Hydraulics and flow modelling of water treatment wetlands constructed on peatlands in Northern Finland

In this study, we evaluated flow structure, effective flow area (Aeff) and effective porosity (θeff) in three peatlands using the stable isotope 18O/16O ratio and tracer tests. We also applied the readily available groundwater modelling MODFLOW code for wetland flow modelling and simulated in one study site how the hydraulic performance of the wetland will be improved by changing the design of the distribution ditch. Preferential flow paths occurred in all three studied wetlands and Aeff varied from 40% to 90% of total wetland area while θeff was 0.75–0.99. Constructed flow models accurately simulated the hydraulic head across wetlands (r2 = 0.95–0.99). Similarities between the flow models and the stable isotope distributions observed in this study suggest possibilities in using MODFLOW to design peatlands. The improvement of the inlet ditch configuration (ditch length/wetland width > 0.45) can prevent or reduce short-circuiting and dead zones in peatlands treating wastewater.