Diatom communities associated with gold mine tailings in the North West province, South Africa

Abstract

There is a continuous increase in environmental pollution through the release of toxic elements and acid mine drainage (AMD) into the environment by anthropogenic activities. This study aimed to determine the effect of gold mine tailings on diatom community composition and abundance in field-based studies and assess the physical and chemical condition of the waterbodies related to mining activities. Diatoms were used as bioindicators for this study due to their fast response to change in their environment, as well as their wide distribution and important role in the food web. For this study, 19 sampling sites were selected in the Klerksdorp-Orkney-Stilfontein-Hartebeesfontein region, which included tailings storage facilities, drainage ditches, return water dams, stormwater outlets, the Africa Explosive Limited dam complex, and streams located upstream from the mining activities. Water quality variables such as phosphate, dissolved oxygen, pH, nitrate and electrical conductivity were measured on-site with portable meters. The cyanide concentrations of the sites were measured and provided by the mining company. Sediment samples were collected and analysed for elemental concentrations using inductively-coupled plasma mass-spectrometry (ICP-MS) after microwave digestion. The diatoms were sampled using standard sampling techniques and was identified and counted using differential interference contrast (DIC) light microscopy. Both the phosphate and nitrate concentrations indicated hypertrophic water conditions which are usually productive but not diverse systems. Often, gold mine effluent has very low pH levels, but the study sites were all weakly acidic to alkaline environments. The results also showed high electrical conductivity at the mining sites. The tailings storage facilities showed adequate removal of cyanide, removing >90% of the total amount of cyanide released by the gold extraction process, although concentrationsfound were still toxic to the biological systems in the water. The physico-chemical parameters in the water that may have had the greatest effect, individually and combined, on the biological systems in the water, included cyanide, dissolved oxygen (DO), NO3- and PO43- . The elements in the sediments which had the greatest toxic potential on the overall biological systems in the water were As, Cr, Cu, Fe, Hg, Mn, Na, Pb, Ni, Se, Sr, and U and Zn, individually, in combination with each other, and with the physico-chemical parameters identified above. Across all study sites 169 species in 49 genera were recorded, with 98 species tolerant to pollution and 71 species less tolerant. A total of 77% of the diatom species that were most commonly encountered were from the order Pennales, classified as Biraphidae. This is because this group is found generally in systems where there are steep physical and chemical gradients (extreme conditions). The results showed low diatom community diversity and abundances, with Craticula buderi, Nitzschia palea, and Achnanthidium sp. as the most commonly occurring species. The alkaline conditions in combination with the high phosphate and nitrate concentrations, made it a more favourable environment for pollution tolerant diatoms to grow. These conditions may explain the relatively high diatom abundances in some seasons at several of the sites. Deformities in diatom cells are morphological changes in the structure of the diatoms, including internal and external alterations, here, presumably caused mainly by high metal concentrations in the sediments but may also be due to other stressors. Most deformities were in the form of irregular striation patterns, abnormal valve outlines, and slightly bent valves. Although the high metal concentrations affected the morphology of the diatom cells, it also caused a change in the composition of diatom communities, with the more tolerant species found at impacted sites and low species diversity and richness. The elements that had the strongest association with deformities in cells were As, Cu, Hg, Ni, and Zn, although the cyanide concentrations might have had a slight effect on the diatom responses. This study showed that diatoms are suitable bio-indicators for the observation of metal pollution at mining sites, although direct causal associations could not be identified for individual agents. The effects seen is therefore likely attributable to a combination of stressors and toxic chemicals unique to affected sites as sites differed greatly. Further research is necessary to investigate some aspects such as smaller diatom size associated with toxic metal concentrations, and the occurrence of metal-sensitive diatoms such as Cyclotella meneghiniana and Cyclotella ocellata found at study sites with high metal concentrations. This study contributed various new questions and research opportunities regarding diatoms at mine-impacted areas.