Application of geophysical borehole logging techniques to examine coastal aquifer palaeohydrogeology

Geophysical logging is shown to be a useful technique to support investigations of coastal aquifer hydrogeology. Formation logging can identify the geological layering and fluid logging can characterize the salinity distribution. The measurements also reveal wellbore flow to be common in coastal boreholes, which can mask the salinity stratification in the aquifer matrix. Geophysical logging can be used to guide water sampling and to provide information on the palaeohydrogeology. In combination with water sampling and age determinations, it has shown modern groundwaters overlying Holocene age groundwaters, in turn overlying groundwaters of Pleistocene age, within 150 m of the surface in some of the aquifers studied. Sea-level change in response to Pleistocene glaciations and deglaciations is recognized as a major control on the salinity of groundwaters and on the development of permeable flow routes in coastal aquifers. The permeable routes that developed by groundwater circulation to older and deeper base levels are now partly or wholly occupied by groundwaters of modern flow systems, and can be the focus for saline intrusion. The effects of Pleistocene sea-level change on aquifer development appear to be worldwide and are being increasingly recognized. Examples are described to illustrate the development of flow horizons in relation to rock layering, structure and base levels.