Askarani, K.K., E.B. Stockwell, K.R. Piontek, and T.C. Sale.
Groundwater Monitoring & Remediation 38(3):43-52(2018)

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 was performed to resolve NSZD-generated energy in terms of W/m², followed by resolution of biodegradation rates by dividing the NSZD energy by the heat of reaction in joules/mol. Temperature data were collected using data loggers, wireless connections, and automated data storage and analysis. Continuous thermal resolution of monthly NSZD rates at a field site indicated that apparent monthly NSZD rates vary through time, ranging from 10,000 to 77,000 L/ha/yr. Temporal variations in observed apparent NSZD rates were attributed to processes governing the conversion of methane (CH₄) to CO₂, as opposed to the actual rates of NSZD. Given a year or more of continuous NSZD rate data, positive and negative biases in apparent NSZD rates are expected 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 supported the validity of both CO₂ trap and thermal monitoring methods. See additional information in 57 slides at http://taep.memberclicks.net/assets/Luncheon/2016/gsi_thermalnszd_taep_9-17-16.pdf.