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Human activities drive changes in freshwater ecosystems by altering biogeochemical cycles. Freshwater networks provide important ecosystem services to human societies by purifying water and serving as an intermediary between terrestrial and marine systems. On high volcanic tropical islands, human activities are compartmentalized by steep terrain that delineates watershed boundaries. Patterns of land use affect adjacent stream ecosystems through runoff of sediment and nutrients, which fluctuates in the tropics as a result of seasonal rainfall. Here, we sought to reveal human impacts on nutrient and sediment regimes of tropical rivers by tracking patterns of river chemistry across a series of watersheds on Moorea, French Polynesia, between 2018 and 2019. Repeated sampling of rivers across a gradient of human activities revealed that water chemistry varied seasonally and with respect to rainfall and land use. In particular, dissolved inorganic nitrogen was more concentrated in rivers of watersheds with higher amounts of land clearing during the rainy season, and total suspended solids and phosphate were higher when recent rainfall was high. Importantly, the water quality of the rivers on Moorea repeatedly exceeded safe water quality standards established for similar high tropical islands in the Pacific. Our results show that differential land use across the landscape can have a substantial impact on the amounts of nutrients and sediment that tropical rivers transport, which on tropical islands could facilitate movement of materials from land to sea as precipitation increases with intensifying climate change.

Dissolved and suspended toxic elements in water discharged from abandoned and active mining areas pose several critical issues, since they represent a threat to the environment. In this work, we investigated the water, suspended particulates, and stream sediments of a 2.1 km long creek (Fosso della Chiusa) that is fed by waters draining the galleries of the abandoned Hg mining area of Abbadia San Salvatore (Mt. Amiata, Tuscany, central Italy). The geochemical results show evidence that the studied matrices are characterized by relatively high concentrations of Hg and As, whereas those of Sb are generally close to or below the instrumental detection limit. Independent of the matrices, the concentration of As decreases from the emergence point to the confluence with the Pagliola creek. In contrast, Hg concentrations display more complex behavior, as water and sediment are mainly characterized by concentrations that significantly increase along the water course. According to the geoaccumulation index (Igeo), sediments belong to Class 6 (extremely contaminated) for Hg. The Igeo of As varies from Class 6, close to the emergence, to Class 2 (moderately contaminated), dropping to Class 0 (uncontaminated) at the confluence with the Pagliola creek. Finally, the total mass load of Hg and As entering the Pagliola creek was computed to be 1.3 and 0.5 kg/year, respectively, when a mean flow rate of 40 L/s was considered. The calculated loads are relatively low, but, when the Fosso della Chiusa drainage basin is taken into account, the specific load is comparable to, or even higher than, those of other mining areas.

Continental weathering plays an important role in regulating atmospheric CO levels. Chemical weathering in glacial areas has become an intensely focused topic in the background of global change compared with other terrestrial weathering systems. However, research on the weathering of the glacial areas in the Yarlung Tsangpo River Basin (YTRB) is still limited. In this article, the major ions of the Chaiqu and Niangqu catchments in the YTRB have been investigated to illustrate the chemical weathering rates and mechanisms of the glacier areas in the YTRB. Ca and HCO dominate the major ions of the Chaiqu and Niangqu rivers, accounting for about 71.3% and 69.2% of the TZ of the Chaiqu (the total cations, TZ = Na + K + Ca + Mg , in µeq/L), and about 64.2% and 62.6% of the TZ of the Niangqu. A Monte Carlo model with six end-members is applied to quantitatively partition the dissolved load sources of the catchments. The results show that the dissolved loads of the Chaiqu and Niangqu rivers are mainly derived from carbonate weathering (accounting for about 62.9% and 79.7% of the TZ , respectively), followed by silicate weathering (about 25.8% and 7.9% of the TZ , respectively). The contributions of precipitation and evaporite to the Chaiqu rivers are about 5.0% and 6.2%, and those to the Niangqu rivers are about 6.3% and 6.2%. The model also calculated the proportion of sulfuric acid weathering in the Chaiqu and Niangqu catchments, which account for about 21.1% and 32.3% of the TZ , respectively. Based on the results calculated by the model, the carbonate and silicate weathering rates in the Chaiqu catchment are about 7.9 and 1.8 ton km a , and in the Niangqu catchment, the rates are about 13.7 and 1.5 ton km a . The associated CO consumption in the Chaiqu catchment is about 4.3 and 4.4 × 10 mol km a , and about 4.3 and 1.3 × 10 mol km a in the Niangqu catchment. The chemical weathering rates of the glacier areas in the YTRB show an increasing trend from upstream to downstream. Studying the weathering rates of glacier catchments in the Tibetan Plateau (TP) reveals that the chemical weathering rates of the temperate glacier catchments are higher than those of the cold glacier catchments and that lithology and runoff are important factors in controlling the chemical weathering of glacier catchments in the TP. The chemical weathering mechanisms of glacier areas in the YTRB were explored through statistical methods, and we found that elevation-dependent climate is the primary control. Lithology and glacial landforms rank second and third, respectively. Our results suggest that, above a certain altitude, climate change caused by tectonic uplift may inhibit chemical weathering. There is a more complex interaction between tectonic uplift, climate, and chemical weathering.

This study analyzed 25 river water samples collected from the Bogacayi River in Antalya, Turkey, to evaluate the potential risk of pollution by heavy metals. Concentrations of As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se, Sr, and V were determined by inductively coupled plasma mass spectrometry (ICP-MS). The method was validated prior to analysis in terms of linearity, limit of detection (LOD), limit of quantification (LOQ), and recovery. In addition, a certified standard (SPS-SW2 surface water) was used to verify method trueness. Method validation data and results obtained from the certified material suggested that the method could be applied to determine elemental compositions of the samples. Although various concentrations of As, Ba, Cd, Cr, Cu, Mn, Ni, Pb, and Sr were found in the samples, no Hg, V, Co, and Se concentrations were found. The highest concentration of Pb, Cd, and As was found in the samples from the 22nd, 16th, and 5th sampling stations, respectively. Concentrations of the studied elements were aligned from high to low as Sr > Ba > Ni > Cr > Cu > Mn > Pb > As > Cd. To evaluate the risk potential of metallic pollution, the data were used to calculate the heavy metal pollution index (HPI). The HPI values were found to be in the range from 7.81 to 43.97 (mean 25.48). Samples from upstream seemed to show lower risk potentials (<15) than those from downstream (>30); however, all HPI values were lower than 100, which is the critical HPI value for drinking safety.

Environmental exposure to active pharmaceutical ingredients (APIs) can have negative effects on the health of ecosystems and humans. While numerous studies have monitored APIs in rivers, these employ different analytical methods, measure different APIs, and have ignored many of the countries of the world. This makes it difficult to quantify the scale of the problem from a global perspective. Furthermore, comparison of the existing data, generated for different studies/regions/continents, is challenging due to the vast differences between the analytical methodologies employed. Here, we present a global-scale study of API pollution in 258 of the world’s rivers, representing the environmental influence of 471.4 million people across 137 geographic regions. Samples were obtained from 1,052 locations in 104 countries (representing all continents and 36 countries not previously studied for API contamination) and analyzed for 61 APIs. Highest cumulative API concentrations were observed in sub-Saharan Africa, south Asia, and South America. The most contaminated sites were in low- to middle-income countries and were associated with areas with poor wastewater and waste management infrastructure and pharmaceutical manufacturing. The most frequently detected APIs were carbamazepine, metformin, and caffeine (a compound also arising from lifestyle use), which were detected at over half of the sites monitored. Concentrations of at least one API at 25.7% of the sampling sites were greater than concentrations considered safe for aquatic organisms, or which are of concern in terms of selection for antimicrobial resistance. Therefore, pharmaceutical pollution poses a global threat to environmental and human health, as well as to delivery of the United Nations Sustainable Development Goals.

Pesticides are ubiquitous environmental pollutants negatively affecting ecosystem and human health 1 , 2 . About 3 Tg of pesticides are used annually in agriculture to protect crops 3 . How much of these pesticides remain on land and reach the aquifer or the ocean is uncertain. Monitoring their environmental fate is challenging, and a detailed picture of their mobility in time and space is largely missing 4 . Here, we develop a process-based model accounting for the hydrology and biogeochemistry of the 92 most used agricultural pesticide active substances to assess their pathways through the principal catchments of the world and draw a near-present picture of the global land and river budgets, including discharge to oceans. Of the 0.94 Tg net annual pesticide input in 2015 used in this study, 82% is biologically degraded, 10% remains as residue in soil and 7.2% leaches below the root zone. Rivers receive 0.73 Gg of pesticides from their drainage at a rate of 10 to more than 100 kg yr −1  km −1 . By contrast to their fate in soil, only 1.1% of pesticides entering rivers are degraded along streams, exceeding safety levels (concentrations >1 μg l − 1 ) in more than 13,000 km of river length, with 0.71 Gg of pesticide active ingredients released to oceans every year. Herbicides represent the prevalent pesticide residue on both land (72%) and river outlets (62%). A global assessment of the mobility of 92 agricultural pesticides from points of application in major agricultural catchments downstream to rivers and oceans identifies flow pathways and pollution hotspots in which monitoring could improve risk mitigation.

The Nakdong River is the longest river in South Korea, and flows through various geological terrains with different land use characteristics; therefore, the geochemistry of its water is expected to be influenced by many factors. In this work, the geochemical characteristics of the Nakdong River were examined, and its chemical compositions, δD, δ 18 O, and δ 13 C DIC values, and 87 Sr/ 86 Sr ratios were determined to investigate the geological and anthropogenic effects on the geochemistry of the Nakdong River water. The obtained concentrations of major ions were strongly affected by both the anthropogenic activity and weathering of the rocks. With increasing the flow distance, the ion concentrations slightly increased; and after the inflow of the Kumho River, which was the largest tributary running through Daegu (the fourth largest city in South Korea), the concentrations of Na and SO 4 ions abruptly increased and decreased again, suggesting the existence of strong anthropogenic effects caused by sewage treatment plants and dyeing industrial complex. Other activities such as agricultural ones also increased the NO 3 concentration. In July, the high precipitation level from tropical cyclones and downpours decreased the ion concentrations as well as the δD and δ 18 O values. The δ 13 C DIC magnitudes showed that the dissolved inorganic carbon mainly originated from mineral weathering upstream, while the oxidation of soil organic materials influenced by agricultural activity became more important downstream. The 87 Sr/ 86 Sr ratios revealed that in the upstream regions, the weathering of granite and gneiss complex was dominant, while in the downstream regions, the weathering of sedimentary rocks became more important. The weathering and anthropogenic effects on the river water chemistry were also demonstrated using statistical analysis, which revealed that the water geochemistry was mostly influenced by the anthropogenic sources, including industrial complex, represented by Na, Cl, and SO 4 . The obtained results show that, as compared to the geochemistry of the Han River (which is also a major river in Korea), the geochemistry of the Nakdong River is more influenced by anthropogenic activities (including agriculture and the industrial complex) due to the different land use.

A two-year study of mercury deposition in the Arctic finds that the main source of mercury is gaseous elemental mercury, which is deposited throughout the year and leads to very high soil mercury levels. Sinking mercury in the Arctic tundra Anthropogenic activities have led to large-scale mercury pollution in the Arctic, but it remains uncertain whether wet deposition of oxidized mercury via precipitation and sea-salt-induced chemical cycling of mercury are responsible for the high Arctic mercury load. This paper presents a mass-balance study of mercury deposition and stable isotope data from the Arctic tundra, and finds that the main source of mercury is in fact derived from gaseous elemental mercury, with only minor contributions from the other two suggested sources. Consistently high soil mercury concentrations derived from gaseous elemental mercury along an inland-to-coastal transect suggest that the Arctic tundra might be a globally important mercury sink and might explain why Arctic rivers annually transport large amounts of mercury to the Arctic Ocean. Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic 1 , 2 , 3 , 4 , 5 , 6 . It has been suggested that sea-salt-induced chemical cycling of Hg (through ‘atmospheric mercury depletion events’, or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg( ii )). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned 2 , 7 . Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide 8 , raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg( ii ) via precipitation or AMDEs. We find that deposition of Hg(0)—the form ubiquitously present in the global atmosphere—occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean 9 , 10 , 11 .

Particulate organic carbon export from the terrestrial biosphere is primarily controlled by physical erosion, and tectonic and climatic forcing of physical erosion may favour biospheric particulate organic carbon sequestration over silicate weathering as a long-term atmospheric carbon dioxide sink. Physical erosion drives of carbon export to the ocean The nature and efficiency of riverine export of particulate organic carbon to the ocean, including the balance between particulate organic carbon derived from the terrestrial biosphere and from rock-derived organic carbon, affects the atmospheric carbon inventory on many different timescales. The fluxes of biospheric and petrogenic organic carbon and the mechanisms controlling particulate organic carbon export, however, remain poorly understood. This paper provides global estimates for biospheric and petrogenic particulate organic carbon fluxes based on data from a suite of river systems. The study also reveals that particulate organic carbon export is primarily controlled by physical erosion and that tectonic and climatic forcing of physical erosion may favour biospheric particulate organic carbon sequestration over silicate weathering as a long-term atmospheric carbon dioxide sink. Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a broad range of timescales 1 , 2 , 3 , 4 , 5 . On geological timescales, the balance between sequestration of POC from the terrestrial biosphere and oxidation of rock-derived (petrogenic) organic carbon sets the magnitude of the atmospheric carbon and oxygen reservoirs 6 , 7 . Over shorter timescales, variations in the rate of exchange between carbon reservoirs, such as soils and marine sediments, also modulate atmospheric carbon dioxide levels 1 . The respective fluxes of biospheric and petrogenic organic carbon are poorly constrained, however, and mechanisms controlling POC export have remained elusive, limiting our ability to predict POC fluxes quantitatively as a result of climatic or tectonic changes. Here we estimate biospheric and petrogenic POC fluxes for a suite of river systems representative of the natural variability in catchment properties. We show that export yields of both biospheric and petrogenic POC are positively related to the yield of suspended sediment, revealing that POC export is mostly controlled by physical erosion. Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of and megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering. We conclude that burial of biospheric POC in marine sediments becomes the dominant long-term atmospheric carbon dioxide sink under enhanced physical erosion.

Methane (CH 4 ) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH 4 emissions from freshwater ecosystems 1 , 2 , providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH 4 emissions remain highly uncertain 3 , 4 . Here we report a spatially explicit global estimate of CH 4 emissions from running waters, accounting for 27.9 (16.7–39.7) Tg CH 4  per year and roughly equal in magnitude to those of other freshwater systems 5 , 6 . Riverine CH 4 emissions are not strongly temperature dependent, with low average activation energy ( E M  = 0.14 eV) compared with that of lakes and wetlands ( E M  = 0.96 eV) 1 . By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land–water connections in regulating CH 4 supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land. A spatially explicit global estimate reveals that land–water connections are important for regulating methane supply to running waters, and that these connections are vulnerable to both climate change and direct human modifications of the land.