Understanding water circulation with tritium tracer in the Tural-Rajwadi geothermal area, India

Abstract

This study investigates the water circulation using tritium measurements in local precipitation, shallow dug wells and thermal springs of the Tural-Rajwadi area, which is one of the potentially productive geothermal areas in India. In the study area, the estimation of tritium-tracer mean transit times (MTTs) in the mixed thermal waters is lacking despite available tritium measurements in 1977 and 2013 and carbon-14 measurements in 2008. As a direct tracer of water movement, tritium radioisotope with a half-life of 12.32 years has been applied to estimate transit times of young waters, especially in the mixture of young and old water components, and tritium sampling was conducted in 2017 to reduce the ambiguity involved with tritium interpretation. For the study area, two scaling factors of latitude effect are established for anthropogenic and ambient tritium levels in precipitation to obtain continuous time-series of tritium in monthly precipitation from the Tokyo area. Annual weighted tritium concentrations in recharge are estimated as a difference between monthly precipitation and evaporation data and are utilized as the tritium input in the convolution integral to obtain tritium-tracer MTTs. Using the exponential piston-flow model (EPM), the representative MTT of shallow groundwater is estimated to be 12.5 ± 4.5 years at TDW-1 well and 15.5 ± 8.5 years at RDW-1 well and uncertainties in MTT due to the EPM ratio and scaling factor are found to be low. For two thermal springs, simulated tritium concentrations of the young groundwater in mixed thermal waters produce unique MTT solutions resulting in MTT of 28.0 ± 12.0 years for THS-1 spring and of 29.5 ± 12.5 years for RHS-1 spring and scaling factor uncertainties are lower than EPM ratio uncertainties, which could be further reduced by an additional tritium sampling. Combining carbon-14 concentrations of 2008 samples with tritium-tracer, the old water MTTs of THS-1 and RHS-1 thermal springs equal to 13500 ± 1500 years and 6900 ± 900 years, respectively, and aggregate MTTs of mixed water are 9189 ± 1023 years for THS-1 spring and 4701 ± 616 years for RHS-1 spring. Using these MTTs with the effective porosity values of 0.01 and 0.03 result in the geothermal reservoir volume of the Tural-Rajwadi area to vary between 347.3 ± 23.3 km3 and 115.8 ± 15.4 km3, respectively, and these volumes could be used to estimate the recharge area with geothermal system depth obtained from deep drilling. In conclusions, we highlight that the environmental tritium provides important information on water circulation in the Tural-Rajwadi area and similar tritium-tracer investigations of young and mixed waters could be conducted in other geothermal areas in Asia.