NAPL dissolution in heterogeneous systems: an experimental investigation in a simple heterogeneous system

Dissolution experiments were conducted in a two-dimensional cell with simple heterogeneous packing consisting of well-defined coarse lenses contaminated by NAPL surrounded by a clean fine sand matrix. Experiments were conducted for a range of conditions by varying the grain size of sand, initial NAPL saturations, and size and number of the coarse lenses. Experimental results show that aqueous phase concentrations at the effluent port were always well below equilibrium concentrations, while concentrations at the local sampling port were at equilibrium for Sn>0.3. Sufficient experimental conditions were tested to suggest three important mechanisms affect the overall mass transfer behavior in this heterogeneous system. (1) Variability in the effective permeabilities affect the relative volume of water flowing through the NAPL source zone and therefore, the extent of dilution of contaminated water with clean water flowing around the source zone. (2) The perimeter surface area of the NAPL source zone affects the total interfacial area for mass transfer. This is an especially important contribution to the overall mass transfer rate during early phases of the experiment when there is little flow of water through this zone. (3) Within the NAPL source zone, decreased mass transfer rates due to limited interfacial area and/or increased aqueous phase flow rates becomes the overall rate-limiting factor in this system for Sn<0.3.