Fate of phosphorus in the flower bulb area, the Netherlands Modeling the contribution of the hydrological sources seepage and agriculture to the PO4 concentration in the Leidse Vaart, using the modeling program SOBEK-Delwaq, and taking into account advection and adsorption.
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In the flower bulb area, phosphorus leaching from agricultural soils poses an important barrier for reaching water quality goals. In this research project, a model was built to estimate the contributions of the hydrological sources seepage and agriculture to the phosphorus load of the surface water, in particular the Leidse Vaart. This should ultimately help to estimate the potential effects of measures on surface water quality. Main question answered is: How is the phosphorus concentration in the Leidse Vaart influenced by the sources seepage and agriculture? The start model was the already existing SOBEK model from the Water Board of Rijnland, which models the contribution by agriculture with the rainfall runoff (RR) module and the main waterways (Dutch: ‘boezemkanalen’) with the channel flow (CF) module. Chosen was to change and extend this model to a SOBEK-Delwaq model. Delwaq is the water quality module of SOBEK that makes it possible to model different substances (e.g. nitrogen, phosphorus) and different processes (e.g. adsorption, mineralization, biological activity). Besides that, Delwaq makes it possible to distinguish between sources of these substances (e.g. seepage, sewage treatment plants). Chosen was to only model the sources seepage and agriculture, only model the substance ortho-phosphate (PO4), and only model the processes advection (by mass balance equation) and adsorption (by Langmuir isotherm). For 2 polders (that were actually not located in the flower bulb area), of which one experiences seepage and one experiences infiltration, a sensitivity analysis was done with a reference scenario that has a PO4 concentration of 3 g PO4/m3 for the source seepage or agriculture. The influence of changes in initial concentration, source concentration, water volume in the polder, and residence time of the substances was tested. As check for adsorption, also the conservative tracer chloride was modeled. From this sensitivity analysis it is concluded that the concentration in the main waterways is especially influenced by the source agriculture: a lot of PO4 is added to the polder channels at the same time in combination with a fast discharge to the main waterways. Therefore PO4 at the top of the water column does not come in contact with the sediment, disallowing adsorption to take place. Because seepage is a continuous source with low concentrations, there is much more mixing in the water column and more interaction of PO4 with the sediment. For the Leidse Vaart, every polder in the flower bulb area (consisting of 69 polders) was modeled with the reference scenario to get an indication of the contributions of seepage and agriculture. A polder was assigned to belong to the flower bulb area when flower bulbs were included in the agricultural crop list, and the polder was located in the west of Rijnland. The results are only meant as a basis for future modeling because of the limited processes included in the model. The results indicate that seepage potentially is a big contributor. However, additional research and modeling is necessary to give definite numbers of its contribution.