Dynamics of Hg and MeHg in the Madeira River basin (Western Amazon) before and after impoundment of a run-of-river hydroelectric dam

The impact of a hydroelectric run-of-river (RoR) dam construction on the dynamics of total mercury (THg) and methylmercury (MeHg) is of interest to the environment and health of human and wild life. We monitored (May 2010 to October 2018) THg and MeHg in the waters and in the suspended particulate matter (SPM) of the Madeira River and its tributaries (before and after dam construction) to evaluate changes upstream and downstream from the Santo Antonio Hydroelectric Dam (SAHD). We collected 2826 samples of water and SPM at sampling stations upstream (UPMD-01, UPMD-02 and UPMD-03) and downstream the Madeira River (DWMD-04, DWMD-05 and DWMD-06), and Tributaries upstream (Branco River, Jaci-Parana River, Jatuarana-I Igarapé, Contra River, Caripunas Igarapé, Ceara Igarapé, and Teotonio Igarapé) and downstream (Jatuarana-II Igarapé and Belmont Igarapé) from the SAHD and monitored water and the total load of SPM. SPM was significantly higher in the Madeira River (median: 178.63 mg.L^-1) than in upstream and downstream tributaries (median: 15.30 mg.L^-1); however, the THg and MeHg concentrations were significantly higher in the tributaries (median: 161.14 ng g^-1 and 9.03 ng g^-1, respectively) than in the mainstream Madeira River (median = 57.06 ng g^-1 and 1.63 ng g^-1, respectively). THg concentration in the water was significantly higher in the mainstream Madeira River (median = 6.51 ng.L^-1) than in the tributaries (median = 2.57 ng.L^-1). However, the percentage of methylation in the tributaries (median = 4.9%) was 4-times higher than in the Madeira River (median: 1.3%). The significantly higher MeHg percentages in the tributaries may indicate natural (hydro-bio-geochemical factors) still predominant in this changing landscape of the Western Amazon. So far, the data suggest that this RoR has not yet impacted the THg and MeHg concentrations.