Over the past few years, new abstractions from the Sherwood Sandstone aquifer in North West England have only been permitted in areas with known saline intrusion subject to the abstractor demonstrating that the new pumping will not cause deep-seated saline groundwater to up-cone. This requirement has necessitated drilling deep boreholes to prove the depth to the fresh–saline groundwater boundary and also to facilitate long-term monitoring. Vertical salinity profiles were defined during borehole construction in two such investigations by monitoring the conductivity of water samples obtained from drilling returns during air-flush drilling and later by the geophysical logging of the completed boreholes. Deep piezometers were constructed allowing samples to be taken from specific depths for chemical analysis, and the results have been compared with the earlier data. These methods successfully defined the broad features of vertical salinity profiles within the aquifer and showed that measurements during drilling provide a practical and economic indication of the position of the interface so that the borehole final depth can be determined.