A TRACER INVESTIGATION INTO THE IMPORTANCE OF FISSURE FLOW IN THE UNSATURATED ZONE OF THE BRITISH UPPER CHALK

The displacement and recovery of surface-applied. (NO3)-N-15, H-2 and Cl- in the unsaturated zone of the British Upper Chalk was followed over a 4 year period. Recovery of Cl- in the chalk matrix under the treated plot was equivalent to 110% of that applied. That of H-2 was 57% in the treated plot. Lateral flow of H-2 was evident from cores taken outside the plot, and if this is included, together with H-2 estimated to have been lost by evapotranspiration, the recovery approached 100%. Recovery of (NO3)-N-15 was 100% 1 year after tracer application, falling to 9.4% after 4 years. No evidence of denitrification was obtained from background NO3 cores, and it was concluded that the shortfall in NO3 recovery was due to uptake by grass from depths of 50-70 cm. The tracer profiles and recoveries were not consistent with significant vertical macrofissure flow, but the lack of seasonal periodicity in background H-2 profiles suggested that some microfissure flow could be occurring. The displacement of the tracer peaks, when considered with the estimates of recharge, indicated that only 50% of the water-filled matrix was conducting flow.