A study at the University of Manitoba investigated the effectiveness of a natural wetland as a remediation system for arsenic contamination derived from mine waste at New Britannia Mine (NBM), Snow Lake, Manitoba, Canada. At this deposit, gold is associated with arsenopyrite which is finely ground and treated with cyanide during processing. Subsequent oxidation of the arsenopyrite in the mine waste is the primary cause of arsenic contamination at the mine site.
Gold was extracted from arsenic-rich ore bodies in Snow Lake, by Nor-Acme Mine from 1949 to 1958 and then by NBM from 1995 to 2002. Nor-Acme left two major sources of arsenic contamination on the mine property, a small tailings area and the arsenopyrite residue stockpile (ARS). Concentrated residue was stockpiled in the ARS in hopes of finding economical means to recover the remaining refractory gold contained in the residue. A later attempt to extract gold from the residue through use of a lined leach pad added to the contamination. Although NBM capped the ARS and revegetated the area of the leach pad in 2000, elevated arsenic concentrations (up to 13 mg/L) have been detected in surface water in a wetland runoff area (RA) on the mine property. These values are well above the World Health Organization guideline of 0.001 mg/L. Water from the RA flows through a wetland toward Snow Lake, the source of drinking water for the Town.
A biogeochemical survey was employed to assess the passage of arsenic from the RA down the flow path toward Snow Lake. The study aimed to identify the distribution of arsenic and iron between surface water, soil and aquatic plants along the flow path, and to determine the mechanism for arsenic sequestration by plants and soil in the wetland. Surface water, soil and common cattail and water sedge plants were collected for total geochemical analyses by ICP-MS. Plants were separated into samples of roots, live shoots and dead shoots. Additional plant samples were squeezed by hydraulic press to extract fluids contained in cell vacuoles. Plant vacuoles and solid envelopes were then separately analyzed for total arsenic using ICP-MS. Root sections were prepared for electron microprobe (EMP) imaging and element mapping of arsenic and iron. In the field, surface water was passed through strong cation and strong anion exchange cartridges to separately retain As(III), As(V), MMA and DMA. Following elution in the laboratory the separate aliquots were analyzed for total arsenic.
Results of the flow path survey revealed the greatest fraction of arsenic in the RA was sequestered in organic soil (~ 4000 mg/kg), followed by plant roots (~ 900 mg/kg in sedge, 800 mg/kg in cattail), dead shoots (~ 800 mg/kg in sedge, 3 mg/kg in cattail), live shoots (~ 40 mg/kg in sedge, 3 mg/kg in cattail) and surface water (~ 1 mg/kg). Total geochemical analyses indicated that arsenic in the system is commonly associated with iron in all sample media. In surface water, arsenate is prevalent over arsenite and ~50% of arsenic existed in a methylated form. Separate analysis of cell vacuoles and envelopes and EMP element mapping showed arsenic to be sequestered with iron within in the cell walls of sedge and cattail roots and shoots.
The study indicated that a natural wetland can be very efficient at sequestering arsenic, reducing the environmentally available concentration by a factor of ten within 200 m and reducing concentrations in surface water well below international guidelines. Understanding the factors controlling sequestration of trace elements and heavy metals in wetlands can prove valuable in efforts to remediate environments influenced by mining and characterizing risk for the remobilization of sequestered elements.
Jill Maxwell, Barbara Sherriff, Elena Khozhina, Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada