Intrinsic remediation of a diesel fuel plume in Goose Bay, Labrador, Canada

Transcription

1 ENVIRONMENTAL POLLUTION Environmental Pollution 103 (1998) 203±210 Intrinsic remediation of a diesel fuel plume in Goose Bay, Labrador, Canada F. Curtis a, *, J. Lammey b a Faculty of Engineering and Applied Science, Memorial University, St. John's, Newfoundland, Canada A1B 3X5 b Serco Facilities Management Inc., PO Box 1012, Station C, Goose Bay, Labrador, Canada A0P 1C0 Received 15 February 1997; accepted 18 June 1998 Abstract This paper begins with a discussion of intrinsic remediation processes. Although intrinsic remediation involves biological, physical and chemical processes, the biological processes are often the most important. A eld investigation consisting of sampling 102 groundwater and three surface water locations is described to characterize the intrinsic remediation occurring at a hydrocarbon contaminated site in Goose Bay, Labrador, Canada. Aerobic bioremediation, denitri cation, ferrous iron reduction and sulphate reduction are occurring. Analysis of isopachs and isopleths resulted in the prediction of the assimilative capacity of groundwater. Intrinsic remediation can reduce the e ects of free oating hydrocarbon, or free product. The e ects of remediation can be optimized by adding nitrate saturated water to the groundwater regime. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Intrinsic remediation; Free product plume; Biodegradation of hydrocarbons; Goose Bay; Labrador 1. Introduction Each year, 3,636,500,000 litres of hydrocarbons are sold in the United States (American Petroleum Institute, 1995). During the manufacture, transportation, storage, use, and disposal of this quantity of product, releases to the environment are inevitable. It is generally recognized that benzene, toluene, ethylbenzene, and the xylene isomers (BTEX) represent the highest risk to biophysical environmental receptors when a hydrocarbon release occurs. Mitigation and/or remediation of hydrocarbons, and BTEX in particular, is a primary problem confronted by the environmental industry and regulators. The application of pump and treat to aquifer remediation projects is limited. Despite the high cost of this technique, it can rarely be employed in isolation to meet regulated clean up criteria. Other techniques, such as air stripping, funnel and gate systems, vapour extraction systems, and bioslurping are of a typically high cost or have limited applications. Due to * Corresponding author. continual scal constraints, a risk management approach to e ectively mitigate or remediate hydrocarbon releases is needed; intrinsic remediation is such an option. This paper discusses how intrinsic remediation can reduce the e ects of free oating hydrocarbon, or free product, on groundwater. This paper begins with a discussion of intrinsic remediation processes. Following, a case study in Goose Bay, Labrador, Canada, discusses sampling and analysis methods of a free product fuel plume and the occurrence, and optimization of intrinsic remediation. 2. Intrinsic remediation Intrinsic remediation (also known as natural attenuation or passive remediation) is the unenhanced, naturally occurring, biological processes (e.g. aerobic and anaerobic biodegradation), physical processes (e.g. dispersion, di usion, dilution by recharge, volatilization) and chemical processes (e.g. sorption and chemical or abiotic reactions) which reduce the total concentration of a contaminant dissolved in groundwater /98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S (98)

2 204 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203± Intrinsic remediation process 3.1. Biological processes Biodegradation typically accounts for the majority of the mass removal of hydrocarbons in intrinsic remediation situations. In aerobic and anaerobic biodegradation, destruction occurs as a result of bacteria oxidizing reduced materials (hydrocarbons) to obtain energy. Their metabolism removes electrons from the hydrocarbon donor via a number of enzyme-catalyzed steps along respiratory or electron transport chains to the nal electron receptor, oxygen for aerobic reactions and a variety of electron acceptors for anaerobic reactions. Metabolized hydrocarbon ends up as new cell mass with the by-products being carbon dioxide, water, and the growth of new microbes. It is generally recognized that microbes capable of degrading hydrocarbons are ubiquitous. Each strain of microbes will use a unique pathway to degrade the hydrocarbon (Ridgeway et al., 1990; Chaudhry, 1994). Therefore, the identi cation of the exact degradation pathway is not vital for demonstrating that biodegradation is occurring. Furthermore, the microbial involvement can be reduced to that of a catalyst (by providing the activation energy). Recognizing this, biodegradation can be simpli ed to a strictly chemical reaction Aerobic biodegradation Aerobic biodegradation of hydrocarbons in general, and all BTEX components in particular, is well understood and well documented in the literature (Chiang et al., 1989; King et al., 1992; Baker and Herson, 1994; Chaudhry, 1994; McAllister et al., 1995). Aerobic bioremediation is biologically preferential to anaerobic bioremediation because it requires less free energy for initiation and yields more energy per reaction (denitri cation being the only exception to this). Due to the rate of aerobic biodegradation it is typically modelled as a transport limited reaction (Rifai et al., 1995b). Stoichiometrically, it can be calculated that 3.1 mg/ litre of oxygen is required to biodegrade 1 mg/litre of hydrocarbons. As groundwater is saturated with dissolved oxygen at 6±12 mg/litre (temperature dependent), fully saturated groundwater can degrade 2±4 mg/litre of hydrocarbons. These calculations do not account for microbial cell growth. If this cell growth is taken into account, the mass of oxygen required to degrade 1 mg of benzene decreases to 1.03 mg Anaerobic biodegradation The rst evidence that was presented that BTEX biodegrades anaerobically was in 1934 (Baker and Herson, 1994). Although slower than aerobic bioremediation, anaerobic bioremediation has many advantages: it does not involve the addition of oxygen with the resultant complications; it provides a wider range of electron acceptors; a greater biomass of microbes is produced compared to aerobic processes; and the microbes are hardier (King et al., 1992). Anaerobic microbes tend to have a high resistance to ph changes and are resistant to high organic loadings and metals (King et al., 1992). Denitri cation is the primary means of anaerobic bioremediation at many sites. Stoichiometrically, 4.8 mg/litre of nitrate is consumed for 1 mg/litre of hydrocarbon degraded. Nitrate is also a transport limited reaction (Rifai et al., 1995a). The augmentation of nitrate in groundwater is superior to oxygen addition. Nitrate may be added by injection wells, in ltration galleries, or surface applications (sprinklers, soaker hoses, etc.). Nitrate addition does not cause precipitation of metals. The solubility of nitrate in groundwater is higher than dissolved oxygen (92 mg/litre vs 6±12 mg/litre) and has higher stability. Despite the higher concentration of nitrate required to degrade a given concentration of hydrocarbons, compared to dissolved oxygen, the higher solubility allows greater degradation from nitrate saturated groundwater than from dissolved oxygen saturated groundwater. The form of nitrogen which is added to the system is not important (Ramstad and Sveum, 1995). The reduction of ferric hydroxide to ferrous iron has an unfavourable hydrocarbon:electron acceptor ratio (1:36 for toluene). Stoichiometrically, 22 mg/litre of ferrous iron is produced for 1 mg/litre of hydrocarbon degraded. Under the appropriate geochemical conditions, sulphanogenesis can occur wherein sulphate is reduced to sul de during the oxidation of BTEX. Stoichiometrically, 4.6 mg/litre of sulphate is consumed for 1 mg/litre of hydrocarbon degraded. Some research has indicated that ethylbenzene cannot be degraded as a result of sulphanogenesis (Salanitro, 1993). After the other substrates discussed above are consumed, methanogenesis can occur. Stoichiometrically, 0.8 mg/litre of methane is produced for 1 mg/litre of hydrocarbon degraded Physical processes The main physical attenuation mechanisms are dispersion, di usion, dilution by recharge, and volatilization. These attenuation mechanisms do not result in a reduction in the total contaminant mass, but in a decrease in concentration Chemical processes The main chemical attenuation mechanisms are chemical and abiotic reactions and sorption. No evidence

3 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203± exists that chemical and abiotic reactions occur in hydrocarbon contaminated groundwater (McAllister and Chiang, 1994). 4. Goose Bay case study Goose Bay, located in Labrador, Canada, has supported a military airport since Located in the subarctic, delivery of fuel is restricted to an annual or semiannual resupply. This occurrence necessitates the shortand long-term storage and handling in excess of 380,000,000 litres of fuel annually. During the life of this facility, releases of hydrocarbons occurred. Since assuming responsibility for the facility in 1989, the Canadian Department of National Defence has aggressively investigated and, where necessary, remediated over 18 free product fuel plumes. To date, in excess of 1200 monitoring and recovery wells have been installed to investigate the free product fuel plumes and land ll sites that are present at the site. The free product fuel plumes range in size from 10,000 to 3,200,000 litres. The largest of these plumes is comprised of Arctic diesel fuel and has a surface area of 38,500 m 2. The release occurred due to a pipeline rupture in This plume is the focus of this study Methodology A eld investigation consisting of sampling 102 groundwater and three surface water locations was developed to characterize the intrinsic remediation that was occurring at the site. The samples were analyzed for a suite of physical and chemical contaminants and characteristics. Analysis of the results allowed for the composition of a series of contaminant isopachs and isopleths, and electron acceptor isopleths. Analysis of these isopachs and isopleths resulted in the prediction of the assimilative capacity of the groundwater. After this value was determined, recommendations were made on the optimization of the intrinsic remediation process Hydrogeological conditions The plume is located on a plateau comprised primarily of medium to ne glacio- uvial sand with discontinuous silt lenses, iron seams and coarse sand. Silty sand dominates below a depth of 35 m. The depth to groundwater is approximately 30 m. Background concentrations of iron and manganese can exceed 10.3 and 0.47 ppm, respectively. The average hydraulic gradient in the area is m/m. Above a depth of 35 m, the measured hydraulic conductivity values range from to cm/s. Below this depth the hydraulic conductivity decreases to cm/s. Vertical hydraulic conductivities are estimated at cm/s Free product reduction mechanisms E ective recovery of the free product commenced in The volume of free product is being reduced by three primary pathways. Free product is being pumped to the surface. Constituents of the free product volatilize and are removed from the soil matrix by a vapour extraction system. This removal creates a concentration gradient which enhances the volatilization of the hydrocarbons. Hydrocarbons also dissolve in the groundwater. As these contaminants are degraded a concentration gradient occurs resulting in more hydrocarbon being dissolved. Therefore, if the degradation of the contaminants in the groundwater can be maximized, the volume of free product removed by this pathway can be increased Site uniqueness factors The investigation of this free product plume is complicated by the presence of four additional free product plumes within 150 m of the plume of interest. The edge of the escarpment is located at the leading edge of the free product plume. A sh bearing surface water body is present at the toe of the escarpment. A signi cant hazardous materials land ll site is present at the leading edge of the free product plume Data collection A total of 105 sampling locations were selected for this investigation. The sites were selected to provide background samples, samples within the plume, and samples downgradient of, or on the perimeter of, the plume. Factors which determined the location of sampling wells were the physical location of the well and the screened interval, the age and condition of the well, and the past use of the well (nitrate addition, water recharge, etc.). Field measurements were taken at each sampling location. Groundwater (surface water) samples were collected at each sampling location for laboratory analysis. Quality assurance/quality control samples including eld blanks, trip blanks, and duplicates were collected. Field measurements collected include: water level, fuel thickness, water temperature, dissolved oxygen, ph, and conductivity. Collection of water level data allows the interpolation of the water elevations and calculations of hydraulic gradients. This information is used to predict the transport component of intrinsic remediation. The presence of free product in a monitoring well is important because it indicates a continual source of dissolved hydrocarbons. Temperature and ph of groundwater in uences the solubility of chemicals and,

4 206 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203±210 therefore, will a ect the fate and transport of contaminants. Stabilization of the conductivity is used to determine when the well is fully developed. Samples are collected for analysis by the laboratory for the following parameters: BTEX, total petroleum hydrocarbons, total extractable hydrocarbons, standard plate count, methane, speciated iron (ferric and ferrous), alkalinity, oxidation/reduction potential, total organic carbon, nitrate, and sulphate. Samples were collected over a 2-week period in June The total cost for the sample analysis was $35, Data analysis The quality assurance/quality control samples indicate, with no signi cant exceptions, that the laboratory results obtained are accurate and precise Free product isopach map The free product thicknesses collected in the eld investigation were corrected to re ect the actual thickness of the product. The results were interpolated and are plotted in Fig. 1. The shape of the plume is in uenced by the groundwater ow patterns. The bulk of the free product in the plume is still near the point of injection into the ground in The isopach lines are not continued past the edge of the escarpment due to the nature of the escarpment. The escarpment contains large quantities of buried waste. This waste a ects the transport of free product and dissolved contaminants. It is not possible to predict the ow of groundwater or free product in this area Contaminant isopleth maps Isopleth maps were created for benzene, toluene, ethylbenzene and total xylenes by interpolating the data. An isopleth map of the sum of benzene, toluene, ethylbenzene and total xylenes concentrations is shown in Fig. 2. The general shape of this isopleth map is expected. The total BTEX concentrations were lower than expected compared to the literature. As previously discussed, there are three main free product reduction mechanisms. The free product removed through total uids pumping was analyzed by specialty laboratories and was generally consistent with fresh product indicating that weathering had not occurred to a signi cant degree. However, the free product thickness has been recorded at values of up to 5 m (in May 1996). The thickness of this plume may have `insulated' a central core from exposure to external forces that cause weathering. This resulted in the preservation of the samples, despite the age of the plume (34 years). However, weathering has occurred at the edges of the plume as a result of the other two main attenuation mechanisms previously discussed. These mechanisms target the volatile components rst. With the removal of the volatiles, the viscosity of the fuel decreases. Thus, a strati cation of the plume is plausible. Fuel at the edges of the free product plume would be severely weathered after exposure to the external forces up to 34 years. This fuel would have a decreased viscosity and would not readily mix with the core of the plume. Instead, it could form a `shell' around the fuel in the core of the plume. The thickness of this shell would increase until a stable concentration gradient for the Fig. 1. Free product isopach map. Fig. 2. Total benzene, toluene, ethylbenzene and xylene isomers (BTEX) isopleth map.

5 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203± volatile components (in this case BTEX) is established. This could explain why lower than expected concentrations of BTEX were found in this investigation. The case histories reported in the literature typically are for smaller plumes that do not exhibit the thickness found in the plume in this study. In addition, the plumes in the literature are generally more recent and are isolated from other areas of concern. Analysis of results obtained from wells screened below the water table can be used to interpret the thickness of the dissolved plume. The dissolved plume resulting from the main free product plume is approximately 15 m thick at the upgradient edge and 25 m thick at the escarpment. The concentrations of BTEX in the surface water downgradient of the plume were low. Non-detectable concentrations of BTEX were obtained in previous investigations. This result may be explained by the rapid volatilization of hydrocarbons from the surface water. precipitating, the concentration of ferrous iron in the surface water decreased to trace levels. Background concentrations of sulphate are 9±11 ppm. In the isopleth for sulphate (Fig. 5), concentrations of sulphate decrease in the areas of the free product plumes when compared to background levels. Sulfanogenesis is the third preferential form of anaerobic biodegradation after denitri cation and iron reduction. As the amounts of bioavailable iron are not consistent it is possible that sulphanogenesis is occurring. In particular, the depression in sulphate concentrations in the main plume directly east of tank 1524 may be indicative of an area of sulphanogenesis. As described in the literature, ethylbenzene is recalcitrant 4.9. Electron acceptor and metabolic by-product isopleth maps The isopleth map for the nitrate concentrations in the wells screened across the water table is shown in Fig. 3. Background concentrations of nitrate are 1.5 ppm. It is evident that the nitrate concentrations rapidly decrease to below detection limits in the presence of free product. The background concentration of ferrous iron is below detection limits. The isopleth of ferrous iron, presented in Fig. 4, indicates that iron reduction is occurring. The highest observed concentration of ferrous iron is 13 ppm. As expected from a visual analysis of the surface water, which indicates that ferric iron is Fig. 4. Ferrous iron isopleth map. Fig. 3. Nitrate isopleth map. Fig. 5. Sulphate isopleth map.

6 208 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203±210 under sulphate reduction. This area of reduced sulphate is consistent with the xylene isopleths. The concentration of methane is below detection limits throughout the study area except for an area located on the edge of the escarpment; this area is not likely to be due to biodegradation of hydrocarbons. Instead this area, part of the south escarpment land ll, contains a large amount of buried waste. The methane in this area is due to these land lled materials. Methanogenesis of hydrocarbons does not occur in the area of study. Analysis of the alkalinity of concentrations indicates that alkalinity increases in the area of the dissolved plume. Alkalinity is a qualitative indicator of biodegradation via aerobic biodegradation, denitri cation, and sulphate reduction. The increase in alkalinity in the area of the dissolved plume is indicative of ongoing intrinsic remediation. Analysis of the isopleths provides strong evidence that intrinsic remediation is occurring. The high number of microbial populations reported in the standard plate count support the presence of microbes in the groundwater. The concentrations of electron acceptors generally decrease where expected and the concentration of metabolic by-products generally increase where expected. This is su cient to show that intrinsic remediation is occurring (Capuano and Johnson, 1996; Buscheck and Alcantar, 1995). The zones of the various attenuation mechanisms are shown in Fig. 6. From this gure it is evident that the majority of the plume, from an areal perspective, is subject to ferrous iron reduction. Zones of denitri cation occur at the perimeter of the plume and in areas where high concentrations of nitrate were added to the groundwater regime. Aerobic respiration takes place at the perimeter of the plume. Sulfate reduction is evident in one zone in the plume. It is likely that iron is not bioavailable in this area. This gure illustrates the vast area that contributes to the amount of iron precipitating in the downgradient surface water body Assimilative capacity calculations The assimilative capacity of groundwater is a quantitative indicator of how much hydrocarbon can be degraded for each litre of groundwater. It can be assumed that each litre of groundwater contains the background concentrations of the electron acceptors (dissolved oxygen, nitrate, sulphate). Further, it can also be assumed that ferric iron is bioavailable to the system to the extent that the maximum observed concentration of ferrous iron can be mobilized. The following mass balance calculations are used to determine the assimilative capacity of the groundwater: 1. BTEX bio,aerobic =0.32(O background O min ); 2. BTEX bio,denitri cation =0.21(N background N min ); 3. BTEX bio,iron reduction =0.05(Fe 2+ max Fe 2+ background); 4. BTEX bio,sulphanogenesis =0.22(S background S min ); and 5. BTEX bio,methanogenesis =1.25(M max M background ). Using these formulas, the total assimilative capacity of the groundwater is 5.68 mg/litre distributed as shown in Table 1. This methodology signi cantly underestimates the total assimilative capacity because it does not take into account microbial cell production. The contribution of denitri cation to the assimilative capacity of the groundwater is low due to the relatively low background concentration of nitrate. If the groundwater was saturated with nitrate (at 92 mg/litre), the assimilative capacity of the groundwater would rise to mg/litre. The calculated assimilative capacity can be compared to the maximum measured concentration of BTEX, as shown in Table 1. As the total assimilative capacity of the groundwater is greater than the maximum BTEX concentration in the plume, it is evident that the Table 1 Assimilative capacity of groundwater Attenuation mechanism BTEX assimilative capacity (mm/litre) Aerobic 2.62 Denitri cation 0.31 Iron reduction 0.65 Sulphanogenesis 2.10 Methanogenesis 0.00 Total assimilative capacity 5.68 Highest observed total BTEX Fig. 6. Zones of attenuation mechanisms. BTEX, benzene, toluene, ethylbenzene and xylene isomers.

7 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203± dissolved plume in this area will not expand. In a typical situation, the dissolved plume can be expected to contract. Due to the speci c circumstances present in Goose Bay, the dissolved plume created by the free product plume is curtailed by a surface water body. It is unlikely that the dissolved plume will contract to a size smaller than the free product plume as a result of the assimilative capacity of the groundwater. Although certain contaminants may not be degraded at equal rates throughout the dissolved plume, most notably ethylbenzene in the sulphate reducing portion of the plume, these contaminants will degrade as the groundwater transports them into another attenuation mechanism's zone of in uence. As a result of the aforementioned theory on the formation of strati cations within the plume, it is unlikely that signi cantly greater amounts of dissolved hydrocarbons can be biologically degraded by increasing the assimilative capacity of the groundwater. Therefore, it is likely that the assimilative capacity of the groundwater will not be exceeded in the future. The saturation of the groundwater with nitrate will reduce the areal extent of the plume subject to ferrous iron reduction. In turn, this will reduce the precipitation of ferric iron in the surface water body with the associated sheries and aesthetic concerns. 5. Conclusions Intrinsic remediation is a risk management option that can signi cantly reduce the costs of remediating or mitigating a contaminated site. Although intrinsic remediation involves a combination of biological, chemical and physical attenuation mechanisms, the biological mechanisms are often the most important. Chemical attenuation mechanisms are rare in hydrocarbon contaminated sites (with the exception of sorption which is not a destructive mechanism) and physical attenuation mechanisms do not result in the destruction of the hydrocarbons. Both aerobic and anaerobic zones are involved in biological attenuation. In most cases, anaerobic bioremediation is often occurring in the majority of the plume. The microbes required to provide the activation energy to the reactions are felt to be ubiquitous. In most cases, the electron acceptor which is used is related to the amount of Gibbs free energy that is released to the microbes by the reaction. The electron acceptor reactions, in preferential order, are: aerobic respiration, denitri cation, ferrous iron reduction, sulphate reduction, and methanogenesis. Analysis of the results obtained in a eld investigation conducted at Goose Bay, Labrador, indicate that aerobic bioremediation, denitri cation, ferrous iron reduction and sulphate reduction are occurring. The methodology employed is e ective and cost e cient. Detailed calculations of the assimilative capacity of the groundwater indicate that 5.68 mg of hydrocarbons will be degraded for each litre of groundwater. This value is to be considered conservative because of the assumptions employed in its calculation, the greatest of which was neglecting microbial growth. The highest BTEX concentration in the dissolved plume was mg/litre. This indicates that the assimilative capacity of the groundwater is su cient to degrade the hydrocarbons that are partitioning from the free product phase. The volume of BTEX contaminants that are partitioning is lower than observed in other cases due to the age of the spill and thickness of the plume. In this understanding of the site, enhancing the assimilative capacity of the groundwater may not result in an increase in the mass of contaminants biodegraded. However, the addition of an alternate electron acceptor, such as nitrate, will reduce the amount of ferrous iron reduction. In turn, this will reduce the amount of ferric iron precipitating in the downgradient surface water body. This investigation indicates that intrinsic remediation is occurring in the free product fuel plume investigated in this study. The e ects of intrinsic remediation (degradation of contaminants and precipitation of iron in the surface water body) can be optimized by adding nitrate saturated water to the groundwater regime. Ongoing monitoring (one year quarterly, thereafter annually) should be conducted to determine the e ects of the addition of nitrate. References American Petroleum Institute, Monthly Statistical Report, Vol. 18, no. 12. American Petroleum Institute, Washington DC. 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8 210 F. Curtis, J. Lammey/Environmental Pollution 103 (1998) 203±210 Shorelines: E ects of Di erent Nitrogen Sources. Presented at the Third International In Situ and On-Site Bioreclamation Symposium, San Diego, April Ridgeway, H.F., Safarik, J., Phipps, D., Carl, P., Clark, D., Identi cation and catabolic activity of well-derived gasoline degrading bacteria from a contaminated aquifer. Applied Environmental Microbiology 56(1), 3565±3575. Rifai, H.S., Borden, R.C., Wilson, J.T., Ward, C.H., 1995a. Intrinsic Bioattenuation for Sub-Surface Restoration. Presented at the Third International In Situ and On-Site Bioreclamation Symposium, San Diego, April Rifai, H.S., Newell, C.J., Miller, R., Ta nder, S., Rounsaville, M., 1995b. Simulation of National Attenuation with Multiple Electron Acceptors. Presented at the Third International in Situ and On-Site Bioreclamation Symposium, San Diego, April Salanitro, J.P., The role of bioattenuation in the management of aromatic hydrocarbon plumes in aquifers. Ground Water Monitoring Review 13(3), 150±161.