Using artificial fluorescent particles as tracers of livestock wastes within an agricultural catchment

Evidence for the movement of agricultural slurry and associated pollutants into surface waters is often anecdotal, particularly with relation to its ‘particulate’ components which receive less attention than ‘bio-available’ soluble phases. To assess the extent of movement of slurry particles artificial fluorescent particles were mixed with slurry and applied to a field sub-catchment within a headwater catchment. Particles were 2–60 μm in diameter and two different densities, 2.7 and 1.2 g cm–3 representing ‘inorganic’ and ‘organic’ material. Water samples from the field and catchment outlet were collected during two storm events following slurry application and analysed for particle and suspended sediment concentrations (SSC). SSC from the field and catchment outlet always formed clockwise hysteresis loops indicating sediment exhaustion and particles of the two densities were always found to be positively correlated. Particles from the field formed clockwise hysteresis loops during the first discharge event after slurry application, but anti-clockwise hysteresis loops during the second monitored event which indicated a depletion of readily mobilisable particles. Particles from the catchment outlet always formed anticlockwise hysteresis loops. Particle size became finer spatially, between field and catchment outlet, and temporally, between successive storm events. The results indicate that slurry particles may be readily transported within catchments but that different areas may contribute to pollutant loads long after the main peak in SSC has passed. The density of the particles did not appear to have any effect on particle transport however the size of the particles may play a more important role in the 2–60 μm range.

Research Highlights
► This study traces the movement of agricultural slurry particles from land to waters.
► Two densities of artificial fluorescent particles were applied to a nested catchment.
► Slurry particles moved from point of application to a stream over some distance.
► Particle density does not appear to affect particle transport.
► Catchment outlet peaks in tracer particles occur after peak discharge.
► Slurry particles may contribute to sediment loads after the main sediment peak.