Karstic features such as swallow holes and dolines are the rule rather than the exception in the Chalk outcrop of southern England, particularly in the region of the Chalk/Tertiary boundary. Previous tracer tests at Bedhampton and Water End have indicated groundwater transport velocities of 2.2 and 5.5km/day. A recent tracer test at Stanford Dingley in Berkshire has indicated comparably high transport velocities in the Chalk: 5.8km/day for peak tracer concentration, and 6.8km/day for tracer breakthrough. In all three cases the swallow hole systems were able to transmit turbid surface run-off contamination over large distances, indicating a short-circuiting of the natural attenuating properties of the unsaturated and saturated zones. In the light of this, the practice of using swallow holes to dispose of agricultural- and road-run-off should be discouraged.