The problem begins with a deceptively simple assumption. For decades, riverbank filtration and managed aquifer recharge systems have been evaluated as if the hydraulic gradient between a river and its aquifer were roughly constant. Water infiltrates, sediments and microbes remove some pollutants, and cleaner water emerges at the well. But field conditions are rarely steady. Storms raise river stage within hours, seasonal recharge resets regional groundwater levels, coastal tides alternate infiltration and discharge over hourly cycles, and human pumping and dam operations bend the flow field at will. Each of these forcings changes the magnitude and direction of water exchange, expands or contracts the active exchange zone, and redistributes the travel and residence times of every water parcel crossing the boundary.

The consequences for organic micropollutants, or OMPs, are profound. When a flood raises the river head above the adjacent groundwater, the prevailing gradient can weaken or reverse entirely. River water loaded with carbamazepine, sulfamethoxazole, benzotriazole, and other persistent compounds is pushed into bank sediments and riparian aquifers that routine conditions never reach. Field monitoring at Danube riverbank filtration transects between 2014 and 2016 captured this directly: two flood events increased riverine loads of these compounds, and groundwater concentrations during flooding could exceed simultaneous river concentrations and extend farther into the aquifer than routine monitoring would suggest. The review is careful to note that this result cannot be read as event-specific degradation or formation from synchronous concentration pairs alone, because the sampled groundwater may reflect antecedent source concentrations, mixing, and a distribution of travel times.

Here the review introduces one of its central integrative concepts: hydrologically realized mobility. Molecular properties such as polarity, charge, and hydrophobicity constrain sorption and persistence, but they do not uniquely predict whether a compound will actually cross the surface water–groundwater interface during a given event. Hydrodynamics determine whether intrinsic mobility is expressed as cross-interface transport. The testable proposition is that the same compound and source pulse will yield different arrival times and exported mass when forcing redistributes pathways, even if chemistry is unchanged. Paired conservative-tracer and OMP breakthrough across contrasting events would support this proposition; unchanged OMP response after demonstrable pathway and river-fraction changes would weaken it. Mobility, in this framing, is an event-specific outcome to be quantified, not a compound property to be assumed.

Once contaminants enter the interface, the event sequence unfolds in stages. During infiltration, the instantaneous contaminant input rate is the product of the surface-water concentration and the normal Darcy flux integrated over the infiltrating boundary. Advection controls the mean direction of transport, while dispersion and media heterogeneity distribute contaminant mass among pathways of different lengths and residence times. A single input pulse can therefore produce distributed arrival times and breakthrough magnitudes. When flow reverses and the river stage falls, infiltrated water drains gradually back to the channel as bank-storage return flow, and some infiltrated river water remains stored in the aquifer long after the storm. Hydrological modeling shows that bank-storage water may return to the river after recession or remain in the aquifer, with event timing changing water ages and return-flow fractions. For OMPs, the corresponding contaminant-mass routes are hypotheses until parent concentrations, fluxes, and stored pools are measured through the event.

Retardation and temporary storage add further complexity. Solid–water partitioning can slow OMP movement relative to the carrier water, and in heterogeneous media reversible sorption and slow mass exchange contribute to retardation and tailing. A field injection experiment resolved depth-dependent pesticide retardation against a conservative tracer, showing that sorption behavior can vary substantially within a single aquifer. But a delayed aqueous tail alone does not demonstrate that OMP mass was stored in sediment or a slow domain. Verifying contaminant storage requires a parent-compound mass balance, a measured sorbed pool, or a release perturbation. The review distinguishes three hydrologic-response classes: mobile–persistent compounds that traverse the interface rapidly or over long distances with little loss, mobile–reactive compounds whose fate depends on reactive exposure and pathway timing, and reversibly retarded compounds that show delayed arrival, tailing, and possible clean-water release. These classes are hypotheses about dominant responses, not fixed compound labels, and a compound may move between classes across sites or hydrologic phases.

Remobilization is where the story turns from storage to secondary source. When a flood recedes, stage declines, or slow pathways reconnect, untransformed OMP mass in slow-exchange domains or on reversible sorption sites may return to the mobile phase. Laboratory evidence is direct: in triplicate riverbed-sediment columns fed with contaminant-free tap water, previously retained compounds entered the effluent, demonstrating remobilization under controlled conditions. The field-scale significance of such release during flood recovery remains untested. Temporary storage can switch the functional role of the interface across event phases: enhanced infiltration creates a temporary sink that suppresses immediate output, whereas untransformed stored mass can become a potential secondary source during recovery. The interface can be described as a potential secondary source only when mass balance, parent compound–transformation product relations, or conservative–reactive tracer comparisons exclude transformation and dilution. Whether delayed release creates an environmentally relevant secondary pulse remains unresolved.

Reaction conditions add a dynamic dimension. Infiltrating surface water supplies dissolved oxygen and electron acceptors, while respiration consumes them along flow paths, creating redox zonation that shifts with temperature, organic carbon supply, and exchange. At the wastewater-impacted River Erpe, suspect screening along preset 35-centimeter hyporheic flow paths retained 130 aqueous trace organic contaminants after quality filters; 58 showed decreases, 63 showed no attenuation, and seven increased. Among the 58 attenuated compounds, 51 had higher estimated rates in the oxic than the suboxic segment. Recent simulated riverbank filtration experiments further show that sequential oxidizing–reducing conditions can improve removal of some recalcitrant OMPs, including irbesartan, benzotriazole, and gabapentin, relative to exclusively oxidizing or reducing conditions, accompanied by distinct redox-driven microbial specialization. The sequence of reaction environments encountered by a contaminant may be more informative than a single redox measurement. The review frames this as a dynamic reaction area: a compound-specific, time-dependent volume where transported OMP mass encounters conditions favorable for transformation. The testable proposition is that OMP transformation rises when event-driven pathway redistribution increases overlap between the OMP pulse and favorable reaction conditions, after conservative transport and mixing are controlled.

Antecedent state, or the memory of the interface, may be the most underappreciated control. Earlier low-flow, high-flow, drought, or stage-fluctuation periods change pore-water composition, redox state, microbial communities, and stored solute mass. Process-based simulations of successive peak flows show that event separation relative to system memory controls whether residence-time effects carry into a later peak. Intermittent-flow OMP columns provide controlled evidence that drying changes microbial composition and transformation routes for selected compounds. A thermo-hydro-biogeochemical model found that low mean stage during drought amplified the effects of high-frequency stage fluctuations on hyporheic temperature and biogeochemical reaction patterns. These sources support a hypothesis of history-dependent OMP fate, but direct event-resolved field tests remain scarce. The testable prediction is that matched current forcing and source inputs yield different OMP breakthrough or recovery when measured pre-event storage or reaction states differ.

The methodological implications are stark. Low-frequency and asynchronous sampling can miss the hydrologic phases that control transport entirely. At one Rhine riverbank filtration site, modeling linked high floods to river-to-well travel times shorter than eight days compared with a routine 25-to-40-day range, while low-flow conditions lengthened modeled residence to about 60 days. A fixed calendar lag may miss the contributing source period. Synchronous surface-water and groundwater concentration ratios are only apparent contrasts, not parcel-based removal fractions or proof of transformation. The review prescribes a hierarchy: constrain hydrodynamics and travel-time distributions first with groundwater levels, electrical conductivity, stable isotopes, and other conservative tracers, and only then estimate compound-specific retardation and reaction parameters. Validation should be partitioned by hydrologic state or independent event, not randomly. A model demonstrates cross-state predictive skill only if it reproduces the hydraulic response, travel-time distribution, arrival time, and OMP breakthrough in independent events without recalibrating key parameters. If dispersion, sorption, or degradation parameters must be adjusted for every flood, the result is event-specific fitting rather than an early-warning model.

Exposure assessment must also change. The review proposes event-resolved metrics: first arrival, peak concentration, exposure duration, cumulative breakthrough mass, and recovery time, each with explicit uncertainty and decision thresholds. At a full-scale riverbank filtration system in the Shepperton gravels, monitored flood inundation raised dissolved organic carbon to approximately 140 percent of baseline and mean well dissolved oxygen to approximately 440 percent of baseline, while micro-organic contaminant detections also increased. Most flood water-quality effects occurred during the first two to three weeks, with a return toward baseline conditions within about six weeks. The end of inundation is not necessarily the end of elevated exposure. Recovery time deserves particular emphasis, and compound- and site-specific decision thresholds and sensitive receptors would be required to extend these transport metrics into a formal health or ecological risk assessment.

The central knowledge gap remains direct event-scale field mass balances linking OMP storage to delayed release. Within the literature corpus assessed, transient water exchange and changing water ages are supported mainly by hydrological studies; direct event-scale field mass balances were not identified. Direct remobilization is demonstrated in a controlled clean-water column test, whereas a field event-scale balance from OMP input through storage and delayed release was not found. The next priority is event-triggered monitoring from rising stage through recovery, paired with time-dependent source–response matching and pathway-resolved reaction measurements. The central question shifts from whether one flood increases an OMP concentration to the following: under which combinations of antecedent state, inter-event interval, and hydrologic disturbance does OMP breakthrough or delayed release exceed a site-defined exposure threshold, and how long does that condition persist after the event? Until independently validated OMP event data are available, this remains a research agenda. But the message is already clear: the riverbank is not a filter. It is a dynamic, history-laden archive that opens and closes with every storm, and we have only just begun to read it.

Subject of Research: Effects of transient surface water–groundwater exchange on the transport of organic micropollutants: temporary storage, retardation, and remobilization

Article Title: Effects of transient surface water–groundwater exchange on the transport of organic micropollutants: temporary storage, retardation, and remobilization

Article References: Effects of transient surface water–groundwater exchange on the transport of organic micropollutants: temporary storage, retardation, and remobilization. (n.d.). https://doi.org/10.3389/frwa.2026.1980968

Image Credits: AI Generated

DOI: 10.3389/frwa.2026.1980968

Keywords: Effects, transient, surface, water, groundwater, exchange, transport, organic, micropollutants, temporary, storage, retardation

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Transient tachypnea of the newborn, despite its name suggesting a fleeting and benign condition, occupies a meaningful position in neonatal medicine precisely because it is so common and because its consequences are not always trivial. The delayed resorption of fetal lung fluid that underlies the disorder is a physiological process that normally accelerates in the hours immediately after birth, driven in large part by epithelial sodium channels that begin actively transporting sodium—and water with it—out of the alveolar spaces. Catecholamines released during labor, particularly adrenaline, are thought to prime this switch from fluid-secreting to fluid-absorbing lung physiology, which helps explain why cesarean delivery without labor is a recognized risk factor: infants who never experience the hormonal surge of labor retain more lung fluid. Any intrauterine or developmental factor that impairs the maturation of these clearance mechanisms, including inflammatory or hypoxic insults, could plausibly shift an infant from the normal trajectory of rapid fluid resorption toward the symptomatic tachypnea that defines TTN. This biological framework provides a coherent rationale for why environmental exposures during pregnancy, which can alter placental function, fetal oxygenation, and airway development, might manifest clinically as impaired neonatal respiratory adaptation.

Carbon monoxide deserves particular attention among the pollutants identified in the Taiwanese analysis, because its association with TTN was both the strongest and the most strikingly elevated of the three signals. Unlike particulate matter, which is a heterogeneous mixture of combustion particles, dust, and secondary aerosols, carbon monoxide is a chemically simple gas with a well-characterized mechanism of harm: it binds hemoglobin with an affinity roughly two hundred times greater than oxygen, forming carboxyhemoglobin and thereby reducing the oxygen-carrying capacity of maternal blood. Because the fetus depends entirely on maternal hemoglobin for oxygen delivery across the placenta, even modest elevations in maternal carboxyhemoglobin can steepen the oxygen gradient that drives placental gas exchange. Fetal hemoglobin itself binds carbon monoxide even more avidly than adult hemoglobin, which means the developing infant may accumulate carboxyhemoglobin disproportionately relative to the mother. Chronic, low-level exposure throughout gestation, of the kind captured by the monthly averaging approach used in this study, could therefore translate into repeated episodes of relative fetal hypoxia, a state known to alter pulmonary vascular tone and potentially to disrupt the surfactant and epithelial transport systems that clear lung fluid after birth.

The traffic-related signature of the findings strengthens this mechanistic interpretation. Carbon monoxide and nitrogen oxides are both emitted predominantly by internal combustion engines, and their concentrations in ambient air track closely with traffic density, congestion patterns, and urban topography. The regional analysis showing that risk elevations were most pronounced in Taipei and New Taipei City is consistent with the geography of these pollutants: the Taipei basin traps vehicular emissions within a densely populated, topographically enclosed airshed, producing higher and more persistent exposures than are typical of the more ventilated coastal and rural regions of the island. Fine particulate matter, by contrast, arises from a broader mixture of sources, including industry, power generation, secondary atmospheric chemistry, and long-range transport, which may explain why its per-unit association with TTN, while statistically significant, was numerically far smaller than that of carbon monoxide. When a pollutant mixture is dominated by traffic sources, the composite exposure signal tends to align most strongly with the individual traffic tracers, and the geographic gradient observed here mirrors exactly that pattern.

The methodological architecture of the study merits consideration when weighing the strength of these associations. A nested case-cohort design embedded within a nationwide claims database offers substantial advantages for rare-outcome research: it preserves the population representativeness of the full cohort while allowing intensive matching and covariate assessment only for the sampled cases and controls. Restricting the analysis to primiparous mothers is an unusual and thoughtful choice, because parity is itself a strong determinant of TTN risk—firstborn infants experience longer labors, different delivery dynamics, and distinct patterns of maternal comorbidity—and restricting to first pregnancies removes this source of confounding entirely rather than merely adjusting for it statistically. Matching on maternal age, neonatal sex, and birth date further controls for the most influential demographic determinants of the outcome, and the four-to-one control ratio provides reasonable statistical power for the multivariable models. The adjustment for gestational age, birth weight, delivery mode, maternal comorbidities, and pregnancy complications addresses the principal clinical pathways through which air pollution might otherwise appear spuriously associated with TTN.

Nevertheless, several limitations inherent to this design shape how the findings should be interpreted. Exposure assessment relied on linking residential postal codes to the nearest of seventy-one monitoring stations, an approach that assigns the same pollutant concentration to every resident of a township and assumes that the nearest station reflects personal exposure. Individual mobility during pregnancy, time spent indoors, use of air filtration, occupational exposures, and local microenvironments such as proximity to major roadways all introduce misclassification that is likely to be non-differential with respect to outcome, which typically biases estimates toward the null. That a clear signal emerged despite this dilution suggests the true underlying associations may be at least as strong as those reported. The monitoring network itself, while extensive, cannot capture street-level variation in traffic exhaust, and the six traffic-oriented stations among the seventy-one may not fully represent the gradients experienced by residents of dense urban corridors. Additionally, claims-based ascertainment of TTN depends on diagnostic coding practices, and although the ICD codes used are standard for the condition, clinical recognition of TTN varies across institutions and may overlap with other causes of mild respiratory distress, introducing some degree of outcome misclassification.

The magnitude of the carbon monoxide association warrants careful scrutiny in this light. An adjusted odds ratio of approximately 1.8 is substantially larger than the modest per-unit elevations seen for nitrogen oxides and fine particulate matter, and it is worth noting that the three pollutants were modeled on different scales, so their coefficients are not directly comparable in absolute terms. It is also possible that carbon monoxide serves as a proxy for a broader traffic pollution mixture whose most damaging components—ultrafine particles, black carbon, and volatile organic compounds—were not individually measured. In multipollutant environments, single-pollutant models can attribute the effects of correlated co-exposures to whichever pollutant is most strongly correlated with the shared source. The consistency of the traffic-related pattern across carbon monoxide, nitrogen oxides, and the geographic stratification, however, lends coherence to the overall conclusion that vehicular emissions are the dominant relevant exposure category.

Placed in a wider evidentiary context, the study aligns with a growing body of literature linking prenatal air pollution to neonatal respiratory morbidity, while addressing a gap that previous work had left largely unfilled. Prior investigations had established associations between maternal exposure to particulate matter and nitrogen dioxide and outcomes such as respiratory distress syndrome, birth asphyxia, and intensive care admission, but TTN specifically had received little population-scale attention, partly because it is often perceived as self-limited and because its diagnosis requires a degree of clinical specificity that administrative datasets do not always afford. By focusing on a large, nationally representative cohort spanning seventeen years of births, the Taiwanese analysis achieves a scale that smaller single-center studies cannot match, and the seventeen-year window captures meaningful temporal variation in ambient pollution levels as vehicle emission standards and fuel quality evolved on the island during that period.

The clinical and public health implications extend in several directions. For obstetric and neonatal practitioners, the findings suggest that maternal residential environment deserves consideration when counseling families about respiratory risk in newborns, particularly in densely trafficked urban settings. For policymakers, the concentration of excess risk in the Taipei basin provides a geographically targeted rationale for emission controls, traffic demand management, and urban planning interventions that reduce maternal exposure near major roadways. Because TTN affects a substantial fraction of newborns and generates considerable healthcare utilization, parental distress, and mother-infant separation in the critical early hours after delivery, even modest reductions in population-level exposure could yield meaningful aggregate benefits. The association with long-term respiratory outcomes reported in prior literature, including childhood asthma and recurrent wheezing, further raises the possibility that TTN serves as an early clinical marker of environmentally induced pulmonary vulnerability, making prenatal air quality a concern that reaches well beyond the neonatal period. Continued research with finer-grained exposure modeling, personal monitoring, and multipollutant analysis will be needed to disentangle which components of the traffic emission mixture drive the observed risk and to quantify the exposure reductions that clinical guidance should target.

Subject of Research: Prenatal air pollution exposure and the risk of transient tachypnea of the newborn

Article Title: Prenatal air pollution exposure and the risk of transient tachypnea of the newborn

Article References: Lin, F.-J., Lin, C.-H., Lin, Y.-H., & Lin, M.-C. (2026). Prenatal air pollution exposure and the risk of transient tachypnea of the newborn. Frontiers in Pediatrics, 14, Article 1941575. https://doi.org/10.3389/fped.2026.1941575

Image Credits: AI Generated

DOI: 10.3389/fped.2026.1941575

Keywords: Prenatal, pollution, exposure, risk, transient, tachypnea, newborn, scientific research