A constructed wetland receiving pasture runoff from a dairy in Tillamook, OR was monitored during the winter of 1997-98 in order to estimate coliform treatment efficiency during winter high flow periods. Monitoring occurred during four sampling periods, each lasting 2 to 4 days. Samples were taken every two hours from the inlet and outlet of each of two parallel wetland cells and analyzed for fecal coliform using the standard membrane filtration technique. Flow into the wetland cells was measured using a chart recorder. Dye tests were conducted for each cell during each sampling period in order to estimate residence time, active cell volume, and qualitatively evaluate the flow regime. Removal was calculated by comparing inlet samples with outlet samples offset by the residence time. This residence time offset method was an attempt to compensate for the changing flows and loads common to storm driven non-point pollution sources. Coliform concentrations and flow rates of the dairy pasture runoff varied widely. Concentrations ranged from 10�� cfu/100mL to more than 10��� cfu/100mL. The highest concentrations typically coincided with the first storm flow peak following manure application. The constructed wetland in this study was able to reduce coliform concentrations in dairy pasture runoff by more than an order of magnitude (98%) during winter storm events. Removals observed during a lower flow period in the fall were significantly lower (78%). A statistical examination of literature data in an attempt to determine the influence of commonly reported parameters on coliform removal had mixed results. Regression modeling suggested that the parameters that most influence coliform removal in wetlands are hydraulic overflow rate (HOR) and inlet coliform concentration. The importance of HOR would appear to suggest that an area-dependent process, such as settling, is the dominant removal mechanism in most wetlands. However, since most wetlands have some form of pretreatment to remove settleable material, it is unlikely that coliform is significantly removed by settling. A theortical examination of coliform removal mechanisms in constructed wetlands suggests that filtration, die-off, and solar ultraviolet disinfection are more likely removal processes. / Graduation date: 1999