Evaluating the sediment–water exchange of hexachlorocyclohexanes (HCHs) in a major lake in North China

A large-scale sampling program was conducted to simultaneously collect surface water, overlying water, pore water and sediment samples at monthly intervals from March to December 2010 from the Baiyangdian Lake, North China to assess the distribution of hexachlorocyclohexanes (HCHs) and determine the net direction of sediment–water exchange. Total HCHs concentrations ranged from 6.30–53.90 ng L−1, 16.59–148.48 ng L−1, 247.93–459.07 ng L−1 and 3.60–12.50 ng g−1 in surface water, overlying water, pore water and sediments, respectively, which was at the intermediate level compared to those of other areas in China. Seasonal variations of HCHs were featured by lower concentrations in April, July and November due to the dilution of HCHs caused by a great deal of fresh water being allocated to the Baiyangdian Lake in these months. Sediment–water fugacity ratios of the HCH isomers were used to predict the direction of the sediment–water exchange of these isomers. The sediment–surface water and sediment–overlying water fugacity ratios of HCH isomers averaged 11.4 and 3.0, which is significantly greater than the equilibrium status (1.0), suggesting that the net flux directions were from the sediment to water and the sediment acted as a secondary source to the water. Sediment–pore water fugacity ratios indicated near equilibrium or escape from pore water to sediment, depending on the isomer and sampling time. The difference of HCH concentrations between sediment and water samples were found to be an important factor affecting the diffusion of HCH from the sediment to water.