The fate and toxicity of nanoplastics (NPs) in the environment is largely determined by their stability. We explored how water composition, nanoplastic size, and surface carboxyl group density influenced the aggregation of polystyrene (PS) NPs in fresh water. Unfunctionalized 200, 300, 500, and 1000 nm PS NPs and 310 nm carboxylated PS NPs with carboxyl group densities of 0.35 and 0.6 mmol g⁻¹ were used to simulate pristine and aged NPs. Natural water matrices tested in this study include synthetic surface water (SSW), water from the Schie canal (Netherlands) and tap water. Suwannee River Natural Organic Matter (SRNOM) was included to mimic organic matter concentrations. In CaCl₂, we found PS NPs are more stable as their size increases with the increase of the critical coagulation concentration (CCC) from 44 mM to 59 mM and 77 mM for NP sizes of 200 nm, 300 nm and 500 nm. Conversely, 1000 nm PS NPs remained stable even at 100 mM CaCl₂. Increasing the carboxyl group density decreased the stability of NPs as a result of the interaction between Ca²⁺ and the carboxyl group. These results were consistent with the mass of Ca²⁺ adsorbed per mass of NPs. The presence of SRNOM decreased the stability of PS NPs via particle bridging facilitated by SRNOM. However, in SSW, Schie water and tap water with low divalent cation concentrations, the hydrodynamic size of PS NPs did not change, even at prolonged durations up to one week. We concluded that PS NPs are unlikely to aggregate in water with low divalent cation concentrations, like natural freshwater bodies. Ecotoxicologists and water treatment engineers will have to consider treating PS NPs as colloidally stable particles as the lack of aggregation in fresh surface water bodies will affect their ecotoxicity and may pose challenges to their removal in water treatment.

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Identifying and determining hydraulic parameters of physically heterogeneous aquifers is pivotal for flow field analysis, contaminant migration and risk assessment. In this research, we applied a novel uniquely sequenced DNA tagged superparamagnetic silica microparticles (SiDNAmag) to quantify hydraulic parameters and associated uncertainties of a heterogeneous sand tank. In the sand tank with lens shaped heterogeneity, we conducted three sets of multi – point injection experiments in unconsolidated (1) homogeneous (zone 0), (2) heterogeneous with a no-conductivity-zone (zone 1), and (3) heterogeneous with a high-conductive-zone (zone 2). From the breakthrough curves (BTC), we estimated the parameters distributions of hydraulic conductivity (k), effective porosity (n_e), longitudinal dispersivity (α_L), transverse vertical (α_TV), and transverse horizontal dispersivities (α_TH) applying Monte Carlo simulation approach for BTC fitting. The estimated parameters and associated uncertainties for each of the heterogeneous sections were further statistically compared (distribution non-specific Mann Whitney U test) these parameter distributions with parameter distributions estimated from the conservative salt tracer. While the time of arrival and time to peak concentration of SiDNAmag and salt in effluent were comparable, peak concentration of SiDNAmag was 1–3 log reduced as compared to the salt tracer due to first order kinetic attachment. Nonetheless, the parameters and associated uncertainty distributions (5 %–95 %) of K, n_e, α_L, α_TV, and α_TH, determined from SiDNAmag BTCs were statistically equivalent to the salt tracer in all three experiment systems. Through our experimental and modelling approach, our work demonstrated that in a coarse to very coarse grain sand medium, with lens shaped heterogeneity, the uniquely sequenced SiDNAmag were a promising tool to identify heterogeneity and determine hydraulic parameters and associated uncertainty distributions.

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Sand filtration systems (SF) are a well-established approach in ensuring the availability of clean water. Understanding the transport properties of colloidal particles within SF systems is of paramount importance for optimizing their performance. This study investigated the potential utilization of silica-encapsulated DNA particles, equipped with a magnetic core to enhance particle separation and quantification efficiency (SiDNAMag). These particles were evaluated as tracers for delineating complex pathways and conducting source tracking within sand filtration (SF) systems for particulate substances. The study focused on exploring the sensitivity of SiDNAMag to solution chemistry, while elucidating the underlying mechanisms governing their transport and retention in sand filtration systems. Laboratory columns and HYDRUS-1D modeling were employed to analyze a range of water chemistry solutions, encompassing NaCl, NaHCO₃, CaCl₂, and MgCl₂, with ionic strengths ranging from 0.1 mM to 20 mM. The results revealed that the transport of DNA-tagged silica particles could be described by a first-order kinetic attachment and detachment rate coefficient. Elevated ionic strengths consistently led to increased particle adhesion and decreased rates of detachment. The sticking efficiencies of SiDNAMag particles exhibited a range of 0.7 to 1. The remarkable adhesive effectiveness can be ascribed to the comparatively low negative charge exhibited by SiDNAMag particles. This leads to the creation of unstable colloids and encourages the aggregation of these colloidal particles, thereby limiting the potential application of these particles as a tracer. In conclusion, this work underlines the potential of SiDNAMag particles as a potential subsurface tracer. However, further research is warranted to investigate strategies for reducing the interaction between these particles and sand, particularly in response to the chemistry of the infiltrated water.

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In this study, we focused on the 3D dispersion of colloids. To our knowledge, we were the first to do so. Thereto, we injected silica encapsulated DNA tagged superparamagnetic particles (SiDNAmag) in a homogeneous coarse grain sand tank. At four downstream locations, SiDNAmag concentrations were determined as a function of time. Longitudinal and transverse dispersivity values and associated uncertainties of SiDNAmag were determined using Monte Carlo modelling approach. The parameter associated uncertainties of hydraulic conductivity as well as of the effective porosity estimated from SiDNAmag breakthrough curves were statistically similar to those estimated from salt tracer breakthrough curves. Further, the SiDNAmag dispersivity uncertainty ranges were then statistically compared with the salt tracer (NaCl, and fluorescein) dispersivities. Our results indicated that time to rise, time of peak concentration and shape of the breakthrough curves of SiDNAmag were similar to those of the salt tracer breakthrough curves. Despite the size difference between the salt tracer molecules and SiDNAmag, size exclusion did not occur, probably due to the large pore throat diameter to SiDNAmag diameter ratio. The median longitudinal dispersivity (α_L) of salt tracer and SiDNAmag were 4.9 and 5.8 × 10⁻⁴ m, respectively. The median ratio of horizontal and vertical transverse dispersivities to α_L, (α_TH /α_L and α_TV /α_L, respectively), for salt tracer and SiDNAmag ranged between 0.52 and 0.56. Through the statistical tests, we concluded that the longitudinal and traverse dispersivities of SiDNAmag were not statistically significantly different from salt tracer in 3 dimensions and could be used to characterize the dispersive properties of the medium we used. Our work contributes to a better understanding of 3D dispersion of SiDNAmag in saturated porous media.

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We investigated the applicability of Silica encapsulated, superparamagnetic DNA particles (SiDNAmag) in determining aquifer hydraulic parameters at different ionic strengths (1 mM, 5 mM, and 20 mM phosphate buffer) of injection suspension. Thereto, in a homogeneous, unconsolidated sand tank we pulse - injected two uniquely sequenced SiDNAmag at two injection points. At 0.5 m and 0.8 m downstream from the injection points, we measured the concentration of SiDNAmags at three vertically distributed and two horizontally distributed sampling locations. We estimated the hydraulic parameter distributions from the SiDNAmag breakthrough curves through a Monte – Carlo approach and compared the parameter distributions with salt tracer breakthrough curves. Our results indicated that at all the ionic strengths, the times of peak concentrations, and the shapes of the breakthrough curves were similar to the salt tracer. As compared to the salt, a 1 – 3 log units reduction in the maximum effluent concentration of SiDNAmag was due to kinetic attachment. The attachment rate reduced from 1 mM to 5 mM phosphate buffer possibly due to competitive adsorption of phosphate onto the favourable attachment sites. SiDNAmag attachment rate further increased in 20 mM buffer suspension, possibly due to the compression of electric double layer and reduction in energy barrier for attachment. The parameter distributions of hydraulic conductivity (k), effective porosity (n_e), longitudinal dispersivity (α_L), vertical transverse dispersivity (α_TV /α_L) and horizontal transverse dispersivity (α_TH /α_L) estimated from the SiDNAmag and the salt tracer breakthrough curves were statistically similar. Our work contributes to the applicability of colloidal SiDNAmags for determining hydraulic parameters at different ionic strength conditions.

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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.

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Green infrastructure drainage systems are innovative treatment units that capture and treat stormwater. Unfortunately, highly polar contaminants remain challenging to remove in conventional biofilters. To overcome treatment limitations, we assessed the transport and removal of stormwater vehicle-related organic contaminants with persistent, mobile, and toxic (in short: PMTs) properties, such as 1H-benzotriazole, NN'-diphenylguanidine, and hexamethoxymethylmelamine (PMT precursor), using batch experiments and continuous-flow sand columns amended with pyrogenic carbonaceous materials, like granulated activated carbon (GAC) or wheat-straw derived biochar. Our results indicated that all investigated contaminants were subjected to nonequilibrium interactions in sand-only and geomedia-amended columns, with kinetic effects upon transport. Experimental breakthrough curves could be well described by a one-site kinetic transport model assuming saturation of sorption sites, which we inferred could occur due to dissolved organic matter fouling. Furthermore, from both batch and column experiments, we found that GAC could remove contaminants significantly better than biochar with higher sorption capacity and faster sorption kinetics. Hexamethoxymethylmelamine, with the lowest organic carbon-water partition coefficient (K_OC) and largest molecular volume among target chemicals, exhibited the lowest affinity in both carbonaceous adsorbents based on estimated sorption parameters. Results suggest that sorption of investigated PMTs was likely driven by steric and hydrophobic effects, and coulombic and other weak intermolecular forces (e.g., London–van der Waals, H-bonding). Results from extrapolating our data to a 1-m depth geomedia-amended sand filter suggested that GAC and biochar could enhance the removal of organic contaminants in biofilters and last for more than one decade. Overall, our work is the first to study treatment alternatives for NN'-diphenylguanidine and hexamethoxymethylmelamine, and contributes to better PMT contaminant removal strategies in environmental applications.

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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 10 ⁷ particle ^–1 nanoplastics. Our results indicate that nanoplastics in low NOM samples (,1 mg·C L ¹) could be successfully quantified. However, in high 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.

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Particle tracers are sometimes used to track sources and sinks of riverine particulate and contaminant transport. A potentially new particle tracer is ~200 nm sized superparamagnetic silica encapsulated DNA (SiDNAFe). The main objective of this research was to understand and quantify the settling and aggregation behaviour of SiDNAFe in river waters based on laboratory settling experiments. Our results indicated, that in quiescent conditions, more than 60% of SiDNAFe settled within 30 h, starting with a rapid settling phase followed by an exponential-like slow settling phase in the three river waters we used (Meuse, Merkske, and Strijbeek) plus MilliQ water. In suspensions of 1000× higher particle concentrations, the hydrodynamic diameter (D_h-DLS) of SiDNAFe increased over time, with its polydispersity index (PDI) positively correlated with particle size. From these observations, we inferred that the rapid SiDNAFe settling was mainly due to homo-aggregation and not due to hetero-aggregation (e.g., with particulate matter present in river water). Incorporating a first-order mass loss term which mimics the exponential phase of the settling in quiescent conditions seems to be an adequate step forward when modelling the transport of SiDNAFe in river injection experiments. Furthermore, we validated the applicability of magnetic separation and up-concentration of SiDNAFe in real river waters, which is an important advantage for carrying out field-scale SiDNAFe tracing experiments.

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Slow Sand Filtration is popular in drinking water treatment for the removal of a wide range of contaminants (e.g., particles, organic matter, and microorganisms). The Schmutzdecke in slow sand filters (SSFs) is known to be essential for pathogen removal, however, this layer is also responsible for increased head loss. Since the role of deeper layers in bacteria and virus removal is poorly understood, this research investigated the removal of E.coli WR1 and PhiX 174 at different depths of a full-scale SSF. Filter material from top (0–5 cm), middle (5–20 cm) and deep (20–35 cm) layers of an established filter was used in an innovative experimental set-up to differentiate physical-chemical and biological removal processes. In the analysis, we distinguished between removal by biological activity, biofilm and just sand. In addition, we modelled processes by a one-side kinetic model. The different layers contributed substantially to overall log removal of E.coli WR1 (1.4–1.7 log₁₀) and PhiX 174 (0.4–0.6 log₁₀). For E.coli WR1, biological activity caused major removal, followed by removal within biofilm and sand, whereas, removal of PhiX 174 mainly occurred within sand, followed by biofilm and biological activity. Narrow pore radii in the top layer obtained by micro-computed tomography scanner suggested enhanced retention of bacteria due to constrained transport. The retention rates of E.coli WR1 and PhiX 174 in top layer were four and five times higher than deeper layers, respectively (k_ret 1.09 min⁻¹ vs 0.26 min⁻¹ for E.coli WR1 and k_ret 0.32 min⁻¹ vs of 0.06 min⁻¹ for PhiX 174). While this higher rate was restricted to the Schmutzdecke alone (top 5 cm), the deeper layers extend to around 1 m in full-scale filters. Therefore, the contribution of deeper layers of established SSFs to the overall log removal of bacteria and viruses is much more substantial than the Schmutzdecke.

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In the terrestrial environment, interactions between natural organic matter (NOM) and colloids can lead to the formation of an environmental corona around colloids, influencing their transport behaviour and, ultimately, their ecotoxicity. We used a synthetically designed colloid tagged with DNA (DNAcol) as a surrogate for natural colloids and investigated its transport in saturated sand columns. We varied the concentrations of NOM and ionic strength (CaCl₂), to better understand the transport and release of DNAcol in porous media under both steady and transient porewater chemistry conditions. In addition, we aimed to understand the main factors that control deposition and release of DNAcol under tested conditions. To induce transient chemistry, we replaced the injection solution containing NOM and/or CaCl₂ with Milli-Q water. The results showed that the deposition rate of DNAcol was inversely proportional to the concentration of NOM. The deposition rate increased significantly even under low ionic strength (CaCl₂) conditions of tested conditions. Notably, the influence of NOM on the transport of DNAcol was most pronounced at the lowest range of [Ca²⁺]/DOC ratios, and the attachment of DNAcol to the sand grains was negligible. Moreover, the results showed while the DLVO theory captured the general trend of experimental results, it significantly underestimated the deposition of DNAcol in the presence of CaCl₂. Under transient porewater chemistry conditions, colloid remobilization was observed upon flushing the column with Milli-Q water, leading to a secondary peak in the breakthrough curves. We observed that under transient porewater chemistry conditions, when the ionic strength of the solution was 10 mM, the magnitude of the remobilization peak was more significant compared to conditions with 1 mM ionic strength. Our work emphasized the complex interplay between water quality on the one hand and deposition and release of colloidal matter in saturated porous media on the other hand.

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Recently, superparamagnetic silica encapsulated DNA microparticles (SiDNAFe) were designed and in various experiments used as a hydrological tracer. We investigated the effect of bed characteristics on the transport behaviour and especially the mass loss of SiDNAFe in open channel injection experiments. Hereto, a series of laboratory injection experiments were conducted with four channel bed conditions (no sediment, fine river sediment, coarse sand, and goethite-coated coarse sand) and two water qualities (tap water and Meuse water). Breakthrough curves (BTCs) were analysed and modelled. Mass loss of SiDNAFe was accounted for as a first-order decay process included in a 1-D advection and dispersion model with transient storage (OTIS). SiDNAFe BTCs could be adequately described by advection and dispersion with or without a first-order decay process. SiDNAFe mass recoveries exhibited a wide range, varying from 50% to 120% from sediment-free conditions to coarse (coated) sediment. In 6 out of 8 cases, SiDNAFe mass recovery was complete. Retention of SiDNAFe was 1–2 orders of magnitude greater than gravitational settling rates, as determined in Tang et al. (Hydrological Processes, e14801, 2023). We reason this was due to grain-scale hyporheic flows and coupled water-sediment-particle interactions. The dispersive behaviour of SiDNAFe generally mimicked that of NaCl tracer. We concluded that SiDNAFe can be used in tracing experiments. However, water quality and sediment characteristics may affect the fate of SiDNAFe in river environments. SiDNAFe is a promising tool for particulate multi-tracing in large rivers.

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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 (<5), high nitrate values (61–190 mg/L as NO₃), and residence times of <30 years. The deeper groundwater system occurred in the saprolite/saprock, had low transmissivity (< 1 × 10⁻⁵ m²/s), lower nitrate values (<20 mg/L as NO₃), 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.

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In recent years, DNA-tagged silica colloids have been used as an environmental tracer. A major advantage of this technique is that the DNA-coding provides an unlimited number of unique tracers without a background concentration. However, little is known about the effects of physio-chemical subsurface properties on the transport behavior of DNA-tagged silica tracers. We are the first to explore the deposition kinetics of this new DNA-tagged silica tracer for different pore water chemistries, flow rates, and sand grain size distributions in a series of saturated sand column experiments in order to predict environmental conditions for which the DNA-tagged silica tracer can best be employed. Our results indicated that the transport of DNA-tagged silica tracer can be well described by first order kinetic attachment and detachment. Because of massive re-entrainment under transient chemistry conditions, we inferred that attachment was primarily in the secondary energy minimum. Based on calculated sticking efficiencies of the DNA-tagged silica tracer to the sand grains, we concluded that a large fraction of the DNA-tagged silica tracer colliding with the sand grain surface did also stick to that surface, when the ionic strength of the system was higher. The experimental results revealed the sensitivity of DNA-tagged silica tracer to both physical and chemical factors. This reduces its applicability as a conservative hydrological tracer for studying subsurface flow paths. Based on our experiments, the DNA-tagged silica tracer is best applicable for studying flow routes and travel times in coarse grained aquifers, with a relatively high flow rate. DNA-tagged silica tracers may also be applied for simulating the transport of engineered or biological colloidal pollution, such as microplastics and pathogens.

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We investigated the transport and retention kinetics of silica encapsulated – silica core double stranded DNA particles (SiDNASi) through 15cm saturated quartz sand columns as a function of a wide range of colloid injection concentrations (C0 = 8.7×102 - 6.6×108 particles ml-1). The breakthrough curves (BTCs) exhibited an overall 2-log increase of maximum relative effluent concentration with increasing C0. Inverse curve fitting, using HYDRUS1D, demonstrated that a 1-site first order kinetic attachment (katt) and detachment (kdet) model sufficed to explain the C0-dependent SiDNASi retention behaviour. With increasing C0, katt log-linearly decreased, which could be expressed as an overall decrease in the single-collector removal efficiency (ƞ). The decrease in ƞ was likely due to increased electrostatic repulsion between aqueous phase- solid phase colloids, formation of shadow zones downstream of deposited colloids and removal of weakly attached colloids from the solid phase (quartz sand) attributing to increased aqueous phase-solid phase intercolloidal collisions as a function of increasing SiDNASi concentration. Our results implied, firstly, that the aqueous phase colloid concentration should be carefully considered in determining colloidal retention behaviour in saturated porous media. Secondly, colloidal transport and retention dynamics in column studies should not be compared without considering colloid influent concentration. Thirdly, our results implied that the applicability of SiDNASi as a conservative subsurface tracer was restricted, since transport distance and retention was colloid concentration dependent. However, the uniqueness of the DNA sequences in SiDNASi imparts the advantage of concurrent use of multiple SiDNASi for flow tracking or porous media characterization.@en

Abstract: This study investigated the occurrence and seasonal variation in concentrations of emerging organic contaminants (EOCs) in shallow groundwater underlying two peri-urban areas of Bwaise (highly urbanised) and Wobulenzi (moderately urbanised) in Uganda. Twenty-six antibiotics, 20 hydrocarbons, including 16 polycyclic aromatic hydrocarbons (PAHs), and 59 pesticides were investigated. Ampicillin and benzylpenicillin were the most frequently detected antibiotics in both areas, although at low concentrations to cause direct harm to human health, but could lead to a proliferation of antibiotic resistance genes. The most frequently detected hydrocarbons in Bwaise were naphthalene and xylene while anthracene and fluoranthene were the most frequent in Wobulenzi, also at low concentrations for ecological impact at long-term exposure. Molecular diagnostic ratios indicated pyrogenic and pyrolytic sources of PAHs in both areas. Cypermethrin (for vermin control) was the most frequent pesticide in Bwaise while metalaxyl (attributed to agriculture) was the most frequent in Wobulenzi. Banned organochlorines (8) were also detected in both areas in low concentrations. The pesticide concentrations between the two areas significantly differed (Z = − 3.558; p < 0.01), attributed to contrasting land-use characteristics. In Wobulenzi (wet season), the total pesticide concentrations at all the locations exceeded the European Community parametric guideline value while 75% of the detected compounds exceeded the individual pesticide guideline value. Thus, the antibiotic and pesticide residues in shallow groundwater underlying both Bwaise and Wobulenzi pose potential adverse ecological effects at long-term exposure. Monitoring of EOCs in both highly and moderately urbanised catchments should be strengthened towards mitigating associated risks. Graphical abstract: [Figure not available: see fulltext.]

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Socio-institutional factors are poorly addressed in the risk assessment of groundwater contamination. This paper contributes to the development of a socio-institutional assessment framework based on a case study of contamination by on-site sanitation (OSS) in an informal settlement of Bwaise (Kampala, Uganda). We conducted a snapshot survey of the recent extent of groundwater contamination by OSS using microbial and hydro-chemical indicators. Through transition arenas and key informant interviews, we investigated the socio-institutional drivers of the contamination. Overall, 14 out of the 17 sampled groundwater sources tested positive for Escherichia coli during the wet season. Nitrate concentrations at four sources exceeded the World Health Organization guideline value (50 mg/L), attributed to OSS. Despite the high contamination, the community highly valued groundwater as an alternative to the intermittent municipal water supply. We deduced six drivers of groundwater contamination, including land-use management, user attributes, governance, infrastructure management, groundwater valuation, and the operating environment (“LUGIVE”). Qualitative indicators for each of the drivers were also construed, and their interlinkages presented in a causal loop diagram, representing a socio-institutional assessment framework. The framework can help policymakers and the community to analyze various socio-institutional control levers to reduce the risk of groundwater contamination by OSS in informal settlements.

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Surface water tracing is a widely used technique to investigate in-stream mass transport including contaminant migration. Recently, a microparticle tracer was developed with unique synthetic DNA encapsulated in an environmentally-friendly silica coating (Si-DNA microparticle). Previous tracing applications of such tracers reported detection and quantification, but a massive loss of tracer mass. However, the transport behavior of these DNA-tagged microparticle tracers has not been rigorously quantified and compared with that of solute tracers. Therefore, we compared the transport behavior of Si-DNA microparticles to the behavior of solute NaCl in 6 different, environmentally representative water types using breakthrough curves (BTCs), obtained from laboratory open channel injection experiments, whereby no Si-DNA microparticle tracer mass was lost. Hereafter, we modelled the BTCs using a 1-D advection-dispersion model with one transient storage zone (OTIS) by calibrating the hydrodynamic dispersion coefficient D and a storage zone exchange rate coefficient. We concluded that the transport behavior of Si-DNA microparticles resembled that of NaCl in surface-water relevant conditions, evidenced by BTCs with a similar range of D; however, the Si-DNA microparticle had a more erratic BTC than its solute counterpart, whereby the scatter increased as a function of water quality complexity. The overall larger confidence interval of D_Si-DNA was attributed to the discrete nature of colloidal particles with a certain particle size distribution and possibly minor shear-induced aggregations. This research established a solid methodological foundation for field application of Si-DNA microparticles in surface water tracing, providing insight in transport behavior of equivalent sized and mass particles in rivers.

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We mapped the double-stranded DNA (dsDNA) virus assemblage in groundwater below sub-Saharan urban poor settlements in Arusha (Tanzania), Dodowa (Ghana), and Kampala (Uganda). Our results indicated that ∼80% of dsDNA virus sequences matched the order of Caudovirales, i.e., indigenous bacteriophages; 1.8% of the dsDNA virus sequences matched those of viral pathogens that infect humans and larger animals, which we defined as so-called above-ground hosts. Within this group, the relative abundances of the genera Chordopoxvirinae, Alphaherpesvirinae, and Betairidovirinae were the highest. Culturable Escherichia coli bacteria were found even in deeper wells, indicating that all water was fecally contaminated. The community assemblages sampled in the cities were statistically significantly different from each other. Dissolved ions, population density, and sanitary status had no significant influence, but pH and latitude did. We concluded that the transport of dsDNA virus in groundwater was location specific but was not determined by input concentrations (i.e., related to population density) or related to groundwater chemistry. We hypothesize that other parameters, like the presence of macropores, cause these variations in these shallow, highly populated, heavily polluted terrestrial groundwater systems. Approximately 34% of Africa has similar hydrogeology, so this may affect many urban areas across the continent.

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We combine satellite imagery, urban growth modelling, groundwater modelling and hydrogeological field expeditions to estimate the potential impacts in 2050 of rapid urbanization and climate change on groundwater in Arusha, Tanzania, and by extension similar areas in Sub-Saharan Africa. Our analysis suggests that a reduction of groundwater recharge by 30–44% will cause groundwater levels to drop by up to 75 m, mainly due to increased evapotranspiration and to an expansion in paved surface. If this scenario becomes reality, we predict that wells will run dry, creating health, social and environmental risks.

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