Effects of wetland restoration on nitrate removal in an irrigated agricultural area: The role of in-stream and off-stream wetlands

•We assessed the efficiency of two different types of restored wetlands in NO3− removal.•In-stream wetlands showed higher NO3− concentrations than off-stream wetlands.•The influence of irrigation return flows could be a key factor for the wetlands performance.•We have developed a model to describe the NO3− removal for each type of wetland in each agricultural period.•This study demonstrated that both in-stream and off-stream wetlands can be adequate as sinks for runoff contaminants is feasible.

Eleven in-stream and five off-stream wetlands were restored in the southern portion of the Flumen River basin, an intensively irrigated agricultural area located in NE Spain, to evaluate their efficiency for nitrate removal and to assess the factors affecting their performance. Samples were taken during different periods of the year during 2011-2014 to evaluate the influence of agricultural activities during the two years following their completion. Nitrate concentration was significantly higher in the in-stream wetlands, showing a clear dilution effect caused by the inputs of the irrigation return flows. The patterns followed by the first-order nitrate removal rate constant were different for in-stream and off-stream wetlands and during irrigation and non-irrigation seasons. During non-irrigation seasons, the nitrate outflowing concentration was found to be negatively correlated (p < 0.01) to the first-order nitrate removal rate constant for in-stream wetlands, indicating that high nitrate inputs may restrict the effectiveness of the wetlands. Temperature and dissolved oxygen were also found to significantly influence the performance of off-stream wetlands during non-irrigation seasons, but only dissolved oxygen promoted the nitrate removal during the irrigation period, indicating the influence of the seasonal factor on nitrate and wetlands dynamics. The results of this study showed that although a longer time is required to achieve optimal wetland development, wetlands can be used as buffer zones that effectively remove nitrates. This study emphasizes the influence of the agricultural seasonality of the factors affecting nitrate removal in wetlands, expanding the information provided by similar studies and validating a model that is applicable to a wide range of agricultural, hydrological and seasonal conditions.