Schwyll Spring, located in an outcrop of Carboniferous Limestone in the Vale of Glamorgan, South Wales, has a catchment area estimated to be some 23 sq km based on water balance calculations. Water tracing indicates a component of the spring discharge is comprised of leakage from rivers which flow across the limestone outcrop. The actual volume of leakage could not be determined due to limited river discharge data, however, spring water chemistry suggest that influent river water does not comprise the major proportion of discharge from Schwyll. Geological information, combined with water tracing results and the water balance has been used to estimate a conjectural groundwater catchment for Schwyll Spring. Based on this area sources of pollution to the spring are assessed, which include those from dispersed recharge, sinking streams and influent rivers.