Stromen van water en stoffen door de bodem en naar de sloten in de Vlietpolder

RIVM has investigated the effects of soil structure and groundwater flow on nutrient transport to soil and surface water in a peat pasture polder near Leyden for their relevancy to national and European legislation on soil and water. Nitrogen and phosphorus concentrations in polder water were found to exceed the limit values for surface water. The land surface level in this polder is 2 m below mean sea level, representing a drop of roughly 3 m due to peat decomposition over many centuries. Groundwater levels are shallow. The rainfall excess of 250 mm.a-1 flows mainly into ditches through a top layer of roughly 2 m. Infiltration to deeper layers is relatively low (25 mm/a). Fertiliser contribution to ditch water is at roughly 10 to 20% for N and P and 30% for SO4; soil processes contribute the remaining parts. Upward diffusion brings 150 kg/ha/a Cl, 7.5 kg/ha/a N and 1.5 kg/ha/a P to shallow layers. SO4 diffuses downward (50 kg/ha/a). Soil decomposition yields 16 kg/ha/a N, 240 kg/ha/a SO4 and 1 kg/ha/a P. Plant growth and sorption cause a further reduction in concentrations in ditch water. Seasonal effects on ditch water quality caused by a time-dependent input were simulated by a model.