Re-use of treated wastewater for irrigation and groundwater recharge: environmental impact assessment based on tracer method at the experimental site in Kinrooi, Belgium

Re-use of treated wastewater is receiving increasing attention as method to reduce water stress resulting from population growth, socio-economic development and climate change. In 2018, the European Commission issued a policy strategy entailing minimum water quality requirements for water re-use for agriculture and aquifer recharge. However, the environmental impact of such solution is yet to be determined.The VUB in collaboration with private and public sector partners set up a field experiment in Kinrooi (Belgium) in which the effects of re-using treated domestic wastewater for sub-irrigation of an agricultural field are monitored. This is an interdisciplinary project which includes analyses of the effects on water quality and quantity in the subsurface saturated and unsaturated zone and nearby surface water, the effects on crops as well as research on the public perception.Within this project, one of the aims is to create an advection-dispersion groundwater transport model to investigate how the chemical composition of the shallow groundwater would change after the treated domestic wastewater is applied through sub-irrigation. Observation data of tracers of the re-used water in the groundwater are needed to calibrate the transport model. Therefore, it is critical to choose a suitable tracer, allowing to unambiguously tell apart the effluent and groundwater end members. Literature suggests the use of chemical properties such as stable isotopes and Cl/Br ratios to use as wastewater tracers. Stable isotopes of hydrogen and oxygen are investigated, but the focus is currently on the use of Cl/Br ratios, which shows promising results. The use of this ratio as tracer is based on the close to ideal conservative behaviour of bromide and chloride ions in water caused by their small size and hydrophilic characteristics. This implies that physical processes such as dilution and evaporation happening in the environment influence the absolute concentrations of the ions but leaves their ratio constant. At the moment, 21 monitoring wells are installed on the field of which 9 monitoring wells have been sampled for data on Cl/Br tracers.In general, the results indicate that finding a suitable tracer is not straightforward because chemical and isotopic compositions of the groundwater and treated wastewater are often similar. Therefore, the research continues to focus on improving the analytical methods used to analyse the currently used tracers (e.g. Cl/Br ratio and stable isotopes) and on the selection of other tracers such as anthropogenic organic compounds (e.g. pharmaceuticals and artificial sweeteners) to quantify the influence of the effluent end member and to enhance modelling performance.