Refinement of the gradient method for the estimation of natural source zone depletion at petroleum contaminated sites

Rates of natural source zone depletion (NSZD) are increasingly being used to aid remedial decision making and light non-aqueous phase liquid (LNAPL) longevity estimates at petroleum release sites. Current NSZD estimate methods, based on analyses of carbon dioxide (CO2) and oxygen (O-2) soil-gas concentration gradients (gradient method) assume linear concentration profiles with depth. This assumption can underestimate the concentration gradients especially above LNAPL sources that are typically characterized by curvilinear or semi-curvilinear O-2 and CO2 concentration profiles. In this work, we proposed a new method that relies on calculating the O-2 and CO2 concentration gradient using a first-order reaction model. The method requires an estimate of the diffusive reaction length that can be easily derived from soil-gas concentration data. A simple step-by-step guide for applying the new method is provided. Nomographs were also developed to facilitate method application. Application of the nomographs using field data from published literature showed that NSZD rates could be underestimated by nearly an order of magnitude if reactivity in the vadose zone is not accounted for. The new method helps refine NSZD rates estimation and improve risk-based decision making at certain petroleum contaminated sites.

Automated summary

This study introduces a new method for estimating NSZD rates by calculating O2 and CO2 concentration gradients using a first-order reaction model. The application of nomographs with field data showed potential underestimation of NSZD rates. The new method helps refine NSZD rate estimation and improve risk-based decision making at certain petroleum contaminated sites.