The redox potential of hot springs in Taiwan

Abstract

Scientists began acquiring the basic of geology, occurrence, water temperature and chemistry of hot springs in Taiwan over a century ago. However, data regarding redox potential and important redox couples still remains limited. This study explores the redox status of hot springs in Taiwan by measuring Eh in the field and by determining the concentrations of commonly found redox couples, i.e., O2/H2O, NO3/NH 4+, and HSVSO4V-2. Water samples were collected at hot spring discharge pools or the heads of water wells using a pump. A total of 11 hot springs located at 9 different locations across Taiwan were surveyed. The measured values of Eh ranging from -23 to -277 mV indicate reducing conditions. Most of the water samples from the hot spring sources contained sulfide and ammonium. In the Tatun Volcano Group, hot springs originating from a mixture of fumarolic gas and stream water contained high concentrations of hydrogen sulfide as the dominant reducing agent. Ammonium, with concentrations ranging from 1 to 55 mg L-1, is another important electron donor. The finding revealed that there were negative Eh measured-values for dissolved oxygen-contained waters, both in the field and in the laboratory. The presence of sulfide or ammonium was also detected in the samples. These results confirm that the Eh sensor displayed a more heightened sensitivity to sulfide and ammonium than dissolved oxygen and nitrate. Hot springs with deep circulations (Samples S1-S4 and M1-M4) lack in oxygen gas and may react with mineral reducers such that they will consequently be in a reducing state rather than oxidizing. Hot spring waters containing dissolved oxygen (S2, S4, and M2) and nitrate (S3, S4, and M2-M4) most likely have mixed with shallow groundwaters. Discussions reveal implications for redox potentials and redox couples for arsenic speciation, disinfection of ammonium-containing hot springs for the spa industry as well as the possibility of using redox potentials and couples as indices for monitoring earthquakes and volcanic activity.