There are three major UK aquifers: the Cretaceous Chalk, Triassic Sherwood Sandstone and Jurassic Lincolnshire Limestone. These supply approximtely 15%, 10% and 5%, respectively, of the UK public water supply. The unsaturated zone in the unconfined parts of the Chalk and Sherwood Sandstone is typically 20 to 50m deep and since the water moves downwards at a rate of 0.7-1.3 m/yr on average, it can take 50yr or more for water infiltrating at the surface to reach the water table. The unsaturated zone of the limestone is shallower, the storage less and there are a greater number of large fissures. This means that the residence time in this aquifer is less, closer to 5-10 years. Numerous studies of the unsaturated zones of these aquifers indicates that they contain regions with high concentrations of nitrate (20-100mg/l NO3-N) in the pore water (Foster et al., 1986). Unless denitrification occurs, this stored nitrate can be expected to contribute eventually to the nitrate load in abstracted waters. The overall objectives of this project is to establish the occurrence and extent of denitrification beneath the rooting zone in relation to geology and other key controlling factors (I) by studying the mass balance of nitrate at sites where repeat drilling has been undertaken; (ii) by measuring key constituents (NO3, NO2, DO, DOC TOC, O2, CO2, bacterial abundane etc) in the unsaturated zone pore water and rock, and (iii) by monitoring the aquifer gases, both free and dissolved, in the unsaturated zone and at the water table for the products of denitrification (N2, N2O, nitrogen isotope fractionation). The project will initially focus on Britain`s two major aquifers, the Chalk and the Sherwood Sandstone, preferably at sites of previous groundwater nitrate research and/or of current research related to Nitrogen Sensitive Areas. Drift cover is likely to have a significant impact on the vulnerability of aquifers to nitrate pollution and will be taken into account in the selection of research sites.