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The Mediterranean Sea (MS) is a semienclosed basin that is considered one of the most oligotrophic seas in the world. In such an environment, inputs of allochthonous nutrients and micronutrients play an important role in sustaining primary productivity. Atmospheric deposition and riverine runoff have been traditionally considered the main external sources of nutrients to the MS, whereas the role of submarine groundwater discharge (SGD) has been largely ignored. However, given the large Mediterranean shore length relative to its surface area, SGD may be a major conveyor of dissolved compounds to the MS. Here, we used a ²²⁸Ra mass balance to demonstrate that the total SGD contributes up to (0.3–4.8)⋅10 ¹² m ³⋅y ⁻¹ to the MS, which appears to be equal or larger by a factor of 16 to the riverine discharge. SGD is also a major source of dissolved inorganic nutrients to the MS, with median annual fluxes of 190⋅10 ⁹, 0.7⋅10 ⁹, and 110⋅10 ⁹ mol for nitrogen, phosphorous, and silica, respectively, which are comparable to riverine and atmospheric inputs. This corroborates the profound implications that SGD may have for the biogeochemical cycles of the MS. Inputs of other dissolved compounds (e.g., iron, carbon) via SGD could also be significant and should be investigated. Significance The Mediterranean Sea (MS) is one of the most oligotrophic seas in the world, and external inputs of nutrients are especially relevant to sustaining primary productivity in this basin. Here we evaluate the role of submarine groundwater discharge (SGD) as a source of nutrients to the entire MS, a pathway that has been largely overlooked. This study demonstrates that SGD is a volumetrically important process in the MS, is of a larger magnitude than riverine discharge, and also represents a major source of dissolved inorganic nitrogen, phosphorous, and silica to the MS.

This study aimed to investigate the presence and persistence of antibiotics in wastewater of four typical pharmaceutical manufactories in China and receiving water bodies and suggest the removal of antibiotics by the wastewater treatment process. It also evaluated the environmental impact of antibiotic residues through wastewater discharge into receiving water bodies. The results indicated that thirteen antibiotics were detected in wastewater samples with concentrations ranging from 57.03 to 726.79 ng/L. Fluoroquinolones and macrolides were the most abundant antibiotic classes found in wastewater samples, accounting for 42.5% and 38.7% of total antibiotic concentrations, respectively, followed by sulfonamides (16.4%) and tetracyclines (2.4%). Erythromycin-H 2 O, lincomycin, ofloxacin, and trimethoprim were the most frequently detected antibiotics; among these antibiotics, the concentration of ofloxacin was the highest in most wastewater samples. No significant difference was found in different treatment processes used to remove antibiotics in wastewater samples. More than 50% of antibiotics were not completely removed with a removal efficiency of less than 70%. The concentration of detected antibiotics in the receiving water bodies was an order of magnitude lower than that in the wastewater sample due to dilution. An environmental risk assessment showed that lincomycin and ofloxacin could pose a high risk at the concentrations detected in effluents and a medium risk in their receiving water bodies, highlighting a potential hazard to the health of the aquatic ecosystem. Overall, The investigation was aimed to determine and monitor the concentration of selected antibiotics in 4 typical PMFs and their receiving water bodies, and to study the removal of these substances in PMFs. This study will provide significant data and findings for future studies on antibiotics-related pollution control and management in water bodies.

The number of studies concerning Submarine Groundwater Discharge (SGD) grew quickly as we entered the 21st century. Many hydrological and oceanographic processes that drive and influence SGD were identified and characterized during this period. These processes included tidal effects on SGD, water and solute fluxes, biogeochemical transformations through the subterranean estuary, and material transport via SGD from land to sea. Here we compile and summarize the significant progress in SGD assessment methodologies, considering both the terrestrial and marine driving forces, and local as well as global evaluations of groundwater discharge with an emphasis on investigations published over the past decade. Our treatment presents the state-of-the-art progress of SGD studies from geophysical, geochemical, bio-ecological, economic, and cultural perspectives. We identify and summarize remaining research questions, make recommendations for future research directions, and discuss potential future challenges, including impacts of climate change on SGD and improved estimates of the global magnitude of SGD.

Lacustrine groundwater discharge (LGD) is an important water and nutrient source for lakes. Despite its importance, high‐resolution quantifying the spatial variability of LGD remains challenging. Particularly, little studies have explored the impact of the interaction zone structure between lakes and aquifers on this variability. Present study presents a high‐resolution quantitative estimation of LGD spatial patterns in an oxbow lake by combining thermal remote sensing with a 222Rn mass balance model. The vertical distribution characteristics of various parameters including lake water temperature, 222Rn concentration, electrical conductivity, and δ18O were examined to elucidate the influence of groundwater on the distribution pattern of lake surface temperature (LST). Regression equations were formulated to correlate LST with lake water 222Rn concentration across different water depth zones, enabling the inverse calculation of the 222Rn concentration in the water of the entire lake. Utilizing a 222Rn mass balance model across all grid points, the LGD rate was determined to vary from 0 to 330.96 mm/d, with an average of 55.02 ± 19.61 mm/d. In shallow water zones, the accumulation of lacustrine sediments has resulted in isolation from confined aquifers, causing LGD to primarily occur as springs in nearshore lake areas. Conversely, the direct connection between the deepwater zone of the lake and the water‐rich confined aquifer has resulted in a higher LGD rate in the lake interior. Present study not only offers a novel approach for quantifying the spatial patterns of LGD but also provides valuable insights for LGD studies conducted in lakes globally. Plain Language Summary Lacustrine Groundwater Discharge (LGD) is a crucial water and nutrient source for lakes, but accurately quantifying its spatial variability is challenging. Little research has explored the impact of interaction zone structures between lakes and aquifers on this variability. This study combines thermal remote sensing with a 222Rn model to estimate LGD spatial patterns in a central Yangtze River basin oxbow lake. Various parameters' vertical distributions were examined to elucidate groundwater's influence on lake surface temperature (LST) distribution. Regression equations correlated LST with lake water 222Rn concentration across different depth zones, enabling inverse calculation of 222Rn concentration in the entire lake. Utilizing a 222Rn mass balance model, LGD rates varied from 0 to 330.96 mm/d. Shallow water zones, isolated from underlying confined aquifers due to lacustrine sediment accumulation, primarily exhibited LGD as nearshore springs. Conversely, direct connections between the lake's deepwater zone and water‐rich confined aquifers led to higher LGD rates in the lake interior. Key Points Novel approach coupling thermal infrared remote sensing and 222Rn model was used for gridded estimation of Lacustrine groundwater discharge (LGD) The spatial distribution patterns of LGD in deep water and shallow water areas are opposite Groundwater‐lake interaction zone structure controls the spatial distribution of LGD

Lacustrine groundwater discharge (LGD) is a vital water and solute source for lakes. However, the understanding of the long‐term temporal variability of LGD remains limited owing to insufficient insights into driving mechanisms, such as climatic and hydrologic changes. In this study, we examined the oxbow lake group in the central Yangtze River (YR) and assessed the LGD rates from 2000 to 2022 using 222Rn combined with meteorological and hydrological data. The findings revealed that groundwater was recharged during the wet season and discharged to the lakes during the dry season. We established a mathematical model to link the LGD rates to the meteorological and hydrological factors of the lakes, which accounted for 98.70% of the LGD rate variance. Using a predictive model combined with meteorological and hydrological data to assess the LGD rate over the past two decades, it was found that in wet years with higher precipitation and higher average YR water levels, the LGD rate was higher. The gradual increase in precipitation during the rising water and wet seasons, along with a slow rise in the YR water levels, will cause the LGD rate to exhibit a slightly increasing trend with fluctuations in the future. This study proposed an innovative approach to investigate the long‐term temporal variation in LGD and identify the weather and hydrological influences on LGD. Plain Language Summary Lacustrine groundwater discharge (LGD) is an important water and substance source for lakes. However, long‐term temporal variability of LGD is poorly understood due to limited knowledge of driving mechanisms such as climatic and hydrologic change. In this study, we focused on the oxbow lake group within central Yangtze River, and estimated LGD rates from 2000 to 2022 with meteorological and hydrological data. A mathematical model is established to correlate the LGD rates with meteorological and hydrological factors of the lakes. The simulated LGD rates in 2000–2022 show that the years with higher precipitation and higher water levels in the Yangtze River correspond to larger LGD rates. Additionally, future prediction suggests continuously increasing LGD rates. This study introduces an innovative approach to explore LGD's long‐term temporal variability and identifies the regulation of weather and hydrology on LGD. Key Points A great relationship (R2 = 0.987) between Lacustrine groundwater discharge (LGD) rates and meteorology and hydrology is established Meteorology and hydrology jointly regulate two‐decadal variability of LGD, with higher LGD during wet years and lower LGD during dry years LGD rates will exhibit a slightly increasing trend with fluctuations in the future under climatic and hydrologic changes

Lateral carbon flux through river networks is an important and poorly understood component of the global carbon budget. This work investigates how temperature and hydrology control the production and export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA. Using field measurements of daily stream discharge, evapotranspiration, and stream DOC concentration, we calibrated the catchment-scale biogeochemical reactive transport model BioRT-Flux-PIHM (Biogeochemical Reactive Transport–Flux–Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash–Sutcliffe efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate and compare the daily DOC production rates (Rp; the sum of the local DOC production rates in individual grid cells) and export rate (Re; the product of the concentration and discharge at the stream outlet, or load). Results showed that daily Rp varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily Re varied by more than 3 orders of magnitude and was strongly associated with variation in discharge and hydrological connectivity. In summer, high temperature and evapotranspiration dried and disconnected hillslopes from the stream, driving Rp to its maximum but Re to its minimum. During this period, the stream only exported DOC from the organic-poor groundwater and from organic-rich soil water in the swales bordering the stream. The DOC produced accumulated in hillslopes and was later flushed out during the wet and cold period (winter and spring) when Re peaked as the stream reconnected with uphill and Rp reached its minimum. The model reproduced the observed concentration–discharge (C–Q) relationship characterized by an unusual flushing–dilution pattern with maximum concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated that this nonlinearity was caused by shifts in the relative contribution of different source waters to the stream under different flow conditions. At low discharge, stream water reflected the chemistry of organic-poor groundwater; at intermediate discharge, stream water was dominated by the organic-rich soil water from swales; at high discharge, the stream reflected uphill soil water with an intermediate DOC concentration. This pattern persisted regardless of the DOC production rate as long as the contribution of deeper groundwater flow remained low (<18 % of the streamflow). When groundwater flow increased above 18 %, comparable amounts of groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C–Q patterns switched to a flushing-only pattern with increasing DOC concentration at high discharge. These results depict a conceptual model that the catchment serves as a producer and storage reservoir for DOC under hot and dry conditions and transitions into a DOC exporter under wet and cold conditions. This study also illustrates how different controls on DOC production and export – temperature and hydrological flow paths, respectively – can create temporal asynchrony at the catchment scale. Future warming and increasing hydrological extremes could accentuate this asynchrony, with DOC production occurring primarily during dry periods and lateral export of DOC dominating in major storm events.

The present work compiles a review on drinking waterborne outbreaks, with the perspective of production and distribution of microbiologically safe water, during 2000–2014. The outbreaks are categorised in raw water contamination, treatment deficiencies and distribution network failure. The main causes for contamination were: for groundwater, intrusion of animal faeces or wastewater due to heavy rain; in surface water, discharge of wastewater into the water source and increased turbidity and colour; at treatment plants, malfunctioning of the disinfection equipment; and for distribution systems, cross-connections, pipe breaks and wastewater intrusion into the network. Pathogens causing the largest number of affected consumers were Cryptosporidium, norovirus, Giardia, Campylobacter, and rotavirus. The largest number of different pathogens was found for the treatment works and the distribution network. The largest number of affected consumers with gastrointestinal illness was for contamination events from a surface water source, while the largest number of individual events occurred for the distribution network.

Despite the increasing Siberian river discharge, the sensitivity of streamflow to climate forcing/permafrost thawing is poorly quantified. Based on the Budyko framework and superposition principles, we detected and attributed the changes in streamflow regimes for the three great Siberian rivers (Ob, Yenisei, and Lena) during 1936–2019. Over the past 84 years, streamflow of Ob, Yenisei and Lena has increased by ∼7.7%, 7.4% and 22.0%, respectively. Intensified precipitation induced by a warming climate is a major contributor to increased annual streamflow. However, winter streamflow appears to be particularly sensitive to temperature. Whilst rising temperature can reduce streamflow via evapotranspiration, it can enhance groundwater discharge to rivers due to permafrost thawing. Currently, every 1 °C rise in temperature likely leads to 6.1%–10.5% increase in groundwater discharge, depending on the permafrost condition. For permafrost-developed basins, the contribution to increased streamflow from thawing permafrost will continue to increase in the context of global warming.

Coastal aquifers are commonly layered, and thus, a clear understanding of groundwater flow and salt transport in layered coastal aquifers is important for managing fresh groundwater. However, the influence of leakage between adjacent aquifers on flow and transport processes remains largely unknown where the influence of tides is considered. This study used laboratory experiments and numerical simulation to examine the processes of flow and transport within a tidal aquifer‐aquitard system (i.e., an unconfined aquifer underlain by a semi‐confined aquifer, with an intervening thin aquitard). The laboratory‐scale observations of the current study are the first observations of offshore fresh groundwater within a semi‐confined coastal aquifer. The numerical and laboratory results are in close agreement, revealing that upward leakage from the semi‐confined aquifer into the saltwater wedge of the overlying unconfined aquifer caused buoyant instabilities to form. The development of freshwater fingers created complex saltwater‐freshwater mixing, leading to mixed saltwater influx‐efflux patterns across the sloping aquifer‐ocean interface. Compared with non‐tidal conditions, tidal forces reduced the net upward leakage from the semi‐confined aquifer to the overlying unconfined aquifer. This increased the horizontal flow toward the sea, which in turn reduced the extent of the saltwater wedge in the semi‐confined aquifer. The higher rates of both fresh and saline submarine groundwater discharge (SGD), caused by tides, led to lower groundwater ages in the semi‐confined aquifer. These findings have important implications for unveiling the complex characteristics of seawater intrusion, SGD and geochemical hotspots within layered coastal aquifers. Plain Language Summary Coastal aquifers contain complex and dynamic hydrological and geochemical processes, which profoundly influence the global water cycle and chemical mass balance. Many coastal aquifers exhibit layerd structures in the form of high‐permeability aquifers alternating with thin aquitards. Inter‐aquifer leakage is a common issue in layered coastal aquifers, but few studies explore its impact on the mixing zone of saltwater wedges. The effects of tides on groundwater dynamics and the seawater extent in layered aquifers also remain poorly known. This study used laboratory experiments and numerical modeling to explore the effects of inter‐aquifer leakage and tides on flow and salinity dynamics within layered aquifer systems. We found that the upward leakage extended the mixing zones from the edges of the saltwater wedge to its interior within unconfined aquifers, leading to mixed saltwater influx‐efflux patterns across the aquifer‐ocean interface. The introduction of tides restricted the seawater extent in semi‐confined aquifers. This is a primary consequence of the tide‐induced increase in the horizontal freshwater flow toward the sea through the semi‐confined aquifer, in addition to the increases in the density‐driven seawater recirculation caused by tides. These findings highlight the important role of inter‐aquifer leakage and tides in layered coastal aquifers. Key Points Leakage from semi‐confined aquifers caused mixed‐convective flow within the saltwater wedge of overlying unconfined aquifers Tidal fluctuations reduced the net upward freshwater leakage, and consequently the extent of seawater, in semi‐confined coastal aquifers Tides increased both fresh and saline submarine groundwater discharge in semi‐confined aquifers, reducing groundwater ages

ABSTRACT Despite the relevance of submarine groundwater discharge (SGD) for ocean biogeochemistry, the microbial dimension of SGD remains poorly understood. SGD can influence marine microbial communities through supplying chemical compounds and microorganisms, and in turn, microbes at the land–ocean transition zone determine the chemistry of the groundwater reaching the ocean. However, compared with inland groundwater, little is known about microbial communities in coastal aquifers. Here, we review the state of the art of the microbial dimension of SGD, with emphasis on prokaryotes, and identify current challenges and future directions. Main challenges include improving the diversity description of groundwater microbiota, characterized by ultrasmall, inactive and novel taxa, and by high ratios of sediment-attached versus free-living cells. Studies should explore microbial dynamics and their role in chemical cycles in coastal aquifers, the bidirectional dispersal of groundwater and seawater microorganisms, and marine bacterioplankton responses to SGD. This will require not only combining sequencing methods, visualization and linking taxonomy to activity but also considering the entire groundwater–marine continuum. Interactions between traditionally independent disciplines (e.g. hydrogeology, microbial ecology) are needed to frame the study of terrestrial and aquatic microorganisms beyond the limits of their presumed habitats, and to foster our understanding of SGD processes and their influence in coastal biogeochemical cycles. The authors review the available literature on the microbial aspects of submarine groundwater discharge, from the freshwater aquifers to the coastal ocean, and identify current challenges and future directions to foster knowledge on microbial ecology at the land–ocean interface.