Evaluation of Release Rate of Nutrients and Hydrogen Sulfide at a Borrow Pit with Whole Sand Capping Using Granulated Coal Ash in Lake Nakaumi

Sand capping using granulated coal ash was carried out for the entire surface of the Hosoioki borrow pit in Lake Nakaumi. The water quality in the borrow pit was measured, and the release rates of nutrients and hydrogen sulfide were determined by the benthic chamber method to estimate the effect of sand capping. NH4-N and PO4-P concentrations of overlying water at the center were reduced after the sand capping; however, the nutrient concentration still changed greatly in the summer. H2S concentrations differed locally, but a declining tendency of the concentration was confirmed in comparison with before sand capping. The mean release rates of NH4-N, PO4-P and H2S were 267 mgN m-2 day-1, 35.0 mgP m-2 day-1 and 227 mgS m-2 day-1, respectively. These rates were reduced by approximately 14%, 22% and 88%, respectively, compared with before sand capping. Since nutrients and H2S were contained in the pore water of sediment, their existence is the cause of decline in the release-suppressing effect. The nutrient-release-suppressing effect was low. On the other hand, the H2S-release-suppressing effect was high and continued for around 2 years under the condition of sedimentation on the granulated coal ash.