Thermal Monitoring of Natural Source Zone Depletion

Abstract

Natural depletion of subsurface petroleum liquids releases energy in the form of heat. The rate of natural source zone depletion (NSZD) can be derived from subsurface temperature data. An energy balance is performed to resolve NSZD‐generated energy in terms of W/m2. Biodegradation rates are resolved by dividing the NSZD energy by the heat of reaction in joules/mol. Required temperature data are collected using data loggers, wireless connections, and automated data storage and analysis. Continuous thermal resolution of monthly NSZD rates at a field site indicates that apparent monthly NSZD rates vary through time, ranging from 10,000 to 77,000 L/ha/year. Temporal variations in observed apparent NSZD rates are attributed to processes governing the conversion of CH4 to CO2, as opposed to the actual rates of NSZD. Given a year or more of continuous NSZD rate data, it is anticipated that positive and negative biases in apparent NSZD rates will average out, and averaged apparent NSZD rates will converge to true NSZD rates. An 8.4% difference between average apparent NSZD rates over a 31‐month period using the thermal monitoring method and seven rounds of CO2 efflux measurements using CO2 traps supports the validity of both CO2 trap and thermal monitoring methods. A promising aspect of thermal monitoring methods is that continuous data provide a rigorous approach to resolving the true mean NSZD rates as compared to temporally sparse CO2 trap NSZD rate measurements. Overall, a vision is advanced of real‐time sensor‐based groundwater monitoring that can provide better data at lower costs and with greater safety, security, and sustainability.

Reply to Comments in Response to Peer‐Reviewed Technical Note “Application of Four Measurement Techniques to Understand Natural Source Zone Depletion Processes at an LNAPL Site” (Kirkman, Zimbron)

“…The subsurface below the deepest thermocouple is a sink for the heat generated by NSZD, and there is no reason to expect a net temperature of zero at this point. Based on our (and others such as Askarani et al 2018) experience, these boundary condition assumptions provide the best results for the background-corrected calculation method.…”

“…In our calculations, we used a fixed zero net temperature at the surface (described in the paper), an approximation to reflect the boundary condition of the atmosphere. This approach is commonly used in NSZD thermal gradient calculations and is used in other peer-reviewed papers (e.g., Askarani et al 2018). The subsurface below the deepest thermocouple is a sink for the heat generated by NSZD, and there is no reason to expect a net temperature of zero at this point.…”

“…However, our objective in "Measurement Techniques" was to report data from four different NSZD methods and the associated data analysis techniques rather than provide an extensive literature review such as Garg et al (2017), a paper with over 90 references. We did not use the methods described in Askarani et al (2018) and Askarani and Sale (2020) or review other commercially-available methods to measure carbon dioxide flux. Finally, it appears Rayner et al 2020 was published in March 2020, two months after we submitted our original paper.…”

Determining groundwater recharge for quantifying PFAS mass discharge from unsaturated source zones

“…There is considerable experience in the contaminant site characterization industry for employing sensors to better understand key processes that drive remediation decisions. One such example is the use of continuous temperature sensors to measure natural source zone depletion (NSZD) rates and other processes (e.g., Askarani et al., 2018; Kulkarni et al., 2022; Sale et el. et al., 2021; Warren & Bekins, 2015).…”

Advanced methods for RNA recovery from petroleum impacted soils

“…NSZD has been well-demonstrated through various lines of evidence, including quantification of CO 2 efflux through the vadose zone using a gas gradient method [5] , the dynamic chamber method [6] , and CO 2 traps [7] . Also, and perhaps more definitively, NSZD has been demonstrated by quantification of the heat generated from biotic oxidation of petroleum hydrocarbons in soil groundwater [8 , 9] . Biotic oxidation of petroleum hydrocarbons, like oxidation of organic composts, produces CO 2 and heat.…”