Quantifying the potential seepage from porphyry copper tailing impoundments using a multi-isotopic approach

Area of Science:

• Environmental Geochemistry
• Hydrogeology
• Isotope Hydrology

Background:

• Porphyry copper mining generates large volumes of tailings, posing risks of groundwater contamination due to seepage.
• Traditional waterproofing is impractical for large tailings impoundments, necessitating hydraulic barriers like pumping wells.
• Determining the origin of pumped water is critical for water rights disputes.

Purpose of the Study:

• To propose and validate stable isotopes as a tool for quantifying tailings seepage into groundwater.
• To assess the effectiveness of hydraulic barriers in mitigating mine tailings impacts.
• To differentiate between mine-impacted water and natural groundwater for water rights allocation.

Main Methods:

• Analysis of stable isotopes in water (δ²H-H₂O, δ¹⁸O-H₂O) and sulfate (δ³⁴S-SO₄²⁻, δ¹⁸O-SO₄²⁻).
• Application of multi-isotopic mixing models (Cl⁻/SO₄²⁻, isotope-ratio-based, etc.).
• Case study at the Quillayes porphyry Cu tailing impoundment in Chile.

Main Results:

• Tailing waters are highly evaporated with elevated sulfate concentrations (~1900 mg L⁻¹).
• Groundwater downstream shows mixing of evaporated tailing water and fresh groundwater.
• Isotope models quantified tailing water contribution to groundwater from 5% to 90% based on proximity to the impoundment.

Conclusions:

• Stable isotopes are a reliable tool for tracing mine tailings seepage and assessing groundwater contamination.
• The study successfully quantified the contribution of tailing water to groundwater, aiding hydraulic barrier evaluation.
• This isotopic approach provides a scientific basis for managing water rights related to mine water extraction.