DIEL CHANGES IN IRON(III) IRON(II) IN EFFLUENT FROM CONSTRUCTED ACID-MINE DRAINAGE TREATMENT WETLANDS

Within the past decade, over 400 wetlands have been constructed to tre at acid coal mine drainage (AMD). This study was undertaken to examine whether the efficiency with which constructed wetlands retain Fe pres ent in AMD changes on a day-night basis. In each of five AMD treatment wetlands, constructed with a different organic substrate (Sphagnum pe at with limestone and fertilizer, Sphagnum peat, sawdust, straw-manure , or mushroom compost), hourly sampling of inflow and outflow waters w as conducted over a 19-h (July 1990) and 24-h period (May 1991). The c hemical composition of the influent AMD was relatively stable over the duration of either sample period. However, a striking diel pattern of shifts in the relative abundances of Fe2+ and Fe3+ in outflow waters was observed for both Sphagnum peat wetlands on both sampling dates, a nd for the straw-manure and mushroom compost wetlands on the May 1991 sampling date. Prior to sunrise, 75 to 100% of the soluble Fe in outfl ow waters was Fe2+ prior to sunset, 62 to 88% of the soluble Fe was Fe 3+. The documented diel shifts in Fe3+ vs. Fe2+ abundance in outflow w aters are greater in magnitude than any previously reported for a natu ral system and suggest that the cycling of Fe in AMD treatment wetland s is more dynamic than previously recognized and that standard daytime only sampling protocols are inadequate for assessing Fe retention-rel ease in AMD treatment wetlands.