Evaluation of the Ion Components for the Estimation of Total Nitrogen Concentration in River Water based on Electrical Conductivity

Abstract

It is generally known that the nitrogen concentration in river water is higher in agricultural areas. However, measurement of the nitrogen concentration is time- and cost- intensive. This study estimated total nitrogen (T-N) concentration based on electrical conductivity (EC) in river water at two agricultural watersheds area which are the Tokachi area, the main land uses are upland and daily farming and the Nemuro area, the main land use is daily farming. The ion components in river water were also considered. The investigation was carried out from August to early September in 2003 to 2006 and in 2012, and T-N concentration, EC and ionic components in river water were measured. In the two areas, there was a positive correlation between T-N concentration and EC in the river water. From this result, T-N concentration was able to be estimated from EC. However, the slope of the regression line differed between the two areas. It is causally related to the effect of the ionic components, because there was a very strong correlation between the concentrations of cationic and anionic components in river water and the EC. In the Tokachi area, SO₄^2- and NO₃^- account for a high percentage of the anionic components. These derive from fertilizer, which correlates positively with T-N concentration. In the Nemuro area, Na⁺ accounts for a high percentage of the cationic components. In comparison with sea salt, the Na⁺ concentration is higher than Cl^- concentration. In addition, there was no correlation between Na⁺ and T-N concentration. In the Nemuro area, the outflow of ion component of geological origin affects EC. In light of the above, T-N concentration was able to be estimated based on EC; however, it is necessary to consider the difference in the concentrations of water quality components for each region.