John Molson

Modeling of multicomponent reactive transport in groundwater: 1. Model development and evaluation

MINTRAN is a new model for simulating transport of multiple thermodynamically reacting chemical substances in groundwater systems. It consists of two main modules, a finite element transport module (PLUME2D), and an equilibrium geochemistry module (MINTEQA2). Making use of the local equilibrium assumption, the inherent chemical nonlinearity is confined to the chemical domain. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient two-step sequential solution algorithm. The advantages of the coupled model include access to the comprehensive geochemical database of MINTEQA2 and the ability to simulate hydrogeological systems with realistic aquifer properties and boundary conditions under complex geochemical conditions. The model is primarily targeted toward groundwater contamination due to acidic mine tailings efiiuents but is potentially also applicable to the full range of geochemical scenarios covered by MINTEQA2. The model is tested with respect to ion exchange chemistry and with respect to precipitation/dissolution chemistry involving multiple sharp fronts. The companion paper presents two-dimensional simulations of heavy metal transport in an acidic mine tailings environment, focusing on environmental implications.

Thermal energy storage in an unconfined aquifer: 2. Model development, validation, and application

A fully three-dimensional numerical model for simulating coupled density-dependent groundwater flow and thermal energy transport is developed and validated. The transport solution is based on a finite element time integration algorithm which generates a symmetric coefficient matrix while retaining second-order accuracy in time. The use of a symmetric conjugate gradient solver for both the flow and transport matrices results in a high degree of computational efficiency. Three-dimensional deformable block elements are used to allow the model to conform to domains with irregular geometry. The thermal transport model is validated against the results of the Borden thermal injection field experiment presented in the companion paper. The model simulations provide an excellent match with the observed temperature distribution over time, with the effects of thermal buoyancy and losses across the ground surface accurately reproduced by the model. The model is shown to be a practical tool for simulating the type of low-temperature thermal transport problems that arise in connection with seasonal aquifer thermal energy storage and ground source heat extraction systems.

Dissolution and mass transfer of multiple organics under field conditions: The Borden emplaced source

The process that transfers mass from a subsurface source zone of residual dense nonaqueous phase liquid (DNAPL) to the flowing groundwater is a controlling factor in determining the time required to dissolve the source by noninvasive means. While mass transfer can be kinetic or equilibrium under laboratory conditions, aqueous concentrations in the field are generally found to be below equilibrium levels. To gain insight into the mass transfer process under field conditions, we simulated the dissolution of the emplaced DNAPL source at the Canadian Forces Base Borden, Ontario, which contains a mixture of three DNAPLs. The simulations clearly show that mass transfer at this site is equilibrium-controlled during the 1000-day observation period and that apparent tailing of one of the organic components is due to its declining solubility, rather than mass transfer kinetics. Flow lines passing through the source are focused in a narrow streamtube downstream of the source, and equilibrium concentrations are therefore observed only at the center of the effluent plume. Since the concentration peaks can be easily missed in the sampling, streamline focusing can explain the low concentrations observed in the field.

Modeling of multicomponent reactive transport in groundwater: 2. Metal mobility in aquifers impacted by acidic mine tailings discharge

1,034,710. Fishing rods. R. G. CATCHPOLE and B. BOCKING. May 28, 1965 [June 1, 1964], No. 22550/64. Heading A1A. In a fishing rod adapted for line-casting, a stationary line-carrying spool 7 is mounted within a rotary cup 2 at the upper end of the rod 1, which is hollow and contains a co-axial inner, rotatable tube 27 secured to the cup and a central, longitudinally-reciprocable, shaft 26 secured to the spool, the cup being provided with a bale arm 8, 8a, which rotates with it, to feed line on to the spool during winding- in. The bale-arm is pivoted on the cup so as to be movable to an inoperative position during casting, but rotation of the cup trips the arm into its operative position through engagement between abutments 18, 20. The tube 27 is rotated by bevel gearing 29, 30 connected to a handle 5 at the lower end of the rod, the crownwheel 29 of the gearing also carrying a crank pin 31 which oscillates the shaft 26 longitudinally by engagement with a grooved follower 33.

Biodegradation modelling of a dissolved gasoline plume applying independent laboratory and field parameters

Abstract Biodegradation of organic contaminants in groundwater is a microscale process which is often observed on scales of 100s of metres or larger. Unfortunately, there are no known equivalent parameters for characterizing the biodegradation process at the macroscale as there are, for example, in the case of hydrodynamic dispersion. Zero- and first-order degradation rates estimated at the laboratory scale by model fitting generally overpredict the rate of biodegradation when applied to the field scale because limited electron acceptor availability and microbial growth are not considered. On the other hand, field-estimated zero- and first-order rates are often not suitable for predicting plume development because they may oversimplify or neglect several key field scale processes, phenomena and characteristics. This study uses the numerical model BIO3D to link the laboratory and field scales by applying laboratory-derived Monod kinetic degradation parameters to simulate a dissolved gasoline field experiment at the Canadian Forces Base (CFB) Borden. All input parameters were derived from independent laboratory and field measurements or taken from the literature a priori to the simulations. The simulated results match the experimental results reasonably well without model calibration. A sensitivity analysis on the most uncertain input parameters showed only a minor influence on the simulation results. Furthermore, it is shown that the flow field, the amount of electron acceptor (oxygen) available, and the Monod kinetic parameters have a significant influence on the simulated results. It is concluded that laboratory-derived Monod kinetic parameters can adequately describe field scale degradation, provided all controlling factors are incorporated in the field scale model. These factors include advective–dispersive transport of multiple contaminants and electron acceptors and large-scale spatial heterogeneities.

Thermal energy storage in an unconfined aquifer: 1. Field Injection Experiment

A thermal injection and storage experiment was conducted to investigate the feasibility of storing thermal energy in shallow unconfined aquifers near the water table. Heated water was injected into a shallow aquifer and plume temperatures were monitored over a 141-day period by means of a dense array of bundle-type piezometers. The highly detailed data, which provide the three-dimensional temperature distribution within the aquifer, give good insight into the physical processes of aquifer thermal energy storage, and provide an excellent basis for the verification of simulation models. The experimental data also allow the physical processes of heat advection, dispersion, retardation, buoyancy and boundary heat loss to be quantified. In a companion paper, a three-dimensional density-dependent groundwater flow and thermal transport model is developed and validated using the results of the thermal injection experiment.

Modelling the closure-related geochemical evolution of groundwater at a former uranium mine

A newly developed reactive transport model was used to evaluate the potential effects of mine closure on the geochemical evolution in the aquifer downgradient from a mine site. The simulations were conducted for the Königstein uranium mine located in Saxony, Germany. During decades of operation, uranium at the former mine site had been extracted by in situ acid leaching of the ore underground, while the mine was maintained in a dewatered condition. One option for decommissioning is to allow the groundwater level to rise to its natural level, flooding the mine workings. As a result, pore water containing high concentrations of dissolved metals, radionuclides, and sulfate may be released. Additional contamination may arise due to the dissolution of minerals contained in the aquifer downgradient of the mine. On the other hand, dissolved metals may be attenuated by reactions within the aquifer. The geochemical processes and interactions involved are highly non-linear and their impact on the quality of the groundwater and surface water downstream of the mine is not always intuitive. The multicomponent reactive transport model MIN3P, which can describe mineral dissolution-precipitation reactions, aqueous complexation, and oxidation-reduction reactions, is shown to be a powerful tool for investigating these processes. The predictive capabilities of the model are, however, limited by the availability of key geochemical parameters such as the presence and quantities of primary and secondary mineral phases. Under these conditions, the model can provide valuable insight by means of sensitivity analyses.

Large‐Scale Dispersion in a Sandy Aquifer: Simulation of Subsurface Transport of Environmental Tritium

Large-scale dispersion in a sandy unconfined aquifer in Denmark was studied by simulating subsurface transport of environmental tritium. Subsurface transport included transport in a moderately deep unsaturated zone and in a relatively long cross section of the aquifer. The tritium data from the site enabled a four-step modeling analysis comprising (1) estimation of tritium content in the infiltration water, (2) transport in the unsaturated zone, (3) estimation of flux-averaged tritium concentration in the recharge water, and (4) transport in the groundwater zone. The groundwater model simulations were sensitive to the longitudinal and transverse dispersivity parameters, αL and αr, as a set of parameters, but a model sensitivity analysis showed that it was not possible to identify a unique set of parameter values. A likely range of variation for the two parameters could be identified: (αL, αT); ∈ [(1 m, 0.005 m); (10 m, 0.0 m)] the two parameters being interdependent in that an increase in αL results in a decrease in αT and vice versa. The reported dispersivities represent a scale of 1000 m, the approximate travel distance from the water table to the observation wells. If the estimated αL can be regarded as being of intermediate reliability following earlier defined criteria, the range or the representative set of values then represent the largest scale of earlier reported values. Including our range of αL in the set of reported dispersivities suggests that αL does not increase indefinitely with scale.

Analyses of water diversion along inclined covers with capillary barrier effects.

Various types of cover systems can be used to control water infiltration into waste disposal sites. One promising option is to combine different types of soil to create a layered cover with capillary barrier effects (CCBE). A CCBE basically involves the placement of a relatively fine-grained soil, which acts as a water-retention layer, over a coarser capillary break material. On slopes, a CCBE promotes lateral water diversion. Inclined CCBEs, however, are relatively complex, as their behaviour is influenced by numerous factors. In this paper, the authors present the key results obtained from a numerical investigation into the response of steeply inclined CCBEs. The study evaluates the behaviour of covers under dry and humid climatic conditions. After a review of the physical processes and background studies, the paper presents simulation results that demonstrate the effect of key factors on the diversion length of covers, including layer thicknesses, material properties, and recharge rates. The results sh...

Humic acid enhanced remediation of an emplaced diesel source in groundwater: 2. Numerical model development and application

A pilot scale experiment for humic acid-enhanced remediation of diesel fuel, described in Part 1 of this series, is numerically simulated in three dimensions. Groundwater flow, enhanced solubilization of the diesel source, and reactive transport of the dissolved contaminants and humic acid carrier are solved with a finite element Galerkin approach. The model (BIONAPL) is calibrated by comparing observed and simulated concentrations of seven diesel fuel components (BTEX and methyl-, dimethyl- and trimethylnaphthalene) over a 1500-day monitoring period. Data from supporting bench scale tests were used to estimate contaminant-carrier binding coefficients and to simulate two-site sorption of the carrier to the aquifer sand. The model accurately reproduced the humic acid-induced 10-fold increase in apparent solubility of trimethylnaphthalene. Solubility increases on the order of 2-5 were simulated for methylnaphthalene and dimethylnaphthalene, respectively. Under the experimental and simulated conditions, the residual 500-ml diesel source was almost completely dissolved and degraded within 5 years. Without humic acid flushing, the simulations show complete source dissolution would take about six times longer.