Total Dissolved Gas Pressure Measurements for Assessing Contaminant Degradation Processes in Groundwater

Abstract

The total dissolved gas pressure (PTDG ) probe has been used in groundwater studies for over a decade, but rarely in assessing contaminant degradation, despite the many degradation reactions that produce or consume dissolved gases. Here we present three studies to demonstrate the application of PTDG measurements to groundwater experiencing contaminant degradation, with discussion of its benefits and limitations. The first study is a pilot‐scale laboratory experiment simulating dissolved ethanol contamination of an anaerobic sand aquifer. Continuous monitoring of PTDG showed the rapid onset of microbial hydrocarbon degradation via denitrification and fermentation. The subsequent formation of a gas phase was revealed when PTDG began mimicking the bubbling pressure (PG *; sum of hydrostatic and atmospheric pressure), fluctuating with atmospheric pressure. Some deviations of PTDG above PG * occurred also, which may hold promise for signalling substantial changes in the rate or type of degradation process (here, the onset of methanogenesis). In the second study, synoptic field measurements at a petroleum plume site demonstrated how elevated PTDG could identify wells with evidence of hydrocarbon degradation (denitrification and/or methanogenesis). And finally, combined field measurements of dissolved oxygen (DO) and PTDG in monitoring wells of a nitrate‐contaminated aquifer (Abbottsford‐Sumas) revealed areas where denitrification was likely occurring. Limitations to PTDG use identified in these studies included the masking of degradation processes by the presence of a gas phase, as when trapped following water table fluctuations or formed from rigorous degradation reactions, and confounded assessment of PTDG patterns from other natural or anthropogenic processes that can also influence groundwater PTDG .

What Can Groundwater Monitoring Tell Us About Gas Migration? A Numerical Modeling Study

“…TDGP probes can be deployed in a monitoring well and used to provide near real-time measurement of total dissolved gas pressure as an indicator for the presence of GM in an aquifer. Due to the ease of use and high temporal resolution, TDGP is gaining popularity as an indicator parameter to be measured during GM investigations (Roy et al 2017(Roy et al , 2022. However, it is noted that TDGP measures the total pressure associated with all dissolved gases present within the surrounding water where it is deployed.…”