A four-year investigation on wet deposition in western Thailand.

The investigation of pH variation and the chemical composition of rainwater have been studied from April 2003 to October 2006 at a rural area of Nakorn Pathom Province, western Thailand. The pH value ranged from 4.0 to 7.8 with an average of 6.1. Out of 319 rain events, 72 events were observed in acidic range (pH < 5.6). The pH of rainwater gradually increased over the sampling period. The volume-weighted mean concentrations (microeq/L) of ion species followed the order: Ca2+ > NH4+ > SO4(2-) > HCO3- > NO3- > Na+ > Cl- > CH3COO- > Mg2+ > K+ > H+ > HCOO- > PO4(3-). Neutralization factor calculations reveal that Ca2+ plays a major role in neutralization processes in this area. Enrichment factor calculations indicate that SO4(2-), K+, Ca2+ and Mg2+ were originated from non-marine sources. The statistical measure principle component analysis indicates influence of various local emission sources e.g., soil, limestone mines, biomass burning and agricultural emissions, on the chemical composition of rainwater.