Regional controls on the hydrochemistry of the Chalk aquifer of East Anglia, UK

The study region is the Chalk (limestone) aquifer of East Anglia, UK, bounded by the North Sea to the north and east, the River Colne in the south, and the limit of Chalk outcrop to the west, around 8000 km 2 . The study documents the variation in Chalk groundwater chemistry over the region, based on analytical data for ∼900 sites, and explores the causes of this variation. The chemistry of the water in the limestone aquifer varies widely, from < 10 to > 600 mg L -1 calcium, 120–660 mg L -1 bicarbonate. Calcium and bicarbonate concentrations are typically higher than in the Chalk elsewhere in the UK. Chloride and sulphate are also higher, but nitrate is often low or absent. The strata overlying the Chalk determine whether the carbonate chemistry has evolved under closed- or open-system conditions, has been affected by leaching from overlying strata during recharge, and whether ion exchange replacement of calcium by sodium has taken place. The equilibrium partial pressure of carbon dioxide has proved to be particularly useful in understanding the origin of fresh groundwaters. Some water is clearly modern recharge water; some water is much older, possibly thousands of years. Brackish water at depth is deduced from circumstantial evidence to be related to a marine inundation at ∼2 Ma. • Overlying deposits control the hydrochemistry of the Chalk aquifer • Glauconitic sands permit ion exchange • Pyrite oxidation occurs in chalky Lowestoft Till Formation • Controls are manifest via calculated carbon dioxide partial pressures PCO2 • Strontium and fluoride are elevated in brackish water of ancient marine origin