Numerical simulation of stable isotope fractionation with density changes in regional-scale confined geothermal aquifers

Abstract

Stable isotope compositions in groundwater are widely used to understand hydrological processes. To infer changes in isotope composition in the subsurface due to interacting processes such as dispersion, chemical reactions, and phase changes, numerical simulations were employed to relate the isotope composition in the groundwater to that in the recharge water. It was early identified in the laboratory that the variation of temperature and pressure can lead to the isotope fractionation in water. This indicated that for geothermal aquifers with fluid density contrasts due to the variation of temperature and pressure, the isotope fractionation among varying-density fluids can occur. However, this effect has not been considered in regional-scale geothermal aquifers previously. This study numerically investigated the relationship between density-contrasts and the distribution of stable isotope in confined aquifers with temperatures up to 180 °C and depths of <3000 m. We found that the magnitude of δ18O fractionation relating to density contrasts can reach 0.02‰ per meter in a 100-m-thick homogeneous aquifer. This magnitude is comparable to the δ18O variation with elevation (~0.25‰ per 100 m) and temperature (0.23‰ per °C) on the land surface. Hence, not accounting for isotope fractionation among varying-density fluids can lead to errors in interpreting the recharge area and recharge temperature. An empirical model was derived that relates the magnitude of δ18O fractionation to the density contrast in geothermal aquifers. This model can help correct errors in estimates of the recharge altitude and temperature due to temperature and pressure variations changes in the aquifers.