Anthropogenic impacts and quantitative sources of nitrate in a rural-urban canal using a combined PMF, δ15N/δ18O-NO3-, and MixSIAR approach.

Nitrate (NO3-) pollution in irrigation canals is of great concern because it threatens canal water use; however, little is known about it at present. Herein, a combination of positive matrix factorization (PMF), isotope tracers, and Mixing Stable Isotope Analysis in R (MixSIAR) was developed to identify anthropogenic impacts and quantitative sources of NO3- in a rural-urban canal in China. The NO3- concentration (0.99-1.93 mg/L) of canal water increased along the flow direction and was higher than the internationally recognized eutrophication risk value in autumn and spring. The inputs of the Fuhe River, NH4+ fertilizer, soil nitrogen, manure & sewage, and rainfall were the main driving factors of canal water NO3- based on principal component analysis and PMF, which was supported by evidence from δ15N/δ18O-NO3-. According to the chemical and isotopic analyses, nitrogen transformation was weak, highlighting the potential of δ15N/δ18O-NO3- to trace NO3- sources in canal water. The MixSIAR and PMF results with a <15% divergence emphasized the predominance of the Fuhe River (contributing >50%) and anthropogenic impacts (NH4+ fertilizer plus manure & sewage, >37%) on NO3- in the entire canal, reflecting the effectiveness of the model analysis. According to the MixSIAR model, (1) higher NO3- concentration in canal water was caused by the general enhancement of human activities in spring and (2) NO3- source contributions were associated with land-use patterns. The high contributions of NH4+ fertilizer and manure & sewage showed inverse spatial variations, suggesting the necessity of reducing excessive fertilizer use in the agricultural area and controlling blind wastewater release in the urban area. These findings provide valuable insights into NO3- dynamics and fate for sustainable management of canal water resources. Nevertheless, long-term chemical and isotopic monitoring with alternative modeling should be strengthened for the accurate evaluation of canal NO3- pollution in future studies.

Silt-size sediment provenance of the Jianggang radial sand ridges in the southwestern Yellow Sea identified by geochemistry of quartz and K-feldspar.

In this paper, the geochemistry of K-feldspar and quartz, combined with MixSIAR and MDS models, was applied to illustrate the provenance of silt-size sediments in the Jianggang RSRs of the Yellow Sea, which profoundly affect the circulation of organic matter. The geochemical distributions revealed an apparent shoreward migration for sediments in the western offshore compared to eastern offshore. The models showed that the onshore RSRs silt-size sediments were mainly from the Yangtze River Mouth and Old Yellow River Delta, with contributions of 43.5 ± 15.3 % and 48.4 ± 13.9 %, respectively. While the offshore RSRs sediments were primarily from the Yangtze River Mouth (61.9 ± 18.0 %). Meanwhile, the transport patterns of silt-size sediments were proposed for the first time. The load calculation further showed that the expansion rate of local coast in 2023-2043 will be lower than in 1997-2017. Therefore, it was recommended to take actions for coastal development.

Pollution source identification of nitrogen and phosphorus in the lower West Main Canal, the Ganfu Plain irrigation district (South China).

The degradation of surface water quality has been a widespread concern around the world. However, irrigation canal water does not attract much attention although it is important to agriculture and population. In this study, a 5-year water quality monitoring of surface water was conducted in the lower West Main Canal of the Ganfu Plain irrigation district to identify the levels and pollution sources of nitrogen and phosphorus.Over 75% of samples had total phosphorus (TP) concentrations of > 0.02 mg/L, and all samples had total nitrogen (TN) concentrations of > 0.2 mg/L, indicating a risk of eutrophication. The concentrations of NO3--N and NH4+-N averagely occupied 57% and 18% of TN, respectively. PCA analysis showed that phosphorus and nitrogen in canal water were associated with meteorological factors, urban life and surface runoff, agricultural cultivation, livestock-poultry breeding, and water-sediment interaction in the wet season, whereas they were affected by meteorological factors, industrial effluent, urban domestic sewage, and livestock-poultry breeding in the dry season. Absolute principal component score-multiple linear regression (APCS-MLR) model results revealed that (1) agricultural cultivation plus livestock-poultry breeding contributed 43.2% of TP in canal water in the wet season, while livestock-poultry breeding contributed 52.9% in the dry season, and (2) domestic sewage plus surface runoff contributed 29.4% of TN in the wet season, while livestock-poultry breeding contributed 45.9% in the dry season. The unidentified sources had significant contributions of > 20% for almost all variables. So further investigations are required for determining unidentified sources, and anthropogenic pollution control is imperative for canal water quality protection.

Pb and O isotopic characteristics of single minerals in the Jianggang radial sand ridges of the southwestern Yellow Sea: Implications for provenance and pathways of silt-size sediments.

Understanding the sediment provenance of the Jianggang radial sand ridges (RSRs) along the Jiangsu coast in the southwestern Yellow Sea is crucial for sustainable coastal development and land resource management. In this study, the provenance and transport patterns of silt-size sediments in the Jianggang RSRs were explored using Quartz oxygen (O) and K-feldspar lead (Pb) isotopic compositions and their large ion lithophile elements (LILEs) concentrations. The Pb-O isotopic compositions and concentrations of most LILEs in RSRs sediments were between those of the Yangtze River Mouth (YTZ), Old Yellow River Delta (OYR), and Modern Yellow River Mouth (MYR). The Pb-O isotopic compositions and typical elemental ratios were similar between the onshore and northwest offshore RSRs sediments, implying shoreward transport of offshore silt-size sediments. Through multidimensional scaling and graphic methods, it was found that the sediments of onshore and offshore RSRs originate mainly from the YTZ and OYR. Furthermore, the MixSIAR model showed that the contributions of the YTZ to onshore and offshore RSRs reached 33 ± 4 % and 36 ± 3 %, respectively. Followed by the OYR with contributions of 36 ± 3 % and 25 ± 8 %, respectively, and the contributions of the MYR and Korean Peninsula were <21 % and 8 %, respectively. Meanwhile, contributions from the Northern Chinese Deserts (approximately 10 %) deserved attention. Through the distribution of indicators, the transport patterns of silt-size sediments were proposed and compared with those of other fractions for the first time. According to the correlation analysis, the change in the area of central Jiangsu coast was mostly affected by terrestrial river supply and coastal mariculture. Therefore, it was necessary to control the scale of river reservoir construction and strengthen mariculture for sustainable land development and management. In future, comprehensive interdisciplinary and large temporal-spatial scales are suggested to further understand the coastal development.

Seasonal variation and provenance of organic matter in the surface sediments of the three gorges reservoir: Stable isotope analysis and implications for agricultural management.

The construction of the Three Gorges Dam has altered the hydrology and increased the trapping of sediment in the reservoir. This has also changed the composition and export of particulate organic matter in the Yangtze River. To understand the seasonal variations and sources of organic matter in sediments, total organic carbon (TOC), total nitrogen (TN), δ13Corg and δ15N in surface sediment samples from the mainstream and tributaries of the Three Gorges Reservoir were measured in the summer (July) and winter (December) of 2017, respectively. The results showed that the concentrations of TOC and TN in the surface sediments of the Three Gorges Reservoir were 0.79 %-1.46 % and 0.07 %-0.13 %, respectively. The ranges of δ13Corg and δ15N were - 26.35 ‰ to-24.70 ‰ and 2.59 ‰ to 5.67 ‰, respectively. According to δ13Corg and the TOC/TN ratio, the source range of organic matter was determined, and the contributions of different organic matter sources were quantified using a Bayesian mixed model. The results showed that soil organic matter and river plankton were the main sources of surface sediment organic matter in summer, whereas soil organic matter and aquatic vascular plants were the main sources in winter. The source of organic matter is related to biological factors in summer, whereas it is mainly caused by hydrodynamic conditions in winter. The analysis of δ15N further reveals that there are obvious external pollutants in the Three Gorges Reservoir, mainly related to artificial nitrogen fertiliser and domestic sewage. This study highlights the influence that soil nitrogen loss may be an important reason for the impact of agricultural non-point source pollution in the reservoir area, showing seasonal differences which were mainly affected by rainfall in summer and controlled by impoundment in winter. Hence, fine nitrogen management is required to reduce pollution in the Three Gorges Reservoir.

Controlling factors and health risks of groundwater chemistry in a typical alpine watershed based on machine learning methods.

Groundwater is a key water resource in alpine watersheds, but its quality is deteriorating due to human activities. The Golmud River watershed is a representative alpine watershed in Northwest China, and it was chosen to explore groundwater chemistry, associated controlling factors, source contributions, and potential health risks. The analysis includes the use of a self-organizing map (SOM), positive matrix factorization (PMF), ionic ratios, and a Monte Carlo simulation. The content of total dissolved solids in phreatic water was higher in the dry season and increased from the mountainous zone to the fine-soil plain-overflowing zone. Additionally, the water type varied from HCO3- to Cl- types whereas confined groundwater was chemically stable and of a HCO3- type. The SOM results showed a visual correlation between the ions in groundwater. The combination of SOM, PMF, and ionic ratios identified water-rock action as a dominant factor of groundwater chemistry. It was also found that Clusters I and III were mainly influenced by silicate weathering (a total contribution of 38.4 %), whereas evaporation was dominant in Cluster VI (a contribution of 32.5 %). Anthropogenic pollution was mainly associated with clusters V and IV and was related to industrial and agricultural activities during the snowmelt and wet seasons, and fluorine deposition formed by residential coal heating during the dry season (contributions of 1.4 % and 23.8 % in Clusters V and IV, respectively). The sudden increases in B3+ and Li+ in Cluster II were due to inputs from small tributaries (a contribution of 3.9 %). The probabilistic health risk assessment showed that fluoride posed a greater non-carcinogenic risk to human health than Sr2+, B3+, and NO3-, and its potential threat to children was more significant during the dry season than in other seasons. It is necessary for local governments to establish urgent fluoride emission control policies within the Golmud River watershed.

The isotopic composition of organic carbon, nitrogen and provenance of organic matter in surface sediment from the Jiangsu tidal flat, southwestern Yellow Sea.

The Jiangsu tidal flat is a significant organic matter reservoir, but quantitative studies of organic matter sources are scarce. In this study, total organic carbon (TOC) and total nitrogen (TN) concentrations and δ13Corg and δ15Ntotal values of surface sediment from Jiangsu tidal flat were investigated for their distributions, influencing factors, and sources of organic matter. TOC and TN were high in the center of study area and correlated well with grain size, indicating hydrodynamic influence on organic matter. High TOC/TN and low δ13Corg and δ15Ntotal in estuaries were characteristic of C3 plants, soil, and fertilizer sources, suggesting source effect on the distribution of organic matter. The MixSIAR model revealed that marine sources were dominant with a contribution reaching 56.9 %, followed by uniform of C3 plants, soil and fertilizer, while domestic sewage was least prominent. This study enriched theories of the biogeochemical cycle and ecological protection in the southwestern Yellow Sea.

Strontium isotopic and hydrochemical characteristics of shallow groundwater and lake water in the Badain Jaran Desert, North China.

Shallow groundwater and lake water are the dominant water resources in the Badain Jaran Desert. There are still controversial hypotheses related to the origin of groundwater in this desert. Few studies have been conducted to explore the Sr provenance of these waters and assess the water-rock interactions using a Sr isotope approach until now. In this text, the Sr isotope data of waters in the hinterland of the Badain Jaran Desert and neighbouring areas are reported. The waters in the Badain Jaran Desert have few links to its surrounding rivers, but could be influenced by the precipitation in the Yabulai Mountains. The 87Sr/86Sr ratio changes constantly, while the Sr2+ concentration of shallow groundwater gradually decreases from Yabulai to the desert hinterland to Gurinai-Guaizihu. Combined with hydrochemical data and hydrodynamic conditions, these results show that the dissolved Sr of waters in the desert hinterland is controlled by the Yabulai precipitation and catchment weathering. They further show that the desert shallow groundwater Sr originates mainly from the Yabulai precipitation (> 94 %), while whole-rock weathering contributes little (< 6 %), as calculated using isotope mass balance equations. Relative Sr contributions to lakes from shallow groundwater and catchment weathering are calculated to be 92.5 and 7.5 %, respectively.

Chemical speciation of phosphorus in surface sediments from the Jiangsu Coast, East China: Influences, provenances and bioavailabilities.

Sixty-one surface sediment samples collected from the Jiangsu Coast (JSC), East China were investigated to explore the influences, provenances and bioavailabilities of P species. Authigenic and detrital P fractions were the dominant species, accounting for 28.53% and 44.04% of the total P content, respectively. Exchangeable, Fe-bound and organic P fractions were biologically available, with an average total of 5.94 µmol/g; this value was governed by grain sizes and the organic matter and carbonate contents. Exchangeable and organic P fractions were transformed between each other, while contributing to the formation of Fe-bound and authigenic P. Phosphorus in the JSC sediments originated mainly from the Yellow River. The unique distribution pattern of P species in the JSC depends on P sources and local environments. These findings improve our understanding of the P cycle and eutrophication in the Jiangsu Coastal Zone.

Base flow in the Yarlungzangbo River, Tibet, maintained by the isotopically-depleted precipitation and groundwater discharge.

The extension-induced rift systems on the Tibetan Plateau (TP) may convey large amount of groundwater to rivers, but sources and flow paths of such groundwater are unknown. The Yarlungzangbo River (YR) is the only large river that traverses the southern Tibetan plateau from west to east, following one major suture zone that is cut by extensional normal faults. The faults could influence the flow paths of groundwater discharging to the river. In this study, O and H isotopes, major ions and 222Rn concentrations are analyzed along the YR, and interpreted in relation to structural geology and tectonics. The YR exhibits an abrupt change of isotopic and chemical compositions along with a large increase in flow where the middle reach intersects NE-SW-trending rifts. Low values of δD and δ18O and high concentrations of major ions and 222Rn in the middle reach show that waters are modified isotopically and chemically by a variety of possible water origins, such as recharge of high-altitude glacier melt and discharge from groundwater. Groundwater contributes 27 to 40% of the river flow in the middle reach. Isotopically-light meltwater from high-altitude glacier melt cannot account for the isotope composition of the present outflow of groundwater. The O and H isotope data in the YR and discharging groundwater can be well explained by the groundwater originated as paleo-precipitation during a cooler time, such as the late Pleistocene to early Holocene. The paleo-groundwater discharge can account for about 36 × 108 m3 water budget unbalance in the middle reach. The study provides the first clear isotope evidence for the source of groundwater discharge into a large river through favorable conduits in large-scale active tensile fault zones and confirms the regional scale of groundwater flow on the Tibetan Plateau. Understanding the characteristics and changes of streamflow and surface-groundwater circulation on the Tibetan Plateau will help to manage water resources under a changing environment.

Evolution of groundwater chemistry in coastal aquifers of the Jiangsu, east China: Insights from a multi-isotope (δ2H, δ18O, 87Sr/86Sr, and δ11B) approach.

Groundwater salinization is currently a very serious and challenging issue in many parts of the world. With an increasing demographic pressure and remarkable changes of water and land uses over the last decades, the multilayer coastal aquifer system of Jiangsu province, east China, was affected by increasing salinization. In this study, we investigate the groundwater salinization process and the salinity sources of the aquifer system in Nantong area (southern part of the Jiangsu coastal plain) using a multi-isotope (δ2H, δ18O, 87Sr/86Sr, and δ11B) approach. The results show that the TDS (total dissolved solids) values in most deep groundwater samples are generally lower than those of the shallow groundwater samples. The TDS of both shallow and deep groundwater increase from western Nantong (inland) to the eastern coastal region of the Yellow Sea. The chemical types transform from Ca-Mg-HCO3 or Mg-Ca-HCO3 to NaCl. The stable hydrogen and oxygen isotopes signatures of the groundwater samples indicate that local precipitation likely acts as the main recharge source of both the shallow and deep confined groundwater systems. The deep groundwater shows more depleted isotopes, suggesting recharging by the precipitation under a cold climate before the Holocene period. The shallow groundwater features heavier water isotopes, indicating recharging source from recent precipitation under a warm climate. The variations in δ11B and 87Sr/86Sr of groundwater samples can be explained by the changes of solute sources. In the inland region (western Nantong), shallow groundwater with higher TDS is mainly caused by evaporation-induced concentration, whereas in coastal areas, seawater intrusion exerts a major influence on the chemical composition of the shallow groundwater. Our results show that that seawater intrusion mainly occurs in eastern and southeastern Nantong area. We also find that hydraulic connection between shallow and deep groundwater is strengthened by continuous overexploitation, and deep groundwater is mixed with shallow groundwater at some points. The mixing between upper saline water and deep freshwater, together with water-rock interactions, likely explain the observed low salinity in deep groundwater in coastal areas. Overall, with growing observations of salty seawater intrusion in the estuary region of the Yangtze River, future efforts are needed to prevent further seawater intrusion as sea level rises and groundwater table declines. In this context, our findings provide key information for groundwater management in other coastal aquifers, east China.

Phosphorus species in bottom sediments of the Three Gorges Reservoir during low and high water level periods.

Eutrophication and algal blooms have recently been found in the backwater areas of some tributaries in the Three Gorges Reservoir (TGR), for which phosphorus (P) is an important driving factor. However, P species in the TGR sediments at different water levels were little known. In this study, five P species of the TGR sediments during the high and low water level periods were analyzed with a sequential extraction method. The total P (TP) concentrations were 714.88 ± 37.86 µg/g and 697.57 ± 111.49 µg/g at the low and high water levels, respectively. The concentrations of P species decreased in the orders detrital P > authigenic P > organic P > iron-bound P > exchangeable P at the low water level and detrital P > organic P > authigenic P > iron-bound P > exchangeable P at the high water level. P in the TGR sediment sourced mainly from the upstream input at the low water level but from the inputs of tributaries and hillslope soils at the high water level. The bioavailable P (BAP) possessed > 29.5% of total P in the two periods. The total storages of total P and BAP were estimated to be about 1.34 × 106 t and 1.77 × 105 t, respectively during 2003-2017. It was further found that the BAP concentration significantly increased from periods I (2003-2009), II (2010-2014), to III (2015-2017), while the deposition flux and storage of BAP were the highest in period II. Our findings provide new insight into the P cycle and benefit eutrophication treatment in the TGR.

Major ion chemistry of a representative river in South-central China: Runoff effects and controlling mechanisms.

The Gan River is a large tributary of the Yangtze River in Jiangxi Province, South-central China. Hydrochemical data for this river were analyzed for the period 1958-2016. Ca2+, Na+ + K+, HCO3-, and SO42- were dominant in river water, and pH and total dissolved solids (TDS) varied from 6.0 to 8.8 and 15.7 to 141 mg/L, respectively. The chemical composition of river water was different between the two periods 1958-1979 and 1980-2016. Monthly yields of all ions were positively correlated with river runoff. Monthly yields of SO42-, NO3-, and Cl- were more positively correlated with river runoff before 1980, indicating non-point sources, while multiple sources were indicated after 1980. Sea salt-sourced Cl- comprised less than 19% of the total Cl- in river water. Weathering of basin rocks with sulfuric acid reflected strengthening of anthropogenic activities after 1980. This was reflected by increases in Cl-/(Na+ + Cl-) and SO42-/(Na+ + Cl-) with gross domestic production, population, coal consumption, fertilizer use, and wastewater discharge. Although water quality in the Gan River makes the water acceptable for drinking according to the World Health Organization standards, increases in Cl- and NO3- concentrations after 1980 are of some concern.

Isotope fractionation of sandy-soil water during evaporation - an experimental study.

Soil samples containing water with known stable isotopic compositions were prepared. The soil water was recovered by using vacuum/heat distillation. The experiments were held under different conditions to control rates of water evaporation and water recovery. Recoveries, δ18O and δ2H values of the soil water were determined. Analyses of the data using a Rayleigh distillation model indicate that under the experimental conditions only loosely bound water is extractable in cases where the recovery is smaller than 100 %. Due to isotopic exchange between vapour and remaining water in the micro channels or capillaries of the soil matrix, isotopic fractionation may take place under near-equilibrium conditions. This causes the observed relationship between δ2H and δ18O of the extracted water samples to have a slope close to 8. The results of this study may indicate that, in arid zones when soil that initially contains water dries out, the slope of the relationship between δ2H and δ18O values should be close to 8. Thus, a smaller slope, as observed by some groundwater and soil water samples in arid zones, may be caused by evaporation of water before the water has entered the unsaturated zone.