This web site is currently being upgraded for an improved user experience. Several web pages will continue to retain the former user interface during this transition period.

Natural Attenuation of Petroleum Hydrocarbons

Printer Friendly View

An Implementation Guidance: May 22, 1996 Introduction Recent guidance from the State Water Resources Control Board (SWRCB) and San Francisco Bay Regional Water Quality Control Board (RWQCB) has allowed natural attenuation or passive bioremediation to be the primary remedial option to be compared against for low risk groundwater cases at UST petroleum release sites. San Mateo County recognizes natural attenuation and has issued this guidance (based upon the ASTM guidance) in an attempt to clarify the needed steps to be taken if this option is selected from the feasibility study. The many factors involved during natural attenuation include: aerobic and anaerobic biodegradation; dispersion; volatilization; and adsorption. Of these, biodegradation is the only component that results in a significant reduction of petroleum mass. Petroleum hydrocarbons and their constituents are generally biodegradable as long as indigenous microorganisms have an adequate supply of nutrients and electron acceptors, and biological activity is not inhibited by substances toxic to the organisms. Aerobic biodegradation tends to occur at the fringe of the dissolved plume and consumes oxygen which, if not replaced, can limit the effectiveness of further aerobic biodegradation. Anaerobic biodegradation is predominant at the core of the plume and occurs much slower than aerobic biodegradation. Site and soil conditions play a significant role in biodegradation efficiencies due to transport of both the impacted water and needed oxygen and nutrients. Under the appropriate conditions, natural attenuation can reduce the potential impact of a petroleum release from being transported to sensitive receptors. However, natural attenuation is not appropriate at all sites. The rates of biodegradation are typically slow and levels may not reach MCL's for many years. Additionally, long term monitoring is needed to demonstrate concentrations are continually decreasing at a rate appropriate to protect potential receptors. This guidance, in addition to other tools such as feasibility studies, will assist you in determining if natural attenuation is appropriate for your site. A check list, adapted from EPA's Guide for Corrective Action Plan Reviewers, is provided to assist you in evaluating the completeness of a Corrective Action Plan and to help focus on areas where additional information may be needed. Site Evaluation and Initial Screening Natural attenuation should only be considered at low-risk groundwater sites contaminated by leaking petroleum fuel tanks, as defined by the RWQCB's January 5, 1996 interim guidance (Appendix A), and a where a feasibility study supports the economics of the long term commitment. Low-risk cases are those that satisfy all of the following: The leak has been stopped and ongoing sources, including free product, have been removed or remediated. The site has been adequately characterized. The dissolved hydrocarbon plume is not migrating. No water wells, deeper drinking water aquifers, surface water, or sensitive receptors are likely to be impacted. The site presents no significant human health risk. The site presents no significant risk to the environment. An initial screening of the site is prudent to evaluate the potential effectiveness of natural attenuation and would include looking at constituent concentrations and the nearby receptors. Constituent Concentrations If initial concentrations are too high, natural attenuation will not reduce concentrations to acceptable levels within a reasonable time frame. The presence of free-product would eliminate this option. The upper limit for concentration is highly dependant on site-specific conditions including the type and concentration of pollution, proximity to potential receptors, and the hydrogeological conditions. Nearby Receptors Since natural attenuation generally allows constituents to migrate farther than if active remedial measures are employed, it is vital to address the potential that individuals or sensitive environmental areas may be impacted. The length of time for pollution to travel to individual receptors can be easily calculated from average hydraulic gradients, conductivity, effective porosity, and distance between the source and the receptor. Typically, travel times of 2 years or more should allow for evaluation purposes and allow sufficient time to implement active remedial options should natural attenuation prove to be ineffective in protecting human health or the environment (EPA 510-B-95-007). Detailed Evaluation of Natural Attenuation Effectiveness Natural Attenuation Mechanisms Biodegradation is considered a destructive mechanism since a net reduction of contaminant mass is achieved. Volatilization, mixing, diffusion and sorption are considered non-destructive since no net loss of contaminant mass occurs. This section will focus on the biodegradation mechanism. However, the other mechanisms should be evaluated to help determine if biodegradation is the predominant mechanism at the site. The rate of oxygen depletion due to microbial metabolism usually exceeds the rate oxygen is replenished to the system. This will typically occur within the core of the plume, forcing anaerobic biodegradation to dominate. Anaerobic systems are slower than aerobic systems. When the oxygen is depleted, an alternative electron acceptor and a microorganism capable of utilizing the alternative electron acceptor must be available for biodegradation to proceed. Anaerobic systems have been shown to effectively reduce toluene concentrations in the field. Degradation of BTEX under conditions of partially depleted oxygen may be possible, but has not been demonstrated. Evaluation of Site Factors 1. Soils Aquifers within soils of higher permeabilities (e.g., sands and gravel) are favorable to biodegradation, however they also provide for faster horizontal and vertical migration of the plume. Soils with lower permeabilities (e.g., clays and silts) increase the time of biodegradation, however migration is also retarded. Regional precipitation also needs to be considered to determine it's affect on leaching potential, oxygen and nutrient replenishment, and effect on groundwater depths. Other site conditions such as preferential pathways must also be addressed. The adsorption potential for various soils will also affect biodegradation and contaminant movement. 2. Groundwater Flow Rate and Aeration Groundwater flow rates are an important factor in the calculation of movement towards an identified receptor. Flow rates will also influence the re-oxygenation process. Systems with low oxygen content can hinder aerobic biodegradation. It is widely accepted that oxygen levels greater than or equal to 2 mg/l in groundwater (2% in soil) are conducive to aerobic biodegradation. Other indications of well-aerated groundwater are shown by the presence of chemicals in their oxidized state (Fe3+ , Mn4+, NO3-, and SO42-). 3. Temperature, pH and Nutrients Extreme temperatures prohibit microbial growth. The optimum temperature range is from 5oC to 45oC. Optimum pH should be 6 to 8. In addition to the need for carbon-rich environments as a food source, levels of nitrogen and phosphorus are necessary for system function and growth. Optimum C:N:P ratios would fall between 100:10:1 and 100:1:0.5. Monitoring 1. Evidence of Natural Attenuation There are several parameters which need to be monitored to determine evidence of natural attenuation. The following table list specific data used to evaluate the effectiveness of natural attenuation. Site Data Used to Evaluate Effectiveness of Natural Attenuation Site Characterization Data Application Direction and gradient of groundwater flow Estimate expected rate of plume migration Hydraulic conductivity Estimate expected rate of plume migration Definition of lithology Understand preferential flow paths Aquifer thickness Estimate volatilization rates and model flow Depth to groundwater Estimate volatilization rates Range of water table fluctuation Evaluate potential source smearing, influence fluctuation on groundwater concentrations, and variation in flow direction and rate Delineation of contaminant source and soluble plume Compare expected extent without natural attenuation to actual extent Date of contaminant release Estimate expected extent of plume migration Historical concentrations along the primary flow path from the source to the leading edge Evaluate status of plume (i.e., steady state, decreasing, migrating) Background D.O. levels upgradient of the source and plume Determine if sufficient D.O. is present for aerobic biodegradation (>1 to 2 mg/l) D.O. levels inside and outside the contaminant plume Identify inverse correlation indicative of aerobic biodegradation Alkalinity, hardness, pH and soluble Fe inside and outside the contaminant plume Evaluate geochemical indicators of natural attenuation Redox potential Determine nature of biologically mediated degradation of contaminants Location of nearest groundwater recharge areas (e.g., canals, retention ponds, catch basins and ditches) Identify areas of natural groundwater aeration Source: Adapted from McAlister and Chiang, 1994 2. Primary Evidence of Aerobic Biodegradation Some of the necessary data will be collected during the course of normal characterization, while some data will be collected specifically to demonstrate the occurrence of natural attenuation. Sampling and analytical methods must remain consistent and monitoring points must be appropriate to individual site conditions. The primary evidence for the occurrence of natural attenuation is generated by monitoring contaminant concentrations over a period of time. If natural attenuation is occurring the plume will migrate slower than expected. Natural attenuation factors may actually begin to shrink the plume. 3. Secondary Evidence of Aerobic Biodegradation A. Dissolved Oxygen The supply of oxygen is an important factor in determining the extent rate of bioremediation. If dissolved oxygen (DO) concentrations exceeding 1 to 2 mg/l are present upgradient of the plume, then groundwater flow will supply the needed DO to maintain aerobic conditions to the plume boundaries. An inverse relationship between DO concentrations and constituent concentrations can be expected where aerobic biodegradation is occurring. B. Geochemical Indicators Aerobic biodegradation of petroleum and it's constituents produces carbon dioxide and organic acids which tend to lower pH and increase alkalinity within the plume. These three indicators provide secondary evidence that aerobic biodegradation is occurring. Anaerobic bioremediation typically causes a reduction of insoluble iron to soluble iron. Therefore soluble iron levels tend to increase immediately downgradient of a source as the DO is depleted. Another indication of anaerobic biodegradation is an increase in methane concentrations. C. Reduction/Oxidation (Redox) Potential Biodegradation rates influence and depend on redox potential. The lower the redox potential, the more reducing and anaerobic the environment. Knowing the redox potential within the plume can also assist in estimating the extent of the plume. Redox potential values taken from within the plume will be lower than values upgradient and outside the plume. Post Remediation Monitoring As with any remedial option, it is important to monitor to ensure the effectiveness of the natural attenuation. If natural attenuation does not appear to be effective, then an alternative active remedial technology will be required. Alternatively, a risk assessment may be acceptable to determine the necessity of a more aggressive remedial approach. Groundwater monitoring is required to ensure the vertical and lateral extent of constituents are evaluated. At a minimum, the groundwater should be monitored for the constituents of concern at the site, dissolved oxygen, carbon dioxide, pH, alkalinity, hardness, soluble iron and the redox potential. Sampling frequency should be quarterly for the first year and may be reduced with concurrence with your San Mateo County site manager. In order to demonstrate that natural attenuation is occurring, a sufficient number of monitoring wells must be appropriately placed as needed. Appropriate monitoring of biodegradation equates to a minimum of four (4) wells in addition to the source area well. Proper well placement includes; includes one (1) well upgradient of the plume and three (3) wells down-gradient of the contaminant source area. The down gradient wells should include; one well close to the source, one further from the source and one well outside the boundary of the plume. Obviously, well placement will depend on several site-specific factors. Monitoring will need to continue until natural attenuation has reduced the constituents below established MCL's or to other performance based levels to be established by SWRCB and RWQCB. Site-specific conditions must also be considered. Checklist: Can Natural Attenuation Be Used At This Site? This checklist can help you to evaluate the completeness of the Corrective Action Plan (CAP) and to identify areas that require closer scrutiny. As you go through the CAP, answer the following questions. If the answer to any of the questions below is no, you may need to request additional information to determine if natural attenuation will accomplish the cleanup goals at the site. 1. Initial Screening Q. Are there no nearby human or sensitive ecological receptors near the site that could be exposed to the petroleum contamination in soil/groundwater? Yes No Q. If potential receptors are present, are they located at a distance that represents a minimum 2-year groundwater travel time? Yes No Q. Are maximum total constituent concentrations in groundwater less than 20,000 to 25,000 ppm TPH? Yes No Q. Are there no nearly potential receptors who could be exposed to contaminated groundwater or vapors? Yes No 2. Detailed Evaluation - Site Factors Affecting Constituent Degradation Q. Are the soils wells aerated, allowing for transfer of oxygen to subsurface soils? Yes No Q. Is the adsorption potential of the constituent/sediment combination high enough to adequately retard constituent migration? Yes No Q. Is the seepage velocity low enough to prevent rapid migration of the constituents? Yes No Q. Is soil oxygen content $ 2 percent and dissolved oxygen content $ 1 to 2 mg/l? Yes No Q. Is moisture available for transport of micro-organisms (soil moisture of 40 to 85 percent field capacity)? Yes No Q. Is the pH of the soil/sediment between 6 and 8? Yes No Q. Are concentrations of heavy metals and other toxic compound below levels that could inhibit microbial activity? Yes No Q. Is rainfall less that 10 inches per year? Yes No Q. Is the climate moderate to warm (i.e., 5E to 45EC)? Yes No Q. Does the soil/sediment have a C:N:P ratio of between 100:1:0.5 to 100:10:1? Yes No 3. Detailed Evaluation - Chemical Constituent Factors Affecting Migration For Those Constituents Requiring The Most Significant Concentration Reduction Q. Are the majority of the hydrocarbon constituents at most slightly soluble in water? Yes No Q Are the majority of the hydrocarbon constituents not highly volatile (as measured by vapor pressure, Henry's Law constant and boiling point)? Yes No Q. Are the Koc and Kd values of constituents high enough to adequately retard migration? Yes No Q. Are the constituents sufficiently biodegradable? Yes No 4. Remedial Monitoring Q. Will Soil/sediment samples be collected? Yes No Q. Will a minimum of four groundwater monitoring wells be sampled? Yes No Q. Is the groundwater monitoring frequency at least quarterly during the first year? Yes No Q. Are the groundwater wells placed to detect the reductions of constituents concentrations in the plume and potential migration of constituents? Yes No