Abstract

 A number of dredging operations have been carried out in the fields (e.g., lakes and reservoirs) minimizing nutrient release from bed sediments. This study examines the effect of the dredging operation on the release of nutrients from polluted bed sediments in laboratory. The sediment cores used in this study were collected from Lake Teganuma, Japan on Apr. 22, 2002.
In the Anaerobic Column test (20 °C, 21 days), a layer of sediments (i.e., 0, 3, 10, and 20 cm from the sediment-water interface) was removed from each core (ID = 10.2 cm, L = 50 cm). The four remaining sediment cores were incubated under anaerobic conditions. The soluble reactive phosphate (SRP) flux rate determined from the control core (0 cm-cut) is 8.4 mg m-2 d-1, while the rate for the 10-cm cut core is approximately 2.0 mg m-2 d-1. In comparison with the control core, SRP released from the 3-, 10-, and 20-cm cut cores are 83.8, 22.7, and 27.7%, respectively. The results suggest that the SRP flux rate decreases with the increasing the depth of sediments dredged.
  In the Sediment Re-suspension Simulation tests, an extent of nutrient (SRP) de-sorption from sediment suspension was evaluated with two layers of sediments (top 0-3 cm and 6-9 cm). The sub samples were mixed separately with distilled water to prepare four different dilutions of sediment suspension (i.e., Vw/Vs = 5, 10, 50, 100; where Vw = volume of distilled water, Vs = sediment volume). After stirring at 350 rpm for 15 min, each sediment suspension was passed through 1.0, 0.45, and 0.2 µm filter, and filtrate was analyzed for SRP. Based on the amount of SRP per unit wet-sediment volume (µg/m3 wet wt), the amounts of SRP released from 0-3 cm layer and 6-9 cm layer of sediments are almost similar. The similarity is also found in different sizes (i.e., 1.0, 0.45, and 0.2 µm) filtrate.
  Conclusions: In the Anaerobic column test, when the top 10-cm (or more) sediment layer was removed from the core, the release rate of SRP decreased more than 70% as compared to the control core (0 cm-cut). However, in the sediment suspension test, we observed no marked differences in the amounts of SRP de-sorbed from the sediment layers with different depths (0-3 and 6-9 cm).