When the liquid residing in a horizontal bed of porous material is displaced by another liquid of different density, the resulting hydrodynamic dispersion is modified by the formation of a tongue of denser liquid undershooting the less dense liquid, a phenomenon known as gravity segregation. An earlier account of gravity segregation contained a substantial error (that of an incorrect frame of reference for flow) and several printing mistakes. In this paper we (i) correct these errors, (ii) extend the analysis to describe the course of breakthrough in beds of rectangular and circular cross-sections, (iii) re-interpret the existing experimental evidence, and (iv) present new experimental results on the vertical and horizontal transport of ionic solutions of different concentrations and densities through inert and reactive porous materials, ballotini, and sepiolite, respectively.

The behaviour of immiscible liquids is predicted by the non-dimensional gravity segregation number, β, segregation becoming more extreme as β increases. With miscible liquids, however, breakthrough starts later and ends earlier then predicted for immiscible liquids, mixing by hydrodynamic dispersion moderating the effect of segregation. Breakthrough curves are well fitted by CXTFIT 2.0; apparent coefficients of hydrodynamic dispersion vary much less with pore-water velocity in horizontal than in vertical flow, but retardation factors are not influenced by orientation. Although a formal analysis of the combined effect of gravity segregation and hydrodynamic dispersion was not possible, the statistically significant inverse relation between β and column Peclet number was explained qualitatively.

Gravity segregation occurs during the intrusion of saline groundwater into coastal aquifers. The simple theory for immiscible displacement overestimates the actual intrusion that occurs with miscible liquids and so provides an effective safety margin.