During the early stages after a petroleum hydrocarbon release, light non-aqueous phase liquid (LNAPL) plumes often form and skimming, vacuum-enhanced skimming, or multi-phase extraction systems are installed to physically remove the LNAPL. The combined benefits of source reduction by active remediation and re-equilibration time (including growth and acclimation of biodegrading microorganisms) often lead to stable LNAPL bodies but with persistent thicknesses in monitoring wells, leading most to believe that active recovery is still warranted, despite low recovery rates and diminished returns. Recent developments have shown that traditional methods to monitor petroleum natural attenuation (NA) can significantly under-estimate the magnitude of NA occurring in LNAPL source zones, primarily because these methods ignore biodegradation occurring above the water table. In response, new methods were developed and are now currently available to monitor and assess NA of LNAPL source zones, referred to as natural source zone depletion (NSZD). This presentation will present the concepts of NSZD and current methods to monitor and quantify NSZD, including the flux chamber, trap, and thermal monitoring methods. NSZD rates from several sites and a hypothetical site will be presented and compared with typical active recovery rates. The findings will show that in most cases for middle to late stage petroleum release sites, LNAPL recovery rates are much lower than natural loss rates, and further recovery is of little benefit. For site owners, quantifying NSZD can be effective tool for site management, allowing resources to be dedicated to higher risk areas, such as dissolved phase plume control and remediation, for example.

Primary Author / Conference Presenter:
Jeffrey Fitzgibbons
Remediation Technologist
Jacobs Engineering
Englewood, Colorado, USA