Agricultural runoff is a major contributor to surface water quality impairment in the Midwestern United States. This impact is particularly noted in drinking water reservoirs in eastern Kansas. Constructed wetlands can act as a water quality buffer, removing and retaining sediments and nutrients in field runoff prior to discharge to receiving waters. A better understanding of the impact of these wetlands at the watershed level, however, is needed for planning and management purposes. This paper combines the results of field studies on several surface flow wetlands with modeling and literature work to establish an expected range for wetland performance for nitrogen, phosphorus and sediment removal under conditions typical of eastern Kansas. A Bayesian analysis combined with a relaxed tank-in-series model was used to extrapolate likely ranges of predicted nutrient and sediment removal from experimental data obtained from three on-field wetlands. The performance of these wetlands was further compared to ‘expected’ performance ranges obtained from a literature survey of wetlands operating in similar geographic conditions and with similar influent quality. This analysis showed underperformance of the current constructed wetland design for nitrogen removal, but good removal of phosphorous and sediment. Improving wetland design and maintenance policies, however, could increase their impact. Finally, the potential impact of more widespread use of treatment wetlands to intercept agricultural runoff and meet nutrient reduction goals was assessed on a watershed scale. These results should aid in future planning for nutrient control of runoff entering Kansas reservoirs.
