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Geochemical behaviour, fate and impacts of Cu, Cd and Zn from mine effluent discharges in Howe Sound

University of British Columbia

The now abandoned mine at Britannia Beach is a source of acidity and heavy metals to Howe Sound. Using evidence from monthly field sample collections over an entire year, the dispersion patterns of Cu, Cd and Zn in the estuary were elucidated. Acid mine drainage discharged to Howe Sound via Britannia Creek was found to mix predominantly with the brackish surface waters, since a pronounced pycnocline inhibits vertical mixing. Deep discharges via a submarine outfall may become trapped at or below ten metres depth during summer, but have been observed to rise to the surface in winter as a result of weaker stratification. Upon mixing with estuarine water, there was generally a transfer of Cu from the dissolved to the particulate phase. Similar removal was observed with Zn, albeit to a much lesser degree. Cd behaved essentially as a conservative element, which permitted its use as a tracer for the mixing of acid mine drainage with seawater. This greatly facilitated interpretation of the field data, since salinity proved to be an unreliable indicator of mixing in this three end-member system of Squamish River, Britannia Creek and seawater. Laboratory mixing experiments demonstrated that the removal of Zn occurred via adsorption onto iron oxyhydroxides, with other processes such as adsorption by flocculated organic matter or by clays playing a less important role. Cu removal was additionally governed to a large extent by direct precipitation of solid phases, most likely Cu(OH)2. Mixing experiments and speciation calculations suggest that Cu, and possibly Zn, may be present to a large degree in colloidal suspension. Metal concentrations in estuarine surface waters ranged between 0.8 - 230, 0.02 - 2.9 and 1.7 - 450 μg/L for Cu, Cd and Zn, respectively at a site some 350 m off the mouth of Britannia Creek. They were found to be controlled by the supply from acid mine drainage, by dilution with Squamish River runoff, and by the extent of removal in the creek mixing zone. All these parameters are strongly seasonal due to their dependence on hydrological conditions. They combined to produce maximum surface metal concentrations in spring and winter, as demonstrated using both field data and model calculations. In a bioassay experiment, Cu concentrations over 6.4 μg/L affected growth of endemic phytoplankton adversely. This level was found to be exceeded in 13 out of 26 field observations in Britannia Bay. In spring, concentrations that could produce detrimental effects on phytoplankton may also extend several kilometres down-estuary.

Thesis/Dissertation

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2009-04-03

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http://hdl.handle.net/2429/6779

Oceanography

University of British Columbia

UBCV

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