The Long's Park Treatment Wetlands and Park Enhancement Project

The City of Lancaster, PA has constructed an innovative water quality improvement project in one of its public parks to capture and treat stormwater runoff as part of its Chesapeake Bay Pollutant Reduction Plan. Runoff quality improved by the project results in reduced nitrogen, phosphorus, and sediment loading to the Little Conestoga Creek in the Lower Susquehanna River Basin, which ultimately contributes to improved water quality in the Bay.

Description: The Long's Park water quality improvement project includes a pump station to convey water from Long's Pond to a constructed natural treatment system (NTS) consisting of a forebay equipped with floating wetland islands (FWI), an iron-enhanced media filter (IEMF), and two constructed marshes that discharge to Long's Pond (Figure 1). NTS are engineered ecosystems that use natural biological, physical, and chemical processes to improve water quality. With lower operating costs, less energy consumption, and fewer residuals produced than conventional active treatment, NTS provide cost-effective solutions for treatment of various types of water inflows including stormwater.

The forebay was designed as an open basin to receive the first flush of stormwater inflows and achieve significant removal of particulate solids and associated pollutants. The FWIs function to improve basin hydraulics by enhancing dispersion and by supplementing solids and nutrient removal through settling, filtering, and uptake processes associated with the suspended root mat of the aquatic plants. In addition, the FWIs are expected to provide an attractive resting, nesting, and feeding habitat for waterfowl and aquatic wildlife like frogs and turtles.

The IEMF receives the outflow from the forebay and promote phosphate removal through iron oxidation. The vertical flow filter media bed consists of pea gravel mixed with iron. Flow from the media filter passes through two constructed marshes in sequence that include deep and shallow zones for enhanced solids retention, denitrification, phosphorus uptake, and ecological habitat including mosquito control. Finally, an outflow pipe from the lower marsh was constructed to convey the outflow from the marshes to Long's Pond. To provide enhanced flushing of Long's Pond, a new pump station pumps approximately 95 liters per minute from the pond to the forebay.

Through engagement with the Long's Park Commission, public recreational use features were incorporated in the form of all-weather access trails to the NTS that wind around the site in addition to a boardwalk that enable public access through the lower marsh. Illustrative educational signs were installed to inform the public of the purpose of the project. Together, these features have created a unique and engaging experience for park visitors, who in turn have enthusiastically embraced the project.

Methodology: The Soil-Plant-Atmosphere-Water (SPAW) water balance model was used to size the Long's Park NTS based on a long-term simulation using climatic data for a 24-year period of record. This period of record was selected to represent potential climatic conditions and included a rainfall event equivalent to the 100-year, 24-hour design storm of 20 cm. The SPAW model provided a water balance for the entire NTS design and ensured the system's design capacity was sufficient for the anticipated stormwater flows.

The SPAW package includes features such as outlet pipes, outlet pumps, and runoff inflows such that a wide variety of pond situations can be described. All hydrologic processes simulated by SPAW are illustrated in Figure 2, and the processes used for the Long's Park NTS design are circled in red. The processes simulated for the NTS design and water balance model include the watershed fields/stormwater runoff, external input (from Long's Pond), evaporation, precipitation, and the outlet pipe.

Results: Through trial and adjustment of the model, the dimensions of the pond, media filter, and marshes were adjusted to ensure sufficient capacity for the design stormwater inflows. Figures 3 through 5 provide the SPAW model simulated hydroperiod for the forebay, upper marsh, and lower marsh. The NTS is projected to reduce stormwater runoff total suspended solids (TSS) and nutrients. Based on the daily water balance model conducted for this system over the 24-year period of record, the annual average inflow the NTS receives is approximately 166,558 liters per day. This total inflow includes stormwater runoff from approximately 3.1 ha of contributing drainage area and a continuous 95 liters per minute flowrate from Long's Pond. This flow was used to predict the water quality performance of the NTS.

The NTS's capacity to improve stormwater quality was evaluated using current approaches to modeling treatment wetlands. The primary removal mechanisms of NTS include solids settling, metal adsorption and precipitation, microbial transformation to gaseous compounds, and plant uptake and burial. The water quality model results indicate that in a typical year, the treatment system provides a reduction of 9 kg of phosphorus, 45kg of nitrate, and 5443 kg of TSS. As shown in Table 1, these reductions result in a 50% reduction of TP, 32% reduction of TN, and 95% reduction of TSS.

This presentation will cover lessons learned from design, construction, and maintenance, as well as qualitative and quantitative performance indicators, including initial water quality data.