Chemical speciation modelling as an aid to the cost-effective evaluation of hazards form mine waste. (WC/99/039).
The abandonment of metalliferous mines around the world has often led to severe pollution problems. The mine water from sulphide hosted, base metal mines is usually very acidic with a large, heavy metal load. British Geological Survey (BGS) is developing software that can be used as a tool to aid in hazard evaluation. The software employs simple chemical and hydrogeological principals to estimate the potential heavy metal load of abandoned mine waters.
To aid the development of the software, geochemical modelling techniques were employed. The modelling code PHREEQC Vl .6 was used to model the oxidation of various primary mineral assemblages to predict likely heavy metal concentrations at discharge points. The modelling required thermodynamic data for minerals and aqueous species that were not available within the PHREEQC thermodynamic database. A collation of available thermodynamic data from several sources produced a new thermodynamic database for the project (named l2CC.dat). However, not all the thermodynamic data required was located, the missing data is identified.
The model data was compared to collated field data. The model data proved useful at low pH conditions i.e. close to discharge point data. However, as the pH was increased the model results drifted from collated data. It is believed that this represents the effect of natural attenuation within the field data, which proved difficult to model. The work shows that discharge point modelling is possible but modelling natural attenuation processes requires more site-specific data.
However, using BGS collated field data, it was possible to produce mathematical equations which express metal concentration as a function of pH, both at the discharge point and downstream of the source where natural attenuation processes occur. The equations are conservative in nature; i.e. they are likely to overestimate metal concentration. They are considered to be applicable for risk assessment.
A hazard assessment approach based around mineralogy and pH is considered feasible. Mineralogy can be used to determine which metals are likely to be found within a particular mine water. The pH measurement can be used to gain a crude estimate of the concentration of heavy metals present using the equations derived from field data.
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