MERCURY CYCLING IN LAKE GORDON AND LAKE PEDDER, TASMANIA (AUSTRALIA). II: CATCHMENT PROCESSES KARL C. BOWLES1,2 , SIMON C. APTE1∗ , WILLIAM A. MAHER2 and DAVID R. BLÜHDORN3 1 Centre for Advanced Analytical Chemistry, CSIRO Energy Technology, PMB 7, Bangor NSW, Australia; 2 Ecochemistry Laboratory, Applied Ecology Research Group, CRC for Freshwater Ecology, University of Canberra, University Drive, Bruce ACT, Australia; 3 Inland Fisheries
Commission, Lampton Ave., Hobart, Tasmania, Australia (currently at Hydro Tasmania, Elizabeth St., Hobart, Tasmania, Australia) (∗ author for correspondence, e-mail:
[email protected], Fax: +61 2 9710 6837)
(Received 26 September 2002; accepted 18 February 2003)
Abstract. The sources and concentrations of total mercury (total Hg) and methylmercury (MeHg) in the upper catchment of the Lake Gordon/Lake Pedder system in Tasmania, Australia were investigated. The catchment area, which contains over 50% wetlands, is located in a temperate region with no obvious point sources of mercury. Surface waters in the region had concentrations of total Hg ranging from 1.2 to 14.4 ng L−1 and MeHg from 1000 m). The Upper Gordon River is the most substantial drainage system in the catchment. Precipitation is very high, measuring between 2–3 m per year (80–120 inches), mainly as rainfall (Nunez, 1978). Rainfall is higher from autumn to spring but still significant throughout
MERCURY IN LAKE GORDON: CATCHMENT PROCESSES
27
TABLE I Vegetative composition of the Lake Gordon and Lake Pedder catchments. Areas (%) were calculated from the Tasmania 1:500 000 Vegetation Map, Forestry Commission Tasmania, 1984 Dominant vegetation
Lake Pedder catchment
Lake Gordon catchment
62.3% 5.1% 16.6% 12.1% 3.9%
53.0% 2.6% 38.0% 4.7% 1.7%
Button-grass plains Wet scrub Eucalyptus forest Rainforest Alpine complex
summer. The weather is unpredictable and snow may fall on the higher slopes in the catchment at any time of year (Volframs, 1978). The average daily temperature ranges are ∼10–20 ◦ C in summer and ∼3–9 ◦ C in winter (Nunez, 1978). The rocks in this region comprise mainly inert Precambrian metamorphic quartzite and schists. Areas of limestone, sandstone, conglomerates and serpentinite also exist in the Lake Gordon and Lake Pedder catchments (Boulter, 1978). Bare rock outcrops are common in the upland areas and leaching of these exposed areas may be a source of minerals to the wetlands and lakes. Osmiridium and chromium deposits have been located in the Adamsfield area to the east of Lake Gordon (Volframs, 1978). Otherwise there are few mineral deposits of significance in the Gordon/Pedder catchment. Restricted drainage in association with the existence of button grass (Gymnoschoenus sphaerocephalus), results in accumulation of organic matter to form acidic peats (Nicolls and Dimmock, 1965) and tea-coloured surface waters. The major vegetation zones in the catchments (Table I) range from temperate rainforest and wet sclerophyll forest, to extensive button grass dominated floodplains, with various scrub regimes between these extremes. The plains are characterised by frequent flooding and wild fires. Open sclerophyll forest occurs typically in raised areas with relatively good drainage and next to creek beds in the button grass plains (Edwards, 1978). Sedges and heath species share the floodplains with button grass. Both catchments are similar in terms of geology and biology although the Lake Gordon catchment contains a greater percentage of forested catchment and less button grass plains. Both contain greater than 50% by area of wetland (button grass plains and wet scrub). 2.2. S URVEY DESCRIPTIONS In June 1996 (early winter) sediment/soil cores and water samples were collected at locations in the Lake Pedder catchment (Figure 1). During February 1997 (late
28
K. C. BOWLES ET AL.
Figure 1. Approximate locations of sampling sites in the Lake Gordon and Lake Pedder catchment areas. Closed circles = rivers; open circles = creeks and run-off; triangles = bogs and swamps; crosses = button grass plain.
summer) samples were collected from both the Lake Gordon and Lake Pedder catchments. In addition, a bog in the nearby Florentine River catchment was also sampled (PC6). The Florentine catchment contains areas of similar geology and biology to the Gordon/Pedder catchments. 2.3. S AMPLING METHODS One litre and 500 mL Teflon FEP bottles (Nalgene, U.S.A.) were used for sampling and storage of water samples for mercury analysis. These were cleaned before use by sequential leaching with 2% detergent (Extran), 10% AR grade nitric acid (BDH, Australia), 50% AR grade nitric acid and 10% ultra-pure grade nitric acid. Bottles were thoroughly rinsed in mercury free water (Milli-Q, Millipore, Australia) after each leaching stage. Water samples were collected using a ‘clean hands, dirty hands’ protocol. River samples were collected whilst wearing plastic waders or by leaning off the riverbank. Samples for dissolved organic carbon (DOC) ana-
MERCURY IN LAKE GORDON: CATCHMENT PROCESSES
29
lysis were stored in 30 mL amber glass vials with an aluminium foil liner under the plastic cap. Samples for the analysis of dissolved mercury species were prepared by filtering the water through pre-ashed glass fibre filters (Whatman GF/F, UK), as soon as possible after sampling (
