Effects of Gravity and Aperture Statistics on DNAPL Entrapment in Fractures

Abstract:

Dense non-aqueous phase liquids (DNAPLs) are an important class of groundwater contaminants. Their migration pathways are particularly difficult to locate in fractured rock aquifers, which are abundant in North America. Over one million people in Southern Ontario alone obtain their drinking water from the Silurian dolostone bedrock. Once a DNAPL is trapped in an aquifer, it becomes a long-term threat to the source water quality. It is imperative to be able to locate and quantify trapped DNAPL to implement appropriate remedial strategies.

This study quantifies volumetric DNAPL entrapment utilizing an invasion percolation (IP) approach to simulate the imbibition of water in a DNAPL-saturated fracture. The relationship between the volumetric fraction of trapped DNAPL, aperture field statistics and fracture orientation was investigated by varying a number of parameters; overall, 34,560 simulation were completed. The standard deviation (σ_b) and mean (μ_b) of the apertures were varied from 0.01-0.3 mm and 0.5-1.5 mm, respectively. The standard deviation (σ_z) and correlation length (λ_z) of the mid aperture field were varied from 0.01-10mm and 5-50mm, respectively. Fracture orientation was varied from 60o above (super-horizontal) to 60o below (sub-horizontal) horizontal.

The results demonstrate that: 1) fractures oriented sub-horizontally permit the complete drainage of DNAPL, though this does not occur for horizontal and super-horizontal fractures; 2) when the coefficient of variation (COV) is larger than 0.1 the fracture orientation has little effect on the volumetric ratio of DNAPL entrapped; and 3) increasing standard deviation of the mid aperture field increases the range of the volumetric ratio of trapped DNAPL.