An experimental approach to estimate groundwater temperature from 18O fractionation

Abstract

18O has long been a useful tool to understand the physical processes operating in hydrosphere and used as geothermometer to estimate paleotemperature. According to the literature review of this research there is no such reported experimental approach, which records 18O fractionation pattern in liquid water system when there is no phase change involved but temperature gradient exist. In this experimental approach, an attempt has been made to fractionate 18O with response to known temperature gradient by simulating a gently dipping confined aquifer system in controlled laboratory condition. The experimental apparatus was designed in such a way that it replicates an impermeable confined aquifer boundary. Initial condition of the model was homogeneous distribution of different isotopomers of water before imposing the temperature gradient. As temperature gradient imposed on the experimental apparatus 18O and 16O will fractionate as per their preferred thermal regime. Depending on the vibrational frequency of 16O–18O and 16O–16O it is likely that 18O will concentrate at lower temperature regime where as 16O will concentrate at higher temperature regime leading to isotopic stratification. Milli-Q water was used for the experiment to fractionate 18O with known temperature gradient. The experiment was to generate a good dataset, which satisfies the physical fractionation of 18O in real life situation in liquid water system with imposed temperature gradient when there is no phase change of water involved. Keeping the findings of this experiment in mind, we can make an inference that for a gently dipping confined aquifer system 18O stratification occurs identifying heat as the primary cause. This temperature dependent 18O fractionation in liquid water system can be farther used to estimate the groundwater temperature with reference to 25 °C benchmark temperature.