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

Abstract

Porphyry-style copper deposits are characterized by low Cu grades and high tonnages, resulting in large mine tailing volumes disposed in impoundments. Due to the mining tailing sizes, waterproofing techniques cannot be applied along the dam base. Therefore, to minimize seepage towards the aquifers, pumping wells are usually installed as hydraulic barriers. Currently, there is a controversy over whether or not the water extracted from hydraulic barriers should be counted as the use of new water rights. Consequently, a growing interest to develop tools to trace and quantify the tailing impacts in groundwater and to determine the water pumped amount subjected to water rights exist. In the present study, isotope data (<em>δ</em>²H-H₂O, <em>δ</em>¹⁸O-H₂O, <em>δ</em>³⁴S-SO₄^2- and <em>δ</em>¹⁸O-SO₄^2-) are proposed as a tool to quantify tailings seepage towards groundwater and to assess hydraulic barriers effectiveness. To illustrate this approach usefulness, the Quillayes porphyry Cu tailing impoundment (Chile) case study is presented. The multi-isotopic approach revealed that tailing waters are highly evaporated showing high SO₄^2- content (~1,900 mg L^-1) derived from primary sulfate ore dissolution, whereas freshwaters, derived from recharge water, have low SO₄^2- contents (10-400 mg L^-1) resulting from the interaction with geogenic sulfides from barren host rock. The <em>δ</em>²H and <em>δ</em>¹⁸O values of groundwater samples collected downstream from the impoundment suggest a mixing at different proportions of highly evaporated water from the mine tailing waters and non-evaporated regional fresh groundwater. Cl^-vs. SO₄^2-, <em>δ</em>³⁴S-SO₄^2- vs. <em>δ</em>¹⁸O-SO₄^2-, <em>δ</em>²H-H₂O vs. <em>δ</em>¹⁸O-H₂O mixing models allowed to determine that groundwater located closer to the impoundment had a mine tailing water contribution from 45 to 90 %, whereas those located farther away had lower contribution (5-25 %). Results confirmed the stable isotope usefulness to determine the water origin and to calculate the hydraulic barrier efficiencies and the pumped water proportions unrelated to the mining tailing subject to the water rights.