About NSZD

How fast does biodegradation occur?

A recent case study found that NSZD-based loss rates measured at several field sites ranged from 700 to 2,800 gallons per acre-year (Garg et al., 2017). Although these rates are larger than expected, they are not high enough to achieve rapid site cleanup. For example, an apparently high NSZD rate of 1,000 gallons per acre-year only amounts to a free petroleum thickness decrease of about 1 mm. If an extraction or cleanup system is pushed to the point that it yields the same results as the natural microbial rate, this could be an indication that nature is achieving more than the active remedy. Sustained monitoring of NSZD rates is, therefore, crucial to the efficient and cost-effective management of active remedies.

What factors affect NSZD rates?

Temperature has a very strong effect on the speed with which microbes break down petroleum products. As a result, NSZD rates at field sites are seasonally dependent, and rates are typically at a maximum in the fall. In general, 35°C to 40°C is the upper temperature tolerance limit for subsurface microbes, while microbial activity may be very slow or completely stopped at temperatures near or below freezing. Site-specific models can help us understand the relationship between temperature and microbial activity by assessing the depth distribution of local soil temperatures; an example of such a model is available for free at BiogenicHeat.com. (As of june 2021 Biogenicheat.com is not active)

Another important factor impacting NSZD rates is the availability of electron acceptors. Some progress has been made by treating dilute dissolved plumes with external electron acceptors, such as sulfate (e.g., Kolhatkar and Schnobrich, 2017). In general, the success of these remedies is limited by the efficiency of contact between the contamination and the electron acceptors.

How are LNAPL natural attenuation (NA), natural source zone depletion (NSZD), and monitored natural attenuation (MNA) related?

According to the ITRC LNAPL update document (ITRC, 2018), natural attenuation (NA) encompasses all natural processes that result in loss or neutralization of the contaminant without human intervention. NA includes both natural source zone depletion (NSZD) and monitored natural attenuation (MNA). NSZD specifically refers to mass loss from the unsaturated source zone caused by both physical and chemical processes, including biodegradation reactions. MNA is used to assess the rate of contaminant removal through the aqueous phase (see USEPA’s OSWER Directive No. 9200-4.17). Because methanogenesis does not require external electron acceptors and its by-products (CO2 and CH4) preferentially partition into the gas phase, fieldmeasured NSZD rates often cannot be explained solely by the consumption of electron acceptors. Mass losses attributed to NSZD mechanisms are much larger than those related to MNA mechanisms (Lundegard and Johnson, 2006).

Can NSZD rates be estimated based on the depletion of external electron acceptors?

Biodegradation reactions can be either aerobic (in the presence of oxygen) or anaerobic (in the absence of oxygen). Methanogenesis (an anaerobic process) is not reliant on external electron acceptors. This makes it impossible to estimate NSZD rates using only the degree of source zone electron acceptor depletion. However, aerobic and methanogenic NSZD pathways both ultimately produce CO2, meaning that in most cases NSZD rates can be accurately estimated using LNAPLderived CO2 fluxes.

Can a site be completely cleaned up by NSZD processes in a few years?

NSZD is a long-term process. For a typical terrestrial petroleum spill, it might take decades for NSZD to remove significant contaminant mass. However, if active mass removal methods have achieved some success, and the remaining contaminant mass poses a low risk to public and environmental health, performing NSZD calculations or studies may inform stakeholders about the usefulness of active and passive remedies in achieving site closure.

Can NSZD monitoring be used at any LNAPL-contaminated site?

In general, yes, NSZD monitoring can be useful at most LNAPLcontaminated sites. NSZD rates can also be used to reinforce the conceptual site model in combination with other sources of data at all stages of the site’s life cycle. For example, LNAPL-derived CO2 fluxes can be used to delineate the extent of the contaminant during early site characterization, or NSZD rates can be used to assess the performance of various active remedies. Keep in mind that some site conditions, such as the presence of gas-impermeable/ wet layers, interfere with the characterization of NSZD processes.

Can NSZD be a final remedy for any LNAPL-contaminated site?

Yes, in the sense that we can sometimes take advantage of NSZD processes to let nature remove the last remnants of a contaminant in the subsurface following implementation of an active remedy.

At how many LNAPL sites has NSZD been monitored to date?

NSZD rate measurements have been taken at hundreds of LNAPL sites and several DNAPL sites across North America. The widespread use of NSZD as a passive remedy and monitoring tool continues to grow every year, and NSZD is also quickly gaining acceptance in Europe and Australia.

Does NSZD affect all LNAPL compounds equally?

This is likely not the case. It has been shown that microbes prefer certain compounds, such as those with lower molecular weights, over others. This means that the composition of the LNAPL contaminant is likely enriched in less biodegradable compounds with time. It is therefore very important that we understand how these compositional changes affect the management of contaminated sites over the long term.