NDLI: Free convective controls on sequestration of salts into low-permeability strata: insights from sand tank laboratory experiments and numerical modelling

Content Provider	Springer Nature Link
Author	Post, Vincent E. A. Simmons, Craig T.
Copyright Year	2009
Abstract	Using sand tank experiments and numerical models, local-scale solute-transport processes associated with free convection in both the region surrounding as well as within discrete low-permeability strata are explored. Different permeability geometries and contrasts between high- and low-permeability regions are tested. Results show that two free convective processes occur at different spatial and temporal scales. In the high-permeability region, salinisation was rapid and occurred predominantly by free convective flow around the low-permeability blocks (interlayer convection). A free convection flow field also became concurrently established within the low-permeability lenses (intralayer convection). It was found that upward vertical flow created by the large-scale interlayer free-convective flow field in the high-permeability region retards salinisation of the lenses as buoyant freshwater displacements oppose the downward penetration of dissolved salts. Salinisation of the low-permeability structures eventually takes place from below as saltwater is dragged upwards. This bottom up convective salinisation process of low permeability strata has not been reported in previous literature. These results demonstrate that variable-density sequestration of solutes driven by a source resident above the low-permeability layer is a complicated function of the geometry of the permeability distribution and the permeability contrast between low- and high-permeability regions.A l’aide d’une cuve à sable et de modèles numériques, on a étudié à l’échelle locale les processus d’écoulement d’une saumure percolant librement dans des couches multiples de faible perméabilité et dans le domaine encaissant. On a testé différentes répartitions et contrastes entre hautes et basses perméabilités. Les résultats montrent que deux processus de convection libre ont lieu dans l’espace et dans le temps. La salinisation est rapide dans le domaine de grande perméabilité et se produit principalement par flux convectif libre autour des lentilles de faible perméabilité (convection transversale). Il s’établit simultanément un flux d’écoulement à l’intérieur des lentilles de faible perméabilité (convection longitudinale). On a découvert que le flux vertical ascendant créé par le flux convectif libre dans le domaine de grande perméabilité retarde la salinisation des lentilles de faible perméabilité, car le flux ascendant de la saumure s’oppose à la migration descendante des sels dissous. Finalement, la salinisation active des horizons peu perméables a lieu par convection lorsque la saumure est mobilisée de bas en haut. Ce processus de salinisation des niveaux peu perméables par convection de bas en haut n’avait pas été signalé jusqu’à présent dans la littérature. Ces résultats montrent que la densité de séquestration de saumures provenant d’une source localisée au-dessus du niveau de faible perméabilité est une fonction complexe dépendant de la distribution des perméabilités et des contrastes de perméabilité entre les différents domaines.Se exploraron los procesos de transporte de soluto, a escala local, asociado con la convección libre, tanto en la región circundante como dentro de estratos discretos de baja permeabilidad, usando experimentos en tanquesde arena y modelos numéricos. Se testearon diferentes geometrías y contrastes de la permeabilidad entre regiones de alta y baja permeabilidad. Los resultados muestran que los procesos convectivos libres ocurren en diferentes escalas espaciales y temporales. En la región de alta permeabilidad, la salinización fue rápida y ocurrió predominantemente por flujo convectivo libre alrededor de bloques de baja permeabilidad (convección intercapa). Un campo de flujo convectivo libre también se estableció concurrentemente dentro de lentes de baja permeabilidad (convección intracapa). Se encontró que el flujo vertical ascendente creado por el campo de flujo convectivo libre a gran escala en la región de alta permeabilidad retarda la salinización de los lentes porque los desplazamientos de agua subterránea flotante se oponen a la penetración descendente de sales disueltas. La salinización de las estructuras de baja permeabilidad finalmente ocurre desde abajo a medida que el agua salada es arrastrada hacia arriba. El proceso de salinización convectiva del fondo hacia arriba de los estratos de baja permeabilidad no ha sido informado en la literatura previa. Estos resultados demuestran que el secuestro de densidad variable de solutos forzado por una fuente residente por encima de la capa de baja permeabilidad es una función complicada de la geometría de la distribución de la permeabilidad y del contraste de las permeabilidades entre las regiones de alta y baja permeabilidad.利用砂槽实验和数值模型研究了与低渗透性地层及其周边区域的自由对流有关的局部尺度的溶质运移过程。根据不同的渗透性几何结构以及高渗和低渗区的比较进行了各种实验。结果表明, 在不同的时空尺度上发生了两种自由对流过程。在高渗区, 盐化作用很快且主要通过低渗区 (夹层对流) 周围的自由对流发生。在低渗透镜体 (夹层对流) 中也同时形成了自由对流场。由高渗区大尺度夹层自由对流场形成的向上垂向流, 阻碍了透镜体的盐化作用, 因为漂浮淡水体的驱替阻止了溶解盐向下渗透。低渗区的盐化作用最终因盐水被向上牵引而从下面发生。这种低渗地层中自下而上的对流盐化过程在以前的文献中还未曾出现过。这些结果显示, 为低渗层之上的盐源驱使的溶质变密度捕获是渗透性几何分布以及高渗区和低渗区之间渗透性差异的复杂函数。São explorados os resultados dos processos de transporte de soluto a escala local associados a convecção independente em zonas circundantes bem como em estratos de baixa permeabilidade, usando experiências em tanques de areia e modelos numéricos. São testadas diferentes geometrias de permeabilidade e de contraste entre zonas de alta e baixa permeabilidade. Os resultados obtidos mostram a ocorrência de dois processos convectivos independentes que ocorrem a diferentes escalas espaciais e temporais. Nas regiões de alta permeabilidade a salinização foi rápida e ocorreu predominantemente por fluxo de convecção independente na vizinhança dos blocos de baixa permeabilidade (convecção intercamadas). Um fluxo convectivo independente foi também identificado no interior das áreas de baixa permeabilidade (convecção intracamadas). Verificou-se que o fluxo ascendente criado pelo campo de fluxo convectivo independente intercamadas de grande escala nas áreas de elevada permeabilidade retarda a salinização das massas lenticulares de água doce, uma vez que estas ascendem devido à sua menor densidade, opondo-se ao processo de afundamento dos sais dissolvidos. A salinização das estruturas de baixa permeabilidade tem eventualmente lugar de baixo para cima, uma vez que a água salgada é arrastada de forma ascendente. Este processo de salinização convectiva ascendente de estratos de baixa permeabilidade não foi referenciado na bibliografia anteriormente ao presente trabalho. Os resultados obtidos demonstram que esta captura de solutos dependente da variação de densidade, provocada por uma fonte sobreposta à camada de baixa permeabilidade, é uma função complexa da geometria da distribuição de permeabilidade e do contraste de permeabilidade entre as zonas de baixa e alta permeabilidade.
Starting Page	39
Ending Page	54
Page Count	16
File Format	PDF
ISSN	14312174
Journal	Hydrogeology Journal
Volume Number	18
Issue Number	1
e-ISSN	14350157
Language	Portuguese
Publisher	Springer-Verlag
Publisher Date	2009-11-04
Publisher Institution	International Association of Hydrogeologists
Publisher Place	Berlin, Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Salinization Groundwater density/viscosity Salt-water/fresh-water relations Laboratory experiments Numerical modelling Waste Water Technology Water Pollution Control Water Management Aquatic Pollution Geology Hydrogeology
Content Type	Text
Resource Type	Article
Subject	Earth and Planetary Sciences Water Science and Technology

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