Spatial and temporal variation in the natural source zone depletion rates of two types of LNAPL sources

Accurate measurement of natural source zone depletion (NSZD) rates is a crucial but challenging task at a large site since NSZD rates usually have large temporal and spatial variation. Few studies focus on the spatial and temporal variability of NSZD rates of different types of light non-aqueous phase liquid (LNAPL). This study has explored the spatial and temporal variability of NSZD rates at one site with two chemically distinct LNAPL bodies (aliphatic-rich naphtha for Source Zone #1 vs aromatic-rich pyrolysis gasoline for Source Zone #2). Higher temperatures result in higher NSZD rates, leading to the highest NSZD rates in summer and the lowest in winter in both Source Zone #1 and #2. Although the LNAPL in Source Zone #1 and #2 have completely different composition, the difference in the area-weighted average NSZD rates between Source Zone #1 and #2 was not statistically significant. Natural soil respiration (NSR) is the dominant source of CO₂ emitted at the land surface throughout all seasons at this site. This suggests that during the growing season (summer), both high mean value and high standard errors of NSR rates compromise the NSZD quantification results based only on simple background correction (using the mean value). This result highlights the significant spatial and temporal variation of NSZD rates in large site and the challenges of using background correction to estimate NSZD rates. It is necessary to consider the importance of data quality to the subsequent data management process.