Baseline groundwater chemistry : the Chalk aquifer of Hampshire

This study characterises the spatial and temporal variations in inorganic chemistry in the Chalk aquifer of Hampshire, spanning the area from Basingstoke in the north-east to Cadnam in the south-west. The investigation aims to assess the likely natural baseline chemistry of the groundwater by identifying the dominant controlling processes (natural and anthropogenic) including from biogeochemical reactions and pollutant inputs. Results have been collected from strategic sampling of 30 groundwater sources, in conjunction with collation of existing groundwater, rainfall, mineralogical and geochemical data. The Hampshire Chalk forms a predominantly anticlinal structure which dips southwards beneath Palaeogene cover into the Hampshire Basin and northwards into the London Basin. It forms a regionally significant aquifer for potable and agricultural use and also provides baseflow to the Rivers Test and Itchen, which are used extensively for fishing. The results show that groundwater is typical of unconfined Chalk groundwater, of Ca-HCO3 type. Variations in the natural chemistry take place with increasing residence time as water moves away from the groundwater divide downgradient beneath the Palaeogene cover. There is a notable effect on groundwater chemistry from the Palaeogene cover in the south of the study area with slightly increased concentrations of Mg, K, SO4, Br, F, Ba, Sr, Li, Rb, Se, Cr, Mo, Ni, U compared to outcrop Chalk groundwater. Human impact is most plainly visible in the distributions of nitrate in the groundwater. These are regionally high, with one analysed source having groundwater above the EC/national drinking-water limit of 11.3 mg L–1 as N and time-series data demonstrating an increase in concentrations in some groundwaters over the last few decades. The widespread presence of nitrate indicates the extent to which the aquifer has been and continues to be influenced by farming practices. Other nitrogen species (NO2 and NH4) and P are present at slightly above concentrations that would be considered baseline in groundwater from the south-east of the study area, particularly the Meon valley. Concentrations of Cu and Zn are also present at relatively high concentrations (up to 31 μg L–1 and 253 μg L–1 respectively) in some groundwaters. This may be due to contamination from wellhead pipework rather than processes leading to accumulation within the aquifer.