Spatial And Temporal Distribution Of Groundwater Hardness And Influencing Factors In The Plain Area Of Hotian Prefecture,Xinjiang

The total hardness of water is mainly composed of a variety of polyvalent metal cations(Ca²⁺,Mg²⁺,etc.)in water.It is one of the main indicators to representing the chemical characteristics of groundwater and an important indicator that used to evaluate groundwater environmental quality.Based on 252 sets of groundwater quality monitoring data in the plain area of Hotian Prefecture,Xinjiang from 2014 to 2018.The Piper trilinear diagram,Gibbs model,factor analysis,saturation index,ion ratio coefficient and reverse hydrogeochemical simulation methods were used to systematically study the influencing factors of the spatial and temporal distribution of groundwater hardness in the study area.This can provide a scientific basis for the prevention and control of local groundwater hardness pollution and the drinking water safety management of residents.The main research contents and conclusions are as follows:（1）The groundwater depth of single structure unconfined groundwater area was 5 to 150 m,and groundwater depth of multilayer structure unconfined groundwater-confined groundwater area was 0 to 150m.The groundwater generally flows from south to north,and the hydraulic gradient of groundwater decreased from south to north.The groundwater in the study area is weakly alkaline.The anions are mainly Cl^-and SO₄^2-,Na⁺was the main cation,and the concentration of Ca²⁺and Mg²⁺were not significantly different.The spatial distribution of Ca²⁺and Mg²⁺plasma in single-structure unconfined groundwater was extremely uneven;in the groundwater of confined water area,the concentration of each ion has the characteristics of confined groundwater>unconfined groundwater in confined groundwater area>unconfined groundwater.At the same time,the content of each ion in the deep confined groundwater（well depth>100 m）was greater than that of the shallow confined groundwater（well depth<100 m）.From 2014 to 2018,the main water chemical type of groundwater in the study area evolved from SO₄·Cl·HCO₃-Mg·Ca·Na type to SO₄·Cl-Mg·Ca type.（2）From 2014 to 2018,the spatial and temporal distribution of the mean value of groundwater hardness in the study area differed significantly.The area of high hardness groundwater（IV,V）showed a trend of increasing first and then decreasing.The hardness of groundwater decreases year by year from south to north in plain area.The groundwater total hardness in the study area in 2018 showed that:horizontally,the high hardness groundwater was mainly distributed in area that near the desert in the north and central part of the study area,and showing a gradually increasing trend along the south to north direction;vertically,the total hardness of single structure phreatic groundwater decreased with the increase of buried depth,while the average value of the total hardness in multi-layer structure phreatic groundwater and confined groundwater fellows the decreasing order of deep confined groundwater(859.9mg·L^-1),phreatic groundwater(628.7mg·L^-1),shallow confined groundwater(471.8mg·L^-1).（3）According to ion sources analysis,the Ca²⁺and Mg²⁺in groundwater total hardness the study area were derived from the weathering dissolution of evaporite and carbonate rock and cation alternate adsorption.The main factors affecting the spatial and temporal distribution of groundwater hardness in the study area are groundwater burial conditions（buried deep in the south and small in the north）,hydrogeochemical effects（evaporation concentration,leaching and alternate cation adsorption）and water environmental chemical factors（salt effect and acid effect）,of which hydrogeochemistry is the dominant factor.The high-hardness groundwater in the study area mainly occurs in Na⁺content of 100～2000mg·L^-1,HCO₃^-content of 50～1000mg·L^-1,K⁺content of 50～100mg·L^-1,and TDS of 800～10000mg·L^-1.In the groundwater environment of high Na⁺,high HCO₃^-,high K⁺and high TDS groundwater environment will cause groundwater TH to rise,there by forming high hardness groundwater.Factor analysis shows that the three factors that affect the chemical composition of groundwater in the study area,especially the total hardness,are:hydrogeological conditions,hydrochemical environment,and human activities（the use of agricultural fertilizer）.The contribution rate is 45.84%,24.80%and 12.08%,respectively.Excessive exploitation of groundwater is an important cause of groundwater hardness increase in the study area.（4）The reverse hydrogeochemical simulation results show that along the groundwater flow path,the ions in water accumulates,rock salt,dolomite and gypsum are dissolving,and calcite precipitates.In the middle and lower reaches of the groundwater flow path,the unconfined groundwater depth became shallower and CO₂ escapes,which was affected by evaporation.The cation exchange adsorption occurs in confined water,and its effect continuously enhanced along the flow path.