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.