Leakage of dense, nonaqueous phase liquids from waste impoundments constructed in fractured rock and clay: theory and case history

Abstract

This paper examines the behaviour of dense, nonaqueous phase liquids (DNAPLs) in fractured media, with an emphasis on waste-disposal ponds constructed in fractured clay and rock. Calculations are presented to estimate the height of DNAPL that may accumulate at the base of a disposal pond prior to initial entry into a water-saturated fracture. This height is found to be a function of the fluid densities, the DNAPL–water interfacial tension, the fracture aperture, and the position of the water table. A numerical model is applied to estimate the steady-state rate of DNAPL leakage from a disposal pond underlain by vertical fractures. This rate of leakage is found to be a function of the spacing of fractures, the fracture aperture, the DNAPL density, and the height of the water table in the formation outside of the impoundment. It is demonstrated that a wide range of leakage rates can occur over a relatively narrow range of parameters. A conceptual analysis of two-phase flow examines the conditions that lead to both uniform and sparse DNAPL migration pathways beneath a disposal pond in fracture networks. A case history is presented as field evidence of the concepts discussed. In particular, the case history demonstrates that sparse DNAPL migration pathways can occur in fractured rock, and that relatively low dissolved phase concentrations can exist in the immediate vicinity of DNAPL source zones. Key words : nonaqueous phase liquids, ground water, fractures, disposal ponds.

Laboratory measurement of capillary pressure‐saturation relationships in a rock fracture

“…Recently, the migration of nonaqueous phase liquids (NA-PLs) in fractures has been recognized as a widespread occurrence throughout North America [Mackay and Cherry, 1989]. Fractures have a profound effect on NAPL distribution in the subsurface because they may provide both horizontal and vertical migration pathways bringing about extensive migration in directions not necessarily coincident with groundwater flow [Kueper et al, 1992]. The study of multiple fluid movement through fractures is also relevant to partially saturated air-water systems as well as the migration of oil in petroleum reservoirs.…”

The use of upward gradients to arrest downward dense, nonaqueous phase liquid (DNAPL) migration in the presence of solubilizing surfactants

“…Although considered immiscible with water from a fluidmechanical point of view, DNAPLs exhibit finite water solubilities that are typically orders of magnitude above allowable drinking water standards. Examples of sites contaminated by DNAPL are described by Poulin and Verge (1993), Kueper et al (1992), and Mackay and Cherry (1989). Figure 1 illustrates a typical DNAPL contamination scenario associated with the release of DNAPL at ground surface.…”

Use of the geotechnical centrifuge as a tool to model dense nonaqueous phase liquid migration in fractures

“…Upon release at the ground surface, DNAPL is able to migrate through the vadose zone and the sand aquifer, following a complex path that is dependent on the DNAPL self‐weight and the heterogeneities of the aquifer and generally uncoupled from the direction of groundwater flow [ Kueper et al , 1992]. At the base of the aquifer, DNAPL can form a pool on top of a fractured bedrock system.…”

“…Once DNAPL enters a fracture system, the probability of encountering the nonaqueous phase liquid in a borehole where a sparse distribution of migration pathways exists will be very low [ Kueper et al , 1992]. Thus the site investigation of a contaminated fracture system will rarely delineate an accurate distribution of DNAPL within the fracture network.…”