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Advances in understanding natural groundwater quality controls in coastal aquifers
Walraevens, K.; Van Camp, M. (2005). Advances in understanding natural groundwater quality controls in coastal aquifers, in: Araguás, L. et al. (Ed.) Groundwater and saline intrusion: selected papers from the 18th Salt Water Intrusion Meeting: 18 SWIM, Cartagena (Spain), 31 May to 3 June 2004. Publicaciones del Instituto Geologico y Minero de España. Hidrogeologia y Aguas Subterraneas, 15: pp. 449-463
In: Araguás, L.; Custodio, E.; Manzano, M. (Ed.) (2005). Groundwater and saline intrusion: selected papers from the 18th Salt Water Intrusion Meeting: 18 SWIM, Cartagena (Spain), 31 May to 3 June 2004. Publicaciones del Instituto Geologico y Minero de España. Hidrogeologia y Aguas Subterraneas, 15. Instituto Geologico y Minero de España: Madrid. ISBN 84-7840-588-7. 766 pp., more
In: Publicaciones del Instituto Geologico y Minero de España. Hidrogeologia y Aguas Subterraneas. Instituto Geologico y Minero de España: Madrid, more

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Document type: Conference paper

Keyword
    Marine
Author keywords
    natural groundwater quality in coastal aquifers, salinization/freshening, cation exchange, reaction-transport modelling

Authors  Top 
  • Walraevens, K., more
  • Van Camp, M., more

Abstract
    Groundwater quality in coastal aquifers is largely influenced by the interaction between the sea and the bordering aquifer systems. This interaction can result in freshening of saline aquifers or salinization of fresh water bodies. In complex cases even both situations can be found in the same aquifer system. While the main mechanism for salinization or freshening is hydrodynamically driven (groundwater flow), also physical and chemical processes within the aquifer will alter groundwater composition. Cation exchange is in many cases an important process to consider; it results in a hydrochemical spectrum of groundwater types reflecting both the hydrodynamical and hydrochemical characteristics of the aquifer.In order to understand the natural groundwater quality and the controlling processes in a coastal aquifer, it is crucial to known both the hydrodynamical and hydrochemical behaviour and the way these are linked together, because usually only the interaction between them can explain the observed quality istributions.Where mixing of fresh and salt water occurs, density-driven flow may become important and change quality distribution, while the groundwater composition itself influences hydrodynamics. An integrated approach of both aspects is indispensable.Modelling is an important tool in understanding how aquifer systems work. Until recently, hydrodynamical and hydrochemical aspects were tackled separately, with different models. Hydrodynamical aspects have been studied with flow models (such as MODFLOW) or a combination of a flow and a solute transport model (such as MT3D) for simulating salt transport, coupled together for incorporating density-driven flow (such as in SEAWAT). On the other hand, hydrochemical aspects have been investigated with hydrogeochemical models based on speciation and a reaction scheme. In hydrogeochemical models, groundwater flow was either not taken into account, or, at best, it was considered in a very simplified approach (along a path-line). Recently, codes have been developed combining three-dimensional flow models fully with the flexibility of an extensible hydrochemical model based on thermodynamic databases, such as PHAST, which combines the well-known PHREEQC model with the HST3D model for flow and transport. Simulations with the PHAST model have shown that it is capable of simulating the whole hydrochemical spectrum of groundwater types in coastal aquifers under freshening and salinizing conditions incorporating cation exchange processes and mineral equilibria such as calcite dissolution. The capabilities of this new generation of models will also allow for including redox components (iron content of the water) or processes such as surface complexation (e.g. sorption onto hydroxides). The future for such models looks bright, because for the first time they will provide the complete groundwater composition as their output.However, models need to be based on good and sufficient field data! Without them the reliability of the models is unknown and their value for predictive purposes is hypothetical. Therefore the advances in modelling should go together with new techniques of sampling, measuring and monitoring and with improved analytical methods. Advances have been made also in these fields. Yet, the most important advance in understanding natural groundwater quality is not related to a specific technical innovation but rests in the minds of hydrogeologists. Indeed, only a profound insight in the combined hydrodynamical and hydrochemical aspects by a refined system analysis can provide the key to really understand what controls groundwater quality, also in coastal aquifers.

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