Constructed stormwater pond and wetland for agricultural runoff treatment

Constructed stormwater pond–wetland systems (PWS) are an edge-of-field practices capable of mitigating agricultural runoff, yet long-term, year-round performance data remain limited. This study evaluated a PWS treating runoff and tile drainage from an 8.5 ha mixed-use basin in northeastern Wisconsin over six years (2018–2024), using year-round monitoring to capture cold-season conditions that could substantially contribute to annual export. The system reduced annual suspended sediment (SS) loads by 44–72 % (110–798 kg SS ha⁻¹ yr⁻¹) and total phosphorus (TP) by 21–48 % (0.3–1.9 kg P ha⁻¹ yr⁻¹), with particulate P reductions of 22–59 %. Dissolved P (DP) reductions were lower (9–27 %), particularly during tile drainage-dominated events. Peak runoff attenuation averaged 86 %, reducing mean event flows from 90 to 12 L s⁻¹ . The majority of annual SS and P loading (66–78 %) occurred outside the growing season, highlighting the importance of PWS for year-round nutrient retention. Particle size analysis showed influent SS was dominated by fine particles (d₅₀ = 2.8–4 µm), limiting sedimentation efficiency. Internal P loading from pond sediments was rare; only 35 % of pond DP measurements exceeded median tile DP, and none exceeded mean surface runoff DP. Results indicate PWS are effective for reducing SS and particulate P from surface runoff but are less effective for DP. Design improvements to enhance fine particle removal may further improve performance. PWS are a critical year-round best management practice for agricultural watersheds, particularly where winter and spring runoff dominate nutrient export.