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

The subject Paper (McHugh et al., 2020) uses carbon dioxide and net thermal signatures to derive conclusions about the rates of natural source zone depletion (NSZD), as well as the location of residual fuel in the formation. We concur that both data sets are indicators of active fuel biodegradation, however, the simplifications of McHugh et al. render its estimates of NSZD rates uncertain and likely over-estimated. We cannot infer what role this degradation capacity may play in site management because: a) the biodegradation evidence is spatially limited and cannot be linked to LNAPL source zones; b) the LNAPL source zones are so poorly understood that we have no mass constraints or balances; and c) this is a very heterogeneous site, in terms of LNAPL source locations, masses, rates, and related subsurface properties. Consequently, much McHugh et al., 2020 amounts to speculative hypotheses and estimates of NSZD that are unbounded by confirmatory data. Several of the McHugh et al. authors prepared a conceptual site model (CSM) report that can be downloaded from the EPA website: https://www.epa.gov/sites/production/files/201907/documents/red_hill_conceptual_site_model_20190630-redacted.pdf. This CSM report incorporates the conclusions of McHugh et al., 2020 as part of a broader interpretation of a generally safe setting with regard to potential aquifer damages being caused by past and future fuel releases because of the assumed large fuel holding and assimilative capacities. Substantial impacts to the aquifer caused by recent fuel releases (May and November 2021) have contaminated drinking water and affected thousands of base residents. These aquifer impact events serve to highlight the importance of adequate technical detail in evaluations, particularly in complex settings like at the subject site. A partial synopsis of these recent fuel release events can be found at: https://www.hawaiipublicradio.org/local-news/2021-12-21/confused-about-the-timeline-for-the-red-hill-fuel-storage-facility-and-contaminated-water-read-this.