An integrated investigation of static, kinetic and water quality testing to predict acid mine drainage from a historical mining related activity into water resources: a case study of Wasbank, uThukela Water Management Area, KwaZulu-Natal, South Africa.

Abstract

Historical and current coal mining-related activities have negatively impacted both the surface and ground resources and the environment globally, causing water pollution, termed acid mine drainage (AMD). Decades of prior surface and underground coal and gold mining have contributed to acid mine drainage extensively. Elevated concentrations of heavy metals and low pH render water resources unsuitable for consumption across the water-use sectors. Currently, the environmental implication of AMD has severely impacted the water resources from an aquatic, domestic, agricultural and livestock perspective. Whilst this has been observed and documented internationally, minimum research attention is placed on small mining towns in South Africa on the prediction of mine drainage from abandoned coal mining sites and its impacts on water resources. Since South Africa being a water-stressed country, and the province of KwaZulu-Natal facing immense water security, quality and quantity issues, the impacts of mine drainage emanating from abandoned mining sites require complete attention to understand the level of environmental risk and impact caused on the water resources and to ensure comprehensive water resource management decisions for the holistic protection of aquatic ecosystem is made. This study employed the use of AMD prediction techniques to determine the impact of mine drainage effluent from an abandoned mine and its impacts on water resources. Static testing (acid-base accounting-ABA), kinetic leach column testing, geochemical, mineralogical analysis of the coal discard (waste rock) and water quality analysis formed part of the integrated investigative research. The study findings indicated ABA and net acid generation (NAG) tests classified the samples as potentially acid-forming (PAF). The samples of all five sampling points (S1-S5) on the abandoned coal discard dump were acidproducing with the following observed, the discarded material had significant sulphur content, positive net acid-producing potential (+NAPP) equivalent to an acid neutralising capacity (ANC)/ maximum potential acidity (MPA) less than 1, (ANC/MPA ratio < 4.5) and the ANC/MPA ratio < 1. Samples S1 to S5, yielded high organic matter and the total carbon percentage ranged from 40.21% to 60.84%. The combination of the NAG and NAPP tests proved to be a better reflection of the site acid mine potential. The kinetic test (leach column testing) was then followed to determine the leachate chemistry of the discard. The test duration was conducted for five samples (five columns) over a 22-week test duration of leachate collection and analyses were made for pH, EC, Na, K, Al, Ca, Cu, Fe, Hg, Ni, Pb, SO4 2- and Cl as well as geochemical analysis. The leachate results were compared to the Department of Water and Sanitation (DWS) Target Water Quality Range (TWQR). The mean pH is indicative of the general acid nature of the leachates. Na, K, Cu, Ni, Zn and Cl are largely within the acceptable mean concentration range, however the other analytes show great variability, often in concentrations that are toxic. The discard was further analysed by conducting ICP trace metal analysis, and X-Ray fluorescence (XRF) for major and trace metal analysis. The findings indicated very high amounts of Ba (453.17ppm), Sr (360.80ppm) and Zr (127.08ppm); moderately high amounts of V and Ce (30 – 99 ppm); moderate amounts (20 -30 ppm) of Cr, Cu, Y and La; and, low Sc, Co, Ni, Rb, Zn, Y, Nb, Mo, La, Pr, Nd, Sm, Eu, Ho, Gd, Tb, Dy, Er, Yb, Lu, Hf, Ta, Th, Pb and U. Evidently some trace elements in coal are radioactive, these included uranium (U) and thorium (Th) of the composite sample. Mineralogical analysis using X-Ray diffraction technique (XRD), identified the following mineral phases: quartz, kaolinite/dickite, illite/muscovite and gypsum with a crystallinity index of 58%. Water quality sampling of the two water resources (Biggargatspruit and Wasbank River) was conducted for a test duration of 28 months. The water quality compliance levels for fitness of use indicated compliance and non-compliance levels to the DWS TWQR, Six parameters, Fe (58%), К (55%), pH (54%), Ca (52%), SO42 (46%), Ni (46%) complied with the DWS TWQR, and 11 parameters were in exceedance of the DWs TWQR that ranged within the chronic and acute effect value range. These include Al (61%), TDS (59%) Cr 6+(55%), Cu (55%), Na (54%), Hg (45%) EC (45%), Zn (36%), Рb (27%), Cl (27%) and Cd (16%). The WQI further indicated the water quality at these sites is unacceptably unfit for domestic use (S2, S3 and S4) to very poor water (S1 and S5). In addition, to determine the inferences in the study from the results, multivariate analysis, the Kruskal-Wallis Test and a post-hoc Mann-Whitney U test were performed. The tests indicated a statistically significant difference across the sampling sites. The findings indicated the successful prediction of AMD from the abandoned coal discard dump, the impacts of the coal discard leachate quality on the water resources, mine waste characterisation, environmental pollution and recommendations for mine water management and resource protection.