Your environmental consultant has just informed you that your site requires remediation to achieve regulatory cleanup criteria. When you hear the word “remediation,” you immediately recall your past experiences that included significant business disruption and a long and costly process to bring your site back to pre-discharge conditions.
While installation and operation of an active remediation system may in fact be warranted, there may be other options to cleanup that take into consideration the appropriate levels of protection of human health and the environment. One such approach is Monitored Natural Attenuation (also referred to as MNA or NAM), which relies on the natural processes in the subsurface to degrade contaminants.
What is NAM?
In 1999 the EPA defined the term “natural attenuation” as a variety of physical, chemical, and/or biological processes that under favorable conditions will act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of hazardous substances in the environment. These in-situ processes include biodegradation, dispersion, dilution, sorption, volatilization, radioactive decay, chemical/biological stabilization, transformation, or destruction of contaminants.
Advantages and Disadvantages of NAM
There are several advantages to considering NAM as the primary cleanup strategy:
- It can be used at all or part of a site including otherwise inaccessible areas,
- It can be used in conjunction with or as a follow up to other active remediation components,
- Implementation provides less disruption of site use, and
- It also can help lower overall cleanup costs, even when considering the long-term costs associated with implementing the monitoring program.
Despite the benefits NAM provides in the remediation strategy, it does have its limitations:
- To properly determine its feasibility there is an up-front cost associated with additional site investigation, and the restoration time may be longer than a remediation system that addresses the entire dissolved impacted plume.
- There is the potential for continued migration of contaminants.
- It is less reliable than other cleanup methods because effectiveness is more dependent on favorable site conditions and site-specific conditions remaining consistent over time.
Your consultant should be considering NAM during the initial stages of site assessment activities and collect the data necessary to adequately delineate the groundwater and soil impacts both horizontally and vertically. They should also be looking at indicator parameters useful in evaluating the ongoing natural attenuation processes at a particular site.
Is NAM Applicable as a Single Strategy?
After a minimum of one year of groundwater data has been collected, a determination can be made as to whether the plume is either stable or shrinking (NAM will not be sole strategy if plume is expanding). Decreasing contaminant concentrations should not be solely the result of plume migration. It should be shown that either chemical or biological degradation is also occurring at the facility.
During the monitoring period, site geochemical indicators (Oxygen, Nitrate, Manganese, Ferric Iron, Sulfate, Carbon Dioxide, Redox Potential, and Alkalinity) in monitoring wells located in the source area, down gradient, and up-gradient will be monitored.
The geochemical indicators in monitoring wells located within and down gradient of the source will be compared to up-gradient or background levels. Changes in these geochemical indicators (either decreases in reactants or increases in metabolic by-products) indicate that biodegradation is occurring.
The NAM Plan
Once it has been established that NAM is an acceptable cleanup alternative, a NAM Plan specifying a groundwater monitoring program that ensures that human health and the environment continues to be protected throughout the cleanup time frame will be prepared. The Plan will establish cleanup goals, an estimated restoration timeframe, and produce milestone targets for contaminant and mass reduction.
The frequency of groundwater sampling is generally conducted quarterly during the first year of implementation, semi-annually for the next two years, and annually in subsequent years based on the data collected during the first three years.
As part of every remediation design strategy your consultant should consider implementation of NAM either as a single source of remediation, as a viable alternative to use in conjunction with another source removal remediation strategy, or as a final polishing step after meeting cleanup milestones from the operation of another remediation system strategy.
ATC has developed and implemented NAM programs at hundreds of sites across the country. Many of the sites have reached cleanup targets after only a few years of monitoring – with both costs and disruption far less than aggressive active remediation programs. Contact us if we can be of assistance.