Comparison of subtropical surface water chemistry between the large Pearl River in China and small mountainous rivers in Taiwan

• The Tropic of Cancer traverses the Pearl River Basin and Taiwan.
• The water chemistry is quite different in these two regions.
• The weathering rate and ion concentrations are higher in Taiwan Rivers.
• Dilution effect dominates water chemistry in the Pearl River Basin at low elevations.

Small mountainous rivers in Oceania typically have higher sediment yields than larger rivers because of their steep gradients and frequent tropical storm-induced high rainfall. The high sediment yields of these small mountainous rivers enable them to transport a large amount of terrigenous material to the oceans. However, data are unfortunately sparse on this topic. In this study, we investigated the water quality and sediment geochemistry of 35 small rivers in Taiwan and compared these data with that of the large Pearl River.Although Taiwan and the Pearl River Basin are both traversed by the Tropic of Cancer, and thus have similar weather patterns, their water chemistry is quite different. The most notable difference is that despite higher rainfall in Taiwan, and hence, a larger dilution effect, the total dissolved solid concentration (TDS), Mg+2, Na++K+, Cl−, and SO4-2 are generally higher in the Taiwanese rivers compared to the Pearl River. On the other hand, the sediment load in Taiwanese rivers is higher compared with the Pearl River, as expected. These observations reflect the higher rate of chemical weathering and denudation in Taiwan due to its steeper terrain and the larger impact of typhoons, hence episodic heavy rains in that region. The pattern of distribution vs. altitude is also different, with higher TDS, Ca+2, Mg+2, SO4-2,HCO3- and pH values at higher altitudes in the Pearl River Basin, whereas these values are lower at higher elevations in Taiwan. We see this difference because the upper reaches of the Pearl River are primarily covered with limestone, whereas the high mountains of Taiwan are mainly composed of slate and schist that are more resistant to chemical weathering and less affected by acid rain compared with limestone. Pollution-related parameters such as non-sea-salt SO4-2(nss-SO4-2), nss-SO4/(Na++K+) and Cl−/(Na++K+) also suggest that rivers in Taiwan are much more polluted than the Pearl River Basin, perhaps due to the widespread burning of coal and plastic-laden refuse in Taiwan.