The impacts of milled peat mining on runoff quality in northern Minnesota were determined using a multiple watershed approach. The frequency distributions of water quality constituents were used to detect whether runoff from a mined bog differed from that of 15 unmined (control) bogs. Peat mining increased water temperature, suspended sediment, specific conductance and concentrations of acidity, iron, sodium, and nitrogen species, although drinking water standards were not exceeded (α= 0.05). The method presented may be applicable for other nonpoint pollution investigations.