Recharge and pollution of the english chalk: Some possible mechanisms

Abstract

Pore water in the Chalk is likely to remain essentially static under the influence of environmental forces with the exception of evapotranspiration. A process of piston displacement of water through networks of microfissures provides a possible mechanism for downward percolation through the unsaturated Chalk. Maintenance of continuous diffusion equilibrium between fissure and pore water throughout downward migration allows the explanation of the observed rates of movement for thermonuclear tritium in the unsaturated zone. Activation of increasingly large fissures with increasing infiltration rates explains observed rapid water-table response under conditions of sustained high inflow, and is consistent with the behaviour of water quality parameters in the saturated aquifer. Whereas in the unsaturated zone it is suggested that 80 to 90% of total flow passes through microfissures, in the saturated aquifer fluid transport is essentially through macrofissures. Diffusion equilibrium between pore water and fissure water in the saturated zone is not therefore expected. Individual macrofissures occasionally are enlarged by solution, making it possible for a large proportion of saturated flow to take place through relatively few preferential flow channels. Although the macrofissures, and particularly enlarged macrofissures, are largely responsible for aquifer transmissivity, microfissure storage may often be highly significant in terms of specific yield.