18O/16O and 2H/1H relationship between rainfall and infiltration water in the English chalk

Full text: Detailed studies of the isotopic variations in rainfall and unsaturated-zone water have been carried out at a research site on arable land in eastern England in order to investigate the mechanisms of water movement. The results also have implications for the interpretation of thermonuclear tritium profiles and the transport of pollutants such as nitrate towards the water table. The isotopic composition of rainfall has been monitored over several years during which four cored profiles of the soil and unsaturated chalk have been taken. Water has been extracted from this drill core at 20 cm intervals by centrifugation. An undisturbed lysimeter on grassland, 5 m deep, has also permitted natural drainage water to be sampled periodically. Two further profiles on permanent grassland permit comparisons for the effects of land use and lithology. Duplicated profiles generally show impressive similarity. The stable-isotope composition of unsaturated-zone interstitial water is virtually uniform below about 4 m depth. Drainage from the lysimeter is mostly homogeneous isotopically and chemically, although there is a rapid hydraulic response to rainfall; infrequent departures from uniformity may represent unusual infiltration events. Above 4 m, the profiles show light isotopic compositions at intermediate depth and marked enrichment of heavy isotopes in the top few tens of centimetres, except in one case in which the opposite occurs. The isotopically heavy water in the top zone shows very little evidence of evaporative modification and is predominantly from unmodified rainfall. The scale of variations and the isotopic balance at this site suggest that the penetration depth by infiltrating rainfall varies greatly with time and season. The presence of a zone of mixing, extending several metres down from the surface, does not permit a 'piston-displacement' mechanism to be applied. Below this zone of mixing, however, the diffusive interchange of water held in fissures and pore spaces with different sizes and hydraulic conductivities ensures an approach to isotopic homogeneity. The results of this study suggest that infiltration below permanent grassland at this site is about 5 per mille δ2H lighter than infiltration below adjacent arable land. There are insufficient data to give a valid comparison between these compositions and the average for rainfall, though it is observed tentatively that both infiltration below grassland and saturated-zone water have isotopic compositions closer to the composition of average rainfall (1979-82) than to that of infiltration below arable land. (author)