Multimethod Analysis of NSZD and Enhanced SZD by Solar-Powered Bioventing at the Guadalupe Restoration Project

Abstract

Active remediation at sites with light non-aqueous phase liquid (LNAPL) often leaves residual hydrocarbons in the subsurface, necessitating long-term management. While much of the effort in recent years has focused on natural source zone depletion (NSZD) as the primary method for demonstrating continued hydrocarbon removal, the same data collection methods can quantify biodegradation enhancements that can sustainably increase the rate of SZD. This approach has been applied at the Guadalupe Restoration Project, one of the first sites at which NSZD measurements and monitoring technology were demonstrated. Sitewide NSZD quantification was conducted using CO₂ efflux measurements and subsurface temperature profiling. The results fell within the range of previously reported estimates that were based on soil-gas profiling in the early 2000s, demonstrating the viability of the new methods at this site. The data collection methods were then deployed during pilot testing of solar-powered bioventing. The system used seven 400-W solar panels to power a regenerative blower that delivered approximately 0.85 cubic meter per minute (30 cfm) air to the LNAPL-impacted vadose soil near the interface with the groundwater table. Soil-gas data indicated an upward fanning of injected air toward ground surface. Elevated temperature due to hydrocarbon oxidation yielded an approximate 10.2 kg day⁻¹ source depletion rate above the baseline NSZD mass removal rate over an approximate 30 m (100 ft) radius of influence, which aligned well with a 8.2 kg day⁻¹ rate estimated from CO₂ efflux measurements. Introduction of O₂ via bioventing substantially increased the LNAPL biodegradation rate from baseline NSZD processes by almost an order of magnitude. The results demonstrate that site management can proceed along a sequenced program that began with aggressive hydraulic recovery of hydrocarbon product, transitions to enhanced SZD in areas with poorly recoverable LNAPL, and then to NSZD without intervention to address residual LNAPL across the full footprint of the LNAPL bodies.