LiORA - NSZD Comparison - Case Study
Summary
A former spill site in the United States has three Soil Sensor locations within ~1 metre of each other. Though they are installed in separate boreholes, they are meant to reflect a gradient in gaseous emissions within the soil. The sensors are installed at 15, 30, and 45 ft (4.57, 9.14, and 13.72 m) belowground.
Here we compare two approaches to calculating NSZD: the first is our typical workflow that treats sensors installed in separate boreholes as distinct Sense units ("separate" hereafter); the second is meant to reflect the rationale for the install configuration where each sensor is treated as a separate depth for the gradient method calculation ("single" hereafter).
NSZD Over Time
We compared NSZD calculated using CH4, C02, 02, and total NSZD derived from all three
gases through time (Figure 1). Mean NSZD was lower when treating each sensor as an individual unit. The differences were mainly due to CH4 (302% greater in single relative to separate), followed by C02 (267% greater), and 02 (94% lower). Total NSZD was
290% greater in single versus separate.
Table 1. Minimum, mean, and maximum NSZD (g m"2 day-1) among gases for the separate and single methods.
NSZD Variability Among Gases
Maximum NSZD values varied less between the methods, with the greatest difference for CH4 (47% greater in single versus separate) and the least for C02 (13% greater) (Figure 2).
Conclusion
This analysis highlights significant differences in gradient-based NSZD calculations when using separate boreholes versus a single-unit approach. The single-unit method resulted in markedly higher mean NSZD estimates, particularly for CH4 and CO2, while O2 was notably lower. Maximum values varied less, though CH4 showed the largest discrepancy. These findings suggest that the choice of calculation method has a substantial impact on NSZD estimates, with the single-unit approach capturing a more integrated representation of subsurface gas flux dynamics. Understanding these differences is critical for accurate site assessments and remediation planning, ensuring that NSZD estimates align with site-specific conditions and monitoring objectives.