17:15 〜 19:15
[MIS22-P12] Modeling Mercury Processes in the Adour Estuary (France) considering Variable Environmental Factors
Mercury (Hg) threatens ecosystems and human health. Aquatic systems are specifically vulnerable to Hg contamination as Hg accumulates in aquatic biota and is repeatedly deposited and discharged in. Estuaries are particularly important in Hg dynamics because they are the primary interface between continental and marine waters. Complex Hg processes are acting in estuaries, combining inputs (e.g., atmospheric dissolution and river and coastal water inflow), outputs (e.g., diffusion and outflow to upstream and the ocean) and internal reactions (e.g., redox reactions, methylation, and demethylation). Estuarine control factors, such as water flow, salinity and suspended particulate matter (SPM), are permanently evolving under the influence of river discharge, tidal cycles, wind, rain and waves. Allowing to individually define the control factors and involved processes, numerical simulations appear as an effective tool for understanding Hg dynamics. Process-based models for estuaries remain sparse, and mostly do not account for short-term responses and combined changes in multiple factors. Therefore, this study aimed to develop an Hg process model considering water flow, salinity, SPM and light as key environmental factors to assess the Hg processes in estuarine water.
The study site was the Adour estuary located in south-west of France. This estuary is characterised an intermittent salt-wedge driven by meso-tides oscillations and a fluctuating river discharge between 80 and 3000 m3 s-1. The Hg concentration in the downstream area is generally higher than in the upstream and coastal areas due to the presence of urban and industrial areas. In this study, the simulation area was defined from river mouth to 5.6 km upstream. A one-dimensional Hg process model was developed based on the mass balance of elemental Hg (Hg0), dissolved inorganic Hg (HgII) and dissolved methylmercury (MeHg), including kinetic equations for Hg reactions. Reaction rates were estimated by multiple regression equations derived from previously reported rates taking into account light, salinity and SPM. Water flows in this study area were estimated using numerical outputs from Defontaine’s high resolution 3D hydrodynamic model (2022). Our Hg model calculated amount of Hg every hour from 10 Sep to 5 Oct (dry season) in 2017. Time series observation of salinity (0.3–33.1 PSU) could be used for this calculation. The averaged SPM in the estuary and Hg concentrations in the upstream and the ocean were set from previous observations (Sharif et al. 2014; Stoichev et al. 2023). Additionally, the light condition was set as an idealized diurnal cycle.
During the modeling period in this study, the total water flow from the river to the ocean was 1,216 million m3, and 862 million m3 of water flowed from the ocean to the river. The estimated range of concentrations was 0.064–0.26 ng L-1, 0.14–0.56 ng L-1 and 0.018–0.032 ng L-1 for Hg0, HgII and MeHg, respectively. These values, except for Hg0, were evaluated as underestimates compared to those previously observed concentrations (Sharif et al. 2014; Stoichev et al. 2023). This result may be related to the presence of sewage treatment plants in or upstream of the study area, which were not included due to the lack of data.
Total Hg input from river and ocean was 581 kg and 282 kg, respectively. Total Hg output to river and ocean was 324 kg and 543 kg, respectively. The net terrestrial Hg load to the ocean was 261 kg. The result of the balance (38 kg) between the input from the river and the output to the ocean found that this estuary became a sink for Hg from the river. As the Hg0 diffusion was only 2 kg, Hg from the river might be accumulated with continuous movement between the estuary and upstream. Although HgII dominated 80% of the input from the river, the ratio in the output to the ocean was decreased due to the strong HgII reduction. The methylation and demethylation results showed that MeHg was not concentrated in this study.
The study site was the Adour estuary located in south-west of France. This estuary is characterised an intermittent salt-wedge driven by meso-tides oscillations and a fluctuating river discharge between 80 and 3000 m3 s-1. The Hg concentration in the downstream area is generally higher than in the upstream and coastal areas due to the presence of urban and industrial areas. In this study, the simulation area was defined from river mouth to 5.6 km upstream. A one-dimensional Hg process model was developed based on the mass balance of elemental Hg (Hg0), dissolved inorganic Hg (HgII) and dissolved methylmercury (MeHg), including kinetic equations for Hg reactions. Reaction rates were estimated by multiple regression equations derived from previously reported rates taking into account light, salinity and SPM. Water flows in this study area were estimated using numerical outputs from Defontaine’s high resolution 3D hydrodynamic model (2022). Our Hg model calculated amount of Hg every hour from 10 Sep to 5 Oct (dry season) in 2017. Time series observation of salinity (0.3–33.1 PSU) could be used for this calculation. The averaged SPM in the estuary and Hg concentrations in the upstream and the ocean were set from previous observations (Sharif et al. 2014; Stoichev et al. 2023). Additionally, the light condition was set as an idealized diurnal cycle.
During the modeling period in this study, the total water flow from the river to the ocean was 1,216 million m3, and 862 million m3 of water flowed from the ocean to the river. The estimated range of concentrations was 0.064–0.26 ng L-1, 0.14–0.56 ng L-1 and 0.018–0.032 ng L-1 for Hg0, HgII and MeHg, respectively. These values, except for Hg0, were evaluated as underestimates compared to those previously observed concentrations (Sharif et al. 2014; Stoichev et al. 2023). This result may be related to the presence of sewage treatment plants in or upstream of the study area, which were not included due to the lack of data.
Total Hg input from river and ocean was 581 kg and 282 kg, respectively. Total Hg output to river and ocean was 324 kg and 543 kg, respectively. The net terrestrial Hg load to the ocean was 261 kg. The result of the balance (38 kg) between the input from the river and the output to the ocean found that this estuary became a sink for Hg from the river. As the Hg0 diffusion was only 2 kg, Hg from the river might be accumulated with continuous movement between the estuary and upstream. Although HgII dominated 80% of the input from the river, the ratio in the output to the ocean was decreased due to the strong HgII reduction. The methylation and demethylation results showed that MeHg was not concentrated in this study.