Impact And Evaluation Of Shallow Groundwater Quality In Karst Mountainous Area Of Guizhou Province

Land use and vegetation coverage changes of various process not only determine the groundwater recharge,but determines the water quality of groundwater.Runoff formation and the way to reach the river runoff,ultimately affect the water quality of water environment.The earth's surface vegetation can improve soil fertility,enhance the function of water and soil conservation.Vegetation and its developed root system and litter humus layer which is formed by the strong water storage capacity,can protect layers from water erosion.It is the center of the karst mountainous area water gathering and storing.The river basin in Puding,with karst terrain strongly develops,Guizhou.In the mid and late 20 th century,agricultural activities and human interference destroyed the forest vegetation.It had gradually replaced by agroforestry ecosystem and agricultural ecosystems.The soil and water quality had droped,especially for shallowly buried shallow groundwater quality,and the water level had fallen.In order to explore the karst mountain seasonal changed characteristics of shallow groundwater,water quality situation and the possible influencing factors,this paper takes the shallow groundwater in Puding County river basin as the research object which is located in the centre of karst particularly strongly developing areas,and low vegetation coverage and frequent agricultural activity areas.To analyze the hydrochemical characteristics and seasonal variation of shallow groundwater,through field investigation and laboratory analysis,the single factor evaluation method and the comprehensive water quality evaluation method are used in the research.It aims to provide a scientific basis and theoretical reference for the conservation and ecologically sustainable development of karst mountain groundwater.The main conclusions are as follows:?1?In the karst river basin of Puding,the mainly naions of shallow groundwater were HCO₃^-,SO₄^2-and Cl^-,accounted for 53%,15% and 3% of the total ion content respectively,and the main cations were Ca²⁺ and Mg²⁺,occupied 19% and 6% of the total ion content separately.The hydrochemical type was mainly HCO₃^-Ca type water.The p H was between 7.51 and 8.23,belonged to slightly alkaline.The variation coefficient of NH₄⁺-N and TP was relatively larger,while the other indexes were stable.Compared with the condition in 1994,shallow groundwater chemistry characteristics undergone large changes,out of the naion HCO₃^-decreased,the content of remaining major ions increased in most areas.?2?In the karst river basin of Puding,Seasonal changes of Cl^-,SO₄^2-,K⁺,and TN followed the order of spring >summer > autumn,while their contents increased during winter.The dynamics of HCO₃^-was in the order of spring?summer<autumn<Winter,and NO₃^--N was summer<spring<autumn<winter.The contents of Na⁺,Mg²⁺ were lower in autumn and winter.The contents of pH,NH₄⁺-N were lower in spring and autumn,and presented“N”type distribution from spring to winter.Hence,The shallow groundwater showed obvious seasonal variation characteristics.HCO₃^-,Na⁺,NO₃^--N,Ca²⁺,TP showed significant seasonal differences?P<0.05?.K⁺?Mg²⁺?Cl^-?TN?NH₄⁺-N?SO₄^2-were different in different seasons,but did not reach significant level?P<0.05?.The chemical properties of the shallow groundwater were effected by agricultural activities,human disturbance and formation lithology,in which the most is typical agricultural activities,followed by human disturbance.Karst mountainous area residents of agricultural activity and human disturbance caused the shallow groundwater quality of N and P concentration increase,the water quality declined.?3?Comprehensive evaluation of water quality showed that water quality in the autumn and winter were better than spring and summer,and the overall water quality was good for all seasons.However,single water quality analysis suggested that specific water indicator exceeded the limit value of IV water in some sampling sites,which mainly contributed to the high Fe and NH₄⁺-N concentrations.The highest quality concentration of Fe was 1.29 mg·L^-1,and the highest concentration of NH₄⁺-N was 0.71 mg·L^-1,the higher concentration of these two ions may relate to the human activities,which has a potential impact of the quality of shallow groundwater.In general,studying the chemical characteristics and water quality of shallow groundwater will provide a scientific basis for protection and management of water resource in karst mountainous areas.?4?There was a significant differences in soil physical and chemical properties between different agricultural land use types in the karst mountain area.The soil quality of vegetable field was worst and the abandoned farmland was better.Soil nutrient reserve was favored after abandonment.In the shallow groundwater of vegetable area,corn area and paddy field area,the average mass concentration of HCO₃^-,SO₄^2-and Cl^-was higher than that of abandoned land,and the average concentration of K⁺,Na⁺,Ca²⁺,Mg²⁺,TN,NH₄⁺-N,TP in shallow groundwater is higher than other land.The results showed that shallow groundwater was sensitive to the soil quality,and the quality of shallow groundwater was positively correlated with the soil quality.In conclusion,in karst mountainous area,the shallow groundwater has obvious seasonal variation characteristics of water quality is good as a whole.It was sensitive to human disturbanceand were closely related to soil quality.In the process of agricultural land to rational utilization of land resources,the ecological engineering construction such as returning farmland to forest,grassland and ecological agriculture should be promoted,effectively prevent and control the shallow groundwater pollution andsoil quality degradation.