Hundreds to thousands of DoD sites are affected by historical releases of light non-aqueous phase liquids (LNAPLs), including fuels, lubricants, and heating oil. Traditionally, costly active treatment technologies (e.g., hydraulic recovery, air sparging, multi-phase extraction, soil vapor extraction [SVE], etc.) have been applied as the presumptive remedy for most LNAPL sites. The overall objective of this project is to demonstrate the use of temperature-based methods to accurately measure natural source zone depletion (NSZD) rates (i.e., gallons of LNAPL degraded per acre per year) in LNAPL source areas. The basic application of temperature-based methods to quantify NSZD rates has already been validated through laboratory and field testing sponsored by industry partners. This demonstration will improve and expand the validation of thermal monitoring of NSZD, resulting in increased regulatory acceptance and reduced cost for application as a site remedy. The specific objectives of this demonstration project are to:

• Demonstrate the use of innovative, inexpensive 2nd generation temperature monitoring systems 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 (i.e., improved background correction);
• Demonstrate (through field deployment) that temperature-based approaches to quantifying NSZD rates are particularly suited for LNAPL source areas located below buildings or other paved surfaces; and
• Compile 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. This project will demonstrate that the thermal monitoring methods for measurement of continuous NSZD rates is less expensive, more accurate (lower uncertainty in NSZD rates), more informative (measures changes in NSZD rates over time), and more broadly applicable (can be applied to LNAPL sources below paved surfaces) than the alternative carbon flux and gradient methods for measuring NSZD rates. A successful demonstration will increase regulatory acceptance of this environmentally sustainable and cost effective technology for remediation of LNAPL source areas.

The ability to accurately quantify NSZD rates will support the transition from active to passive remediation technologies at many DoD sites with LNAPL source areas and will eliminate the need to install new active remediation systems at other sites. Traditional costs for operating active treatment systems for sites with residual LNAPL range from $100K to several million dollars per site for a potential total DoD cost of more than one billion dollars over the next ten years. A conservative estimate of potential cost savings by improving response actions and reducing delays in remedy selection is $100 – 500M over the next ten years across the DoD.

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.