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Water chemistry of Blue Lake, Central Otago
Blue Lake is an abandoned alluvial gold mine located at the town of St. Bathans, in Central Otago. The mine was worked sporadically up to the 1940s, and was then abandoned and subsequently filled with water. The resulting pit lake is now considered a local amenity for swimming and boating, in an area with low population and little human development.
Blue Lake was excavated at the west-sloping boundary between basement rocks and younger sediments. The basement rocks consist of greywacke and argillite similar to that which occurs throughout Canterbury to the north. These rocks are generally hard and resistant to erosion, although the argillite breaks into small fragments in cliffs. Near the gold-bearing boundary, the basement rocks have been extensively altered by groundwater to form soft clay.
The greywacke basement rocks are overlain by 10-20 million year old river and lake sediments, including some lignite. At Blue Lake, these sediments are mainly white conglomerate consisting of quartz pebbles. The quartz conglomerate contains alluvial gold, particularly near the base of the formation where it rests on the altered basement rocks. Hence, gold mining at Blue Lake was focussed on this portion of the formation, and the miners followed the dipping strata downwards to the west, exposing the basement alteration surface in the process. Mining eventually became uneconomic as the gold-bearing stratum was followed downwards, and when it ceased, the mine filled with water to form Blue Lake.
The oxygen and hydrogen isotope ratios of water in nearby rivers are typical of precipitation of the area. They lie close to the meteoric (rain) water values for Central Otago and the relatively light values (d18O near -11‰) reflects the rain shadow setting for this precipitation. In contrast, Blue Lake water has distinctly heavier d18O and d2H ratios (near -7‰ and –55‰, respectively). The 5‰ rise in d18O requires at least two-fold evaporative concentration of lake waters. There has also been a 3-5 fold evaporative enrichment of the major cations: Na+, K+, Mg2+, and Ca2+ compared to river waters.
Localised acidification in surface and ground waters occurs during oxidation of pyrite. This acidification causes metals to dissolve from the surrounding rocks. Most metals are mobilised by oxidation of pyrite in basement argillite. This results in the lake waters being enriched in dissolved Zn compared to nearby stream waters which are dominated by dissolved Cu. Iron and Zn may be partly removed from solution by adsorption on to settling particulate iron oxyhydroxides, but lake bottom sediments show only minor evidence of such metal enrichment.
The major processes affecting the chemical composition of the major ions and trace metals of Blue Lake are summarised in the diagram below. These are all natural processes, which occur within the lake catchment itself. Evaporative concentration and input of trace metals by surface and ground waters due to geochemical weathering of sulfide minerals are the main processes. In general, metal enrichment in lake waters and bottom sediments will probably continue to occur into the future. While the observed dissolved trace metal concentrations for Zn and Cu in Blue Lake water (50 and 20 nmol/L, respectively) are near the threshold for incipient toxicity of aqueous ecosystems, level of dissolved Ni (44 nmol/L) is significantly lower and is not a potential issue. The bioavailability of the dissolved Zn, Ni and Cu in Blue Lake is at present unknown.
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