Large-scale tank experiments simulating soil aquifer treatment – Assessing the attenuation potential of emerging organic compounds and nutrients during managed aquifer recharge

Managed aquifer recharge (MAR) via infiltration basins to replenish aquifers is an important part of the integrated water resource management (IWRM) toolbox. Soil aquifer treatment managed aquifer recharge (SAT-MAR) basins are used to improve water quality during infiltration. However, SAT-MAR can also pose the risk of contaminating the aquifer, by infiltrating treated wastewater effluent, which may still contain high concentrations of e.g., nutrients (N and P) and emerging organic compounds (EOCs), e.g., pharmaceuticals. In order to assess these potential risks and to be able to take measures, it is important to understand the SAT-MAR system. In this context, it is necessary to study the degradation and sorption capacity of natural conditions as well as modified regimes, e.g., by incorporating reactive layers. While laboratory column experiments are widely used and provide detailed process understanding under controlled conditions, transferring the results to field size and conditions remains challenging. On the other end, in-situ field experiments give great insights into real systems while they often study only one SAT-MAR site under distinct environmental settings which hinders to transfer knowledge to other sites. One way to bridge this gap between the two scales is through large tank experiments. However, there are few such large tank experiments in research on MAR that seek to combine the representativeness of in-situ experiments with the controlled characteristics of laboratory column studies.

Therefore, we designed and conducted a large-scale tank experiment consisting of three tank replicates for the purpose of analyzing SAT infiltration basins using treated wastewater effluent under controlled conditions. The three tanks are packed with fine sand and comprise a vadose zone as well as a saturated zone. The vadose zone of two tanks incorporates a mixed layer of biochar/fine sand as reactive layer, while the third tank consists solely of fine sand and acts as reference. The tanks are equipped with various sensors (high resolution oxidation-reduction potential, water pressure, soil moisture content, electrical conductivity, water pressure, and temperature). To be able to measure the concentration of solutes along the flow path, several suction cups and small-diameter wells allow sampling in the vadose zone and saturated zone, respectively. The infiltrating water in this study is treated wastewater while the groundwater flowing continuously in the lower part of the tank consists of local groundwater. A set of six EOCs (carbamazepine, diclofenac, ibuprofen, naproxen, gemfibrozil, and triclosan) act as model substances as they cover a wide range of physicochemical parameters and degradation potentials.

Preliminary results are presented on the influence of operational regimes and reactive barriers on the attenuation of EOCs, as well as on nutrients, dissolved organic carbon, and major ions in both the vadose zone and groundwater.

Acknowledgement: This work is presented within the framework of the project MARSoluT (www.marsolut-itn.eu), a four-year Marie Skłodowska-Curie Actions (MSCA) Innovative Training Network (ITN) funded by the European Commission (Grant Agreement 814066).