Soil Gas Gradient Method for Estimating Natural Attenuation Rates

This technical report discusses the soil gas gradient method for estimating natural attenuation rates of non-aqueous phase liquids (NAPL) and specific chemicals of concern (COCs). It outlines the progress in understanding the processes responsible for NAPL depletion and the development of measurement methods for natural source zone depletion (NSZD) rates. The report details various methods categorized under the ASTM Standard Guide for estimating natural attenuation rates, focusing on the soil gas gradient method's application for bulk depletion rates and COC-specific rates. The proposed approach utilizes a simplified analytical 1-D solution to estimate COC-specific natural attenuation rates based on soil gas concentration profiles. Preliminary results indicate a correlation between this method and existing technologies like BioVapor. The report also discusses the applicability of this approach in assessing site conditions and highlights the advantages and limitations of using the soil gas gradient method for estimating natural attenuation rates over time.