Prevalence of Polyhalogenated Carbazole in an Anthropogenically Impacted Soil Groundwater System

It has been proven that highly halogenated conditions could favor the generation of PHCZs in laboratory experiments, which has not been confirmed in the environment. In this study, carbazoles (CZs) and eight polyhalogenated carbazoles (PHCZs) were investigated in the groundwater-surface soil system along the coast of Laizhou Bay, around the main halogen producing areas, covering chemical enterprises, agriculture, aquaculture, and salt fields. The concentrations of PHCZs in surface soil and groundwater samples range from 0.15 to 1327.94 ng/g dry weight (dw) (mean: 80.06 ng/g dw) and 0.12-10710.56 ng/L (mean: 365.14 ng/L), respectively, much higher than those detected in other areas in China and abroad. The primary contaminant is 1,3,6,8-tetrabromocarbazole (1,3,6,8-BCZ), with a concentration range of nondetectable (N.D.)-10706.69 ng/L (detection rate, 95.79%) in groundwater and N.D.-818.52 ng/g dw (detection rate, 97.62%) in soil samples. The characteristics of the dominant position of 1,3,6,8-BCZ in the environment are different from those of other results from China, and its groundwater concentration is thousands of times higher than that of other water samples, indicating that highly halogenated environmental conditions also largely promote the generation of highly halogenated PHCZs, e.g., 1,3,6,8-BCZ. This may be the same reason for the identification of some PHCZ derivatives and mixed-substituted carbazoles in the studied area through suspected targeted screening. More than half of the soil and groundwater samples are considered to have moderate or high carcinogenic risk (CR_T > 10^-4), and 11 of the 95 groundwater samples are considered to have high health risk (HQ_T ≥ 1). The mobility of groundwater improves the migration possibility of PHCZs from enterprises to residential areas to induce direct toxic effects, and the high concentration and risk levels of PHCZs in agricultural and aquaculture areas at present may induce bioaccumulation in agricultural and sideline products; therefore, the occurrence of PHCZs in the environment deserves urgent concern.