MÅLING AF NÆRINGSSTOFFER FRA LAND – TASK 1.1:Målingskvalitet - rapport til Kystvandråd Rødsand og Bredningen

The report’s analysis covers the discharge and nitrogen concentrations from the catchments supplying pumping stations 61000010 and 61000015, which merge immediately after measurement and are subsequently monitored at station 61000089 during the period 2021–2024. The purpose is to assess the quality of the measurements, as there is suspicion that water is being pumped across catchment boundaries. It is expected that catchment-adjusted discharge should be relatively similar across the stations, since geological and hydrological conditions are broadly comparable, given the close geographic proximity of the catchments. The data show substantial and fluctuating differences between the stations from year to year. Particularly noteworthy is that, in some years, the smaller catchment exhibits higher absolute discharge than the larger one, which is hydrologically unlikely. Furthermore, the combined discharge from the two upstream channels (61000010 and 61000015) does not always (some time far from) correspond to the downstream measurement (61000089), as it should. Both observations point to significant issues in the discharge estimates and indicate that water is being pumped across catchment boundaries. The data also reveals large variations in the measured nitrogen concentrations, with the relative magnitude of concentrations between 61000010 and 61000015 changing markedly from month to month. This likewise suggests problems with measurement and accounting. One possible solution is to treat the two pumping catchments as a single combined catchment. This would eliminate issues related to water being transferred from one catchment to another. The challenge, however, is that concentrations are measured at different locations, making it difficult to determine which concentration should be applied, or how to combine the two. Overall, the measurement data are considered highly uncertain, and the results should be used with great caution.