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An intradunal wetland within the Indiana Dunes National Lakeshore on the south shore of Lake Michigan was flooded for 15 years by seepage from fly-ash settling ponds located adjacent to the park. Studies were undertaken to determine the effects of the seepage on water chemistry in the flooded wetlands. These water chemistry conditions have been correlated to ongoing studies of soil contamination and secondary succession in the wetland basin following cessation of seepage. The seepage increased the concentrations of calcium, potassium, sulfate, aluminum, boron, iron, manganese, molybdenum, nickel, strontium, and zinc in ground water and surface water downgradient from the settling ponds. Chemical interactions with aquifer materials, particularly organic matter, significantly limit the transport of aluminum, iron, nickel, and zinc in this system. The organic soils of the dewatered wetland basin now contain elevated concentrations of aluminum, boron, manganese, and zinc that are potentially phytotoxic under the low pH (<4) conditions that exist. Plant growth and secondary succession were affected by the soil chemistry of the dewatered wetlands.