Wetland Photochemistry As A Major Control On The Transport Of Metals In An Acid Mine Drainage Impacted Watershed
Duren, Sabre M 1 ; McKnight, Diane M 2
1 CVEN, University of Colorado at Boulder
2 CVEN, University of Colorado at Boulder
To date, few studies have focused on iron photochemistry in acid mine drainage impacted wetlands, although several studies have addressed iron photochemistry in acid mine drainage streams and lakes. Wetlands produce DOM, which supports photochemical reactions and metal speciation. In addition, the cycling of iron has been correlated to the cycling of DOM, which can catalyze Fe3+ photoreduction and oxidation of Fe2+. If wetlands control the cycling of DOM and metals, photochemistry may have a major influence on the chemistry of receiving waters. While Fe2+ is produced by photoreduction, it is also consumed in the photo-Fenton reaction. H2O2 can be produced through photolysis of DOM in the presence of ultraviolet light and O2. The rates of superoxide radical and H2O2 formation are functions of DOM concentration and reactivity and ultraviolet light intensity.
A diel study was performed on October 14, 2011 in a wetland system located downstream of Pennsylvania Mine in Summit County, Colorado to quantify the concentrations and reaction rates of DOC, H2O2, Fe2+/Fe3+, and other metals of interest. Ten samples were collected during daylight hours and 5 samples were collected after dark. The results confirmed that photochemistry is a major control on the oxidation and reduction of iron in AMD-impacted wetlands. At midday the dissolved iron and H2O2 concentrations reached a maximum and then decreased in the afternoon. The corresponding iron oxide concentrations are a major variable for trace metal transport. The diel fluctuations of dissolved iron concentrations driven by changing light intensity were associated with nearly identical trends in the concentrations of 25 different metals. In addition to metals commonly found in AMD streams (Al, As, Cd, Cu, Ni, Mn, Pb, and Zn), these metals included a number of rare earth metals (Dy, Er, Eu, Gd, Ge, Ho, La, Lu, Nd, Pr, Sm, Tb, Tm, U, Y, Yb, and Zr) some of which occurred in concentrations exceeding 200 ug/L.
The data collected during the experiment confirmed the role of photochemistry in controlling the oxidation and reduction of iron, and the effect iron speciation has on other metal concentrations in the wetland. Wetlands are “hot spots” for iron-dissolved organic matter (DOM) photochemistry because the shallow waters are influenced by high light intensity and high DOM concentrations in slow moving waters with residence time for reactions to take place. The results of this study have important implications for AMD modeling and transport studies. Application of iron redox biogeochemistry to reactive solute transport modeling may improve predictive capabilities of various trace metal and solute interactions incorporated with the cycling of iron within AMD streams. Further, model improvement of iron cycling may enable more accurate remediation predictions for AMD streams.
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