An investigation of metal enrichment in Triassic Sandstones and porewaters below an effluent spreading site, West Midlands, England

Abstract

Abstract Soil and sandstone samples were taken from below a sewage effluent spreading site at Whittington Hall, near Stourbridge. The sandstones belong to the Triassic Sherwood Sandstone Group, whole rock samples were analysed for major and trace elements, and porewaters (extracted by high speed centrifuge) for trace metals. The elements found to be enriched in the sediments, at shallow depths (generally not greater than 1 m) were, in order, Zn, Pb, Cu, Cr, Ni and Sr. Systematic decreases with depth were not recorded for the elements Mn, V, Rb, Y and Zr. Retention of the metals is believed to be a combination of filtration of particulate matter and reaction with the sediment, particularly the oxyhydroxide material and possibly smectites. The observed metal enrichment corresponds with the effluent composition and in the case of Zn there would appear to be an approximate balance between the total input into the system and the amount retained in near-surface sediment. The borehole samples were frozen on site in order to suppress microbial changes in the porewaters. Metal concentrations in the porewaters were not, in general, affected but significant differences were noted for Fe, Mn and Ca. At depth in the unsaturated and saturated zones porewater concentrations approach the ‘natural’ background concentration. These decreases in concentrations take place below the zone of sediment-porewater reaction based on whole-rock analyses and are therefore attributed to progressive dilution of the porewater and groundwater.