Chloride in the Chalk aquifer of East Anglia, UK. Part 1: distribution of chloride in brackish and saline groundwater

The Chalk aquifer of East Anglia is heavily exploited as a groundwater source. During exploration and exploitation, it has become clear that some of the groundwater in the Chalk is brackish and unsuitable for drinking (>250 mg l −1 chloride). Data comprising chemical analyses of pumped water samples and borehole depth samples (more than 900 results in total), and downhole geophysical surveys have been used to assemble maps of the distribution of chloride areally and with depth. Brackish or saline water is probably present at depth in the Chalk, even where shallow freshwater is present, over much of the area where Crag Group marine sediments have been deposited. The depth below surface at which brackish water is encountered varies between 50 and more than 100 m, and the elevation of the transition varies from below −91 to +16 m above ordnance datum (AOD). From its chemistry, it is deduced that most of the brackish water is of ancient marine origin but that there has been localized intrusion of modern seawater in coastal areas, especially around Ipswich. Saline water most probably entered the aquifer during Crag Group times (late Pliocene–early Pleistocene) and its chemistry has been modified subsequently. The most saline old water that has been encountered has full marine salinity.