Application of fate and transport models to evaluate the efficiency of a Cr(VI) remediation pump and treat system

Nkosi, Sifiso Collen
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Groundwater treatment by chemical precipitation is a popular form of remediation at mines that are in operation. The water quality status at the implementation of the PAT in this study was compared to the water quality status after a six-month period of active remediation. Chromium is very important as an industrial metal owing to its numerous uses in a variety of industries. The objective of the remedial action is to intercept the Cr(VI) plume, abstract contaminated groundwater and chemically treat it on the surface. The long-term (15-year) objective is to eventually reduce Cr(VI) concentrations in the aquifer(s) to below 0.05 mg/ℓ. The PAT system was implemented as a mediumterm (5-year) strategy to intercept the Cr(VI) contamination plume during migration to prevent it from negatively impacting on groundwater users downstream of the mine. In the vicinity of the three PAT systems’ abstracting wells, water levels declined by an average of 2 m compared to the same period in 2014. Periodical fluctuations in the fractured aquifer are reflective of the influence of fractures on groundwater flow. In the aquifer, hydrochemical signatures show evidence of mixing between the primary and secondary aquifers. The treatment system has been successful in reducing Cr(VI) to Cr(III) and precipitating Cr(OH)3. The treatment system was designed to treat Cr(VI), other elevated constituents and generally high dissolved ions are not treated in this remedial process. Sulphate concentrations increase after treatment as a result of the addition of Fe(II)SO4 for chromate contamination treatment purposes. The simulated reaction path shows that the transformation of CrO4 2- to Cr2O3 in the treatment system is not immediate. The Cr(VI) to Cr(III) transformation is irreversible, this is beneficial as the water is abstracted from more reducing conditions, and the treatment ponds are open to the atmosphere thus the conditions following dosing with Fe(II)SO4 are oxic and chromate complexes are stable over a wider range of Eh-pH conditions than Cr(III) compounds. This ensures that the efficiency of the dosing system is not reversed in Settling Pond B. The modelled flow paths are similar to the inferred flow vectors in the plume capture zone. Fracture flow is the dominant type of flow, the fault zones and dykes create high permeability conduits to flow. Flow paths are parallel to fault lines or the lateral dimension of dykes; flow occurs along fractures and deformation zones. The reduction of Cr(VI) concentrations in some of the peripheral sampling points indicates that the PAT system has been successful in capturing the chromate contaminated water through pumping. Keywords: Hexavalent chromium, groundwater pollution, remediation, pump-and-treat, fractured aquifers
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science in hydrogeology, School of Geosciences. Johannesburg, 23 May 2016.
Nkosi, Sifiso Collen (2016) Application of fate and transport models to evaluate the efficiency of a Cr(VI) remediation pump and treat system, University of Witwatersrand, Johannesburg, <>