Characterizing the immiscible transport properties of diesel and water in peat soil

Extensive pipeline and railway corridors crossing Canadian peatlands make them vulnerable to hydrocarbon spills, potentially impairing ecosystem health, so it is important to be able to forecast hydrocarbon fate and transport within and beyond the peatland. The redistribution of hydrocarbon liquids in groundwater systems are controlled by the multiphase flow characteristics of the aquifer material including capillary pressure-saturation-relative permeability (P_c-S-k_r) relations. However, these relations have never been characterized for the hydrocarbon-water phases in peat. To address this, the flow and transport of diesel and water in peat soils were examined through a number of one dimensional vertical immiscible displacement tests, in which diesel was percolated into peat pore space displacing peat water, leading to a two-phase flow regime. Inverse modelling simulations using both Brooks and Corey's and power law relative permeability models, matched the data of the immiscible displacement tests well. Irreducible water saturation (S_wirr) and the curvature of water relative permeability relation increased with peat bulk density. The residual diesel saturation (S_Nr) ranged between 0.3% and 17% and increased with bulk density of peat. In a given peat, S_Nr was a function of saturation history and increased with increasing maximum diesel saturation. The receding contact angles of water in water-air systems and diesel in diesel-air systems, respectively, were 51.7° and 61.2°, showing that the wetting tendency of peat in the air imbibition condition is toward the draining liquid. These experiments advance our knowledge on the behavior of hydrocarbons in peat, and can improve numerical modelling of hydrocarbon transport after a spill.