Process Enhancement of a Cr (VI) Remediation Method to Minimize the Hazardous By-products in the Treated Water

Abstract

A Cr (VI) contamination at the study area is related to the leaking baghouse slimes dam which was decommissioned 18 years ago, in order to manage and control excessive Cr(VI) contamination impacting the aquifer, and thereafter impacting the off-site receptors. The baghouse slimes dam was decommissioned in the year 2000 and covered with polyethylene thermoplastic to prevent leaching prompted by rain water and to prevent Cr oxidation. Cr(VI) contaminated groundwater is pumped from the underlying aquifer system to the two existing surface treatment settling ponds and thereafter the water is treated by means of dosing system (FeSO4) and electrochemical or reducing system. These treatment systems are effective in managing or controlling the Cr(VI) contamination. However, neither system can work effectively independently. Hence, the installed liner is another mechanism that was implemented to enhance the remediation systems at the study area. The main purpose of this project is to determine the effect of the polyethylene thermoplastic liner on the seepage water quality observed in the monitoring boreholes. The plume movement from the source area to the impact monitoring boreholes was modelled in order to ascertain the plume mass, plume movement during abstraction and the plume capture zone. The water chemistry data was used to achieve the objective of the study. The definite decreasing Cr(VI) concentrations in nine (9) of seventeen (17) existing monitoring boreholes were observed from the year 2000 to 2017. This confirms that the installed liner was capable of ceasing the recharge in the slimes dam and minimizing the chromium oxidation. Six (6) of the seventeen (17) monitoring boreholes showed fluctuating concentrations, which could be attributed to interaction of groundwater chemistry and geological formation. The remaining two (2) monitoring boreholes showed an increasing trend, that stipulates that the plume is localised within that area and the Cr(VI) concentrations trend in these boreholes indicate a potential gradual decrease. Based on the Mann-Kendall trend analyses result, the installed liner is proved to be effective on the seepage water quality. Given that the plume has reached the impact boreholes, it was deemed necessary to ascertain plume movement, plume mass and the plume capture zone via transport and numerical model using Groundwater Modelling System (GMS10.0). A conceptual site model was used to develop the numerical and transport model for the study area. GMS runs modular finite difference codes, using modflow in conjunction with its several packages to formulate three-dimensional flow conditions in the vicinity of the baghouse slimes dam. The model was subjected to both calibration and sensitivity analysis. The calibration results indicated a few outliers, which can be attributed to pumping activities taking place at the study area and they were within an acceptable error. Sensitivity analysis indicated certainty for most stressed parameters. Only two off-set values were evident for recharge and hydraulic conductivity of layer number two. Based on the modelling results, the movement of the plume is controlled by sorption process. The plume was characterised by means of considering and disregarding sorption. This exercise proved that the plume is moving rapidly when sorption is not considered. The plume mass was also calculated with and without sorption and a difference in mass was observed. When sorption is considered the mass over time is larger than when it is not considered. This proved that when sorption is not considered in the numerical or transport model, inaccurate predictions of plume mass and movement can be calculated and this can lead to negative impacts on plans and finances of the mine.