Application of water chemistry as a hydrological tracer in a volcano catchment area: A case study of the Tatun Volcano Group, North Taiwan

In this paper, water chemistry is successfully applied to elucidate hydrological processes through the use of natural tracers in a hydrological system. The concept of a natural tracer is principally based on water–rock interaction. In this case, a volcanic watershed in the Tatun Volcano Group is examined with the hydrochemistry of the Peihuang Creek system analyzed in terms of acidic hydrothermal water. The application of principal component analysis demonstrates that the hydrochemistry of Peihuang Creek is dominated by mixing among three end members, Lujiaoken seep water, Matsao seep water and shallow circulated water. Conservative ions, such as halogens, reveal that recharge of shallow circulated water with low ionic concentration is dominant in the mountain area and gradually becomes insignificant in the plains area. Rare earth elements also confirm this derivation. In addition, rare earth elements demonstrate parallel pattern along the tributaries, which implies that the fractionation of rare earth elements is not considerable and dilution is the major factor attenuating the concentrations of rare earth elements. Therefore, the constant slope of REE pattern allows for semi-quantitative estimation of mixing proportion of the two major tributaries. The results show that Lujiaoken Creek supplies about 50% of waters to the downstream Peihuang Creek. Comparing commonly used nature tracers, such as Cl− and environmental isotopes, this study demonstrates that rare earth elements have the advantage of very low background concentrations and easily defined sources if fractionation is not considerable. Under this circumstance, the calculation of water mixing is applicable.