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Comparison of measured dissolved metal concentrations with the Dutch water quality objectives points at a potential risk for copper, nickel and zinc. However, application of bioavailability corrections derived with BLM into the risk assessment, reveals that actual location specific risks are not expected for copper and nickel at both locations (Asten and Den Bosch) and for zinc at Den Bosch. Only in case of zinc at Asten actual risks may occur. In this refinement preliminary European water quality objectives and background concentrations are also considered.
There is no clear differences in metal concentrations between the sampling locations upstream and downstream from the WWTP influent point of the streams at Asten and Den Bosch. Therefore exceedance of the MPCs can not be ascribed to WWTP effluent contributions in Asten and Den Bosch.
In order to comply with the requirements of the WFD daughter directive EC (2006) 397 conform the monitoring and evaluation of prioritary substances it is recommended to intensify the monitoring van the three metals in the surfacewater during the year in order to derive an annual average or 90 percentile metal concentration values. These values should be compared with the annual average environmental quality standards (AA-EQS). The most important recommendation from the present study is to measured relevant water chemistry parameters concomitant to the dissolved metal concentrations in order to be able to calculate the location specific bioavailability of the metals and apply these in a risk assessment. |