<p>It took several consecutive years of devastating droughts sweeping through Europe, causing substantial economic losses, for many to realise how urging it is to improve the water directives, making critical sectors like agriculture more resilient to a changing climate. Shrinking water supplies and growing demand further forced stakeholders to seek alternative sources, drawing their attention to projects previously considered economically unjustified. Therefore, water re-use and reclamation became one of the EU&#8217;s priorities fulfilling the ambitions of the European Green Deal to implement circular water management strategies. To facilitate the transition and support new legislation, in-depth research in the feasibility and environmental impacts of aquifer recharge with reclaimed wastewater is essential. The GROW project investigates this issue on multiple levels, among which the effect of reclamation of wastewater through aquifer recharge on local and regional scale groundwater levels.</p><p>At the experimental site in Kinrooi, Belgium, the groundwater levels are closely monitored with automatic submersible data loggers installed in 21 monitoring wells distributed on the investigated agricultural field and its vicinity. Data on water levels in the underlying Quaternary, porous aquifer are collected hourly and are verified through monthly manual measurements taken to ensure an unhindered operation of the infrastructure.</p><p>A distributed, transient-state flow model is used to simulate the groundwater table&#8217;s response to the effluent sub-irrigation at the desired rate. The model&#8217;s flexibility also allows making predictions of the aquifer behaviour under changing climatic conditions by augmenting the soil-water balance model with revised weather data. The model&#8217;s performance is tested against the high temporal resolution dataset obtained from the monitoring network. Attention is also paid to the experiment&#8217;s effect on the water levels in the adjacent hydrological network, while the effluent is partly rerouted from the hitherto used surface water discharge to the sub-irrigation system.</p><p>The data collected in our experiment is used to determine the capability of the aquifer to store and recover the reclaimed wastewater during drought periods. That would reduce the demand for traditional, inefficient surface irrigation and increase the climate resilience of the agricultural sector in Flanders. Together with data from similar projects carried out throughout Europe, our results can be used to facilitate long-expected EU legislation enabling circular water use. To support this process, we also investigate the impact of the re-use of treated wastewater for agriculture on groundwater quality and the public perception of this sensitive issue.</p>
<p>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.</p><p>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.</p><p>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.</p><p>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.</p>
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