Renovation of a failed constructed wetland treating acid mine

Acid mine drainage (AMD) from aban- doned underground mines significantly impairs wa- ter quality in the Jones Branch watershed in McCreary Co., Kentucky, USA. A 1022-m 2 surface- flow wetland was constructed in 1989 to reduce the AMD effects, however, the system failed after six months due to insufficient utilization of the treat- ment area, inadequate alkalinity production and metal overloading. In an attempt to improve treat- ment efficiencies, a renovation project was designed incorporating two anoxic limestone drains (ALDs) and a series of anaerobic subsurface drains that promote vertical flow of mine water through a suc- cessive alkalinity producing system (SAPS) of li- mestone beds overlain by organic compost. Analy- tical results from the 19-month post-renovation pe- riod are very encouraging. Mean iron concentra- tions have decreased from 787 to 39 mg l -1 , pH in- creased from 3.38 to 6.46 and acidity has been re- duced from 2244 to 199 mg l -1 (CaCO3 equivalent). Mass removal rates averaged 98% for Al, 95% for Fe, 94% for acidity, 55% for sulfate and 49% for Mn during the study period. The results indicate that increased alkalinity production from limestone dissolution and longer residence time have contrib- uted to sufficient buffering and metal retention. The combination of ALDs and SAPS technologies used in the renovation and the sequence in which they were implemented within the wetland system proved to be an adequate and very promising de- sign for the treatment of this and other sources of high metal load AMD.