<p>Water balance closure using purely remote sensing products was difficult to achieve until the launch of Gravity Recovery and Climate Experiment (GRACE) satellites in 2002. The accurate quantification of<strong> </strong>water cycle components (precipitation, evapotranspiration, runoff, and terrestrial water storage) over a large-scale basin is an important step in improving the understanding of the water balance and the response of the basin to different hydrologic extremes. The Upper Blue Nile (UBN) basin contributes about 60% of the streamflow to the main Nile River annually, and hundreds of millions of people heavily rely on the Nile River. Thus, accurate quantification of the hydrological cycle fluxes will help manage the water resources in an effective, sustainable manner. Hydrometeorological data is lacking; nevertheless, remote sensing data provides an alternative approach to estimating the water cycle components. However, prior to incorporating these products into the water budget calculation, their performance over the studied basin should be assessed. In this study, we aim to estimate runoff from the water budget equation and diagnose the estimated runoff with the Eldiem gauge records at the outlet of the UBN basin for the 2003–2014 period. We evaluate the water cycle components for seven rainfall products (CHIRPSv2, CRU TS4.06, ERA5, TRMM 3B43 V7, GPM, CFSR, and SM2RAIN-CCI), three evapotranspiration products (GLEAM, MOD16, and PLM), and two terrestrial water storage solutions (GRACE JPL MASCON, and Spherical Harmonic (SH) products). The Overall Unified Metric (OUM) approach is adopted to choose the best performing combination among the 42 combination scenarios. The OUM is an approach based on summing up the rankings given for the error and linear fit metrics—namely, R<sup>2</sup>, slope, y-intercept, RMSE, MAE, and PBIAS. Among the 42 combinations, the best rainfall, TWS, and ET combination performance products to estimate runoff are SM2RAIN-CCI, GLEAM, and GRACE SH, respectively. The statistical results for the six chosen metrics are R<sup>2</sup> = 0.7, slope = 1.6, y-intercept = - 0.5 cm, RMSE = 3 cm, MAE = 2.8 cm, and PBIAS = 36%. The 95% confidence bound of the combination scenarios was found to be able to bracket the runoff during the dry season, but the runoff was overestimated during the rainy season. The uncertainty analysis revealed that all the combinations were able to estimate the seasonal trend variation, but closing the water balance equation was not achieved. This deviation in closing the water budget equation might be attributed to the uncertainty associated with satellites, the limitation of land surface models to account for anthropogenic activities, and the coarse resolution of GRACE. Additionally, the signal processing uncertainties and the different algorithm assumptions of the remote sensing products may also have an influence. Further studies are needed to improve the reliability of the remote sensing product for the water budget closure, especially for applications on ungauged basins. Moreover, advancement in satellites will lead to accurate estimates in the near future.</p>

<p>Per- and Polyfluoroalkyl Substances (PFAS) are chemicals used for many domestic and industrial purposes related to their physicochemical properties. However, those same properties make them mobile and persistent in the environment, and on top of that, they are toxic and can affect human health in the short and long term, as they are bio-accumulative. Many processes govern the transport of PFAS in the surface waters and groundwater, e.g., sorption, biodegradation, co-transport, and transformation. Monitoring PFAS at different locations can help understand these processes and provide datasets to calibrate and validate reactive transport models simulating PFAS fate and transport. This study compares PFAS presence and distribution in river water and groundwater at two Danube river sites. One site is characterized by a steady water level in the river and natural flow from the river to the groundwater, with a clogging layer at the aquifer-river interface. In contrast, the other site has a more dynamic water level in the river, several pumping wells affecting water infiltration rates, and lacks a clogging layer.</p> <p>Samples were collected monthly for 12 months at the static study site and 8 months at the dynamic study site. Targeted analysis for 32 PFAS compounds has been carried out using liquid chromatography mass spectrometry (LCMS). The concentrations of the compounds were generally less than 20 ng/l, and most of the compounds were lower than the limit of quantification/detection. The results show that 3H-perfluoro-3-[(3-methoxypropoxy) propanic acid] (ADONA) had the highest concentration at the two sites, both in the river and groundwater. The longer chain PFAS exhibited a slight reduction in concentration from the river towards groundwater due to, most likely, sorption, while the shorter chain did not. The 6:2 FTS precursor was detected in the river but not in the groundwater. For some substances, the concentrations were higher in the groundwater compared to the river, indicating either background water influence, a transformation of PFAS, different transport routes (e.g., accumulation over time), or longer flow paths. Longer chain lengths, greater than 9 carbon atoms, were never detected above the limit of quantification in the river and groundwater. More PFAS compounds were detected at the dynamic study site than at the static one, even though, it is located further downstream, indicating nearby PFAS sources or/and influents along the river course. It is worth mentioning that large wastewater treatment plants are discharging their effluent downstream of the static site, in addition to sewer overflows from large cities in between. The PFAS concentrations in the river and groundwater during one high-flow event showed little difference compared to the ones during basic monthly monitoring at both study sites, however, another high flow event is needed to confirm this observation.</p>

<p>PFAS are of emerging concern due to their high environmental persistence, human health effects, and bioaccumulation attributed to their chemical properties. These chemical properties make them preferable for many industrial and domestic purposes. In turn, PFAS are emitted from a vast amount of sources and eventually reach the surface water and groundwater environments. The most critical pathways for groundwater are infiltration via the unsaturated zone and riverbank filtration. Natural filtration of PFAS reduces the risk of PFAS contamination, and sorption is considered the most crucial removal mechanism of PFAS from saturated porous media. This study aims to better understand the sorption processes and factors affecting the affinity of soil to sorb different types of PFAS. A thorough understanding of these processes is needed to model PFAS fate and transport in groundwater and estimate human health's impact. To meet this aim, we conducted a literature survey involving PFAS sorption behavior to soil and external sorbents in batch and column experimental studies and monitoring studies in the field.</p><p>PFAS tail group are hydrophobic organic chemicals. Hydrophobic interactions are thus one of the main sorption mechanisms in groundwater, especially when the soil has a higher organic carbon content. Several studies have found that the retention of PFAS is increased with the increase in PFAS hydrophobicity and the amount of organic matter in the soil. Another important forces affecting the interaction of PFAS with soil are the electrostatic forces. Many PFAS are present in the environment in their anion form and bond to positively charged soil surfaces. Soils with a negative charge surface can repel PFAS and reduce retention. Other minor processes such as the hydrogen bond and PFAS functional group forming complexes can increase the sorption to soil. Soil properties and solution chemistry significantly affect these forces and bonds and can either reduce or increase the affinity of PFAS sorption to soil. The presence of co-contaminants and nonaqueous phase liquids in groundwater further affects these processes.</p><p>Difficulties in degrading PFAS compounds led to alternative ways of remediation, such as stabilizing PFAS in soil by employing sorption processes. With the gained knowledge, external sorption enhancers can be used to increase the PFAS sorption besides altering the solution chemistry to maximize the retention and stabilization of PFAS in soil.</p><p>The dynamics of the sorption process are affected by preferential flow and intra-sorbent diffusion, leading to rate-limited sorption effects. These dynamics are essential for the model selection and estimating the time needed for the clean-up during remediation.</p>