Filter Marshes: A Nature-Based Solution to Improving Water Quality

Introduction Filter marshes are a nature-based solution of constructed wetlands to reduce surface water pollution and improve water quality. They are an effective best management practice helping communities meet regulatory standards for surface water and watershed management, while improving the quality of life within communities. This presentation focuses on the design of filter marshes and how these small, decentralized stormwater management systems aid communities in meeting nutrient-based total maximum daily loads (TMDLs). Objectives The objectives of this presentation are to provide basic guidance on the modeling and design of filter marsh systems, demonstrate how filter marsh design elements can address surface water contamination within watersheds, and increase awareness of sustainable water treatment technologies. Status Surface water contamination is a widespread and expensive problem spurring the establishment of standards for surface water and watershed management. One solution for achieving nutrient level and suspended solids reductions in water bodies is filter marshes — constructed wetlands addressing surface water contamination by mitigating flood impacts, providing storage for surface water, and filtering nutrients from surface water runoff that, in excess, pollute lakes, streams, and wetlands. This presentation presents a case study and basis of design for a filter marsh system in southwest Florida. Filter marsh design consists of a combination of deep and shallow water bodies with a large variety of plant communities providing wildlife habitat. They can temporarily retain a large percentage of stormwater runoff and gradually release it, allowing suspended sediments and attached pollutants to be removed through natural sedimentation alongside biological and physiochemical processes. These smaller, decentralized filter marsh treatment systems are viable alternatives to advanced treatment technologies, which tend to be expensive and difficult to operate. A significant benefit to communities is also meeting nutrient-based water quality total maximum daily loads (TMDLs) which specifically target Total Nitrogen (TN) and Total Phosphorous (TP). Methodology The case study for this presentation is the Palm Creek Filter Marsh project in Lee County, Florida. Florida is among a growing group of states instituting numeric water quality standards for freshwater and estuary water bodies. This project includes the development of water control structures conveying water from Palm Creek through a series of vegetated filter marsh wetlands prior to discharging into the tidal Caloosahatchee River, which is impaired for dissolved oxygen and nutrients. This project also features boardwalks through the filter marsh and adjacent trails enhancing community interaction and public access while improving the quality of life for residents. The project was designed in 2023 to provide approximately 15 acre-ft of storage within the filter marsh system. Controlled by a gate system within Palm Creek, water is conveyed into and flows through the cellular treatment network before returning to the channel. The filter marsh system is divided into six (6) different cells of varying depths — each connected by wet berms that which serve as walking paths during the dry season. Shallow cells are planted with wetland vegetation and provide nutrient uptake through phytoremediation. Deeper cells serve as settling basins with limited wetland vegetation. The design also features removable stop logs integrated within the weir structure to provide additional water conveyance through Palm Creek during storm events. The project is currently being permitted with construction anticipated to commence in mid-late 2024. Findings and Significance The long-term objective of this project is to implement dynamic, effective water quality enhancement for Lee County's designated impaired water bodies, with nutrient reduction as the primary focus. The design removal efficiencies include removing 4,574 lbs/yr of TN and 597lbs/yr of TP. TN and TP removal are important to the County in meeting their overall reduction goals and improving water quality, as the County withdraws surface water from the Caloosahatchee River for potable water treatment. In addition to providing water quality treatment, the design features a parking lot, boardwalks, and a walking trail to engage the community and provide educational experiences surrounding constructed wetland systems.