A Multiproxy Analysis of Rainwater Chemistry and Moisture Sources in Borneo

Abstract Rainfall variability in tropical region reflects complex interactions between atmosphere, terrestrial and oceans. Hence, this study aims to understand the geochemical processes, which control the rainwater composition and its sources in Ba’kelalan region of Northeast Borneo. Rainwater samples were collected on a daily and monthly basis at different topographic elevations in Ba’kelalan during northeast monsoon. The collected samples were analysed for physico-chemical parameters, including trace metals and stable isotopes using standard protocols. Higher amount of rainfall was recorded during January 2019, particularly in the upstream of the study area (i.e., total of 2,290 mL). The interpretation of data reveals that the acidic to near acidic (4.6 to 6.2) nature of rainwater supports the dissolution of ions. Ca 2+ , Na + , K + , HCO 3 − and Cl − are the dominant ions in the rainwater. K + showed higher concentration (1.5 to 5.9 mg/L) among the cations, whereas HCO 3 − (8.1 to 10.8 mg/L) was more prevalent among the anions. The results indicated that sea salt from the marine source controls Na + , K + and Cl − concentrations, whereas Ca 2+ and Mg 2+ originate from terrestrial soil dust. Isotopic composition reflecting more depleted signatures (δ 18 O, − 8.57 ‰ to − 7.86 ‰; δ 2 H, − 47.50 ‰ to − 40.71 ‰) than the adjoining regions. The moisture was mainly derived from the oceanic and terrestrial sources with respect to different sampling periods. This is the first baseline data on the rainwater chemistry and micro-climatic variations in Ba’kelalan region, providing significant insights into the geochemical characteristics of rainwater in Borneo. Graphical Abstract This is the first attempt to understand the spatial and temporal variation of rainwater chemistry and its controlling mechanisms in Ba’kelalan region of Northeast Borneo. In this study, multiple parameters were analysed in the rainwater samples such as insitu measurements, major ions, nutrients, trace metals and stable isotopes. The data was analysed by using an integrated approach of hydrochemistry, statistics, geochemical modelling, isotopes and air mass backward trajectory transport model. The results find the compositional variability in the precipitation with respect to the sampling periods and locations. This study identifies the possible geochemical processes controls the presence of elements in the precipitation, and also determined the moisture source in this region. The outcome of this study brings a new knowledge on rainwater dynamics in Ba’kelalan, which helps for the sustainable management of rainwater and climate adaptation in this region.