Water quality dynamics of irrigation reservoirs in series at a production plant nursery

The dynamics of sediment, phosphorus, and nitrogen were characterized in a treatment train consisting of a vegetated channel followed by two irrigation reservoirs (RR1 and RR2) at a retail/production plant nursery in South Carolina’s Piedmont Ecoregion. Water quality and hydrologic data (flow rates, rainfall, irrigation events) were collected to evaluate seasonal variation in treatment capacity. Nominal hydraulic retention times (HRTs) were generally longer in winter and shorter during active production months, reducing treatment capacity when irrigation flows and nutrient loads were highest. Irrigation dominated hydrology in summer and fall, while rainfall was the primary driver in winter and spring. Concentrations of total suspended solids (TSS), phosphate (PO₄-P), and dissolved inorganic nitrogen (DIN) declined sequentially from the vegetated channel through RR1 to RR2. Average removal rates in RR1 and RR2 were 189 ± 106 and 32.9 ± 18.7 g m⁻² d⁻¹ for TSS, 580 ± 310 and 58.7 ± 30.9 mg m⁻² d⁻¹ for DIN, and 51.3 ± 24.4 and 9.19 ± 6.17 mg m⁻² d⁻¹ for PO₄-P, respectively. Removal was highest in spring and summer and lowest in winter, when internal loading and reduced microbial activity likely limited performance. Despite its larger size, RR2 showed more variable treatment, suggesting that treatment efficiency is influenced more by hydraulic design and influent loading than basin size alone. These findings support the dual role of irrigation reservoirs in water quality improvement and water security, emphasizing the need for design strategies that optimize both treatment and storage functions. • Retention reservoir hydraulic retention time longer in winter and shorter during production months • Summer and fall hydrology regulated by irrigation • Winter and spring hydrology regulated by rainfall • Sequential declines in total suspended solids, nitrogen, and phosphorus as water flowed through reservoir series • Total suspended solids, nitrogen, and phosphorus removal highest in spring and summer