Mercury content of soils in the vicinity of A past-producing mercury mine, central British Columbia, Canada

A. Plouffe, G.E.M. Hall, and P. Pelchat (Geological Survey of Canada, Ottawa, Ontario, Canada K1A 0E8; corresponding author: aplouffe@nrcan.gc.ca)

ABSTRACT

Humus, B-horizon, and C-horizon soil samples collected in a region of past mercury mining activity in central British Columbia are used to differentiate between naturally elevated mercury concentrations in soils and sediments from mercury derived from anthropogenic loading (mining and ore processing). Near the Pinchi Lake mercury mine, there is a net mercury enrichment in humus compared to the underlying sediments which is thought to be partly related to anthropogenic loading. However, part of this mercury enrichment in humus could be natural because enrichment was also observed at sites removed from the influence of the mine. Preliminary speciation results suggest that the anthropogenic signal is present in both the labile and non-labile phases of the humus portion of soils.

INTRODUCTION

The Pinchi fault region located in central British Columbia, Canada (Fig. 1) is known to be naturally enriched in mercury because of the abundance of cinnabar (mercury sulphide-HgS) mineralization in bedrock (Armstrong, 1966). The mercury mineralization is reflected in glacial sediments along the fault because advancing glaciers eroded mineralized bedrock and transported the debris along their path (Plouffe, 1995; 1998). Two mercury mines were active in the region in the 1940's and one of them (Pinchi Lake mine) reopened from 1968 to 1975.

The objective of this paper is to report on current research focussed on differentiating the natural mercury enrichment (high mercury concentrations in bedrock and in mineral sediment transported by glaciers) from the anthropogenic mercury loading (atmospheric transport related to the mining activity). Humus, B-horizon and C-horizon soil samples were collected along sampling transects in the region (Fig. 1). Only the results of the Pinchi Lake mine locality are discussed here. This project is being conducted as part of the Geological Survey of Canada (GSC) Metals in the Environment (MITE) initiative.

METHODS

Samples were collected in hand-dug pits located ca. 100 metres from nearest forestry roads to avoid dust contamination derived from road beds. Sampling sites at 52 and 87 km to the northwest of Pinchi Lake mine are located in a region with high mercury concentrations in glacial sediments (Plouffe, 1995). These two sites are used as reference sites because they are thought to lie beyond the area of contamination related to the mining activity. Humus, B- and C-horizon samples were collected at every site. Humus samples consist of the dark organic rich layer developed at the top of soil profiles. Humus thickness averages 5 to 10 cm in the region. The B-horizon samples were collected at an average depth of 15 cm in the reddish brown oxidized sediments that underlie the humus. C-horizon samples were collected at a minimum depth of 1 m in relatively unweathered glacial sediments. All samples were sieved to less than 2 mm prior to geochemical analyses.

Figure 1. Sample site locations within the study area. Circles are centered on Pinchi Lake mine and indicate distance from the mine site. Inset: study area location in British Columbia (BC). Wind direction data were obtained at Fort St. James, 22 km south of Pinchi Lake mine (Environment Canada Climate Data Services; data from 1979 to 1987). Arrow thickness proportional to percent frequency of occurrences.

Two leaches were applied in sequence to differentiate metals held in labile from non-labile phases in humus and mineral soil (B- and C-horizons) samples. They included a pyrophosphate leach (0.1 M Na₄P₂O₇) (labile phase) followed by an aqua-regia leach (HCl-HNO₃) (non-labile phase) for the humus samples and a hot hydroxylamine leach (0.25 M NH₂OH.HCl in 0.25M HCl) (labile phase) followed by aqua-regia leach (non-labile phase) for the B- and C-horizon samples. Total mercury concentrations as reported here refer to the addition of both mercury determinations following the two leaches. Mercury analyses were done by cold-vapor atomic absorption (CV-AA) and inductively coupled plasma mass spectrometry (ICP-MS) in a commercial laboratory and at the Analytical Chemistry Laboratory of the GSC. Analytical precision is ± 19% (calculated following Garrett's (1969) method) and the analytical error is significantly smaller than the overall data variability.

RESULTS AND DISCUSSION

At increasing distance from Pinchi Lake mine, there is a net decrease in total mercury concentrations in humus (Fig. 2). Furthermore, at 52 and 87 km, where there are large mercury concentrations in the C-horizon (290 and 140 ppb, respectively) there is no noticeable enrichment in the humus. Based on these observations, the high mercury concentrations in humus near the mine are thought to be related in part to anthropogenic loading resulting from past mining activities. The strong relationship between mercury concentrations and distance from the mine observed in humus is not apparent in the B-horizon. This indicate that mercury has not significantly migrated down to the B-horizon from the enriched humus.

Figure 2. Total mercury concentrations in humus, B-horizon, and C-horizon versus distance from Pinchi Lake mine. Sample locations are shown on Figure 1.

Figure 3. The ratio total Hg humus / total Hg C-horizon versus distance from Pinchi Lake mine. Sample locations are shown on Figure 1.

Some of the high mercury levels in humus near Pinchi Lake mine could be due to the high concentrations in the underlying sediments (C-horizon). Mercury in soils is taken up by plants and is returned to the soil surface following plant decay (Warren et al., 1966; 1983; Andersson, 1979). The relationship between mercury concentrations in humus and the underlying sediments is illustrated by an enrichment factor (total Hg _humus / total Hg _C-horizon) calculated for every site and plotted against the distance from the Pinchi Lake mineralization (Fig. 3). The enrichment factor constantly decreases with increasing distance from the mine (up to 20-40 km) with one exception at 73 km. The high ratio observed at 73 km from the mine is related to the low mercury concentration in the C-horizon (Fig. 2). Nevertheless, it suggests that enrichment can occur in the humus even on sediments with low mercury content . Therefore, some of the enrichment in humus near the mine could be natural. Mercury is strongly held to organic matter and can be cycled from the mineral soil by plants as indicated above. In addition, mercury gas that escapes along faults could be absorbed onto organic matter (Jonasson 1970; Rasmussen 1993). Both of these processes could contribute to the strong enrichment observed in humus in the Pinchi fault region.

Figure 4. Mercury concentrations in humus determined following a pyrophosphate leach versus distance from Pinchi Lake mine. Sample locations are shown on Figure 1.

Preliminary results of the sequential leach analyses suggest that the majority of mercury in mineral soils and humus is held in non-labile phases. This supports conclusions reached by Plouffe (1997) who indicated that mercury in the glacial sediments is dominantly present as cinnabar which is considered a non-labile mineral. Nevertheless, part of the anthropogenic signal near Pinchi Lake mine seems to be present in a labile phase, because there is a relationship between the mercury concentrations in humus determined following a pyrophosphate leach and distance from the mine (Fig. 4). However, the data set is incomplete and more analyses are required to support this hypothesis.

There is an enrichment of mercury in humus compared to the underlying sediments near the past-producing Pinchi Lake mine which is thought to be related to a combination of anthropogenic loading derived from past mining activities and natural recycling of mercury in sediments by plants. Future research will concentrate on the characterization of the forms of mercury in humus notably with the use of the scanning electron microscope.

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