Spatial-temporal trends and correlations from a large natural source zone depletion (NSZD) research project at a site with LNAPL

Abstract

Results are reported from the third phase of an extensive Natural Source Zone Depletion (NSZD) research project with NSZD measurements at 12 locations with up to four years of monitoring using two measurement technologies: Carbon Traps and the temperature based, Thermal NSZD technology. Additionally, location-specific site characterization data was compiled to evaluate factors impacting NSZD rates. The two methods both resulted in site wide average NSZD rates in the low hundreds of gallons of light non-aqueous phase liquid (LNAPL) biodegraded per acre per year (gal/acre/year): 280 gal/acre/year for Thermal NSZD measurements and 360 gal/acre/year for Carbon Trap measurements. Based on the temporal and spatial variability in the NSZD data, these values were considered to be relatively consistent. In addition, these data support the case that NSZD is actively removing LNAPL at this site. Annual temporal rates generally varied by a factor of about two during the two- to four-year monitoring period, and the spatial distribution of NSZD rates across the site for the two methods ranged from 80 to 890 gal/acre/year. While the site-wide NSZD rates were relatively consistent, the location-specific Carbon Trap and Thermal NSZD data only had a weak correlation with a linear regression r2 of 0.4. The correlation between the spatial distribution of the NSZD rates and nine specific lithological, concentration, LNAPL distribution, and LNAPL composition factors showed strong correlations with only two of the factors: (1) a positive correlation to the fraction of LNAPL in the saturated zone as indicated by a rapid optical screening tool (ROST); and (2) a negative correlation to locations with significant clay in the geologic boring log. Overall, these results confirmed the observations from other NSZD field studies that showed significant temporal, spatial, and methodological variation in the NSZD rates at LNAPL sites. More importantly, these results support the conclusion that NSZD is active even for submerged LNAPL.