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Antimony in stream waters, Queen Charlotte Walkway, Marlborough Sounds
The Marlborough Sounds are a complex set of marine waterways formed as rising sea level drowned river valleys. The Queen Charlotte Walkway is a popular walk around the margins of Queen Charlotte Sound. The walkway passes Endeavour Inlet, one of Captain Cook’s favourite anchorages. Endeavour Inlet was also the site of some historic mining for antimony. The Endeavour Inlet area is steep and rugged, with little soil cover on bedrock hillsides. The area is naturally coated with forest, and this forest has largely regenerated after extensive deforestation during mining. The mines were developed in a vein system that occurs in a narrow (200-500 m wide) zone that trends northwest from Endeavour Inlet. This mineralised zone is traceable from valley to ridge crest, with a vertical extent of 700 m. Veins occur with c. 100 m spacing, and individual veins are traceable for tens of metres laterally and vertically. Veins are typically c. 1 m thick, but pinch and swell along strike and down dip. The veins are dominated by quartz and quartz-cemented rock breccias, and have up to 50% stibnite, Sb2S3, which was the principal target for the mining. Arsenopyrite (FeAsS) is a common accessory mineral, with minor pyrite. Gold is present but was at subeconomic levels. Mining for stibnite began in the Endeavour Inlet area in 1874, and continued for c. 30 years. Smelting problems and low antimony prices ensured that the operation was not profitable, and the site was abandoned by 1907. Almost all mining was conducted from underground tunnels (adits) cut into the steep hillsides. The adits are up to 200 m long and typically <2 m wide and <2 m high. Two of these adits collect shallow groundwater which flows out the adit entrances and into a nearby stream. There are significant dissolved metalloid (As and Sb) concentrations in these adit discharge waters.
Natural groundwater entering the adit at the top of the mineralised catchment had 190 mg/L Sb and 10 mg/L As. The amount of arsenic increased along the adit as the water interacted with arsenopyrite-bearing rocks and debris (up to 2000 mg/kg As, 500 mg/kg Sb) on the adit floor. Sb(III) was below 14 mg/L, and there was no detectable As(III). Antimony content remained near constant in the adit but increased outside the adit because of interaction with stibnite-rich debris. Dilution of these waters occurred in a nearby natural stream, which carried <30 mg/second Sb and <10 mg/second As away from the site. An adit 500 m downstream was developed in a lower, more arsenopyrite-rich portion of the mineralised system with debris containing up to 15000 mg/kg As and 5000 mg/kg Sb. Water from this adit had up to 200 mg/L Sb and 1650 mg/L As. Dilution and adsorption decreased dissolved metalloid contents.
River water near to the Queen Charlotte Walkway contains low metalloid concentrations, even near an old antimony smelter site. Sb concentrations were about 25 mg/L, while As concentrations fluctuated around 8 mg /L. Arsenic and antimony concentrations in the river running alongside the smelter site exhibited no significant changes from upstream to downstream of the smelter site. Levels of both metalloids were elevated above recommended trigger values for maintenance of ecosystem health: Sb trigger value is 9 m/L, and As trigger value is 1 mg/L. The dissolved As is lower than recommended drinking water limit (10 mg/L). However, Sb is significantly higher than recommended drinking water limit (3 mg/L).
Water metalloid concentrations and flow rates can be used to provide estimates of metalloid flux through the natural stream that drains into Endeavour Inlet, as indicated in the diagram below. The top adit contributes <30 mg/second Sb and <10 mg/second As, and the lower adit discharges have negligible effects on metalloid fluxes. Total metalloid flux from the catchment, measured near the Queen Charlotte Walkway, is about 14000 mg/second Sb and 5000 mg/second As, which is about three orders or magnitude greater than observed mine discharges to the catchment. Highest flux occurred in September as water tables rose in the winter.
Nearly all the metalloid flux is derived by natural groundwater and surface water interaction with mineralised rock. This interaction between water and mineralised rock is enhanced in this area because the catchment runs subparallel to the mineralised zone which controls the Sb-bearing veins. Surface and ground water in contact with Sb and As rich portions of this mineralised zone readily dissolve these metalloids. Groundwater entering the top adit near the head of the catchment already had metalloid concentrations higher than the river at the bottom of the catchment. Likewise, surface water passing the lower adit already had antimony concentrations higher than discharges from that adit. Most of the dissolved Sb and As reaching the Queen Charlotte Walkway is apparently dissolved from the rock in the lower part of the valley. Further information
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