Non-aqueous phase liquid spills in freezing and thawing soils: Critical analysis of pore-scale processes

The frequent use of non-aqueous phase liquids (NAPLs) in cold regions creates serious risks of soil and groundwater contamination. NAPL contaminants can stay in soil for long times due to their entrapment by strong interfacial forces, resulting in a source of pollution caused by their slow dissolution in groundwater over decades. The presence of these contaminants in ice-containing soils creates a four-phase problem, which must be fully understood to design/develop or improve remediation techniques in cold climate regions. In this review, the fate and transport of NAPL contaminants in periodically freezing-thawing and frozen soils is discussed, with emphasis on pore-scale processes. Three topics are identified for future research focus: (a) study of the dynamics of NAPLs during freezing and thawing of soils using non-destructive imaging techniques, and the effect of various factors, including wettability, pore size, consolidation of porous matrix, and fluid properties; (b) investigations of the fate and transport of NAPL contaminants in frozen soils with different wettabilities, and the effect of the spatial distribution of ice clusters on NAPL retention and movement; and (c) pore-scale modeling of the fate and transport of NAPL spills in freezing-thawing and frozen soils. This will lead us towards a complete pore-scale understanding of NAPL spills in cold climate soils, and their fate and transport over time.