Variation characteristics of water table and water content in vadose zone under disturbance of coal mining

Abstract: Human activities commonly dominate the water source, such as moisture conditions, even for the surface subsidence, fissure zone, more severe groundwater exploitation, and complex precipitation in the semi-arid areas. This study aims to determine the variation of groundwater level and water content in the vadose zone in typical mining areas of Northern Shaanxi Province in China. The relationship between precipitation and groundwater was established by in-situ observation, statistical and wavelet analysis from the perspective of the water cycle. Some parameters were also measured, including the precipitation, groundwater table and soil moisture content in the vadose zone under the disturbance of coal mining. Particularly, the vadose zone dominated the infiltration, evapotranspiration, groundwater recharge and discharge in the water cycle. The interaction mechanism was also proposed for the response of groundwater level and water content to the changes in precipitation in both unmined areas and mining areas. The self-test system, groundwater observation wells and sensors were used to evaluate the rainfall, phreatic water level, soil moisture, and water potential in the mining areas. The soil water movement was monitored in the depths of about 10, 20, 30, 40, 50, 80, 100, and 150 cm in the unmined areas and in the depths of about 10, 20, 30, and 50 cm in the mining areas. After that, an evaluation was made on the variation of groundwater level under the disturbance of coal mining, the trend of soil moisture content with the depth, and the relationship between soil moisture content and rainfall response before and after coal mining. The results show that there was an outstanding response of groundwater level to precipitation in the unmined areas, where the absolute value of correlation coefficient reached more than 0.7. There was also a lag of about four or five months. There was no effect of small intensity of precipitation (like 50 mm) on the soil water content near the depth below 100 cm in the unmined area, but the larger precipitation (60 mm) infiltrated the depth below 100 cm. The groundwater level continued to decline, indicating the weak relationship between groundwater level and precipitation after the disturbance of coal mining for about several years. The moisture decreased with the increase of depth in the vadose zone, indicating the gentle fluctuation range of dynamic change. The soil moisture was less vulnerable to the external influence, as the farther away from the ground. There was an outstandingly different trend of soil moisture content with the rainfall at different depths, although no significant response of water content to precipitation at the same depth. In addition, the maximum soil moisture content in the mining area occurred earlier than that in the unmined area, indicating a reduced range overall. In particular, the precipitation increased significantly in July and August. The reason was that the fissures caused by coal mining shortened the precipitation time to recharge the shallow groundwater. There was no response at the depth below 50 cm to the small intensity of precipitation (like 30 mm). In general, the coal mining caused the drop in water level, resulting in the thickening of the vadose zone, whereas, the loss of water increased in the vadose zone, resulting in the rainfall infiltration of groundwater recharge decreased, further aggravating the drop of water level. Accordingly, the findings can provide theoretical support for the water cycle mechanism under coal mining.