Citation:

Nietch, C., B. Hawley, A. Safwat, J. Christensen, Matthew Heberling, J. McManus, R. McClatchey, H. Lubbers, N. Smucker, E. Onderak, AND S. Macy. Implementing constructed wetlands for nutrient reduction at watershed scale: Opportunity to link models and real-world execution. JOURNAL OF SOIL AND WATER CONSERVATION. Soil and Water Conservation Society, 79(3):113-131, (2024). https://doi.org/10.2489/jswc.2024.00077

Impact/Purpose:

Much of the research on the design and performance of constructed wetlands has been conducted at the unit-level, where one wetland or a series of cells comprising a wetland treatment system is evaluated by in-out type measurement and modeling.Determining how multiple wetland installations across a landscape combine to effect watershed-level nutrient loads has largely been approached as a modeling exercise and is less well understood  Real world examples that measure the effectiveness of multiple constructed wetlands in the aggregate are needed because there may be barriers to implementation at the small scale or emergent properties at the larger scale that are otherwise difficult to account for with modeling exercises. This study presents the actions that the East Fork Watershed Cooperative, a watershed partnership, has undertaken to build wetlands for intercepting excess nutrients from row crops (soybean and corn), which are the dominant source of nutrient pollution in the Upper East Fork of the Little Miami River Watershed (UEFW), the case study system. It fits within a bigger context of understanding the cost effectiveness of mitigating nutrient pollution using a variety of agricultural conservation practices (ACPs). The watershed-scale constructed wetland implementation framework included, 1) SWAT modeling for the planning-level assessment of the cost effectiveness of alternative practices for meeting watershed nutrient reduction targets; 2) identifying suitable locations for wetland construction using the Agricultural Conservation Practice Framework tool; 3) incorporating demonstration sites of innovative designs meant to optimize nutrient removal and validate their performance, and 4) returning to the watershed scale with the real and projected expenditures to assess the feasibility of meeting nutrient reduction goals given the original estimates. This paper discusses what we know to date about the realized cost-effectiveness of a decade of effort trying to implement constructed wetlands in the UEFW.

Description:

  The negative effects of nutrient pollution in streams, rivers, and downstream waterbodies remain widespread global problems. Understanding the cost-effectiveness of different strategies for mitigating nutrient pollution is critical to making informed decisions and defining expectations that best utilize limited resources, which is a research priority for the US Environmental Protection Agency. To this end, we modeled nutrient management practices including residue management, cover crops, filter strips, grassed waterways, constructed wetlands, and reducing fertilizer in the upper East Fork of the Little Miami River, an 892 km2 watershed in southwestern Ohio, United States. The watershed is 64% agriculture with 422 km2 of row crops contributing an estimated 71% of the system’s nutrient load. The six practices were modeled to treat row crop area, and among them, constructed wetlands ranked highest for their low costs per kilogram of nutrient removed. To meet a 42% phosphorus (P) reduction target for row crops, the model results suggested that the runoff from 85.5% of the row crop area would need to be treated by the equivalent of 3.61 km2 of constructed wetlands at an estimated cost of US$2.4 million annually (or US$48.5 million over a 20-year life cycle). This prompted a series of projects designed to understand the feasibility (defined in terms of build, treatment, and cost potential) of retrofitting the system with the necessary extent of constructed wetlands. The practicalities of building this wetland coverage into the system, while leading to innovation in unit-level design, has highlighted the difficulty of achieving the nutrient reduction target with wetlands alone. Approximately US$1.2 million have been spent on constructing 0.032 km2 of wetlands thus far and a feasibility analysis suggests a cost of US$38 million for an additional 0.409 km2. However, the combined expenditures would only achieve an estimated 13% of the required treatment. The results highlight the potential effectiveness of innovative design strategies for nutrient reduction and the importance of considering realistic field-scale build opportunities, which include accounting for acceptance among landowners, in watershed-scale nutrient reduction simulations using constructed wetlands.

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