Hydrochemical Evolution Model Of Groundwater In The Alluvial Plain Of The Middle And Lower Yellow River

Under the dual action of climate change and human activities,the chemical evolution of groundwater becomes more and more complex,and the role of human activities in the chemical evolution of groundwater becomes increasingly strong,which destroys the original model of groundwater chemical evolution and presents diversified development.The People’s Victory Canal Irrigation area is located on the alluvial plain of the middle and lower reaches of the Yellow River.In the early stage,the water diverted from the Yellow River is sufficient,so agricultural irrigation mainly uses the water from the Yellow River.In the later stage,the water diverted from the Yellow River decreases,and the groundwater becomes the main water supply for agricultural irrigation in the irrigation area.At the same time,groundwater runoff conditions in the irrigation area change,and groundwater chemistry also changes,gradually from quantitative change to qualitative change,into a new groundwater hydrochemical evolution model.Therefore,the temporal and spatial evolution characteristics of groundwater hydrochemistry in the irrigated area from 1987 to 2022 were analyzed.The effect of atmospheric precipitation,Yellow River diversion irrigation,groundwater depth and runoff conditions on the hydrochemical evolution of groundwater were studied.The hydrochemical evolution model of groundwater is revealed,which aims to provide scientific basis for rational utilization of groundwater and healthy development of water resources system in irrigation area.The main research contents and conclusions are as follows:（1）Parameter statistics,GIS spatial analysis model and Sniff map have been used to analyze the spatio-temporal evolution characteristics of groundwater chemistry in irrigation areas from 1987 to 2022.The evolution characteristics and differences of groundwater chemical components in different regions and at different times are described.The result showed that the change trend of major anions,cations and hydrochemical types in groundwater from 1987 to 2022 showed regional and phased characteristics.The average concentration of ions in the groundwater with high salinity is higher than that in the groundwater with low salinity.In the low salinity area,the average concentration and salinity of ions in groundwater decreased gradually from1987 to 2001,and the average concentration of HCO₃^-,Cl^-,SO₄^2-ions increased gradually from 2002 to 2022.From 1987 to 2001,the hydrochemical type of groundwater in the low salinity area changed from HCO₃·SO₄-Na·Mg and HCO₃-Na·Mg to HCO₃·Cl-Na·Mg.From 2006 to 2022,the hydrochemical type of groundwater in the low salinity area showed regionalization and diversification.In the high salinity area,the average concentrations of HCO₃^-,Ca²⁺,and Mg²⁺ions decreased gradually from 1987 to 2001,and the main hydrochemical types of groundwater were SO₄·HCO₃-Na·Mg,SO₄·Cl-Na·Mg and Cl·SO₄·HCO₃-Na·Mg.（2）The hydrogeochemical process of groundwater flow was analyzed by Gibbs diagram and ion proportion diagram.The effects of atmospheric precipitation,Yellow River diversion irrigation,groundwater depth and runoff on groundwater hydrochemical evolution were considered.The attribution analysis of natural and human activities in the process of the change of groundwater hydrochemical characteristics.The results show that the formation of groundwater hydrochemistry is mainly controlled by rock weathering,and the high salinity area is also affected by evaporation and concentration.Groundwater ions are mainly derived from the dissolution of silicite,carbonate,gypsum and salt rock.In the low salinity area,the decrease of ion concentration in groundwater by precipitation recharge is the main effect on the chemical composition of groundwater.From 1987 to 2001,there was a large amount of water diverted from the Yellow River,which made the ion concentration of groundwater close to that of the Yellow River.Precipitation and Yellow River water will dilute the concentration of groundwater ions in low salinity areas.When the supply amount is large,the depth decreases and the ion concentration decreases.With the decrease of supply,the depth increases and the ion concentration increases.The sampling point of Xiazhuang in the central region formed a falling funnel from 2001,and the groundwater flow direction changed from the head of the canal to the northwest region from 1996 to 2001,to the head of the canal and the northwest region to the funnel region of Xiazhuang.The high salinity groundwater flows to the funnel area,which increases the concentration of ions along the way,especially SO₄^2-,Na⁺+K⁺ions.In the high salinity area,the leaching of soil soluble salt into the aquifer with precipitation is the main effect on the chemical composition of groundwater in the high salinity area.When the water supply of the Yellow River is large,the groundwater ion concentration of the Yellow River decreases by charging the groundwater sampling points close to the canal system.When the Yellow River water supply is small,the Yellow River water recharge is not the main factor affecting the change of groundwater ion concentration.There is a negative correlation between groundwater ion concentration and depth.Groundwater ion concentration and depth are affected by leaching of soil salt and extraction amount in addition to recharge factors.（3）The groundwater system in the People’s Victory Canal Irrigation area is divided into four evolution models by considering the performance characteristics,internal evolution and external influence factors.The characteristics of groundwater chemistry include the change of ion concentration,the spatio-temporal evolution of the classification results of groundwater chemistry by self-organizing mapping neural network（SOM）,and the types of groundwater chemistry.The internal evolution includes ion origin and hydrochemical evolution.The external influencing factors include landform,groundwater runoff conditions,atmospheric precipitation,and the amount of water diverted from the Yellow River.Namely,the depression of the Back River of the Ancient Yellow River-Evaporation concentration-Precipitation leaching model,the beach of Ancient Yellow River-Irrigation supply mode of Yellow River diversion model,the beach of the Ancient Yellow River-Irrigation supply for Yellow River diversion-Runoff model,the Ancient Yellow River beach-Irrigation supply for Yellow River diversion-Weak runoff model.