A New Insight into the Influencing Factors of Natural Attenuation of Chlorinated Hydrocarbons Contaminated Groundwater: A Long-Term Field Study of a Retired Pesticide Site (Abstract)
Fan, T., M. Yang, Q. Li, Y. Zhou, F. Xia, Y. Chen, L. Yang, D. Ding, S. Zhang, X. Zhang, R. Yu, and S. Deng. | Journal of Hazardous Materials 439:129595(2022)

A study monitored contaminants environmental factors and microbial community from 2016-2021 at a former pesticide site undergoing natural attenuation in Jiangsu Province. Groundwater was severely contaminated with chlorinated hydrocarbons. The spatial variation of contaminants, including chlorinated ethenes and ethanes, indicated that the site could be divided into a source area, diffusion area, and an end-of-diffusion area, where organohalide-respiring bacteria (OHRB) were detected. Contaminants and environmental factors influenced the OHRB community structure, which explained 7.6% and 33.2% of the variation. The abundances of obligate and facultative OHRB were affected in opposite ways by pollutants and environmental factors. Dehalococcoides and Dehalogenimonas in obligate OHRB were significantly inhibited by sulfate (r = -0.448, p < 0.05). The spatial-temporal characteristics of pollutants and the reveal of microbial community structure and its restricting factors in different areas establish the foundation for strengthening the implementation of natural attenuation.

Application of Monitored Natural Attenuation and Risk-Based Corrective Action at a Chlorinated-Hydrocarbon Contaminated Site for Risk Management
Dai W.C., T.T. Tsai, C.M. Kao, Y.M. Chang, and H.C. His.
Research Journal of Chemistry and Environment 16(3):87-97(2012)

This paper presents a case study of the feasibility of MNA for risk reduction of TCE and 1,1-DCE at a spill site. The objectives were to (1) evaluate the possible exposure routes and human health risks from the contaminated groundwater using tiered risk assessment approach; (2) apply the BIOCHLOR model to assess the effectiveness of natural attenuation for the contaminant plume; (3) apply probability and Monte Carlo analyses to develop more practical remediation goals; and (4) develop a realistic streamlined process and risk-based decision-making strategies for site management.

Assessment of Post Remediation Performance of a Biobarrier Oxygen Injection System at a Methyl Tert-Butyl Ether (MTBE)-Contaminated Site, Marine Corps Base Camp Pendleton, San Diego, California
Neil, K., T. Chaudhry, K.H. Kucharzyk, H.V. Rectanus, C. Bartling, P. Chang, and S. Rosansky.
ESTCP Project ER-201588, 284 pp, 2017

This project was conducted to evaluate the long-term performance of natural attenuation of MTBE after shutdown of a biobarrier system. The long-term impact of the biobarrier system on formation permeability was assessed via slug tests. In addition to evaluating data collected using conventional monitoring techniques, this project applied metagenomics and metaproteomics to improve the understanding of long-term impacts of the remedy on biodegradation at the site.

Benzene Degradation in Contaminated Aquifers: Enhancing Natural Attenuation by Injecting Nitrate (Abstract)
Muller, C., K. Knoller, R. Lucas, S. Kleinsteuber, R. Trabitzsch, H. Weiß, R. Stollberg, H.H. Richnow, and C. Vogt. Journal of Contaminant Hydrology 238:103759(2021)

A field experiment was conducted to stimulate benzene biodegradation by injecting nitrate (NO₃⁻) into a sulfidic benzene plume. The experiment aimed to recycle sulfate by nitrate-dependent sulfide oxidation and use NO₃⁻ as direct electron acceptor for benzene oxidation. Over 60 days, 6.74 tons of sodium nitrate were injected into the lower aquifer. Biogeochemical changes within the benzene plume were monitored for more than one year. Nitrate was microbiologically consumed, but benzene was not biodegraded by sulfate-reducing consortia. However, small carbon isotope fractionation of benzene suggested the presence of in situ benzene biodegradation processes in the plume, probably supported by nitrate. In conclusion, nitrate injection resulted in changing redox conditions and recycling of sulfate in the benzene plume due to microbial oxidation of reduced sulfur species, potentially leading to conditions favorable for in situ benzene biodegradation.

Challenges in Planning for Groundwater Remedy Transition at a Complex Site
O'Steen, W.N. and R.O. Howard, Jr. U.S. EPA Region 4, 12 pp, 2014

Complex groundwater contamination sites require comprehensive, structured groundwater monitoring in planning for transition to a new groundwater remedy. This paper provides as an example the Medley Farm Superfund site, a former waste solvent dump located in South Carolina. PCE, TCE, and their degradation products in the groundwater were addressed with pump and treat from 1995 to 2004. Between October 2004 and April 2012, injection of a lactate solution to promote enhanced reductive dechlorination (ERD) was conducted on multiple occasions, with positive responses in hydrogeochemistry and groundwater quality. In 2012, EPA issued an amended ROD, changing the groundwater remedy to ERD. MNA was selected as a contingency remedy in anticipation that as cleanup progresses, ERD may transition to MNA. Restructuring the site's monitoring and data evaluation program will enable EPA to discern treatment effects more clearly and facilitate MNA evaluation.

Characterizing Natural Degradation of Tetrachloroethene (PCE) Using A Multidisciplinary Approach
Kesson, S.A., C.J. Sparrenbom, C.J. Paul, R. Jansson, and H. Holmstrand.
Ambio 50:1074-1088(2021)

Conventional groundwater analysis combined with compound-specific isotope data of carbon, microbial DNA analysis, and geoelectrical tomography techniques was used to investigate chlorinated solvents originating from a former drycleaning site in Sweden. The zone where natural degradation occurred was identified in the transition between two geological units, an observation confirmed by all methods. The change in hydraulic conductivity in this transition may have facilitated biofilm formation and microbial activity. The examination of the impact of geological conditions on the biotransformation process was facilitated by the unique combination of the applied methods. The extended array of investigation methods was thus beneficial with the potential to reduce remediation.

Delineation and Characterization of the Borden MTBE Plume: An Evaluation of Eight Years of Natural Attenuation Processes
1998. American Petroleum Institute (API). Publication no. 4668.

Evaluation of the Protocol for the Natural Attenuation of Chlorinated Solvents: Case Study at the Twin Cities Army Ammunition Plant
2001. Wilson, John T. (U.S. EPA, Subsurface Protection and Remediation Div., Ada, OK); Don H. Kampbell; Mark Ferrey (Site Remediation Section, Minnesota Pollution Control Agency, St. Paul); Paul Estuesta. Report No: EPA 600-R-01-025, 49 pp.

Expedited Approach to a Carbon Tetrachloride Spill Interim Remedial Action
1998. C. Cowdery; A. Primrose; J. Uhland; N. Castaneda. RFP-5191, NTIS: DE98003480, 8 pp.

Monitored natural attenuation was selected as an interim measure for a carbon tetrachloride spill at a Rocky Flats site where source removal or in situ treatment could not be implemented due to the surrounding infrastructure.

Field, Laboratory and Modeling Evidence for Strong Attenuation of a Cr(VI) Plume in a Mudstone Aquifer Due to Matrix Diffusion and Reaction Processes
Chapman, S., B. Parker, T. Al, R. Wilkin, D. Cutt, K. Mishkin, and S. Nelson. Soil Systems 5(18)(2021)

Both conventional and high-resolution field and laboratory methods were used to investigate processes that attenuate a Cr(VI) plume in sedimentary bedrock. Cr(VI) concentrations in the plume at a former industrial facility declined by more than three orders of magnitude over 900 m downgradient. Internal plume concentrations generally are stable or declining due to diffusive and reactive transport in the low-permeability matrix as fluxes from the source dissipate due to natural depletion and active remediation. The strong attenuation is attributed to diffusion from mobile groundwater in fractures to immobile porewater in the rock matrix and reactions causing transformation of aqueous Cr(VI) to low-solubility Cr(III) precipitates. Field characterization data for the fracture network and matrix properties were used to inform 2-D numerical model simulations that quantify attenuation due to diffusion and reaction processes. The field, laboratory, and modeling evidence demonstrate effects of matrix diffusion and reaction processes causing strong attenuation of a Cr(VI) plume in a sedimentary bedrock aquifer.

Five-Year Enhanced Natural Attenuation of Historically Coal-Tar-Contaminated Soil: Analysis of Polycyclic Aromatic Hydrocarbon and Phenol Contents
Telesinski1, A. and A. Kiepas-Kokot. ǀ International Journal of Environmental Research and Public Health 18:2265(2021)

A study assessed concentrations of phenols and PAHs following enhanced natural attenuation (ENA) at an industrial waste area where coal tar was processed. Soil in the investigation area was formed from a layer of uncompacted fill. Twelve sampling points were established to collect soil samples. A previous study did not detect heavy metals, BTEX, or cyanide concentrations though PAHs and phenols were detected at concentrations higher than permitted by Polish norms. Repeated analyses of phenols and PAHs were conducted in 2020 to determine ENA effectiveness. Results showed that ENA efficiently degraded phenols and naphthalene; concentrations were not elevated compared to the standards for industrial waste areas. The three- and four-ring hydrocarbons were degraded at a lower intensity. Based on the mean decrease in content following five years of ENA, the compounds can be arranged in the following order: phenols > naphthalene > phenanthrene > fluoranthene > benzo(a)anthracene > chrysene > anthracene.

Improving Long-Term Monitoring of Contaminated Groundwater at Sites Where Attenuation-Based Remedies are Deployed (Abstract)
Denham, M.E., M.B Amidon, H.M. Wainwright, B. Dafflon, J. Ajo-Franklin, and C.A. Eddy-Dilek. | Environmental Management 66:1142-1161(2020)

A long-term monitoring strategy is proposed for contaminated groundwater with residual contamination on the subsurface. The strategy focuses on measuring the hydrological and geochemical parameters that control attenuation or remobilization of contaminants while de-emphasizing contaminant-concentration measurements. The approach was demonstrated at a site in South Carolina where groundwater is contaminated by several radionuclides, and a comprehensive enhanced attenuation remedy was implemented to minimize discharge of contamination to surface water. The proposed long-term monitoring strategy combines subsurface and surface measurements using spectroscopic tools, geophysical tools, and sensors to monitor the parameters controlling contaminant attenuation. This approach can detect the possibility of contaminant remobilization from engineered and natural attenuation zones, allowing potential adverse changes to be mitigated before contaminant attenuation is reversed.

Long-Term Evaluation of Hydrocarbon Degradation Rates Within the Source Zone Under Natural and Enhanced Attenuation for Site Closure Purposes
Schneider, M.R., A.C.C. Bortolassi, A.U. Soriano, M.P.M. Baessa, M.L.B. da Silva, and A.J. Giachini. ǀ Remediation 34(3):e21778(2024)

Four independent controlled-release sites containing a particular fraction of ethanol in gasoline (10%, 24%, 25%, and 85% [v/v]) were subjected to monitored natural attenuation (MNA) alone (E24 and E85) and nitrate (E25) and sulfate (E10) biostimulation near the source zone, and monitored for site closure for ~ 18 years. Both natural source zone depletion and biostimulation reduced benzene concentrations to acceptable remediation target levels. Benzene biodegradation rates (Λs) were one order of magnitude higher than those reported in the literature for diluted concentrations in groundwater plumes. The decay rate of benzene varied between 0.45 and 1.75/year and was strongly influenced by the mass of ethanol. Compared to biostimulation with nitrate (half-life of 1.52 years), MNA alone resulted in faster benzene removal rates (half-life of 0.96 years). Anaerobic biostimulation with sulfate had negligible effects on BTEX biodegradation compared to natural attenuation alone. The highest ethanol concentration (E85) led to faster benzene removal (with a half-life of 0.40 years), which was even higher than MNA under lower ethanol concentrations (E24). Benzene half-life was 2.2-fold slower at the site with the lowest ethanol (E10) compared to E85, indicating that the negative effects of ethanol on BTEX biodegradation appeared to be short-lived and may need to be reevaluated over the long term. Benzene decay rates were ~6 times slower in the source zone than those obtained for groundwater plumes, emphasizing the importance of targeting the LNAPL after plume retreat. Findings can assist practitioners in better predicting the lines of evidence for BTEX bioremediation and remediation cleanup times at a lower cost in the presence of ethanol and serve as a guide to help determine whether biostimulation is necessary to expedite source zone remediation for site closure. Longer Abstract

Monitored Natural Attenuation (MNA) of Contaminated Soils: State of the Art in Europe — A Critical Evaluation
Declercq, I., V. Cappuyns, and Y. Duclos.
Science of the Total Environment 426:393-405(2012)

This paper presents a snapshot of the state of the art for MNA implementation in six European countries, including a comparison between countries, an overview of some existing MNA cases, and relevant aspects related to return on experience. Although MNA is currently being applied in most of the study countries, some (e.g., Finland) still do not recognize it as an official remediation technique. This study also illustrates the differences in MNA approach that exist between countries, as well as the differences in application levels.

Monitored Natural Attenuation of Perchlorate in Groundwater: ESTCP Cost and Performance Report
ESTCP, Project ER-200428, 51 pp, 2010

To test the processes and methods needed to obtain lines of evidence to support MNA as a remedy for perchlorate-contaminated groundwater, this project applied the tiered approach developed and described in the 2008 perchlorate MNA protocol to two field demonstration sites in Maryland, the first at the Naval Surface Warfare Center, Indian Head, and the second at a manufacturing facility in Elkton where perchlorate commingled with TCE. Indian Head Site Demonstration; Elkton Site Demonstration

Multimethod Analysis of NSZD and Enhanced SZD by Solar-Powered Bioventing at the Guadalupe Restoration Project
McAlexander, B., E.J. Daniels, N. Sihota, J. Eichert, and C. Smith. Groundwater Monitoring & Remediation 44(2):72-85(2024)

An approach combining natural source zone depletion (NSZD) measurements and monitoring technology was applied at the Guadalupe Restoration Project to quantify biodegradation enhancements and sustainably increase the rate of source zone depletion. NSZD quantification was conducted using CO₂ efflux measurements and subsurface temperature profiling. Results fell within the range of previously reported estimates based on soil-gas profiling in the early 2000s, demonstrating the viability of the new methods. Data collection methods were then deployed during pilot testing of solar-powered bioventing. The system used seven 400-W solar panels to power a regenerative blower that delivered ~0.85 m³/minute (30 cfm) air to the LNAPL-impacted vadose soil near the interface with the groundwater table. Soil-gas data indicated an upward fanning of injected air toward ground surface. Results demonstrate that site management can proceed along a sequenced program that began with aggressive hydraulic recovery of hydrocarbon product, transitions to enhanced SZD in areas with poorly recoverable LNAPL, and then to NSZD without intervention to address residual LNAPL across the full footprint of the LNAPL bodies.

NATO/CCMS Pilot Study for the Evaluation of Demonstrated and Emerging Technologies for the Treatment and Clean Up of Contaminated Land and Groundwater (Phase III) 1999 Special Session: Monitored Natural Attenuation
EPA 542-R-99-008, 1999

This publication represents the proceedings of the Monitored Natural Attention Session in May 1999. This session was chaired by Fran Kremer, Ph.D. of the U. S. Environmental Protection Agency and Anja Sinke, Ph.D. from the Dutch TNO Institute of Environmental Science. It was presented at the second meeting of the Phase III Pilot Study.

Natural Attenuation Potential of Vinyl Chloride and Butyl Acrylate Released in the East Palestine, Ohio Train Derailment Accident
Chen, G., S. Rosolina, E. Padilla-Crespo, G. He, Q. Chen, A. Arosemena, B.E. Rosado-Maldonado, C.M. Swift, P.B. Coelho, A.J. Whelton, D. Taggart and F.E. Loffler. Environmental Science & Technology 58(40):17743-17755(2024

The natural attenuation potential of VC and BA was assessed at the East Palestine, Ohio train derailment by quantifying biodegradation biomarker genes and conducting microcosm treatability studies on streambed sediment, surface water, and private well water samples collected 128 and 276 days post-accident. qPCR detected the aerobic VC degradation biomarkers etnC in ∼40% and etnE in ∼27% of sediments collected in both sampling campaigns in abundances reaching 105 gene copies/g. The 16S rRNA genes of Dehalococcoides and Dehalogenimonas were detected in 50 and 64% of sediment samples collected 128 days post-accident and in 63 and 88% of sediment samples collected 276 days post-accident, respectively, in abundances reaching 107 cells/g. Elevated detection frequencies of VC degradation biomarker genes were measured immediately downstream of the accident site in Sulphur Run. Aerobic VC degradation occurred in all sediment microcosms and coincided with increases of etnC/etnE genes and Mycobacterium, a genus comprising aerobic VC degraders. The conversion of VC to ethene and an increased abundance of VC reductive dechlorination biomarker genes were observed in microcosms established with sediments collected from Sulphur Run. All anoxic microcosms rapidly degraded BA to innocuous products with intermediate formation of n-butanol and acrylate. Results indicate that microbiomes in the East Palestine watershed have natural attenuation capacity for VC and BA.

Natural Attenuation Processes Control Groundwater Contamination in the Chernobyl Exclusion Zone: Evidence from 35 Years of Radiological Monitoring
Bugai, D., S. Kireev, M.A. Hoque, Y. Kubko, and J. Smith.
Scientific Reports 12 18215(2022)

Groundwater measurements covering key aquifers collected from the Chernobyl Exclusion Zone (CEZ) are presented and analyzed over 35 years since the accident. The CEZ contains most radionuclides released by the accident in nuclear fuel particle form. The data shows that ⁹⁰Sr remains mobile in the subsurface environment, while groundwater concentrations of ¹³⁷Cs, Pu isotopes, and ²⁴¹Am are relatively low, and are not a radiological concern. During the last two decades, ⁹⁰Sr and ¹³⁷Cs levels have declined or remained stable in most monitoring locations. This is due to natural attenuation driven by gradual exhaustion of the fuel particle source, geochemical evolution of groundwater downstream from waste dumps, and radionuclide retention in surface soil due to absorption and bio-cycling. Decommissioning of the cooling pond and construction of the 'New safe confinement' over Unit 4 (damaged reactor) also favored better groundwater protection close to the Chernobyl plant site. Data from confined and unconfined aquifers and rivers showed low radiological risks from groundwater contamination outside the CEZ and to on-site "self-settlers." Though several groundwater contamination hot spots remain in the vicinity of Unit 4, "Red Forest" waste trenches and surface water bodies with contaminated bottom sediments, the findings support a monitored natural attenuation approach to groundwater management in the CEZ.

Natural Attenuation and Biostimulation for In Situ Treatment of 1,2-Dibromoethane (EDB)
Koster van Groos, P., P. Hatzinger, G. Lavorgna, P. Philip, and T. Kuder. ESTCP Project ER-201331, 782 pp, 2022

The goals of this project were to improve understanding of EDB attenuation, particularly novel compound-specific isotope analysis tools, and determine whether biostimulation or bioaugmentation could effectively enhance in situ treatment of EDB. Improved methods to measure carbon isotope composition with low EDB concentrations were developed and applied. Differences in the isotopic composition of EDB among field samples provided valuable insights into EDB degradation processes. A lactate-based anaerobic in situ bioremediation approach was also applied in an impacted source area for chlorinated VOCs. The ISB effort aimed to demonstrate that higher EDB concentration source areas can be treated when attenuation processes are insufficient to protect receptors.

Natural Attenuation of Chlorinated Solvents: Performance and Cost Results From Multiple Air Force Demonstration Sites. Technology Demonstration Technical Summary Report
1999. Air Force Center for Environmental Excellence Technology Transfer Division, 101 pp.

Natural Attenuation of Chlorinated Volatile Organic Compounds in a Freshwater Tidal Wetland, Aberdeen Proving Ground, Maryland
1997. M.M. Lorah; L.D. Olsen; B.L. Smith; M.A. Johnson; W.B. Fleck, USGS. Publication WRI 97-4171, 108 pp.

Natural Attenuation of Fuel Hydrocarbons: Performance and Cost Results From Multiple Air Force Demonstration Sites. Technology Demonstration Technical Summary Report
1999. Air Force Center for Environmental Excellence Technology Transfer Division, 67 pp.

Natural Attenuation of Trichloroethene at the St. Joseph, Michigan, Superfund Site [video]
EPA 600-V-95-001, 1995

Request Video from SRIC (580-436-8651).

Natural Attenuation of a Chlorinated Ethene Plume Discharging to a Stream: Integrated Assessment of Hydrogeological, Chemical and Microbial Interactions
Ottosen, C.B., V. Ronde, U.S. McKnight, M.D. Annable, M.M. Broholm, J.F. Devlin, et al.
Water Research 186:116332(2020)

Several methods were combined in a multi-scale interdisciplinary in situ approach to assess and quantify the near-stream attenuation of a plume, primarily consisting of cDCE and VC that is discharging to a stream. Monitoring was conducted over seven years. At first, the site exhibited limited degradation from 2012-16 despite seemingly optimal conditions but presented notable degradation levels in 2019. A conceptualization of flow, transport, and processes clarified that hydrogeology was the main control on the natural attenuation, as short residence times of 0.5-37 days restricted the time in which dechlorination could occur. Longer Abstract

Natural Source Zone Depletion (NSZD) Insights From Over 15 Years of Research and Measurements: A Multi-Site Study
Kulkarni, P.R., K.L. Walker, C. J. Newell, K.K. Askarani, Y. Li, and T.E. McHugh.
Water Research 225:119170(2022)

Site-average natural source zone depletion (NSZD) rates from 40 petroleum LNAPL source zone sites from researchers, project reports, and scientific papers to compile data on general site location; LNAPL fuel type; measurement method, number of locations, and number of measurements per location; and calculated site-average NSZD rate in L/ha/yr per site and the associated measurement method. The resulting dataset showed site-average NSZD rates that ranged from 650 to 152,000 L/ha/yr (70 to 16,250 gal/acre/yr), with a median value of 9,540 L/ha/yr (1,020 gal/acre/yr). The median site-average NSZD rate by type of fuel spill did not show a statistically significant difference between fuel types. When comparing the different NSZD measurement methods applied to the same sites, the site-average NSZD rates differed by a factor of up to 4.8 (i.e., ratio of faster rate to slower rate), with a median difference of 2.1. No clear bias was observed between NSZD rate measurement methods. At four sites with NSZD rate calculations by season, NSZD rates were typically higher during summer and fall compared to winter and spring. For these sites, Q₁₀ values (a measure of the increase in NSZD rate associated with a 10°C increase in temperature) ranged from 0.8-15.1, with a median of 2.2. Results suggest that increasing mean annual soil temperature at a site using engineered methods could potentially increase the biodegradation rate (e.g., an increase of 10°C could double the NSZD rate). For five sites with site-average NSZD rates for multiple years, average NSZD rates varied by 1.1 to 4.9 times across years. Evaluating the NSZD rates suggests that measurable NSZD occurs across a broad range of LNAPL sites. Although NSZD rates vary across sites, fuel type is not the primary factor explaining observed differences in rates. Longer Abstract

Prairie NSZD: A Comparison of NSZD Rates Across Nine Sites Using New High-Data Density Technology
Mamet, S., N. Higgs, and S. Siciliano | Remediation Technologies Symposium East, 1-3 June, Niagara Falls, Ontario, 36 slides, 2022

An economical and robust sensor suite ("Soil Sense") was developed to facilitate the construction of networks within and surrounding LNAPL plumes. These networks allow high-resolution spatial and temporal quantification of LNAPL plume dynamics and biological processes associated with natural attenuation (e.g., methanogenesis, methanotrophy). Natural source zone depletion (NSZD) data from nine sites across Saskatchewan and Alberta were analyzed. Gas flux (CO₂, O₂, CH₄) data, pressure, air and soil temperatures, relative humidity, and petroleum hydrocarbon vapor concentrations were collected at 30-minute intervals. NSZD rates varied as a function of site and LNAPL plume characteristics. The high-temporal resolution data generated robust estimates of areal plume extent, volume, mass, NSZD rates, and time to closure. While NSZD rates slowed during soil freeze-up, they were non-zero, which affords users a more nuanced and robust picture of natural attenuation at sites over seasons to years.

Proceedings: SNOWMAN Network Conference on Monitored Natural Attenuation, November 7, 2011, Salon du Relais, Paris

Six separate reports concerning the use of MNA in Belgium, Denmark, France, Germany, the Netherlands, and Sweden are included in the proceedings. A summary report inventories the significant differences observed among the countries in MNA definitions and implementation.

Pump and Treat and In Situ Bioremediation of Contaminated Groundwater at the French Ltd. Superfund Site, Crosby, Texas: Cost and Performance Report
1998. Federal Remediation Technologies Roundtable. 16 pp.

Region 5 Monitored Natural Attenuation Report

This report describes a natural attenuation field study conducted jointly by the U. S. EPA Region 5, Wisconsin Department of Natural Resources (WDNR), the Illinois Environmental Protection Agency (IEPA), and Amoco Corporation (Amoco) since October 1994. To obtain the appendices to this report, please contact alvarez.gilberto@epa.gov.

Remote Monitoring of Natural Source Zone Depletion Using Temperature Data to Support Long-Term Passive Management Strategies

The specific objectives of this demonstration program were to: (1) demonstrate the use of innovative, inexpensive 2nd generation temperature monitoring systems to improve data quality and reduce costs; (2) demonstrate improved methods to separate the heat signal associated with biodegradation of petroleum from seasonal and other sources of temperature fluctuations in soils; (3) demonstrate that temperature based approaches to quantifying natural source zone depletion (NSZD) rates are particularly suited for LNAPL source areas located below paved surfaces; and (4) compile results from monitoring of NSZD at many sites and utilize these results to i) document the range of NSZD rates and ii) identify site factors that may be predictive of higher or lower NSZD rates at individual sites. NSZD was documented to occur at all sites across 40 sites where NSZD rates have been measured; rates did not vary by fuel type. NSZD was also documented at two demonstration sites under paved and unpaved locations. Different methods used to quantify rates yielded a range of rates generally within an order of magnitude. While offering some clear advantages, additional work may be required to fully validate 2nd generation monitoring equipment and background correction methods, especially to resolve short-term NSZD rates. The primary cost driver is the cost of the temperature sensor stations. While one-time measurements of NSZD may be cheaper for single measurements, the temperature-based methods offer clear cost advantages at sites where long-term monitoring is required or advantageous. The results of these two demonstration sites, a review of 40 sites in the published literature, and recent guidance documents indicate the presence and magnitude of NSZD processes and provide further guidance for reliable methods to quantify NSZD rates. The 2nd generation monitoring sensors and communication equipment can be procured from S3NSE Technologies as newly commercialized, custom-built equipment. Final Report

Site-Specific Work Plan [Monitored Natural Attenuation]: Brown & Bryant Superfund Site, 600 South Derby Street, Arvin, California
U.S. Army Corps of Engineers (USACE), 68 pp, 2012

This plan describes the actual performance of the natural attenuation remedy. It is expected that by using a predictive model, the rate of attenuation of the contaminants of concern (1,2-dichloropropane, 1,3-dichloropropane, 1,2,3-trichloropropane, chloroform, Dinoseb, 1,2-dibromo-3-chloropropane, and ethylene dibromide) can be described, thus providing a measure to gauge progress. If monitoring data indicate that the contaminant levels do not continue to decline in accordance with expectation as defined by this model, USACE and EPA will reconsider the remedy decision.

Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater, Vol. II
1999. T. Wiedemeier; J.T. Wilson; D.H. Kampbell; R.N. Miller; J.E. Hansen, Air Force Center for Environmental Excellence (AFCEE) Technology Transfer Division, Brooks AFB, San Antonio, TX. Volume 2, ADA324247, 343 pp.

Volume II presents the results of intrinsic remediation demonstrations at Hill AFB, Utah, and Patrick AFB, Florida. Engineering evaluation/cost analyses (EE/CAs) were conducted to evaluate the use of intrinsic remediation (natural attenuation) with long-term monitoring (LTM) for remediation of BTEX contamination dissolved in ground water.

The Collaborative Monitored Natural Attenuation (CMNA) of Soil and Groundwater Pollution in Large Petrochemical Enterprises: A Case Study
Song. Q., Z. Xue, H. Wu, Y. Zhai, T. Lu, X. Du, J. Zheng, H. Chen, and R. Zuo.
Environmental Research 216(Part 4):114816(2022)

Collaborative monitored natural attenuation (CMNA) was measured in soil and groundwater at a large in-service petrochemical enterprise in northeast China to remediate combined contaminants and reduce environmental risks. Contaminate distribution was determined based on a detailed investigation, and targeted contaminants in soil and groundwater were screened. The spatiotemporal variations of targeted contaminants and relative microbial responses were explored during the CMNA process. CMNA efficiency at the initial stage was evaluated by calculating the natural attenuation rate constant. The targeted contaminants in soil and groundwater were 2,2′,5,5′-tetrachlorobiphenyl (2,2′,5,5′-TCB) and petroleum hydrocarbons (C10∼C40), and 1,2-DCA, respectively. Concentrations of all targeted contaminants decreased continuously during four years of monitoring. Targeted contaminants played a dominant role in microbial species' variability in soil and groundwater, increasing the relative abundance of petroleum tolerant/biodegradation bacteria, such as Actinobacteria, Proteobacteria, and Acidobacteriota. The average natural attenuation rate constant of 2,2′,5,5′-TCB, and C10∼C40 in soil was 0.0012/1 and 0.0010/, respectively, meeting the screening value after four years of attenuation. The average natural attenuation rate constant of 1,2-DCA was 0.0004/d, which needs strengthening measures to improve the attenuation efficiency. Longer Abstract

Tracking NSZD Mass Removal Rates Over Decades: Site-Wide and Local Scale Assessment of Mass Removal at a Legacy Petroleum Site
Davis, G.B., J.L. Rayner, M.J. Donn, C.D. Johnston, R. Lukatelich, A. King, T.P. Bastow, and E. Bekele. | Journal of Contaminant Hydrology 248:104007(2022)

Site-specific gasoline and diesel NSZD rates are reported from sites undergoing NSZD over 21-26 years. NSZD rates were estimated in 1994, 2006 and 2020 for diesel and in 1999, 2009 and 2020 for gasoline using depth profiles of soil gases (oxygen, carbon dioxide, methane and volatiles) above LNAPL. Each date also had soil-core mass estimates, which were used with NSZD rates to estimate the longevity for LNAPL presence. Site-wide coring (in 1992, 2002 and 2007) estimated LNAPL mass reductions of 12,000 t. For diesel NSZD, the ratio of NSZD rates for 2006 (16,000-49,000 L/ha/y) to those in 2020 (2600-14,000 L/ha/y) was ~3-6. By 2020, the 1994 diesel NSZD rates would have predicted the entire removal of measured mass (16-42 kg/m²). For gasoline, NSZD rates in 1999 were extremely high (50,000-270,000 L/ha/y) but 9-27 times lower (5800-10,000 L/ha/y) a decade later. The gasoline NSZD rates in 1999 predicted near complete mass removal in 2-12 years, but 10-11 kg/m² was measured 10 and 21 years later, which is 26% of the initial mass in 1999. The outcomes substantiate the need to understand NSZD rate changes over the lifetime of LNAPL-impacted sites.

USGS Research on Variability in Composition of an Oil Spill After More Than 30 Years of Natural Attenuation
U.S. Geological Survey

The objective of the project is to improve the understanding of the mobilization, transport, and fate of crude oil in the shallow subsurface. The U.S. Geological Survey Toxic Substances Hydrology Program began an interdisciplinary research project in 1983 at the site of a crude-oil spill near Bemidji, Minnesota.