Influence of Pond Age on Phosphorus Water Column-Sediment Interactions in Coastal Stormwater Ponds

Effective management of coastal stormwater ponds (SWP) to improve water quality requires an understanding of pond age influences on nutrient dynamics. To reduce eutrophication and regulate nutrient loading in receiving water bodies within a catchment area, coastal SWPs are essential. This study aimed to assess the effect of SWP age on internal sediment phosphorus (P) dynamics and ascertain whether benthic sediments function as a sink or source of water column P over time. Specifically, the objectives were to (1) assess the influence of pond age on water quality in the pond, and (2) understand the behavior of pond sediment as source or sink as influenced by pond age. The study examined nine wet SWP in residential areas of Charleston, South Carolina (SC). Through various interrelated approaches such as water quality analysis, equilibrium phosphorus concentration (EPC0) analysis, assessment of sediment characteristics, and land use characteristics, the linkages between the adsorption/desorption of P and age of the SWP was investigated. The SWP were categorized as 0-5 years (young), 5- 15 years (mid-age), and 15+ years (old) SWP. The spatial and temporal variability of SWP water column physiochemical and nutrient concentrations were compared in each pond category using ANOVA, and regression models were developed to observe how sediment characteristics and soluble reactive phosphorus (SRP) influence EPC0. Results suggested water column water quality varied across pond age. These variations were influenced by several interrelated factors. Older SWP exhibited higher total phosphorus and total nitrogen concentrations. Additionally, the study revealed that sediment in all SWPs acted as source of P. This means sediments in the SWP release P that was adsorbed back into the water column. Results indicated that the interaction between particle size and SRP influence EPC0 in old age pond and should be considered when considering P management strategies. These findings will help inform coastal SWP management strategies such as forebay sizing to specific pond conditions. By addressing the factors that influence P release, such as SRP levels and sediment, we can reduce the likelihood of harmful algal blooms and maintain better water quality in ponds.