The Future of Petroleum Hydrocarbon Remediation: Site Closure Through Enhancement of In Situ Biological Degradation

Transcription

1 The Future of Petroleum Hydrocarbon Remediation: Site Closure Through Enhancement of In Situ Biological Degradation Abstract By Peter Palmer, P.E., ARCADIS G&M, Inc. Sharon Hall, P.E., ARCADIS G&M, Inc. Joe Darby, ARCADIS G&M, Inc. Monitored Natural Attenuation (MNA) is the most important advancement in the field of remediation during the 1990 s. We have all come to understand it as the process that is responsible for removing the majority of the organic compounds at our contaminated sites. This is especially true for petroleum hydrocarbon sites. The problem has been that while this technology has been perceived as very cost effective, a majority of these projects have not reached closure within a reasonable and cost effective time frame. The reoccurring costs of a MNA program include the anticipated monitoring, reporting and agency interaction costs, and also the transactional costs associated with management (internal and consulting) of the project by the property owner while these projects remain open. There is mounting evidence that there is a more cost effective method to achieve site closure at many petroleum hydrocarbon sites. Several methods have been developed that can stimulate the rate at which the natural bacteria degrade the petroleum hydrocarbons. While these methods can cost significant amounts of money, the project reaches closure at a much faster rate than MNA. When the total costs of the project with and without stimulation are compared, the most cost effective process is often enhanced biodegradation. The main methods of stimulation involve providing alternate electron acceptors to the natural bacteria. Bacteria are more energy efficient when using oxygen or nitrate as their final electron acceptor than when using sulfate or other anaerobic pathways. This energy efficiency can translate into faster biological reactions and faster closures. Several case histories will be provided that show the results of adding oxygen and nitrate to enhance the natural bacterial actions at petroleum-contaminated sites. Cost analysis from these applications show that the up front expenditures for the enhancement processes can be recouped in 1 to 3 years compared to a MNA strategy.

2 Introduction During the 1990 s, consultants have been trying to persuade regulators that a MNA approach is practical and can be more cost effective than active remedial approaches. In many cases this is true; however, the total project costs using a MNA approach can still be excessive. This is especially true when all client and/or property owner costs are considered. Many times when consultants determine project costs, the annual costs will include sampling, analysis, report writing, and project management. However, there are other costs that are not always considered that will be referred to as transactional costs. These costs are very real and include the following: Management of the consultant Communications with regulators Management and maintenance of permits Legal fees Corporate internal communications Administrative project support Management and storage of documents Third party liabilities associated with contaminants crossing property boundaries Environmental corporate liability associated with a contaminated site (which must be reported on corporate annual reports, this represents a financial liability that affects a corporations overall worth and potentially their stock price) Although some of these costs are more difficult to quantify than others, they all represent real costs. As long as a project is alive, these costs will be realized by the client and should be considered when evaluating project costs. When we add up all of these transactional costs over a long period of time (10 to 20 years or more) they become very significant. It is obvious to see that reductions in a project life can represent significant project savings. The focus of this paper is to discuss methods to reduce the lifecycle of MNA projects. One of the most effective means of reducing the lifecycle of a MNA project is to enhance the biodegradation that is most likely already occurring. This enhancement can be achieved by the addition of electron acceptors to the aquifer. The net effect will be to increase bacterial activity and reduced times to reach project closures thus lowering the overall project costs. Enhancement of In Situ Biological Degradation MNA is a passive remedial approach that relies on natural processes to reduce the contaminant concentrations within an aquifer. The natural processes include biodegradation, dispersion, dilution, sorption, volatilization, and abiotic degradation. Of all these process, biodegradation is the primary process that results in the most significant reduction of organic compounds in groundwater.

3 Biodegradation Enhancements Methods The basic bacterial reaction involved in the biodegradation process is well understood and can be summarized as follows: Electron donor + Electron acceptor Metabolic byproducts + Energy In hydrocarbon remediation, the electron donor is the food source for bacteria, which in this situation the petroleum hydrocarbon constituents serve as this food source. The electron acceptor can be several naturally occurring (or injected) chemical compounds within the aquifer. Not all bacteria can use all of the electron acceptors, and not all compounds can be degraded by all of the bacterial enzyme pathways. However, because most natural environments include a wide variety of types of bacteria, we can use the following methods to stimulate a portion of the bacterial population in order to enhance the degradation that we are trying to achieve. There are two main categories for biological enhancement: aerobic and anaerobic. Figure 1 shows the relative energy that bacteria can derive from using different electron acceptors (Nyer, 2000). The important point to understand from Figure 1 is the relative energy that bacteria gains by the use of different electron acceptors. Each electron acceptor degrades the same amount of organics, but the bacteria are better off using the acceptor that provides the most energy. Therefore, the bacteria would rather use O 2 than NO 3, because oxygen provides 60 percent more energy than nitrate. Similarly, the natural bacteria would rather use NO 3 than SO 4. Not only would they rather use a particular electron acceptor, but they also grow faster with the higher energy electron acceptors. In aquifers, bacteria will utilize the electron acceptors that are available. It is routinely noted in many contaminated aquifers, that there is a reduction in the oxygen and nitrate available within the contaminant plume when compared to the background levels. Generally, the order of electron acceptors in Figure 1 is the order that the electron acceptors will be used by the bacteria, that is from the most efficient to the least efficient. Since these higher efficiency electron acceptors are the first to go, the bacteria have to rely on the more abundant, but less efficient electron acceptors such as sulfate, which is typically available at higher concentrations under natural conditions. Therefore, MNA processes are typically occurring using more abundant but less efficient electron acceptors such as sulfate. If the more energy efficient electron acceptors listed on Figure 1 can be supplied, the bacteria would use them preferentially, first using oxygen and then using nitrate to degrade the compounds at a faster rate. For petroleum hydrocarbon remediation, environmental consultants typically default to using O 2 for stimulation of natural bacteria by creating an aerobic environment. As we will demonstrate in this article, there are many circumstances where the application of nitrate is a more cost effective electron acceptor to stimulate the natural bacteria to degrade petroleum hydrocarbon compounds. Although oxygen provides more energy than nitrates, the limitation in using oxygen as the preferred electron acceptor is that its solubility is limited to 8 mg/l (depending upon injection depth and oxygen source) and quickly consumed by the natural bacteria. Therefore, the treatment area is usually limited using O 2 at sites where there is limited groundwater velocity to transport the oxygen at significant concentrations beyond the point of injection. However, nitrates can be delivered at much higher concentrations of

4 10,000 mg/l or greater based on its higher solubility in water than oxygen. Using oxygen and nitrate stimulations are most effective for petroleum hydrocarbons and lower molecular weight organics. Practical Application Methods For Adding Oxygen and Nitrate To Impacted Groundwater In most applications for petroleum hydrocarbons, the emphasis is to add oxygen or nitrate to enhance bioremediation. Oxygen can be added using several techniques, some more intensive than others. Some of the oxygen delivery methods are summarized below. Oxygen Delivery Methods Air sparging injection of compressed air into the aquifer. Vapor extraction system using a low-vacuum blower to provide oxygen laden atmospheric air to the vadose zone. Vacuum enhanced recovery using a high vacuum blower to pull both liquids and vapors from the aquifer and vadose zone, increasing the air flow rate through the affected zone. Oxygen release materials solid materials that produce slow release molecular oxygen when applied in wells or directly injected in the aquifer. Hydrogen peroxide applied in low concentrations (500 to 1,000 mg/l) within wells or trenches to serve as an oxygen source. Pure oxygen sparging - similar to air sparging, but pure oxygen is used. Oxygen diffusion pure oxygen is diffused through hollow microporous membranes that are placed in wells to diffuse oxygen. Oxygen electrolytic generation these are oxygen generators that are placed down into wells to create molecular oxygen by dissociating oxygen and hydrogen using an electrolytic cell. Nitrate can be applied using various nitrate compounds and by various methods. Generally, ammonium nitrate is used because it is very inexpensive and readily available in large quantities. Ammonium nitrate is used as an agricultural fertilizer and is applied as a solid and/or liquid to agricultural fields. Sodium nitrate is also a viable alternative for nitrate applications. Nitrate can be applied using the following methods depending upon the site-specific conditions. Nitrate Delivery Methods Surface application apply solids material directly to the ground surface and use a spray irrigation system to deliver the nitrates into the subsurface. Trenches a nitrate solution can be injected into infiltration trenches. Injection wells a nitrate solution can be pumped directly into the aquifer via wells. Although oxygen is the most efficient electron acceptor as shown on Figure 1, it is more expensive to apply than nitrates due to the associated capital expenditures and more costly proprietary chemicals or processes. Nitrates are very inexpensive and in most cases easily to apply; however, the user must be careful not to over apply. Nitrate

5 is a regulated compound with a drinking water maximum contaminant level of 10 mg/l as N and in many states; ammonia is also a regulated compound in groundwater. Care must be taken to avoid excessive down gradient concentrations by applying more nitrate than required by the natural bacteria for the complete degradation of the petroleum hydrocarbons. When applying nitrates, the injections are completed in batch solutions where only 10 percent of the total quantity of nitrate is applied at one time. Case Studies Using the Injection of Electron Acceptors Case Study #1 Stanely Works, Pittsburg, California At the Stanely Works site, there was a leaking underground storage tank containing xylene. The investigation work was completed in the 1980 s and a significant free product plume was discovered with as much as three feet of free product on the water table. A pump-and-treat system was installed in the early 1990 s that was effective in providing hydraulic control and preventing the xylene plume from moving off-site; however, the xylene product recovery was slow. In 1995, the size of the free phase xylene plume was approximately one acre, as shown on Figure 2. It was decided to evaluate the effects of adding an electron acceptor to increase the biodegradation at the site. Starting in 1996 through 1998, nitrate (ammonium nitrate) was surface applied to a large area around the facility. Solid ammonium nitrate was spread on the land surface and a spray irrigation system was used to dissolve the ammonium nitrate and carry it to the water table. By 2001, all xylene concentrations were reduced below 10 mg/l with the exception of the former tank area. The limits of the free phase xylene plume are shown on Figure 2. Case Study #2 Active Petroleum Bulk Storage facility, Green Bay, Wisconsin At the Green Bay site, there were two different areas on site that were contaminated by gasoline releases. The two areas were not far from each other and the magnitude of the impacted area and the geology were very similar. Two different remediation techniques were used for each area. In the first area, a biosparge system was installed that included low flow air injection into the groundwater. The purpose of this system was to add oxygen to the formation and increase the biodegradation of the BTEX compounds. In the second area, an enhanced MNA approach was used with the electron acceptor nitrate being added to the formation. Dry ammonium nitrate was surface applied and watered in using a spray irrigation system. The first system, the biosparge system, operated for over three years until closure was achieved. BTEX concentrations were reduced from 6,000 ug/l to 200 ug/l during the three years of system operation. The second system, the ammonium nitrate system, also reached closure in three years. Benzene concentrations over the three-year period are shown in Figure 3. Although both systems achieved closure in a similar amount of time, the cost for the ammonium nitrate system was significantly less since no continuously operating equipment was required; capital, labor, and utility costs were significantly lower. Both systems were successful in reaching closure within an acceptable amount of time and both added electron acceptors to the aquifer to speed biodegradation of the BTEX compounds.

6 Economic Analysis of Biodegradation Enhancement To demonstrate the financial impacts of enhancing the biodegradation of an MNA project, a hypothetical case is presented. The hypothetical case involves a hydrocarbon plume from a gasoline storage tank area that is stable and is naturally biodegrading. The contaminants are decreasing; however, it is desirable to increase the biodegradation rate to achieve site closure within a more reasonable time period. Let us look at increasing the rate of biodegradation at this site by stimulating the bacteria with oxygen. The site in question has had ongoing assessment and remediation (including source removal) activities for a period of 14 years. Constituents of concern are BTEX, MTBE and naphthalene, contained in both shallow and deep surficial aquifers at concentrations exceeding state groundwater Cleanup Target Levels (CTLs). The saving grace of this site is that the plume is stationary and is contained within site property boundaries. The difficulty at this site is that the plume is located primarily beneath an active automobile sales and service facility. Site investigations over the past 5 years have shown that anaerobic conditions prevail within the plume, with aerobic conditions at the leading edge. While dissolved hydrocarbon concentrations have decreased in the shallow surficial aquifer during this period, benzene and MTBE concentrations have increased in the deep stratum wells. Thus anaerobic degradation processes are occurring, although at a rate insufficient to reach site rehabilitation within a reasonable and cost effective time frame. An aggressive remediation strategy was developed that entailed injection of air into the affected area, thereby enhancing the dissolved oxygen availability in order to utilize the faster aerobic degradation pathway. Given the location of the plume beneath the building, however, air sparging alone was not possible due to the potential buildup of explosive gases beneath the structure. A series of horizontal vapor recovery wells were installed within the building to mitigate this situation. The resulting system did two things. First it allowed establishment of an aerobic environment to facilitate hydrocarbon degradation. Second, it allowed volatilization and transfer of contaminants from the dissolved and adsorbed phases to the vapor phase within the zone of aeration, thus increasing mass removal. Vapor phase treatment was achieved using granular activated carbon (GAC). Recall that this site has undergone remediation activities for 14 years. With the exception of one source removal event, the site has simply been monitored for groundwater hydrocarbon concentrations. After all that time, contaminant levels were still well above state groundwater CTLs. Monitoring costs for this site have been $30,000 per year. This cost includes sampling, analytical, reporting, and project management. The transactional costs are estimated to be another 20%, bringing the total annual cost to be $36,000 per year. To continue with MNA at this site for another 14 years would cost $500,000, and there is no reasonable assurance that the contaminant levels will be at or below state CTLs. Given the extent and degree of contamination at this site, it is anticipated that petroleum constituent concentrations will be reduced below state default natural attenuation levels after two years of active remediation using air sparging and vapor extraction. The estimated cost of implementing this remediation system, two years of operation, and one year of post active remediation monitoring is $360,000 (this includes transactional costs).

7 In this hypothetical case the system operates for 24 months and reported concentrations have already decreased to below detection levels for vapor-phase BTEX. Groundwater BTEX, MTBE, and naphthalene concentrations have been reduced to below state groundwater CTLs in all but one well, where trace levels were reported outside of the influence area of the air sparge system. Consequently, the initial 3-year projection for achieving MNA levels is well within reach. Although the initial costs for the air sparging and vapor extraction system are significant, bringing the site close to clean up in such a short period of time by enhancing subsurface conditions more than justifies the expense. Conclusions In the early 1990's mass removal followed by a lengthy MNA program was a common approach for remediation projects. Now that some time has passed and we have a better understanding of natural attenuation processes, we see that many MNA projects are not reaching site-specific cleanup goals within an acceptable amount of time. As discussed, the MNA annual project costs are not just limited to sampling, analytical, reporting, and project management, but transactional costs that the client or property owner bears must also be considered. The limitation of the MNA approach was that natural environments generally do not have sufficient availability of the higher efficiency electron acceptors to completely degrade the petroleum hydrocarbons. There are new approaches to enhancing the natural degradation processes for site remediation. In 2000 and beyond, our focus now is achieving site closure by mass removal, followed by enhanced in situ biological degradation. The focus is to provide the high efficiency electron acceptors, oxygen and nitrate, to the impacted areas. Using this alternative approach, we can now achieve site closure more cost effectively and within a more reasonable time frame than a typical MNA approach. References Nyer, Evan K., 2001, In Situ Treatment Technology 2 nd Edition, Lewis publishers/crc Press, Inc., Boca Raton, FL Biographical Sketches Peter Palmer, P.E. is a Senior Vice President and Remediation Director with ARCADIS G&M, Inv. where he is responsible for the profitability and growth of the Remediation Business Practice. He also oversees the firm s innovative remediation technology demonstration program, which includes projects currently being performed for ESTCP, AFCEE, and DOE. Sharon Hall, R.E.M., R.G. is a Project Manager with ARCADIS with 20 years of petroleum industry experience that includes 13 years of environmental experience. Projects she has managed are soil and groundwater investigation and remediation for hydrocarbon, saltwater, metals, nitrates and NORM. She has managed projects in Texas and New Mexico and also provides litigation support for ARCADIS clients. Joe Darby is a Project Engineer with ARCADIS and has 15 years of experience in environmental engineering predominantly related to groundwater and soil remediation. He has been involved in the planning, design, construction, start-up, and operations of pilot and full-scale systems under various regulatory programs around the United States.

8 Their address is: ARCADIS 3903 Northdale Blvd., Suite 120 Tampa, Florida Fax:

9 1/20 2 /H 2 0(+0.82,2e - ) Fe 3 +/Fe 2+ (+0.76, 1e - ) NO 3 - /N 2 (+0.74, 5e - ) N0 3 - /N0 2 - (+0.42, 2e - ) FUMARATE/SUCCINATE (+0.02, 2e - ) E 0 h volt PARACOCCUS DENITRIFICANS PARACOCCUS DENITRIFICANS S0 2-4 /H 2 S (-0.022, 8e - ) CO 2 /ACETATE (-0.28, 8e - ) CO 2 /METHANOL (-0.38, 6e - ) 2H + /H 2 (-0.42, 2e - ) CO 2 /GLUCOSE (-0.43, 24e - ) DESULFOVIBRIO ENTEROBACTER Figure 1. Energy Tower for Different Electron Acceptors in Biodegradation.

10 3000 BENZENE (ug/l) Period of Nitrate Application 0 12/1/1997 6/1/ /1/1998 6/1/ /1/1999 6/1/ /1/2000 DATE Figure 2. Case Study #1 - Ammonium Nitrate Application for Xylene Removal

11 Figure 3. Case Study #2 Ammonium Nitrate Application for BTEX Removal. Green Bay, Wisconsin Figure 3. Case Study #2 Ammonium Nitrate Application for BTEX Removal. Green Bay, Wisconsin