WRRCTR No.160 Chemical Constituents of Rainfall at Different Locations on Oahu, Hawaii

Abstract

Rainfall sampling, which began in 1981 before the 1982 to 1983 El Niño and continued into 1984 on Oahu in Hawaii, represented sites with widely different rainfall amounts. Samples stored under refrigeration prior to analysis were analyzed for pH and for the major cations, chloride and sulfate, and the nitrogen series. For the stored samples, the rainiest site was acidic with median pH 4.9, and rainfall weighted pH 4.77. Samples analyzed immediately on receipt at the laboratory had median acidity of pH 4.2 and rain-weighted pH 4.22. Samples that included sane cloud water had acidity with a median pH 5.1 and rainfall weighted pH 5.17. Samples taken before and after the eruption of El Chichon in April 1982 showed no apparent effect of the stratospheric sulfur on rainfall acidity. Electrical conductivity of the rainfall was equivalent to cation concentrations of 1 to 10 milliequivalents per liter. Chloride, a major cation, decreased with distance from the ocean sources. Sulfate values in the rainwater increased during southerly flow when Kilauea volcano was erupting, so that the sulfate to chloride ratio increased to tenfold that for seawater. Organic nitrogen forms made up about 40% of the total nitrogen content of the rainfall, and calculated organic-N loadings from the annual rainfall had values from 4.39 to 1.62 kg/ha/yr. The pH values of 4.2 to about 5.0 support the contention that the acidity in naturally occurring rainfall in remote areas has a pH slightly below 5.0.