Geochemical implication of 87Sr/86Sr ratio of high-temperature deep groundwater in a fractured granite aquifer

Abstract

The Dongrae thermal water area located at the southeast margin of the Korean Peninsula is one of the oldest hot springs in Korea and has been used in spas since the 9th Century. In this paper, a geochemical significance of 87Sr/86Sr ratios from deep thermal water, groundwater, surface water and rainwater in the Dongrae area is discussed. The bedrock of this thermal water-bearing aquifer is composed of Mesozoic granitoids. For temperatures up to 71°C, the thermal water is Na-Cl type, whereas shallow cold groundwater is Ca(-Na)-HCO3 type. 87Sr/86Sr ratios of the thermal water are in the 0.705651 ± 11-0.705696 ± 12 range and have remained nearly unchanged over the past 4 years (2004-2007). 87Sr/86Sr ratios of the shallow cold groundwater, surface water and rainwater range from 0.705781 ± 26 to 0.705789 ± 12, 0.706700 ± 14 and 0.707375 ± 11 respectively. 87Sr/86Sr ratios of the thermal water in the Dongrae area are lower than those of groundwater, surface water, rain water as well as aquifer bearing granite. These Sr isotopic signatures indicate that the circulation rate between thermal water and current meteoric water, including groundwater, surface water and rain water in the Dongrae area should be very slow. Therefore, the thermal water might be derived from a high temperature paleo-groundwater reservoir rather than from circulation of young meteoric water heated by current heat sources. Our data show that 87Sr/86Sr ratios may become an important time lag indicator for the groundwater cycle between deep and shallow groundwater in a fractured granite aquifer system.