Natural source zone depletion (NSZD), the natural processes that contribute to subsurface-LNAPL depletion, may in some cases remove more hydrocarbons from light non-aqueous phase liquids (LNAPL) in the subsurface at contaminated sites than active, engineered remediation systems. Awareness of NSZD’s contributions to LNAPL mass removal at contaminated sites is growing as NSZD becomes better understood among regulators and site owners and operators.
NSZD background
NSZD processes that remove hydrocarbons from LNAPL include volatilization, dissolution, and biodegradation that occur in both the vadose zone (above groundwater) and within groundwater (Johnson et. al 2006). These processes typically occur across the entire smear zone footprint. NSZD is an important environmental process for many petroleum sites and is influenced by temperature and soil moisture (McAlexander and Sihota 2019; Sihota et al. 2016). Especially for large LNAPL sites, NSZD provides substantial hydrocarbon removal that is sustained year after year (Eichert et al. 2017; Sihota et Sihota et al. 2018).
Figure 1: NSZD processes. Image from Sihota and Mayer, 2011
NSZD data collection involves one or more techniques that have been developed to measure signatures produced by NSZD processes (Figure 1). The most widely used NSZD data collection approaches involve measurement of surface CO2-efflux, subsurface soil vapor concentrations, or subsurface temperatures. Raw data collected using any of these techniques may be converted into NSZD rates.
NSZD awareness gains momentum
While NSZD may be a powerhouse in passive LNAPL removal, its abilities often go unnoticed or are underutilized because few sites measure NSZD to fully understand the hydrocarbon removal being achieved without active remediation.
Recently, though, NSZD has become more accepted by regulators. For example, the Interstate Technology and Regulatory Council (ITRC) included NSZD as one of the 21 remediation systems in its LNAPL Technical/Regulatory Guidance Document. This publication combined with ITRC trainings (staffed in part by Trihydro) fosters an improved understanding of how NSZD can work in combination with an active remedial system for subsurface LNAPL cleanup. In addition, stakeholders are recognizing that NSZD can be a standalone remedy for long-term LNAPL site management.
NSZD that has been incorporated into the LNAPL management strategy for challenging petroleum sites has done more than just provide a mechanism for confirming sustained hydrocarbon removal. NSZD incorporation has also provided a site-specific understanding of processes that might be enhanced, should this be desired. Such enhancements are becoming part of integrated “green” remediation solutions that minimize energy input while maximizing environmental benefit.
How does NSZD compare to active, engineered systems for LNAPL mass removal?
Trihydro has collected NSZD data over several years at multiple sites in different states and has found that NSZD is often a major component of removing LNAPL mass in the subsurface. In fact, we have found NSZD often exceeds the LNAPL mass removal rates when compared to active, engineered systems, such as pump and treat, air sparging, or soil vapor extraction. This is especially true if the active remediation system is reaching its technological endpoint, where the LNAPL removal curve approaches asymptotic conditions.
How can NSZD be used at remediation sites?
NSZD can be used at an LNAPL remediation site as:
- A relatively simple data-collection tool that allows site managers to quantify and claim credit for LNAPL depletion due to natural degradation processes that are already occurring in the subsurface
- A benchmark to compare LNAPL removal by NSZD to the LNAPL removal by an active engineered system
- A final step in a remedial system treatment when active, engineered systems have reached their technological endpoints
- The primary remedy for a portion of the site, while other portions of the site have an active remediation system
- The primary remedy for an entire site if risks to receptors have been managed and are acceptable
- A remedy for the fringe of the smear-zone footprint, or locations where mobile LNAPL is no longer present but where smear-zone exists
Often NSZD is not the sole remedy for the early part of LNAPL remediation at a site. Even so, NSZD can be a component of the remedy at most LNAPL sites, even at the beginning. This is because it is important to understand how much LNAPL mass is being removed by an active remedial system(s) and compare it to LNAPL mass being removed by NSZD. This helps stakeholders determine how NSZD fits into a broad remedial strategy. Furthermore, NSZD data collection techniques may be used to evaluate/optimize other remediation technologies, many of which leave measurable signatures (e.g., CO2 flux or heat) that are similar to those produced via NSZD processes.
What are some best practices in measuring NSZD effectiveness?
When measuring NSZD at large sites, one measurement per several acres density may be needed to measure a site-wide average hydrocarbon removal rate (Eichert et al. 2017; Sihota et al. 2018). This can equate to many measurements (often more than 20, and in some cases more than 100) for some sites. The dynamic closed chamber instrument can be used to collect this number of measurements in just a few days. Smaller, less complex sites can have fewer measurement points, but it is a good idea to confirm statistically meaningful data are collected.
At sites without the four seasons, NSZD rates may be steady across the course of one year (Makay et al. 2018). At other sites, NSZD rates tend to be highest in summer and lowest in winter, with measurements in the fall considered to be the best approximator of the annual average (Eichert et al. 2017, Sihota et al. 2016) because stakeholders interested in characterizing environmental factors that influence NSZD at their LNAPL sites can establish “stations” for taking NSZD measurements more frequently without having to characterize the entire site (McAlexander and Sihota 2019).
Finally, it is important to set good metrics upfront with regulators on when an active remediation system will be discontinued. This can occur when LNAPL removal reaches an asymptotic recovery curve, or an untenable cost-per-gallon of LNAPL recovered is reached. Once the metric is reached, a transition to a different remedial system, or transition to NSZD, can be justified.
What’s next?
As NSZD research continues, the following questions are being explored:
- What is the long-term trajectory for NSZD? Will natural hydrocarbon removal rates eventually decrease as less degradable components of LNAPL persist or might NSZD rates actually increase as LNAPL becomes more accessible to groundwater and soil gases?
- How will stakeholders integrate NSZD, and its accompanying impact on greenhouse gas emissions, to broader site management1?
- What are the best “green remediation” methods to enhance the NSZD processes in the subsurface?
- How do NSZD processes affect the LNAPL composition over time; does NSZD preferentially remove more soluble/volatile LNAPL constituents (e.g., benzene)?
Trihydro is actively researching these questions and collaborating in nationwide research with others in the industry.
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1: Additional reading: