Hydrochemical evolution of the East Midlands Triassic sandstone aquifer, England

Abstract

A detailed hydrogeochemical interpretation has been made of the red-bed, Sherwood Sandstone (Triassic) aquifer of the East Midlands of England, on which a radiocarbon model has been used to establish a chronology of groundwaters along the flow line. Groundwater evolution is dominated by carbonate and sulphate mineral solution and precipitation and by redox reactions; cation exchange reactions are negligible and the hydrogeochemistry has evolved in the absence of residual saline water. Three distinct zones are recognised: 1. I. Recent groundwater (mainly < 100 y), oxidising, dominated by congruent dissolution of dolomite, and by inputs from atmospheric and anthropogenic sources. 2. II. Groundwater age 10 3–10 4 y, predominantly reducing, dominated by incongruent dissolution of dolomite. 3. III. Groundwater age > 10 4 y, reducing conditions, approach to isotopic and chemical equilibrium in the carbonate system, dominance of gypsum dissolution. The minor and trace element variation can be interpreted in relation to the principal geochemical reactions and recharge history. Na +, Cl −, F − and B are controlled by atmospheric inputs. Sr 2+ and Mn 2+ trends can be explained by carbonate and sulphate mineral dissolution; Ba 2+ is derived from carbonates but controlled by barite equilibrium. Fe, SO 4 2−, NO 3 − and U are strongly influenced by redox reactions. The concentrations of transition metals and other elements remain very low. Only Li + and K + are considered to represent additions by reaction of silicate minerals. The extent of direct anthropogenic influence is probably limited to enhanced SO 4 −, NO 3 2− and possibly Cu, Zn and Pb in Zone I.