Fresh- and salt-water intrusion: laboratory experiments and geochemical transport modelling
Beekman, H.E.; Appelo, C.A.J. (1989). Fresh- and salt-water intrusion: laboratory experiments and geochemical transport modelling. Natuurwet. Tijdschr. 70(1-4): 247-255
In: Natuurwetenschappelijk Tijdschrift. L. Walschot/Natuur- en Geneeskundige Vennootschap: Gent. ISSN 0770-1748, more
Also appears in:
De Breuck, W.; Walschot, L. (Ed.) (1989). Proceedings of the 10th Salt-Water Intrusion Meeting Ghent (Belgium), 16-20 May 1988. Natuurwetenschappelijk Tijdschrift, 70(1-4). Natuurwetenschappelijk Tijdschrift: Gent. 408 pp., more
Buffer reactions between sediment and groundwater occur if amounts of exchangeable cations are relatively high compared to concentrations in water. Characteristic concentrations of elements develop in both time and space during dispersive intrusion of fresh and salt water as a result of cation exchange, and may be used to derive paleohydrological information (e.g. location of historical fresh- and salt-water interfaces). Experimental data on fresh- and salt-water intrusions are rarely available. This is considered a limiting factor for multicomponent transport modelling. Column experiments were performed at varying concentration levels in order to obtain such information . Fresh- and salt-water intrusions were repeatedly simulated, with good reproducibility, by percolating a sediment filled column with both sea-water (once diluted) and fresh groundwater. The material used comprised a fine-grained unconsolidated sand from Holocene perimarine gully-deposits in the western part of The Netherlands. Calculated dispersivity values were found to be three times larger during the fresh-water intrusion in sea-water equilibrated sediment than for the reverse situation. Additional experiments with synthetic solutions indicated a chemical explanation for these differences. Selectivity coefficients were determined for fresh- , diluted sea-water and the synthetic sodium-calcium solutions. Using calculated selectivity coefficients and dispersivity values as input, a geochemical transport model was applied to simulate the multicomponent ion exchange during fresh- and sea-water intrusions. The results show that sea-water intrusion in fresh-water equilibrated sediment can only be modelled when the selectivity of calcium over magnesium decreases by a factor of two during the breakthrough. In the case of fresh-water intrusion in sea-water equilibrated sediment. the appropriateness of fit is less sensitive to selectivity changes during breakthrough and calibration is thus easier to perform.
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