Understanding pesticide dynamics at the catchment scale for the management of drinking water reservoirs

Pesticides applied on farmland are a source of pollution for the surrounding areas and in particular for local watercourses. During rainfall events pesticides are transported away from the area of application towards the rivers via surface runoff and attached to eroded sediment. Where such local watercourses supply water to drinking-water reservoirs, a variety of different pesticides ultimately end up in the reservoirs. However, the timing of pesticide arrival and their concentrations in the reservoir, vary throughout the year as a function of both rainfall variability and the timing and type of pesticide application on fields. Because at present there is no legislative requirement to record pesticide application on fields, companies such as Wessex Water have no information on the seasonal dynamics within the catchment that will affect the pesticide levels within their reservoirs. Hence, monitoring and managing pesticide levels within the reservoirs are a serious problem with important implications for public health. The project will adopt a field-based methodology in which the catchment hydrological and erosion responses will be characterised within a hierarchical approach. The study site will be the Durleigh catchment which is one of Wessex's lowland agricultural catchments and one which is most prone to pollution from pesticides. The first part of the work will involve a pilot study in which classes of persistent pesticides will be identified from water and sediment samples from the rivers in relation to classes found in the reservoir. Any existing available data on pesticides, land use and hydrology will also be collated and synthesised for a broader historical understanding of the pesticide dynamics within the catchment. The second part of the study will be carried out over 12 months and will involve spatial sampling of river water and riverbed sediment every two weeks. Wessex already carries out routine sampling of the reservoir water every two weeks. Both water and sediment samples will be analysed for the classes of pesticides identified in the pilot study and related to the reservoir pesticide levels. The third part of the project will build on the data collected in the previous year to identify catchment 'hotspots' of pesticide response and, over a 6-month period, carry out within those a more detailed characterisation of pesticide transport, particularly within individual rainfall events and at the individual farm system scale. A particularly innovative aspect of the project is the comparison between the dissolved and sediment-bound pesticide transport mechanisms in relation to catchment processes. Currently, Wessex Water has no information on the role of sediment-bound pesticides within their catchments but there are serious implications of the potential for this component, through desorption mechanisms, to act as a sustained source of pesticide pollution in the reservoir long after their initial mobilisation from the fields. The synthesis of these data will provide a much needed detailed understanding of catchment dynamics in relation to pesticide transport mechanisms and how these affect the pesticide dynamics within the Durleigh drinking-water reservoir. This understanding will lead to better management practices of the reservoir and ultimately to the design of mitigation measures at the farm level. The project combines different strands of investigation into an innovative and achievable course of doctoral training with great practical application to a pressing pollution and water-quality problem. The student will benefit from the different expertise offered by the supervisors both at the University of Bristol and Wessex Water. The student will benefit from establishing contacts with Wessex Water as well from the training opportunities offered at Wessex. The pesticide analyses of the water samples will be carried out in the Wessex labs and the student will have training in analytical techniques.