Jan Willem Foppen

Urban aquifers are a potential alternative, with which to cover the rising demand for freshwater. However, they are commonly polluted by a wide range of anthropogenic substances, including rare earth elements (REEs). In this context, it is essential to investigate the presence and behaviour of these pollutants, in order to safely use urban groundwater. An example of these contaminants is Gadolinium, which can reach the environment after it has been used for medical purposes. Gadolinium anomalies were detected in hydraulically connected surface and subsurface water bodies in Barcelona, northeast Spain. A striking factor in this study, is that Gadolinium pollution in groundwater was less frequent and more attenuated than in surface water, suggesting that Gadolinium attenuation may be prevalent in the subsurface. Batch experiments were conducted and modelled to ascertain processes affecting Gadolinium in the sampled aquifers. Experimental results have (i) demonstrated that the concentration of dissolved Gadolinium decreased 90% in 24 days, and (ii) suggested that sorption plays a key role in Gadolinium attenuation. The modelling of the experiments provides essential parameters, with which to predict the behaviour of Gadolinium. Our results have significant implications for understanding the evolution of anthropogenic Gadolinium in aquifers in general, which is of paramount importance for determining where groundwater can be safely used after its natural or artificial infiltration.

Nanoplastics are detected in surface water, yet accurately quantifying their particle number concentrations remains a significant challenge. In this study, we tested the applicability of a gold-labelling method to quantify nanoplastics in natural organic matter (NOM) containing water matrices. Gelatin-coated gold nanoparticles (Au-gel NPs) form conjugates with nanoplastics via electrostatic interaction which produces peak signals which can be translated into particle number concentration using single-particle inductively coupled plasma–mass spectrometry (SP-ICP-MS). We used water samples with various NOM concentrations, with and without the addition of 1 × 107 particle–1 nanoplastics. Our results indicate that nanoplastics in low NOM samples (15 mg·C L−1), only 13–19% of added nanoplastics were successfully quantified. Further digestion to remove NOM yielded only 10% of spiked nanoplastics. This discrepancy in high NOM samples could likely be attributed to the competition between nanoplastics and NOM existing in the water sample to bind with Au-gel NPs. Our study highlights the suitability of the Au-gel labelling method for quantifying nanoplastics in low NOM water samples. Nevertheless, further optimization, including pre-digestion steps, is essential to apply this method for high NOM water samples effectively.

The use of artificial DNA (artDNA) in hydrological applications is becoming increasingly popular, either in dissolved form (dissolved artDNA) or encapsulated and protected by a layer (encDNA). DNA can be detected even at low concentrations and offers the ability to create numerous uniquely identifiable DNA labels, making it ideal for a wide range of multi‐tracer applications. A literature review revealed that in streams, the breakthrough curve of artDNA is visually similar to that of a conservative tracer in terms of time to rise, time to peak, and dispersion coefficient. In saturated porous or fractured media, the time of first arrivals and time to peak of artDNA are usually earlier than that of a conservative tracer, indicating size exclusion of both dissolved artDNA and encDNA. Transport in subsurface media can be described by one‐site or two‐site kinetic attachment. The recovery of artDNA in environmental systems is always less than 100% due to adsorption and decay. Although the processes responsible for both are known, yet they cannot be quantified or predicted in mass balance approaches. Despite these limitations, artDNA can be used in various hydrological applications in environmental studies and engineering. Finally, attention should focus on the use of rapid detection of DNA tracers in the field, on upscaling of DNA production, and on increasing the efficiency of the DNA encapsulation process.This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality Science of Water > Methods

Artificial DNA as a tracer in environmental applications has received increased attention in environmental science. In the last few years, we have been looking at the transport of silica encapsulated DNA particles (SiDNA), which we injected instantaneously or as a function of time in various saturated groundwater and surface water laboratory set-ups. These included batches, columns, sand tanks, open pipes, trenches, flumes, etc. The overarching aim of all these experiments was to understand SiDNA transport behaviour, to quantify the mass balance and to assess tracer-like capabilities of SiDNA. Our work indicated that in most applications, the shape of the breakthrough curve in terms of time to rise and time to peak were similar to the breakthrough curve of a conservative tracer. Specifically, SiDNA could be used to quantify dispersion in surface water transport, and to determine aquifer parameters, like hydraulic conductivity and porosity in multi-tracer experiments. However, this was accompanied by some uncertainty as in most applications, injected mass recoveries were less than 100% due to losses as a result of settling, river bed interactions, interactions with particulate matter (in surface water applications), straining, kinetic attachment and detachment (in groundwater applications). We conclude that SiDNA can be used when mass balance issues are relatively unimportant, for instance in case of complex flow path analyses or source tracking applications, whereby encapsulated artificial DNA with different DNA strands can be injected in several locations or can be added to the source. Currently, we think large scale field applications of SiDNA are still limited, due to required specific knowledge and analytical infrastructure, relatively high costs and limited SiDNA production scale. Once these issues are tackled, a truly unique multi-tracer will enrich the toolbox of hydrologists.

*Corresponding author: j.foppen@un-ihe.org© 2022 IWA Publishing2022

Weathered basement aquifers are vital sources of drinking water in Africa. In order to better understand their role in the urban water balance, in a weathered basement aquifer in Kampala, Uganda, this study installed a transect of monitoring piezometers, carried out spring flow and high-frequency groundwater level monitoring, slug tests and hydrochemical analyses, including stable isotopes and groundwater residence time indicators. Findings showed a typical weathered basement aquifer with a 20–50-m thickness. Groundwater recharge was 3–50 mm/year, occurring during sustained rainfall. Recharge to a deep groundwater system within the saprock was slow and prolonged, while recharge to the springs on the valley slopes was quick and episodic, responding rapidly to precipitation. Springs discharged shallow groundwater, mixed with wastewater infiltrating from onsite sanitation practices and contributions from the deeper aquifer and were characterised by low flow rates (< 0.001 m³/s), low pH (3), and residence times of −5 m²/s), lower nitrate values (3), pH 6–6.5 and longer residence times (40–60 years). Confined groundwater conditions in the valleys were created by the presence of clay-rich alluvium and gave rise to artesian conditions where groundwater had lower nitrate concentrations. The findings provide new insights into weathered basement aquifers in the urban tropics and show that small-scale abstractions are more sustainable in the deeper groundwater system in the valleys, where confined conditions are present.