Study of diel hydrochemical variation in a volcanic watershed using principal component analysis: Tatun Volcano Group, North Taiwan

Heavy metal contamination commonly appears in mining areas and volcanic watersheds due to the acidic drainage water. Under the hydrochemical condition, the solar photocycle would result in changes of the water temperature, in photosynthesis and in iron photoreduction, which leads to substantial hydrochemical fluctuations, especially for heavy metals. It is important to consider the daily variations in water quality when developing a hydrochemical monitoring plan for an area with highly developed agriculture, such as the Tatun Volcano Group watershed area in this study. The results show that the water chemistry is highly complicated by both solar photocycles and hydrochemical fluctuations in the upstream area. Using principal component analysis, the contributions from the two factors can be successfully separated. During the daytime, the photocycle results in the formation of aluminum hydroxide, which can remove heavy metals from water. Consequently, the content of heavy metals, including As, Cu, Ni, Co and Ba, increases after sunset and can reach a maximum before sunrise, while Fe behaves inversely due to the photoreduction. The variation of As during a diel cycle can reach 97%. However, the content of most of the heavy metals during diel cycle is incomparable with those in the earlier studies due to the formation of aluminum hydroxide instead of iron hydroxide. The other significant factor, hydrochemical fluctuation, can explain the variation of major components in water including Cl, SO₄. Rare earth elements (REEs) were also analyzed and can be an excellent natural tracer in this study. The distribution of REEs shows a depletion of light REEs and an even normalized concentration of middle and heavy REEs. It is theorized that the REEs in the water in this study derive mainly from the reservoir rock of geothermal water. In a hydrochemical monitoring plan, REEs can be an indicator for identifying an anthropogenic source.