Borehole catchments need to be defined for groundwater protection in general and for the control of nitrates in particular. There are rarely enough data to make an accurate definition. The catchment of a borehole which draws from the glacial infill aquifer in a Shropshire valley has been studied, with emphasis on how sensitive the catchment is to variations in hydrological and hydrogeological parameters. The amount and distribution of recharge are the most important controls on the catchment. In this example, 250f the borehole abstraction is derived from valley side recharge, that is runoff from the valley sides which recharges the margins of the aquifer. Thus part of the catchment lies off the aquifer. Despite the limited data available, a zone which probably accounts for about 650f the borehole's abstraction can be defined, which would be sufficient to justify a nitrate control scheme. The complete catchment cannot be accurately defined however and the area covered by the possible range of catchment boundaries is 25% larger than the ‘best’ estimate. Additional field data would be needed to narrow this range significantly.