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On the upper reaches of the Ishite River, Ehime Prefecture, Japan, long-term changes in the water quality were investigated to clarify the effects of acid rain on the chemistry of streams flowing out of a moderately acid-tolerant catchment of brown forest soil over coarse-grained granodiorite. The chemical composition of bulk rainwater was determined in the catchment over two time periods, 1986–1987 and 1997–1998, in which the annual mean pH was c. 4.6. From January 1978 to December 2003, several sets of surveys were carried out at three sampling stations along the main stream, i.e., W1 (214 m a.m.s.l., 56.3 km in the catchment area), W2 (445 m) above which there was no village or arable land, and W3 (530 m). A monthly survey of pH at W1 (n = 312) indicated that pH decreased gradually from c. 8.4 in 1978 to c. 7.5 in 1991, then gradually rose to c. 8.1 in 2003. Similar trends were found at W2 and W3. Another survey series that was carried out every three months from February 1985 to November 2003 at W1 (n = 76) indicated that among strong acid anions (SO4 , Cl and NO3 ), SO4 2− concentrations decreased gradually throughout the survey period. Base cations (Ca, Mg, Na and K) and dissolved silica indicated no clear long-term increase or decrease. A similar situation was found at W3. The third survey series, i.e., a monthly survey at W1 from February 1993 to December 2003 (n = 131) covering HCO3 − concentrations and the partial pressure of CO2 gas (Pco2) indicated that HCO3 − and surplus base cations over strong acid anions resulted in the weakly alkaline water. A factor analysis showed that pH levels fluctuated depending on Pco2 rather than HCO3 −

Long-term effects of acid rain on the water quality of a stream flowing out of moderately acid-tolerant brown forest soil: a study of the Ishite River