Long-term Decrease of pH of River and Lake Water in the Upper-most Stream Part of the Mountainous Region in Central Japan

Decrease of pH in Past 30 Years in Relation with Acid Rain

Abstract

Annual changes of the pH of upstream river and lake water in the mountainous region in central Japan was investigated by using 32-years (1972-2003) worth of records. Monthly pH at 27 water quality monitoring points (22 points along 17 rivers and 5 in lakes) with negligible anthropogenic pollution effects and precipitation pH at 21 air pollution sampling sites from 1972 to 2003 were analyzed statistically and the characteristics of long-term trends and distribution, along with their relationship, were investigated.
Acid rain monitoring data and atmospheric deposit gauge data indicated that precipitation pH in Nagano Prefecture ranged from 4.8 to 5.3 for the years 1972-2003. Annual mean precipitation pH levels were lower in the northeastern part of Nagano Prefecture, since it was easily affected by the long-range transport of acidic pollutants from the Tokyo metropolitan area and the Asian Continent. The central part of Nagano Prefecture, on the other hand, was not affected as much, with the annual mean precipitation pH level being higher. Seasonal changes of precipitation pH in Nagano Prefecture were attributed to the emission of acidic pollutants in local and neighboring areas, transport of pollutants from extensive emission source areas in the warm season, transboundary transport of acidic pollutants from the Continent in wintertime, and transport of Kosa (Asian dust) in springtime.
In the past 30 years, pH values of upstream river and lake water showed significant decreases (level of significance α= 0.01: 12points, 0.05: 3points) at 15 out of the 27 points. The decreasing pH trend of Hime-kawa, Aokiko, Takase-gawa, Kizaki-ko, Azusa-gawa and Matsu-kawa, where watershed bedrock was made of acidic rock such as granite and rhyolite, was confirmed to be ΔpH= 0.3-0.7 for the 30 years, even after the previous reports (Kurita et al., 1990, 1993). In addition, Nakatsuna-ko, Narai-gawa, Yomase-gawa, Koshibu-gawa Nakatsu-gawa, Tagawa and Susobana gawa also showed decreasing pH tendencies, which had not been reported in the previous papers. The estimated pH decreases of these rivers over the 30 years were 0.3-0.8. In most of these rivers the long-term decreases of pH values were closely related to alkalinity (HCO₃^- concentration) data. This suggests that the long-term pH decreases were caused by acid rain. Of the 15 points where the decreases of freshwater pH were significant, the lowest pH= 6.3 in the year of 2003 was observed in Yomase-gawa. The next lowest pH points were Kizaki-ko (bottom), Nakatsu-gawa, Nakatuna-ko, Aoki-ko, and Takase-gawa, the pH being in the range of 6.4-6.8. In the case of Yomase-gawa, the low concentration of HCO₃^- was due to the inflow of hot spring water with low alkalinity, which reduced the acid neutralization capacity; thus causing a remarkable decline of the river pH in the early snow melting season.
These findings suggest that, in the upstream areas of the central mountainous region, pH values of a number of rivers and lakes with low acid neutralization capacity have been decreasing over the years, and that the decreasing pH areas are extending gradually from areas that lie over acidic bedrock to other areas.