Modeling the Dynamics of Mixture Toxicity and Effects of Organic Micropollutants in a Small River under Unsteady Flow Conditions.

The presence of anthropogenic organic micropollutants in rivers poses a long-term threat to surface water quality. To describe and quantify the in-stream fate of single micropollutants, the advection-dispersion-reaction (ADR) equation has been used previously. Understanding the dynamics of the mixture effects and cytotoxicity that are cumulatively caused by micropollutant mixtures along their flow path in rivers requires a new concept. Thus, we extended the ADR equation from single micropollutants to defined mixtures and then to the measured mixture effects of micropollutants extracted from the same river water samples. Effects (single and mixture) are expressed as effect units and toxic units, the inverse of effect concentrations and inhibitory concentrations, respectively, quantified using a panel of in vitro bioassays. We performed a Lagrangian sampling campaign under unsteady flow, collecting river water that was impacted by a wastewater treatment plant (WWTP) effluent. To reduce the computational time, the solution of the ADR equation was expressed by a convolution-based reactive transport approach, which was used to simulate the dynamics of the effects. The dissipation dynamics of the individual micropollutants were reproduced by the deterministic model following first-order kinetics. The dynamics of experimental mixture effects without known compositions were captured by the model ensemble obtained through Bayesian calibration. The highly fluctuating WWTP effluent discharge dominated the temporal patterns of the effect fluxes in the river. Minor inputs likely from surface runoff and pesticide diffusion might contribute to the general effect and cytotoxicity pattern but could not be confirmed by the model-based analysis of the available effect and chemical data.

H2 S Dosimetry by CuO: Towards Stable Sensors by Unravelling the Underlying Solid-State Chemistry.

The precise detection of the toxic gas H2 S requires reliable sensitivity and specificity of sensors even at minute concentrations of as low as 10 ppm, the value corresponding to typical exposure limits. CuO can be used for H2 S dosimetry, based on the formation of conductive CuS and the concomitant significant increase in conductance. In theory, at elevated temperature the reaction is reversed and CuO is formed, ideally enabling repeated and long-term use of one sensor. Yet, the performance of CuO tends to drop upon cycling. Utilizing defined CuO nanorods we thoroughly elucidated the associated detrimental chemical changes directly on the sensors, by Raman and electron microscopy analysis of each step during sensing (CuO→CuS) and regeneration (CuS→CuO) cycles. We find the decrease in the sensing performance is mainly caused by the irreversible formation of CuSO4 during regeneration. The findings allowed us to develop strategies to reduce CuSO4 formation and thus to substantially maintain the sensing stability even for repeated cycles. We achieved CuO-based dosimeters possessing a response time of a few minutes only, even for 10 ppm H2 S, and prolonged life-time.

Separation of Photochemical and Non-Photochemical Diurnal In-Stream Attenuation of Micropollutants.

For a better process understanding of in-stream attenuation of trace organic contaminants (TrOCs), quantitative comparisons between field studies under different environmental conditions and controlled laboratory experiments are important to separate different processes. However, this is hampered by the challenge to transfer kinetics from the laboratory to different field conditions due to the lack of good quantitative measures to account for different boundary conditions. For phototransformation, in situ light conditions in a river are difficult to determine because light is reduced, for instance, by absorption, scattering on suspended particles, and shading effects. In this study, we present an approach to separate photochemical from non-photochemical diurnal in-stream attenuation based on rate constants relative to diclofenac, as a reference compound, to account for the difference in the in situ light conditions combined with laboratory experiments. 12 out of 45 detected target TrOCs showed a diurnal attenuation at a selected river stretch. A non-photochemical process, potentially biotransformation, was responsible for the diurnal attenuation of bisoprolol, metoprolol, O-desmethylvenlafaxine, tramadol, and venlafaxine. Attenuation of amisulpride, flufenamic acid, hydrochlorothiazide, naproxen, and xipamide can be quantitatively explained by phototransformation, partially for sotalol. Attenuation rate constants of hydrochlorothiazide at different field sites from this study and from published data range over 2 orders of magnitude. Differences can be quantitatively explained by different light exposures but not by water chemical parameters.

Suspended Particulate Matter-A Source or Sink for Chemical Mixtures of Organic Micropollutants in a Small River under Baseflow Conditions?

Suspended particulate matter (SPM) plays an important role in the fate of organic micropollutants in rivers during rain events, when sediments are remobilized and turbid runoff components enter the rivers. Under baseflow conditions, the SPM concentration is low and the contribution of SPM-bound contaminants to the overall risk of organic contaminants in rivers is assumed to be negligible. To challenge this assumption, we explored if SPM may act as a source or sink for all or specific groups of organic chemicals in a small river. The concentrations of over 600 contaminants and the mixture effects stemming from all chemicals in in vitro bioassays were measured for river water, SPM, and the surface sediment after solid-phase extraction or exhaustive solvent extraction. The bioavailable fractions of chemicals and mixture effects were estimated after passive equilibrium sampling of enriched SPM slurries and sediments in the lab. Dissolved compounds dominated the total chemical burden in the water column (water plus SPM) of the river, whereas SPM-bound chemicals contributed up to 46% of the effect burden even if the SPM concentration in rivers was merely 1 mg/L. The equilibrium between water and SPM was still not reached under low-flow conditions with SPM as a source of water contamination. The ratios of SPM-associated to sediment-associated neutral and hydrophobic chemicals as well as the ratios of the mixture effects expressed as bioanalytical equivalent concentrations were close to 1, suggesting that the surface sediment can be used as a proxy for SPM under baseflow conditions when the sampling of a large amount of water to obtain sufficient SPM cannot be realized.

Temporal and spatial variable in-stream attenuation of selected pharmaceuticals.

Organic micropollutants enter rivers mainly with discharges of wastewater treatment plants (WWTP) and pose a risk to aquatic ecosystems and water quality. A considerable knowledge gap exists for disentangling overlapping processes and driving conditions that control the fate of these pollutants. Thus, the aim of this study was to identify the driving parameters for attenuation of selected pharmaceuticals (carbamazepine, diclofenac, tramadol and venlafaxine) under field conditions. The presented study was performed at a small river (Ammer River, mean discharge 0.87 m3 s-1) which is hydrologically complex due to karstification, numerous artificial discharges, and engineered modifications of the channel. We applied a Lagrangian sampling scheme at two sequential river reaches. In general, for the investigated compounds and over the length of the tested reaches, the absolute net attenuation representative for 24 h was low (≤ 23% net attenuation), yet calculated half-lives were comparable to literature. Photodegradation is specifically relevant for the first river reach characterized by a higher net attenuation of the photosensitive compound diclofenac (14.5% ±11.3%) compared to the second section (9.8% ±13.7%). This is likely due to a spatial difference in canopy shading, which is supported by significant correlations (R2 ≥ 0.8) of the temporally changing 'temperature' and 'solar radiation' with time-specific degradation rate constants of photosensitive compounds for consecutive hourly water parcels. In general, the presented spatially and temporally resolved approach is a suitable tool to determine the attenuation of organic micropollutants and to narrow down the interpretation of net attenuation to a few reasonable processes.

Influence of Emission Sources and Tributaries on the Spatial and Temporal Patterns of Micropollutant Mixtures and Associated Effects in a Small River.

Organic micropollutants of anthropogenic origin in river waters may impair aquatic ecosystem health and drinking water quality. To evaluate micropollutant fate and turnover on a catchment scale, information on input source characteristics as well as spatial and temporal variability is required. The influence of tributaries from agricultural and urban areas and the input of wastewater were investigated by grab and Lagrangian sampling under base flow conditions within a 7.7-km-long stretch of the Ammer River (southwest Germany) using target screening for 83 organic micropollutants and 4 in vitro bioassays with environmentally relevant modes of action. In total, 9 pesticides and transformation products, 13 pharmaceuticals, and 6 industrial and household chemicals were detected. Further, aryl hydrocarbon receptor induction, peroxisome proliferator-activated receptor activity, estrogenicity, and oxidative stress response were measured in the river. The vast majority of the compounds and mixture effects were introduced by the effluent of a wastewater-treatment plant, which contributed 50% of the total flow rate of the river on the sampling day. The tributaries contributed little to the overall load of organic micropollutants and mixture effects because of their relatively low discharge but showed a different chemical and toxicological pattern from the Ammer River, though a comparison to effect-based trigger values pointed toward unacceptable surface water quality in the main stem and in some of the tributaries. Chemical analysis and in vitro bioassays covered different windows of analyte properties but reflected the same picture. Environ Toxicol Chem 2020;39:1382-1391. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Designing field-based investigations of organic micropollutant fate in rivers.

Organic micropollutants in rivers are emitted via diffuse and point sources like from agricultural practice or wastewater treatment plants (WWTP). Extensive laboratory and field experiments have been conducted to understand emissions and fate of these pollutants in freshwaters. Nevertheless, data is often difficult to compare since common protocols for appropriate approaches are largely missing. Thus, interpretation of the observed changes in substance concentrations and of the underlying fate of these compounds downstream of the chemical input into the river is still challenging. To narrow this research gap, (1) process understanding and (2) measurement approaches for field-based investigations are critically reviewed in this article. The review includes, on the one hand, processes that change the volume of the water (hydrological processes) and, on the other hand, processes that affect the substance mass within the water (distribution and transformation). Environmental boundary conditions for the purpose of better comparability of different attenuation studies, as well as promising state-of-the-art measurement approaches from different disciplines, are presented. This overview helps to develop a tailored procedure to assess turnover mechanisms of organic micropollutants under field conditions. In this respect, further research needs to standardize interdisciplinary approaches to increase the informative value of collected data.