Balance of S in a constructed wetland built to treat acid mine drainage, Idaho Springs, Colorado, U.S.A

The wetland constructed at the Big Five Tunnel in Idaho Springs, Colorado was designed to remove, passively, heavy metals from acid mine drainage. In optimizing the design of such a wetland, an improved understanding of the chemical processes operating there was required, particularly SO 4 2− reduction and sulfide precipitation. For this purpose, field and laboratory data were collected to study the balance of S in the system. Field data collected included water analyses of the mine drainage and wetland effluents and measurements of H 2S gas emissions from the wetland. The concentration of sulfide in the wetland effluent ranged from 10 −4 to 10 −3 mol/l. The average rates of H 2S emission from the surface of the substrate were 150 nmol/cm 2/d in the summer and 0.17 and 0.35 nmol/cm 2/d in the winter. This maximum estimated loss of sulfide was not significant in reducing the amount of sulfide available for precipitation with metals. Sequential extraction experiments for S on wetland substrates showed that acid volatile sulfides (AVS) increased with time in the wetland substrate. A serum bottle experiment was conducted to study the S balance in the Big Five wetland by quantitatively measuring the amount of S in different phases as microbial SO 4 2− reduction progressed. The increase in AVS reasonably balanced the decrease in SO 4 2− concentration in the experiment, suggesting that the decrease in SO 4 2− concentration represented the amount of SO 4 2− reduced and that nearly all of the sulfide produced was precipitated as AVS. Sulfide precipitation was determined to be the primary metal removal process in the wetland system and amorphous FeS is the primary iron sulfide formed in the substrate.