Groundwater Flow System in a Permafrost Area in the Upper Reaches of Heihe River

English Abstract:

Heihe River is the second largest inland river in China, and the permafrost zone in the upper reaches is a main water flow producing region for Heihe River. However, few hydrogeological investigations were conducted and understanding about groundwater flow system in the source areas of major rivers reminds almost blank. This study aims to characterize the groundwater flow system in a typical permafrost zone at Hulugou catchment in the upper reaches of Heihe basin. A hydrogeological approach was used based on the integration of hydrogeological survey, field drilling, groundwater level and temperature data monitoring. In the summer of 2014, 21 boreholes were drilled at four sites, separately located at seasonally frozen zone and permafrost zone. At each site, 5 to 7 boreholes with different depth were drilled and the deepest one was as deep as 30m. Taking this method of drilling is to monitor subsurface ground temperature and take groundwater samples at different depth. Data shows that seasonally freezing zone mainly distributed in the piedmont plain of the catchment, where the altitude is below 3,400 meters above sea level. From June to October, groundwater flow system is mainly affected by the porosity and permeability of sediment layers at this area. Water within 2 meters below ground surface has become frozen in the winter and functioned as impermeable layer for infiltration recharge. Permafrost is mainly distributed in the mountainous area above 3,400 m above sea level. The temperature-log data shows that the depth between 2m and 17m below the surface is freezing layer and the depth between 0m to 2m is active layer, which leads to a weak hydraulic relationship between supra- and sub- permafrost water. The supra-permafrost water table in active layer is near ground surface in summer. During and after the precipitation events, the water table increases and reaches to the ground surface, and is likely to produce surface runoff. Unfrozen layer under the base of permafrost is with low soil moisture. More hydrogeochemical and isotopic data has been collected to further identify the origin of the sub-permafrost water, and groundwater and surface water interaction. The seasonal freezing and thawing process has significant impacts on groundwater flow and its contribution to Heihe River.