Grantee Research Project Results

Final Report: Effect of Spatial Heterogeneity on the Natural Bioattenuation of Dissolved Hydrocarbons

EPA Grant Number: R825418
Title: Effect of Spatial Heterogeneity on the Natural Bioattenuation of Dissolved Hydrocarbons
Investigators: Barlaz, Morton A. , Borden, Robert C.
Institution: North Carolina State University
EPA Project Officer: Hahn, Intaek
Project Period: October 28, 1996 through October 27, 1999 (Extended to April 30, 2000)
Project Amount: $449,052
RFA: Environmental Fate and Treatment of Toxics and Hazardous Wastes (1996) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals

Objective:

The overall objective of this research was to improve our understanding of processes controlling the natural bioattenuation of dissolved hydrocarbons in groundwater. Field and laboratory studies were conducted in two petroleum contaminated aquifers to improve our ability to: (1) estimate the effect of natural bioattenuation processes on the fate and transport of hazardous organics in groundwater under ambient conditions; and (2) estimate the risk of these contaminants to the public and environmental receptors.

Summary/Accomplishments (Outputs/Outcomes):

Monitored natural attenuation (MNA) is being widely adopted as a corrective measure at sites where groundwater is contaminated with petroleum hydrocarbons including benzene, toluene, ethylbenzene, and xylene isomers (BTEX). The widespread adoption of this technology is due to several factors.

1. Laboratory studies have shown that microorganisms are present at a substantial number of contaminated sites that can biodegrade one or more BTEX components under anaerobic conditions using nitrate, ferric iron, sulfate and/or carbon dioxide as terminal electron acceptors.
2. Field studies have shown that many of the dissolved hydrocarbon plumes are less than a few hundred meters in length and appear to be stable or shrinking in size.
3. The U.S. Environmental Protection Agency (EPA) has published guidance on the use of MNA at Superfund, RCRA, and UST sites. This guidance recommends a three-tiered characterization approach to estimate with an acceptable level of confidence both the rate of contaminant attenuation and the anticipated time required to achieve remediation objectives.

The three tiers of evidence listed in the EPA guidance are summarized below.

Tier 1 Groundwater and/or soil chemistry data that demonstrates a clear trend of decreasing contaminant mass and/or concentration.

In practice, dissolve hydrocarbon plumes are typically defined in sufficient detail to determine the rate and direction of groundwater flow and detect changes in contaminant concentration with distance from the source. Contaminant degradation as a function of travel time from the source is then calculated after first correcting for the effects of dilution and/or dispersion using a mathematical model or a "non-degradable" tracer such as trimethylbenzene.

Tier 2 Hydrogeologic and geochemical data that can be used to demonstrate indirectly the types(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.

In practice, the most common approach for indirectly demonstrating natural attenuation processes at petroleum sites is to monitor for differences in the concentration of major electron acceptors and donors (oxygen, nitrate, ferrous iron, sulfate and methane) between background and contaminated wells. These differences are then used as indicators of contaminant biodegradation and are used to calculate the total mass of contaminant degraded, typically assuming that 100 percent of the change in electron acceptor or donor concentration is due to biodegradation of the regulated contaminant.

Tier 3 Data from field or microcosm studies which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern.

In practice, field or microcosm studies that directly demonstrate attenuation of the particular contaminant of concern have NOT been required at most petroleum sites.

Pope AFB-Evaluation of Current Approaches for Monitoring Bioattenuation

In the first phase of this project, indirect measures of contaminant bioattenuation (changes in contaminant and electron acceptor/donor concentration) were compared with more direct measures of contaminant attenuation (e.g., laboratory microcosms) to evaluate the efficacy and reliability of these two approaches. This work was conducted at a petroleum-contaminated site located at Pope Air Force Base (AFB) near Fayetteville, NC. The site was used as a Fire Training & Protection Area from the 1950s to 1990 resulting in the release of substantial volumes of JP-4 and other flammable liquids to the subsurface.

Field Monitoring Data. Monitoring data collected over a 12-month period showed strong evidence of contaminant biodegradation. The concentration of dissolved electron acceptors (oxygen and sulfate) decreased and the concentration of reduced products (ferrous iron and methane) increased with distance from the source suggesting oxidation of organic contaminants in the subsurface. The raw and dilution corrected concentration data showed substantial reductions in BTEX concentration with distance from the source, suggesting that all of the BTEX components were biodegrading under ambient conditions.

To further evaluate the potential for anaerobic biodegradation of BTEX at this site, sediment samples were collected aseptically at multiple locations within the plume and in background uncontaminated areas and assayed to determine the geochemical characteristics of the sediment and characterize the in situ microbial community. These results provided additional indirect evidence of the potential for anaerobic biodegradation of contaminants in the subsurface at this site.

Direct Evidence of Contaminant Biodegradation. Laboratory microcosms were constructed using sediment from three locations within the plume (source, fringe and downgradient) and treated with a variety of amendments to study BTEX biodegradation under iron reducing, sulfate reducing and methanogenic conditions. Observed changes in electron acceptors and donors (Fe[III], SO₄ and CH₄) indicated vigorous microbial activity in source and downgradient area sediments. However, benzene, ethylbenzene and the xylene isomers did not biodegrade in these microcosms and the observed toluene loss accounted for a small fraction of the observed changes in electron acceptors/donors. This indicates that some other organic substrate was being degraded in these sediments. During operation of the Fire Training & Protection Area, a wide variety of materials were released in this area (i.e., JP-4, diesel fuel and fire fighting foam) and are still present in the aquifer. The high level of biological activity in the aquifer is associated with biodegradation of these other organic materials, not the target compounds (BTEX).

Summary. At many sites where Monitored Natural Attenuation is being considered, changes in electron acceptor and donor concentrations in groundwater are used as very important (possibly primary) indicators of microbial activity against the target compounds. Results from this work clearly show the problems with this approach. Changes in electron acceptor and donor concentration only show microbial activity-they do NOT indicate what compounds are being biotransformed.

At this time, it is not clear whether biodegradation processes are significantly reducing the migration of benzene, ethylbenzene, and/or the xylene isomers (BEX) in the contaminated aquifer at Pope AFB. The microcosm results suggest BEX is not likely to biodegrade under in situ conditions. In contrast, the groundwater monitoring data suggest that BEX concentrations in ground water are being reduced by some process other than dilution or sorption.

What is needed is an accurate, reliable method of determining if the target compounds are degrading and for estimating plume scale biodegradation rates. Microcosms studies provide direct evidence of the potential for contaminant biodegradation. However, there are always questions about how well any laboratory study conducted using a few discrete sediment samples can represent large-scale plume behavior. Field scale ground water monitoring results are clearly representative of large-scale plume behavior. However, it is often very difficult to infer what specific processes are causing a decline in contaminant concentration because of the difficulties in precisely defining and monitoring transport pathways.

Rocky Point-Development of Large Scale Columns for Estimating In Situ Biodegradation Rates

In the second phase of this project, we developed and applied a procedure using large diameter in situ columns to determine if target compounds are degrading and to estimate plume scale biodegradation rates. Results from these in situ columns were then compared to results from laboratory microcosms and a detailed hydrogeologic, geochemical and microbiological characterization to evaluate the representativeness and reliability of this procedure. This work was conducted at a gasoline-contaminated aquifer near Rocky Point, NC.

Large Diameter In Situ Column Protocol. A field technique was developed for measuring in situ biodegradation rates at multiple depths with minimal disturbance of the subsurface ecosystem. Two ft diameter carbon steel pipe was driven into the ground to isolate a large volume of undisturbed aquifer material preventing dilution with background groundwater. A stainless steel multilevel sampler (MLS) was then installed in the center of each column to allow injection and removal of groundwater and tracers at discrete intervals. Three large diameter test columns were installed in an equilateral triangle (10 ft on each side) approximately 300 ft downgradient from the original gasoline release at the Rocky Point site. Two of the test columns were operated as live experiments while the injection fluid for the third column was amended with formaldehyde to inhibit biological activity. Effective first order loss rates were calculated at each sampling point after correcting for dilution using a non-reactive tracer (NaBr).

Biodegradation was statistically significant (alpha = 0.05) at one or more depths in the first live column for benzene, toluene, ethylbenzene, m- , p-xylene, and mesitylene. In the second live column, many of these compounds also were rapidly depleted. However, bromide declined very rapidly in this column (reason unknown) and statistically valid degradation rates could not be calculated. Vertically averaged first-order decay rates in the first column were 0.004 benzene d^-1, 0.09 toluene d^-1, 0.01 ethylbenzene d^-1, 0.02 m-p-xylene d^-1, 0.01 mesitylene d^-1 and 0.01 total BTEX d^-1. There also were significant variations in the decay rate between different monitoring depths (ports).

Geochemical and Microbial Characterization. Split spoon samples were obtained from within the first live column and at three locations intermediate between the three columns. These samples were used for microcosm construction and to evaluate the effect of spatial variations in soil characteristics, geochemistry, and microbial populations on contaminant degradation rates. Toluene biodegradation rates in the microcosm somewhat lower than the toluene degradation rates observed in the in situ columns. The sediment analyses showed considerable variations in sediment properties and microbial populations both within a core and between cores. However, these variations did not appear to correlate with variations in BTEX degradation rates.

Summary. A field technique was developed for measuring in situ biodegradation rates at multiple depths. The technique appears to provide reliable, statistically valid estimates of biodegradation rates under representative in situ conditions. Application of the technique to a petroleum contaminated field site in Rocky Point, NC, generated biodegradation rate estimates somewhat higher than laboratory microcosm rates. The in situ rates also showed significant variation with depth. However, these variations in degradation rates could not be correlated with sediment characteristics or microbial populations. Future modeling studies will be conducted to compare rates measured using the in situ column procedure with large-scale plume behavior.

Journal Articles on this Report : 4 Displayed | Download in RIS Format

Publications Views

Other project views:	All 17 publications	6 publications in selected types	All 4 journal articles

Publications

Type	Citation	Project	Document Sources
Journal Article	Borden RC, Daniel RA, LeBrun IV LE, Davis CW. Intrinsic biodegradation of MTBE and BTEX in a gasoline-contaminated aquifer. Water Resources Research 1997;33(5):1105-1115.	R825418 (Final)	Full-text: AGU - Full Text PDF Exit Abstract: AGU - Abstract HTML Exit
Journal Article	Hunt MJ, Shafer MB, Barlaz MA, Borden RC. Anaerobic biodegradation of alkylbenzenes in laboratory microcosms representing ambient conditions. Bioremediation Journal 1997;1(1):53-64.	R825418 (Final)	Abstract: Taylor & Francis - Abstract HTML Exit
Journal Article	Hunt MJ, Borden RC, Barlaz MA. Determining anaerobic BTEX decay rates in a contaminated aquifer. Journal of Hydrologic Engineering 1998;3(4):285-293.	R825418 (1998) R825418 (Final)	Abstract: ASCE - Abstract HTML Exit
Journal Article	Kota S, Borden RC, Barlaz MA. Influence of protozoan grazing on contaminant biodegradation. FEMS Microbiology Ecology 1999;29(2):179-189.	R825418 (Final)	not available

Supplemental Keywords:

monitored natural attenuation, MNA, hydrocarbon, anaerobic., RFA, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Ecosystem/Assessment/Indicators, Ecosystem Protection, exploratory research environmental biology, Chemical Mixtures - Environmental Exposure & Risk, Contaminated Sediments, Ecological Effects - Environmental Exposure & Risk, Hazardous Waste, Ecological Effects - Human Health, Ecological Risk Assessment, Hazardous, Groundwater remediation, Ecological Indicators, treatment residuals, ecological exposure, risk assessment, petroleum contaminants, ecological receptors, biodegradation, spatial heterogeneity, contaminated sediment, hazardous organic contaminants, dissolved hydrocarbons, bioattenuation, ecological impacts, risk assessments, aquifers, hydrocarbons, ecosystem impacts

Progress and Final Reports:

Original Abstract

1997 Progress Report

1998 Progress Report

1999