Measurement and characteristics of radon flow in an extremely arid region based on a closed-system earth-air model.

Radon (222Rn) is a radioactive gas that occurs naturally in the soil and is harmful to the environment and health. However, the measuring the amount of radon flowing is challenging. This study reveals the mechanism responsible for radon transportation and concentration variation, the main driving forces acting, and the key factors operating in the vadose zone. In this study, two separate holes were used to monitor the amount of earth-air and radon flowing in and out of the soil in the extremely arid region in China where the Mogao Grottoes are located. Using a closed-system model, the quantity, characteristics, and regularity of the flow of earth-air and radon were thus determined on daily and yearly timescales. The same patterns of variation in earth-air flow and radon concentration were found at the two sites, both depending on the variation in the atmospheric pressure (AP). When the AP decreases, earth-air flows out from the soil with a high radon concentration. Conversely, when the AP increases, earth-air enters into the soil with a low radon concentration. Thus, radon is continuously emitted from the soil. The concentration of radon in the earth-air is proportional to the rate of flow of earth-air and therefore increases as the AP decreases. The radon emission also varies with the seasonal variation in temperature and AP, which is high in summer and low in winter. On a daily timescale, the radon varies in a bimodal manner. Therefore, the net amount of radon emitted from the soil is positively correlated with the amplitude of the AP fluctuation, temperature, soil porosity, and thickness of the vadose zone. The atmospheric pumping is the main driving force responsible for the radon emission. However, the surface closure, landform, cracks, faults, grain size, pore structure, soil adsorption, basal uranium/radium, salts, wind, lunar cycle, latitude and altitude have important effects on the number of radon emission. As such, it provides a scientific basis for the effective utilization of radon and prevention of its emission from soil.

Quantity and spatial distribution of earth-air activity in Cave 108 of the Dunhuang Mogao Grottoes.

Monitoring of earth-air humidity in soil has revealed extensive rising/falling activity on the land. Alternation of dry and wet air is the main root cause deterioration in cave wall paintings. However, the quantity and spatial distribution of earth-air activity in caves remain unclear. This study used air flowmeters to monitor the earth-air outflow at 136 points on the west, south, and north walls of Cave 108 in the Dunhuang Mogao Grottoes, and to determine its spatial distribution. Results showed that under the influence of atmospheric fluctuation, there is frequent air exchange between the atmosphere and the earth-air, and that the daily outflow of earth-air from Cave 108 is 50.893 m3/d. Earth-air flux is negatively correlated with atmospheric pressure, and the fluctuation of atmospheric pressure is the main driving force of earth-air activity. The average outflow of earth-air from the ground of Cave 108 is 0.109 L/(min·m2). Apart from that, the overall spatial distribution of earth-air quantity showed the following pattern: ground > west wall > north wall > south wall. What's more, the outflow of earth-air is inversely proportional to the wall height, while proportional to the depth of the cave embedded in the cliff. In such case, sealing the cave floor with concrete will heighten earth-air flow on the walls, thereby quickening the deterioration process of wall paintings. This study unveiled the deterioration mechanism of wall paintings in the context of earth-air conditions, thus establishing a foundation for the scientific protection of wall paintings.