Andrew Frampton

Permafrost degradation and subsurface-flow changes caused by surface warming trends

Change dynamics of permafrost thaw, and associated changes in subsurface flow and seepage into surface water, are analysed for different warming trends in soil temperature at the ground surface with a three-phase two-component flow system coupled to heat transport. Changes in annual, seasonal and extreme flows are analysed for three warming-temperature trends, representing simplified climate-change scenarios. The results support previous studies of reduced temporal variability of groundwater flow across all investigated trends. Decreased intra-annual flow variability may thus serve as an early indicator of permafrost degradation before longer-term changes in mean flows are notable. This is advantageous since hydrological data are considerably easier to obtain, may be available in longer time series, and generally reflect larger-scale conditions than direct permafrost observations. The results further show that permafrost degradation first leads to increasing water discharge, which then decreases as the permafrost degradation progresses further to total thaw. The most pronounced changes occur for minimum annual flows. The configuration considered represents subsurface discharge from a generic heterogeneous soil-type domain.RésuméLe changement de dynamiques du dégel du permafrost et les changements associés des écoulements souterrains et des exfiltration vers les eaux de surface sont analysés pour différentes tendances de réchauffement de la température du sol à la surface selon un système d’écoulement triphasé à deux composantes couplé au transport de la chaleur. Les changements des écoulements annuels, saisonniers et extrêmes sont analysés pour trois tendances d’élévation de la température représentant des scénarios simplifiés de changement climatique. Les résultats sont cohérents avec des études antérieures concernant la réduction de la variabilité temporelle des écoulements souterrains pour toutes les tendances simulées. La diminution de la variabilité des écoulements inter annuels peut ainsi servir d’indicateur précoce de la dégradation du permafrost avant que l’effet des changements sur le long terme soit notable sur les écoulements moyens. Ceci représente un avantage du fait que les données hydrologiques sont beaucoup plus faciles à obtenir, peuvent être disponibles pour de longues séries temporelles et reflètent généralement des évolutions à plus grande échelle que des observations directes sur le permafrost. Les résultats de plus montrent que la dégradation du permafrost conduit d’abord à une augmentation des débits dans les zones de décharge, puis à leur diminution lors de la progression de la dégradation du permafrost jusqu’au dégel complet. Les modifications les plus marquées se produisent pour les écoulements annuels minimums. La configuration considérée concerne le débit d’écoulement souterrain dans les zones de décharge pour un sol type générique hétérogène.ResumenSe analiza la dinámica del cambio en el descongelamiento del permafrost, y los cambios asociados al flujo subsuperficial y a la infiltración del agua superficial, para diferentes tendencias de calentamiento en la temperatura de los suelos en la superficie del terreno con tres fases, dos componentes del sistema de flujo acoplados con el transporte de calor. Los cambios en los flujos anual, estacional y extremos son analizados para tres tendencias de las temperaturas de calentamiento representando escenarios simplificados de cambios climáticos. Los resultados se apoyan en estudios previos de la reducida variabilidad temporal del flujo de agua subterránea a través de todas las tendencias investigadas. La disminución de la variabilidad del flujo interanual puede así servir como un indicador temprano de la degradación del permafrost antes que los cambios a largo plazo en los flujos medios sean notables. Esto es ventajoso ya que los datos hidrológicos son considerablemente más fáciles de obtener, pueden estar disponibles en series de tiempo más largas, y generalmente reflejan condiciones de una escala mayor que las observaciones directas del permafrost. Los resultados además muestran que la degradación del permafrost primero lleva a un aumento en la descarga del agua, que luego disminuye cuando la degradación del permafrost progresa más allá del descongelamiento total. Los cambios más pronunciados ocurren para los flujos mínimos anuales. La configuración considerada representa la descarga subsuperficial a partir de un dominio genérico de un suelo de tipo heterogéneo.摘要本文通过耦合热交换三相、二组份流动系统探讨地表面土壤温度不同增温条件下永久冻土消融,以及与之相关的地下径流改变及地表水渗漏的变化动力学。将气候变化场景简化为三种不同的增温趋势,分析多年、季节性以及极端的径流变化。所有研究趋势下的结果均支持地下水径流时间变化减少这一先前研究。因此在年均径流长期变化量显著前,年内径流变化量减少可作为永久冻土退化的一个早期信号。因为水文数据相对容易获得,可能存在长时间序列,比起直接的冻土测量,通常更能反映大规模的条件,因此更为有利。结果进一步表明,永久冻土退化首先导致排泄量增加,然后随着永久冻土进一步解冻,排泄量减少。最显著的变化发生在年均径流量最小的时候。考虑的结构代表了来自各向异性土壤类型主导的地下排泄区。ResumoA mudança da dinâmica do degelo no permafrost e as correspondentes alterações no escoamento subsuperficial e na drenagem para as águas superficiais são analisadas para diferentes tendências de aquecimento da temperatura do solo à superfície do terreno com um sistema de fluxo, acoplado a transporte de calor, para três fases e duas componentes. São analisadas as mudanças dos escoamentos anuais, sazonais e extremos para três tendências de aumento de temperaturas, representando cenários simplificados de alterações climáticas. Os resultados apoiam estudos anteriores que mostram a reduzida variabilidade temporal do escoamento de água, atravessando todas as tendências investigadas. A diminuição da variabilidade do escoamento intra-anual pode, portanto, servir como um indicador precoce da degradação do permafrost, antes de se notarem mudanças nos escoamentos médios a prazos mais longos. Isto é vantajoso uma vez que os dados hidrológicos são consideravelmente mais fáceis de obter, podendo estar disponíveis séries temporais mais longas e, em geral, refletindo uma maior escala temporal que as condições de observação direta do permafrost. Os resultados mostram ainda que a primeira degradação do permafrost leva ao aumento da descarga de água, a qual, em seguida, diminui à medida que a degradação do permafrost progride mais para o descongelamento total. As mudanças mais acentuadas ocorrem para os escoamentos anuais mínimos. A configuração considerada representa a descarga subsuperficial proveniente de um domínio genérico de solo heterogéneo.

Transport and retention from single to multiple fractures in crystalline rock at Äspö (Sweden): 2. Fracture network simulations and generic retention model.

Hydrogeologic characterization of crystalline rock formations on the field scale is important for many applications but still presents a multitude of challenges [Neuman, 2005]. In this work we use ...

Upscaling particle transport in discrete fracture networks: 2. Reactive tracers

Understanding groundwater flow systems and how these control transport is an essential part in assessing the suitability of subsurface environments as hosts for storage of toxic waste. Therefore it is important to be able to integrate knowledge obtained from field characterisation of the subsurface with methods which can be used to evaluate and predict possible impact on surrounding environments.In this thesis I investigate the characteristics of flow and transport in discrete fracture networks by analysing Eulerian and Lagrangian descriptions within a stochastic framework. The analysis is conducted through numerical flow and transport simulations configured according to available field data, combined with independent theoretical analytic and semi-analytic methods which are able to reveal insight to relevant constitutive properties. It is shown that numerical simulations conducted with the discrete fracture network approach can be both conditioned and confirmed against field measurable quantities, and the developed theoretical methods are evaluated against results obtained from simulation. Thereby, a methodology which can provide links between field measurable quantities and tracer discharge is presented, developed and evaluated. It is shown to be robust with respect to underlying assumptions used for flow configurations.In particular, a specific sampling algorithm for obtaining a Lagrangian description of transport based on a Eulerian description of flow is proposed, evaluated and shown to be robust for the cases considered, providing accurate replications. Also a generalisation of both the advection-dispersion solution and the one-sided stable distribution is shown to be able to evaluate advective transport quantities, and combined with a Lagrangian retention model it is shown to be a fairly accurate and robust method for upscaling distributions, enabling predictions of transport in terms of tracer discharge. Evaluation of transport is also conducted against the advective-dispersion assumption, where results indicate advective transport is generally non-Fickian for the fracture networks and domain scales considered, but not necessarily anomalous. Additionally, the impact certain model assumptions have on tracer discharge are analysed. For example, transport is evaluated for assumptions regarding injection mode, fracture network heterogeneity, relationship between aperture and transmissivity, relationship between transmissivity and size, as well as scale and modelling dimension. In relation to hydraulic testing and flow analysis, a method for conditioning fracture transmissivity from field measurements of flow by simulation is developed and evaluated against homogenisation assumptions commonly used in field applications. Results indicate the homogenisation assumption generally fails for current interpretations of field data.

Inference of field-scale fracture transmissivities in crystalline rock using flow log measurements

Inference of field-scale fracture transmissivities in crystalline rock using flow log measurements

Solute transport and retention in three‐dimensional fracture networks

Resolving the hydrodynamic control of retention is an important step in predictive modeling of transport of sorbing tracers in fractured rock. The statistics of the transport resistance parameter beta [T/L] and the related effective active specific surface area s(f) [1/L] are studied in a crystalline rock volume on a 100 m scale. Groundwater flow and advective transport are based on generic boundary conditions and realistic discrete fracture networks inferred from the Laxemar site, southeast Sweden. The overall statistics of beta are consistent with statistics of the water residence time tau; the moments of beta vary linearly with distance, at least up to 100 m. The correlation between log tau and log beta is predominantly linear, however, there is significant dispersion; the parameter s(f) strongly depends on the assumed hydraulic law (theoretical cubic or empirical quadratic). Fast and slow trajectories/segments in the network determine the shape of the beta distribution that cannot be reproduced by infinitely divisible model over the entire range; the low value range and median can be reproduced reasonably well with the tempered one-sided stable density using the exponent in the range 0.35-0.7. The low percentiles of the beta distribution seems to converge to a Fickian type of behavior from a 50 to 100 m scale.

An indirect assessment on the impact of connectivity of conductivity classes upon longitudinal asymptotic macrodispersivity

Solute transport takes place in heterogeneous porous formations, with the log conductivity, Y = ln K, modeled as a stationary random space function of given univariate normal probability density fu ...

Thermal effects of groundwater flow through subarctic fens: A case study based on field observations and numerical modeling

Modeling and observation of ground temperature dynamics are the main tools for understanding current permafrost thermal regimes and projecting future thaw. Until recently, most studies on permafrost have focused on vertical ground heat fluxes. Groundwater can transport heat in both lateral and vertical directions but its influence on ground temperatures at local scales in permafrost environments is not well understood. In this study we combine field observations from a subarctic fen in the sporadic permafrost zone with numerical simulations of coupled water and thermal fluxes. At the Tavvavuoma study site in northern Sweden, ground temperature profiles and groundwater levels were observed in boreholes. These observations were used to set up one- and two-dimensional simulations down to 2 m depth across a gradient of permafrost conditions within and surrounding the fen. Two-dimensional scenarios representing the fen under various hydraulic gradients were developed to quantify the influence of groundwater flow on ground temperature. Our observations suggest that lateral groundwater flow significantly affects ground temperatures. This is corroborated by modeling results that show seasonal ground ice melts 1 month earlier when a lateral groundwater flux is present. Further, although the thermal regime may be dominated by vertically conducted heat fluxes during most of the year, isolated high groundwater flow rate events such as the spring freshet are potentially important for ground temperatures. As sporadic permafrost environments often contain substantial portions of unfrozen ground with active groundwater flow paths, knowledge of this heat transport mechanism is important for understanding permafrost dynamics in these environments.

Impact of degrading permafrost on subsurface solute transport pathways and travel times

Subsurface solute transport under surface warming and degrading permafrost conditions is studied using a physically based model of coupled cryotic and hydrogeological flow processes combined with a particle tracking method. Changes in the subsurface water and inert solute pathways and travel times are analyzed for different modeled geological configurations. For all simulated cases, the minimum and mean travel times increase nonlinearly with warming irrespective of geological configuration and heterogeneity structure. The timing of the start of increase in travel time depends on heterogeneity structure, combined with the rate of permafrost degradation that also depends on material thermal and hydrogeological properties. The travel time changes depend on combined warming effects of: i) increase in pathway length due to deepening of the active layer, ii) reduced transport velocities due to a shift from horizontal saturated groundwater flow near the surface to vertical water percolation deeper into the subsurface, and iii) pathway length increase and temporary immobilization caused by cryosuction-induced seasonal freeze cycles.

Experimental and theoretical study of the spread of fluid from a point source on an inclined incontinence bed-pad

Abstract The spread of fluid from a localized source on to a flat fibrous sheet is studied. The sheet is inclined at an angle, α, to the horizontal, and the areal flux of the fluid released is Q a. A new experimental study is described where the dimensions of the wetted region are measured as a function of time t, Q a and α (>0). The down-slope length, Y, grows according to Y ∼ (Q a t )2/3(sin α)1/3; for high discharge rates and low angles of inclination, the cross-slope width, X, grows as ∼ (Q a t )1/2, while for low discharge rates or high angles of inclination, the cross-slope transport is dominated by infiltration and X ∼ 2(2K sΦ∗t)1/2, where K s is the saturated permeability and Φ∗ is the characteristic value of capillary pressure. A scaling analysis of the underlying non-linear advection diffusion equation describing the infiltration process confirms many of the salient features of the flow observed. Good agreement is observed between the collapse of the numerical solutions and experimental results. The broader implications of these results for incontinence bed-pad research are briefly discussed.

MODELING TWO-PHASE-FLOW INTERACTIONS ACROSS A BENTONITE CLAY AND FRACTURED ROCK INTERFACE

Abstract Deep geological repositories are generally considered as suitable environments for final disposal of spent nuclear fuel. In the Swedish and Finnish repository design concept, canisters are to be placed in deep underground tunnels in sparsely fractured crystalline bedrock, in deposition holes in which each canister is embedded with an expansive bentonite-clay-mixture buffer. A set of semigeneric two-dimensional radially symmetric TOUGH2 simulations are conducted to investigate the multiphase dynamics and interactions between water and air in a bentonite-rock environment. The main objective is to identify how sensitive saturation times of bentonite are to the geometry of the rock fractures and to commonly adopted simplifications in the unsaturated flow description such as Richards assumptions. Results show that the location of the intersection between the fracture system and the deposition hole is a key factor affecting saturation times. A potential long-lasting desaturation of the rock matrix close to the bentonite-rock interface is also identified extending up to 10 cm inside the rock. Two-phase-flow models predict systematically longer saturation times compared to a simplified Richards approximation, which is frequently used to represent unsaturated flows. The discrepancy diverges considerably as full saturation is approached.