DISSOLVED INORGANIC CARBON EVOLUTION IN ACID MINE DRAINAGE IMPACTED WATERS: INSIGHTS FROM LABORATORY REACTORS

AMD impacted water samples were collected from a spring, select stream stations, and groundwater and allowed to evolve in contact with the atmosphere in the laboratory. The objective was to determine how dissolved inorganic carbon (DIC) and the carbon isotope ratio of DIC (δ¹³C_DIC) evolve in acid mine impacted stream water in the absence of in-stream processes (e.g. aquatic photosynthesis). Over time, the spring, stream water, and one groundwater sample showed decreases in Fe²⁺, pH, alkalinity, and DIC, while the δ¹³C_DIC increased and SO₄^2- remained nearly constant. One of the groundwater samples showed an increase in pH despite decreases in Fe²⁺, alkalinity, and DIC, increase in the δ¹³C_DIC and nearly constant SO₄^2-. The decreases in Fe²⁺ in the samples were concomitant with a general increase in dissolved oxygen. This suggest that Fe²⁺ from AMD impact was oxidized to Fe³⁺ and subsequently hydrolyzed to Fe(OH)₃(s). This reaction produces protons which explain the temporal decrease in pH with decrease in Fe²⁺. The DIC loss pattern for water samples with decreasing pH was characteristic and showed rapid decline at lower pH which we attribute to pH induced loss. In these samples, the δ¹³C_DIC increased between 1 to 4 ⁰/₀₀. This is in contrast to the groundwater sample which showed a systematic increase in pH while DIC and alkalinity decreased, due to equilibration with atmospheric CO₂. For this groundwater sample, the δ¹³C increased ~9⁰/₀₀. The decrease in Fe²⁺ and pH coupled with decrease in alkalinity, DIC and enrichment of δ¹³C in the water samples impacted by AMD suggest that AMD evolution enhances CO₂ exsolution. The results of this experiment showed that AMD evolution in receiving streams and the nature of CO₂ loss from AMD impact can be determined from Fe²⁺, pH, DIC, and δ¹³C_DIC.