Tracing urban sewage pollution in Chaohu Lake (China) using linear alkylbenzenes (LABs) as a molecular marker

Linear alkylbenzenes (LABs) were determined in 61 surface sediment samples from Chaohu Lake, one of the largest and most euthrophication shallow lakes in China. Amongst these samples, thirty-five were collected with a 0.05 × 0.05° latitude/longitude resolution and 26 surface sediments were obtained at three main estuaries of inflowing rivers. The total concentration of LABs (defined as ΣLAB) ranged from 18.5 to 5720 ng g(-1) dry weight. Concentrations of ΣLAB in the western region of the lake were much higher than those in the eastern region, and the highest value was at a location in the estuary of Nanfei River, indicating that substantial amounts of municipal wastewater were discharged to Chaohu Lake via riverine runoff. Low ratio of the concentrations of internal isomers versus external isomers of C(12)-LABs ((6-C(12)+5-C(12))/(4-C(12)+3-C(12)+2-C(12)) (defined as I/E) suggested that only 16% of LABs were biodegraded on average in the sediment. Therefore untreated/inadequately treated sewage was discharged into the lake. Log-based concentrations of ΣLAB were positively correlated with log-based I/E ratios. On the other hand, negative correlative relationships were found between the log-based concentrations of ΣLAB and both log-based ΣC(13)-LAB/ΣC(12)-LAB and (5-C(13)+5-C(12))/(5-C(11)+5-C(10)) ratios (referred as L/S ratio), probably implicating that the ΣC(13)-LAB/ΣC(12)-LAB and L/S ratio should be used with caution. The total amount of ΣLAB annually deposited to Chaohu Lake's sediment was estimated at 0.2 tons yr(-1), accounting for 2.4% of the total amount of LABs contained in detergents used within the entire watershed. Therefore, the majority of LABs discharged from the watershed still remained in the environmental media.