Catchment liming as alternative to use of liquid sodium silicate in the tributary Litleåna, River Lygna, Vest-Agder County

Summary

The salmon river Lygna in southern Norway is acidified, and has been deacidified with calciumcarbonate by use of dosing equipment since 1991. In the tributary Litleåna, which enters Lygna in the anadromous part, liquid sodium silicate has been used for rapid detoxification of inorganic aluminium (Al) since 2011. Due to technical challenges and high costs catchment liming will be considered as alternative to silicate in the future. In this report the scientific, technical and economic aspects of both methods have been evaluated. Whereas silicate binds rapidly with inorganic Al, catchment liming results in retention of Al in the soil and prevents it from leaching to surface waters. Liquid silicate is continuously added from dosing equipment regulated by both upstream pH and water flow. Catchment liming on the other hand is carried out as a onetime operation for an effect over several years, e.g. 20 years. Liming on land, rather than in surface waters, demands careful spreading to avoid liming on areas of barren rocks and certain sensitive marsh types. Use of coarse-grained dolomite with the particle size distribution of 0.2-2 mm is recommended. Based on the assumptions made in this report catchment liming will be significantly less expensive than use of liquid silicate and also significantly less demanding when it comes to technical and administrative issues.