Changes in groundwater chemistry due to rising groundwater levels in the London Basin between 1963 and 1994

Commencing in the last century, groundwater abstraction has modified groundwater flow and lowered groundwater levels in the Chalk aquifer of the London Basin. Since the 1960s, a decline in pumping has caused a constant rise in groundwater levels and groundwater is now re-entering strata which had formerly been dewatered. In order to assess the effects of the changed hydraulic conditions on the chemical composition of groundwater. Chalk groundwater samples along a transect from Watford to Croydon across the London Basin were analysed for several chemical parameters in 1994. and compared with results from a similar hydrochemical study reported in 1963. A general deterioration in groundwater quality was identified. Much higher sulphate and alkali and alkaline earth metal concentrations were found in almost the entire study area and are associated with the reflushing of formerly dewatered strata. In the Chalk outcrop areas, high nitrate, sulphate, chloride and sodium concentrations indicate anthropogenic aquifer pollution. Several chemical and physical processes that control the groundwater chemistry down-gradient towards the centre of the Basin were identified. Finally, five groundwater zones established in 1963 on the basis of hydrochemistry were revised to produce three groundwater zones.