Rate controls on the chemical weathering of natural polymineralic material. II. Rate-controlling mechanisms and mineral sources and sinks for element release from four UK mine sites, and implications for comparison of laboratory and field scale weathering studies

Abstract

Predictions of mine-related water pollution are often based on laboratory assays of mine-site material. However, many of the factors that control the rate of element release from a site, such as pH, water-rock ratio, the presence of secondary minerals, particle size, and the relative roles of surface-kinetic and mineral equilibria processes can exhibit considerable variation between small-scale laboratory experiments and large-scale field sites. Monthly monitoring of mine effluent and analysis of natural geological material from four very different mine sites have been used to determine the factors that control the rate of element release and mineral sources and sinks for major elements and for the contaminant metals Zn, Pb, and Cu. The sites are: a coal spoil tip; a limestone-hosted Pb mine, abandoned forthe last 200 a; a coal mine; and a slate-hosted Cu mine that was abandoned 150 a ago. Hydrogeological analysis of these sites has been performed to allow field fluxes of elements suitable for comparison with laboratory results to be calculated. Hydrogeological and mineral equilibrium control of element fluxes are common at the field sites, far more so than in laboratory studies. This is attributed to long residence times and low water-rock ratios at the field sites. The high water stor-ativity at many mine sites, and the formation of soluble secondary minerals that can efficiently adsorb metals onto their surfaces provides a large potential source of pollution. This can be released rapidly if conditions change significantly, as in,for example, the case of flooding or disturbance.