The deterioration of the quality of superficial and subsurface waters by motorway organic pollution is a problem of great concern nowadays. The absorption of road runoff waters through sinkhole clay filled in a karstic area has been proposed as a way to the attenuation pollution. The groundwaters near of surface are very vulnerable at this pollution. The study of the storm basin of Belle-Croix (RN 20 Loiret) indicates a diminution of DOC levels through the first meter of infiltration in sinkhole, and then little change occurs down 5 meters. However only the > 10 KD (10 nm) organic fraction is retained. This fraction is weakly fluorescent and should correspond to black carbon (BC) matter associated to clays as it is suggested by Electron Microscopy in Transmission EMT observation. The < 10 KD fraction which are the more fluorescent account for up to 80 % of total DOC at 5 meter depth, at the contact with the limestone.