Abstract
Rainwater was collected during June through September at northern Thailand (four sites in Chaing Mai Province, three sites in Lamphorn Province, four sites in Mae Moh basin and three sites in Lampang Province) by bulk and wet only fractional sampler. Average pH and electric conductivity (EC) of precipitation collected by bulk sampler were 6.30 and 20.3μS/cm respectively. Sulfate was a dominant chemical component in anions and calcium in cations. Obtained multiple regression equation by stepwise method was pH=0.055 [Mg2+] +0.027 [K+] -0.015 [SO42-] +6.23 (n=56, R=0.8), variation of pH was explained these three major ions. The lowest pH (3.80) was monitored in Mae Moh basin where was a large lignite coal power plant. On the contrast, average pH and EC of early 1mm precipitation stage collected by wet only fractional sampler were 6.24 and 17.1μS/cm respectively. Moreover at August 17, though pH of precipitation was neutral, abnormal precipitation of ionic composition (EC=300μS/cm, SO42-=2083 and Ca2+ =1863μEq/l) was monitored at Mae Moh basin. Therefore, it was presumed that an occurrence of a certain accident was caused by waste gas treatment facility of Mae Moh power plant. The ionic balance of rainwater calculated from three anions (Cl-, NO3- and SO42-) and six cations (H+, Na+, NH4+ K+ Mg2+ and Ca2+) was relatively poor. Then it is presumed another existence of anions such as HCO3- in the rainwater due to normal pH of precipitation. On the other hand, the pH and EC of precipitation in Yokkaichi in Japan on the same method collected during 1990 through 1992 (wet only sampler), and 1992 through1994 (bulk sampler), where were big industrial complex, were 4.55 (bulk) and 4.34 (wet only fractional), 37μS/cm and 77.3μS/cm.
