Nitrogen Output from Residential Stormwater Pond during Wet Season.

Stormwater runoff carries excessive nitrogen (N) from urban land resulting in water quality degradation in urban waters. This excess of N in water bodies leads to eutrophication, hypoxia, and harmful algal bloom thus stormwater ponds has been constructed as one of the effort to reduce N input. Our study objectives were to investigate composition and loading of N from residential stormwater pond outflow to the water bodies. Pond outflow samples (n=141) were collected during wet season (May to September 2016) from an outlet of stormwater wet pond that has been installed with flowmeter equipped with an autosampler. Samples were analyzed for various N forms [ammonium (NH3–N), nitrate (NO3–N), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON)]. A total of 13 storm events were captured during the wet season. Total rainfall recorded throughout the wet season was 107.4 cm that generated 5.6 million L of pond outflow. Total pond outflow throughout the 13 sampling events was 3.7 million L (66% of total outflow over wet season) and carried 3.5 kg of total N. Organic nitrogen was the dominant forms at all 13 events with DON as the major contributor (73%), followed by PON (10%), NH3–N (10%), and NO3–N (7%). Our results suggest that PON from plant materials such as leaves, and grass clippings that settle in the pond likely contributed to the DON leaving the pond outflow to waters bodies. A portion of DON pool leaving the stormwater pond might be bioavailable and can be a potential source of N for algae thus lead to water quality degradation in the urban coastal ecosystem. Therefore, urban stormwater best management practices (BMP) should consider all N forms to ensure an effectiveness of N control in preserving water quality in urban waters.