Study On Distribution And Migration Of Pahs Of Different Forms In Karst System

Karst groundwater is a major water resource in karst area,and it is related to people’s daily life and economic development.However,there are more and more pollution originated with human activities with the rapid development of society and economy.As vulnerabilities in karst areas,the pollutants migrate easily from the earth’s surface to the underground,and pollute the water in aquifers.Polycyclic aromatic hydrocarbons(PAHs)are persistent pollutants that degrade difficultly,volatilize strongly and accumulate organisms,and has received widespread attention with carcinogenesis,teratogenesis and mutagenesis characteristics.PAHs in aquatic ecosystem are mainly absorbed into DOM,colloidal substance or suspended particulate matter and which may pose a threat to the karst ecological environment during the transport of the PAHs by groundwater.So studying the composition,migration distribution and pollution levels of PAHs in karst environment,and analyzing their sources and environmental behaviors can help to provide references for prevention and control of pollutants in karst ecosystem.In this study,an underground river system in Chongqing City,China was selected as the study object.Adopting field investigation,location sampling and analytical test methods(GS-MS)in order to(1)study their vertical distribution,composition and sources in soils,(2)analyze the concentration variations,migration characteristics,compositions and pollution levels of different forms PAHs in soil water and karst groundwater(3)and discuss its relevance between soil water and karst groundwater.It is found that:(1)Soil samples collected from the karst groundwater system were analyzed for 16 polycyclic aromatic hydrocarbons(PAHs).the concentration range of ∑PAHs(total PAHs concentrations)and ∑PAHscarc(total PAHscarc concentrations)was 43.17~2127.96ng/g,11.84~1414.14ng/g respectively in catchment soils,and their concentrations in Xinlicun soil were obviously lower than those in Banbianjie and Shiyuanzi soil.The vertical distribution of totalPAHs concentrations among different soils exhibited large difference,but the total PAHs concentrations mainly enricheded in shallow soil layer(0-20cm).And the ∑PAHscarc concentrations also had a similar vertical distribution patterns with ∑PAHscarc.Shallow soil layers(0-20cm)showed a proportionally higher amount of high-ring PAHs,whereas low-ring and mid-ring PAH concentrations exhibited a relative uniform distribution model among different depth of soils.High-ring PAHs(especially 5-ring)showed a proportionally higher amount in different soils,but which corresponding decreased with the depth of soil and the amounts of low-ring and mid-ring PAHs were corresponding increased.Two physicochemical properties(pH and TOC)of the soils were characterized,in order to estimate the factors responsible for PAH sorption.Soil pH in Xinlicun soil produced a great influence on the vertical distribution of PAHs contents,and a correlation of the extent of PAH sorption onto soil material and the content of soil total organic carbon(TOC)in Banbianjie and Shiyuanzi soils.Source analysis indicated that fossil fuel combustion,biomass burning and vehicular emissions were the principal sources while low temperature conversion or leakage of petroleum products made moderate contribution.(2)Apart from the DaA,15 dissolved PAHs have been detected among the 16 species nominated on the EPA priority list in the catchment soil waters,what’s more,the detectable rate of Nap is 100%.The variation of total concentrations of PAHs and PAHscarc in the soil waters were105.90~4696.08ng/L and ND~1388.79ng/L,respectively.The very high concentration of dissolved PAHs found at Xinlicun followed by Banbianjie and Shiyuanzi,but the concentration of PAHscarc in Xinlicun was obviously lower than those in Banbianjie and Shiyuanzi.The results showed that a similar vertical migration pattern of dissolved PAHs in soil waters was found between Xinlicun and Shiyuanzi,the total concentrations of dissolved PAHs was increased by the depth of soil and the increased amplitude was obvious in 50-80 cm soil layer than that in 20-50 cm,and which exhibited more obvious during the dry season.The concentration of dissolved PAHs at Banbianijie sampling site exhibited considerable variation among different months in vertical direction and which resulted from the heavy rainfall erosion.As the dissolved PAHs migrate from20 cm to 80 cm in soil profile the percentage of low-ring(especially,3-ring)was the dominant composition and continued to increase,that is because low-ring PAHs has low hydrophobicity and more be easily dissolved into waters,and which have stronger migration ability,what’s more,no evidence was found that DOC in soil water can affect the concentration of dissolved PAHs for the moment.In general,soil waters was moderately polluted by dissolved PAHs,especially by Bap and Phe.In addition,the total concentration of colloid PAHs in soil water at Shiyuanzi was correspondingly increased by the soil depth,especially in the 50-80 cm soil layer.The totalconcentration of colloid PAHs at Banbianjie was increased from 20 to 50 cm,whereas which decreased from 50 to 80 cm.And no obvious pattern was found at Xinlicun where the total concentration of colloid PAHs in soil water varied largely among different months in vertical direction.Colloid and dissolved PAHs at Xinlicun shared similar composition pattern,unlike dissolved PAHs,the composition of colloid PAHs changed different during the vertical migration process,the proportion of low-ring PAH was decreased from 20 to 80 cm soil layer,but the proportion of high-ring PAH was increased.(3)Apart from the Ant,15 PAH compounds have been detected among the 16 species nominated on the EPA priority list in karst groundwater river,what’s more,Nap,Flu,Phe,Fla and Pyr had high detection rate in different forms and 7 kinds of carcinogenic PAH monomers was low detection rate.In addition,total dissolved and colloid PAHs have similar concentration,with the average concentration of 532.74 ng/L and 479.11ng/L,respectively,and the carcinogenic PAHs was focused on suspended particulate matter.Total concentration of PAHs in karst ground-river in dissolved,colloid and particulate forms showed high values in February,2016,which may result from human activities during Chinese Spring period.Low-ring(especially,3-ring)was the dominant composition of PAHs among different forms in karst ground-river.However,the percentage of mid and high-ring PAHs in particulate matters was obviously higher than those in dissolved and colloid matters.From the Shidengzi sampling site to the outlet of karst ground-river,concentrations and compositions of different forms of PAHs in karst ground-river changed a lot because of the supply water and the effect of different degradation of PAHs components along the route.For further analysis of dissolved PAHs,free PAHs were the constituents of dissolved PAHs and the lower ring of the PAHs in waters and the stronger bioavailability.In general,karst ground-river waters was lightly polluted by dissolved PAHs,especially by Phe,InP and Fla.By analyzing the composition of dissolved PAHs and correlation of PAH compound between karst ground-river waters and soil waters,it can be found that PAHs in soil waters were the important sources of karst ground-river waters.