3-3. pH
Figure 3-9 shows the change in pH in the Hyakumakidani stream over 24 years, from July 1998 to December 2022. No significant changes have been observed over the past 24 years, with values around 5.2 and a minimum value of 4.4. The adverse effects of low pH on fish reproduction and mortality have been well documented, since acid rain caused the acidification of river and lake waters in Europe and North America (Wright & Snekvik, 1978, Baker, 1990, Wignington et al., 1990), and the threshold level of pH could be 5 for acid-sensitive fish, such as trout and salmon— especially their fry (Beamish et al., 1975, Farmer et al., 1980). Since all of the streams on Kureha Hill are small-scale streams with steep slopes, no fish live there; however, the water quality of the Hyakumakidani stream is crucial for fish.
Figure 3-10 shows the change in pH with flow rate in the Hyakumakidani stream, showing that pH decreased as flow rate increased. Taking the H+ concentration into consideration, when the pH value is reduced from 5.2 to 4.4, which corresponds with the average value and the lowest value, H+ is increased by 34 μeq/L. On the contrary, the nitrate concentration varied more than 150 μeq/L, indicating that a neutralizing process took place to suppress change in the H+ concentration.
Figure 3-11 shows the change in the acid-neutralizing capacity (ANC) of the stream water against the flow rate. With an increase in the flow rate, the ANC decreased, indicating that the ANC was consumed to neutralize H+associated with nitrate. However, the ANC value in the Hyakumakidani stream was originally low due to the geology and was not sufficient to neutralize the entire increase in H+ associated with nitrate.
Figures 3-12 and 3-13 show the relationship between the sum of cations and chloride concentrations and the flow rate, respectively. No relationship was found, indicating that these ions did not contribute to neutralization.
Figure 3-14 shows the relationship between sulfate concentration and flow rate. The sulfate concentration comparatively decreased according to the increase in the flow rate. The decrease in the sulfate concentration combined with the decrease in ANC is the reason that the H+ concentration did not increase due to the increased nitrate concentration in the high flow rate.
Figure 3-15 illustrates the inverse relationships between nitrate concentrations and sulfate concentrations for the years 1998–2007, 2008–2017, and 2018–2022. The decrease in sulfate concentration prevents acidification due to the increasing nitrate concentration; however, the sulfate concentration at the specific nitrate concentration gradually decreased over time, which means that sulfate’s neutralizing capacity gradually decreased.