I.C.P analytical techniques applied to the hydrogeochemistry of the southern Lincolnshire Limestone aquifer

Abstract

The hydrogeochemistry of the southern Lincolnshire Limestone is investigated employing inductively coupled plasma-optical emission spectrometry (ICP-OES) as the primary analytical technique. Two simultaneous, multi-element instruments were used. The principles of the technique and comparisons of the hardware, operating conditions and routine sample preparation methods are outlined and realistic levels of precision and detection limits evaluated. The range of determinations in the groundwaters was extended by the development of novel sample treatment techniques including the determination of dissolved sulphide down to the 1 level using a gas-liquid separator, and the determination of rare earth elements in groundwater evaporation residues. The methods were applied to the analysis of groundwaters and rocks of the Lincolnshire Limestone in southern Lincolnshire. The geology, hydrogeology and hydrochemistry of the aquifer are reviewed and the sample collection methods described, stressing the precautions taken to avoid contamination. The Lincolnshire Limestone is an aquifer of considerable heterogeneity, argillaceous limestones are intercalated with pure oolites and it is confined by predominantly clastic formations. The major element chemistry of the groundwaters follow a down-dip trend between calcium-bicarbonate-sulphate waters near outcrop and saline, non-potable waters in the east. The groundwaters are sub-divided into zones based on the processes of calcite dissolution, ion exchange and the mixing of recharge and saline interstitial waters. Minor and trace element behaviour is controlled by a combination of oxidation-reduction reactions, adsorption on clay minerals and organic matter, mineral solubilities, complex formation andgroundwater pH. Ultimately the low levels of most trace metals are limited by their low abundance in the limestone. Temporal hydrochemical trends are identified, and the influence of man, with reference to the down-gradient migration of agrichemical pollutants (e.g. nitrates) is assessed. Fluctuations in the movement of the potable/saline water interface are noted and theories on the origin of the saline waters reviewed.