SHORT-TERM VARIATION IN THE CONCENTRATION OF SELECTED IONS WITHIN INDIVIDUAL TROPICAL RAINSTORMS

Rainwater was collected at the campus of the University of Brunei Daru ssalam in Bandar Seri Begawan, Brunei Darussalam, using a funnel-in-bo ttle sampler. Polypropylene bottles were changed at intervals during r ainstorm events. The pH and conductivity were determined immediately a fter collection on aliquots of the sample. Samples were refrigerated a t 5 degrees C for subsequent chemical analysis. Analyses for Na, Mg, C a, Zn and Fe were carried out by means of inductively coupled plasma a tomic emission spectroscopy (ICP-AES); Cu and Mn were analysed by grap hite furnace atomic absorption spectroscopy (GFAAS); K was analysed us ing flame atomic emission spectroscopy (FAES); and Cl-, NO3- and SO42- were analysed by ion chromatography (IC). Concentration versus time p rofiles are reported for three rainstorm events. All ions exhibited a decrease in concentration during the rainstorm. The first sample conta ined the highest concentration of ions, consistent with a ''first-flus h'' effect. The contribution of the initial stages of the shower to th e total quantity of ion deposited during the entire rainstorm is quite overwhelming; in many cases 20 to 30% of the mass was deposited in le ss than 5% of rainstorm duration. On the other hand, the pH and conduc tivity variation during rainstorms did not exhibit a consistent patter n.