SPATIAL AND TEMPORAL TRENDS IN ATMOSPHERIC MERCURY DEPOSITION TO SOUTHERN PERU

Artisanal and Small-Scale Gold Mining (ASGM) is the second largest source of mercury (Hg) to the atmosphere after coal combustion. ASGM operations in the Madre de Dios region of southeastern Peru have grown exponentially with the price of gold over the past ~10 years. The local effects of ASGM in Madre de Dios have recently become evident by elevated Hg levels in people near the mining area, but the regional and global fate of these Hg emissions remain unknown. In addition, historic and pre-colonial use of Hg is documented in parts of central Peru, but little is known about the timing and spatial extent of its deposition in southern Peru.

Here we assess spatial and temporal trends in atmospheric Hg deposition using lake sediments collected from southern Peru in 2011. The sampled lakes (n = 15) are all down-wind and to the west of the major ASGM practices in Madre de Dios. The lakes span two degrees of latitude (13 –15 °S), two degrees of longitude (70 – 72 °W), and two kilometers of elevation (3 – 5 km asl). Hg concentrations in surface (0 – 2 cm) sediments from thirteen lakes are significantly correlated with longitude (r = 0.89, p < 0.01) and elevation (r = 0.59, p < 0.05), indicating that both proximity to ASGM sites and orography influence Hg transport and deposition in this region. Dated cores from four lakes exhibit an overall increasing trend in Hg fluxes over the past 400 years and most prominently over the past 100 years. However, some cores display larger-than-modern peaks in Hg concentration and flux during pre-industrial time, most notably in a core from lake “PLS-8” from A.D. 1700 – 1800 and in a core from lake “Yanacocha” from B.C. 1000 – 500. These site-specific post-industrial enrichments, likely from small-scale mining within or near the catchment, illustrate the need for Holocene records to establish true natural Hg fluxes. In addition, such high Hg concentrations in certain pre-industrial sediments may pose a hazard if re-mobilized. Modern Hg fluxes from the four studied lakes (3.5 – 7.8 µg m^-2 a^-1) are comparable to fluxes from other remote closed-basin lakes from around the world, which suggests that Hg from ASGM in southeastern Peru is not primarily deposited to nearby downwind regions; rather, these Hg emissions are likely entering the global atmospheric pool.