Electrical Resistance Heating of Groundwater Impacted by Chlorinated Solvents in Heterogeneous Sand

Abstract

Remediation of sites impacted by dense nonaqueous phase liquids (DNAPLs) is an ongoing, costly challenge where technological improvement is needed to allow for cost-effective cleanup. Remedial processes using in situ thermal technologies have received increasing attention in recent years. Electrical resistance heating (ERH) is a commonly applied thermal technology in the industry. ERH applications are used at heterogeneous sites, where permeability contrasts can significantly impact the heating process along with the movement of DNAPL and gas phases. To further understand this, a series of intermediate-scale laboratory experiments were completed in a two-dimensional flow cell examining the impacts on subsurface processes occurring during ERH. Groundwater velocity variations within a homogenous coarse sand were shown to limit the heating rates during ERH. Additionally, experiments entailed the heating 270 mL of trichloroethene (TCE), chloroform (CF), and tetrachloroethene (PCE) pools in a medium-grained silica sand with a coarse-grained sand lens located above the DNAPL pools. Spatial and temporal distributions of temperature, gas saturations, and aqueous DNAPL concentrations were collected and observed using a distribution of thermocouples, front-face image capture, and post treatment soil sampling. In each experiment, temperatures increased above DNAPL-water co-boiling plateaus creating estimated gas volumes of 131 L, 114 L and 215 L that originated from the TCE, CF and PCE pools respectively. Produced gas migrated vertically, entered the coarse lens and became trapped beneath a capillary barrier laterally spreading outside the heated treatment zone where 26 - 56% of injected DNAPL condensed back into a DNAPL phase. These findings demonstrate that layered heterogeneity with higher permeability lenses can potentially facilitate the transport of contaminants outside the treatment zone by mobilization and condensing of gas phases during ERH applications.