MAGNITUDE AND SOURCES OF HG CONTAMINATION WITHIN ALLUVIAL SEDIMENTS OF THE ESSEQUIBO AND MAZARUNI RIVERS, GUYANA

Small and medium scale mining operations in Guyana increased dramatically during the late 1980s and early 1990s. The majority of these mining operations utilize mercury (Hg) amalgamation in the recovery process, raising the question as to the significance of Hg inputs to river systems from mining activities. In March and April, 2001, 168 samples were collected from floodplain, sand bar, and channel bed deposits along a 350 km reach of the Mazaruni River and a 160 km reach of the Essequibo River. The floodplain deposits are characterized by highly weathered (lateritic) soils overlain by 0-1 m of unweathered or only slightly weathered sediment. The latter materials have formed by vertical accretion during overbank flooding within the past few decades. Deposition of the lateritic materials pre-date Hg amalgamation mining and, therefore, represent background concentrations of Hg in alluvial deposits. Both types of floodplain deposits were sampled. Trends in the geochemical data suggest that Hg concentrations in the post-mining alluvial sediments reflect the recent influx of anthropogenic Hg to the rivers. These trends include (1) Hg concentrations in approximately half of the sand bar, channel bed, and floodplain surface deposits that exceed the maximum Hg concentration (0.080 µg/g) observed within the lateritic soils, (2) Hg concentrations that systematically increase toward the surface of the floodplain (i.e., in younger sediments), and (3) Hg concentrations that are elevated downstream of tributaries characterized by extensive mining operations, or in areas characterized by local mining activities. Recent investigations in parts of Amazonia where deforestation is significant have found that Hg from amalgamation mining represents a relatively small portion of the total Hg load to riverine environments, the majority derived from the erosion of Hg enriched upland soils. The geochemical data collected from the Essequibo and Mazaruni Rivers suggest that Hg from mining may be more a significant source in Guyana where large-scale deforestation is limited. However, it is unclear whether the increased Hg represents the direct input associated with the amalgamation process or Hg associated with the erosion of soils and sediments that accompany mining activities.