Innovations in nature-based remediation of petroleum sites

NSZD refers to a combination of different processes that reduce petroleum contaminant mass in the subsurface. These processes include dissolution and volatilisation of LNAPL constituents, as well as direct biodegradation of LNAPL, soil vapour, and dissolved phase through microbial activity in the unsaturated and saturated zones. As a result, NSZD is a main contributor to LNAPL body stability.

There are complex combinations of interacting attenuating mechanisms during NSZD, and it's important to recognise these and consider them early in the project lifecycle for optimal design and implementation of remediation practices.

Typically, natural source zone depletion of LNAPL contamination proceeds anaerobically under methanogenic conditions, mainly through direct LNAPL biodegradation, which generates methane (CH₄) and carbon dioxide (CO₂). The generated methane will be subsequently oxidized to carbon dioxide (CO₂) via an exothermic process in the vadose zone.  Since gaseous CO₂ will be the ultimate by-product of LNAPL mineralisation, one typical method for screening for NSZD processes is to perform a survey of surficial CO₂ efflux within the petroleum-impacted area. Subsurface soil gas gradient monitoring is another technique involving multi-level gas measurements (CH₄, CO₂, O₂). Additionally, the exothermic CH₄ oxidation reaction in the vadose zone presents another potential line of evidence of NSZD.  Relating estimates of heat flux above background levels to the heats of reaction for the NSZD processes can be used to approximate NSZD rates.

Using a combination of different measurement methods provides multiple lines of evidence of NSZD activity at a site.

NSZD rates provide an end point metric to indicate when engineered remediation has reached technical impracticability and it’s time to transition to nature-based remediation. It’s important to factor this metric into remedial decision-making.

LNAPL transmissivity (T_n) provides a scientific, standardised approach to evaluate LNAPL recoverability across a site. Thus, it also provides a key line of evidence for mobility of LNAPL at the site. Evaluation of LNAPL transmissivity (T_n) involves a “bail down” test as per ASTM Method E2856-13, where LNAPL is removed quickly from a well, and recharge is measured. After analysis, results are compared to an established de minimis criteria to determine potential hydraulic recoverability. This process can also be used to indicate when recovery efforts reach the limit of practicability beyond which there would be no technical benefits for engineered remediation at a particular site.

This focus on transmissivity is different to previously established practices that are based recoverability from a well on apparent LNAPL thickness. Research has found no link between apparent LNAPL thickness in a well and transmissivity/recoverability of LNAPL.

Measurements of LNAPL transmissivity and NSZD can thus be used for decision-making and design of remedial actions, such as when it may be appropriate to discontinue LNAPL recovery, and what other remediation or management options could be feasible. Many technical guidance documents have now been updated to reflect this shift towards more effective and sustainable remediation practices. Our knowledge of LNAPL and NSZD will continue to evolve. For example, new models are being developed to update the fundamentals and improve predictions such as estimation of remedial timeframe via NSZD. GHD continues to follow advancements in this area as well as contributing to guidance updates.