Recent studies of sediment capping and flocculation for nutrient stabilisation

Water quality in the Rotorua Lakes has declined in the past 30 to 40 years due to increasing nutrient loads, specifically nitrogen and phosphorus. In situ restoration techniques, including sediment capping and flocculation, have been developed to attempt to reduce internal nutrient loads, which can be comparable in magnitude to external loads in eutrophic lakes. The aim of this report is to summarise the current state of knowledge, and to documents some recent studies of sediment capping and flocculation techniques designed to remove nutrients from the water column by retaining them permanently within the bottom sediments. Experimental set-ups for testing the efficacy of different materials range from conventional batch adsorption studies to sediment reactor experiments. Natural ecosystems have also been simulated with mesocosms in Lake Okaro. A full scale application of aluminium sulphate (alum) in Lake Okar. A full scale application of aluminium sulphate (alum) in Lake Okaro has also been intensively monitored. The different studies have provided information on the restoration potential of some sediment capping agents and flocculants, but many questions still remain. Given the current state of knowledge it is not possible to confirm a priori the circumstances under which a whole lake trial would be successful. Future research should be carried out on the follow foci: • Establishment of chronic or acute toxic effects of the adsorbent materials. • Effect of treatment applications on benthic biota, particularly capping materials that may alter physical characteristics of the sediments. • Application of computer models for the purpose of both hindcasting and better understanding effects of application of an adsorbent to a lake, and for the purpose of predicting the changes in trophic status. • Application techniques and costs, particularly in view of potential for quite radical changes in source of adsorbent materials, grain sizes and methods of application for a single adsorbent