulators who ultimately must agree that the technology has a reasonable chance to reduce the contaminant(s) at the site to acceptable levels. This paper describes the information that regulators need to make their decision. Basically, this information comprises descriptions of the site, the specific cleanup process, and the overall approach to site cleanup. The paper also answers the questions of who, what, when, where, and how in the context of bioremediation on the basis of my 24 years of experience as a regulator. INTRODUCTION During the past 20 years, various companies and individuals have developed or claim to have developed biological treatment processes that could cleanup various wastes generated by human activities. These wastes include polychlorinated biphenyls (PCBs), crude oil, refined crude oil products, crude oil wastes, and DDT, to name a few. One problem that the proponents of such treatment technology face is state and federal regulations. It is often hard for the regulated community to understand what is required to ensure that the regulator will approve a proposed treatment process.

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This paper describes what a regulator looks for in a proposal to cleanup (remediate) a site to legal standards. The guidance provided here is a condensation of the requirements of the many statutes and regulations used by the Michigan Department of Natural Resources. The paper reflects my view, after 24 years as a regulator, of what information needs to be routinely provided to evaluate a cleanup technology before it is applied to a particular site. Complex sites with unique or unusual features may have to be characterized in greater detail before a cleanup technology can be chosen. Also, the regulated community (potentially responsible parties) must realize that the cleanup process itself is but one facet of the overall site cleanup. To gain approval for implementation of a cleanup process, the responsible party should supply information that includes: a description in three dimensions of the site and of the type and extent of contamination, a detailed description of the cleanup process(es) to be applied to the site, and a detailed description of the approach to overall site cleanup. SITE DESCRIPTION The site description should specifically identify the types and amounts of chemical(s) released to the soil and ground water and other phases of the site environment. The description should also include estimates of the rate of movement of the contaminants through the various phases of the environment and of where they are likely to end up. The regulator's response to a given situation depends strongly on the rate of transport and the likely fate of the contaminants. The site is the three-dimensional area contaminated by the chemicals that have been released. The site is not limited to legal property boundaries. In fact, it usually involves more than one property owner, and the owners may not all be responsible for the contamination. The site description should also include the vertical, horizontal, and lateral extent of contamination, which includes: soil type(s), permeability, porosity of the soils and/or aquifer, and concentrations of contaminants in soil; if appropriate, depth to ground water, rate and direction of flow, concentrations of contaminants in ground water, and concentrations of naturally occurring or other compounds (inorganic or organic) that may interfere with the treatment process; if appropriate, concentrations of contaminants in the air, prevailing wind direction, and nearest human receptors; and

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concentrations of contaminants in surface waters and sediments. In any site description the regulator will place great emphasis on identifying the location of the source(s) of contamination. Removal of these sources, or hot spots (identified by an adequate site investigation), is the most effective way to limit migration of chemicals off site. In addition, elimination of the source of the contamination as early as possible is one of the most cost-effective ways to limit future cleanup costs. A site description should also describe the process that caused the release. This is important because the regulator is required to determine the full extent of the type of contamination at the site. If the material released is gasoline, for example, it is very important to know whether it is leaded or unleaded and whether it came from a hole in a tank; an overfilled tank; or faulty pipes, valves, or other fittings. If the release is described as crude oil, it is important to know if brine, condensate, or other materials are present as well. The description of the cause of the release allows the regulator to identify its source and thus the most highly contaminated areas of the site. PROCESS DESCRIPTION The responsible party should provide a detailed description of the treatment process to be used. The engineer who is accustomed to describing an activated carbon process should provide the same level of detail for a biological process. The description should show how the process chosen will contain, destroy, or remove the contaminants to meet legal standards. If biological treatment is chosen, the regulator must be given data that show the ability of the organisms present in or added to the contaminated area to safely and effectively treat the chemical(s) on the site. When living organisms are proposed to cleanup a site, the regulator expects to see a detailed description of the organisms' requirements for oxygen, nutrients, temperature, moisture, and pH. We must be sure the organism will thrive long enough to treat the chemicals to legal cleanup standards. In addition, if an anaerobic treatment scenario (such as one using iron or sulfur) is proposed, the regulator needs to know that native microbes are capable of the proposed metabolism and that ambient or added nutrients will be available in amounts likely to allow effective treatment but not likely to cause rapid plugging of the delivery wells and/or the soils.

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We must be able to determine that the use of bacteria in the soils and ground water (if unsuccessful) will not prevent other treatment technologies from being applied. Use of organisms without adequate information or controls in the past has resulted in severe plugging problems in ground water monitoring wells and/or the aquifer itself. Such loss of permeability not only prevents delivery of the nutrients and oxygen necessary to sustain biological activity to cleanup the soils or aquifer but may seriously hamper use of other technologies. OVERALL SITE CLEANUP DESCRIPTION A very important part of the description of the overall approach to site cleanup is the method(s) to be used to prevent movement of the contaminants farther off site through the soil or to or through the ground water or other medium. Containment to prevent further spread of the chemicals is as important in the regulator's mind as any other part of the cleanup. The regulator needs a complete description of the steps to be taken to prevent further movement of the chemicals through the soil, air, ground water, or surface water. For example, contaminated ground water may be moving down-gradient at 15 cm per day. Purge and capture wells would have to be installed to pump this contaminated ground water back upgradient to the treatment system to prevent further movement of the contaminants off site. If the water is discharged to the ground surface via an infiltration bed, and if it contains volatile organic chemicals (VOCs) that would be released, the responsible party needs to demonstrate adequate control of VOC discharge to the air. The description also should cover equipment necessary to achieve the cleanup. With biological treatment systems, equipment may be needed for adjustment of the pH of the ground water, removal of iron or other interfering substances before treatment, oxygen/air delivery or oxygen reduction, and identification and monitoring of tracers and nutrients added. For example, if the proposal is to use aerobic bacterial decomposition of the contaminant(s) and the contamination exists to a depth of 15 m below ground water surface in soils with a permeability of 10-7 cm/s, the regulator will be interested in how the responsible party intends to deliver oxygen or air and related nutrients to the organisms. Also necessary is a description of the monitoring procedures to be used to show that the cleanup system is operating properly. When using biological systems, the responsible party must show that the organisms are, in fact, doing the job. For example, if an aerobic process is used, the level of oxygen in and around the plume of

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contamination in the ground water will have to be monitored to ensure that the organisms have sufficient oxygen to decompose the chemicals in the ground water. This type of monitoring may be in addition to or in place of simply monitoring for the contaminant itself. In addition, if nutrients are added, they may also be contaminants and require monitoring. Nitrate, for example, is a chemical of concern that may have to be added to a biological treatment system as a nutrient or may be proposed as an electron acceptor in an anaerobic treatment process. In Michigan the drinking water supplies may not contain more than 5 mg/1 of nitrate. Therefore, if nitrate is used, the regulatory agency will require that it be monitored in addition to other monitoring requirements. CONCLUSION A regulator looks for the data necessary to determine that a proposed treatment technology, if properly installed and operated, will reduce the contaminant concentrations in the soil and water to legally mandated limits. In this sense the use of biological treatment systems calls for the same level of investigation, demonstration of effectiveness, and monitoring as any conventional system.

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An Industry's Perspective on Intrinsic Bioremediation Joseph P. Salanitro Shell Development Company Houston, Texas SUMMARY Laboratory and field evidence is now sufficient to demonstrate that soil microorganisms in aquifers are responsible for a significant portion of the attenuation of aromatic compounds—benzene, toluene, ethylbenzene, and xylenes (BTEX)—from fuel spills to the subsurface environment. Most subsoils contain indigenous microbes that can biodegrade low levels of BTEX (ppb to low ppm), given enough dissolved oxygen in the ground water. With adequate site characterization, analysis, and monitoring, this type of intrinsic bioremediation can shrink plumes and control the migration of hydrocarbons. In situ biodegradation processes, properly monitored, should be considered practical, cost-effective alternatives for managing low-risk, hydrocarbon-contaminated ground waters that are unlikely to affect drinking water wells. PROBLEM IDENTIFICATION Accidental releases of fuels from underground storage tanks over the past 10 to 20 years have been responsible for the presence of hydrocarbons, mainly water-soluble aromatic compounds (benzene, toluene, ethylbenzene, and xylenes, or BTEX), in aquifers. In most states, government agencies have required the regulated industry to

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restore ground water at such sites to drinking water (health) standards—for example, 1 to 5 parts per billion (ppb) benzene (Marencik, 1991). Corrective actions taken include removal of free product and contaminated soil, site assessments (soil borings and monitoring wells), and determination of the extent of contamination in subsoils and ground water. For a majority of the sites, the ground water hydrocarbon (BTEX) levels are low, on the order of 100 to 1000 ppb. Higher levels are often associated with soil and ground water samples taken near the spill area. Technologies that have been used to control migration of hydrocarbon plumes or to remediate subsurface soils include soil venting (vadose zone) and sparging (saturated zone) and ground water extraction and treatment (pump and treat) (Mackay and Cherry, 1989; Newman and Martinson, 1992). In addition to these operations, extensive soil and ground water surveying must be done to assess the extent of contamination and the effectiveness of the cleanup method. Current estimates for site assessment and in situ or ex situ restoration of subsoils and ground water to health standard criteria indicate that these operations may be costly ($500,000 to $2 million per site) and not cost-effective and that they may not achieve restoration within time periods of years or decades (Travis and Doty, 1990). Many contaminated ground waters (e.g., at fuel service station sites) are in shallow aquifer zones, are not used directly for human consumption, and do not even affect downstream drinking water wells. Furthermore, good field evidence indicates that plumes in these ground waters reach a stable condition in which contaminants of concern (BTEX) are biodegraded at some rate by indigenous hydrocarbon-utilizing soil bacteria. This type of unassisted in situ biodegradation has been termed natural attenuation or intrinsic bioremediation. Industry has been confronted with very large operating and cleanup costs for subsoils and ground water in the restoration of underground fuel storage tank sites to drinking water standards. Where thorough site characterization warrants its use, intrinsic bioremediation offers a way to manage non-migrating or shrinking BTEX plumes in low-risk aquifers that do not affect drinking water wells. Evidence that this natural process is occurring has been obtained from laboratory and field observations. EVIDENCE FOR INTRINSIC BIOREMEDIATION OF AROMATIC HYDROCARBONS IN It is now widely recognized that the most significant factor in the time-dependent decrease of BTEX compounds in aquifers is degrada-

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tion by soil microbes. Studies reported for laboratory slurry microcosms of subsoil and ground water show that microbes in many soils inherently biodegrade aromatic hydrocarbons at varying rates (5 to 50 percent per day) (Barker et al., 1987; Chiang et al., 1989; Gilham et al., 1990; Hutchins et al., 1991; Kemblowski et al., 1987; Major et al., 1988; and Thomas et al., 1990). These biodecay rates are usually first order; they occur with low levels of hydrocarbon (50 to 10,000 ppb); and they are rapid with adequate dissolved oxygen (e.g., 2 to 3 mg oxygen per milligram of hydrocarbon). Field estimates of hydrocarbon biodegradation rates calculated from fate and transport models using data from upstream and downstream monitoring wells have shown that plume BTEX compounds usually decrease at rates of 0.5 to 1.5 percent per day (Barker et al., 1987; Chiang et al., 1989; Kemblowski et al., 1987; Rifai et al., 1988; and Wilson et al., 1991). Laboratory and field data suggest that in a well-studied sandy aquifer a minimum, or threshold, level (≥1 to 2 ppm) of dissolved oxygen may be required to sustain hydrocarbon degradation (Chiang et al., 1989). It should be emphasized that laboratory and field data have confirmed that all BTEX compounds can be biodegraded under aerobic conditions (dissolved oxygen in ground water) in aquifer subsoils in which oxygen is the terminal electron acceptor. Soil microcosm experiments or enrichments of aquifer material have shown that toluene and xylenes can be degraded by microbes under iron-reducing, denitrifying, and sulfate-reducing (anaerobic or very low dissolved oxygen) conditions when ferric ion (Fe3+), nitrate ion (NO3-), and sulfate ion (SO42), respectively, serve as electron acceptors (Beller et al., 1992; Edwards et al., 1992; Hutchins, 1991; Lovley et al., 1989; and Zeyer et al., 1986). Field evidence is insufficient, however, to demonstrate that BTEX is biodegraded under anaerobic conditions in an aquifer. LEVELS OF INTRINSIC ATTENUATION IN GROUND WATER Evidence from site characterization, ground water monitoring, and modeling at field sites suggests that there may be two levels of intrinsic bioremediation. Figure 1 shows these aspects of a plume in which one is stabilized (A) and the other is reducing (B) in size and extent of contamination. In Figure 1 A the hydrogeological features indicate that ground water velocity (also BTEX and dissolved oxygen) and recharge are slow because of low permeability of the aquifer subsoil. Dissolved oxygen is low within the plume (e.g., <1 ppm). Oxygen is detected in monitoring wells at the edges and is respon-

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FIGURE 1 Levels of intrinsic bioremediation in aquifers. sible for the biodegradation of low levels (ppb) of BTEX. Another indirect indicator of soil microbial degradation in aquifers low in dissolved oxygen may be the presence of dissolved ferrous ion (Fe2+) above background well levels. It is known that various ferric oxides in soil can be used (as electron acceptors) by anaerobic iron-reducing bacteria to completely metabolize some aromatic compounds, such as toluene and phenol (Lovley et al., 1989). Therefore, when dissolved oxygen is low, ferric iron may substitute for oxygen, and this biodegradation process may result in elevated concentrations of ferrous ion in ground water. At the next level of intrinsic bioremediation, plumes noticeably shrink over time, with significant decreases in shape and extent (Figure 1B). This type of plume behavior is observed in aquifers that usually are more permeable (e.g., sandy subsoils), that exhibit higher ground water velocities, and that are higher in dissolved oxygen (higher aquifer reaeration rate) in many monitoring wells. Published examples of plumes undergoing significant intrinsic attenuation of BTEX are those at the Borden (Barker et al., 1987), Traverse City (Rifai et al., 1988; Wilson et al., 1991), and Michigan gas plant (Chiang et al., 1989) sites. Monitoring wells at the periphery show significantly higher dissolved oxygen (e.g., ≥1 ppm) and lower BTEX concentrations, which are consistent with a predominantly biodegradation-driven mass reduction in the aquifer. Examination of monitoring well BTEX levels within the flow path of upstream and downstream segments may also match the biodecay rates (about 1 percent per day) calculated from fate and transport models for BTEX and dissolved oxygen

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Remediation of Ground Water at the Denver Site Table 2 compares the reduction in concentrations of benzene and total BTEX compounds in ground water that was achieved by in situ bioremediation at the site in Denver. Monitoring wells MW-1 and MW-8 (shown in Figure 6) are in areas with oily-phase hydrocarbons. Well MW-2A is just outside the region with oily-phase hydrocarbon. Well MW-3 is a significant distance from the region with oily-phase hydrocarbon; it sampled the plume of contaminated ground water that moved away from the spill. Before remediation, concentrations in wells MW-1 and MW-8 were equivalent. Well MW-1 was closest to the nutrient recharge gallery, and the aquifer surrounding MW-1 was completely remediated; BTEX compounds were undetectable in ground water. In well MW-8, immediately adjacent to the point of release, the concentration of benzene was reduced at least one order of magnitude, and the concentrations of benzene and BTEX compounds in well MW-3 were also reduced an order of magnitude. It is of particular interest that significant concentrations of benzene or total BTEX never developed in the pumped recovery well (RW-1). The BTEX compounds were monitored twice a month from July 1989 to March 1992. Benzene was detected only twice, at a concentration of 2 µg/1. The other BTEX compounds were never detected. Water from contaminated flow paths sampled by MW-3 was probably diluted by uncontaminated water from other flow paths to RW-1 (compare Figure 7). This behavior illustrates the contrast in contaminant concentrations between passive monitoring wells and pumped wells. TABLE 2 Reduction in Concentration (µg/1) of Hydrocarbon Contaminants in Ground Water Achieved by In Situ Bioremediation   Benzene Total BTEX Well Before During After Before During After MW-1 220 <1 <1 2030 164 <6 MW-8 180 130 16 1800 331 34 MW-2A ? 11 0.8 ? 1200 13 MW-3 11 5 2 1200 820 46 RW-1 <1 2 <1 <1 2 <1

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Remediation of Subsurface Material at the Denver Site Water in the monitoring wells and the recirculation well contained low concentrations of contaminants by March 1992. Active remediation was terminated, and the site entered a period of postremediation monitoring. In June 1992 core samples were taken from the aquifer to determine the extent of hydrocarbon contamination remaining and whether a plume of contamination could return once active remediation ceased. The site was cored along a transect downgradient of the release. The transect extended laterally from clean material, through part of the spill, into clean material on the other side. In each borehole, continuous cores extended vertically from clean material above the spill, through the spill, to clean material below. The cores were extracted and analyzed for total petroleum hydrocarbons and for the concentrations of individual BTEX compounds. The relationships between the land surface, the water table, the region containing hydrocarbons, and the bedrock are presented in Figure 8. Significant amounts of hydrocarbons remain within a narrow interval, approximately 0.6 m thick, near the water table. The total saturated thickness of the aquifer was approximately 6 m. At the time of sampling the elevation of the water table was 1610 m (5280.5 ft) above mean sea level, and all the hydrocarbons were below the water table. The highest concentrations of hydrocarbons at the Denver site were obtained in samples from the borehole (D) closest to the work pit. Table 3 presents the vertical distribution of BTEX compounds and total petroleum hydrocarbons (TPH) in borehole D. The material in the interior of the spill had higher proportions of BTEX compounds. Table 4 makes the same comparison at the most contaminated depth interval along the transect. Material closer to the spill had higher concentrations of TPH and greater relative proportions of BTEX compounds. Figure 9 plots the percentage of BTEX in the residual oil after bioremediation against the total content of hydrocarbon. Obviously, the materials with lower residual concentrations of hydrocarbons are more extensively weathered. Infiltration of hydrogen peroxide and mineral nutrients at an aviation gasoline spill in Michigan preferentially removed BTEX compounds from the oily-phase gasoline, leaving a total petroleum hydrocarbon residual low in aromatic hydrocarbons (Wilson et al., in press). At the Denver site, apparently, a cortex of material that has been physi-

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FIGURE 8 Cross section showing the vertical relationship of the land surface, water table, residual hydrocarbon after bioremediation, and the lower confining layer of the aquifer. The cross section runs through core boreholes depicted in Figure 6. TABLE 3 Vertical Extent of Total BTEX Compounds and Total Petroleum Hydrocarbons (mg/kg) at Borehole D, the Most Contaminated Borehole in the Transect (Figure 8) Elevationa TPH BTEX Benzene Color and Texture 1609.711 to 1609.458 <44 <1 <0.2 Brown sand 1609.458 to 1609.354 227 5.1 <0.2 Brown sand 1609.354 to 1609.230 860 101 <0.2 Black sand 1609.230 to 1609.101 1176 206 4.3 Black sand 1609.101 to 1609.050 294 27 0.68 Black sand 1609.050 to 1608.949 273 7.4 0.26 Black sand 1608.949 to 1608.821 <34 <1 <0.2 Black sand 1608.821 to 1608.492 <24 <1 <0.2 Brown to yellow sand a Meters above sea level.

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TABLE 4 Lateral Distribution of Total BTEX Compounds and Total Petroleum Hydrocarbons (mg/kg) Along the Transect (Figure 8) at the Most Contaminated Depth Interval Borehole TPH BTEX Benzene B 167 0.8 <0.2 C 156 3.5 <0.2 D 1176 260 4.3 E 156 3.5 0.06 FIGURE 9 Relationship between extent of hydrocarbon contamination (concentration of total petroleum hydrocarbons) and the extent of biological and chemical weathering (reduction in percentage of BTEX compounds in total petroleum hydrocarbons) in core material after bioremediation at the Denver site.

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cally and biologically weathered surrounds a central core of material that has not been depleted of BTEX compounds. The concentration of an individual petroleum hydrocarbon in solution in ground water in contact with oily-phase hydrocarbon can be predicted by Raoult's law. The solution concentration in water should be proportional to the mole fraction of the hydrocarbon in the oily phase. Assume that the weathered material is weathered because it is in effective contact with moving ground water that supplied nutrients and electron acceptors, and the residual is not weathered because it was not in effective contact and the supply of nutrients and electron acceptors was inadequate. If partitioning between moving ground water and the weathered oily residual controls the concentration of hydrocarbons in the water, the 10-fold reduction in concentrations of benzene and BTEX compounds seen in the weathered core material (Table 3) would produce the 10-fold reduction in concentrations of benzene and BTEX compounds seen in the monitoring wells (Table 2). Do Mass Transfer Effects Limit Development of a Plume? The usual expectation for in situ bioremediation is total removal of the contaminant from the subsurface environment. The extent of remediation more commonly achieved is removal of the contaminant from the circulated ground water. In situ bioremediation merely accelerates the natural physical and biological weathering processes that occur in the subsurface. The oily material in most intimate contact with the circulated ground water is weathered to the greatest extent. After extensive remediation of the more transmissive regions, the release of contaminants to the circulated ground water is controlled by diffusion and slow advection from the subsurface material that still contains significant quantities of contaminants. The relationships are presented in Figure 10. In such circumstances the disposition of contamination in the ground water can only be understood as a dynamic system. This release may best be described through the chemical engineering concept of a mass transfer coefficient. As the circulated water passes through the weathered spill, a certain quantity of hydrocarbon is transferred to the water; the amount transferred is directly proportional to the exposure time of the water in the contaminated area. If the circulated water contains enough nutrients and electron acceptor to meet the demand of the contaminants transferred from the fine-textured material, the plume will be destroyed by biological activity as rapidly as it is produced. When active remediation is stopped, the concentration of elec-

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FIGURE 10 Schematic representation of in situ bioremediation in heterogeneous geological material. A fresh release (panel A) is rapidly weathered (panel B) due to increased flow of water and increased concentration of electron acceptor. As weathering progresses, aromatic hydrocarbons such as the BTEX compounds are restricted to regions with low hydraulic conductivity (panel C). After bioremediation the flux of aromatic hydrocarbons from the residual core to the moving ground water is controlled by mass transport limitations. The extent of the plume produced is controlled by the supply of electron acceptor (panel D). Although greatly attenuated, the plume may be regenerated under ambient conditions.

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TABLE 5 Contrast in Conditions During Active In Situ Bioremediation and Conditions at the Termination of Remediation at a Spill from an Underground Storage Tank in Denver Colorado Parameter During Active Remediation Ambient Conditions After Remediation Introduced concentration of oxygen 470 mg/l 5.5 mg/l Hydraulic gradient 0.097 m/m 0.0012 m/m Interstitial flow velocity 2.4 m/day 0.03 m/day Travel time of water across the spill 20 days to recovery well 1500 days to monitoring well Maximum oxygen demand supported 20 mg/l per day 0.004 mg/l per day tron acceptor returns to ambient conditions in the aquifer, and the hydraulic gradient returns to the normal condition. As a result, the residence time of water in the spill area is longer, and the total amount of hydrocarbon transferred to the water is greater, although the supply of electron acceptor for biological destruction of the hydrocarbon is less (compare panels C and D in Figure 10). These relationships are well illustrated by the performance of bioremediation at the Denver site (Table 5). The hydraulic conductivity in the depth interval containing the hydrocarbons is approximately 8.5 m per day (David Szlag, University of Colorado, Boulder, personal communication). The distance from the nutrient recharge gallery to the recovery well is 45 m. Assume that the distance from the upgradient edge of the spill, through the spill, to monitoring wells downgradient at the property boundary is also 45 m. Assuming an effective porosity of 0.35, Darcy's law can be used to estimate the interstitial flow velocity of the ground water and its residence time along the flow path. When active remediation was terminated, the residence time of water was 75 times longer, and the supply of oxygen was 85 times less than conditions during active remediation (compare Table 5). In June 1992 core material from the spill was assayed for the potential rate of oxygen consumption. Core material was dewatered by placing it in a Buchner funnel and applying a partial vacuum. Then the core material was sealed in a glass mason jar. After 24 hours, a sample of air in contact with the core material was passed through an oxygen-indicating tube to estimate oxygen consumption potential. Acclimated material exerted oxygen demands of 6 to more than 36 mg/kg core material per day (Table 6), equivalent to 40 to more than 240 mg/l ground water per day. At these rates, the oxy-

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TABLE 6 Relationship Between the Potential Oxygen Uptake Rate (mg oxygen per kg per day) of Freshly Collected Core Material and the Location of the Core Material with Respect to the Interval Contaminated with Hydrocarbons   Position of Borehole in the Transect Location in Borehole A B C D E F Just Above     <4   7.4   Within 15.5 >30 >36   >34 23.5 Just Below 6.0 <3 5.7 7.3   21 gen supplied as hydrogen peroxide during active remediation would be consumed in 2 to 12 days. In the assay the microbial consumption of oxygen was faster than the rate of supply during active remediation. If the microbes in the aquifer expressed the potential rate of oxygen consumption, oxygen would have been depleted before the recharge water moved across the spill. In the absence of oxygen, BTEX compounds would have partitioned to the ground water and should have been detected in the monitoring wells. In fact, oxygen concentrations between 2 and 5 mg/l were always present in water produced by the recovery well, and BTEX compounds were virtually absent. Oxygen consumption must have been limited by mass transfer of hydrocarbon to the ground water circulated through the spill. Relationship to Siting and Sampling Monitoring Wells No established procedures exist for determining under ambient conditions whether the mass transfer of hydrocarbons from oily residual material will exceed the supply of oxygen or other natural electron acceptors. As a result, it is impossible to predict if natural bioremediation will prevent the regeneration of a plume, or if a plume of contaminated ground water will regenerate and at what concentrations. Ground water moving under the natural gradient must be allowed to travel all the way through the spill and then to the monitoring wells before it is possible to determine whether mass transfer effects will reestablish a plume. An assessment of natural hydrologic conditions at a site will be necessary to intelligently locate compliance monitoring wells and determine an appropriate schedule of monitoring. Required are an un-

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derstanding of the average natural hydraulic gradient and the hydraulic conductivity in the depth interval containing residual hydrocarbon. This information can be used to predict the velocity and trajectory of potential plumes of contaminated water. The frequency of monitoring can be adjusted to reflect the expected time required for ground water to travel through the area containing residual hydrocarbon to the point of compliance. CONCLUSIONS When oily-phase materials are to be remediated, core analyses are required to estimate the total mass of contaminant subject to remediation. Headspace analyses in the filed can be used to screen core samples to identify those that deserve further analysis in the laboratory. If properly benchmarked by a limited number of laboratory analyses, field headspace techniques can provide a rapid and affordable estimate of total contaminant concentrations. Simple ground water flow models can estimate the volume of water circulated through a spill during in situ bioremediation. This information can be coupled with simple partitioning theory to estimate the apparent attenuation due to dilution. In situ bioremediation frequently leaves a residual of weathered oily-phase material. Partitioning theory can be used to predict the concentrations of BTEX compounds in ground water in contact with the weathered oily residual. After extensive in situ bioremediation, pockets of fine-textured material may still contain high concentrations of contaminants. Mass transfer effects control the access of residual organic contaminants to moving ground water. Under proper conditions, natural biodegradation supported by ambient concentrations of electron acceptors and mineral nutrients may destroy organic contaminants as fast as they escape from the oily-phase residual. At the present state of science, only long-term monitoring can determine if natural biodegradation will prevent the regeneration of a plume of contaminated ground water. ACKNOWLEDGMENTS This work was supported by the United States Air Force through Interagency Agreement RW 57935114 between the Armstrong Labo-

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ratory Environics Directorate (U.S. Air Force) and the R. S. Kerr Laboratory (U.S. Environmental Protection Agency). This work was also supported by the U.S. Environmental Protection Agency through the Bioremediation Field Initiative. It has not been subjected to agency review and therefore does not necessarily reflect the views of the agency, and no official endorsement should be inferred. REFERENCES Bell, C. E. 1990. State-by-state summary of cleanup standards. Soils: Analysis, Monitoring, Remediation (Nov.-Dec.):10-16. Clark, I. 1979. Practical Geostatistics. London: Applied Science Publishers. Downs, W. C., S. R. Hutchins, J. T. Wilson, R. H. Douglas, and D. J. Hendrix. 1989. Pilot project on biorestoration of fuel-contaminated aquifer using nitrate: part I—field design and ground water modeling. Pp. 219-233 in Proceedings, Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Restoration, Nov. 15-17, 1989, Houston. Dublin, Ohio: National Water Well Association. Downs, W. C., S. R. Hutchins, J. T. Wilson, R. H. Douglas, and D. J. Hendrix. In press. Nitrate-mediated biodegradation of BTEX in JP-4 contaminated soil and ground water. In Bioremediation: Field Experiences, P. E. Flathman, D. E. Jerger, and J. H. Exner, eds. Chelsea, Mich.: Lewis Publishers. Hutchins, S. R., G. W. Sewell, D. A. Kovacs, and G. A. Smith. 1991a. Biodegradation of aromatic hydrocarbons by aquifer microorganisms under denitrifying conditions. Environmental Science and Technology 25(1):68-76. Hutchins, S. R., W. C. Downs, J. T. Wilson, G. B. Smith, D. A. Kovacs, D. D. Fine, R. H. Douglass, and D. J. Hendrix. 1991b. Effect of nitrate addition on biorestoration of fuel-contaminated aquifer: field demonstration. Ground Water 29(4):571-580. Kampbell, D. H., and M. L. Cook. 1992. Core assay method for fuel contamination during drilling operations. Pp. 139-140 in Subsurface Restoration Conference Proceedings. Houston: Rice University, National Center for Ground Water Research. Kennedy, L. G., and S. R. Hutchins. 1992. Applied geologic, microbiological, and engineering constraints of in situ BTEX bioremediation. Remediation 3(1):83-108. Nelson, C., R. J. Hicks, and S. D. Andrews. In press. In situ bioremediation: an integrated system approach. In Bioremediation: Field Experiences, P. E. Flathman, D. E. Jerger, and J. H. Exner, eds. Chelsea, Mich.: Lewis Publishers. Robbins, G. A., R. D. Bristol, and V. D. Roe. 1989. A field screening method for gasoline contamination using a polyethylene bag sampling system. Ground Water Monitoring Review (Fall):87-97. Siegrist, R. L., and P. D. Jenssen. 1990. Evaluation of Sampling Method Effects on Volatile Organic Compound Measurements in Contaminated Soils. Environmental Science and Technology 24(9):1387-1392. Smith, J. H., J. C. Harper, and H. Jaber. 1981. Analysis and Environmental Fate of Air Force Distillate and High Density Fuels. Report ESL-TR-81-54. Tyndall Air Force Base, Fla.: Engineering and Services Laboratory, Air Force Engineering and Services Center. Wilson, J. T., J. M. Armstrong, and H. Rifai. In press. A full scale field demonstration on the use of hydrogen peroxide for in situ bioremediation of an aviation gaso-

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line-contaminated aquifer. In Bioremediation: Field Experiences, P. E. Flathman, D. E. Jerger, and J. H. Exner, eds. Chelsea, Mich.: Lewis Publishers. Zapico, M. M., S. Vales, and J. A. Cherry. 1987. A wireline piston core barrel for sampling cohesionless sand and gravel below the water table. Ground Water Monitoring Review 7(3):74-82.