Accumulation and transport of ammonium in aquitards in the Pearl River Delta (China) in the last 10,000 years: Conceptual and numerical models [Accumulation et transport d'ammonium dans des aquitards du delta de la Rivière des Perles (Chine) durant les 10,000 dernières années: Modèles conceptuels et numériques]

摘要

Generation, accumulation and migration of ammonium in the coastal aquifer-aquitard system of the Pearl River Delta, China, are a result of complex and long-term processes including sea-level changes, shoreline migration, sedimentation, decomposition of organic matter, solute-transport processes, and chemical reaction during the Holocene. Simulation of long-term ammonium distribution is not well addressed in the literature due to the difficulties in quantifying the complex geochemical processes. Salinity and ammonium profiles in the aquitards at two typical sites were obtained from geochemical analyses of soil and water samples. One-dimensional numerical models were used to integrate present knowledge of stratigraphy, historical evolution of the geological system during the Holocene, and the processes related to the generation and migration of ammonium. The ammonium generation and accumulation were approximated by an exponential function, and a moving boundary condition was used to reflect the sea-level changes during the Holocene. The observed salinity profiles were used to estimate flow and dispersion parameters, then the observed ammonium profiles were used to estimate the reactive parameters related to ammonium generation. The reasonably good match between the simulated and observed results demonstrates that the models can capture the dominant processes that control the generation and movement of ammonium.