Spatio-Temporal Evolution Of Hydrochemical Components And Human Health Risk Assessment Of Groundwater In Guanzhong Plain

Guanzhong Plain is located in the deep interior and belongs to the arid and semi-arid area of Northwest China.With the fragile ecosystems,it is sensitive to natural environment and human activities.In recent years,due to the improvement of strategic position and rapid economic development,the problem of water quality has become increasingly prominent in Guanzhong Plain.As a traditional agricultural irrigation area,total irrigation areas in Guanzhong Plain are about 15,600 km²,which occupies 77.6%of the whole plain.The intensive agricultural activities have exerted great pressure on the fragile groundwater environment.Additionally,the hydrogeological conditions of Guanzhong Plain are complex due to the vast land,and numerous hydrogeochemical environmental problems,such as endemic diseases,bitter and brackish water,etc.,have posed a serious threat to the health of local residents.In order to solve these problems,it is necessary to ascertain the spatio-temporal evolution of regional groundwater chemical compositions and deeply understand the influence of nature and human activities on groundwater environment.Based on extensive literature review,combined with field investigation and indoor analysis,the author systematically elucidated the spatial and temporal variation of groundwater chemistry,nature background values,the pollution state of groundwater,the effects of irrigation on groundwater quality and human health risk in Guanzhong Plain.The aim is to provide scientific basis for rational utilization of groundwater resources and promote the harmony between human and nature.The main conclusions and highlights of this paper are shown as follows:(1)Based on 892 samples of groundwater quality collected in 2000,2012 and 2015,the spatio-temporal distribution characteristics of hydrochemical types in Guanzhong Plain were analyzed by the Shukalev classification and illustrated with the GIS spatial analysis model in detail.The application of GIS spatial analysis model overcame the shortcomings of the conventional mapping that cannot reflect the ordering of ion content and the limited expression of hydrochemical types.The key conclusions are shown as follows:With the number of hydrochemical types increasing from 48 in 2000 to 76 in 2012 and the appearance of NO₃^-type water,groundwater chemical field in Guanzhong Plain has changed obviously and groundwater is tending to be complex and diverse.Groundwater quality in the south of Wei River is better than that in the north of Wei River.The hydrochemistry types in the south of Wei River are mainly HCO₃-Ca,HCO₃-Ca·Mg.The hydrochemistry types in the north of Wei River gradually become complex from west to east:groundwater in the west of Qishui River is mainly HCO₃-Ca and HCO₃-Ca·Mg types;groundwater in the west of Jing River is mainly HCO₃-Na,HCO₃-Na·Ca,HCO₃-Na·Mg types,and Mg-type water has increased since2012;groundwater in the east of Jing River mostly belongs to mixed type,and Cl-type and SO₄-type water have increased since 2012.The characteristics of groundwater in the south of the Wei River are mainly determined by water-rock interaction.In the north of the Wei River,both agricultural activities and water-rock interaction have significant influences on groundwater located in the west of the Jing River,while groundwater in the east of the Jing River is more susceptible to be affected by the evaporation and human activities.(2)The concept of groundwater natural background value(NBV)is redefined with considering the spatio-temporal variations caused by groundwater flow.Groundwater natural background value is defined as“the concentration of groundwater chemical components in a region during one certain period,which is corresponding to no or only very minor anthropogenic disturbtion.”Groundwater natural background values in Guanzhong Plain is calculated from the perspective of time and space for the first time.The Guanzhong Plain is divided into 10 groundwater environment units(GEU)based on local hydrogeological conditions,and NBV of each GEU are separately calculated by various mathematical statistics methods.The results show that NBVs vary along the groundwater flow and NBVs of Cl^-and SO₄^2-gradually increase from upstream to downstream.Influenced by long-term irrigation activities,NBVs of most ions in the Jinghuiqu and Jiaokou Irrigation districts are very high.In terms of temporal characteristics,due to the influence of water-rock interaction and anthropogenic activities,NBVs of most ions tend to increase,especially in the north of the Wei River.The F-value method was used to evaluate the background water quality.Results show that the background water quality in the south of the Wei River was good as a whole,but the background water quality in most areas of the north of Weihe River was poor or extremely poor.(3)The response mechanism of hydrochemistry to irrigation in Guanzhong Plain was studied by synthetically applying water-salt balance,isotope technology,End-element mixed model and hydrogeochemical simulation technology.Results show that groundwater quantity in Jinghuiqu Irrigation district is negative balance but in the state of salt accumulation,while groundwater quantity in Baoyang irrigation districts is in balance status but salt discharge.Poor runoff conditions,high salt contents in supply water and soil are the principal factors causing the continuous increase of groundwater salinity in Jinghuiqu Irrigation district.In the groundwater discharge areas with flat terrain and shallow groundwater depth,it is easy to accumulate salt with high contribution of irrigation water.However,groundwater desalination mainly occurs in the region with a high-speed water cycle.According to the results noted above,we believe that it is not a high-efficient measure to improve soil salinization by blindly reducing groundwater levels or increasing the ratio of well irrigation.In fact,well irrigation sites should be selected and evaluated reasonably based on the comprehensive consideration of groundwater quantity and quality and the regional hydrogeological condition,so as to realize the sustainable development of water resources in the irrigation districts.(4)The health risks of children and adults in Guanzhong Plain were thoroughly evaluated with four pollutant indexes(As?F^-?Cr⁶⁺and NO₃^-).The spatial distribution of human health risks in the region are characterized by using GIS technology,which provides scientific support for the risk prevention and control of groundwater as well as groundwater pollution according to local conditions.Based on the uncertainty and sensitivity analysis by Monte Carlo method,it is determined that Cr⁶⁺and F^-have the highest contribution rate to the carcinogenic risk and non-carcinogenic risk respectively,which are the hazardous substances that need priority treatment in the shallow groundwater of Guanzhong plain.