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Weizmann Institute of Science (1985)

Hydrogeological effects of changes of permeability in coastal aquifers due to water-rock interactions

Goldenberg Lior C.

Titre : Hydrogeological effects of changes of permeability in coastal aquifers due to water-rock interactions

Auteur : Goldenberg Lior C.

Etablissement de soutenance : Weizmann Institute of Science

Grade : Doctor of Philosophy (PhD) 1985

Résumé partiel
Groundwater flow in a coastal aquifer may be Influenced in a specific manner by water-rock interactions. Under a constant hydraulic head, departures from a steady-state flow may appear when the microstructure of the aquifer undergoes diagenetic changes. Porosity- and connectivityproperties which influence hydraulic conductivity (H.C.)- alter over prolonged periods due to processes such as precipitation and dissolution. A rapid diagenetic process was observed to take place at the seawaterfresh groundwater interface zone. There, a fast decrease in H.C. occurred when seawater was flushed by fresh groundwater in sediments taken from a ’good aquifer’. The hydraulic conductivity dropped between 1/10 to 1/1000 of the original values. Subsequent flushing with seawater restored the H.C. only in a slight degree, if at all. This phenomenon was attributed to the presence of minute quantities of clay minerals in the sediments. The influence of small amounts of clay minerals on the H.C. was investigated by laboratory experiments. Admixture of up to 1.5% by weight of clay minerals to sand did not cause any measurable decrease of hydraulic conductivity for seawater. Increasing the clay fraction from 1.5% to 10% decrease hydraulic conductivity by one order of magnitude. Montmorillonite caused the strongest decrease ; kaolinite and illite were only half as effective as montmorillonlte. A large hydraulic conductivity de- crease occurred when seawater flowing through the artificial sand-montmorillonite mixtures was flushed by fresh groundwater. This effect is expressed as the relationship : log K s 0.295 - 1.32 « C where K is the hydraulic conductivity (cm/sec) C is the amount (t) of clay in the sediment. However, flushing with freshwater did not measurably affect the hydraulic conductivity of an illlte-sand or kaollnite-sand mixture. The explanation for this phenomenon is the different capabilities of the clay types to adsorb various quantities of water between their platelets, which Induces an in situ gel-droplet formation. The process of gel formation is governed by the chemical characteristics of the flowing water. These clay structures, situated in the constrictions of the pores, decrease the connectivity of the system. The ’plate-by-plate’ mechanism of decrease of hydraulic conductivity by the creation of gel-droplets by platelets which arrive one at a time to a seawater-fresh water interface, was investigated. As little montmorllIonite as 5*10 J % of the quantity of sand packed in the experimental columns was found to decrease the hydraulic conductivity by 3 orders of magnitude. Such a situation may be expected in an Interface zone that undergoes deflocculation, where dispersion of clay arrangements occur. In a characteristic situation at an interface, when flow of seawater was exchanged for fresh water, hydraulic conductivity of the entire sample fluctuated, prior to a lower value of H.C. becoming stabilized.

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