Hydrogeochemistry of aquifer storage and recovery in the Lower Greensand (London, UK) for seasonal and drought public supply

Investigations are in progress to determine the potential for ASR, in the Cretaceous Lower Greensand aquifer at Horton Kirby, to meet demand during droughts of up to 2 years, whilst also meeting normal seasonal demands. The sands and sandstones are glauconitic and ferruginous so an understanding of the hydro geochemistry is needed to predict the responses to injection of aerobic water from the overlying Chalk aquifer. Pumped and pore water from the aquifers have been characterised, microcosm experiments undertaken and the results used to constrain geochemical modelling. The transmissivity of the 23 m thick aquifer is calculated to be 45 m2/d and it contains Ca-HCO3 type groundwater with a pH of 7.6 and a SEC of 329 μS/cm. Concentrations of Fe(total) and Mn exceed the prescribed concentration value (PCV). The water to be injected is also a Ca-HCO3 type with a pH of 7.4 and a SEC of 537 μS/cm, is aerobic and contains elevated concentrations of nitrate (22 mg/l), but not in excess of limits. Likely impacts of ASR are reactions with Fe-minerals (including small quantities of pyrite) resulting in an increase in dissolved iron and sulphate, and removal of injected nitrate through reduction.