Land characterisation for soil-based constructed wetlands: Adapting investigation methods to design objectives

Abstract

Buffer zones between wastewater treatment plants and receiving water bodies have recently gained interest in France. These soil-based constructed wetland (SBCW) systems receive treated wastewater and may have various designs aiming to mimic ‘natural’ wetlands. Research is needed to assess the treatment efficiency of such systems. To this aim, a comprehensive study is carried out to understand the fate of water, conventional pollutants (suspended solids, organic carbon, ammonium, and phosphates), micro-pollutants that are refractory to up-stream biological treatment, and pathogens. Special attention must be paid to understand the fate of the infiltrated treated wastewater in the field where systems are built, in order to ensure their long-term operation and to protect the underground water bodies. To address these issues, we propose a comprehensive strategy combining successive steps using either geological or hydrological methods. It provides the following prominent information for a proper design of SBCW: (1) the number and the location of the different soil layers; (2) the infiltration capacity of each layer; (3) the water table depth. The paper presents a successful application of the proposed strategy to evaluate the fate of the infiltrated treated wastewater before the implementation of a semi-industrial scale SBCW in Bègles (France). Moreover, methods used for long-term efficiency assessment are introduced.