Executive Summary
Phosphorus and nitrogen are naturally occurring elements that, in excess, contribute to poor water quality in aquatic ecosystems worldwide. Strategies have been developed to mitigate nutrient runoff from agricultural fields, a major source of excessive nutrient loads. Treatment wetlands1 – created or re-established systems that are man-made and designed to accomplish a pollutant reduction goal – provide one strategy for nutrient management, but there remains a wide range of questions and concerns about their effectiveness and role within nutrient management programs, relative to other strategies.
The Nature Conservancy engaged partners to explore these issues by conducting a preliminary assessment of the scientific literature, as well as relevant policies and programs. Interviews were conducted with experts in this field on the effectiveness of using wetlands to reduce phosphorus and nitrogen loads to downstream waters as part of a nutrient management strategy in agricultural systems. The objective of this report is to outline the findings from the science and policy literature review and provide recommendations for moving forward with this strategy.
Numerous variables influence the effectiveness of a treatment wetland to reduce phosphorus and nitrogen, including hydraulic loading, wetland age, season, temperature, and inflow concentration of nutrients. These variables have been assessed in this report, and associated recommendations have been made toward improving site selection, design, and construction. However, due to the complexity and heterogeneity of treatment wetlands as well as inconsistencies in data reporting, assigning specific nutrient reduction amounts to any given treatment wetland remains problematic. More research is needed to fully understand the mechanisms driving phosphorus and nitrogen reduction by a wetland, especially to influence the current policies and programs that do not currently credit wetlands for their treatment capabilities.
This study leads us to three major conclusions:
- First, the implementation of existing best management practices and reduction of nitrogen and phosphorus applications beyond crop needs, may reduce nutrient loading to surface waters in agricultural watersheds. This will likely result in water quality improvements, and a decrease of time and effort to develop new technologies and strategies if loads were reduced at the source.
- Second, ensuring effective implementation of existing policies will also aid water quality improvement, especially if supported through expanded and standardized monitoring and assessment efforts.
- Finally, understanding the potential role of treatment wetlands as part of agricultural nutrient management strategies will take a multidisciplinary approach with engineers, conservationists, ecologists, policy makers, and biologists working together to design and construct the most efficient management practices to reduce phosphorus and nitrogen loading to surface waters.
Image: Phosphorus cycle of soluble and particulate phosphorus. Source: Reddy et al. 1999
