4. Discussion
The high percentage of chlorine consumption in pond water occurred during the first 2 days of chlorine exposure in both treatments of low and high chlorine doses. This could be largely related to the process of chlorine degradation, which rate depends on the type of compounds present in the pond water and sediment. As chlorine react first with reactive organic compounds (more oxidizable matters) and then with less reactive organic compounds or inorganic decomposition of chlorine (Wong and Davidson, 1977). The presence of greater concentration of organic and nitrogenous compounds in the top soil layer of 0–0.3 cm consumed more chlorine than in layers of 0.9–1.2 and 1.8–2.1 cm. Negative correlation (r=0.83) was found between the chlorine demand with concentration of organic carbon and TKN in soil layers 0–0.3 cm in all chlorine treatments at the end of the experiment support this explanation. Second, it could be related to the initial concentration of chlorine applied. In treatment with higher chlorine concentration applied, the more chlorine demand was resulted which was partly because of the loss of the undissolved chlorine through precipitating and adsorbing to the surface of sediment. Acher et al. (1997) stated that the efficiency of chlorine application was governed by the chlorine dose and the contact time. These explanations are also the reason for the presence of free and total residual chlorine in low chlorine dose at the end of the experiment. There should be no free and total residual chlorine in chlorine dose at 300 mg Cl2 l−1 since soil sediment could consumed more than 1500 mg Cl2 l−1.