Fluid flow and diagenesis in the East Midlands Triassic sandstone aquifer

Abstract

Summary The flow system, hydrochemistry and isotope hydrology of the freshwater aquifer in the E Midlands Triassic sandstone are reviewed. Isotopic data suggest a mean flow velocity of around 0.6 m.y. over the last 30 k.y., in contrast with the present-day velocity of around 0.2 m.y. The difference is attributed to changes in hydraulic gradients over this period; fluctuations in recharge and gradients would have influenced meteoric water influx and diagenesis over the last 10 7 years or more. The diagenetic modifications to the arkosic sandstone are described and are divided into those taking place before and during compaction, probably with saline pore water, and those occurring as a result of meteoric water influx following uplift and denudation. The relation between the kinetics of diagenetic reactions and the groundwater flow velocity is discussed. Relevant experimental data for reaction kinetics are summarized. The hydrochemistry and flow velocity of the present regime are used to deduce mass transfers within and outside the aquifer. These are equivalent to uniform porosity increases of only about 0.17fb3e69cer million years for each of the dolomite and K-feldspar dissolution reactions. Both these dissolution reactions are incongruent, and precipitate calcite and kaolinite respectively. The spatial arrangement of reactant and product minerals in incongruent reactions is expected to reflect interplay between dissolution/precipitation kinetics and water flow rate. Scanning electron micrographs show the proximity of K-feldspar and kaolinite. Constraints can be placed on the flow rates responsible by considering chemical balances for aluminium and silicon and their maximum production rates by feldspar dissolution. This approach involves many simplifications and assumptions, but with further experimental kinetic data it offers the possibility of deducing information on past flow systems from detailed measurements of diagenetic mineralogy and geochemistry in clastic sediments.