Hydrogeochemistry and Isotopic Study of the Origin of Underground Mine Water at Golden Star Bogoso/Prestea Limited (New Century Mines)

Abstract

Hydrogeochemical study of the origin of underground mine water at Golden Star Bogoso/ Prestea Limited (New Century Mines) was carried out to determine the geochemical processes controlling surface water, groundwater and underground mine water and to identify the source of recharge to the underground water in the mines; investigate the hydrochemical facies of groundwater in the various aquifers (gallaries), and identify the relationship between surface water and groundwater, and underground mine water using hydrogeochemistry and stable isotope technique. Water from seventeen (17) surface water bodies, ten (10) hand-dug wells, eleven (11) boreholes and twenty-one (21) underground mine water samples were collected within Prestea in the Prestea-Huni Valley district of Western Region of Ghana between October 2013 and March 2014.The objectives of the study was achieved through the determination of pH, temperature, TDS, salinity, alkalinity, electrical conductivity, anions (SO42-, HCO3-, Cl-, PO43-, NO3-), major cations (Ca2+, Mg2+, Na+, K+) and trace elements (As, Cd, Cu, Fe, Mn, Pb and Zn). Arsenic,(As), was determined by Hydride Generation Atomic Absorption Spectrometry (HG-AAS). Levels of Cd, Cu, Fe, Mn, Mg, Pb and Zn were determined by Flame Atomic Absorption Spectrometry (FASS). The contents of Na+ and K+ were determined by Flame Photometry. Measurement of the levels of SO42-, PO43- and NO3- were achieved by UV-Visible spectrophotometry. Titrimetry was used for the determination of alkalinity, HCO3- and Cl-. Temperature, pH, Conductivity, Salinity and Total Dissolved Solids (TDS) of the surface water, groundwater and underground mine water were all assessed. The stable isotopes (δ2H and δ18O) compositions of the waters were determined using the Liquid-Water Isotope Analyzer [based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) via laser absorption]. The determined Levels of major cations (in ranges, meq/L) for the various waters were as follows: Surface Water: Ca [0.64-33.00], Mg [0.44-18.87], Na [0.12-5.59] and K [0.01-0.24]; Groundwater: Ca [1.01-5.39], Mg [0.76-2.32], Na [0.61-3.57] and K [0.11-3.57]; Underground mine water: Ca [5.91-43.00], Mg [3.61-14.50], Na [0.28-4.14] and K [0.83-0.96];.The determined Levels of major anions (in ranges, meq/L) for the various waters were as follows: Surface Water: SO4 [3.00-49.30], HCO3 [0.94-9.02], Cl [0.64-3.28], PO4 [0.01-3.06] and NO3 [0.01-0.17]; Groundwater: SO4 [2.15-5.96], HCO3 [0.28-2.38], Cl [0.69-3.89], PO4 [0.01-2.00] and NO3 [0.01-1.36]; Underground mine water SO4 [5.48-8.98], HCO3 [2.22-8.02], Cl [0.16-6.90], PO4 [0.10-3.23] and NO3 [0.08-0.43]. Strong correlation between HCO3- and SO42- demonstrate the formation of H2SO4 while groundwater and underground mine water traversing through pyrite bearing shale may dissolve heavy metals into groundwater. The variation of surface water, groundwater and underground mine water hydrogeochemistry revealed the dominance of Ca2+ > Mg2+ > Na+ > K+ as relative abundance of cation, and the dominance of relative anions SO42- > HCO3- > Cl- > PO43-> NO3-. The Piper trilinear diagram indicated water types from GSBPL (NCM) corresponded to the NaCl or Na - HCO3 – Cl, Ca – Mg – SO4 - Cl (water type) and mixed water type Ca – Mg – HCO3 – Cl. The study approach includes geochemical analysis and conventional graphical plots (Bivariate) of the hydrochemical data to assess the geochemical evaluation of processes. The result suggested different natural hydrogeochemical processes such as simple ion exchange, leaching and dissolution, evaporation and oxidation-reduction are the key factors. The influence on the species of groundwater and underground mine water chemistry is the leaching and dissolution of weathered rock with limited cation ion (reverse ion exchange) at few locations. Evaporation and oxidation-reduction are not among the major hydrochemical processes controlling the hydrochemistry of groundwater and underground mine water. Gibbs diagram also confirms the dominant process as the leaching and dissolution of minerals. Stable isotope composition measurements (in ranges ‰) for surface water [δ18O (-2.16 to +0.47); δ2H(-6.66 to +3.02)], hand-dug wells [δ18O (-2.28 to -0.87); δ2H(-6.48 to +0.58)], boreholes [δ18O (-2.23 to -0.48); δ2H(-8.88 to -1.68)] and underground mine water [δ18O (-2.22 to -0.95 ); δ2H(-8.54 to -1.00)]. This result indicates that stable isotopes (δ18O and δ2H) in the water samples from the GSBPL (NCM) clustered closely along the Global Meteoric Water Line (GMWL), suggesting an integrative and rapid recharge from meteoric origin (rainfall). Consequently, majority of the water samples are mainly meteoric water with little or no isotopic variations. Finally, recharging relationships were identified for these water samples; groundwater at Himan [borehole (HBH2)] and River Ankobra [ARaW (DS)]; surface water at Prestea [PUWF (DS)] and underground mine water at Prestea Central Shaft [UD-17F]; and surface water at Ankobra [ASP] and Prestea [PUWM(MS)], a borehole at Prestea [PBH6] and underground mine water at Prestea Central Shaft [UD-17B].