Hydrochemistry process and microweathering behaviour of sandstone heritages in the Nankan Grotto, China: Insights from field micro-observations and water–rock interaction experiments

Geochemical analyses of groundwater samples and water‒rock interaction experiments were conducted to elucidate the hydrochemical process of the water‒rock interactions in the Nankan mountainside. Additionally, the mineralogy and microstructure of field sandstone samples were analysed to explore the microweathering behaviour of minerals in the Nankan Grotto. The results indicated that the cations in the groundwater followed the concentration order of Ca²⁺  > Mg²⁺ ≈ Na⁺  >  > K⁺, while the anion concentrations were in the order of HCO₃⁻  >  > NO₃⁻  > SO₄²⁻ ≈ Cl⁻. The Piper, Gibbs, and ion correlation diagrams suggested that the groundwater was characterized as HCO₃-Ca type. Three stages, the calcite dissolution stage (Stage I), feldspar weathering stage (Stage II), and faint‒weathering stage (Stage III) were observed in the water‒rock interaction experiments. Based on microscopic petrographical analysis, the dissolution of calcite released bulk Ca²⁺ and HCO₃⁻, and this was the primary hydrochemical process that accounted for the HCO₃-Ca type groundwater. The weathering of biotite to chlorite-(Mg) introduced Mg²⁺ and K⁺. The weathering of albite and plagioclase-TIMA to sericite or other clay minerals released Na⁺ and Ca²⁺ into the solution. Slight weathering of K-feldspar released minor amounts of K⁺ into the solution. The abovementioned weathering processes resulted in the poor cementation and loose microstructure of the sandstone, leading to the macroscopic deterioration of the sandstone heritage site. Our research revealed the deterioration mechanisms of the sandstone in the Nankan Grotto, which paves the way for the subsequent conservation of sandstone heritage sites.