Fate and transport of free-phase and dissolved-phase hydrocarbons in peat and peatlands: developing a conceptual model

Abstract

Physical and chemical, and pore-scale to field-scale properties of peat soil affect the migration of non-aqueous phase liquids (NAPLs) and dissolved-phase solutes in contaminated peatlands in a way not anticipated based on the current understanding derived from their behavior in mineral soil systems. Peat pore surface wettability, which is determined by pore surface chemistry, has strong hysteresis and shows hydrophilic and hydrophobic behaviors during water drainage from and water imbibition into pore spaces. This leads to high residual NAPL saturation in pore spaces. Systematic reduction of pore radius size with depth associated with greater peat decomposition in a typical peat profile leads to an increase in NAPL-entry capillary pressure and a reduction in peat permeability with increasing depth. The former leads to stronger capillarity in deeper peat horizons, causing resistance to NAPL percolation compared to that in shallow ones; along with decreased permeability with depth, this results in higher NAPL mobility in shallower peat. The cumulative effect is preferential horizontal migration of NAPL in shallow less decomposed peat horizons. Occupation of peat macro-pores by NAPL dramatically decreases the effective water permeability and leads to lower rates of water infiltration and groundwater discharge in the contaminated area. With respect to dissolved-phase hydrocarbons, the dual porosity structure of peat soil, which exhibits larger pores and higher effective porosity near the surface, also favours preferential transport in the surface peat layer, and leads to increasing solute retardation with depth. In addition, adsorption of hydrocarbon solutes onto natural dissolved organic carbon present in peat pore water influences the effective adsorption of hydrocarbon solutes onto peat by reducing the apparent adsorption partitioning coefficient. The concepts and evidence presented in this manuscript suggest both free-phase and dissolved-phase hydrocarbon have restricted mobility in peatlands. On this basis, in large peatlands where ecological function is not notably impacted, a case can be made for allowing natural processes to degrade the hydrocarbon, rather than the common response of digging up and disposing of contaminated soils, which destroys the ecosystem function.