Environmental factors regulating river water quality

in the regenerated Ishite River below a diversion weir

Abstract

In an upper reach of the Ishite River, the stream water is completely diverted at a weir except under flood conditions, and is regenerated below the weir by groundwater. To investigate environmental factors regulating the water quality of the regenerated stream, a 6-year monthly survey was undertaken 2.4 km below the weir between 10:00 and 14:00 from January 1992 to December 1997. Using 51 samples under base-flow conditions, a factor analysis suggested that the water quality was mainly controlled by four factors. Factor 1 accounted for 22.4% of the total variance and correlated positively with Ca²⁺ and Mg²⁺ concentrations and C_B-C_A (surplus strong base concentration). Since the factor 1 score correlated negatively with the flow-rate (γ=-0.618, p<0.001), Ca²⁺ and Mg²⁺ concentrations seemed to be controlled by the flow-rate. Factor 2 correlated positively with Na⁺ and K⁺ concentrations and C_B-C_A, and was responsible for 17.4% of the total variance. Factor 3 correlated positively with SO₄^2- and Cl^- concentrations, and factor 4 correlated positively with pH and negatively with NO₃-, accounting for 15.4% and 13.3% of the total variance, respectively.
Multiple regression analyses showed that both Ca²⁺ and Mg²⁺ concentrations were controlled by flow-rate and water temperature. The flow-rate reduced both concentrations, but the temperature increased Ca²⁺ concentration and reduced Mg²⁺ concentration, resulting in an increase in the Ca/Mg ratio.