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Seawater and fresh groundwater were made to flow through columns filled with samples from a ‘good aquifer’ that contained 5% or less clay minerals. Hydraulic conductivities decreased very sharply whenever seawater was flushed by fresh groundwater. The extent of the decrease depended mainly on the percentage of clays and reached maximum values of 10−1 to 10−3 of the original hydraulic conductivity. Subsequent flushing with seawater restored hydraulic conductivity only in a slight measure, if at all. The mechanism of this process was investigated by using CaCl2 and NaCl solutions with the ionic strengths of seawater and fresh groundwater. It appears that the decrease of hydraulic conductivity is caused by an incipient stage of gel formation. The small water‐clay configurations that are formed during this stage behave as practically rigid particles and close the bottlenecks between adjacent pores. The major implications of these findings for groundwater hydrology are as follows: (1) In natural coastal aquifers the seawater‐freshwater interface is likely to undergo shifts on a semigeologic time scale. If the aquifer contains even a small percentage of clays, these shifts may create an almost impermeable boundary in the zone of the interface. Hitherto existing theories on movements of the interface regard permeability as constant. (2) The attempt to repel a fresh water‐seawater interface by the injection of fresh water may create a practically impermeable boundary within a relatively short time, and thus defeat its own purpose.

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