Geothermics 58 (2015) 62–74 Contents lists available at ScienceDirect Geothermics jo ur nal homep age: www.elsevier.com/locate/geothermics Use of mean residence time and flowrate of thermal waters to evaluate the volume of reservoir water contributing to the natural discharge and the related geothermal reservoir volume. Application to Northern Thailand hot springs Carmine Apollaro a , Giovanni Vespasiano a , Rosanna De Rosa a , Luigi Marini b,∗ a Department of Biology, Ecology and Earth Sciences (DIBEST), University of Calabria, Via Ponte Bucci 4, Cubo 15B, I-87036 Arcavacata di Rende, CS, Italy b Luigi Marini, Consultant in Applied Geochemistry, Via A. Fratti 253, I-55049 Viareggio, LU, Italy a r t i c l e i n f o Article history: Received 6 May 2015 Received in revised form 8 September 2015 Accepted 17 September 2015 Available online 29 September 2015 Keywords: Thermal water Mean fluid residence time Flowrate Northern Thailand Reservoir volume a b s t r a c t Tritium-based mean residence time,  w , and flowrate, Q w , of thermal waters from eleven different sites of Northern Thailand were used to evaluate the volume of reservoir waters contributing to the natural discharge, V w , and the volume of the related geothermal reservoirs, V res , assuming steady-state conditions and using simple relationships. Measured 3 H contents of the thermal waters of interest, apart from San Kamphaeng, range from 0.4 to 1.3 TU. Based on the well-mixed reservoir model, mean fluid residence times between 500 and 1700 years are obtained. For most thermal waters of Northern Thailand, calculated V w range from 0.13 to 0.68 km 3 , whereas V res vary from 13 to 68 km 3 , assuming an effective porosity of 0.01. The only exception is San Kamphaeng, that has a high discharge leading to a calculated V w of 2.2 km 3 and V res of 220 km 3 , although its mean fluid residence time of 955 years is within the range of other Northern Thailand thermal waters. Even though the uncertainties on both V w and V res are relatively high, the simple approach used here may be used to evaluate geothermal resources during the exploration stage. The obtained V res may be used in the volume method of geothermal resource assessment (which is usually utilized to evaluate the productive potential of unexploited and minimally exploited geothermal fields) and may be compared with the V res estimated by means of other approaches. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction At steady state, the mean residence time of a thermal water in a geothermal reservoir,  w , and the flowrate of the thermal water discharged at the surface, Q w , are linked through the well-known relation (e.g., Kazemi et al., 2006 and references therein): Q w = V w  w , (1) where V w is the volume of reservoir water contributing to the natu- ral discharge. This reservoir water comprises both the mobile water present in fissures and the stagnant, or quasi-stagnant, water occur- ring in the porous matrix (Zuber et al., 1995). Assuming a suitable value for the effective porosity, , the volume of the geothermal ∗ Corresponding author. Tel.: +39 584 945230. E-mail address: luigimarini@appliedgeochemistry.it (L. Marini). reservoir hosting the naturally discharging water is given by the following equation: V res = V w + V r = V w  (2) where: V res = total volume of rock and fluids contained in pore space; V r = volume of solid non-porous rock of the reservoir; V w = volume of water in the voids. Use of Eqs. (1) and (2) allows one to evaluate the geothermal resource volumes, to a first approximation. In spite of their useful- ness and simplicity, Eqs. (1) and (2) are rarely used in geothermal studies, as far as we know, although there are some noteworthy exceptions (e.g., Goff et al., 1991; Zuber et al., 1995; Shevenell and Goff, 1995; Vespasiano, 2015). Residence time and flowrate are generally not discussed together in geochemical investigations. In this communication we present an example of application of Eqs. (1) and (2) to estimate the volume of reservoir water contributing to the natural discharge, using geochemical data http://dx.doi.org/10.1016/j.geothermics.2015.09.006 0375-6505/© 2015 Elsevier Ltd. All rights reserved.