THE UNSATURATED ZONE: KEY TO THE PAST, GUIDE TO THE FUTURE

For nitrate in particular, the chemistry of water in the unsaturated zone tends to reflect past inputs at the surface (soil zone). Nitrate profiles (concentration vs depth) give some kind of record of these losses. These tend to be dominated by fertiliser losses from agricultural land and natural atmosphere/soil/plant/microbial/hydrological processes elsewhere. Unsaturated zone nitrate profiles therefore provide a useful and quantitative insight into these past processes, albeit subject to the ravages of dispersion and the complexity of the real world.

They also provide a guide to the future evolution of groundwater quality. Unsaturated zone profiling gives an early warning of things to come since the unsaturated zone contains the store of solutes that will be delivered to the saturated zone some time in the future.

The Water Framework Directive (WFD) of the European Union was passed in 2001 and is a major piece of environmental legislation aimed at (i) protecting aquatic ecosystems by achieving ‘good status’ by 2015 and (ii) ensuring the progressive reduction of anthropogenic groundwater pollution (reversal of trends). The WFD has led to an increased emphasis on the monitoring of groundwater quality, the assessment of pollutant trends and prediction of the future evolution of groundwater quality. Nitrate is likely to be the most problematic pollutant in groundwater. Because of the long delays built into the groundwater system, increased monitoring of nitrate in the unsaturated zone will be necessary in order to demonstrate early on the effectiveness of any mitigation measures put in place.

Examples will be given of (i) methods of sampling the unsaturated zone pore water (centrifuging, squeezing, skimming) for nitrate etc.; (ii) the impact of different land uses (including woodland); (iii) spatial and temporal variability and the statistical analysis of trends in groundwater quality; (iv) future modelling requirements.