Diffuse nitrate (NO3) contamination from intense agriculture adversely impacts freshwater ecosystems, and can also result in nitrate concentrations exceeding limits set in drinking water regulation, when receiving surface waters are used for drinking water production. Implementation of near-natural mitigation zones such as reactive swales or wetlands have been proven to be promising measures to reduce nitrate loads in agricultural drainage waters. However, the behavior of these systems at low temperatures and its dependence on system design has not been well known until now. In this study, the behavior of a full-scale (length: 45 m) reactive swale treating drainage water from an agricultural watershed in Brittany (France), with high nitrate concentrations in the receiving river, was monitored for one season (6 months). As flow in this full-size field system is usually restricted to winter and spring months (December–May), it usually operates at low water temperatures of 5–10 °C. Tracer tests revealed shorter than designed retention times due to high inflows and preferential flow in the swale. Results show a correlation between residence time and nitrate reduction with low removal (<10%) for short residence times (<0.1 day), increasing to >25% at residence times >10 h (0.4 day). Performance was compared to results of two technical-scale reactive swales (length: 8 m) operated for 1.5 years with two different residence times (0.4 and 2.5 days), situated at a test site of the German Federal Environmental Agency in Berlin (Germany). Similar nitrate reduction was observed for comparable temperature and residence time, showing that up-scaling is a suitable approach to transferring knowledge gathered from technical-scale experiments to field conditions. For the design of new mitigation systems, one recommendation is to investigate carefully the expected inflow volumes in advance to ensure a sufficient residence time for effective nitrate reduction at low temperatures.

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