Constraining the mean transit time (MTT) of relatively modern thermal waters in Deccan volcanic geothermal area, India using tritium tracer

Abstract

Constraining the mean transit time (MTT) of thermal waters is essential to understand the sustainability and renewal rate of geothermal systems. In this study, tritium concentrations of samples from five thermal water manifestations, three shallow groundwater wells (dug wells) and one deep groundwater (borehole) are analysed at regular time intervals for three years to estimate the MTT of the thermal and non-thermal waters in the Deccan Volcanic geothermal province by using lumped parameter models. The tritium concentrations of the thermal waters are found to vary between 0.77 and 2.77 TU (±0.0.23 TU at 1σ) which are lower than those of the shallow non-thermal groundwater (5.4–7.18 TU) (±0.40 TU at 1σ) whereas the tritium value for the deep groundwater ranges between 2.30 and 2.78 TU (±0.27 TU at 1σ). Among different lumped parameter models, PFM (piston flow model), EMM (exponential mixing model) and EPM (exponential piston flow model) are used to estimate the MTT of the groundwater and thermal waters in the study area. The historical records of weighted mean concentration of tritium in precipitation at the nearby Tural-Rajwadi geothermal area is taken as input function in the modelling procedure. The PFM and EMM models represent the lower and upper case of the EPM model mixing, respectively whereas the values obtained from the EPM models provide the actual range of mean transit times. The overall uncertainty in the MTT estimation has also been computed. The MTT of the younger fraction of the thermal waters in Ganeshpuri thermal water (HS-1) and Akloli thermal water (HS-7) are found to be 70 ± 5 and 90 ± 14 years old respectively. The MTT of both the shallow groundwater (DW-1 and DW-3) samples is estimated to be ~1.5 years whereas the MTT of the deep groundwater (BH-1) is 39 ± 6 years. Thermal springs in this area undergo mixing with non-thermal water during its ascent towards the surface as a result binary mixing model has been applied to find the MTT of the mixed thermal spring. The aggregate MTT of HS-1 turns out to be 7860 ± 870 years. This study of MTT estimation can be used for accessing the sustainability and renewability of these thermal waters compared to other thermal areas in Deccan volcanic region.