Abstract
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 km2 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 (SO42-, Cl- and NO3-), SO42- concentrations decreased gradually throughout the survey period. Base cations (Ca2+, Mg2+, 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- concentrations, which were c. 105 times greater than H+ concentrations. An increase or decrease in Pco2 seemed to have greater effects on pH than on HCO3- concentrations, due to the changes in the same amount of H+ and HCO3- through the dissociation of H2CO3. Based on the literature, concentrations of base cations at W1 in the 1980s and 1990s were greater than those between 1949 and 1950, and were accompanied with increases in both SO42- and NO3- presumably supplied by acid rain. The acid rain appeared to hasten the velocity of mineral weathering in the catchment soil. From 1950 to the 1980s-1990s, almost no increase in HCO3- concentration and only a slight increase in pH were observed. In an outline, hypothetical changes in CO2 production in the soil depending on the stand age of the forest agreed with changes in the stream pH.
