Vulnerability And Contamination Risk Assessment Of Karst Groundwater

Groundwater is the main drinking water resource in many parts of China.Contamination resulting from industry,urbanization and agriculture poses a threat to water quality.Wise management strategies are thus required to find a balance between groundwater protection on the one hand and economic activities on the other hand.The task of balancing groundwater protection and economic activities is particularly challenging for karst groundwater,which is estimated as supplying water to one quarter of the world's population.Karst aquifers are particularly vulnerable,as contaminants can easily enter the subsurface through thin soils and via swallow holes.Karst systems are more complicated than sand and gravel aquifers,because of their high degree of heterogeneity and the resulting duality of recharge(autogenic vs.allogenic) infiltration (diffuse vs.point) and porosity(conduits vs.fissured matrix).So it's very necessary to do the vulnerability assessment of karst groundwater."Vulnerability of groundwater to contaminants " which is designated by Margat in 1968 is a new conception of hydrogeology.There are many arguments about the means of this new conception during the forty years.The generally accepted definition is:groundwater vulnerability is possible for the contaminants move to some parts of aquifer system.And there are two kinds of groundwater vulnerabilities:The first one is called as intrinsic vulnerability which is used to define the vulnerability of groundwater to contaminants generated by human activities.It takes into account the inherent geological,hydrological,hydrogeological characteristics of the area,but is independent of the nature of contaminants;Another one is named as specific vulnerability,which is additionally considered the vulnerability of groundwater to a particular contaminant or some kinds of human activities.Up to now,many methods have been proposed for vulnerability mapping,such as hydrogeological method,GOD,DRASTIC,SINTACS,EPIK,PI,COP,AVI,ISIS,REKS,Irish method,German Method.But few are suitable to karst area.Pan-European approach is one of the more advanced methods.Pan-European approach to Intrinsic karst groundwater vulnerability mapping consider four factors.The overlying layers(O) may provide some degree of protection to the groundwater.In karst areas,however,allogenic recharge may bypass these layers.Therefore,the concentration of flow(C) also has to be considered.The precipitation regime(P) is important when comparing groundwater vulnerability in different climatic regions but less relevant for vulnerability mapping at a more local scale.The K factor describes the hydraulic properties of the karst aquifer. In this research,based on the hydrogeology survey and the characters in this research areas,I have finished the vulnerability and risk assessment of Qing MuGuan karst valley by the improved Slovenia method and the improved Vietnam method.Slovenia method takes into account four factors:C,O,P,K.It consider the texture and thickness of soils,lithology and fracturation and thickness of unsaturated bedrock,swallow hole,sinking stream,Vegetation,slope,surface karst features,number of days with precipitation 20-80mm/24h,number of days with precipitation＞80mm/24h,transport time,characteristics of aquifer media,information of conduit system.Slovenia method needs more data so it is difficulties for many developing countries.We take a simplified methodology of Pan-European approach(Vietnam method).Vulnerability is assessed by two factors:the overlying layers(O) and the concentration of flow(C).The risk map is obtained by putting the vulnerability map together and a simplified hazard assessment.This method can be applied in different climate and hydrogeological settings.For the intrinsic vulnerability assessment and four second class factors -texture and thickness of soils,feature of water system,dominant flow process.Further more,we take the land use for the simplified hazard assessment.There are three new ideas in this paper.1).Karst valley in east part of Sichuan is selected as my research area.Two new methods of European model is introduced and applied in this research.2).The improvement of Slovenia method is more focus on the vegetables and slopes.There are two different kinds of scenarios to be discussed:one is inside the catchment area of swallow hole,the vegetables and gentle slopes can give the groundwater better protection.The other is in the the rest aquifers,vegetables and gentle slopes may cause the vulnerability of the groundwater.3).The improvement of Vietnam method consider the catchment of slope runoff around the spring when we subdivide the land surface into different types of zones.