Geochemistry of mercury in tropical swamps impacted by gold mining

•Methylation is higher in the dry season in tropical swamps sediments with lesser T-Hg.•Mercury is mainly associated (33–43%) to the organic fraction in both marshes.•The organic Hg is higher during the dry, whereas elemental Hg is during flooding.•Elemental Hg is related with anthropogenic activities in upstream gold mining sites.•High amount (>15%) of labile Hg species poses a potential risk to biota and humans. Artisanal and small-scale gold mining (ASGM) poses a serious threat to the local environment. Colombia has very active ASGM activities, where mercury (Hg) ends in piles of mining waste, soils, and waterways. In this study, we assessed Hg speciation and bioavailability in sediments of two tropical swamps, impacted by ASGM. In Ayapel swamp, total Hg (T-Hg) concentrations in sediments ranged between 145 and 313ngg−1 dry weight (dw) (mean: 235±49ngg−1 dw), whereas Grande Achi swamp levels are 3-fold higher (range: 543–1021ngg−1 dw; mean: 722±145ngg−1 dw). Even though lower levels of Hg were found in Ayapel, methylation was found to be significantly higher than in Grande Achi, and it is significantly higher in the dry than in the rainy season for both swamps. This increased methylation is linked to the statistically significant correlation between T-Hg, MeHg and organic matter in the Ayapel swamp. In fact, Hg content in both swamps is mainly associated to the organic fraction (Hg-o), with a higher statistically significant difference in Ayapel (43±5%) compared to Grande Achi (33±5%). On the other hand, a significant percentage (30±6%) of elemental Hg fraction (Hg-e) was found in Grande Achi, directly related with Hg released during the gold recovery process from upstream ASGM sites. The percentage of the bioavailable fraction (Hg-w and Hg-h) is elevated (up to 15%), indicating a potential risk to the aquatic environment and human health because these labile Hg species could enter the water column and bioaccumulate in biota.