Modeling the dynamic changes in concentrations of γ-hexachlorocyclohexane (γ-HCH) in Tianjin region from 1953 to 2020

A level IV fugacity model was used to simulate the dynamic changes of γ-hexachlorocyclohexane (γ-HCH) concentrations in environmental media in Tianjin, China. A similar model (level III) was previously used and validated under steady state conditions; this paper explores its dynamic behavior. Application of the level IV fugacity model has been validated using independently observed γ-HCH concentrations in various media during the early 1980s and during 2001. Sensitivity analysis was conducted using coefficient-of-variation normalized sensitivity coefficients. The model was also subject to uncertainty analysis using Monte Carlo simulation. It was found that concentrations of γ-HCH reached within 95% of their steady-state levels in all media after less than 15 years. Around one order-of-magnitude decreases in γ-HCH concentrations in various media occurred between 1993 and 2001. We project that γ-HCH concentrations will decrease another 1.7–1.9 orders of magnitude to reach 1.1 (0.9–1.2)×10−14, 2.7 (2.5–3.0)×10−10, 1.2 (1.1–1.3)×10−7, and 6.1 (5.4–6.8)×10−8 mol/m3, in air, water, soil, and sediment, respectively, by 2020. The sensitivities and true uncertainty of the model are discussed.