The source of rock moisture in a sandstone cave in arid northern China: rainfall infiltration or vapor condensation?

During summers with intensive rainfall, water droplets and/or rock moisture are observed in some of the ancient caves of the Yungang Grottoes, northern China. There is debate on whether the water is derived from infiltrating precipitation or from condensation of water vapor, and this restricts the choice of appropriate methods to alleviate weathering of the stone carvings. By monitoring soil water content at the top of the vadose zone and analyzing responses to rainfall events, it is found that the silt overlying the rock mass plays the role of barrier to rainfall infiltration. By monitoring rock moisture in a cave wall at shallow depth, using frequency domain reflectometry, it was found that variations in rock moisture are directly controlled by the fluctuating vapor concentration. Moreover, there is a significant linear relationship between the magnitude of rock wetting (moisture addition) and the cumulative difference in vapor concentration between the inside and outside of the cave. Thus, the condensation of water vapor, which leads to decreased vapor concentration in the cave atmosphere, is the source of the rock moisture. Also, the period associated with rapidly increasing rock moisture and the highest rock moisture corresponds to the period with occurrence of water droplets in some caves. This study confirms that the water vapor is an important source of water which is responsible for weathering in caves, and indicates that the approach of estimating groundwater recharge based on cave drip rates should be used with caution, especially in arid environments.