G. van der Kamp

Use of vertical profiles of δ18O to constrain estimates of hydraulic conductivity in a thick, unfractured aquitard

Vertical distributions of δ18O were used to constrain hydraulic conductivity (K) in a thick, unfractured aquitard. Previous investigations utilizing permeability tests at a variety of scales [Keller et al., 1989] indicated that bulk K at the site ranges from 3.5× 0−11 m/s to 1.2×10−10 m/s. Advective displacements calculated on the basis of these K values lead to a wide range of predicted distributions of natural tracers after 11,000 years. Comparison of δ18O distributions and simulated advection-diffusion profiles provide a more precise constraint on advective displacement and hence on K. The simulations, cast in terms of a Peclet number (P = V2T/D*) ranging in value from 0 to 12, indicate that transport is diffusion controlled and that K is actually less than 1.2×10−11 m/s. P greater than 0.1 (K > 1.2×10−11 m/s) overestimates transport. This result indicates that conventional permeability tests in very tight media tend to overestimate K. In practical terms, advective transport at the site is negligible, and transport can be simulated with diffusion only (P = O).

The Radial Diffusion Method: 1. Using intact cores to determine isotopic composition, chemistry, and effective porosities for groundwater in aquitards

A new method is described for the determination of the isotopic composition and chemistry of groundwater in aquitards. The effective porosities for each component can be determined concurrently. The method is based on diffusive exchange of water and solutes between groundwater in a cylindrical core of aquitard material and water in a reservoir placed along the axis of the cylinder. The method was tested for 2H, Cl, and SO4in an aquitard consisting of clay-rich glacial till. The δ2H in aquitard core samples as determined by radial diffusion agreed closely with δ2H in groundwater obtained from corresponding piezometers installed in the aquitard. For the concentrations of Cl and SO4 the agreement was less close but still satisfactory. The effective porosities were equal to the total porosities for 2H and were less than the total porosities for Cl and SO4 ions.

Hydrogeochemistry of a Clayey Till: 1. Spatial Variability

The spatial variability of hydrogeochemical processes was studied within an 18-m-thick unit of clayey till beneath a flat prairie landscape. Models of major element mass transfer, constrained by chemical, mineralogical, and isotopic measurements on waters and solids, were developed for two end-member water quality types occurring within the unit. The observed pattern of chemical variation, which is predominantly lateral on a scale of tens of meters, is caused by microtopographically controlled differences in the ratio of vertical water flux to oxidation rate associated with depression-focused recharge. Such variation is to be expected where dynamic flow regimes develop in thin surficial tills containing chemically reduced constituents. Observed depletions of major elements from oxidized zones of the till support the mass transfer models and demonstrate that the time scales of persistence of hydrogeochemical variability in the till and of water quality evolution in the aquifer beneath range from 104 to 105 year. Na and S concentrations in such aquifers will depend primarily on ratios of influxes of end-member water quality types from the till above, while Ca, Mg, and inorganic C concentrations will remain high as long as partial pressures of CO2 in the till remain high. CO2 generation in the till is discussed in the companion paper (Keller, this issue).

The Radial Diffusion Method: 2. A semianalytical model for the determination of effective diffusion coefficients, porosity, and adsorption

A model for interpreting diffusional transport in porous geological materials is developed. The model is based on a laboratory method described in a companion paper [van der Kamp et al., this issue] by which radial diffusion from or into a cylindrical reservoir in a core-sized sample is measured. The model accounts for radial diffusion, mass balance in the reservoir, linear adsorption, decay or transformation, and periodic abstraction of samples. The model is derived using the Laplace transform method for both finite and semi-infinite domains. For conditions where solute concentrations equilibrate (i.e., in finite diameter samples), a simple expression is derived that can be used to interpret the results for effective porosity and a retardation factor. It is demonstrated that the method can provide independent measures of the effective diffusion coefficient, adsorption, and effective porosity when the results are interpreted using the model. Several real and hypothetical diffusion experiments are presented to illustrate the use of the model.

Monitoring of total soil moisture on a scale of hectares using groundwater piezometers

A method has been developed for continuous measurement of total soil moisture on a scale of hectares. The method is based on accurate measurement of groundwater pressures in thick clay formations and depends on the principle that changes of mechanical load result in changes of groundwater pressure. The resulting data are similar to those obtained by conventional weighing lysimeters, but on a much larger scale and with no significant hydrologic disturbance of the site. Several years of field testing show the method to be practicable.

Hydrogeological processes in seasonally frozen northern latitudes: understanding, gaps and challenges

The groundwater regime in seasonally frozen regions of the world exhibits distinct behavior. This paper presents an overview of flow and associated heat and solute transport processes in the subsurface, from the soil/vadose zone, through groundwater recharge to groundwater discharge processes in these areas. Theoretical developments, field studies and model development are considered. An illustrative conceptual model of the system is presented. From a groundwater perspective, the dominant effect is the extent of hydraulic isolation between the water above and that below the near-surface frozen zone. The spatial and temporal occurrences of this isolation are seasonally variable and may also be modified under a future changing climate. A good qualitative conceptual understanding of the system has been developed over numerous decades of study. A major gap is the inability to effectively monitor processes in the field, particularly unfrozen water content during freezing conditions. Modeling of field-scale behavior represents a major challenge, even while physically based models continue to improve. It is suggested that progress can be made by combining well-designed field experiments with modeling studies. A major motivation for improving quantification of these processes derives from the need to better predict the impacts of a future changing climate.RésuméLe régime de l’eau souterraine dans les régions gelées de façon saisonnière fait apparaître différents comportements. Cet article présente une vue d’ensemble du flux et des processus de transport de chaleur et de solutés en sub-surface, depuis la zone sol/vadose, à travers des processus de recharge-décharge dans ces zones. Des développements théoriques, études de terrain et développement de modèles sont considérés. Un modèle conceptuel est présenté pour illustrer. Si l’on considère l’eau de nappe, l’effet dominant est l’importance de l’isolation hydraulique entre l’eau sur et l’eau sous la sub-surface gelée. Dans le temps et dans l’espace cette isolation varie avec les saisons et pourra aussi être modifiée par un futur changement du climat. Une bonne compréhension conceptuelle qualitative du système a été développée durant de nombreuses décades d’étude. Une lacune majeure est l’impossibilité de contrôler effectivement le processus sur le terrain, particulièrement la teneur en eau libre durant les périodes de gel. La modélisation du comportement à l’échelle du terrain représente un défi majeur, même si les modèles à base physique continuent de s’améliorer. On suggère que des progrès peuvent être faits en combinant des expériences de terrain bien conçues et des études de modélisation. Une motivation majeure pour améliorer la quantification de ces processus dérive du besoin de mieux prévoir les impacts d’un futur changement climatique.ResumenEl régimen de agua subterránea en regiones estacionalmente congeladas del mundo exhibe comportamientos distintivos. Este trabajo presenta un panorama del flujo y los procesos asociados de transporte de calor y soluto en el subsuelo, a partir de la zona vadosa / suelo, a través de los procesos de recarga a la descarga del agua subterránea en estas áreas. Se consideran los desarrollos teóricos, los estudios de campo y los modelos desarrollados. Se presenta un modelo conceptual ilustrativo del sistema. Desde la perspectiva del agua subterránea, el efecto dominante es el grado de aislamiento entre el agua por encima y por debajo de la zona congelada próxima a la superficie. La presencia espacial y temporal de este aislamiento es variable estacionalmente y pueden también ser modificadas bajo un futuro cambio climático. Un buen entendimiento cualitativo conceptual del sistema se desarrolló durante numerosas décadas de estudio. Una dificultad mayor es la incapacidad para monitorear efectivamente los procesos en el campo, particularmente el contenido de agua no congelada durante las condiciones de congelamiento. El comportamiento modelado a escala de campo representa un desafío mayor, aun cuando los modelos de bases físicas pueden seguir mejorando. Se sugiere que se puede avanzar mediante la combinación de experimentos de campo bien diseñados con estudios de modelos. Una motivación mayor para mejorar la cuantificación de estos procesos deriva de la necesidad de predecir mejor los impactos de un futuro cambio climático.摘要世界上季节性冻土区的地下水动态呈现出不同的行为特征。本文对这些地区的地面以下,从土壤/渗流区通过地下水的补给到地下水的排泄过程的水流运动和相关的热、溶质运移过程作了一个概述。本文综合考虑了理论的发展,场地的研究和模型的发展。另外,文中还展示了一个解释说明性的系统概念模型。从地下水的角度来说,占优势的效应是近地表的冻土区上、下的地下水间的水力隔绝程度。时间、空间上这种隔绝的存在是随着季节变化的,可以在未来变化的气候下进行模拟。通过几十年的研究,对季节性冻土系统的较好的定性概念性认识已经建立起来了。一个主要的不足在于无法有效地监测场地的过程,特别是在严寒条件下不冻水的成分。甚至当基于物理的模型在继续向前发展时,场地尺度上的行为模拟仍是一个主要的挑战。文中指出,可以通过将设计精良的场地试验与模型研究相结合来获得突破。提高这些过程的定量化的主要动机来源于更好地对未来气候变化影响的预测的需求。ResumoO regime de águas subterrâneas em regiões do mundo sazonalmente geladas exibe comportamento distinto. Este artigo apresenta uma visão geral dos processos subsuperficiais de fluxo e de transporte de calor e de soluto associados, desde o solo/zona vadosa, através dos processos de recarga de águas subterrâneas, até à sua descarga. São considerados desenvolvimentos teóricos, estudos de campo e desenvolvimento de modelos. É apresentado um modelo concetual ilustrativo do sistema. Da perspetiva das águas subterrâneas, o efeito dominante é a extensão do isolamento hidráulico entre as águas acima e abaixo da zona gelada próxima da superfície. As ocorrências espaciais e temporais destes isolamentos são variáveis sazonalmente e podem também ser modificadas num clima futuro em mudança. Ao longo de numerosas décadas de estudo, desenvolveu-se uma boa compreensão concetual qualitativa do sistema. Uma grande lacuna é a incapacidade de monitorizar eficazmente os processos no terreno, particularmente o teor de água não solidificada durante condições de congelação. A modelação do comportamento à escala do terreno representa um desafio maior, mesmo enquanto os modelos físicos continuam a melhorar. Sugere-se que se pode conseguir uma boa progressão combinando experiências de terreno bem concebidas com estudos de modelação. Uma motivação principal para melhorar a quantificação destes processos deriva da necessidade de prever melhor os impactes de alterações climáticas futuras.

A multiscale approach to determine hydraulic conductivity in thick claystone aquitards using field, laboratory, and numerical modeling methods: DETERMINING HYDRAULIC CONDUCTIVITY IN CLAYSTONE AQUITARDS

Characterizing the hydraulic conductivity (K) of aquitards is difficult due to technical and logistical difficulties associated with field-based methods as well as the cost and challenge of collecting representative and competent core samples for laboratory analysis. The objective of this study was to produce a multiscale comparison of vertical and horizontal hydraulic conductivity (Kv and Kh, respectively) of a regionally extensive Cretaceous clay-rich aquitard in southern Saskatchewan. Ten vibrating wire pressure transducers were lowered into place at depths between 25 and 325 m, then the annular was space was filled with a cement-bentonite grout. The in situ Kh was estimated at the location of each transducer by simulating the early-time pore pressure measurements following setting of the grout using a 2-D axisymmetric, finite element, numerical model. Core samples were collected during drilling for conventional laboratory testing for Kv to compare with the transducer-determined in situ Kh. Results highlight the importance of scale and consideration of the presence of possible secondary features (e.g., fractures) in the aquitard. The proximity of the transducers to an active potash mine ( 1 km) where depressurization of an underlying aquifer resulted in drawdown through the aquitard provided a unique opportunity to model the current hydraulic head profile using both the Kh and Kv estimates. Results indicate that the transducer-determined Kh estimates would allow for the development of the current hydraulic head distribution, and that simulating the pore pressure recovery can be used to estimate moderately low in situ Kh (<10 211 m s).