Fate of Linear Alkylbenzenesulfonates in a Lake Estuary

The concentration of linear alkylbenzenesulfonates(LAS)in the Lake Teganuma estuary indicated a seasonal variation and a downstream decrease. The downstream decrease in the studied section varied between 28% and 100% seasonally, and the average was 66(±25)% during 1987-1988. As to explain the decrease, we investigated biodegradation of LAS in water, diffusive exchange of dissolved phase LAS through the sediment/water interface, and settling of adsorbed phase LAS. Adsorption experiments were performed between 0.3-9 ppm of the initial concentrations of LAS. The experimental equation used to estimate the adsorption coefficient of LAS was verified by using adsorption data from field observations. The averaged value 2.4×10⁻⁵ cm² sec⁻¹ for the diffusion coefficient in the sediment interstitial water was obtained from the experiments using the sediment core samples. The biodegradation rate constant (between 0.06 and 3.33 day⁻¹) in water and the diffusive flux rate (between −0.157 and 0.022 mg m⁻² hour⁻¹) from water to sediment were obtained from field observations and experiments. Based on this information, a mathematical model was applied to simulate the downstream decrease of LAS in water. With this model, nearly 10% of the total LAS from the river was found to be transported to sediment both in summer and winter. The settling flux of LAS in the lake estuary was ten times greater than the diffusive flux. The settling flux was ten times smaller than the disappearing flux by biodegradation in summer but was the same level in winter.