Biogenic heat results from exothermic microbial activity associated with Natural Source Zone Depletion (NSZD). The products (including heat) generated from the breakdown of a representative petroleum compound are shown below:
C₈H₁₈ + 12.5 O₂ + 8 CO₂ + 9 H₂O ΔH = 10.7 kcal per g C₈H₁₈
Mineralization processes occurring at a LNAPL-contaminated site are shown in the conceptual site model diagram on the right. The reaction proceeds directly under aerobic conditions, or through a two-step sequence of anaerobic methanogenesis followed by methane oxidation. These two reaction pathways are found in different geochemical zones. The conceptual site model includes soil gas transport of reactants and products (including heat) involved in these biodegradation reactions.
Biodegradation reactions are exothermic, and their rates are strongly dependent on local soil temperatures. Understanding biogenic heat is thus fundamental to conceptualizing NSZD. Monitoring NZSD using soil and groundwater temperatures is considered an innovative method by a recent API guidance document.
What is Bio-Therm?
Historically, lab studies have been the primary method for understanding microbial activity (NSZD). Extrapolating these results to the field-scale has proven difficult because of the influence of site-specific conditions, which vary spatially and temporally.
Bio-Therm quantitatively links the heat produced from NSZD reactions (i.e., mineralization) to local soil temperatures and to heat transfer processes.
This web-based model creates a virtual contaminated site with the purpose of predicting NSZD rates. It is a central repository of both field data (ambient and groundwater temperatures, soil temperatures, and measured NSZD rates) and lab data (NSZD rates and their temperature dependence).
Bio-Therm Has Multiple Applications
• Estimate the seasonality of NSZD rates
• Help formulate and validate LNAPL conceptual site models
• Reconcile field data with site-specific conditions
• Account for temporal variability in the design of NSZD monitoring plans
• Evaluate the feasibility and basis for design of thermally enhanced NSZD systems
• Develop and validate methodologies to accurately monitor NSZD using soil temperatures
Monitoring NSZD Using Soil Temperatures Without a Background Correction:
Soil and groundwater temperatures can indicate the occurrence of NSZD. By using Bio-Therm, we have demonstrated that thermal gradient-based NSZD rates calculated using short-term soil temperature measurements are prone to high error rates caused by temporal ambient temperature fluctuations. Such errors are not likely to be adequately accounted for by the background correction (the subtraction of soil temperatures at an unimpacted location from those at a contaminated location). Bio-Therm’s patent pending approach (US Patent 62/151,564) uses high-frequency soil temperature measurements to calculate the long-term heat balance, which cancels the error caused by short-term temperature fluctuations.
Bio-Therm’s method does not need a background location control measurement.
Bio-Therm predicts NSZD rate fluctuations based on the contaminant profile and temporal soil temperature changes. The graphic on the right illustrates a predicted three-year correlation between soil temperatures and NSZD rates, given a site-specific contaminant distribution.