Many Department of Defense (DoD) sites are affected by historical releases of light non-aqueous phase liquids (LNAPL), with costly active technologies traditionally being applied as the presumptive remedy for most LNAPL sites. Recently, monitoring of natural source zone depletion (NSZD) has emerged as a passive remedy approach that offers the potential for greater rates of LNAPL destruction when compared to active remedies, a more sustainable remediation approach, and lower long-term costs. The specific objectives of this demonstration program were to: demonstrate the use of innovative, inexpensive second generation temperature monitoring systems developed by Colorado State University to improve data quality and reduce costs;demonstrate improved methods to separate the heat signal associated with biodegradation of petroleum from seasonal and other sources of temperature fluctuations in soils;demonstrate that temperature based approaches to quantifying NSZD rates are particularly suited for LNAPL source areas located below paved surfaces; andcompile results from monitoring of NSZD at many sites and utilize these results to i) document the range of NSZD rates and ii) identify site factors that may be predictive of higher or lower NSZD rates at individual sites.

Analogous to the generation of heat from a compost pile, the biological degradation of petroleum in the subsurface generates heat. Specifically, NSZD generates the same amount of heat per volume of petroleum degraded as produced during combustion of petroleum (for example, the same heat as combustion of petroleum in an oil furnace used for home heating). At remediation sites, this heat signature allows use of an innovative temperature-based technology to quantify biologically-mediated depletion of LNAPL in the subsurface. The breakthrough method for the conversion of generated heat to NSZD rates is based on thermodynamic equations and was originally developed by Colorado State University. In short, 1) vertically-spaced temperature sensors are used to measure soil temperature from ground surface down to the LNAPL source area, 2) this temperature profile is used to determine the heat flux away from the biological reaction zone, and 3) this heat flux is used to calculate the amount of petroleum being degraded (i.e., the volume of petroleum per unit time required to account for the amount of heat being generated). Because continuous temperature monitoring is very inexpensive, thermal monitoring of NSZD provides a more accurate and much more detailed record of NSZD rates compared to the alternative carbon flux or soil gas gradient methods.

Across 40 sites where NSZD rates have been measured by various parties, NSZD was documented to occur at all sites. The measured NSZD rates did not vary by fuel type. NSZD was also documented at the two demonstration sites under both paved and unpaved locations. Different methods used to quantify rates yielded a range of rates that were generally within an order of magnitude. While offering some clear advantages, additional work may be required to fully validate second generation monitoring equipment and background correction methods, especially to resolve short-term NSZD rates (e.g., monthly or seasonal). The primary cost driver is the cost of the temperature sensor stations. While one-time measurements of NSZD, such as Carbon Traps, may be cheaper for single measurements, the temperature-based methods offer clear cost advantages at sites where long-term monitoring is required or advantageous.

There are no known implementation issues for temperature-based methods, which have been validated previously and discussed in multiple guidance documents and standards around the world. Despite general acceptance of NSZD as a scientific process, and the efficacy of different measurement methods, regulatory acceptance remains a concern in some locales. The results of these two demonstration sites, a review of 40 sites in the published literature, and recent guidance documents (e.g., Interstate Technology and Regulatory Council 2018; American Society for Testing and Materials E3361-22 2022) indicate the presence and magnitude of NSZD processes and provide further guidance for reliable methods to quantify NSZD rates. The second generation monitoring sensors and communication equipment can be procured from S3NSE Technologies as newly commercialized, custom-built equipment. Colorado State University Research Foundation currently owns the patent (Sale et al. 2015; US Patent No. 10,094,719) for devices and methods for measuring the thermal flux and estimating the NSZD rate. (Project Completion - 2023)

Poonam, R.K., K.L. Walker, C.J. Newell, K.K. Askarani, Y. Li, and T.E. McHugh. 2022. Natural Source Zone Depletion (NSZD) Insights from Over 15 years of Research and Measurements: A Multi-Site Study. Water Research, 225:119170. doi.org/10.1016/j.watres.2022.119170.