ROLE OF FLOODPLAIN SEDIMENTS IN REDUCING THE NITRATE CONCENTRATION OFSUBSURFACE RUN-OFF - A CASE-STUDY IN THE COTSWOLDS, UK

Discharge of groundwater from a limestone aquifer through floodplain s ediments is associated with a large decrease in the nitrate concentrat ion of the water. Results are presented to show that only a small amou nt of this reduction is caused by dilution of groundwater by water alr eady present within the floodplain sediments; most of the effect is an active reduction process, most probably biological denitrification. T he nitrate reduction process appears to operate independently of surfa ce vegetation type and tends to be focused in specific regions of the floodplain where sediments are anaerobic and carbon-rich. The results suggest that active denitrification can operate throughout the winter, when nitrate concentrations in groundwater are at their highest and t hat the process remains effective even during periods of maximum run-o ff. The results show that undrained floodplains can be used as buffer zones to protect surface waters from groundwater polluted with agricul turally derived nitrate.