| Longdong Loess Plateau is an important national energy industry base.As surface water is heavily polluted,groundwater has become the main water resource to support the economy development.In recent years,with the increasing groundwater exploitation,a series of problems have been caused,such as the decline of groundwater level and serious groundwater pollution.At present,the researches of groundwater evolution process and recharge mechanisms of Longdong Loess Plateau are still relatively weak,and can not be used for guiding the protection of groundwater resources.With the development of social economy and ecological construction,the rational and sustainable utilization of groundwater resources on Longdong Loess Plateau is urgently needed.Therefore,how to correctly understand the mechanisms of groundwater circulation and its response to the dual influence of climate change and human activities is one of the key issues for the sustainable and scientific development of the loess plateau area.This study based on field investigation of hydrological geological conditions,collected 52 water samples in Longdong Plateau,integrated the use of hydrochemical and isotope technology,analyzed the hydrogeochemical evolution of groundwater of Longdong Loess P lateau,explored the characteristics of hydrogen and oxygen isotope of atmospheric precipitation,surface water and groundwater,revealed the mutual transformation between precipitation,surface water,groundwater and groundwater recharge mechanism.By using various14C age correction models and the reversed hydrogeochemical simulation,the age of regional groundwater was determined.The main results are as follows:The distribution of chemical ions of groundwater in Dongzhi Tableland is quite different from that along Malian River.The groundwater in Dongzhi Tableland is alkaline.Ca2+is the main cation,followed by Na+and K+,and Mg2+is the least.The anion is mainly HCO3-.The ions are low and evenly distributed spatially,and the groundwater type of Dongzhi Tableland is HCO3-Ca-Na type.Dissolution of carbonate minerals is the main water-rock interaction in the aquifer of Dongzhi Tableland,along with eluviation and cation exchange.The influence of evaporation and concentration to groundwater is weak.However,groundwater along Malian River is alkaline.The cations are mainly Na+and K+,and the anions are mainly Cl-and SO42-.The salinity is high and the water quality is poor.Spatially,TDS decreases from northwest to southeast,and the water quality becomes slightly better.The hydrochemical type of groundwater is from Huan County SO4-HCO3-Na-Ca type to Qingcheng County Cl-HCO3-Na-Ca type.Dissolution of halite,gypsum and sodium sulfate is the main water-rock interaction in the aquifer under Malian River.?18O value of groundwater is between-11.70-9.05,and?2H is between-86.15-68.95.The heavy isotopes of groundwater are relatively depleted,far less than that of modern atmospheric precipitation(?18O of-7.1and?2H of-46.3),which means the groundwater receives little recharge from modern precipitation and more charge from paleo precipitation when it was humid and cold.Except for special points,hydrogen and oxygen isotope values of groundwater have little change with depth.The?18O value of surface water is between-9.44-8.70,with mean value of-9.01,and the?2H value is between-72.03-66.28,with mean value of-69.26.The hydrogen and oxygen isotopes have little difference with those of groundwater.In addition to runoff formed by rainfall,the surface water in the middle and lower reaches of Malian River mainly receives the recharge from groundwater on both sides of the valley,while the source of the Malian River mainly receives local precipitation recharge.14C results show that the age of groundwater is between 7000 and 25000 years.Combined with the characteristics of hydrogen and oxygen isotope distribut ion,it is believed that there is paleoprecipitation recharge to groundwater during the late Pleistocene and Holocene humid periods.Various models consider different groundwater systems and have different results.The results of Vogel model,Tamers model,CMB chemical mass balance model and F-G model are similar.The correction results of Pearson model are quite different from other models,perhaps because it does not consider other CO2 source,and the carbon exchange between soil carbon dioxide and the dissolved inorganic carbon in groundwater.Netpath XL1.4 was used to carry out the reversed hydrogeochemistry simulation of G22 to G26 flow path,and the simulation value was similar to the measured value,indicating that the reversed model was applicable.The simulation result of starting point of 14C is 27.4294 pmc;the simulation result of migration time is 16041 years;the groundwater age of G22 is about 8000 years;therefore,the groundwater age of G26 is about 24041 years.By comparing the age results of Vogel Model,Tamers Model,CMB Chemical Mass Balance Model and F-G Model in G26?2428125788 years?,it is considered that the conclusion of reversed simulation is consistent with the results of 14C correction models. |
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