Pesticide penetration and groundwater recharge on a chalk hillslope in southern England

Pesticide penetration and groundwater recharge monitoring on a chalk hillslope in southern England identified very different mechanisms of contaminant transport related to the water table depth. On the interfluve, where the groundwater was 18-20 m below the surface, matrix flow only through the deep chalk unsaturated zone was observed. In contrast, the dry valley bottom with a shallow (4-6 m deep) water table experienced rapid preferential water/contaminant transport, as well as matrix flow, to the groundwater. Unsaturated soil/chalk hydrology demonstrated the importance of the shallow groundwater capillary rise in keeping the unsaturated profile well supplied with water resulting in a reduced storage capacity for rainfall pulses; little water is required to wet the shallow profile before rapid preferential flow events occur. Storage sites on chalk surfaces and at "contact points" remain empty where the groundwater is deeper and rainfall pulses are attenuated as these sites absorb the downward water fluxes. Consequently preferential events here are rare. The maximum potential mass load of pesticide that might be transported to the groundwater in the preferential events in 1996-7 was determined from bromide tracer studies. Calculations under the worst case scenario showed no more than 0.13 % of the applied pesticide would be transported to the water table rapidly at the shallow groundwater site. A more realistic estimate was between 0.04 % and 0.07 % of the applied pesticide. The work indicates contamination of groundwater by pesticides applied following normal agricultural practice over unconfined chalk can occur, however the major pollution sites are likely to be restricted to the dry valley areas at least in the short term.