Distribution And Causes Of Natural Inferior Karst Groundwater In Guizhou

Since 2007,Guizhou Province has built 4,694 pumped wells in order to solve the problem of drinking water safety in rural areas.Due to the lack of research on the geochemical background of dolomite and the spatial distribution of inferior karst groundwater,some pumped wells have sufficient water,but the water quality exceeds"Water Quality Standard"(GBT-14848-2017)Class III standard,which does not meet the requirements for drinking.By collecting 4067 water quality reports from Guizhou Province's 2007-2015 pumped wells and national groundwater monitoring project monitoring sites,Using Map GIS,Arc GIS and other software,using Piper three-line diagram method,Shukarev classification method,mathematical statistics method,ion ratio method,single factor evaluation,Nemero index method,fuzzy mathematical evaluation and other methods and theories,the groundwater quality,Systematic research on the characteristics of water chemistry,the spatial distribution of natural inferior groundwater and the cause of formation.The following results have been achieved:(1)The average content of cations in the study area is Ca²⁺>Mg²⁺>Na⁺>K⁺,and the average content of anions is HCO₃^->SO₄^2->NO₃^->Cl^->F^-.The chemical types of groundwater are mainly HCO₃-Ca·Mg,HCO₃-Ca HCO₃·SO₄-Ca and HCO₃·SO₄-Ca·Mg.Using Gibbs model diagram and ion ratio method to analyze,it is concluded that rock weathering is the main factor affecting the chemical composition of groundwater.Ca CO₃and Mg CO₃in limestone and dolomite are the main sources of the major components of groundwater Ca²⁺,Mg²⁺,and HCO^3-.SO₄^2-comes from the dissolution of gypsum and anhydrite in the dolomite formation.(2)Using single factor evaluation method,Nemero index method and fuzzy comprehensive evaluation method to evaluate the water quality data of pumped wells in the study area,it is concluded that the proportion of groundwater quality grades I??is 81.44%-90.93%,and the proportion of water quality grades IV and V A total of9.07%-18.56%of the total proportion of the class,the main over-standard components are 21 items such as Al,Fe,Mn.Through the comparative analysis of the water quality evaluation process and calculation results,the fuzzy comprehensive evaluation method is more suitable for the study area.(2)Natural inferior karst groundwater can be divided into three categories:total sulfate hardness and TDS exceeding standard,aluminum iron manganese exceeding standard,and fluoride exceeding standard.On the plane,according to lithofacies paleogeography,sedimentary environment,rock lithology and mineral composition,the study area can be divided into 6 types of natural inferior karst groundwater areas.Groundwater has obvious zoning characteristics in the vertical direction.When the underground burial depth is greater than 100m,the deeper the depth,the higher the gypsum salt content.(3)Combining lithofacies paleogeography,sedimentary environment,rock lithology and mineral composition,etc.,it is preliminarily divided into 6 types of natural inferior karst groundwater areas in different periods according to chronological strata:Upper Sinian strata(?),the First to Fourth Cambrian Strata(?),the upper Devonian Strata(?),the Lower Carboniferous Strata(?),the Permian Yangxin Strata(?),the lower and Middle Triassic Strata(?).(4)The formation of natural inferior karst groundwater is closely related to the rock geochemical background of the water-bearing rock layer of the groundwater system.It is mainly distributed in the dolomite strata deposited in a closed-semi-closed sedimentary environment.The excessive substances mainly come from gypsum,fluorite and cryolite in the dolomite.The formation of natural inferior karst groundwater is also related to the type of groundwater system and hydrodynamic conditions.There are very few natural inferior karst groundwater in karst spring systems and underground river systems with strong hydrodynamic conditions;dispersed drainage systems and deep confined groundwater with weak hydrodynamic conditions It is the main distribution area of natural inferior karst groundwater.