An investigation into the effects of diffusion on salinity distribution beneath the Fraser River Delta, Canada

Bridger, David W.; Allen, D. M.

doi:10.1007/s10040-006-0060-1

Cited by 11 publications

(5 citation statements)

References 23 publications

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“…Studies of the expansion and contraction in coastal groundwater systems, in response to the Quaternary changes in global sea level, have so far mainly been focused on the mode of seawater intrusion into existing geologic layers with variable permeability. Where the sea floods high-permeability layers, a relatively fast horizontal and vertical downward salinization will occur [27][28][29] , whereas salinization of low-permeability muds and clays will be slow and dominated by diffusion 24,30 . We infer that another important salinity source in delta systems is the re-distribution of trapped saline water in marine aquitards.…”

Section: Broader Impactsmentioning

confidence: 99%

Groundwater salinity influenced by Holocene seawater trapped in incised valleys in the Red River delta plain

et al. 2017

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Salty and brackish groundwater has been observed at least 100 km inland in some aquifers contained within Quaternary delta plains. This phenomenon limits access to fresh groundwater resources, particularly in the densely populated deltas of Southeast Asia. However, the causes of inland salinity are unclear. Here we present borehole and geophysical data that show that in the Red River delta plain of Vietnam, salty and brackish groundwater primarily occurs in incised valleys that were formed during sea-level lowstands during the Pleistocene. During the mid-Holocene, these valleys were filled with fine-grained marine deposits containing trapped seawater. We conduct groundwater flow simulations that show that the age, thickness, and permeability of the marine sediments are the primary controls on the leaching of salty porewater into the freshwater aquifer. We find that salty groundwater originating from this trapped seawater is still present in Holocene-aged sediments with low permeability, and a ects groundwater salinity in adjacent aquifers. In contrast, trapped seawater from all Pleistocene-aged sediments has been leached. We identify a number of brackish to saline delta aquifers elsewhere in Asia and throughout the world that have a similar sedimentary history, and thus are likely to be influenced by this leaching process.T he coastal zone can be defined to encompass the continental shelf (water depth <200 m), and the adjacent 100 km of land from the coastline 1 , which hosts an estimated 20-40% of the global population and 20 of the world's 33 megacities 1,2 . The stresses on water resources in the densely populated coastal zones are thus high, and are likely to be compounded by future sea-level rise, although the impact of this on the groundwater resource is still debated 3 . Groundwater constitutes a significant proportion of the water resources in the coastal zone, but is frequently salty or brackish due to mixing with recent or palaeo-seawater 4 . Therefore, a better hydrogeological understanding of the processes controlling the distribution of fresh, brackish and saltwater in the coastal zone is warranted for the utilization of deltaic water resources.Processes resulting in high-salinity groundwater in coastal aquifers are: recent natural or pumped induced saltwater intrusion 5-7 , salinization due to intrusion from rivers by surfacegroundwater interactions 8-11 and the occurrence of palaeo-saltwater, generated under climatic and hydraulic conditions prevailing during the Quaternary period 12 .The ∼100 kyr Milankovitch cycles have during the Quaternary period generated large ice caps on continents and led to a sealevel lowerings of 120 to 140 m below the present level 13,14 . These sea-level lowerings have caused the erosion of valley structures into Pleistocene delta plain sediments 15 , which during Holocene transgressions were filled up with marine sediments 16,17 . The valleyfill sediments tend to constitute aquifers when they are coarsegrained 16 , and aquitards with salty porewater when they are fineg...

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Section: Broader Impactsmentioning

confidence: 99%

Groundwater salinity influenced by Holocene seawater trapped in incised valleys in the Red River delta plain

et al. 2017

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“…Many previous groundwater modeling studies focusing on coastal deltas have used 1D or 2D models that evaluate vertical groundwater flow through a column or along a longitudinal cross‐section of the delta (Bridger & Allen, 2006; Delsman et al., 2014; Larsen et al., 2017; Van Pham et al., 2019). The lack of spatially varying lateral heterogeneity and anisotropy within deltas results in a limited understanding of how permeability structures vary across the delta plain and how this impacts horizontal groundwater flow through a delta.…”

Section: Discussionmentioning

confidence: 99%

Sensitivity of Subsurface Permeability in Coastal Deltas to Their Morphodynamic and Geomorphic Characteristics

Anderson,

Allen,

Venditti

2023

Water Resources Research

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The amount of fresh water moving through coastal deltas worldwide is controlled by the complex subsurface structures within a delta. Morphodynamic influences produced by the feeding river, waves, and tides, in addition to sea level transgressions and regressions, have resulted in deltaic aquifers that are highly heterogeneous. We use 171 unique two‐dimensional morphodynamic models to explore the range of subsurface permeability, hydraulic gradient, and groundwater flux within three end‐member delta types (fluvial, wave, and tidal). We quantify the connectiveness of the subsurface permeability and estimate the horizontal heterogeneity and anisotropy of the permeability. A distance‐based generalized sensitivity analysis is used to investigate the impact morphodynamic influences (fluvial, wave, and tidal), basin conditions (sediment concentration and bathymetric gradient), and geomorphic characteristics (number of channels, shape of the delta plain, and shoreline rugosity) have on the subsurface permeability, hydraulic gradient, and connectivity. We find that the median permeability in deltaic landforms is 4.0 × 10−12 m2 (relating to a hydraulic conductivity of 2.1 × 10−5 m/s), the average hydraulic gradient is 3.9 × 10−4, and the mean specific discharge is 1.3 × 10−8 m/s. High permeability bodies are highly connected and are associated with channelization. Subsurface permeability, hydraulic gradient, and the connectiveness of high permeability areas are sensitive to morphodynamic influences (fluvial, wave, and tidal) and the geomorphic characteristics (number of channels and shoreline rugosity) within a delta. Since morphodynamic influences and geomorphic characteristics are easily identified by looking at the surface of the delta, we suggest that the deltaic subsurface can be characterized by identifying features on the delta surface.

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“…Examples can be found in the literature of saltwater intrusion into coastal aquifers of Canada, and of these cases described, several relate to intrusion that occurs naturally (Carr 1969;Carr and van der Kamp 1969;van der Kamp 1981;Dakin et al 1983;Neilson-Welch and Smith 2001;Bridger and Allen 2006;Allen and Suchy 2001). An exception is the intrusion of saltwater into the sandstone aquifers of Prince Edward Island (Tremblay et al 1973) and nearby Magdalen Islands (Comte and Banton 2006), both of which are caused by groundwater pumping.…”

Section: Sensitive Areas In Canadamentioning

confidence: 94%

Submarine groundwater discharge (SGWD): an unseen yet potentially important coastal phenomenon in Canada

2011

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Both terrestrial and fresh or marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows from confined aquifers further out on the shelf. The terrestrial and oceanic forces overlap spatially, so measured fluid advection through coastal sediments may be a result of composite forcing. ''Subsurface/submarine groundwater discharge'' (SGWD) can be defined as any and all flow of water on continental margins from the seabed to the coast, regardless of fluid composition or driving force. SGWD is typically characterized by low specific flow rates, making detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may substantially increase the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coast. Terrestrially derived fluids represent a pathway for the flux of new material to the coast. This may result in diffuse pollution in areas where the discharge of contaminated groundwater occurs. This review presents an historical context of SGWD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. This review concludes that, to fully characterize marine geochemical budgets, one must give due consideration to SGWD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGWD, pollutants to evaluate these fluxes more precisely to Canadian coast.

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An investigation into the effects of diffusion on salinity distribution beneath the Fraser River Delta, Canada

Cited by 11 publications

References 23 publications

Groundwater salinity influenced by Holocene seawater trapped in incised valleys in the Red River delta plain

Groundwater salinity influenced by Holocene seawater trapped in incised valleys in the Red River delta plain

Sensitivity of Subsurface Permeability in Coastal Deltas to Their Morphodynamic and Geomorphic Characteristics

Submarine groundwater discharge (SGWD): an unseen yet potentially important coastal phenomenon in Canada

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