Effects of wetland construction on water quality in a semi-arid catchment degraded by intensive agricultural use

Many factors can influence the improvement of water quality in surface-flow constructed wetlands (SFW). To test if water quality was improved, especially in nutrient and salt content, after passage through SFW, 11 wetland plots of various sizes (50, 200, 800 and 5000 m2) were established within constructed wetlands on agricultural soils in the Ebro River basin (NE Spain) that had been affected by salinization. A set of 15 water quality parameters (e.g., nutrients, salts, sediments, and alkalinity) was obtained from samples collected at the inflow and outflow of the wetlands during the first 4 years after the wetlands were constructed. NO3-N retention rates were as high as 99% in the largest (5000 m2) wetlands. After 4 years, total phosphorus was still being released from the wetlands but not salts. Over the same period, in small wetlands (50, 200, and 800 m2), retention rate relative to the input of NO3-N increased from 40% to almost 60%. Retention of NO3-N amounted to up to 500 g N m−2 per year, for an average load concentration at inflow of ∼20 mg l−1. Release of Na+ declined from 16% to 0–2% by volume, for an average load concentration at inflow of ∼70 mg l−1. At the current retention rate of NO3-N (76–227 g m−2 per year), 1.5–4% of the catchment should be converted into wetlands to optimize the elimination of NO3-N.