Experimental observations of saltwater up-coning

SummaryThe proper management of coastal aquifers commonly requires an understanding of the behaviour of saltwater plumes underlying production wells. The current understanding of saltwater up-coning (i.e. the pumping-induced rise in the freshwater–saltwater interface) is based predominantly on theoretical constructs of salinity and flow responses to groundwater pumping, plus a limited number of field-scale and laboratory investigations. There is a need to produce direct observations of saltwater up-coning, given the considerable and inherent uncertainties associated with the prediction of density-dependent flow and dispersive solute transport behaviour. In this study, time-series observations of saltwater up-coning were made using controlled sand-tank experiments of a two-dimensional aquifer cross-section. A range of mixed convection ratios was tested by invoking different groundwater abstraction rates and saltwater densities. The temporal development of up-coning was characterised in terms of the height of the interface apex, the width of the salt plume, and the salinity of the pumping well. Experimental results were compared to an existing sharp-interface analytical solution and close agreement was obtained for two of the cases. Higher freshwater–saltwater density differences and lower pumping appeared to induce the largest deviations from theoretical up-coning. Dispersive transport processes dominated during phases of near-well up-coning, causing gradual increases in pumped water salinity, although plumes became non-dispersive after the interface made contact with the bore. An interesting departure from the classical up-coning shape was obtained for the situation of highest saltwater density and lowest pumping, in which up-coning occurred as an almost horizontal interface for the majority of the experiment, eventually producing a double-apex shape as the interface neared the well. The experimental results could form a useful basis for the testing of density-dependent groundwater flow and transport models, since they offer controlled laboratory analogues of saltwater up-coning.