| Located in the eastern part of Sichuan Basin,the paralleled ridge-valley of East Sichuan is the most orderly region with northeast-southwest strike in China,and is also the most prominent folded mountain belt in the world.It is called the three major folded mountain systems in the world together with the Appalachian Mountains and the Andean-Rockies Mountains in America.For geographers,the landform combination of ridges and valleys parallel to each other is a very representative and attractive geographical phenomenon.In paralleled ridge-valley of East Sichuan,soluble carbonate rocks are widely distributed in the mountain ranges.A dual spatial structure of“soil in the upstairs and water in the downstairs”subject to hydrogeology is formed.Karst landform is sensitive to the change of external environment,and the circulation of pollutants in the pedosphere and hydrosphere is accelerated.Therefore,compared with non-karst areas,the ecological environment in karst areas is more fragile.With the advancement of industrialization and urbanization,the emissions of polycyclic aromatic hydrocarbons(PAHs)are increasing day by day.Once entering karst environment,PAHs will quickly migrate and spread in soil and water,causing deterioration of groundwater quality and posing a serious threat to ecological safety and human health.In recent years,more severe man-land contradictions and ever-expanding pollution discharge channels in paralleled ridge-valley of East Sichuan,bring persistent pollution to the already fragile karst environment.Given the importance of ecological status about paralleled ridge-valley of East Sichuan,this study takes Zhongliangshan karst trough valley as an example to explore the distribution characteristics and migration patterns of PAHs in karst soil and aquifer system,which can not only fill in gaps in study about PAHs pollution of the study area,but also provide basic data for further researching on PAHs pollution in karst area.In this study,Zhongliangshan karst trough valley in paralleled ridge-valley of East Sichuan was chosen to explore pollution level,component characteristics,source,partition and migration behavior of 16 priority PAHs by quantification of gas chromatography-mass spectrometry in karst soil and water,based on karst hydrogeological and pollution source investigation,as well as sampling regularly.It was found that:(1)The mean content of PAHs in topsoils significantly affected by surface cover of the study area was 348 ng·L-1 and high-molecular-weight PAHs were predominant,specifically shown as cultivated land A(466 ng·g-1)>bamboo forest land B(466 ng·g-1)>cultivated land B(448 ng·g-1)>shrub land A(282 ng·g-1)>shrub land B(216 ng·g-1)>bamboo forest land A(209 ng·g-1);on the whole,the PAHs in topsoils were at a slightly polluted level.Our results of characteristic ratio method and principal component analysis revealed that PAHs in topsoils of the study area were mainly produced by combustion sources;hierarchical cluster analysis revealed that PAHs were mainly from near-source pollution,and the source of PAHs in cultivated land A was special due to the impact of strong human activities.(2)The pollution level of monomer PAHs was divided by Nemerow index method,and it was concluded that the topsoils in the study area had a certain degree of ecological risk.Our results of toxicity equivalent quality revealed that seven carcinogenic PAHs in topsoils were the main contributors to soil toxicity in the study area,and the TEQBa Pvalue in topsoils of bamboo forest land A,shrub land A and shrub land B did not exceed the Dutch soil quality evaluation standards,while bamboo forest land B,cultivated land A and cultivated land B exceeded the standard,indicating a relatively large toxicity risk in topsoils.Our results of ecological risk quotient revealed that monomer PAHs in topsoils of the study area were generally at medium risk level,and a few were at high risk level,and the RQNCs of?PAHs in each topsoil ranged from8.8 to 19.6,indicating a low risk level.In general,there was a certain degree of ecological risk in soil of Zhongliangshan karst trough valley,and it was necessary to strengthen the management of PAHs pollution in the study area.(3)In the study area,the content range of PAHs in soil profiles of bamboo forest land A,bamboo forest land B,shrub land A,shrub land B,cultivated land A and cultivated land B were 47.6-209,41.0-518,33.3-282,48.1-258,57.2-954 and 40.4-448ng·L-1,and the mean content were 115,204,140,106,359 and 175 ng·L-1 respectively,indicating that there was PAHs pollution with different degrees in soil profiles.The migration of PAHs in soil profiles was mainly affected by soil physicochemical properties(such as moisture content,TOC content and p H value)and soil layer thickness.Compared with bamboo forest land B and cultivated land B,soil water in bamboo forest land A and cultivated land A with thin soil layer moved faster,which promoted vertical migration of low-molecular-weight PAHs in soils;the dissolved amount of low-molecular-weight PAHs increased due to the high soil moisture content in shrub land B,for which the migration capacity of low-molecular-weight PAHs was stronger than that in shrub field A with thin soil layer.Affected by the adsorption of high TOC content,high-molecular-weight PAHs were mainly enriched in surface soil layer,and the adsorption behavior of TOC on high-molecular-weight PAHs was related to the soil p H value.In addition,the study revealed that soil particle size was not an important factor affecting PAHs migration.(4)The monthly variation characteristics of PAHs in surficial karst spring,shallow groundwater,karst subterranean river,tunnel drainage and surface karst lake in the study area were mainly affected by the surrounding environment and seasonal change.At the sampling points of EII,EII,SI,GI,GII,GIII,TGW1,TGW2,TGW3,and LIII,the mean concentration of dissolved PAHs were 246,216,327,235,330,289,342,270,266 and 233,colloidal PAHs were 384,450,388,595,440,390,377,422,329 and 388ng·L-1,and particulate PAHs were 365,459,421,383,490,388,458,490,396 and 367ng·L-1 respectively;for component characteristics of PAHs,karst water was enriched with low-molecular-weight PAHs,in which 3-ring PAHs accounted for the largest proportion,and 6-ring PAHs were not detected.(5)The partition coefficient(Kp)of PAHs in karst water varied greatly from 1133to 113985 L·kg-1,and the Kp value of Ant was the largest,indicating that Ant prefered to partition from aqueous phase to particulated phase.Our results of the study on the factors affecting the partition behavior of PAHs revealed that low-molecular-weight dissolved PAHs were mainly controlled by DOC,while high-molecular-weight particulated PAHs were mainly controlled by POC;compared with DOM-associated PAHs,freely dissolved PAHs in water accounted for a larger proportion of total dissolved PAHs.In addition,the partition behavior of Nap between particulated and aqueous phases was significantly affected by water temperature.The partition characteristics of PAHs with different forms in water were as follows:total PAHs and low-molecular-weight PAHs tended to partition to colloids and suspended particulate matter,while high-molecular-weight PAHs tended to partition to suspended particulate matter;for total dissolved PAHs,low-molecular-weight PAHs tended to exist in water as free state,while high-molecular-weight PAHs tended to partition to DOC.(6)During the vertical migration of PAHs from karst unsaturated zone to deep saturated zone,the total concentration,forms and components of PAHs changed significantly during dry and rainy season in the Fenghuang subterranean river basin,Longju subterranean river basin and Qingfeng subterranean river basin;the migration behavior of PAHs was mainly affected by physicochemical properties of PAHs,internal and external environmental factors of karst aquifer,etc;the different regulation functions of karst fissures and pores,shafts and ponors on PAHs pollutants led to different migration behaviors of PAHs;tunnel construction could enhance the hydrodynamics of karst aquifer,which improved the migration efficiency of PAHs.The analysis results of PAHs profile revealed that the PAHs in karst subterranean river could migrate to tunnel outlet along the fractured rock strata;the enrichment ability of low-molecular-weight PAHs was weak within the thin soil layer in karst area,for which more low-molecular-weight PAHs migrated to karst subterranean river through karst unsaturated zone,while the enrichment ability of high-molecular-weight PAHs was strong in karst soil with high TOC content,for which the protection of overlying soil reduced the pollution of high-molecular-weight PAHs to karst subterranean river;surface karst lake was an important source of PAHs pollution due to its recharge relationship with karst underground rivers,and no obvious adsorption and degradation occurred during the migration of PAHs from LIII to GIII through karst unsaturated zone. |
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