Natural Source Zone Depletion (NSZD) refers to microbial and physical processes that reduce hydrocarbon content in the subsurface. Microorganisms degrade hydrocarbons deep in the soil and rapidly deplete available oxygen and other electron acceptors in the soil and groundwater.  At that point, microorganisms begin to ferment hydrocarbons, producing organic acids. These organic acids are used by a group of microorganisms called methanogens, which consume these organic acids and release methane. The methane migrates towards the upper layers of soil, where another group of organisms, called methanotrophs, consume the methane and release carbon dioxide as a product.

Estimating the NSZD links the methane, carbon dioxide, and oxygen profiles to a hydrocarbon depletion rate. As recommended by guidance documents, the hydrocarbon depletion rate is calculated using specific ratios between methane production and hydrocarbon depletion. Typically, a representative hydrocarbon such as hexane or octane is used to provide stakeholders with a volumetric depletion rate, thereby ensuring the accuracy and reliability of the validation process.

Natural Source Zone Depletion (NSZD) extends the earlier concept, Monitored Natural Attenuation (MNA), to the entire soil profile. MNA only considered groundwater and typically accounted for 10 to 30 percent of degradation occurring in a hydrocarbon plume. Several investigators found that NSZD rates are a reasonable approximation in laboratory and field demonstration sites. However, despite this convincing work, practitioners often struggle with NSZD rates that imply sites should naturally clean up in a relatively short period.  

Thus, practitioners are left with a difficult question:

If my site has such high NSZD rates, why has it persisted for 50 years?

Environmental Material Science developed an autonomous soil sensor that measures the gradient of methane, carbon dioxide and oxygen above a hydrocarbon plume. The SoilSense sensor estimates the gradient every 30 minutes and uses soil moisture and temperature measurements to correct for changes in soil diffusion rates.  

The SoilSense also measures dissolved hydrocarbons in the soil vapour phase. Up to approximately 10,000 ppm, soil vapour phase hydrocarbon concentrations can be used to estimate total semi-volatile hydrocarbons. Thus, paired with the estimates of NSZD from the gas gradient, the SoilSense estimates hydrocarbon concentrations in the subsurface.  

Across 3 years and over 19 sites, LiORA collected 4.6 million measurements of 22 variables and then assessed these 101 million data points to address a depletion measurement question:

Are hydrocarbon depletion estimates from guidance accurate estimates of observed hydrocarbon plume reduction?

Figure 1. Comparison of Petroleum Hydrocarbon (PHC) depletion rate to Natural Source Zone Depletion (NSZD) across 19 sites over 3 years.  Points represent 4.2 million estimates of 22 variables.

The trend line across all 19 sites is PHC Depletion = 0.013 NSZD Rate – 0.0003, P<0.01.

Our data set demonstrates that NSZD reliably estimates PHC Depletion rates but is consistently positively biased. Our observation confirms most practitioners’ instincts that NSZD rates based only on gas gradients were likely too high at their site. It also supports the guidance recommendations that practitioners use complementary lines of evidence for their NSZD estimates, such as:

1. ¹⁴CO₂ analysis of evolved CO₂
2. Thermal profile analysis
3. Hydrocarbon composition analysis

Sources of uncertainty in this analysis

Conclusions

Natural Source Zone Depletion approaches are valid for estimating hydrocarbon depletion in cold regions. Investigators should use:

1. direct estimates of hydrocarbon depletion, such as that used by LiORA’s Soil Sense; or
2. multiple lines of evidence as outlined in guidance documents.