P.J. Van Geel

Laboratory and model simulations of a LNAPL spill in a variably-saturated sand, 1. Laboratory experiment and image analysis techniques

Abstract A two-dimensional, multiphase flow experiment was conducted in the laboratory. The saturation distribution of a lighter than water non-aqueous-phase liquid (LNAPL) as it migrated through a variably-saturated sand medium was determined using image analysis techniques. The pressures in the water and LNAPL phases were measured using hydrophilic and hydrophobic porous cups connected to a series of pressure transducers and a data acquisition system. The LNAPL inflow and water outflow were also recorded. A series of capillary pressure-saturation experiments were conducted for each two-phase system. The capillary pressures recorded by the pressure transducers were used to calculate the pphase saturations based on a fully hysteretic capillary pressure-saturation algorithm. The static equilibrium capillary pressure-saturation relationships proved to be invalid immediately ahead of the LNAPL front. The capillary pressure-saturation relationship appeared to be dynamic for short periods of time as the LNAPL front arrived at each transducer location. The presence of an entrapped air phase as the LNAPL migrated through the unsaturated zone clearly affected the migration of the LNAPL and the maximum LNAPL saturation reached in the sand medium.

The importance of fluid entrapment, saturation hysteresis and residual saturations on the distribution of a lighter-than-water non-aqueous phase liquid in a variably saturated sand medium

Abstract A two-dimensional, multiphase flow experiment was conducted in the laboratory and numerically modelled using a finite difference multiphase flow code. Heptane, a lighter-than-water non-aqueous phase liquid (LNAPL), was spilled on a variably saturated sand medium. After the spill was completed, the LNAPL was allowed to distribute itself above the capillary fringe. The water pressure at the base of the experimental box was subsequently raised and lowered on two separate occasions to simulate fluctuating water table conditions. The water and LNAPL pressures were measured using hydrophillic and hydrophobic porous cups connected to pressure transducers and a data acquisition system. The laboratory spill was modelled using a hysteretic and non-hysteretic multiphase flow code. A comparison of the experimental data to the model results illustrates the effects and importance of fluid entrapment and saturation hysteresis.

A proposed model to include a residual NAPL saturation in a hysteretic capillary pressure-saturation relationship

A residual non-aqueous phase liquid (NAPL) present in the vadose zone can act as a contaminant source for many years as the compounds of concern partition to infiltrating groundwater and air contained in the soil voids. Current pressure-saturation-relative permeability relationships do not include a residual NAPL saturation term in their formulation. This paper presents the results of series of two- and three-phase pressure cell experiments conducted to evaluate the residual NAPL saturation and its impact on the pressure-saturation relationship. A model was proposed to incorporate a residual NAPL saturation term into an existing hysteretic three-phase parametric model developed by Parker and Lenhard [Water Resour. Res. 23(12) (1987) 2187], Lenhard and Parker [Water Resour. Res. 23(12) (1987) 2197] and Lenhard [J. Contam. Hydrol. 9 (1992) 243]. The experimental results indicated that the magnitude of the residual NAPL saturation was a function of the maximum total liquid saturation reached and the water saturation. The proposed model to incorporate a residual NAPL saturation term is similar in form to the entrapment model proposed by Parker and Lenhard, which was based on an expression presented by Land [Soc. Pet. Eng. J. (June 1968) 149].

Laboratory and model simulations of a LNAPL spill in a variably-saturated sand, 2. Comparison of laboratory and model results

Abstract A two-dimensional, multiphase flow experiment was modelled using a finite-difference, multiphase flow and transport code. The laboratory experiment consisted of a lighter-than-water, non-aqueous-phase liquid (LNAPL) spill in a variably-saturated sand. The numerical model allows a non-hysteretic, a partially hysteretic and a fully hysteretic solution. The fully hysteretic algorithm accounts for fluid entrapment, saturation hysteresis and hysteresis in the relative permeability terms. The partially hysteretic model allows air-phase entrapment within the LNAPL as the LNAPL migrates through the unsaturated zone. The fully hysteretic model results were compared to the laboratory pressures and saturations. The importance of hysteresis is clearly illustrated; however, the inclusion of hysteresis substantially increases the computation and storage requirements. The partially hysteretic model predicts the movement of the LNAPL front relatively well but does not account for the hysteretic conditions which persist during the redistribution of the LNAPL after the spill was complete. The laboratory experiment was modelled using an implicit in pressure, explicit in saturation (IMPES) solution and a fully implicit solution. The fully hysteretic IMPES solution with two-point upstream weighting of the relative permeability terms resulted in the best representation of the experimental data for the model scenarios evaluated.

Arsenic Pollution of a Loam Soil: Retention Form and Decontamination

A study was conducted to assess the retention form of arsenic in soil and to evaluate the use of phosphate for releasing it from the soil. In this study, a loam soil was artificially polluted with arsenate at pH 5.5, which is one of the pH values at which maximum arsenic adsorption occurred. The soil was kept for 2.5 months under wet conditions to allow for stabilization. The soil was maintained under aerobic condition and losses of arsenic by volatilization were determined to be minimal. The soil was then sequentially extracted with a series of chemicals to identify the soil fractions in which the arsenic was bound. The percentage of arsenic found in the Fe bound-exchangeable, reducible-residual, Al bound exchangeable, residual, calcium bound exchangeable, and easily exchangeable forms was 31.6, 27.3, 25.2, 5.5, 4.9, and 4.7%, respectively. A batch experiment showed that at 20°C, 80% of the bound arsenic was removed by phosphate in the pH range of 5 to 7. A power function model was found to fit the data with a desorption rate constant of 402 mg/kg As h−1.

Modeling cometabolic biodegradation of organic compounds in biofilms

A dynamic model that describes the biodegradation of secondary substrates through oxygenase reactions in biofilms has been developed. The model incorporates intracellular reducing power, in the form of NADH, as a link between the rate of utilization of primary substrates and the biodegradation of secondary substrates. The reaction kinetics were integrated into a diffusive biofilm model with internal and external mass transfer limitations and then combined into a continuous flow reactor model. Preliminary evaluation of the model has demonstrated that mass transfer limitations substantively reduced the removal of the secondary substrate as compared to a suspended growth reactor with an equivalent mass of biomass. A sensitivity analysis revealed that model predictions of the removal of secondary substrate were highly sensitive to the parameters describing the availability of reducing power and to the compound-specific biodegradation kinetic parameters.

Long Term Monitoring of Hydrocarbon Contamination Using Multi-Level Vapor Phase Piezometers

This study evaluated the feasibility of supplementing groundwater monitoring protocols by assessing the vadose zone for the extent of residual subsurface contamination. The study also characterized the response of the soil gas signatures with respect to different soil types and degrees of contamination. A field study was conducted at a former gasoline vending station located in Ottawa, Canada. The current state of contamination was determined by analysis of soil samples taken from boreholes. A series of 10 nested soil gas wells with monitoring depths of 0.75, 1.5, 2.25 and 3.0 m were then installed. Using these wells, soil gas surveys were performed at regular intervals over an extended period to quantify Gaseous TPH (TPH g ), oxygen and carbon dioxide concentrations in the soil gas. Results indicate that soil gas wells located near the source term exhibited characteristic soil gas signatures and significant fluctuations in TPH g , oxygen, and carbon dioxide concentrations with time. Soil gas wells located beyond the soil contamination demonstrated limited correlation between TPH g , oxygen and carbon dioxide concentrations and decreased seasonal variability.

Application of a Biodegradation-Redox Model for Predicting Bioremediation Performance

A coupled biodegradation-redox reaction model (BioRedox) is used to show that the placement of a low-permeability cap over an unlined landfill will significantly alter redox conditions in the underlying aquifer, and may adversely influence the intrinsic bioremediation of chlorinated ethenes in the leachate plume. The importance of bioavailability considerations when evaluating redox-altering schemes is discussed.