Methods to derive natural background concentrations of metals in surface water and application of two methods in a case study

In this report, by Deltares and KWR, an inventory and evaluation of methods to derive natural background concentrations of metals in surface water are described. Six available methods are discerned of which three are evaluated: the clean streams approach, the sediment approach and the origin of surface water approach. A fourth method, the monitoring data approach, is recommended for quality control purposes. For open sea the clean stream approach is considered useful, for large Dutch rivers the sediment approach is recommended. For estuaries and coastal water the best method depends on the relative contribution of sea water and river water. For Dutch regional waters the background concentration may be based on the natural composition of groundwater, but due to lack of data the monitoring data approach is recommended.