The leachates generated by mining wastes have been widely studied, regulated and identified as the main environmental concern for both groundwater and surface water pollution (e.g. Ochieng et al., 2010). However, only acid leachates are considered environmentally harmful, while neutral and alkaline mine drainage (NAMD) is mostly conceived as harmless effluent. Recent studies are showcasing that alkaline leachates can be toxic to humans and the environment (Gomes et al., 2016). But nowadays, there is poor regulation and international consensus about how to proceed when mine wastes produce NAMD.
In order to demonstrate the limitations of current regulations and protocols, the present study compares the results of different leaching tests performed to waste rock dump (WRD) and tailing samples from Phalaborwa Industrial Complex (South Africa), with the characteristics of the groundwater underneath of them.
14 sample were analysed by acid rock drainage classification tests (ARD-CT) such as paste pH-EC, Static Net Acid Generation (NAG), Acid Base Accounting (ABA) and textural Acid Rock Drainage Index (ARDI). Despite some discrepancies with a feldspathic pyroxenite sample (according to NAG, it would fall under the classification of Potential Acid Forming-Low Capacity), the rest of the samples were classified as non-acid producing or even acid consuming. Consequently, no further analysis was required for them, according to protocols (e.g. Usher et al., 2003).
Complementary tests were conducted to assess their potential toxicity, bioavailability and mobility of trace metals. The results from the Toxic Characteristic Leaching Procedure (TCLP) showed that all the samples are bellow toxicity thresholds (US EPA, 1998). Whereas Leachable Concentration Tests (LCT) and sequential extraction (bioavailable (F1) + reducible (F2)) exceeded the South African limits for inert waste classification on all the tailing samples and 60% of the WRD samples. In addition, when LCT was correlated with TCT, as described in GNR 635 (2013), all the samples were classified as waste type 3 non-harmful. In contrast, the analysis of groundwater samples showed TDS up to 8200 mg/L (2697 mg/L, on average); sulphate up to 6040 mg/L (1267 mg/L, on average); and Cl up to 1465 mg/L (329.8 mg/L, on average). In fact, these results correspond with high salts concentration (Ca, Na, Mg, sulphate, fluoride and chlorides) detected during the analyses of the leachates. Due to the high salinity, the groundwater is not suitable for farm stock, irrigation nor drinking water (SANS 241:2011).
While specific mine waste characterizations (ARD-CT) are based on leachates’ acidity, thresholds for waste landfill classification are focused on toxic trace metals (more likely to be released under acidic conditions). There are no tests nor thresholds established that targets saline leachates prediction and its adverse impact on groundwater.