Natural Source Zone Depletion
Natural Source Zone Depletion (NSZD) describes processes that result in the transformation of LNAPL contaminants into inorganic dead-end products (e.g., carbon dioxide, or CO₂). These transformation processes are called mineralization.
Most mineralization processes are driven by microbes and result in the oxidation of petroleum compounds into CO₂. This oxidation requires external electron acceptors such as oxygen, which drives aerobic biodegradation. Under anaerobic conditions, possible electron acceptors are sulfate, nitrate, and metal oxides.
Often, sub-surface LNAPL is anaerobically transformed into methane (CH₄) and CO₂ without the use of external electron acceptors in a process called methanogenesis. Both CH₄ and CO₂ are mobile in the sub-surface. As CH₄moves upwards into shallow depths where oxygen is readily available, it undergoes rapid aerobic biodegradation, which produces more CO₂.
Recent and ongoing research suggests that LNAPL NSZD rates can be significant. LNAPL degradation rates at hydrocarbon spill sites range from hundreds to thousands of gallons per acre per year; these rates are similar to those achieved by active remediation efforts (such as hydraulic recovery). Therefore, NSZD should be considered a viable remedial option and/or a benchmark for other remediation efforts.
NOT considering NSZD
Quantifying NSZD rates can be a valuable tool in determining the appropriate timing, scale, and aggressiveness of LNAPL remediation efforts.
If NSZD is ignored, the total contaminant mass is considered constant throughout time unless remediation takes place. Following remediation, the total contaminant mass is estimated to be equal to the original contaminant mass minus the mass reduction by remediation.
NSZD has been shown to occur at most petroleum-contaminated sites, and different techniques are available for measuring the rate at which NSZD occurs. The most common methods are based on the measurement of the CO₂ flux out of the soil (efflux) and are described in a recent American Petroleum Industry (API) guidance document on Petroleum Natural Source Zone Depletion.
These methods for quantitating NSZD are important, as they provide a baseline for evaluating active remedies.
The Interstate Technology and Regulatory Council (ITRC) recommends including NSZD as part of a comprehensive remedial site plan. For more information, download the NSZD guidance document