EXPERIMENTAL SIMULATION OF TEMPORAL VARIATIONS IN DISSOLVED INORGANIC CARBON AND δ13CDIC EVOLUTION OF DISCHARGE FROM NEUTRALIZED MINE TAILINGS

The neutralization of carbonate-rich mine tailings releases high amounts of CO₂ that may be trapped in groundwater systems, and released rapidly to the atmosphere when the groundwater discharges on the surface. In this study, we measured dissolved inorganic carbon (DIC), major ions, and stable carbon isotope ratios δ¹³C of DIC from carbonate-rich mine tailings, that produce neutral mine drainage. We investigated the temporal evolution of DIC and δ¹³C_DIC released from the neutralized carbonate-rich tailings in a laboratory leaching experiment. The goal was to simulate DIC evolution and the resulting changes in stable carbon isotopes of DIC that arise when groundwater is released from the subsurface from neutral mine drainage in carbonate-rich environments. The objective was (1) to investigate DIC and δ¹³C_DIC evolution during the outgassing of CO_2(aq) from the neutral tailings discharge (2) temporal changes in δ¹³C_DIC during CO_2(g) outgassing. From the experimental data, rapid decreases in DIC and increasing pH and alkalinity was observed in the first 48 hours of leaching carbonate rich tailings waste with excess sulfuric acid. The DIC decrease is due to loss of excess CO_2(aq) released from the leachate as reactor was opened to atmosphere. The DIC decrease from CO_2(aq) loss, maybe analogous to CO_2(aq) loss as groundwater charged with DIC is discharge to the surface at a seep or source point. We relate the temporal change in DIC from the reactor over time to decreases that would be observed in field settings in the DIC as groundwater discharge flows downstream. The mechanism of excess CO_2(g) loss from the reactor is kinetic and this will lead to a kinetic δ¹³C_DIC enrichment (from initial -1.9permil to final 2.5permil after 172 hours of sampling). the highest enrichment ~ 5.0 permil occurs between the first 100hours and then remains more or less constant to end of the experiment at 172 hours. The magnitude of the δ¹³C_DIC enrichment depends on the initial concentration of excess CO_2(aq) in the discharge water and the amount plus rate of CO_2(g) loss from the discharge. Carbon evolution in neutral mine drainage has potential implications in our understanding of CO_2(g) released from carbonate-rich mining tailings environments.