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The young water fraction of streamflow (Fyw), an important hydrological variable, has been calculated for the first time, for a monsoon‐fed coastal catchment in northern Vietnam. Oxygen stable isotopes (δ18O) from six river sites in the Day River Basin (DRB) were analysed monthly, between January 2015 and December 2018. River δ18O signatures showed sine wave variability, reflecting the amount effect and tropical (dry‐rainy) seasonality of the region. The δ18O composition of precipitation ranged from −12.67 to +1.68‰, with a mean value of −5.14‰, and in‐streamflow signatures ranged from −11.63 to −1.37‰ with a mean of −5.02‰. Fractions of young water (Fyw) were calculated from the unweighted and flow‐weighted δ18O composition of samples. Unweighted Fyw ranged between 29 ± 8% and 82 ± 21% with a mean value of 51 ± 19%, and was not significantly different from flow‐weighted Fyw (range between 33 ± 25% and 92 ± 73%, mean 52 ± 36%). Both unweighted and flow‐weighted Fyw were highest in the middle of stream and lowest in downstream sites, capturing the impacts of landuse changes, hydrology and human activities in the catchment. Our calculations imply that more than a half of rainwater reaches the DRB river mainstream within the first 3 months. The Fyw is much higher than the global average (of one‐third) and insensitive to discharge due to the combination of a humid catchment with high rainfall, low storage capacity, flat landscape and an intensive drainage system in the DRB. Also the low discharge sensitivity of Fyw in the DRB implies that the regional hydrology is severely altered by humans. [ABSTRACT FROM AUTHOR]

• NO 3 − is formed from nitrification of soil leachate, domestic waste, and NH 4 + fertilizers. • Denitrification occurs at heavily impacted domestic waste sites. • The Red River is autotrophic, despite being undersaturated in dissolved oxygen. • Manure and septic waste contribute 50 % of NO 3 − in the middle-lower section during the dry period. • High loading of fertilizer-leached NO 3 − is driven by agricultural activities. In many places around the world, anthropogenic activities have resulted in nitrate (NO 3 −) pollution and changes in the metabolic state of aquatic ecosystems. Here we combined stable isotope and physico-chemical monitoring to assess the sources of NO 3 − and the overall metabolic state within the Red River delta, Vietnam. River water stable isotope compositions (δ18O-H 2 O) ranged between −11.2 and −2.7 ‰, δ18O-NO 3 − between −7.1 and + 29.7 ‰ and δ15N-NO 3 − between −3.9 and + 14.0 ‰. We identified the dominant NO 3 − sources as: 1) soil leachate, 2) domestic waste flushed from urban areas, and 3) NH 4 + fertilizers washed from paddy fields. The relative impact of each source depends on geographical location within the delta and the time of year, due to dilution and concentration effects during wet and dry seasons. The primary NO 3 −source upstream is natural soil leachates, predominantly from tributaries connected to the Red River's main stream. Within the middle-lower section of Red River delta, urban pollution from manure and septic waste reaches as high as 50 % of the total NO 3 − load during dry season. NO 3 − leached from fertilizers is also high at sites in the middle of the delta, related to agricultural activities. Dissolved oxygen isotope (δ18O-O 2) values calculated from δ18O-H 2 O and δ18O-NO 3 − values indicate that the aquatic metabolism is net autotrophic (oxygen from primary production exceeds consumption by respiration), but high inputs of biodegradable organic matter from untreated domestic waste and high rates of sediment oxygen demand (SOD) and chemical oxygen demand (COD) have resulted in the whole river system becoming undersaturated in oxygen. High NO 3 − loads and low DO saturation are of critical concern and require mitigation practices to improve water quality for millions of people. [ABSTRACT FROM AUTHOR]

• First calculations of suspended metal pollution for the red river Delta, vietnam. • The global flux contribution is <0.2% and not a major source of oceanic pollution. • The delta is largely a sink of pollutants with a high level of deposition (∼50%). • 'Hotspots' of pollution due to dissolved organic matter, monsoons, and salinity. • Predicted increases in severe storms, saline intrusion and human activity can shift sink to source. Due to the depositional environment, river deltas are said to act as filters and sinks for pollutants. However, many deltas are also densely populated and rapidly urbanizing, creating new and increased sources of pollutants. These sources pose the risk of tipping these environments from pollution sinks to sources, to the world's oceans. We provide detailed seasonal and annual assessments of metal contaminants in riverine suspended particulate matter (SPM) across the densely populated Red River Delta (RRD), Vietnam. The global contributions of elements from the RRD are all <0.2% with many elemental fluxes <0.01%, suggesting the RRD is not a major source of elemental pollution to the ocean. However, 'hotspots' of metal pollution due to human activity and the impacts of tropical storm Son Tinh (July 2018) exceed both national level regulations and international measures of toxicity (e.g. enrichment factors). There is widespread 'extreme pollution' of Cd (enrichment factor >40) and concentrations of As higher than national regulation limits (>17 mg/Kg) at all sites other than one upstream, agricultural-dominated tributary in the dry season. These 'hotspots' are characterised by high inputs of organic matter (e.g. manure fertiliser and urban wastewater), which influences elemental mobility in the particulate and dissolved phases, and are potentially significant sources of pollution downstream. In addition, in the marine and fresh water mixing zone, salinity effects metal complexation with organic matter increasing metals in the particulate phase. Our calculations indicate that the delta is currently acting as a pollutant sink (as determined by high levels of pollutant deposition ∼50%). However, increased in-washing of pollutants and future projected increases in monsoon intensity, saline intrusion, and human activity could shift the delta to become a source of toxic metals. We show the importance of monitoring environmental parameters (primarily dissolved organic matter and salinity) in the RRD to assess the risk of transport and accumulation of toxic metals in the delta sediments, which can lead to net-increases in anthropogenic pollution in the coastal zone and the incorporation of toxic elements in the food chain. [ABSTRACT FROM AUTHOR]