2Natural Attenuation The biodegradation, dispersion, dilution, sorption, volatilization, and/or chemical andbiochemical stabilization of contaminants toeffectively reduce contaminant toxicity,mobility, or volume to levels that are protectiveof human health and the ecosystem(US EPA ORD, OSWER)
3Natural AttenuationNaturally occurring processes in soil and ground water that act without human intervention to reduce the mass, toxicity, mobility, volume or concentrations of contaminantsBiodegradation, dispersion, dilution, absorption, volatilization, and abiotic reactions
4Evidence of Natural Attenuation Plume Length Should Be =Seepage Velocityx Time÷ Retardation FactorPlume Length Is .....ShorterThinnerAppears not to be moving
5Natural Attenuation A Do-Nothing Approach? Requires quantitative assessment of aplume’s behavior -amount, extent, and rate of travel,as well as long-term evidence of attenuation
6RequirementsSite assessment - hydrogeology, geochemistry, microbiologyHigh tech approaches - sampling, analytical, modeling techniquesPrediction of plume behaviorMay be combined with source/hot spot controlContainment of dissolved plumeA risk management strategy
7Fate of Organic Contaminants in the Subsurface VolatilizationSorptionAbiotic TransformationsHydrolysisBiodegradationAdvectionDilution
8Advection Contaminants transported with ground water flow No effect on contaminant concentrationNo net loss of contaminant mass
9Dispersion Mechanical and hydraulic mixing Decreases contaminant concentration in center of plume – increases concentration on edgesNo net loss of contaminant mass
10SorptionPartitioning of contaminants between aqueous phase and solid aquifer matrixDecreases dissolved contaminant concentration until sorption capacity is reachedNo net loss of contaminant mass
11VolatilizationMovement of contaminants from aqueous phase in saturated zone to vapor phase in unsaturated zoneCan result in net loss of contaminant mass from the aquiferRarely a significant attenuation mechanism except in capillary zone
12Dissolution (Leachability) Transfer of contaminants from NAPL phase to aqueous phaseMost significant physical process controlling extent of the plumePlume stable or expanding until residual (source) contaminants removedNo net loss of contaminant mass
13Abiotic Transformations Reactions such as hydrolysis and dehalogenationReduces aqueous concentrations of contaminantsReduction of contaminant mass
14Biotic Transformations Aerobic and anaerobic biodegradationReduces aqueous concentrations of contaminantReduction of contaminant massMost significant process resulting in reduction of contaminant mass in a system
15When to Enhance?Bioremediation – enhancement of natural attenuation processesConditions in the subsurface not conducive for degradationDissolved oxygen concentration - too high or too lowLow electron donor concentrationToo much mass for Natural AttenuationShort time frameMany processes available for bioremediation
16When 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 timeframeOften used in conjunction with other active measuresOSWER Directive , 1997
18MNA AdvantagesReduce potential for waste generation and human exposure during ex situ treatmentLess intrusiveCan be used with, or after, other remediation approachesReduction in cost
19MNA DisadvantagesIncreased time frame for remediation and monitoring (institutional controls)Requires more complex and costly site characterizationIncomplete attenuation may result in increased toxicity (especially chlorinated solvents)Potential for continued contaminant migration and cross-media transferPublic acceptance
20Critical Questions in MNA How long will the plume extend?How long will it take for the contamination to disappear?Mass transfer vs. fate processesFuture land useNatural resource damage assessment
21Multi-Site Studies (Newell and Connor, API) 0 ft200 ft400 ft600 ft800 ft1000 ftBTEX plumes at 42retail LUST sites213 ft x 150 ftChlorinated ethene(PCE, TCE, DCE, or VC)plumes at 88 sites1000 ft x 500 ftOther chlorinated solventplumes (TCA, DCA) at29 sites500 ft x 350 ftChloride, salt waterplumes at 25 sites700 ft x 500 ft
22EPA Lines of EvidenceHistorical groundwater and/or soil chemistry data that demonstrate clear trends of decreasing contaminant mass (concentration) that is not the result of migrationHydrogeologic and geochemical data that are indirect indicators of attenuation mechanismsData from field and microcosm studies that directly demonstrate certain attenuation mechanisms
23Assessment of MNA Potential Source evaluationcomponentsdistributionloading ratePlume evaluationcontaminantsgroundwater chemistrymetabolic productsaquifer characteristicsSite characterization and data evaluationSite conceptual modelFootprint analysisLab Studies and Modeling
24Shrinking 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 SECTIONTIME 1PLAN VIEWMW-9MW-5MW-1TIME 2TIME 3WHEN ? Mass loading rate < attenuation rate, resulting in reduced plume mass in water-bearing unit.WHY ? Shrinking plume is evidence of natural attenuation.
25Stable 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 SECTIONTIME 1PLAN VIEWMW-5MW-9TIME 2MW-1TIME 3WHEN ? Mass loading rate = attenuation rate, resulting in plume stabilization.WHY ? Stable plume is evidence of natural attenuation.
26Expanding 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 SECTIONTIME 1PLAN VIEWMW-9MW-1MW-5TIME 2TIME 3WHEN ? 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.
27Effort for Site Characterization and Data Interpretation NRC,2000
28Conceptual Models of Source Distribution Must determine source information during site characterization to use MNAa) Aqueous Phase Release to Saturated Zoneb) NAPL Release in Vadose Zone Onlyc) LNAPL Release to Water Tabled) DNAPL Release to Saturated Zone
30Lab Studies and Modeling Lab studies or “microcosms”Must mimic site conditionsData may be useful for rate estimatesRequires appropriate expertise throughoutModelingBudget analysis on electron donor and electron acceptorScreening modelsBIOCHLOR, etc.Complex modelsMODFLOW, etc.
31MNA Observations Attenuation occurs at all sites Effectiveness dominated by mass reduction mechanisms, usually biodegradationRate and extent of biodegradation controlled by site specific conditionsAcceptance of MNA requires considerable analysis and monitoringTools for incorporating natural attenuation into groundwater management strategies continue to improve
32What 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?