A paradigm shift in methods to monitor petroleum natural attenuation

Traditional methods to monitor petroleum natural attenuation (NA) focus on the saturated zone and monitoring dissolved constituents for contaminant concentration trends and electron acceptor and biodegradation by-product changes across the impacted zone (NRC, 2000). Recent developments in NA monitoring approaches have shown, however, that traditional methods can significantly under-estimate the magnitude of NA occurring in light non-aqueous phase liquid (LNAPL) source zones. This is primarily due to the fact that traditional methods miss a large portion of the biodegradation by-products that manifest themselves as changes in vadose zone gases (Molins et al., 2010). Because the magnitude of these gaseous by-products often exceed solubility and vent upwards, they are not accounted by traditional methods which focus only on constituent concentrations in groundwater. To address this short-coming, new methods were developed and are now currently available to fully monitor and assess NA of LNAPL source zones. The methods are derived from nearly a decade of university research and are currently being used to estimate natural source zone depletion (NSZD) rates on many sites worldwide (Johnson et al., 2006; Sihota et al., 2011, and McCoy et al., 2014). (author)