Temporal And Spatial Hydrochemical Distribution,Genetic Mechanism And Health Risk Of Groundwater Resource In The Chongqing Parallel Ridge And Valley Area

Chongqing’s main urban area is densely populated and industrially developed,with prominent groundwater environmental safety problems.Because it is located in a special tectonic environment of the parallel ridge and valley area,the red layer of the trough and valley and the karst area of the peaks and valleys are distributed among each other,and the hydrogeological structure is difficult to be accurately portrayed,which adds difficulty to the study of the groundwater environment in the parallel ridge and valley area of Chongqing.To date,there is a lack of research on the groundwater environment in the main urban area of Chongqing,and the water quality status is still unknown,and the groundwater health risk is unknown.In this study,we identified the regional geological and hydrogeological conditions based on borehole and monitoring well data to address the deficiencies of groundwater environment studies in the parallel Ridge Valley area of Chongqing.FEFLOW software was used to construct a three-dimensional hydrogeological model of the parallel valley area in Chongqing,and the spatial and temporal distribution characteristics of the groundwater dynamic and chemical fields in the parallel valley area were analyzed by integrating field monitoring,experimental tests and numerical simulations to reveal the formation mechanism of groundwater chemical components.The main findings areThe main conclusions are.1.The distribution of soluble rocks and changes in cross-cutting valleys and microtopography are the main factors affecting the burial depth and runoff of groundwater in the parallel ridge and valley areaThe main urban area of Chongqing is characterized by tight back-slope and wide and slow oblique,which shows a special parallel valley geomorphology with the lithology of Triassic and Permian soluble rock strata distributed in the peak-ridge area and the Jurassic red layer distributed in the trough valley area,and the loose accumulation of the fourth system is mainly distributed on both sides of modern rivers or hills and mountain slopes.The groundwater gradually changes from Ca-HCO₃type to Ca-SO₄type from the axis of the back slope to the direction of the two flanks,and the Cl^-component in the groundwater is higher in the sloping area.The groundwater dynamic change characteristics seasonal rainfall changes obviously,and the water level and rainfall are proportional to each other.The groundwater level and depth of burial gradually decrease from north to south,and from the core of the back slope of the Peak Range to the trough and valley.The distribution of soluble rocks is the main factor affecting the burial depth and runoff of groundwater in the Fengling area,and the change of cross-cutting valley and microtopography is one of the main factors affecting the drainage of groundwater runoff in the trough valley.2.tectonic geomorphology and lithological changes are influenced the hydrogeological characteristics of the parallel ridge valley areaThe Fengling area in the study area is mainly located in the backslope core of Guanyinxia backslope,Longwangdong backslope,Tongluoxia backslope,South Hot Spring backslope and Mingyue Mountain backslope.The karst water shows a certain change pattern between the watershed and the deep-cut valley,and the water level depth and hydraulic slope are increasing.The stratigraphy of the trough and valley area in the study area is mainly Jurassic strata,and its lithology is mainly a set of red mudstone dominated by the overall poor water content of the strata,the flow rate of springs is generally lower than 4L/s,and the degree of water richness is low.At the same time,from north to south,as the stratum gradually changes from sandstone phase to mudstone,the water-richness has the characteristic of gradually decreasing from north to south.3.Characteristics of spatial and temporal distribution of groundwater chemical field in the parallel ridge and valley area and the genesis mechanismBased on the groundwater chemical data set obtained from the test,the spatial and temporal variation characteristics of groundwater chemical types and components in the study area were analyzed by using software such as SPSS and Arc GIS.The water chemical composition of the Feng,Kui and Ping phases of the Fengling area is influenced by lithological control,and the mountain ridge karst water is mainly Ca-HCO₃type water,which is influenced by the dissolution of carbonate minerals;the water chemical components of the red layer groundwater are mainly influenced by the dissolution of silicate（sodium feldspar）minerals and positive cation exchange.The results of nitrogen and oxygen isotope mapping,Bayesian analysis and dissolution transport simulation show that the source of NO₃^-content of groundwater in the red layer area of the trough valley is the superimposed mixture of soil nitrogen and chemical fertilizer,and the karst groundwater samples of the peak ridge show a trend toward the source of manure and domestic sewage,while the surface water shows the characteristics of both.In the red layer area of the trough valley is mainly a human living area,which leads to feces and domestic sewage as the main reason for the NO₃^-content exceeding the standard in this area,while the karst area of the Peak Ridge is mainly an agricultural activity area,so fertilizer and soil nitrogen become the main source of NO₃^-in this area.4.Results of groundwater quality evaluation and health risk study in the parallel ridge valley areaThe overall groundwater quality condition in the study area is good,and the percentage of samples meeting the drinking water standard is 78.39%.In the abundant water period,the areas with high Brown’s water quality index（EWQI）are mainly distributed in the south west of the study area,with local high value areas in the central and west,and the high non-carcinogenic risk of the total health risk index（HI）is concentrated in the south west and east of the study area,with the second highest non-carcinogenic risk in the west and north west;in the dry water period,the high values of Brown’s water quality index（EWQI）are mainly scattered in the north,west and north west;in the In the flat water period,the higher values of Brown’s water quality index（EWQI）are mainly concentrated in the south west,and the non-carcinogenic risk in the west and north west is the second highest.The southern and western parts of the study area are the key areas for future groundwater environmental protection and management.