Assessment of the Transportation of Abandoned Mine Water Contaminants to the Surrounding Aquifers. A Case of Mlolongo Open-Pit Lake, Kenya

This study investigated the water quality of Open-pit Lake and the potential transportation of contaminants to the surrounding aquifers over time. The objectives included determining key water quality parameters, and modelling contaminants' spatial distribution and migration to understand their impact on the surrounding aquifers. Among the measured chemical water quality parameters, calcium (Ca), potassium(K), aluminium (Al), sulphates (SO₄ ^²⁻ ), chlorides (Cl ^⁻ ), nitrite (NO₂ ^⁻ ), nitrate (NO₃ ^⁻ ), and nitrogen (N) were within WHO threshold recommended limits. However, sodium (Na), ammonia (NH₄ ^⁺ ), manganese (Mn), fluoride (F ^⁻ ), carbonates (CO _₃ ^2⁻ ), iron (Fe), and phosphates (PO ₄ ³⁻ ) exceed WHO recommended threshold limits. The potential migration and transportation of contaminants were then modelled using MODFLOW and MT3DMS codes. The hydrological, geological, and concentration of contaminants were used as input parameters for the Processing MODFLOW Pro and MT3DMS models. The spatial distribution and travel distance of the selected three contaminants; sodium (Na), carbonates (CO _₃ ^2⁻ ), and nitrate (NO₃ ^⁻ ) from Mlolongo Open-pit Lake to the surrounding aquifers was simulated. The findings show that the spatial distribution and travel distance increase with time, with shallow aquifers being more vulnerable than deeper aquifers. In addition, the projection of the concentration of contaminants in the surrounding aquifers shows that the concentration of the contaminant increases with time and stabilizes after some time due to dilution and increased travel time. The Processing MODFLOW Pro validation results show a Mean Root Squared Error (RMSE) of 0.2240, indicating a minimal difference between the modelling results and the actual observed values, and thus, a lower modelling error.