The utilization of passive treatment systems to mitigate the effects of acid mine drainage and acidic leachate discharge is a recent innovation in the restoration of aquatic ecosystems. During the construction of U.S. Route 48 (presently Interstate Route 68) and the Maryland Route 219 Interchange in Garrett County, Maryland, in approximately 1973, sulfide-bearing rock material was utilized as valley fill and for embankments on the eastern side of Keysers Ridge. The placement of this material affected the headwater areas of two tributaries to Lake Louise, an impoundment of Puzzley Run. The movement of water through the material induced biological and chemical processes to occur, resulting in acidic leachate discharge to the tributary streams. Degradation of the aquatic ecosystems in the tributaries and Lake Louise was documented in 1975. Watershed studies have since identified aluminum leaching, an artifact of the acidic leachate, as the probable source of impairment. The Maryland State Highway Administration constructed two passive treatment systems in 1996 employing successive alkalinity-producing technology to remediate the effects of the acidic leachate discharge. The ultimate objective of the passive treatment was to initiate the recovery process of Lake Louise and its affected tributaries. Extensive water quality analyses, phytoplankton and zooplankton community assessment, fish bioassays, and fish repatriation commenced in 1997, and will be continued through 2005. The biological sampling has documented improvements in the lake phytoplankton and zooplankton communities, as well as survival and growth of rainbow trout (Oncorhyncus mykiss) and brook trout (Salvelinus fontinalis) stockings. Young-of-the year brook trout were collected during 2002 in an impacted tributary, indicating the return of water quality and habitat conditions which support natural reproduction. Inflow, effluent, and biological monitoring completed to date have provided insight on the effectiveness and performance of these passive treatment systems for the restoration of the coldwater ecosystem.