Biodegradation and Natural Attenuation

Natural Attenuation The biodegradation, dispersion, dilution, sorption, volatilization, and/or chemical and biochemical stabilization of contaminants to effectively reduce contaminant toxicity, mobility, or volume to levels that are protective of human health and the ecosystem (US EPA ORD, OSWER)

Natural Attenuation Naturally occurring processes in soil and ground water that act without human intervention to reduce the mass, toxicity, mobility, volume or concentrations of contaminants Biodegradation, dispersion, dilution, absorption, volatilization, and abiotic reactions

Evidence of Natural Attenuation Plume Length Should Be = Seepage Velocity x Time ÷ Retardation Factor Plume Length Is ..... Shorter Thinner Appears not to be moving

Natural Attenuation A Do-Nothing Approach? Requires quantitative assessment of a plume’s behavior - amount, extent, and rate of travel, as well as long-term evidence of attenuation

Requirements Site assessment - hydrogeology, geochemistry, microbiology High tech approaches - sampling, analytical, modeling techniques Prediction of plume behavior May be combined with source/hot spot control Containment of dissolved plume A risk management strategy

Fate of Organic Contaminants in the Subsurface Volatilization Sorption Abiotic Transformations Hydrolysis Biodegradation Advection Dilution

Advection Contaminants transported with ground water flow No effect on contaminant concentration No net loss of contaminant mass

Dispersion Mechanical and hydraulic mixing Decreases contaminant concentration in center of plume – increases concentration on edges No net loss of contaminant mass

Sorption Partitioning of contaminants between aqueous phase and solid aquifer matrix Decreases dissolved contaminant concentration until sorption capacity is reached No net loss of contaminant mass

Volatilization Movement of contaminants from aqueous phase in saturated zone to vapor phase in unsaturated zone Can result in net loss of contaminant mass from the aquifer Rarely a significant attenuation mechanism except in capillary zone

Dissolution (Leachability) Transfer of contaminants from NAPL phase to aqueous phase Most significant physical process controlling extent of the plume Plume stable or expanding until residual (source) contaminants removed No net loss of contaminant mass

Abiotic Transformations Reactions such as hydrolysis and dehalogenation Reduces aqueous concentrations of contaminants Reduction of contaminant mass

Biotic Transformations Aerobic and anaerobic biodegradation Reduces aqueous concentrations of contaminant Reduction of contaminant mass Most significant process resulting in reduction of contaminant mass in a system

When to Enhance? Bioremediation – enhancement of natural attenuation processes Conditions in the subsurface not conducive for degradation Dissolved oxygen concentration - too high or too low Low electron donor concentration Too much mass for Natural Attenuation Short time frame Many processes available for bioremediation

When Not to Enhance? Use Monitored Natural Attenuation (MNA) MNA is reliance on natural attenuation processes to achieve site-specific remedial objectives in a time frame that is reasonable compared to other methods. Appropriate only when demonstrated capable of meeting objectives in acceptable timeframe Often used in conjunction with other active measures OSWER Directive 9200.4-17, 1997

Observation of Natural Attenuation

MNA Advantages Reduce potential for waste generation and human exposure during ex situ treatment Less intrusive Can be used with, or after, other remediation approaches Reduction in cost

MNA Disadvantages Increased time frame for remediation and monitoring (institutional controls) Requires more complex and costly site characterization Incomplete attenuation may result in increased toxicity (especially chlorinated solvents) Potential for continued contaminant migration and cross-media transfer Public acceptance

Critical Questions in MNA How long will the plume extend? How long will it take for the contamination to disappear? Mass transfer vs. fate processes Future land use Natural resource damage assessment

Multi-Site Studies (Newell and Connor, API) 0 ft 200 ft 400 ft 600 ft 800 ft 1000 ft BTEX plumes at 42 retail LUST sites 213 ft x 150 ft Chlorinated ethene (PCE, TCE, DCE, or VC) plumes at 88 sites 1000 ft x 500 ft Other chlorinated solvent plumes (TCA, DCA) at 29 sites 500 ft x 350 ft Chloride, salt water plumes at 25 sites 700 ft x 500 ft

EPA Lines of Evidence Historical groundwater and/or soil chemistry data that demonstrate clear trends of decreasing contaminant mass (concentration) that is not the result of migration Hydrogeologic and geochemical data that are indirect indicators of attenuation mechanisms Data from field and microcosm studies that directly demonstrate certain attenuation mechanisms

Assessment of MNA Potential Source evaluation components distribution loading rate Plume evaluation contaminants groundwater chemistry metabolic products aquifer characteristics Site characterization and data evaluation Site conceptual model Footprint analysis Lab Studies and Modeling

Shrinking Ground Water Plume Affected Ground Water “Solute plume margin is receding back toward the source area over time and the concentrations at points within the plume are decreasing over time.” CROSS SECTION TIME 1 PLAN VIEW MW-9 MW-5 MW-1 TIME 2 TIME 3 WHEN ? Mass loading rate < attenuation rate, resulting in reduced plume mass in water-bearing unit. WHY ? Shrinking plume is evidence of natural attenuation.

Stable Groundwater Plume Affected Ground Water “ Solute plume margin is stationary over time and concentrations at points within the plume are relatively uniform over time or may decrease over time.” CROSS SECTION TIME 1 PLAN VIEW MW-5 MW-9 TIME 2 MW-1 TIME 3 WHEN ? Mass loading rate = attenuation rate, resulting in plume stabilization. WHY ? Stable plume is evidence of natural attenuation.

Expanding Plume TIME 1 TIME 2 TIME 3 Affected Ground Water “Solute plume margin is continuing to move outward or down-gradient from the source area.” CROSS SECTION TIME 1 PLAN VIEW MW-9 MW-1 MW-5 TIME 2 TIME 3 WHEN ? Mass loading rate > attenuation rate, resulting in increased plume mass in water-bearing unit. WHY ? Expanding plume may be evidence of natural attenuation if expansion is less than expected based on ground water flow.

Effort for Site Characterization and Data Interpretation NRC,2000

Conceptual Models of Source Distribution Must determine source information during site characterization to use MNA a) Aqueous Phase Release to Saturated Zone b) NAPL Release in Vadose Zone Only c) LNAPL Release to Water Table d) DNAPL Release to Saturated Zone

Natural Attenuation Footprints NRC, 2000

Lab Studies and Modeling Lab studies or “microcosms” Must mimic site conditions Data may be useful for rate estimates Requires appropriate expertise throughout Modeling Budget analysis on electron donor and electron acceptor Screening models BIOCHLOR, etc. Complex models MODFLOW, etc.

MNA Observations Attenuation occurs at all sites Effectiveness dominated by mass reduction mechanisms, usually biodegradation Rate and extent of biodegradation controlled by site specific conditions Acceptance of MNA requires considerable analysis and monitoring Tools for incorporating natural attenuation into groundwater management strategies continue to improve

What We Don’t Know When do you give up on natural attenuation? What do you pump into an aquifer and why? In what form do you add supplements to enhance bioattenuation? Liquid, gas or solid? How long should you wait to see a response after enhancement of bioattenuation? Are there compounds, classes of compounds, or aquifers that require bioaugmentation?