Hydrogeochemical weathering of reservoir-bank heritage sites: Water-rock interaction mechanism at Bingling Temple Grottoes, NW China

Abstract

The Bingling Temple Grottoes, a UNESCO World Heritage site, are undergoing accelerated weathering due to changes in hydrogeochemical conditions following the construction of a nearby reservoir. This study examines the chemical composition and evolutionary trends of three water types—reservoir water, spring water, and fissure water—all of which impact the grottoes. For the first time, we integrate 55 sets of historical data and on-site field data collected from the Bingling Temple Grottoes. We also propose a research framework that combines hydrochemical indicators with software-based calculations. The results reveal the following: (1) A distinct hydrochemical differentiation: reservoir water is primarily of the HCO₃-Ca type, while spring and fissure waters have evolved from SO₄-HCO₃-Ca to mixed HCO₃-SO₄-Na types due to prolonged feldspar dissolution. (2) Fissure water exhibits the highest concentrations of Na⁺, Ca²⁺, and SO₄²⁻, driven by sustained water-rock interactions, whereas reservoir water remains low in mineral saturation. (3) HCO₃⁻ plays a critical role in gypsum dissolution and enhances rock weathering under alkaline environments. (4) Seasonal water level fluctuations intensify ion exchange and salt precipitation, posing a direct threat to the preservation of murals and sculptures. This study provides the first comprehensive assessment of reservoir-regulated hydrogeochemical processes affecting grotto heritage sites. It highlights how changes in water chemistry, particularly bicarbonate dynamics, can exacerbate the deterioration of sandstone relics. Our findings offer a scientific foundation for developing strategies to mitigate hydrological risks to cultural heritage sites located near reservoir banks.