Underestimated Natural Source Zone Depletion (NSZD) Rates – Biogenic Heat Method

By: Andrew Fowler, Ph.D., Senior Geologist

Quantification of natural source zone depletion (NSZD) rates at remediation sites can play a key role in selecting an effective remediation strategy for a site. NSZD assessments typically focus on the vadose zone where over 90% of NSZD mass removal occurs1. Methods used to assess NSZD in the vadose zone include use of passive gas traps, dynamic closed chamber systems, gas gradient assessments of nested soil gas probes, or the biogenic heat method.

Until recently, biogenic heat was considered a cost-effective method as it may be implemented in existing monitoring well networks using relatively inexpensive equipment2.

The thermal conductivity (KT) of soil is a key factor used to calculate NSZD rates in the unsaturated zone using the biogenic heat method. Moisture has a profound effect on soil KT values by affecting the heat balance through heat loss by evaporation and radiation, and heat advection with moisture migration3. These effects are frequently overlooked or oftentimes assumed to be negligible in biogenic heat NSZD assessments. A study by Janssen, H., Carmeliet, J. and Hens, H., 2004, provides evidence that oversimplifying subsurface heat loss mechanisms may lead to underestimation of NSZD rates using the biogenic heat method3.

The approach is unique because most assessments of heat loss to soil do not account for the coupling effects of between soil heat and soil moisture transfer are taken into account – and these effects are shown to be notable. Consequently, the approach of identifying factors that underestimate NSZD methods promoted in current guidance documents provides a strong case for the need of more flexibility related to the treatment of data in biogenic heat NSZD studies. Accounting for the factors that underestimate NSZD in biogenic heat programs may lead to long term cost savings by realizing earlier active remediation endpoints.

What these findings may mean for your site:

  • NSZD rates calculated by the biogenic heat method may be underestimated if variations in soil moisture are not considered.
  • When assessing biogenic heat data, methods for determining thermal conductivity should account for soil moisture.
  • Comparing in-situ soil thermal conductivity measurements using a hand-held device with default values is a nominal cost that in some instances, may prove to be a worthwhile investment.

In summary, the use of biogenetic heat at specific sites could result in reduced time to reach remediation goals, cost decreases, and improvements to site schedules.

To discuss NSZD and Biogenic Heat Methods in more detail, EHS Support’s team of subject matter experts are here to help:

Andrew Fowler, Ph.D., Senior Geologist

Nigel Goulding, Chief Technical Officer, Co-Managing Director

Tom Silverman, Remediation Services Group Leader

Sources:

1 ITRC, 2018, Light non-aqueous phase liquids (LNAPL) document update, LNAPL-3, Interstate Technology & Regulatory Council, LNAPL Update Team, Washington, USA.

2Sweeney, R.E. and Ririe, G.T. (2014). Temperature as a Tool to Evaluate Aerobic Biodegradation in Hydrocarbon Contaminated Soil. Groundwater Monitoring & Remediation 34(3).

3Janssen, H., Carmeliet, J. and Hens, H., 2004. The influence of soil moisture transfer on building heat loss via the ground. Building and Environment, 39(7), pp.825-836.

Illustration: https://lnapl-3.itrcweb.org/appendix-b-natural-source-zone-depletion-nszd-appendix/

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