Metal Mine Tailings and Sludge CO-Deposition in a Tailings Pond

In the Lower Cell of the Heath Steele mine tailings arealocated 50 km north of the city of Miramichi, New Brunswick,Canada, tailings and lime treatment sludge were co-depositedand the effluent occasionally exceeded regulatory dischargelimits for total suspended solids (TSS), especially on verywindy days. A combined field and laboratory investigationwas undertaken during 1998-1999 to identify the cause of thehigh TSS. The methodology included field measurement of windvelocity and sampling of bed tailings, sludge,suspended sediment, and the water cover. The samples werethen subjected to various laboratory examinations using x-raydiffraction, scanning electron microscopy interfacedwith energy dispersion x-ray spectroscopy, critical shearstress measurements, and water chemistry analysis. Thesuspended sediment was found to be composed primarily ofcalcite and metal hydroxides derived from the bed sludge inthe cell. Only a very small amount of tailings (less than5%) was detected in the suspended sediment. The sludge,which covered the tailings in the shallow western section ofthe Lower Cell at depths less than 1 m, was a loose, low-densitymaterial with a low critical shear stress (approximately 0.058 Pa). In shallow water cover (less than 1 m), calculated bed shearstress mobilized by wind-induced waves and return currents exceeded the critical shear on a number of occasions, resultingin resuspension of the sludge and hence high TSS. Although occasional elevated zinc concentrations appeared to follow a similar pattern to high TSS, there was no evidence that thesuspended sludge sediments would release metals into the watercover, due to the high pH of the Lower Cell water cover. Since the tailings did not resuspend significantly, it was clear thatat water cover depth less than 1 m, sludge was eroded instead oftailings, and thus provided a barrier against tailings resuspension. As part of the final closure scheme for the HeathSteele tailings area, sludge and tailings were dredged and relocated from areas where the water cover was < 1 m to deeperwater cover areas to ensure that the effluent met required totalsuspended solids discharge criteria.