Manganese is a problem at the Ni River Water Treatment Plant in Spotsylvania County, Virginia. The Ni River Reservoir (the water source) is a eutrophic reservoir. In the summer, the dissolved oxygen decreases to near or zero at depths greater than two meters. As a result, soluble manganese increases to levels of nearly 6.0 mg/L at the bottom. It is released from the sediments under anaerobic conditions. Total organic carbon levels ranging from 4.0 to 7.25 mg/L were noted with increasing depth. Plant profiles were developed to indicate the performance of the sedimentation and filtration units in reducing manganese concentration. Essentially, all the particulate manganese was removed by sedimentation, and some removal of soluble manganese was evident. The filters removed additional soluble manganese. Soluble manganese removal probably was due to the adsorption of manganese on solid manganese dioxide in the sludge blanket and on the filter media with subsequent further oxidation. Ozone was effective at a dose of approximately 5 mg/L. Chlorine and chlorine dioxide were marginally effective as pretreatments at dosages of 5 and 2 rng/L, respectively. Potassium permanganate proved effective at dosages of 0.5 to 0.625 mg/L (1.5 to 1.9 times the theoretical requirement). Aeration proved effective in reducing levels of approximately 0.1 mg/L to below the secondary maximum contaminant level (0.05 mg/L) and, in addition, somewhat effective in reducing a concentration of nearly 2 mg/L by 31 percent. Aeration appears to be a viable means of reducing the anaerobic conditions in the reservoir that lead to the high soluble manganese concentrations. / M.S.