Research articleTreated acid mine drainage and stream recovery: Downstream impacts on benthic macroinvertebrate communities in relation to multispecies toxicity bioassays
Introduction
Globally, land use activities and anthropogenic pollution associated with mining, agriculture and industry, have resulted in multiple pressures on freshwater ecosystems, with severe loss of biodiversity and ecological functioning. Remediation efforts to minimize the effects of AMD on stream ecosystems are occurring worldwide (Gunn et al., 2010), and has become a rather large and profitable business (Bernhardt et al., 2005). Monitoring the success of remedial actions usually involves both chemical and biological sampling (DeNicola and Stapleton, 2014), with responses of in-stream communities superior to that of chemical measurements (Adams et al., 2002; Gunn et al., 2010; Kruse et al., 2013). The reduction in diversity and abundance of macroinvertebrates by acid mine drainage (AMD) is well established and commonly used as ecological indicators (Gray, 1998; Chambers and Messinger, 2001; He et al., 2015). To date, only a few studies have examined the impacts of pH-treated AMD on macroinvertebrates, with mixed results. DeNicola and Stapleton (2002) observed reduced macroinvertebrate density as a result of AMD exposure and subsequent increase after AMD treatment system installation. Nevertheless, the increased macroinvertebrate densities observed after treatment were not comparable to controls at most sampling sites and taxa richness remained low (DeNicola and Stapleton, 2014; DeNicola and Stapleton, 2016; Gunn et al., 2010). In contrast, Perrin et al. (1992) reported no effect of treated AMD on macroinvertebrate numbers or number of taxa. In particular, treated AMD will tend to be diluted as it moves farther downstream, gradually alleviating toxic effects on biota (Oberholster et al., 2013). According to Covich et al. (1999) macroinvertebrates appeared to be more sensitive to treated AMD shown by their decline in diversity. This observation leads to dramatic changes in understanding organic matter processing and nutrient cycling due to the large occurrence of primarily predators. Many of the discrepancies between reported studies can be attributed to varying levels of AMD concentrations and the type of AMD treatment.
The ultimate goal when treating AMD is to improve the ecological health of a water body (Kruse et al., 2013). Traditionally, alkaline addition treatment is designed to increase the AMD to pH > 6.5 and to maintain net alkaline conditions in the stream. Yet, several studies (Cravotta and Bilger, 2001; Keener and Sharpe, 2005; McClurg et al., 2007) showed that neutral pH and net-alkaline conditions are not always successful in achieving biological recovery. Potential biological recovery and treatment success downstream are poorly understood (Gunn et al., 2010; Kruse et al., 2013; He et al., 2015). The addition of toxicity tests to evaluate stream water quality of streams affected by treated AMD will assist in assessing biological recovery of these waters. According to Gerhardt et al. (2004), this is important because rapid bioassessment methods based on macroinvertebrates represent an overall summation parameter integrating several effects on aquatic biota, such as toxic effects, habitat degradation and physical disturbance, while rapid toxicity tests can add value in the assessment and ranking of stream sites. A single bioassay is unlikely to be responsive to all possible toxicants (Toussaint et al., 1995). A multi-trophic battery of bioassays for the evaluation of complex environmental samples and toxic mixtures has been widely recommended as superior to a single bioassay (Clarke and Barrick, 1990; Rojickova-Padrtovz et al., 1998; Baran and Tarnawski, 2015). Repetto et al. (2001) as well as Kokkali and Van Delft (2014) proposed a battery of assays with a great variety of endpoints (e.g., bacterium, invertebrate, plant, and algae) as it improves the sensitivity to a variety of environmental stressors. The plant bioassays (A. cepa, and L. sativa) are fast and simple methods to assess the phytotoxicity of substances or matrices of environmental concern based on inhibition of root growth and seed germination, respectively (Roccotiello et al., 2011; Silveira et al., 2017). Geremias et al. (2012) successfully used A. cepa as bio-indicator to test the efficacy of treating acid mine drainage resulting from coal mining wastes. Similarly, the D. magna bioassay is highly sensitive to environmental changes (Fischer et al., 2011; Wojtal-Frankiewicz, 2012), especially metal toxicity (Poynton et al., 2007; Okamoto et al., 2015) and acidification resulting from anthropogenic pollution and global climate change (Locke, 1992; Locke and Sprules, 2000).
Besides concerns for the natural environment, the possible harmful effects of AMD on humans has been raised (UNEP, 2010; Steyn and Genthe, 2011; IHRC, 2016). The Ames test, making use of Salmonella typhimurium, is a biological assay to assess the mutagenic potential of chemical compounds in the DNA of the test organism, and by extension pose a risk of cancer in humans (Mortelmans and Zeiger, 2000). The liver is one of the main detoxifying organs of the human body. The human and/or rat primary hepatocytes or permanent human liver HepG2 or HepaRG cell lines are therefore commonly used in toxicity and clinical drug screening (Schoonen et al., 2011). For the current study, an in vitro bioassay making use of the human liver cancer cell line (HepG2) in addition to the aforementioned assays were used to examine the ability of this battery of tests to assess the downstream impacts of pH-treated AMD in relation to in-stream macroinvertebrates.
Around the world, ecotoxicology are increasingly used to assess impacts of mining waste or remedial activities on aquatic ecosystems or human health either through multi-species toxicity testing in the laboratory, or observing biological effects in situ. Short and long term studies to assess the success of such mitigation and remediation efforts are however limited. The objective of the current study was to use the Tweelopie Spruit1 as case study to:
- (a)
determine the effects of pH-treated AMD and examine possible recovery of macroinvertebrate families' distribution over longitudinal distances; and
- (b)
in relation to (a) above, expose a battery of static bioassays to assess toxicity of the treated AMD impacted stream relative to multiple stressors.
Section snippets
Study area and site selection
The study area (Fig. 1) is located close to the town of Krugersdorp, west of the city of Johannesburg in the Gauteng Province of South Africa in the gold mining area of the Witwatersrand (also referred to as the Western Basin). Besides mining, land use practices in the area are predominantly agriculture (e.g., farms as well as agricultural small holdings) surrounded by peri-urban and urban land. The study area has a complex geology. The Witwatersrand Supergroup, overlain by the Ventersdorp
Physical and chemical water quality
The mean physical and chemical water quality (2012–2013) data is summarised in Table 2 (Supplementary Figure S1 shows the mean chemical composition of the water samples from the different study sites in a Piper diagram). Sites 1–3 represent water directly impacted by AMD. As described by Akcil and Koldas (2006), the alkaline treatment (neutralisation) process does not affect sulphate levels; evident from the high sulphate (SO42−) concentrations (2721 mg l−1) still present at S1 to S3 (Table 2;
Conclusion
While the Tweelopie stream was still severely impacted for kilometres following active alkaline treatment, the treatment was effective in reducing the physical-chemical water quality of the stream. However, there seems to be a delay in biological recovery of the system directly downstream. Providing conclusive results on impacts associated solely from alkaline treated AMD was unfortunately not possible. The Tweelopie/Bloubank stream was moderately to severely degraded by multiple anthropogenic
Competing interests
The authors declare that we have no competing financial, professional or personal interests that might have influenced the performance or presentation of the work described in this manuscript.
Acknowledgements
Our gratitude is extended to the CSIR SRP AMD project team who assisted with sampling (2011–2013), with special thanks to Mr Phil Hobbs for his invaluable input and assistance with sampling site identification. Credit also to the CSIR team who took the photographs.
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