Studies on the Release of Ammonium Nitrogen from the Bottom Sediments in Freshwater Regions

Abstract

In order to evaluate the effect of the bottom sediments on the overlying water quality in an aquatic environment, it is important to clarify the processes of movement and transformation of various substances in the surface layer of the sediments. The porosity of the sediments in this layer shows a gradual decrease with depth and biological activities of the transformation of substances are active. In this paper, the author proposed a model for movement and the transformation of nitrogenous compounds in sediments, undergoing compaction and metabolic processes. This model was applied to the vertical distributions of ammonium nitrogen and particulate organic nitrogen in the sediments of the north basin of Lake Biwa and the following results were obtained : The vertical distribution of ammonium nitrogen in the interstitial water of the sediments was explained in terms of the rate of ammonium nitrogen generation in the sediments and the generation rate was estimated. The vertical distribution of organic nitrogen in the sediments was determined by the ratio of the coefficient of the first-order kinetics for the decomposition of particulated organic nitrogen to the sedimentation rate of particles on the bottom sediment surface (α/U0), the value of which was 5 (/cm). Under the assumption that organic nitrogen are decomposed into ammonium nitrogen and accumulated, the ratio of the diffusion coefficient of ammonium nitrogen in the interstitial water to the sedimentation rate (D/U0) was estimated to be 2, 000 (cm).It was shown from the model that almost half of the particulated organic nitrogen deposited on the bottom sediments returned to the overlying water in the form of ammonium nitrogen. The sediments, therefore, play an important role for the overlying water by acting as the sink of particulated organic nitrogen and as the source of ammonium nitrogen in the water region.