Preliminary study on contaminant of polycyclic aromatic hydrocarbons in vulnerable karst aquifer in a heavy industry district

One heavy industry district (steel and fertilizer plants) in the karst area of southwestern China was selected as the study area. The groundwater and soil samples in the hydrogeological units were collected, from which 16 polycyclic aromatic hydrocarbons (PAHs) prioritised by USEPA were analysed using GC/MS method. The results show that total concentration of the PAHs in groundwater ranges from 1135.79 to 1361.26 ng/L, with dominant species as phenanthrene, anthracene, naphthalene, chrysene and pyrene. Compared with other regions, groundwater pollution due to the PAHs in the study area is of a middle level. According to National Recommended Water Quality Standards of priority toxic pollutants (2009 USEPA)，it is found that benzo[a]anthracene, chrysene, benzo[b]fluoranthene, and benzo[k]fluoranthene exceed the standard. The figures of PAHs characteristic ratio suggest that the PAHs sources in the karst aquifer were derived from incompletely burning and coking of coal, which are similar to contaminant sources discharged by steel and fertilizer plants. The karstic groundwater in the downstream areas have been highly polluted; and it appears that the PAHs concentration in the discharge area is a higher than that of runoff area. Impact of the PHAs on the karst aquifers is sever, which can be attributed to the following two aspects. On the one hand, common presence of karstic depressions, dense fissures developed in the karst, weathered dolomite and fault fracture zones provides a favorable condition for PAHs migration in the karst aquifers which are high vulnerable to the contaminant. Meanwhile, large quantity of groundwater in source areas has been extracted for industrial use, which speeds up the infiltration of pollutants into the karst aquifers. On the other hand, the surface water level in the study area has been raised owing to the construction of the Honghua hydropower station dam which stores huge amount of water in the reservoir; this in turn elevates the groundwater level and slows down its discharge to the stream system. As a result, the self-purification ability of PAHs in the karst aquifer decreases. The location of pollution sources in the groundwater system is also one of the factors that impact of the PAHs migration in karst aquifer. Risk assessment of groundwater suggests that the content of phenanthrene, anthracene, pyrene, benzo[a]anthracene and benzo[b]fluoranthene in the karst groundwater is at a very high risk grade. Thus, it is suggested that local government should take immediate intervention to reduce this pollution risk.