| Presently, groundwater contamination assessment or managementwerebased on point-in-time measurements of contaminant concentration. But, the concentration of chlorinated hydrocarbons distributed uniformly in different areas of groundwater, so the concentration was not able to make sure whether the distribution of contaminants is stable or not,optimize existing remedial actions, and forecast the risks at various points of potential exposure. Aiming at this problem,some scholars proposed new indicators to evaluate site environment—mass flux, which can reflectthetotalmass discharge rate to groundwater and surfacewater. Such information is useful formanagement of contaminated sites. Furthermore, contaminant fluxes measured at locations down-gradient of sourcezones permit verification of remediation technology performance, assessment of natural attenuationrates andevaluation of environmental risks, optimize the site conceptual model.Mass flux is morereasonable than concentration of pollutants to evaluate the impact of contaminanton receptors.Besides,based on the importance of flux, several methods of mass flux estimationcommonly used abroad were comparedand then thetransect method was selected as the method of estimating mass fluxby combining with the site characteristicsand experimental conditions.This study got the following conclusions through literatureand case analysis:(1) The topography of contaminated sites in china was complex and the study to flux was rare. In addition, combining the characteristics of different fluxes estimating methods and the distribution of contaminantsin actual site of this study, the transect method wasselectedto estimate the flux of contaminants.(2) One contaminated sitepolluted by chlorinated hydrocarbon was used tobe the case study. Firstly, the distribution of 1,2-dichloroethane(DCA), vinyl(VC) and chloroform(CF) concentration was analyzed, the results showed that the concentration of contaminants changed consistentlyin aquifer of monitoring point GW64, the concentration in deep layer(3.0-3.5m) washigher than it in shallowlayer(0-0.15m) and higher than it in intermediate layer(0.15-3.0m).The difference of contaminant concentrations at various aquifers of different monitoring points was great.Then, the mass flux toolkit was chosento assessedthe mass flux of 1,2-DCA, CF, VC in two different sections downstream of the source zone. The results showed that the flux of 1,2-dichloroethane and vinyl chloride were 0.356 g/day,0.209 g/day in cross-section one and the flux of them were 0.547 g/day,0.461 g/day in cross-sectional two, respectively.The flux of 1,2-dichloroethane and vinyl chloride were higher than it ofchloroform, which was 0.050 g/day in cross-section oneand was 0.011 g/day in cross-sectional two. The fluxes of pollutants at different distances from the source changed differently. Theflux of chloroform was smaller with thefarther distances from the contaminated source; however, it was showed thatthe cross-section was farther from sources of pollution, the flux of 1,2-dichloroethane and vinyl chloride werelarger, namely, the flux in cross-section oneis less than that in cross-section two.(3) There was some uncertainty during flux calculation, the impact of the sampling interval and the interpolation method on uncertainty was analyzed in this study. The results showed that the fluxestimatedbylinear interpolation and nearest neighbor was higher thanitestimatedby log transformationinterpolation. In addition, the larger intervalof vertical sampling was carried out, the greater effect on the flux uncertainty was appeared.The influence ofeach individual flux on the total flux was analyzedby cross-validation method, if every single flux representeda highpercentage, then the uncertainty in the calculation was high and additional monitoring points should be added to reduce the uncertainty. The results showed that the flux of 0.15-3 m intervals had a greater impact on the total flux, and its maximum couldbe reached 99.7%. |
PDF Full Text Request