Environ. Sci. Technol. 2005, 39, 4361-4368
Historical Trends in Mercury Sedimentation and Mixing in the Strait of Georgia, Canada SOPHIA C. JOHANNESSEN,* ROBIE W. MACDONALD, AND K. MAGNUS EEK† Institute of Ocean Sciences, 9860 W. Saanich Rd., P.O. Box 6000 Sidney, British Columbia V8L 4B2, Canada
Seventeen sediment cores collected in the Strait of Georgia reveal a history of mercury contamination that began in the 1860s and include episodic contamination during World War II and in the late 1960s. Surface sediment mercury concentrations ranged from 60 to 420 ng/g dry weight and the current fluxes to sediments are estimated at 5-181 ng cm-2 a-1. In one location in Port Moody Arm, a Hg spill seems to have sufficiently poisoned the sediments to eliminate biomixing for about 20 years. Although the surface concentration of Hg is likely to decrease at all stations in coming years, sites in the industrialized Vancouver Harbor and Port Moody Arm will continue to be sufficiently contaminated to endanger benthic organisms. Variations in sedimentation and mixing rates among sites result in surface sediment Hg concentration patterns that do not reflect accurately the distribution of Hg flux. In particular, the concentration of Hg in sediments near the mouth of the Fraser River is low, despite the high Hg load of that river, because of dilution by other particles. A preliminary Hg budget indicates that most of the Hg enters the Strait of Georgia via the Fraser River (2090 kg a-1), and that, while burial in Strait of Georgia sediments is a major sink (1800 kg a-1), there may be a significant outflow of Hg through Juan de Fuca Strait (∼3400 kg a-1).
Introduction Mercury is a persistent, global contaminant that cycles among the atmosphere, ocean, soil, and sediments (review by 1). It is a powerful neurotoxin, and, in its methylated form, it biomagnifies in the food chain. Mercury occurs naturally in the earth’s crust and enters the aquatic environment and the atmosphere by weathering and volatilization (2). However, for about the last 100 years in North America and 500 years in parts of Europe (e.g., 3), human activities, such as mining, have also released mercury into the atmosphere and ocean. As much as two-thirds (4, 5) or, according to a more recent estimate, half (6) of the mercury in the atmosphere may now be anthropogenic. Contaminant mercury travels through the atmosphere to remote locations (e.g., 7), and storms can carry it across the Pacific Ocean from Asia to the west coast of North America (8). The Strait of Georgia, an inland sea located between Vancouver Island and mainland British Columbia, Canada (Figure 1), has received contaminant mercury from distant * Corresponding author phone: (250)363-6616; fax: (250)363-6310; e-mail:
[email protected]. † Magnus Eek has recently moved to the California Institute of Technology 10.1021/es040535i CCC: $30.25 Published on Web 05/05/2005
2005 American Chemical Society
and local sources. Local sources include a chlor-alkali plant, pulp mills, mine tailings, shipyards, and municipal outfalls (9-12). The most notorious discharge of contaminant Hg was from a chlor-alkali plant at the head of Howe Sound, which released as much as 20 kg/d of Hg from 1965 to 1970 (13). Being highly particle-active, mercury tends to accumulate in sediments. There has been some disagreement in the literature over the mobility of Hg in sediments (e.g., 14, 7). Lockhart et al. (15) reconciled the arguments for and against mercury’s mobility by calculating that in most lake and coastal marine sediments the upward, redox-driven diffusion of Hg was much slower than the downward flux because of sedimentation, while in sediments that accumulated very slowly (e.g., Arctic basin sedimentation rate ∼1 mm/100 years; 16), upward diffusion was probably significant. The average sedimentation rate in the Strait of Georgia is about 1 cm/ year (17), high enough to swamp redox-driven diffusion. The 1965-1970 chlor-alkali plant contamination episode has previously been identified in sediments from the Strait of Georgia (13, 11). However, the 17 sediment cores analyzed in detail here reveal a much longer history of Hg contamination, dating back to the placer gold mining of the 1860s and including a peak that coincided with World War II and another whose source remains unknown. Here, we describe the history of local Hg contamination using mixing models to interpret sediment core records and present a preliminary budget for modern Hg fluxes in the Strait of Georgia and adjacent inlets.
Methods Sample Collection and Dating. Sediment box cores were collected in the Strait of Georgia, Vancouver Harbor, and Howe Sound from 1990 to 2001 (Figure 1, Table 1). The cores were sectioned aboard ship into 1-cm intervals for the top 10 cm, 2-cm intervals for the next 10 cm, and 5-cm intervals for the remainder of the core. The outermost 1 cm of the core was discarded to avoid contamination of the samples by smearing during the core collection. Each sample was homogenized in a 500-mL glass jar with a stainless steel spatula and then further subsampled into smaller containers dedicated for different analyses. 210Pb and 226Ra were measured in the subsamples by Flett Research Ltd. (Winnipeg, Canada; counting errors
