Isotopes to define natural water

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SRK News | Issue 55: Mine Water Management

José Miguel García, Senior Hydrogeologist
Osamu Suzuki, Principal Hydraulic Engineer

Water isotopes (2H and 18O) are stable compounds and their principal fractionation comes from the evaporation and condensation process. In groundwater, the isotope variations and changes just come from the mixing of water of different origins.

In this project, a three-year quarterly water sampling program was oriented to know the presence and the percentage of natural water pumped by the hydraulic barrier downstream of a leaking tailings dam. The isotope content and water chemistry were analysed from water pumped by wells, surface water and tailings in order to characterise the natural, tailings and mixed water. This definition is required to estimate the amount of natural groundwater that is being pumped without water rights.

The isotope content shows that the water stored in the tailings dam has evaporated, so its isotopic content is more enriched than the natural water (Figure 1). This situation allows differentiating the isotopes end members from the dam (PR3A) and natural groundwater (ABQ6A1), which will be used to estimate the natural water proportion downstream of the tailings dam.

The figure below shows the end members and the “mixing line”. The groundwater isotopic content between both end members and near the mixing line indicates the existence of a mixture in the groundwater sampled. The proportion of the natural water is calculated using an analytical equation of the binary mix.

Results show that the percentage of natural water is higher when the distance to the dam increases. Water samples taken near the dam (ABQ6D and ABQ9) have no natural water because their isotopic content is similar to the groundwater stored in the dam.

The isotopic content of some groundwater samples (ABQ2, ABQ2NX, ABQ6AX) is in a line parallel to the Global Meteoric Line (GML); therefore, these points have not received any flow from the dam.  This result indicates the hydraulic barrier is preventing the advance of water filtration from the dam to the downstream where public wells are located.

Water chemistry is used to support the percentage of the natural water in each point. The natural water is fresh water, calcium bicarbonate type, and 40% sulfate in anions. The water dam is slightly saline, calcium sulfate type, and 90% sulfate in anions. The mixingwater is between these water types and sulfate percentage.


 

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