Salt transport and weathering processes in a sandstone cultural relic, North China

摘要

Salt crystallization in pore spaces will usually cause major damage to natural sandstone. Salt distribution and movement are hard to detect in the field within a small scale. Understanding salt generation and transportation in semi-arid regions can help to protect natural cultural relics and control the salt weathering damage. Salt transportation behavior and deposition processes in Yungang Grotto were studied through a detailed hydrochemistry analysis within the context of the atmosphere-rock-water environmental system. Changes of salt types in different media were analyzed according to the sequence of water and salt transportation, integrating the results of atmospheric dry and wet deposition, soil in the aeration area and sandstone, and salt on the rock surface. The study shows the generation of salt is related to vertical transport in the soil and rock weathering zone. Salt transportation originates from soil cover, passes through the sandstone weathering zone, and finally accumulates in the grottoes. Due to coal burning pollution, acid deposits leading to sulfate bind to the soil. Salt generation in soil is the result of neutralization among sulfate, carbonate, and carbonate minerals, leading to cations of soluble salt that give priority to bonding with Ca2+ and Mg2+, while the anions are mainly HCO3- and SO42-. Salt in sandstone weathered crust is not only impacted by soil, but also carbonate dissolution from CO2-rich water, its inverse process, and feldspar dissolution. Rock surfaces in the inner grottoes suffer intense evaporation, resulting in salt accumulation, and carbonate and sulfate mineral deposition or dissolution, producing the salt type of MgSO4 or NaSO4, while HCO3- and Ca2+ are heavily consumed. The result can provide a scientific basis for engineering measures that reduce the hazard of salt accumulation.