Sequential isotope and solute profiling in the unsaturated zone of British Chalk

The evolution, over periods of up to nine years, of tritium, nitrate and other pore-water profiles in the Chalk unsaturated zone at sites in three areas in Britain is presented and evaluated in terms of transport mechanisms. Etablissement des profils séquentiels des isotopes et des matières en solution dans le zone non saturée de la partie britannique de la couche de chaux Résumé On présente l'évolution, sur des periods allant jusqu'à neuf ans des profils du tritium, des nitrates at d'autres produits solubles dans l'eau des pores dans la zone non saturée de la chaux à un certain nombre de sites dans trois zones et on analyse cette évolution en termes de mécanisme de transport. INTRODUCTION Background The Chalk is the most important aquifer in Britain, yielding about 4000 Ml day" of groundwater mainly for potable supplies. The extensive recharge areas (Fig. 1) commonly have intensively cultivated permeable soils. Hydrogeological research has established that the progressive changes in arable farming practice since the 1960s, aimed at increasing productivity, are having a profound effect on the quality of infiltrating rainfall (Foster & Young, 1980; Foster et al, 1982, 1985 and 1986). Rising nitrate and pesticide concentrations in groundwater are a consequence of particular concern but mechanisms of downward solute transport through the unsaturated zone are complex, making it difficult to predict future trends precisely. Sequential re-profiling of the Chalk unsaturated zone at sites in Dorset, Cambridge and Norfolk has been carried out in an attempt to improve the understanding of solute movement and the prediction of future groundwater quality. The profiles were obtained by core-drilling, with sampling of pore-water by high-speed centrifugation or vacuum distillation (Foster et al, 1986). The lateral variation between profiles from boreholes drilled in the *Present address: Mananga Agricultural Management Centre, PO Box 20, Mhlume, Swaziland. "Present address: Pan-American Center for Sanitary Engineering & Environmental Science (CEPIS), Casilla Postal 4337, Lima 100, Peru. Open for discussion until I August 1989 79 A. K. Geake & S. S. D. Foster 80 Fig. 1 Outcrop of the Chalk and location of research sites. same field at the same time has been shown generally to be minor. Thus they can be taken as reasonably representative of local conditions (Foster & Bath, 1983). The present paper focuses on pore-water profiles of chloride, nitrate, and isotopes of oxygen and hydrogen. Chloride is a stable, non-reactive ion, with very limited uptake by most plants in humid climates. Chloride profiles can therefore represent a useful tool in the interpretation of unsaturated zone solute transport, if chloride inputs from rainfall, potash fertiliser and other agricultural additives are known, and especially if these have a distinctive historical pattern. The transport of nitrate is likely to be similar to that of chloride, except that denitrification or nitrification within the Chalk unsaturated zone cannot be discounted since the presence of both the required bacteria and suitable substrates has been shown (Whitelaw & Rees, 1980; Whitelaw & Edwards, 1980). Use of isotopic methods The determination of tritium (H) profiles aids the evaluation of unsaturated zone solute transport mechanisms because of its unique temporal distribution in rainfall and infiltration. It can thus be used as an analogue for 81 Isotope and solute profiling in British Chalk non-reactive solutes of similar physicochemical behaviour whose temporal input functions are less easy to estimate. Many, but not all, tritium profiles show a distinct peak originating from fallout in rainfall in 1963-1965 following atmospheric testing of thermonuclear weapons. The peak has moved slowly downwards with time, but has been preserved despite attenuation through radioactive decay (tritium having a half-life of 12.4 years) and the effects of hydrodynamic dispersion and molecular diffusion (Fig. 2a). Consideration of the tritium mass balance, however, shows that profiles must be interpreted with caution (Foster & Smith-Carington, 1980).