Kerry T.B. MacQuarrie

Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers. I. Model formulation and performance

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. The objectives of this study are to develop and assess the performance of a mechanistic flow and reactive transport model which couples the most relevant physical, geochemical and biochemical processes involved in wastewater plume evolution in sandy aquifers. The numerical model solves for variably saturated groundwater flow and reactive transport of multiple carbon- and nitrogen-containing species in a three-dimensional porous medium. The reactive transport equations are solved using the Strang splitting method which is shown to be accurate for Monod and first- and second-order kinetic reactions, and two to four times more efficient than sequential iterative splitting. The reaction system is formulated as a fully kinetic chemistry problem, which allows for the use of several special-purpose ordinary differential equation (ODE) solvers. For reaction systems containing both fast and slow kinetic reactions, such as the combined nitrogen-carbon system, it is found that a specialized stiff explicit solver fails to obtain a solution. An implicit solver is more robust and its computational performance is improved by scaling of the fastest reaction rates. The model is used to simulate wastewater migration in a 1-m-long unsaturated column and the results show significant oxidation of dissolved organic carbon (DOC), the generation of nitrate by nitrification, and a slight decrease in pH.

Climate change impacts on the temperature and magnitude of groundwater discharge from shallow, unconfined aquifers

Cold groundwater discharge to streams and rivers can provide critical thermal refuge for threatened salmonids and other aquatic species during warm summer periods. Climate change may influence groundwater temperature and flow rates, which may in turn impact riverine ecosystems. This study evaluates the potential impact of climate change on the timing, magnitude, and temperature of groundwater discharge from small, unconfined aquifers that undergo seasonal freezing and thawing. Seven downscaled climate scenarios for 2046–2065 were utilized to drive surficial water and energy balance models (HELP3 and ForHyM2) to obtain future projections for daily ground surface temperature and groundwater recharge. These future surface conditions were then applied as boundary conditions to drive subsurface simulations of variably saturated groundwater flow and energy transport. The subsurface simulations were performed with the U.S. Geological Survey finite element model SUTRA that was recently modified to include the dynamic freeze-thaw process. The SUTRA simulations indicate a potential rise in the magnitude (up to 34%) and temperature (up to 3.6°C) of groundwater discharge to the adjacent river during the summer months due to projected increases in air temperature and precipitation. The thermal response of groundwater to climate change is shown to be strongly dependent on the aquifer dimensions. Thus, the simulations demonstrate that the thermal sensitivity of aquifers and baseflow-dominated streams to decadal climate change may be more complex than previously thought. Furthermore, the results indicate that the probability of exceeding critical temperature thresholds within groundwater-sourced thermal refugia may significantly increase under the most extreme climate scenarios.

Numerical simulation of a fine-grained denitrification layer for removing septic system nitrate from shallow groundwater

One of the most common methods to dispose of domestic wastewater involves the release of septic effluent from drains located in the unsaturated zone. Nitrogen from such systems is currently of concern because of nitrate contamination of drinking water supplies and eutrophication of coastal waters. It has been proposed that adding labile carbon sources to septic distribution fields could enhance heterotrophic denitrification and thus reduce nitrate concentrations in shallow groundwater. In this study, a numerical model which solves for variably saturated flow and reactive transport of multiple species is employed to investigate the performance of a drain field design that incorporates a fine-grained denitrification layer. The hydrogeological scenario simulated is an unconfined sand aquifer. The model results suggest that the denitrification layer, supplemented with labile organic carbon, may be an effective means to eliminate nitrogen loading to shallow groundwater. It is also shown that in noncalcareous aquifers, the denitrification reaction may provide sufficient buffering capacity to maintain near neutral pH conditions beneath and down gradient of the drain field. Leaching of excess dissolved organic carbon (DOC) from the denitrification layer is problematic, and causes an anaerobic plume to develop in simulations where the water table is less than 5-6 m below ground surface; this anaerobic plume may lead to other down gradient changes in groundwater quality. A drain field and denitrification layer of smaller dimensions is shown to be just as effective for reducing nitrate, but has the benefit of reducing the excess DOC leached from the layer. This configuration will minimize the impact of wastewater disposal in areas where the water table is as shallow as 3.5 m.

Field and modeling evidence for a stagnant flow zone in the upper meter of sandy phreatic aquifers

Abstract A stagnant flow zone, where effective hydraulic conductivities are reduced by entrapped bubbles, may be ubiquitous in the upper meter of the saturated zone in phreatic sand aquifers. It is proposed that the stagnant zone acts as a thin semi-aquitard within which groundwater flow paths tend toward the vertical. Once groundwater reaches the underlying active flow zone, the flow paths are mainly horizontal in accordance with the aquifer hydraulic gradients. The presence of stagnant zones in shallow groundwater is inferred from field and modeling data for three sand aquifer sites in southern Ontario, Canada. Field data from an agricultural nitrate monitoring study and the vertical transport of a ground-applied bromide tracer show clear evidence of a stagnant zone influence on solute distributions. More direct evidence is provided by the analysis of data from multi-point tracer tests (akin to borehole tracer tests). The inclusion of a stagnant zone in a numerical simulation of a septic-effluent plume developed in an unconfined aquifer markedly improves the agreement between the simulated and field results. Potential mechanisms for the origin of bubbles in the stagnant zone are discussed. Since many contaminants traverse the upper meter of the saturated zone, an increased understanding of the stagnant zone is important for monitoring and prediction of groundwater contamination.

Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers. II. Model application to a field site.

A multicomponent reactive transport model as presented by MacQuarrie and Sudicky [MacQuarrie, K.T.B., Sudicky, E.A., this volume. Multicomponent simulation of wastewater-derived nitrogen and carbon in shallow unconfined aquifers: I. Model formulation and performance, J. Contam. Hydrol.] is applied to a well-studied wastewater plume in a sandy aquifer near Cambridge, Ontario. Domestic wastewater is released into the unsaturated zone via a drain field at a depth of about 0.8 m. The physical transport parameters for the model are obtained by simulating a non-reactive solute, while kinetic input data for the nitrogen and carbon reaction network are obtained from the literature. The model shows that the wastewater-loading rate has little influence on the moisture content in the unsaturated zone, thus oxygen diffusion in the air phase is an important transport mechanism. The model results are in general agreement with the field-determined moisture and oxygen profiles near the drain field. The simulation results show that oxidation of ammonium and dissolved organic carbon (DOC) goes to completion in the 1.5-m distance between the drain field and the water table, and that calcite dissolution limits the pH reduction to about 0.2 units. The model-predicted nitrate concentrations in the core of the plume are in the range of 20-25 mg N/l and are in good agreement with the field data. Overall, the results for the major reactive species from the model simulation agree well with the geochemical data obtained below the drain field and it is concluded that the major physical and biochemical processes have been correctly captured in the current model formulation.

Dissolution of creosote in groundwater: an experimental and modeling investigation

Abstract The dissolution of industrial creosote with water was investigated using a small physical model (generator column) in the laboratory. The column was packed with a creosote residual of 10% and the effluent was monitored for ten target polycyclic aromatic hydrocarbons (PAHs) including naphthalene, phenanthrene and benzo[a]pyrene. Tests were also conducted with variable contact time to evaluate mass-transfer rates and the rate of approach to equilibrium dissolution. For most components equilibrium between creosote and water was attained after a contact time of 60 h. During shorter contact periods, lower aqueous concentrations of PAHs were attained. However, the ratios of these concentrations were in proportion to their effective solubilities which were calculated using Raoults law and the creosote compositional data. Comparison of the laboratory data with results from an equilibrium model indicated that creosote dissolution could not be described by equilibrium relationships between component concentrations in the two phases at the experimental groundwater velocities. As well as the equilibrium model, a kinetic model was used in an attempt to stimulate the flushout data. It is suggested that increasing mass-transfer rate limitations between the two phases and within the non-aqueous-phase liquid (NAPL) as dissolution proceeded accounted for the differences between the kinetic model and experimental results. Both the physical and mathematical models showed that the dissolution of creosote dense non-aqueous-phase liquid (DNAPL) would initially result in high concentrations of components having high effective solubilities. Complete depletion of creosote by dissolution alone, assuming equilibrium, would require a water/creosote volume ratio of > 270, 000. Therefore, water flushing as a remedial measure will be relatively ineffective for removing residual creosote. The properties of creosote and the aqueous phase, time of contact between the two phases, the degree of contact and the groundwater velocity are important factors in controlling mass transfer between the two phases.

An evaluation of the relative importance of the effects of climate change and groundwater extraction on seawater intrusion in coastal aquifers in Atlantic Canada

To investigate the relative importance of projected sea-level rise, climate change effects on recharge, and groundwater extraction on seawater intrusion in important coastal aquifers in Atlantic Canada, a three-dimensional numerical model of density-dependent groundwater flow coupled with solute transport was developed for the Richibucto region of New Brunswick. The model was used, with an efficient 2k factorial design approach, to perform simulations for the period 2011–2100. The results of the factorial analyses indicate that the relative importance of the three factors investigated varies depending on the model location considered. The effect of declining recharge is most significant at shallow to intermediate depths along the freshwater–seawater transition zone, while the effect of increasing pumping rates dominates at a location relatively close to the well field. The effect of sea-level rise is shown to be significant only at the much deeper inland toe of the transition zone. The spatial variation in importance is related to how different model boundary conditions influence freshwater flow at the different locations within the model domain. This investigation indicates that sea-level rise has the least significant effect (of the three factors considered) on future seawater intrusion in sandstone aquifers in the Richibucto region.RésuméPour étudier l’importance relative de la montée prévue du niveau de la mer, des effets du changement climatique sur la recharge et du prélèvement d’eau souterraine sur l’intrusion d’eau marine dans les aquifères côtiers importants du Canada atlantique, un modèle numérique tridimensionnel d’écoulement, intégrant les variations de densité et couplé avec le transport en solution, a été développé pour la région de Richibucto au Nouveau Brunswick. Ce modèle a été utilisé pour réaliser des simulations pour la période 2011–2100, avec une approche factorielle 2k efficace. Les résultats des analyses factorielles indiquent que l’importance relative des trois facteurs étudiés varie en fonction de la localisation du modèle considéré. L’effet de la baisse de la recharge est plus significatif à des profondeurs faibles ou intermédiaires le long de la zone de transition eau douce–eau salée, tandis que l’effet de l’augmentation des débits de pompage domine à proximité du champ de captage. On montre que l’effet de la montée du niveau de la mer est seulement significatif à terre au niveau le plus profond de la zone de transition. La variation spatiale de l’impact dépend de la manière dont les conditions aux limites du modèle influencent l’écoulement d’eau douce en différents points du domaine modélisé. Ce travail indique que la montée du niveau de la mer a le plus faible effet (parmi les trois facteurs considérés) sur l’intrusion future d’eau de mer dans l’aquifère des grès de la région de Richibucto.ResumenPara investigar la importancia relativa del ascenso proyectado del nivel del mar, los efectos de cambios climáticos en la recarga, de la extracción de agua subterránea sobre la intrusión de agua de mar en un importante acuífero costero en Atlantic Canada, se desarrolló un modelo numérico tridimensional de flujo subterráneo dependiente de la densidad acoplado con transporte de soluto para la región de Richibucto de New Brunswick. El modelo se usó, con un enfoque factorial eficiente de diseño 2k, para realizar simulaciones para el período 2011–2100. Los resultados del análisis factorial indican que la importancia relativa de los tres factores investigados varía en función de la ubicación considerada del modelo. Los efectos de la declinación de la recarga es más significativo en profundidades intermedias y someras a lo largo de la zona de transición de agua dulce–agua de mar, mientras que el efecto de ritmos crecientes de bombeo dominan en una ubicación relativamente cercana al campo de pozos. El efecto de ascenso del nivel del mar se muestra significativo solamente en el extremo más profundo tierra adentro de la zona de transición. La variación espacial de la importancia está relacionada a como las diferentes condiciones de contorno de los modelos influyen en el flujo de agua dulce en los diferentes sitios dentro del dominio del modelo. Esta investigación indica que el ascenso del nivel del mar tiene el menor efecto significativo (de los tres factores considerados) en la futura intrusión de agua de agua los en acuíferos de areniscas en la región de Richibucto.ResumoPara investigar a importância relativa da projetada subida do nível do mar, dos efeitos das alterações climáticas na recarga e a extração de água subterrânea na intrusão salina em importantes aquíferos costeiros do Canadá atlântico, foi desenvolvido para a região de Richibucto, em New Brunswick, um modelo numérico tridimensional de fluxo de água subterrânea dependente da densidade e acoplado a transporte de solutos. O modelo foi usado, com uma abordagem fatorial 2k eficiente, para realizar simulações para o período 2011–2100. Os resultados da análise fatorial indicam que a importância relativa dos três fatores investigados varia na dependência da localização considerada no modelo. O efeito da diminuição da recarga é mais significativo nas baixas a intermédias profundidades ao longo da zona de transição água doce–água salgada, enquanto que o efeito do incremento das taxas de bombeamento dominam numa localização relativamente perto do campo de captações. Mostra-se que o efeito da subida do nível do mar só é significativo no muito mais profundo pé da zona de transição, no lado terrestre. A variação espacial da importância está relacionada com a forma como as diferentes condições de fronteira do modelo influenciam o fluxo de água doce em diferentes locais dentro do domínio do modelo. Esta investigação indica que a subida do nível do mar tem o menor efeito significativo (dos três fatores considerados) na futura intrusão salina nos aquíferos de arenito na região de Richibucto.

The importance of conceptual models in the reactive transport simulation of oxygen ingress in sparsely fractured crystalline rock

Redox evolution in sparsely fractured crystalline rocks is a key, and largely unresolved, issue when assessing the geochemical suitability of deep geological repositories for nuclear waste. Redox zonation created by the influx of oxygenated waters has previously been simulated using reactive transport models that have incorporated a variety of processes, resulting in predictions for the depth of oxygen penetration that may vary greatly. An assessment and direct comparison of the various underlying conceptual models are therefore needed. In this work a reactive transport model that considers multiple processes in an integrated manner is used to investigate the ingress of oxygen for both single fracture and fracture zone scenarios. It is shown that the depth of dissolved oxygen migration is greatly influenced by the a priori assumptions that are made in the conceptual models. For example, the ability of oxygen to access and react with minerals in the rock matrix may be of paramount importance for single fracture conceptual models. For fracture zone systems, the abundance and reactivity of minerals within the fractures and thin matrix slabs between the fractures appear to provide key controls on O(2) attenuation. The findings point to the need for improved understanding of the coupling between the key transport-reaction feedbacks to determine which conceptual models are most suitable and to provide guidance for which parameters should be targeted in field and laboratory investigations.

Parallelization of MIN3P-THCm: A high performance computational framework for subsurface flow and reactive transport simulation

Abstract This paper presents the development of ParMIN3P-THCm, a parallel version of the reactive transport code MIN3P-THCm, which can run efficiently on machines ranging from desktop PCs to supercomputers. Parallelization of ParMIN3P-THCm was achieved through the domain decomposition method based on the PETSc library. The code has been developed from the ground up for parallel scalability and has been tested for up to 768 processors with problem sizes up to 100 million unknowns, showing strong scalability in modeling large-scale reactive transport problems. The total speedup tends to be ideal and near linear up to 768 processors when the degrees of freedom per processor is larger than 8000–15,000, depending on the relative complexity of the reactive transport and flow problems. The improved code efficiency allows refining of the model discretization in both space and time and will facilitate 3D simulations that were impractical to carry out with the sequential version of MIN3P-THCm.

Numerical Simulation of Active Heat Injection and Anomalous Seepage near an Earth Dam-Concrete Interface

AbstractActive heat injection and temperature monitoring are methods that may have potential for the detection of anomalous seepage at the interface between earth dams and concrete structures. A finite-element modeling investigation was conducted to assess the feasibility of using such a seepage-detection approach. Anomalous seepage at the interface between a clay and concrete structure was simulated by increasing the hydraulic conductivity in a 3-m-thick zone and imposing a range of hydraulic gradients. The results indicate that the heat-injection location would need to be less than 1.0 m from the interface to ensure seepage-related temperature deviations of 1.0°C or greater. Provided the heat-injection location is within 1.0 m of the interface, it is concluded that the interpretation of temperature data collected prior to, during, and following heat injection should be a feasible method for identifying locations of anomalous seepage. A simulation with realistic dam geometry, including seasonally varying...