Multi-isotope calibration for inductively coupled plasma mass spectrometry.

Multi-isotope calibration (MICal) is a novel approach to calibration for inductively coupled plasma mass spectrometry (ICP-MS). In MICal, only two calibration solutions are required: solution A, composed of 50% v v-1 of sample and 50% v v-1 of a standard solution containing the analytes, and solution B, composed of 50% v v-1 of sample and 50% v v-1 of a blank solution. MICal is based on monitoring the signal intensities of several isotopes of the same analyte in solutions A and B. By plotting the analytical signals from solution A in the x-axis, and from solution B in the y-axis, the analyte concentration in the sample is calculated using the slope of that graph and the concentration of the reference standard added to solution A. As both solutions contain the same amount of sample, matrix-matching is easily achieved. In this proof-of-concept study, MICal was applied to the determination of Ba, Cd, Se, Sn, and Zn in seven certified reference materials with different matrices (e.g., plant materials, flours, and water). In most cases, MICal results presented no statistical difference from the certified values at a 95% confidence level. The new strategy was also compared with traditional calibration methods such as external calibration, internal standardization and standard additions, and recoveries were generally better for MICal. This is a simple, accurate, and fast alternative method for matrix-matching calibration in ICP-MS. Graphical abstract Multi-isotope calibration: fast and innovative matrix-matching calibration for ICP-MS.

Contrasting nitrogen transport patterns in subtropical basins revealed by combined multiple isotopic analyzes and hydrological simulations.

Although enhancing the knowledge of nitrogen (N) dynamics in aquatic systems is crucial for basin N management, there is still a lack of theories on the patterns of basin N sources and transport because of the intricate influence of human activities, climatic conditions, landscape patterns, and topography on the trajectory of basin N. To shed new light on the patterns of basin N sources and transport in the Chinese subtropical monsoon region, this study provides a comprehensive approach combining multiple isotopes and hydrological model based on monthly records of hydro-chemistry and isotopes (18O-NO3- /15N-NO-3 and 18O-H2O /2H-H2O) for river water, groundwater and rainfall in three basins over multiple years. Our observations of hydro-chemistry showed that fluvial N levels in highly urbanized basins (3.05 ± 1.42 mg·L-1) were the highest and were characterized by higher levels in the dry season. In the agricultural basin, fluvial N levels in February and March were approximately 1.9 times higher than those in the other months. The fluvial N load was higher in agricultural basins (0.624-0.728 T N km -2 y -1) than in urban basins (0.558 T N km -2 y -1), primarily because of variations in sewage treatment rates and fertilizer application. In highly urbanized basin, manure and sewage (46.9 %) were the dominant sources of fluvial N, which were discharged into rivers after treatment. In the plain agricultural basin, a substantial portion of diffused residential sewage leaches into aquifers and is stored. In the hilly agro-forest mixed basin, the high baseflow coefficient (75.8 %) and the key role of groundwater N, mainly from soil N (27.3 %), chemical fertilizers (20.2 %), manure and sewage (46.6 %), to fluvial N (26.5 %) indicated that a high proportion of the N sources leached into the aquifer and were then transported to rivers. For the first time, this study integrated multiple methods to substantiate the proposed typical patterns of N sources and transport within the basins. These findings have significant implications for tailored basin-specific N management strategies.

The distinct phases of fresh-seawater mixing intricately regulate the nitrogen transformation processes in a high run-off estuary: Insight from multi-isotopes and microbial function analysis.

Excessive anthropogenic nitrogen inputs lead to the accumulation of nitrogen, and significantly impact the nitrogen transformation processes in estuaries. However, the governing of nitrogen during its transport from terrestrial to estuary under the influence of diverse human activities and hydrodynamic environments, particularly in the fresh-seawater mixing zone, remains insufficient researched and lack of basis. To address this gap, we employed multi-isotopes, including δ15N-NO3-, δ18O-NO3-, δ15N-NH4+, and δ15N-PN, as well as microbial function analysis, to investigate the nitrogen transformation processes in the Pearl River Estuary (PRE), a highly anthropogenic and terrestrial estuary. Principle component analysis (PCA) confirmed that the PRE could clearly partitioned into three zone, e.g., terrestrial area (T zone), mixing area (M zone) and seawater area (S zone), in terms of nitrogen transportation and transformation processes. The δ15N-NO3- (3.38±0.60‰) and δ18O-NO3- (6.35±2.45‰) results in the inner estuary (T area) indicate that NO3-attributed to the domestic sewage and groundwater discharge in the river outlets lead to a higher nitrification rate in the outlets of the Pearl River than in the reaching and seawater intrusion areas, although nitrate is rapidly diluted by seawater after entering the estuary. The transformation of nitrogen in the T zone was under significant nitrogen fixation (0.61 ± 0.22 %) and nitrification processes (0.0043 ± 0.0032 %) (presumably driven by Exiguobacterium sp. (14.1 %) and Cyanobium_PCC-6307 (8.1 %)). In contrast, relatively low δ15N-NO3- (6.83 ± 1.24‰) and high δ18O-NO3- (22.13±6.01‰) imply that atmospheric deposition has increased its contribution to seawater nitrate and denitrification (0.53±0.13 %) was enhanced by phytoplankton/bacterial (such as Psychrobacter sp. and Rhodococcus) in the S zone. The assimilation of NH4 results from the ammonification of NO3- reduces δ15N-NH4+ (5.36 ± 1.49‰) and is then absorbed by particulate nitrogen (PN). The retention of nitrogen when fresh-seawater mixing enhances the elevation of δ15N-NH4+ (8.19 ± 2.19‰) and assimilation of NH4+, leading to an increase in PN and δ15N-PN (6.91 ± 1.52‰) from biological biomass (mainly Psychrobacter sp. and Rhodococcus). The results of this research demonstrate a clear and comprehensive characterization of the nitrogen transformation process in an anthropogenic dominated estuary, highlighting its importance for regulating the nitrogen dissipation in the fresh-seawater mixing process in estuarine ecosystems.

Differentiation of natural and anthropogenic contaminant sources using isotopic and microbial signatures in a heavily cultivated coastal area.

Hydrogeochemical and multiple isotope data for groundwater samples were obtained and interpreted to discriminate anthropogenic and natural contaminant sources in a coastal aquifer underlying a heavily cultivated watershed in Hwaseong, South Korea. The local aquifers are vulnerable to contamination, due to high anthropogenic N inputs and the location close to the ocean facilitating seawater intrusion. Thus, to effectively control the groundwater quality in the study area, it is necessary to differentiate between anthropogenic and natural contaminant sources. The concentrations of NO3-N in the groundwater ranged between 0.14 and 45.6 mg/L in August 2015 and 0.2-39.6 mg/L in March 2016. High concentrations of Cl- (388-1107 mg/L) and a high electrical conductivity (1027-2715 µS/cm) were observed in the study area, suggesting that the groundwater was affected by seawater intrusion. Furthermore, δ15N-NO3-, δ34S-SO42- values and 87Sr/86Sr of groundwater were determined to reveal the origins of the natural and anthropogenic contaminants and the groundwater mean residence times (MRT) and 87Sr/86Sr ratios were used to assess the hydrogeochemical processes along the flow path in the study area. Young groundwater was affected by an anthropogenic contamination source with contributions of 26-46% adding nitrate to the aquifer, whereas old groundwater was impacted by mixing with seawater with contributions of 10-20% with low concentrations of NO3-N, but elevated concentrations of chloride and sulfate. Recently recharged uncontaminated groundwater showed oxic conditions with a diverse microbial community structure, whereas young groundwater contaminated by anthropogenic sources showed a less diverse microbial community structure. The results of this study suggest that multiple isotopes combined with groundwater MRT and microbial data can be applied to distinguish natural and anthropogenic contaminant sources in a groundwater system.

Use of multiple isotopes to evaluate nitrate dynamics in groundwater under the barrier effect of underground cutoff walls.

Underground cutoff walls are useful in conserving groundwater and preventing seawater intrusion in coastal regions. However, the environmental effects of human activities on groundwater quality in the upstream area of the underground cutoff wall over the long term are not clear. In this study, combined analysis of multiple isotopes (δ15N-NO3-, δ18O-NO3-, δ2H, and δ18O-H2O) and nitrate concentrations was used to assess the effect of underground cutoff walls on nitrogen dynamics in groundwater in an agricultural area in China. We sampled groundwater wells in the upstream and downstream areas of the underground walls in April, July, and September. The results indicated that the underground cutoff walls hampered the horizontal groundwater flow, making the upstream groundwater a closed system, which led to an increase in the nitrate concentration and accelerated nitrification processes. Manure was the main nitrate source in the upstream groundwater, and its levels in the groundwater were similar during the three seasons, indicating that there was no difference in the nitrate sources in the upstream groundwater among the three seasons. Hence, further management measures for manure application may be critical for groundwater protection in the upstream area of underground cutoff walls.

Contamination patterns in river water from rural Beijing: A hydrochemical and multiple stable isotope study.

The pollution of urban river has reached a critical level. In the present study, the hydrochemical composition of local surface water collected during two seasons from the rural area around urban Beijing, China, was examined. Concentrations of selected cations and anions reveal the temporal and spatial distribution of anthropogenic and natural pollution. Multiple stable isotopes (δ15Nnitrate, δ18Onitrate, δ34Ssulfate, δ18Osulfate, Δ33Ssulfate) analyses were applied for detecting of specific hydrochemical processes and identifying the major sources of contamination. Twenty-eight percent of the river water samples from the wet season and 34% from the dry season exhibit a minor water quality of class IV (poor) and below due to high pH values as well as high nitrate, sulfate and fluoride concentrations. This water is hazardous for human health and the aquatic ecology. The seemingly better river water quality in the wet season is caused by higher precipitation and, hence, dilution of the water constituents. Multiple isotopes identify urban wastewater as the prime pollution source. The higher fraction of deteriorated river water in the southern rural area (42%) compared to the north (24%) points to a higher contribution of urban effluents to the downstream rivers in the rural south. It can, thus, be concluded that less discharge of wastewater to the local surface and subsurface environments by better controls of public and private sewage disposal combined with the renewal of leaking sewer pipes would reduce the risk of anthropogenic contamination of river water.

The impact of an underground cut-off wall on nutrient dynamics in groundwater in the lower Wang River watershed, China.

Underground cut-off walls in coastal regions are mainly used to prevent saltwater intrusion, but their impact on nutrient dynamics in groundwater is not clear. In this study, a combined analysis of multiple isotopes ([Formula: see text]) and nitrogen and phosphorus concentrations is used in order to assess the impact of the underground cut-off walls on the nutrient dynamics in groundwater in the lower Wang River watershed, China. Compared with the nitrogen and phosphorus concentrations in groundwater downstream of the underground cut-off walls, high [Formula: see text] and total dissolved nitrogen concentrations and similar concentration levels of [Formula: see text] and total dissolved phosphorus are found in groundwater upstream of the underground cut-off walls. The isotopic data indicated the probable occurrence of denitrification and nitrification processes in groundwater upstream, whereas the fingerprint of these processes was not shown in groundwater downstream. The management of fertilizer application is critical to control nitrogen concentrations in groundwater restricted by the underground cut-off walls.