Abstract
We have measured the concentrarion of nitrate-nitrogen (called as ’concentration’ hereinafter) in tap water as a proxy of the concentration in the Yodo river. We examined the data with some data which are obtained from public offices, and found as follows. (1) Concentration in the Yodo river depends on the discharge at the Seta weir, i.e., the concentration decreases when the discharge increases. Water in the Biwa lake whose concentration is relatively low contributes to the decreasing. (2) On the other hand, when the discharge increases, the load of nitrate-nitrogen in the Yodo river often increases. Large amount of discharge contributes to the increasing. (3) Concentration in the tap water, namely, that in the Yodo river decreases when temperature is relatively high. The possible causes are that plants and denitrifying bacteria are active when temperature is high.
