Author response to technical commentary: Direct aerobic NSZD of a basalt vadose zone LNAPL source in Hawaii, McHugh et al., JCH #235 (2020)

Our Paper uses two independent methods (carbon dioxide flux and heat flux) to measure the rates of natural source zone depletion (NSZD) at a petroleum release site in Hawaii. The two methods yielded estimates of the NSZD rate that agreed within a factor of 2. We disagree with the technical commentors (Beckett et al., 2022). Specifically, the available data indicate that the observed NSZD is not occurring through a two-stage process of methane generation at the water table followed by methane oxidation in the vadose zone; rather direct aerobic oxidation of LNAPL in the vadose zone is the simplest and most likely explanation for the observed heat generation. In addition, the agreement between the two independent NSZD rate measurement methods and the temporal consistency in the measured heat flux across two field events provides confidence that the NSZD rates presented in the paper are not greatly over or under-estimated.

Automated summary

Our study measures the rates of natural source zone depletion (NSZD) at a petroleum release site in Hawaii using two independent methods. The results show that the NSZD rate estimates from the two methods are consistent within a factor of 2. We found that the observed NSZD is most likely due to the direct aerobic oxidation of LNAPL in the vadose zone, rather than a two-stage process of methane generation and oxidation.