Calcium Isotopes in Hot Spring Water: Constraints on the Evolution of Groundwater

The geochemical evolution of water-rock reactions is controlled by many processes before exposed to surface. Identifying the hot spring evolution and its controlling processes has always been an unsolved problem.

Hot springs carry plenty of information about the groundwater movement and the water-rock interaction, which provide a chance to understand the evolution of groundwater. Calcium isotopes (δ^44/40Ca) are useful for tracing water-rock interactions because secondary minerals preferentially uptake ⁴⁰Ca, resulting in waters are enriched in heavy Ca isotopes. Based on these theories, water samples were collected from hot springs in Sichuan and Yunnan provinces, and the relationship between the behavior of Ca isotopes with groundwater evolution was studied.

Results show that both elemental and Ca stable isotope values of hot spring water are in a wide range. According to cationic concentration, water samples could be divided into two groups: Ca-rich water (Ca>Si) and Si-rich water (Si>Ca). Si-rich water might come from silicate aquifer, with larger variation of δ^44/40Ca values (0.45‰ 2.16‰) comparing with global large rivers. Si-rich water samples with high fluorion concentration had lower isotopic compositions (0.45‰ 0.78‰), which might be caused by fluorion fluid inflow. However, the Ca isotope values of other Si-rich water sample were inversely proportional to Ca concentration, suggesting calcite precipitated during groundwater evolution. On the other hand, Ca-rich water might come from carbonate aquifer, one of them with highly soluble component (Na, Cl, SO₄) owned δ^44/40Ca value (1.12‰) similar with that of the nearby river water(1.01‰), could represent groundwater at the early stage of evolution. With the evolution of groundwater, highly soluble components moved out and insoluble components (Ca and Si) increased, so did the δ^44/40Ca value, suggesting groundwater is at late stage of evolution which had suffered from calcite deposition and fluid mixing. This study highlights that Ca isotopes combined with water chemistry data could help recognizing the evolution of groundwater.