Geochemical and mineralogical properties of the Lower Callovian (Jurassic) Kellaways Sand, variations in trace element concentrations and implications for hydrogeological risk assessment

Abstract

This paper provides details of the trace element and total organic carbon (TOC) content of the Lower Callovian Kellaways Sand. Heavy metals are associated with mineral phases such as pyrite, iron and manganese oxyhydroxides, organic matter and clay minerals, and correlations are varied. Using a multi-disciplinary approach and geochemical comparisons it has been shown that the Kellaways Sand is particularly enriched in Cr. Cr concentration data suggest two separate components, one bound with Ni and MnO, and the other occurring as an accessory resistate phase Fe–Ti–Cr oxide (ulvöspinel) with up to 4.7 wt% Cr 2 O 3 . The closest correlations between matrices are Zn and Ni with Fe 2 O 3 ; V, Ni, Rb and Ga with MnO, MgO and K 2 O; Ni and MnO with TOC; and Cd with Zn. Average concentrations for trace elements and TOC are generally enriched in both the Oxford and Kellaways clays. Variable concentrations of SO 4 in associated groundwaters indicate that oxidation of FeS 2 influences the ranges and concentrations of trace elements observed through Eh and pH controlled adsorption or desorption reactions. The natural and variable concentrations of the metals that also occur in associated groundwaters (including Hg, Cd, Ni, As and Pb) have important implications for the associated, risk-based hydrogeological assessments completed for landfill sites contained within the Oxford Clay, and affect the choice of priority determinants significantly for the monitoring control and trigger levels.