Hailong Li

Coastal aquifers are threatened by seawater intrusion (SWI) from sea‐level rise (SLR) and storm surges. Vulnerability is highest in urban areas, where dense populations rely on fresh groundwater and saltwater can damage subsurface infrastructure. The effects of deep building foundations on transient SWI and recovery remain insufficiently understood. Three‐dimensional variable‐density simulations show that, at quasi‐steady state, low‐permeability foundations displace the interface slightly landward by reducing the effective flow cross‐section. Under SLR, foundations slow intrusion once saltwater reaches the foundation zone. Under storm surges, buildings with full‐depth foundations in the low‐permeability aquifer reduce peak whole‐aquifer excess salt mass by 38.8% and prolong its 90% flushing time by 41 years relative to the case without foundations. Aquifer permeability controls foundation‐zone salinization timing and post‐surge recovery rates. By stalling saltwater and impeding flushing, foundations may accelerate their corrosion, even without groundwater pumping. As coastal cities expand, the presence of foundations may exacerbate stresses on urban freshwater resources and infrastructure.

Elevated ammonium concentrations in deltaic groundwater pose a widespread environmental challenge, yet the microbial mechanisms linking depositional history to ammonium dynamics remain poorly understood. The Pearl River Delta, with the highest naturally occurring groundwater ammonium concentrations documented globally, provides a unique natural system to investigate these processes. Here, by integrating geochemical and metagenomic data, we show that fermentation-related genes are the most prevalent across all depositional zones, suggesting fermentation as the potential primary pathway for ammonium production, with the functional potential declining as sedimentary organic matter becomes increasingly recalcitrant with sediment age. Secondary mechanisms shift from nitrate reduction to nitrite ammonification across terrestrial-to-marine-dominated zones, reflecting salinity-driven metabolic partitioning. Notably, the marine-derived genus Brevirhabdus emerges as a key taxon linking depositional history to present-day biogeochemistry, demonstrating remarkable metabolic versatility. These findings demonstrate that paleo-depositional and hydrogeological evolution fundamentally shape microbial landscapes and dictate groundwater quality in deltaic systems worldwide.

Evaporation exerts a dominant influence on groundwater‐lake interactions in arid environments, yet its coupling with redox dynamics and trace element mobilization remains poorly understood. Here, we combine stable isotope ( δ ² H, δ ¹⁸ O) and trace element concentration analyses of manganese (Mn), arsenic (As), and barium (Ba) to quantify evaporation intensity and evaluate its geochemical effects in the Badain Jaran Desert, China. These trace elements were selected because they are commonly enriched in arid regions and highly sensitive to both evaporative concentration and redox transformations, posing potential risks to human health. High‐resolution sampling along two groundwater transects at South Sumujaran Lake reveals distinct isotopic and geochemical patterns. Deuterium excess ( d‐excess ) and evaporation coefficients jointly indicate that evaporative fractionation is stronger in the oxidation‐dominated eastern transect, where evaporation drives Mn and Ba accumulation, whereas in the reduction‐dominated western transect, redox‐controlled mobilization and desorption lead to As and Ba enrichment. At the regional scale, Bayesian isotope modeling of lakes across the Badain Jaran Desert indicates that pre‐evaporation δ ¹⁸ O decreases and the evaporation coefficient increases from southeast to northwest, reflecting regional gradients in precipitation, continentality, and aridity. The lakes are sustained primarily by local and mountain‐block groundwater recharge, with about 91% occurring under reducing conditions that enhance trace element mobility. These findings provide a process‐based understanding of how coupled evaporative and redox processes regulate isotopic evolution and solute dynamics in desert aquifers and highlight the growing vulnerability of groundwater quality in arid regions under intensified evaporative stress.

Mangrove ecosystems are vital intertidal transition zones where nutrient cycling sustains productivity and function. However, studies on the spatiotemporal distribution, transport fluxes, and controlling mechanisms of nutrient cycling in mangroves under land–sea interactions remain limited. In this study, we investigated the spatiotemporal distributions and fluxes via submarine groundwater discharge (SGD) of dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicon (DSi) with different seasons (autumn, winter and summer) and depths in a subtropical mangrove wetland. Research results indicated that mangrove ecosystems exhibited overall phosphorous limitation, especially in summer. Moreover, the spatiotemporal patterns and cycling of nutrients were influenced by land–sea interactions, mangrove development, sediment composition, and microbial processes, showing pronounced spatial heterogeneity. Higher nutrient fluxes from submarine groundwater discharge occurred in summer, and groundwater end‐member selection significantly influenced flux estimates. Nitrogen cycling shifted along the groundwater flow path from dissimilatory nitrate reduction to ammonium (DNRA) to nitrification in fringe mangroves, and from nitrification to denitrification and anammox in interior mangroves, driving nitrogen loss. Nitrogen loss was enhanced in winter, whereas nitrogen fixation was strengthened in summer. Seasonal changes further regulated phosphorus cycling, such that its release peaked in winter, while mobilization was offset by biological uptake in summer. These findings enhance our understanding of biogeochemical cycling in mangrove groundwater and support coastal mangrove ecosystem management.

Submarine groundwater discharge (SGD) derived nutrient inputs have been extensively documented. However, SGD‐derived carbon fluxes remain largely unconstrained, representing a critical gap in most coastal carbon budgets. Here, we resolve SGD and dissolved carbon budgets in the Pearl River Estuary (PRE), the largest estuary in Southern China surrounded by the world's largest urban conglomerate. Broadly‐defined SGD contributes 89%–96% of the dissolved inorganic carbon (DIC) pool (2–4 times riverine inputs) and 20%–70% of the dissolved organic carbon (DOC) fluxes of the PRE. SGD transports DIC exceeding total alkalinity (TAlk) by 2.7–7 times, potentially driving pH decline and acidification of nearshore waters. Groundwater p CO ₂ values are 10–36 times higher than estuarine waters. SGD‐derived DOC mineralization can decrease estuary water pH by 0.04–0.16 units and increase CO ₂ by 6.0–90.0 μmol L ⁻¹ , affecting local coral populations and benthic organisms. SGD also reduces seawater dissolved oxygen (DO) by 12–150 μmol L ⁻¹ and fuels the development of hypoxic zones. Overall, SGD regionally intensifies seawater hypoxia and acidification, creating challenging conditions for coral reef survival in an already stressed ecosystem. Our findings demonstrate that SGD should be integrated into carbon budgets and ecological assessments of the land‐ocean continuum.

Salt marshes are crucial for blue carbon sequestration and the mitigation of climate change. While submarine groundwater discharge (SGD) is increasingly recognized as an important pathway for carbon export in salt marsh tidal creek systems, current research predominantly relies on point‐scale, time‐series observations of creek water. Investigations employing extensive spatial coverage and depth‐resolved sampling within creek networks remain limited. This study implemented a sampling campaign across spring‐neap tidal cycles encompassing a wide area of a tidal creek and collecting groundwater samples from multiple depths at the creek bottom within the North Inlet‐Winyah Bay National Estuarine Research Reserve System in South Carolina, United States. By employing radium isotopes ( ²²⁶ Ra and ²²⁸ Ra) as tracers, we quantified SGD and associated carbon fluxes and elucidated the biogeochemical transformation processes occurring within the marsh‐creek basin. The results reveal substantial blue carbon exports via creek bottom SGD: a dissolved inorganic carbon (DIC) flux of 241 ± 61 mmol/m ² /d and net negative fluxes of dissolved organic carbon (−22 ± 5 mmol/m ² /d) and total nitrogen (−0.98 ± 0.27 mmol/m ² /d). DIC as the predominant form of carbon export via SGD is comparable to the carbon sequestration capacity of primary productivity and surpasses the net carbon burial of the marsh. A comparative analysis across the globe indicates that tidal creeks are a significant component influencing the coastal carbon budget balance process. These findings further highlight the critical role of groundwater‐transported carbon fluxes in the coastal carbon cycle, particularly within tidal creek basins, and emphasize the necessity of integrating groundwater processes into blue carbon assessments to effectively address climate change.