Natural Attenuation Monitoring: Letting nature take its course (but only under close scrutiny)

When a property experiences any type of contamination, the effects of that contamination will change over time, depending upon the characteristics of the contaminant and the subsurface environment.

On some sites, conditions underground will tend to contain contaminants, while other sites may be more transmissive, allowing pollutants to seep into groundwater aquifers.

A variety of factors can also act on the contaminants themselves.  Many contaminants undergo biological degradation as they are consumed by naturally-occurring microbes.  This process is largely dependent on groundwater chemistry, specifically the presence of oxygen, nitrate, manganese, iron, and sulfate. Contaminant plumes may also undergo reductions in concentration through dilution and dispersion to the point that they pose little environmental threat.

These natural processes can actually reduce the concentrations of contaminants in the soil or groundwater across a period of months or years unaided — an approach known as Natural Attenuation. 

However, Natural Attenuation must be closely monitored to discover whether those natural processes are having a significant enough effect for the site to reach necessary Groundwater Cleanup Target Levels (GCTLs).  In some cases Natural Attenuation can be reasonably effective and economical, but in other situations it’s simply not enough.

Natural Attenuation Monitoring (NAM) can gauge whether conditions are improving at a suitable pace…or whether more active measures need to be taken.

How Fast is Fast Enough?

In 2015, ERMI was selected by the Florida Department of Environmental Protection as one of two firms to complete a Natural Attenuation Monitoring (NAM) optimization study on 86 sites in the Petroleum Restoration Program (PRP) for over 17 years.

One major goal of the study was to determine whether FDEP funding was being effectively used to achieve site cleanup goals.  ERMI completed the study on FDEP-funded cleanup sites located across Florida to determine whether those sites were reaching closure endpoints in a timely manner—and if not, determine why not. 

ERMI completed conceptual site models for each facility and researched the various sites’ histories, identified their water and rock characteristics, and established a statistical trend based on groundwater monitoring data.

Over the course of six months, ERMI worked with FDEP site managers and environmental consultants to review and analyze laboratory analytical data.  The following data points summarize our investigation:

86 Total Sites Evaluated – 65 Linear Regression Analyses Completed

  • 66% of the site evaluated indicated they would take more than 10 years to achieve GCTLs
  • 17% of the sites evaluated indicated they would reach GCTLs in less than two years.

Results showed that overall, continuing to use NAM on these historical discharges was not achieving remediation goals in reasonable time frames .  Based on a review of the data and the results of our regression analyses, ERMI placed the sites into three categories for future work:

  • On 45% of the sites evaluated, we recommended Site Assessment/Limited Scope Remedial Action Plan
  • On 31% of the sites, we recommended Modified NAM. (Modified NAM supplements natural processes with more direct methods to stabilize properties—worksite efforts ranging from removing contaminated soil to adding other chemicals to a site to counteract existing pollutants.)
  • 32% of the sites lacked sufficient data to conduct a NAM evaluation

Additionally, we recommended Limited-Scope Remedial Action Plans (LSRAP) and/or modified NAM for approximately 76% of the sites in our study.  

With this research and information, ERMI provided FDEP with a solid technical assessment of which sites could safely continue the cost-saving NAM method, and which sites warranted consideration of additional remediation.

Our broad-based experience with NAM projects over the years equips ERMI to provide informed guidance on the prospects for NAM methods on both public- and private-sector properties with either historical or newer releases.