Characterizing the transport of hydrocarbon contaminants in peat soils and peatlands
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Widespread transportation corridors crossing Canadian peatlands make these landscapes vulnerable to hydrocarbon spills. After a spill happens, free hydrocarbon spreads in the peat layer forming a free-phase plume. Water soluble compounds of the free-phase plume then partition into the pore water and the flowing aqueous phase forming a dissolved-phase plume. These plumes threaten peatland ecosystem health and impose risk to aquatic systems located nearby the contaminated area. For this reason, environmental scientists should be able to predict the behavior of hydrocarbon contaminants and the temporal evolution of the hydrocarbon plumes. Properties of peat soils control the fate and transport of the spilled non-aqueous phase liquids (NAPL) and dissolved-phase hydrocarbon solutes in contaminated peatlands. Since the fate and transport of these contaminants in peat has received little attention, there is insufficient knowledge of parameters governing their mobility. The cumulative effect of processes including dissolution, advection, and dispersion, diffusion into immobile water, adsorption onto soil matrix, volatilization, biodegradation, and other transformation processes determines the temporal evolution of contaminants in aquifers. The physical, hydraulic, and chemical properties of the aquifer soil and the hydrological, thermal, biological, and geochemical characteristics of the aquifer determine the rates and the relative dominance of abovementioned processes. It is well established that peat physical and hydraulic properties including its porosity, hydraulic conductivity, and average pore radius size vary systematically with peat depth. Also, peat decomposition and humification modifies the chemical composition of the peat matrix. However, the effect such systematic variations in peat has on the redistribution of hydrocarbon contaminants has not been investigated. Multiphase flow characteristics of peat including capillary pressure-saturation-relative permeability (Pc-S-kr) relations control the redistribution of free-phase hydrocarbon in a peatland. These relations will be functions of peat type and its physical properties. The functionality of Pc-S-kr relations and residual NAPL (diesel) saturation (SNr) with peat type were examined in two types of peat in which SNr ranged between 0.3-17% and increased with peat bulk density. In a given peat, SNr was a function of saturation history and increased with increasing maximum diesel saturation. Irreducible water saturation, which is the saturation at which aqueous phase stops moving, and the curvature of water kr-S curves both were a function of peat type, and increased with peat bulk density. The results suggested that the kr-S relations of water derived from unsaturated hydraulic conductivity of peat (in the presence of air) might be a good estimate of the water kr-S relation in presence of NAPL. Although the functionality Pc-S-kr relations to peat depth was not determined in this study, conceptually, it is expected that the reduction of pore radius typically taking place down the peat profile leads to 1) reduction of peat hydraulic conductivity with depth, 2) increase in NAPL-entry capillary pressure and water retention with depth, which cumulatively could cause a preferential migration of NAPL in shallower peat layers after a pressurized release of NAPL. In this condition, the exchange of gases between the source zone and the atmosphere happening due to wind or water table fluctuations may efficiently 1) drain contaminated soil-gas, and 2) promote aerobic conditions in the contaminated area. The water table fluctuation, however, might enhance the lateral redistribution of the free-phase plume. The retardation of dissolved hydrocarbons is dominantly controlled by their adsorption onto the soil. The adsorption of benzene and toluene, as two of the most toxic and mobile dissolved organic compounds present in petroleum liquids, and their dependency on peat depth were explored. The linear adsorption isotherms for benzene and toluene were obtained with adsorption coefficients ranging from 16.2-48.7 L/kg and 31.6-48.7 L/kg, respectively. In the experiments, the benzene and toluene adsorption coefficients were not constant along the peat profile and varied with peat depth. The variations of toluene adsorption correlated with typical variations of cellulose and humic acid characteristic of a peat matrix. The organic carbon adsorption coefficient (KOC) obtained for benzene in peat was equal and higher than the average benzene KOC reported in literature for soils with low organic carbon content (fOC). However, toluene KOC was 10-50% less than the average value which suggests that using the average value might overestimate toluene retardation and underestimate its mobility down-gradient of the spill zone. The competition between benzene and toluene adsorption was insignificant, suggesting that individual adsorption coefficients could be used to study the adsorption of individual contaminants in a multi-solute problem. The adsorption studies showed adsorption of benzene and toluene at the equilibrium condition. However, the adsorption model parameters that control the chemical equilibrium during contaminant transport remained unknown. Besides, the effect of mobile-immobile mass transfer, which takes place due to the dual-porosity pore structure of peat, on the retardation of dissolved hydrocarbons in the inactive pores, were not known. To address these, miscible (solute) transport experiments were conducted showing that the mass transfer rate between mobile and immobile zones of peat could be sufficiently high to establish physical-equilibrium between mobile and immobile zones of peat pore space. The results also showed that the relatively slow kinetics of adsorption could cause chemical non-equilibrium between the aqueous phase and adsorbed phase, leading to decreased adsorptive retardation in high discharge conditions. The retardation factor of benzene increased with depth and degree of peat decomposition. This coupled with the typical reduction of hydraulic conductivity with depth could cause a preferential redistribution of dissolved-contaminants in shallow peat layers in a contaminated peatland. This study is the first study that characterizes the fate and transport of hydrocarbon contaminants in peat at the laboratory-scale and with specific focus on peat properties. Although scale-dependent phenomena such as field-scale heterogeneities might impose additional complexities to the fate and transport processes, the scale-independent parameters obtained in this study including adsorption partitioning coefficients and adsorption kinetics parameters, as well as residual NAPL saturation, irreducible water saturation, and water relative permeability relations have increased our understanding on the transport of free-phase and dissolved-phase hydrocarbons in in peat. The results can help predict the temporal evolution of the hydrocarbon plumes after a spill. The results also can help in assessing the risk after an oil spill accident and for evaluating the appropriateness of potential remediation plans.
Cite this version of the work
Behrad Gharedaghloo (2018). Characterizing the transport of hydrocarbon contaminants in peat soils and peatlands. UWSpace. http://hdl.handle.net/10012/13703