The management of sediment, soil and water in the Netherlands dates back to the first settlements in the lower Northern and Western parts of the Netherlands. Around 500 B.C. farmers constructed ‘terps’ (artificial dwelling mounds) to protect against floodwater. The Romans (50 B.C. – 250 A.C.) reshaped natural waterways, to improve transport by ship. This also meant that river embankments were constructed and waterways and harbours had to be dredged. With the construction of river dikes in the period 700 – 1200 A.C. the sediment challenge began. The peatlands behind the dikes dried out, creating land below the average sea level. Dikes and dike maintenance therefore became crucial and since 1255 A.C. the first official governmental bodies, public utility boards (waterschappen) were created. These public utility boards together with the Dutch government, provinces and cities are still responsible for sediment and water quantity and quality. With this timeframe in mind, the scope of this thesis, examining developments in sediment and water quality management in the Netherlands over the past 25 years (1993 – 2018) is relatively short. A critical focus during these past 25 years was the heritage of industrial pollution of waterways and sediments from the early twentieth century. During the past 25 years changes took place in the way risks of contaminants in sediment and water were evaluated, partly due to scientific progress on the ecotoxicological impact of contaminants. As important as the scientific progress were the policy and legislation changes. These changes are driven by a broader spectrum of societal needs, like safety against flooding, scarcity of public funds and the need to change to a more circular economy using sediment as a resource. The goal of this thesis is to help water managers to understand the mechanisms that change the ecotoxicological risks in their water and sediment systems, providing tools that go beyond the legislation requirements to assess these risks.

In this study the spatial distribution of dissolved metals in surface water is studied at nine locations in Lake Ketelmeer (the Netherlands). The measured dissolved metal concentrations are combined with the local water quality parameters for salinity, pH, alkalinity and DOC to calculate a FIAM Free Ion Activity Model (FIAM) and the Biotic Ligand Model (BLM) based bioavailable metal concentration. The BLM model is used for Cu, Ni, Pb and Zn and the FIAM model for Cd, Cr, Cu, Ni, Pb and Zn. To be able to compare the dissolved metal concentration with the FIAM or BLM based bioavailable metal concentration, an accepted reference standard can be used which is also corrected for the bioavailable concentration. Here the Water Framework Directive (WFD) Annual Average Quality Standard (AA-QS) is used, corrected for the FIAM and BLM based bioavailable metal concentration under reference conditions. This yielded a site specific Risk Characterization Ratio (RCRFIAM/RCRBLM). The FIAM model shows an exceedance of the site specific AA-QS for Cu (RCRFIAM of 1.8) and Pb (RCRFIAM of 1.5) in the northern middle part of the lake. This is due to a lower pH in this part of the lake. The BLM model was inconclusive with regard to spatial trends for Cu and Ni due to out of boundary conditions for the model. For locations where the BLM model was within the model boundary conditions, the RCRBLM could be as high as 7.5 for Cu and 3.2 for Ni. The main water quality parameter causing the high RCRBLM was the low DOC concentration. To establish if the locally increased RCR for Cu and Pb (FIAM) or Cu and Ni (BLM) poses an ecotoxicological risk to organisms the multi substances Potentially Affected Fraction (ms-PAF) model is used. The FIAM based ms-PAF indicates that the northern middle part of the lake has the highest chronic metal exposure risk, with an ms-PAF of 27%. The BLM based ms-PAF has a maximum of 45%, but lacks a spatial trend due to the missing BLM corrected Cu and Ni concentrations for some locations.