Assessment of mine spillage effects on receiving waters: hydrodynamic and water quality studies on Bacanuchi river, Mexico.

Abstract

This research focuses on the development of a hydrodynamic and water quality model to simulate the spillage event that occurred in the “Buenavista del Cobre” copper mine in the Bacanuchi river basin in 2014. This incident remains ambiguous since there is a lack of an initial estimation of the spilled volume as well as a model that describes the hydrodynamics of the spill and its impact on water quality. To address this, a hydrodynamic and water quality model was developed. The hydrodynamic model was set up and run in HEC-RAS. The model presented stability issues associated to steep slopes, low flows and hydraulic jumps, which limited the application of HEC-RAS in the first sections of the Tinajas stream and the Sonora river. Nevertheless, the model provided an estimated travel time for the spill from the Tinajas stream to the city of Arizpe of 10-13 hours. Additionally, a Python script using different reservoir capacity curves, initial volumes and outflow pipe diameters was developed to estimate the spilled volume of enriched copper solution. The results estimated the volume between 42,000-45,000 m3. Regarding water quality, the model was set up and run in AQUASIM with parameters selected based on concentration levels observed in samples of the spilled solution. These include Fe, Al, Cu, Mn, Zn, Pb, Cd, Cr, and As. CaCO3, SO4-2, CO3-2, and OH- were also considered due to their presence in the river, as well as their reactive potential with metals under varying pH conditions and level concentrations. Notably, concentrations of all metallic elements exceeded the official permissible limits for drinking water and human use (NOM-127-SSA1-2021) for 3 to 5 hours along the Tinajas stream and Bacanuchi river. The model identified that the contaminant plume behaved as piston flow while travelling along the river system. Other findings include the low propensity of precipitation of the metals due to the acidic conditions created in the river as a consequence of the spill, potentially extending contamination to the Bacanuchi and Sonora aquifers. Although these models provide a tool to assess the spill’s dynamics and impacts along the Tinajas streams and Bacanuchi rivers, limitations including data availability, computational constraints, and time were involved. Future research could include the influence of hurricanes Norberto and Odile on the spill's hydrodynamic and water quality effects, employ a modelling software capable of handling channels with steep gradients, low flows, and hydraulic jumps to represent the Sonora river's dynamics, as well as extending the water quality model to the river , and develop a groundwater model for a comprehensive assessment of the spill's impact on the basin.