Experimental investigation on effects of acid/base waters on the bottom sediment of Kaita Cove (Hiroshima, Japan)

The decomposition of organic matter existing in bottom sediment produces reduced substances, and this has an influence on the water environment. Recently, it has been pointed out that the water environment can be improved after covering the bottom sediment with alkaline material. In this study, we experimentally investigate the effects of acid and base waters (hydrogen peroxide and calcium oxide solutions, respectively) on bottom sediment. The bottom sediment of Kaita Cove (Hiroshima, Japan) was mixed and stirred with the acid or base water, and then the dissolved carbon content (DCC), the pH, and the ammonium nitrogen (NH4–N) of the overlying solution were analyzed along with the particle size distribution, particulate carbon content (PCC), and particulate nitrogen content (PNC) of the sediment. It was found that particulate organic matter was decomposed under acid water conditions, leading to large decreases in PCC and PNC, and to large increases in pH, DCC, and NH4–N. Importantly, there were no variations in PCC, PNC, or particle size under base water conditions. However, there were increases in NH4–N, and large amounts of DCC remained in the overlying solution. It is evident from the experimental results that base water conditions enhanced both the elution of nutrient salts and the dissolved organic matter from the sediment, but retarded the decomposition of organic matter. These are considered as important factors associated with the improvement of water environments.