Mechanisms of water storage and flow in the unsaturated zone of the Chalk aquifer


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June 2000


It is known that the volumes of water draining from some Chalk catchments in recessions are significantly greater than can be explained by gravity drainage from fissure porosity. The most likely source of this water is slow drainage from the matric porosity above the water table. This paper suggests that the location of this storage is in the irregularities on the fissure surfaces. Studies of resin-impregnated blocks of chalk failed to reveal the presence of hydrologically significant microfissures or macropores that could be the source of additional drainage. Ultrasonic measurements at varying values of effective stress show no indication of changes in the acoustic transmission properties that could be attributed to closure of microfissures. Drainage and mercury-intrusion measurements indicate that there is significant storage on block surfaces. The drainage measurements confirm that the storage is of the order of magnitude (about 0.3%) needed to explain the slow drainage. This additional storage explains why the water table is slow to respond after recharge events, and why the Chalk is resilient to drought. The presence of storage in the unsaturated zone implies that the position of the water table may not be an accurate indicator of groundwater resources. The concept of filling and draining of irregularities on fissure surfaces also leads to a new model for the generation of fissure flow in the unsaturated zone of the Chalk. This suggests that fissure flow can be generated at any depth in the profile, and in a sequence of uniform vertical permeability is likely to originate near the water table rather than high in the unsaturated zone.


Chalk, Unsaturated Zone, Storage, Recharge, Fissure Flow, Matrix

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Journal of Hydrology, Vol. 233, no. 1--4 (2000-06-12).