Bridging constructed wetlands and riparian buffer zones: A hybrid subsurface flow system for wastewater treatment

We developed an Intermittent Horizontal Subsurface Flow (IHSSF) system using WWTP effluent to promote a nature-based solution for tertiary treatment in a riparian forest. The objectives were to assess the nutrient removal rates, the impact on groundwater table and soil (0–40 cm depth), and evaluate the overall system performance. A treated zone was established alongside a control zone, with the effluent applied from the hillslope to the near-stream zone, constituting two sampling areas. We found relevant nutrient removal rates for NH₄ + -N (39–65 %) and SRP (45–75 %), moderate for NO₃ − -N (18–34 %) and variable for DOC (−0.02 % - 19 %). The groundwater table showed a temporary increase in electrical conductivity close to the effluent source, though SRP increased up to 1.5 mg/L in the treated zone. Regarding soil, the effluent caused minor pH increases and a moderate increase in soil electrical conductivity attributed to the salts of the WWTP effluent. The exchangeable sodium percentage increased but it remained under the sodic threshold. In addition, there was an increase of NH 4 + -N in topsoil. These findings provide valuable insights for applying this approach in riparian zones of Mediterranean regions, where WWTP effluent discharges into low-flow and intermittent streams represent a huge ecological threat. • Innovative nature-based solution for WWTP effluent tertiary treatment. • Study confirms riparian soils as an effective compartment to receive WWTP effluent. • Effective NH₄ + -N and SRP removal; moderate NO₃ − -N and variable DOC removal. • Limited groundwater impact in the riparian zone where WWTP effluent was discharged. • Minor soil impact: stable pH, low EC, and ESP below sodic threshold.