Chloride in the Chalk aquifer of East Anglia, UK. Part 2: causes of elevated chloride in freshwater

Fresh groundwaters (<250 mg l −1 Cl) in the Chalk of East Anglia have a range of chloride concentrations that are higher than the 10–20 mg l −1 level that is conventionally expected from the evaporative concentration of rainfall chloride, with a median concentration of 46 mg l −1 being observed. This is also higher than the typical median chloride concentration in other English aquifers (i.e. 20–30 mg l −1 ). Three possible explanations are explored: increased evaporative concentration in a region with very low effective rainfall, agricultural inputs, and the effects of old saline water known to be present at depth. Saline intrusion is discounted as a significant cause. Nitrate and potassium are possible indicators of agricultural impact, with chloride being associated with the latter, but neither is an entirely satisfactory indicator. De-nitrification is possibly widespread. The three processes considered are not mutually exclusive and none is decisively eliminated but the balance of evidence favours the influence of saline water at depth, from which chloride is diffusing up into the active aquifer. There are clearly some observations affected by agricultural input but evidence for the effect of increased evaporative concentration is weak.