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Natural metalliferous springs, Coromandel Peninsula
Rocks that are enriched in gold are also commonly enriched in other metals. On the Coromandel Peninsula, copper, lead, zinc and cadmium (Cu, Pb, Zn, and Cd) are particularly common. Gold mines developed in this area potentially have elevated Cu, Pb, Zn, and Cd in their discharge waters. However, these metal enrichments are natural, and the metal-enriched rocks become exposed to rainwater at and near the surface by natural erosion processes, without involvement of mining. Hence, waters discharging from metal-rich rocks commonly contain elevated metal concentrations. This naturally high metal background has to be taken into account when using the water for domestic or agricultural purposes, or for planning mine site environmental monitoring. One of the problems in determining the naturally elevated metal background in streams in undeveloped areas is access to roadless terrain for sampling, and the associated issues of getting the samples back to the laboratory in a timely manner. An alternative approach is to do the analytical work on the site in a remote area. This requires portable equipment, most of which does not operate with low detection limits. In this study, we developed portable anodic-stripping voltammetry equipment and associated support equipment so that it would all fit into the back-packs of a small survey team.
The square wave voltammograms were recorded using a PalmSens Electrochemical Sensor Interface (Palm instruments BV, The Netherlands) in combination with a Compaq iPAQ Pocket PC running PalmScan v1.30. Data was stored on the iPAQ and downloaded when back in the base camp. The batteries supplied with the PalmSens (two AA nickel hydride 1.2V/1750mAh) were sufficient to power the potentiostat for a full day of analytical work. An additional battery extension pack was used to supply enough power for the iPAQ. The PalmSens is waterproof and the iPAQ was made waterproof by using a rugged case (standard accessory). The weight of the voltammetric field instrumentation is below 1 kg. Including chemicals and labware (metal standards, MilliQ, acids, electrodes, cell and micro-pipettes) no more than 4-5 kg have to be carried into the field to cover a whole day session.
The chosen study area is in the Waioumu area of the Coromandel Peninsula. The area has a long history of small scale underground mining, and mineralised rocks are widespread. A natural acid spring was intersected by the road to the Monowai mine, c. 30 m above Paroquet Stream. Spring waters flowed over the road and built up a crust of brown hydrated iron oxide. When the road fell into disuse, this deposit built up to form an apron up to 10 m across, cascading down the hillside in the regenerating forest. Severe rainfall events occurred in the Waiomu area in June 2002, resulting in extensive flooding in all streams, and numerous landslides on valley sides.
A landslide developed across the abandoned road at the natural spring site, removing part of the apron of hydrated iron oxide and the underlying surface deposits (up to 6 m thick) on which the road was built. The landslide margins are defined by a steep headwall, steep to near-vertical sides, and a toe region of hummocky rubble.The landslide disrupted the natural spring. Field work for our study was conducted soon after the landslide formed. At this time, spring waters were initiating new brown hydrated iron oxide crusts on the walls and floor of the landslide so that discharge points were visible in the landslide area.
The natural springs contain significant levels of Cu, Pb, Zn and Cd in acid waters. Of particular note was the very wide range of compositions of the waters in close proximity in the small landslide area. The relative amounts of metals in these waters are related to the metal contents of the rocks immediately beneath the surface at the discharge site, and large variations can occur on the metre scale. The analyses show that naturally elevated metal concentrations occur in waters discharging from the mineralised rocks. The natural springs examined in this study contribute only trivial proportions of the total flux of Cu, Pb and Cd discharging in surface waters from the catchment. However, the springs could contribute up to 10% of the zinc flux from the area.
Our study has shown that field analysis of natural waters for labile Cu, Pb, Zn, and Cd is possible down to the µg/L level using anodic stripping voltammetry. This field technique is truly portable enabling analysis at the remotest site. Immediate on-site analysis avoids many of the sample preservation issues that arise when transporting samples to a laboratory. The technique allows more systematic examination of a site, as results are determined before sampling has finished, and additional key samples can be obtained. In this study, we identified four different water compositions within metres of each other by repeated targeted sampling at the site. This targeted sampling is not possible when samples are taken back to a laboratory, rather than analysed in the field. More information
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