Slamhalter i flodmynningar 2018- 2020: Utvärdering av halter och möjligheter att använda turbiditet som proxy

Suspended solids concentration in rivers, streams and lakes is an important water quality parameter and is included in criteria for determining ecological status in surface water bodies in many European countries. The amount of particles can be measured in different ways, e.g. through filtration of a water sample followed by weighing the amount of particles on the filter (termed suspended solids) or indirectly using optical methods where light scattering is measured (termed turbidity). At the moment, turbidity is a part of the national monitoring programme for river mouths, while suspended solids is not. During the years 2018-2020, a project to sample and analyze suspended solids monthly in all river mouths was carried out, to see if it can be motivated to include in the monitoring programme. In some cases there is a good relationship between turbidity and suspended solids, where turbidity can be used as a proxy for suspended solids, and turbidity is already included in the monitoring programme. This report has evaluated the data that was collected during the years 2018-2020 for suspended solids, to investigate where there are high loads and concentrations of suspended solids, and where it may be motivated to include measurements of suspended solids in the monitoring programme. The possibility to use turbidity as a proxy for suspended solids has also been investigated. We have been able to identify a number of sites with high concentrations of suspended solids and sites with a considerable yearly load to the sea. For several stations a good relationship between suspended solids and turbidity was identified, where turbidity potentially could be used as a proxy for suspended solids. There is a relationship between R2 -values and amount of clay and arable land in the catchment. Over a certain percentage (2.5% clay content and ca 12% arable land) the R2 -values are consistently high, while for lower percentages there is a higher spread in the R2 -values. We are presenting a list of stations with either a high median concentration of suspended solids or a high load to the sea, and a measurement of the relationship between turbidity and suspended solids. We believe that an expanded sampling can be motivated in a number of stations, with a focus on stations with a high proportion of arable land or clay, where the risk of erosion and increased losses is higher.