A large hydrochemical data-set for the East Yorkshire Chalk aquifer has been assessed. Controls on the distribution of water qualities reflect: water-rock interactions (affecting especially the carbonate system and associated geochemistry); effects of land-use change (especially where the aquifer is unconfined); saline intrusion and aquifer refreshening (including ion exchange effects); and overexploitation (in the semi-confined and confined zones of the aquifer). Both Sr and I prove useful indicators of groundwater esidence times, and I/Cl ratios characterize two sources of saline waters. The hydrochemical evidence clearly reveals the importance both of recent management decisions and palaeohydrogeology in determining the evolution and distribution of groundwater salinity within the artesian and confined zones of the aquifer. Waters encountered in the aquifer are identified as complex (and potentially dynamic) mixtures between recent recharge waters, modern seawater, and ancient seawater which entered the aquifer many millennia ago.