EFFECT OF REDOX CONDITIONS ON ARSENIC MOBILITY AND THE FORMATION OF ARSENIC SULFIDES IN A WETLAND AT TERRA MINE, NORTHWEST TERRITORIES

Elevated arsenic concentrations in surface water from storing mine tailings in lakes can have a negative impact on local and downstream vegetation and aquatic life. At Terra Mine, an abandoned silver and copper mine in the Northwest Territories, tailings storage in Ho-Hum Lake has resulted in dissolved arsenic concentrations of 56-72 µg/L, exceeding the 5 µg/L maximum guideline for aquatic life. A natural wetland located downstream appears to be attenuating arsenic from surface water. To understand the effectiveness of the wetland to sequester arsenic from the water, sediment mineralogy and water chemistry was analyzed to determine how arsenic is being stored in the sediments and whether this is a stable method for long term storage. Surface and pore water was analyzed for arsenic speciation and chemistry to identify how it is moving through the wetland in the dissolved phase. To determine how arsenic is bound to the sediments, sediment cores were collected and analyzed for bulk chemistry, arsenic speciation and mineralogy using XANES and ESEM.

Dissolved arsenic concentrations are higher in the spring than late summer in all surface and pore water samples. Arsenic concentrations in the water increase downstream from the tailings lake in the spring, while concentrations in the surface water are relatively constant by late summer. This implies that arsenic is being released from the sediments in the spring, and is not efficiently removed from the water in late summer. At most sample sites, arsenic concentrations in the sediments increase over the summer. In late summer, arsenic is predominantly stored in the sediments as arsenic sulfides due to decreased redox potential and the abundance of sulfur in the system, and is clearly visible in ESEM images and XANES analysis. These sulfides may be susceptible to oxidation if the redox potential increases due to cyclic seasonal changes, remobilizing arsenic in the spring and decreasing concentrations in the sediments. These observations suggest that the wetland is not sequestering dissolved arsenic from the surface water in significant quantity. Arsenic that is retained in the sediments as secondary arsenic sulfide is vulnerable to reoxidation and release as a result of seasonal variation in redox conditions.