Groundwater Salinization Mechanism And Interaction Processes Of The Evaporation And Salt Accumulation In An Arid Inland Basin

Manas River Basin?MRB?is a typical mountains-oases-desert arid inland river basin in Northwest China and one of the most important economic agricultural production areas.It is characterized by arid climate with hot summers,sparse rainfall and freezing winters.Population in the oases had increased from 55,600 in 1949 to1,000,000 in 2010,and agricultural areas increased from 31×10³hm² to 211×10³hm²,accompanying with the increasing water demands.Therefore,groundwater is exploited for irrigation and domestic demands owing to the limited surface water resources in the oases.In previous investigation,the groundwater were found brackish and saline,however,the mechanism of groundwater salinization has not been determined,which affect the utilization and managements of the regional water resources.Meanwhile,saline soil is widely distributed in this area due to the strong evaporation and agricultural activities,which will further lead groundwater salinization and restrict the evaporation processes from soil and phreatic water as feedback.Thus,it is still a challenge to evaluate the evaporation in saline soil and saline phreatic water.The objectives of this study are to qualitatively determine the groundwater salinization mechanism through the groundwater hydrochemicals and isotopic analyses,and quantitatively evaporation contribution ratios to the groundwater salinity;calculate the evaporation rates in phreatic water and soil,evaluate the relationship between the evaporation rates and salts accumulation through previous research collection,field investigation,field and laboratory experiments and numerically modeling,based on hydrogeochemistry and solute transport theory.This study could provide insights into the groundwater salinization and water cycle,and help prevent water-table decline,establish reasonable exploitation plan and irrigation regime,and better preserve groundwater resources.Therefore,we carried out the following works.?1?Water samples in the Quaternary aquifers,Manas River and other surface water bodies were collected.Water hydrochemistry,stable isotopes and radioactive isotopes were measured,and therefore groundwater flow system and groundwater salinization processes were determined.We also used a deuterium excess analytical method to quantitatively calculate the evaporation contribution ratios to the groundwater salinization.?2?Porewater samples in the Quaternary aquitards were collected through the drilling.The hydrochemistry and isotopic characteristics of the porewater samples were determined to analyze the origins of the saline porewater and its influence on the groundwater salinization processes.?3?Taking the middle alluvial-fluvial plain and desert area as the study area,Bromide tracer tests were carried on to calculate the evaporation rates in phreatic water,and micro-lysimeters were used to measure the soil evaporation rates.Simultaneously,the dynamic soil water content,temperature,and EC were monitored by the 5TE sensors for improving the irrigation regimes.?4?In the laboratory conditions,saline water evaporation processes in porous medium were carried out to investigate the interactions between evaporation and salts accumulation,different NaCl concentrations and MgSO₄ concentrations solutions were used as the saline water in this study.Hereafter,numerical modeling method was used to further study the evaporation-salts accumulation mechanism and the feedback of salts on the evaporation processes.Conclusions are as follows:?1?Groundwater and perennial natural lake salinization remains a major problem in the MRB.Hydrogeochemical and isotopic characteristics revealed groundwater salinization processes in this typical mountain–oasis–desert,arid inland basin.Groundwater in the piedmont area had low salinity associated with fast freshwater infiltration and low evaporation;the average salinity increased along the groundwater flow because of mineral dissolution,evapotranspiration,and agricultural activities.The salinization processes are linked to recharge,migration,and discharge processes but driven by complex controlling mechanisms.The ³H and ¹⁴C contents in some groundwater samples suggested that groundwater received modern water recharge from irrigation and channel leakage with flushed soil evaporites in the middle alluvial–diluvial plain.Surface water was critically affected by evaporation from the isotopic signals.However,direct evaporation was not the dominant process on groundwater salinization in this arid basin,only 1-59%groundwater loss and0.14-49.85%contribution ratios to salinity in the basin scale was reported by using the d_ex analytical method.?2?The salinization of pore water mainly attribute to the minerals dissolution owing to the abundance of CaSO₄ crystals in the aquitards?0-11.5m and 29-49m?.It reveals the prevalence of arid paleoclimate and strong evapoconcentration effects on the paleo-lakes.Owing to the depleted isotopes in pore water at the depth of 29-49m,the initial pore water must be replaced.?3?Combined with the groundwater hydrodynamic and hydrogeochemistry,a conceptual mode of the groundwater salinization processes based on the comprehensive information was proposed.This mode can provide insights into groundwater resources exploitation and management.Porewater released from aquitards because of overexploitation is an additional factor affecting salinization of middle and deep groundwater in the depression zone?30%?.?4?The average evaporation rates in phreatic water between May and August is0.04-2.26 mm/day through the Bromide tracer tests.It behaves a nonlinear relationship between evaporation rates and salts contents in vadose zone.In the study area,evaporation rates reached up to the maximum value after the precipitation.When the soil water content reduced,the rates would decrease in the desert area and non-saline soil.It seems the low water content is the major factor that restricts the soil evaporation rates.While in the saline soils,the salts content in the soils and salts crusts on the land surface will reduce the soil evaporation rates.?5?The laboratory tests showed that the concentrations of solutions could affect the evaporation rates in saturated porous medium during the steady evaporation processes.The higher concentrations in the solution,the lower evaporation rates will happen in the porous medium.At some concentration,the NaCl solution in porous medium could badly lower the evaporation rates than the MgSO₄ solution.And at the beginning of the evaporation processes,the MgSO₄ solution could promote the evaporation rates due to the easily weathered salts crystals.?6?The interactions processes between evaporation and salts accumulation had been modeled through Hydrus-1d and SUTRA 2.2 codes.Evaporation from phreatic water induces the salts accumulation and precipitates on the soil surface.Velocities of salt crust growth and the resistance increased at the early stage of evaporation processes,and then slow down mainly due to the salts crust?0-10.0mm/d?.