Close to the outlet pipe (CP), the highest percentage of THg as MMHg occurred in subsurface sediments between 1 and 3 cm depth (Fig. 4c) while at FP site, this percentage did not really vary with depth ranging from 0.24% to 0.85% (FP1-2, Fig. 4f). The percentage of THg as MMHg along the upper 4 cm was significantly higher (p-value < 0.01) in CP (n = 12) than FP (n = 8) site. Maxima of 4.5% and 5.5% were found for CP1 and CP3, respectively at 1–2 cm while CP2 reached a maximum value of 6.2% at 2–3 cm. In porewater, dissolved percentage of THg as MMHg at CP site followed the same trend as for solid Riluzole and ranged from 0.1% to 9.1%, with the highest value found at 2–3 cm depth. This indicates an equilibration between solid- and aqueous-phase, suggesting hypertension MMHg concentrations are constrained by the production of MMHg. It is further interesting to note that solid percentage of THg as MMHg (Fig. 4c) were not related to dissolved THg (Fig. 5), indicating that the amount of dissolved IHg is not the limiting factor for methylation in sediments but the speciation of Hg or the bacterial activity. The percentage of THg as MMHg was thus higher in subsurface sediments at CP although their OM and Hg contents are similar to FP. If we consider percentage of THg as MMHg as a proxy for the net Hg methylation rate (Drott et al., 2008), our results demonstrate that the ferruginous conditions that prevail around the outlet pipe promote the formation of MMHg compared to further away where sulfate-reducing conditions are taking over.