Epsilon Archive for Student Projects

Abstract

Constructed wetlands are widely promoted as edge‑of‑field measures to reduce nutrient and sediment losses from agricultural catchments, yet their event‑scale retention performance remains poorly defined. Conventional monitoring approaches, based on grab sampling or flow‑proportional sampling, are costly and typically lack the temporal resolution. That is needed to capture the short‑lived, high‑concentration pulses that can drive the bulk of annual nutrient transport.
In this study, high‑frequency (HF) in situ sensors measured turbidity, fluorescent dissolved organic matter (fDOM) and nitrate and were deployed at the inlet and outlet of a small phosphorus retention wetland in a tile‑drained clay catchment in east‑central Sweden. Fortnightly grab samples were collected and used to establish site‑specific turbidity–total phosphorus (TP) and fDOM–dissolved organic carbon (DOC) regression relationships, enabling continuous concentration and load estimates throughout the monitored period. Six events, spanning early winter to spring snowmelt, were identified and analysed with respect to hydraulic loading, inlet–outlet concentration dynamics, concentration‑discharge hysteresis, and hydraulic and concentration lags.
Turbidity proved a strong proxy for TP (R² = 0.93, n = 13), while the fDOM–DOC relationship was weaker (R² = 0.23, n = 15) and less reliable for load estimation. The wetland acted as a TP sink during four of the six events, while two events showed net P export, indicating that retention performance varied considerably between events, although the event definition was not representative for all actual field conditions. Hydraulic lags (Q-lag) between inlet and outlet peaks deviated from the theoretical water residence time (τth) in several events, indicating short‑circuiting and variable effective wetland volume. Concentration‑discharge hysteresis differed between inlet and outlet and between events, revealing distinct differences in source dynamics and internal sediment mobilisation. Nitrate monitoring could not be completed quantitatively due to sensor drift.
These findings demonstrate that HF sensor‑based monitoring can resolve event‑scale processes in agricultural wetlands that remain invisible to conventional sampling, and that hydraulic loads (HL) and effective volume (Veff) are critical determinants of short‑term TP retention performance.