Enhanced DNAPL Transport in a Sand Core during Dynamic Stress Stimulation

Abstract

Extraction of dense, nonaqueous-phase liquid (DNAPL) contaminants trapped in groundwater aquifers is a major problem in environmental remediation because existing field techniques, such as pump and treat, have limited effectiveness. We present new laboratory experimental evidence that low-frequency (100 Hz or less) stress waves could increase the mobility and transport of DNAPL by an order of magnitude or more during pump and treat. This unproven technology could decrease the time and cost of groundwater remediation efforts and reduce the final amount of contaminant trapped in an aquifer. A sand core was confined in a specialized flow apparatus and contaminated with a small amount of trichloroethylene (TCE). Steady-state water flow was then initiated through the core, and effluent TCE concentrations were measured using gas chromatography (GC). At various stages of water flow, axial stress cycling at 25–100 Hz was applied by mechanical coupling of a magnetostrictive actuator to the sand core's outlet face. During most of the stimulated runs pure-phase TCE droplets were produced in the effluent and the effective TCE concentration increased by as much as a factor of 20. Numerous flow runs were performed to investigate the effects of stimulation frequency, amplitude, and duration, as well as water flow rate and background effluent TCE concentration. Calibrated accelerometer and pore pressure measurements allowed determination of the stimulation threshold parameters required to achieve enhanced TCE production. The physical mechanisms responsible for the observed phenomenon are not fully understood. Further research is required before this promising new technology can be applied reliably to contaminated groundwater aquifers.