Monitored Natural Attenuation (MNA) of Chlorinated Solvents in the Environment

Chlorinated solvents are a large group of chemical compounds commonly used as cleaners and degreasers in the commercial, manufacturing, and military industries.  Their chlorine-containing structure helps to efficiently dissolve organic materials like fats and greases.  They were widely used in the 20th century in dry cleaning operations, automotive servicing, aerosol propellants, paint removers and metal and circuitry processing.  Two of the well-known chlorinated compounds are tetrachloroethene (PCE), and trichloroethene (TCE).  The history of PCE dates back to the early 1800’s when PCE was first synthesized.  In the 1930’s, PCE and TCE were introduced as dry cleaning solvents, replacing petroleum compounds that were in use since late 19th century.  The use of TCE was phased out of the dry cleaning industry in the 1950’s, but was still widely used in the electronics, defense, chemical, rail, adhesive, automobile, textile and food processing industries.  Federal Regulations played an increasing role in the use and handling of chlorinated solvents with the 1977 Clean Water Act, 1980 Resource Conservation and Recovery Act and the 1990 Clean Air Act.  Health effects from exposure to chlorinated compounds include chronic skin problems and damage to the nervous system, reproductive system, kidneys and liver.

Today, chlorinated solvents are one of the most common and widespread contaminants encountered in the environmental remediation industry.  Monitored natural attenuation (MNA) can serve as an effective remedial action as chlorinates degrade in the environment naturally under a variety of conditions.  Natural attenuation is defined by the U.S. Environmental Protection Agency as “naturally-occurring processes in the soil and groundwater environment that act without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants in those media.”   Understanding their chemical properties, structure and behavior in the subsurface is key to defining the nature and extent, as well as guiding a successful remediation through MNA.

Monitored natural attenuation involves:

  • Characterizing the fate and transport of the chlorinated solvents to evaluate the nature and extent of the natural attenuation processes;
  • Ensuring that these processes reduce the mass, toxicity and/or mobility of subsurface contamination in a way that reduces risk to human health and the environment to acceptable levels;
  • Evaluating the factors that will affect the long-term performance of natural attenuation; and
  • Monitoring of the natural processes to ensure their continued effectiveness.

 

MNA should only be employed when the chlorinated plume is defined and there is evidence that it is stable or reducing.  Additionally, there should be no unacceptable risk to exposure points, such as drinking water wells and the source of the contamination has been identified and controlled.

 

 

Fate and Transport of Chlorinated Solvents

Chlorinated compounds have a relatively high mobility in the subsurface and a density greater than water.  These properties allow the free-phase liquid to move through unsaturated soil easily and sink below the groundwater table when it reaches saturated soils.  In addition, most chlorinated solvents have limited solubility in water, which contributes to their prominence as groundwater pollutants.  Because they do not readily dissolve in the groundwater, they have a tendency to occur in the subsurface as separate phase liquids.  Unlike gasoline compounds in which the separate or non-aqueous phase liquid (NAPL) floats above the water table, NAPL formed from chlorinated solvents sinks below the groundwater table as a result of their density.  If large quantities of chlorinated compounds are released, this dense non-aqueous phase liquid, or DNAPL, can travel to great depths and form pools of free phase liquid above a low or non-permeable surface such as a clay layer or the top of bedrock.  DNAPL can then serve as a continuing source of groundwater impacts and be difficult to remove from the deep subsurface.

Chlorinated solvents evaporate readily in the environment and low concentrations of chlorinates in the groundwater, even those approaching clean-up standards, can still volatilize and pose a risk to of vapor intrusion into the indoor air.  Additionally, the relationship between groundwater concentrations and concentrations in the soil gas is not straight forward and can be strongly dependent on soil moisture content.  Vapor plumes can migrate long distances from the source of contamination as the degradation process of chlorinates can be a much slower process than petroleum pollutants.

Natural Attenuation Process

Chlorinated solvents naturally attenuate through different processes.  These include:

  • Reductive dechlorination, a process that favors highly chlorinated compounds, such as PCE and TCE, and occurs in an oxygen depleted (anaerobic) environment;
  • Oxidation, a process that occurs in oxygen enriched (aerobic) environments, whereby the chlorinated compound is directly used as an electron donor, or food source, for microorganisms; and
  • Co-metabolism, an aerobic process whereby the chlorinated compound in converted into another compound while microorganisms use other carbon compounds as a food source.

The breakdown of PCE goes through five stages of degradation until the end products (carbon dioxide, water and chlorine) are produced.  The degradation of PCE and TCE is primarily through reductive dechlorination.  It is a microbially remediated process under anaerobic conditions whereby the compound is converted to a lesser chorinated compound by serving as an electron acceptor replacing chlorine atoms with hydrogen atoms.  With PCE and TCE, the intermediate, lesser chlorinated chemicals produced are toxic compounds such as 1,2-Dichloroethene (DCE) and vinyl chloride, which is a known carcinogen.  However, if oxygen is introduced by in-situ remediation techniques, or the conditions at the remediation site are conductive to an aerobic environment, DCE and vinyl chloride can quickly degrade through the oxidation process, resulting in the further breakdown of that product into the end products.

In some cases, where there are other carbon sources available, such as petroleum compounds, methane, propane, or ethane, and an oxygen rich environment, the chlorinated compounds serve as the secondary food source and are co-metabolized into their daughter compounds.

Monitored Natural Attenuation (MNA)

Since incomplete degradation may mean the production of toxic daughter compounds, proper monitoring over time along with a thorough understanding of the site’s hydrogeology is critical to remediating chlorinated hydrocarbons through the use of MNA.  The three common lines of evidence used to demonstrate the effectiveness of MNA as a remedial action are the reduction of contaminant concentrations from groundwater laboratory data, field geochemical data measured during groundwater sampling, and laboratory analysis for microbial activity.

Two of the field geochemical indicators include dissolved oxygen and oxidation-reduction potential, along with temperature, pH and specific conductivity.  The latter three are mainly used to ensure the groundwater being sampled is representative of the aquifer being monitored.  Dissolved oxygen concentrations are used to indicate whether aerobic or anaerobic degradation processes are controlling natural attenuation processes. Biodegradation is the fastest and most efficient degradation process for lesser chlorinated compounds (i.e., DCE) when occurring under aerobic conditions.  Oxygen consumption provides the greatest amount of energy to microbes during metabolism.  Generally, close to the source of the plume, oxygen can be used up, resulting in an anaerobic environment.   Therefore, if the dissolved oxygen measurement is lower inside the plume than outside, it may indicate that biodegradation is occurring.

The oxidation-reduction potential of groundwater, or redox, is a measure of the relative tendency of a solution to accept or donate electrons. Generally, a positive value indicates that the solution is oxidizing (aerobic) while a negative value indicates that the solution is chemically reducing (anaerobic). Groundwater in a high oxidative state indicates that PCE will not dechlorinate, but the intermediate compounds will oxidize.   If the oxidation-reduction potential measurements taken outside the plume are higher than the measurements in the plume, it is an indication that biodegradation may be occurring. Dissolved oxygen and oxidation reduction potential readings should be in agreement.

The main driving force of redox is the availability of a carbon source.  The subsurface may contain a naturally occurring carbon source, such a peat, but most commonly, anthropogenic sources, such as a petroleum spill, can provide a carbon source.   At MNA sites, the carbon source should be identified in order to ensure it can sustain MNA for the time period required to reduce the chlorinated concentrations below the required standards set to protect the health of humans and the environment.

Monitored Natural Attenuation as a Remedial Action

As stated previously, the use of MNA as a remedial action should only be considered when the chlorinated plume is stable or reducing and the limits of the plume are defined both vertically and horizontally throughout the site.  MNA is not a step back and watch approach.  It is an active solution that involves the monitoring of laboratory and geochemical field indicators combined with modelling and prediction of degradation rates.

In many cases, MNA is less costly of a solution than other remedies. MNA causes minimal disturbance to the site, adjacent properties and the environment and is not limited by the site’s buildings or infrastructure as some remedial technologies are.  MNA can be used in conjunction with other technologies, and the major advantage of implementing MNA is that it gives the environmental consultant a more complete understanding of the dynamic and changing biochemical processes specific to each site.

References:

Morrison, Robert D., Murphy, Brian L., 2010.  Environmental Forensics: Contaminant Specific Guide, Academic Press, pp. 260-267

 

Environmental Protection Agency.  March 2012.  Petroleum Hydrocarbons and Chlorinated Solvents Differ In Their Potential For Vapor Intrusion.

https://www.epa.gov/sites/production/files/2014-09/documents/pvicvi.pdf

 

Henry, Susan M., Warner, Scott D.  2002.  Chlorinated Solvent and DNAPL Remediation: An Overview of Physical, Chemical, and Biological Processes, in: Henry, Susan M., Hardcastle, Calvin H., Warner, Scott D (Eds.), Chlorinated Solvent and DNAPL Remediation: Innovative Strategies for Subsurface Cleanup.  American Chemical Society, pp. 1-20.

 

Wisconsin Department of Natural Resources.  October 2014.  Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation.

http://dnr.wi.gov/files/pdf/pubs/rr/rr699.pdf

 

Interstate Technology and Regulatory Cooperation Work Group.  September 1999.  Natural Attenuation of Chlorinated Solvents in Groundwater:  Principles and Practices.

 

Colorado Department of Public Health and Environment.  2014.  Fact Sheet: Dry Cleaners and PCE.https://www.colorado.gov/pacific/sites/default/files/HM_hw-drycleaner-pce-factsheet.pdf

 

Colorado Department of Labor and Employment, Divison of Oil and Public Safety.  June 17, 2002.  Monitored Natural Attenuation in Groundwater Guidance Document.

https://www.colorado.gov/pacific/sites/default/files/MNA%20DOC%2006-17-02.pdf

 

Cwiertny, David M. and Scherer, Michelle M. 2010. Chlorinated Solvent Chemistry:  Structures, Nomenclature and Properties, in: H.F. Stroo and C.H. Ward (eds.), In Situ Remediation of Chlorinated Solvent Plumes.  Springer Science and Business Media LLC. pp. 29-37.