In situ redox potential measurements as a monitoring technique for hot spring water quality

Previous studies on the redox potential of hot springs have indicated that redox potential sensors display a more heightened sensitivity to the HS⁻/SO₄ ²⁻ redox couple, and that redox potential is not related to many different redox couples. However, previous studies have not focused on long-term, continuous measurement of hot spring aquifers. Therefore, three continuous in situ redox potential (Eh) measurements, each lasting approximately 1 month, were conducted using a platinum electrode in a 32 °C hot spring aquifer. Measured Eh values were −353, −343, and −329 mV in the three monitoring campaigns. The chemical properties of water sampled from the aquifer indicate that S²⁻ concentrations ranged from 0.04 to 0.19 mg/L and Fe concentrations were below the quantification limit (<0.003 mg/L). Comparison of measured Eh values and those calculated using HS⁻/SO₄ ²⁻ and S²⁻/SO₄ ²⁻ redox couples show an agreement within 0.02–0.05 V. This indicates the effectiveness of long-term, continuous, in situ redox potential measurements as a monitoring technique for hot spring aquifers.