Manas River Basin (MRB) is a typical mountains-oases-desert arid inland river basin in the Northwest China. Its oases plain is an important economic agricultural production area. With the increase of population and land reclamation in the MRB, water shortage problem is getting seriously in the past decades. The continuous decline of groundwater level, groundwater salinization and environmental degradation are threatening sustainable development of the local economy. It is significant to understand the mechanism of groundwater salinization for water utilization, soil salinization controlling and environmental protection in the MRB.
Based on field investigation and system analysis of regional groundwater flow, surface water, aquifer groundwater and aquitard porewater in the MRB were collected and their hydrochemistry, stable and radioactive isotopes were measured. Soil water content, temperature and EC were monitored by using the 5TE sensors and soil evaporation rates were monitored by using the micro-lysimeters in the field. Artificial bromide tracers were used to evaluate groundwater evaporation rates. Soil column experiments and numerical simulation of flow and salt transport were carried out to understand the interaction processes of evaporation and salt accumulation.
The results show that the salinization or total dissolved solids of groundwater are different at different location of the MRB. The hydrogeochemical and isotopic characteristics revealed groundwater salinity in the piedmont area was low with fast freshwater infiltration and low evaporation. The average salinity increased along the groundwater flow paths because of mineral dissolution, evapotranspiration, and agricultural activities. Groundwater received modern water recharge from irrigation and channel leakage with flushed soil evaporites in the middle alluvial–diluvial plain. Direct evaporation occupied 1-59% of groundwater loss and contributed 0.14-49.85% to salinity in different aquifers at different location based on the deuterium excess method. The saline porewater released from aquitards due to overexploitation is an important factor for groundwater salinization of the middle and deep aquifers in the depression zone. The average evaporation rates of phreatic water between May and August of 2017 was 0.04-2.26 mm/day estimated by Bromide tracing. Evaporation rates reached up to the maximum value after precipitation. When soil water content reduced, the rates decreased in the desert area and non-saline soils. The low water content is the major factor that restricts soil evaporation rates. While in the saline soils, the salt content in the soils and salt crusts on the land surface reduce the soil evaporation rates. The results of soil column experiment showed that the solution concentrations affected the evaporation rates in saturated soils. The higher solution concentrations, the lower evaporation rates happened. Evaporation from saturated zone resulted in salt precipitates and salt crust on the soil surface. The evaporation rates increased at the early stage, and then slowed down due to the salts crust growth.