Abstract
Electrical conductivity (EC) and geochemical data were interpreted to determine the nature, origin and distribution of salinity in pore waters of sediments in a deltaic environment. The role of diffusion as a mechanism for transporting saline water within lower permeability prodelta and delta slope sediments is specifically investigated. Characteristic vertical salinity profiles at several different regions of the Fraser River Delta, British Columbia (Canada) are identified, including relatively shallow salinity zones in areas currently and historically near main river channels, and deeper salinity zones reaching up to 300 m depth in delta-front and inland areas. Comparison of salinity profiles with the results of a simple salt transport model suggests that diffusion may be a significant mechanism controlling the observed distribution of salinity in current or former estuarine areas of the delta.Density-effects were found not to be significant given the low permeability of the silt through which the salt is diffusing; however, in similar environments with higher permeability sediments, density effects may be significant. In inland and delta front areas, salinity extends to a considerable depth in the silts, beyond what would appear to be possible by diffusion alone, and points to a connate origin.
Resumé
La conductivité électrique (EC) et des données géochimiques ont été interprétées pour déterminer la nature, l’origine et la distribution de la salinité dans l’eau souterraine des sédiments d’un environnement deltaïque. Le rôle de la diffusion, en tant que mécanisme de transport de l’eau salée au travers des perméabilités plus faibles du prodelta et du delta, est plus particulièrement investiguée. Les profiles verticaux de salinité dans différentes parties du Delta de la Fraser, Colombie Britannique (Canada) ont été identifiés, incluant les zones de salinité de sub-surface proches des lits mineurs actuels et historiques, ainsi que les zones de salinité plus profonde (300 m) sur le front du delta et à l’intérieur des terres. La comparaison des profils de salinité et d’un simple modèle de transport de sels, suggère que la diffusion pourrait être un mécanisme significatif du contrôle de la salinité dans les zones des estuaires actuels ou passés du delta. Les effets de densité ne sont pas significatifs au regard de la faible conductivité hydraulique du silt à travers duquel le sel diffuse ; néanmoins, dans des environnements similaires et des sédiments perméables plus importants, les effets de la densité seraient plus importants. Dans les zones de front du delta et de l’intérieur des terres, la salinité s’étend à des profondeurs considérables dans les silts, au-delà d’un point qui ne serait justifié que par la diffusion ou la présence d’eaux connées.
Resumen
Se han interpretado datos de conductividad eléctrica y geoquímicos para determinar la naturaleza, origen y distribución de la salinidad en aguas intersticiales de sedimentos en un ambiente deltaico. Se investiga en detalle el papel de la difusión como un mecanismo para transportar agua salada en un ambiente prodelta de baja permeabilidad y sedimentos en ladera del delta. Se identifican perfiles verticales característicos de salinidad en varias regiones del Delta Rio Fraser, Columbia Británica (Canada) incluyendo zonas salinas relativamente someras en áreas que histórica y actualmente se ubican cerca de los canales fluviales principales, y zonas de salinidad más profunda que alcanzan hasta 300m de profundidad en áreas tierra adentro y en la parte frontal del delta. La comparación de los perfiles de salinidad con los resultados de un modelo simple de transporte de sales sugieren que la difusión puede ser un mecanismo significativo que controla la distribución observada de la salinidad en estuarios actuales o antiguos del delta. Se encontró que los efectos de la densidad no son significativos dada la baja permeabilidad del limo a través del cual se mueve la sal; sin embargo, en ambientes similares con sedimentos de mayor permeabilidad los efectos de la densidad si pueden ser significativos. Tierra adentro y en los frentes deltaicos la salinidad se extiende a considerable profundidad en los limos, más allá de lo que pudiera ser factible en base al mecanismo de difusión por sí solo, lo que indica un origen connato.
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Bridger, D.W., Allen, D.M. An investigation into the effects of diffusion on salinity distribution beneath the Fraser River Delta, Canada. Hydrogeol J 14, 1423–1442 (2006). https://doi.org/10.1007/s10040-006-0060-1
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DOI: https://doi.org/10.1007/s10040-006-0060-1