Sampling

The different sampling techniques used for waters, sediments, tailings and plants collection are presented below including their pre-treatment and storage conditions prior sample preparation steps. In situ measurements conditions are also discussed.

5.3.1 Water sampling

The water samples were collected according to commonly accepted sampling procedures (Tutu, 2004 and references therein; USEPA, 2007b; Quevauviller, 2001). Water samples were collected as duplicate samples at each site into acid-washed and conditioned polypropylene (PP) one litre bottles. The recommended “clean hands–dirty hands” procedure was used during sampling in order to discriminate the contamination risk (Montgomery et al., 1995).

Surface waters were collected in dams and wetlands “by hand” directly in the sampling bottle using nitrile gloves. The bottle had to be opened and closed under water to avoid mixing with the surface microlayer or oxidation of sample (Stoichev et al., 2006). In streams, samples were taken in the main stream flow away from the banks and a point sampler consisting of an aluminium rod and a PP cup was used to collect the sample. The sampling bottles were rinsed with the site water immediately before sampling and the rinsed water was discarded away from the sampling point. The goal of this procedure was to condition or equilibrate the sampling equipment to the sample environment and to help ensure that all cleaning-solution residues had been removed before sampling (Tutu, 2006; Stoichev et al., 2006).

The PP bottles were filled with water leaving no air space and the field parameters such as temperature (T), pH, electrical conductivity (Ec) and redox potential (Eh) were measured in situ before tightly closing the containers to prevent any leakage (figure 5.1). Each bottle was then marked with the date of sampling and sample description, placed into cooler boxes and transported to the laboratory. Global Positioning System (GPS) coordinates were also taken at each sampling point and were used for mapping with the

Geographic Information System (GIS) software (ArcGIS 9.x, USA). Quality control, or blank, samples were used to assess the level of contamination and to quantify any background concentrations.

Field parameters measurements were carried out with a portable kit (WTW multi- parameter instrument pH/Cond 340i and ORP, Germany) equipped with a pH electrode, an integrated temperature probe, a standard conductivity cell and an oxidation-reduction potential probe. The meters were calibrated and tested prior to sampling using standard buffer solutions according to the manufacturer’s instructions.

Redox potentials were obtained from Pt electrodes versus Ag/AgCl and all reported potentials were corrected relative to the standard hydrogen electrode (SHE).

Figure 5.1 In Situ measurements of physico-chemical parameters of a water sample

Once in the laboratory, each sample was divided into two parts: the one was filtered under vacuum with 0.45 µm filter papers (Millipore) and used for the anions (Cl-, F-, NO3-, NO2-, PO43- and SO42-) determination by ion chromatography; the other was unfiltered and acidified with 1% (v/v) HCl suprapure (37%, Sigma Aldrich) and then analysed for mercury and other metals using ICP techniques. The samples were transferred in PTFE (or Teflon®) bottles or in borosilicate bottles with PTFE-lined caps and stored at 4oC until analysis.

5.3.2 Sediment sampling

Sediment (surficial and bulk) samples were collected from adjacent to gold tailings facilities, and from dams, streams and wetlands. Background samples were also collected in order to determine the contrast between background mercury concentrations and contamination.

For stratigraphic (profile) studies, piston corer (auger) or Polyvinylchloride (PVC) cores of 50 cm height were used for sample collection. In a few cases, pits were excavated to a maximum depth of 3 m and samples were collected at 20 cm depth intervals after scrapping off the outer layers of the sediment profile. Geochemical (field) parameters of the sediments were also determined, mainly by inserting the appropriate probe into the slurry in situ. Where the slurry was not immediately available, it was made by mixing a portion of the sediment (about 50 g) to about 50 ml of deionised water and the measurements taken of the resulting slurry (Tutu, 2006). The GPS coordinates of the sampling points were also measured. Collected samples were then stored in double plastic bags in the dark within cold boxes.

In the laboratory, samples were first frozen at -18°C to increase the stability of MeHg+ (Parker and Bloom, 2005) and PVC cores were sliced into portions of 2 to 5 cm thickness using a saw equipped with a clean stainless steel blade. The sediments were sieved, when necessary, with Nylon sieves to eliminate stones and other gross particles. After homogenization, a representative portion of each sample was freeze-dried (Varekamp et al., 2000) in a lyophilizer (Labconco, USA) at -40°C for 24 to 48 hours and the dried samples were pulverised with acid-washed pestles and mortars and then stored at 4°C in acid-cleaned polystyrene bottles for further preparation (figure 5.2).

Figure 5.2 Sediment core sampling (1) and pre-conditioning steps which included: dissection of the PVC core (2), sample drying in a lyophilizer (3) and storage in acid-

washed polystyrene bottles (4) .

5.3.3 Tailings sampling

Tailings samples were collected both vertically and laterally. Near surface samples were collected by means of shovels (after scrapping off the oxidised layers) while the subsurface samples were collected by means of PVC cores (figure 5.3). The methods used depended largely on the accessibility of the sampling points. The samples were then treated in the same way as for sediments.

5.3.4 Plants collection

Samples of plant tissues (algaes, twigs and leaves) were collected randomly from wetlands, creeks, ponds or from plants growing on the tailings dumps and tailings footprints for phytoremediation purposes. To minimize the risk of contamination, plastic gloves were used to collect the samples (figure 5.4) and hand separation of the plant tissues from other material. Samples were rinsed with deionized water to remove eventual metals attached at the surface and were kept in polyethylene plastic bag, then frozen and dry-frozen at -40°C (Cabanero et al., 2002) within hours of collection. After being ground to a homogeneous powder with the help of liquid nitrogen (Heller and Weber, 1998), the samples were stored in the dark, to avoid photodegradation, cleaned polystyrene bottles (Yu and Yan, 2003).

Figure 5.4 Collection of algae in a creek