BROMIDE TRANSPORT AT 2 TILE-DRAINED FIELD SITES

Within comparative studies on solute movement in tile drained agricult ural fields in Schleswig-Holstein a bromide field test was conducted a t two field sites. The soil of site 'Bokhorst' shows both spatial and profile heterogeneity with significant clay and silt contents (loam) w hereas site 'Hohn' had a homogeneous particle size distribution within the profile with sand being the dominating particle fraction (sand) b ut with neglible spatial variability. Rainfall amount, drain dis charg e and bromide concentrations in drain outflow were monitored over a 6 months period. A rapid response of drain discharge on rain events was observed for the loamy site but not for the sand. The simple water bal ance yielded +8.7 mm of drain discharge for site Bokhorst (loam) and - 43 mm for the sandy site Hohn (sand). Largest bromide concentrations w ere detected shortly after chemical application at the loamy site. The bromide loss curve was characterized by simultaneous in- and decrease of flow rate and bromide concentrations. This solute behaviour was na med 'event-dependent'. Bromide occurred in drain outflow at the sandy site not before 30 mm discharge and maximum concentrations were observ ed at the end of the monitoring period, Mass balance calculations reve aled that 70.5% (loam) and 33% (sand) of the applied solute mass were transported from the fields via the tile drains. A two dimensional, ad vection dispersion equation based simulation model was applied to pred ict solute behaviour at site Bokhorst. Computed bromide concentrations were not in line with the observed preferential breakthrough curve. C onsequences of preferential transport conditions in tile-drained field soils on surface water quality are briefly discussed, especially conc erning the displacement of plantprotective agents and methodical diffi culties.