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Sedimentary tracers of sewage inputs to the southern Strait of Georgia

University of British Columbia

Sewage effluent has been discharged at depth to the Strait of Georgia just south of Vancouver since 1988, following primary treatment at the Iona Island Sewage Treatment Plant. The discharge is accomplished through a pair of semi-parallel diffusers between water depths of ~ 70 m and ~ 105 m that lie orthogonal to the bottom contours on the foreslope of the Fraser River Delta. Prior to the research reported here, knowledge was limited as to the transport pathways and depositional fate of the particulate matter in the effluent. Thus, the objective of this work is to investigate and apply a suite of sewage-specific tracers, in particular silver, stable carbon and nitrogen isotopes, and more commonly-considered contaminant metals (Cu, Pb and Zn) in an effort to define better the depositional footprint of the Iona sewage in southern Georgia Strait. Silver has the highest relative enrichment of any metal in sewage, and is expected to be an ideal candidate as a sewage tracer. That this is the case here is borne out by a well-defined band of high Ag concentrations (~ 400-800 ng/g) in sediments that extends along the foreslope north of the outfall by ~ 6 km and south by ~ 1 km. This pattern reflects transport of the effluent plume predominantly to the north with little lateral dispersion. The accumulation of silver in this area is unlikely to be due to authigenic precipitation of Ag2S or other sulphide phases, but is instead attributed to direct deposition of a very small amount of particulate sewage in which the silver content is highly enriched. Very high silver concentrations (1, 400 ng/g) are also observed in an area of Sturgeon Bank close to the Iona Island plant, where direct discharge of sewage occurred prior to 1988. In contrast, very low levels (~ 50 ng/g) are found in coarse-grained lag deposits in shallow regions of the delta. Sediments in the deep central basin of the Strait of Georgia host concentrations on the order of 130 ng/g. Significant enrichments of manganese oxides in the surface sediments of some cores from the central Strait are not matched by enhanced silver concentrations, suggesting that silver is minimally associated with manganese oxide phases in marine sediments. Neither stable carbon arid nitrogen isotope distributions nor concentration patterns for Cu, Pb and Zn show any associations in the Strait of Georgia with the pattern of sewage deposition defined by the silver data. The lack of sensitivity of these two groups of potential tracers is attributed to a coupling between low signal to background ratios and the substantial discharge of sediments (some 20 x 10^ m^/year) from the Fraser River which dwarfs sewage-specific inputs of the trace metals and the isotopes. Thus, of the various tracers investigated in this research, silver is uniquely appropriate.

Thesis/Dissertation

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2009-04-27

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http://hdl.handle.net/2429/7607

Oceanography

University of British Columbia

UBCV

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