The impacts of natural processes and anthropogenic input on riverine nitrate (NO-3) could be identified by NO-3 concentrations and nitrogen and oxygen isotope ratios (δ15N-NO-3 and δ18O-NO-3); however, the effects of variable land use on riverine NO-3 sources and transformations remain unclear. In particular, the human impacts on riverine NO-3 in mountain areas are still unknown. The Yihe River and Luohe River were used to elucidate this question due to their spatially heterogeneous land use. Hydrochemical compositions, water isotope ratios (δD-H2O and δ18O-H2O), and δ15N-NO-3 and δ18O-NO-3 values were utilized to constrain the NO-3 sources and transformations affected by different land use types. The results indicated that ① the mean nitrate concentrations in the Yihe River and Luohe River waters were 6.57 and 9.29 mg·L-1, the mean values of δ15N-NO-3 were 9.6‰ and 10.4‰, and the average δ18O-NO-3 values were -2.2‰ and -2.7‰, respectively. Based on the analysis of δ15N-NO-3 and δ18O-NO-3 values, the NO-3 in the Yihe and Luohe Rivers were derived from multiple sources, and nitrogen removal existed in the Luohe River, but the biological removal in the Yihe River was weak. ② The contributions of different nitrate sources were calculated using a Bayesian isotope mixing model (BIMM) based on δ15N-NO-3 and δ18O-NO-3 values of river water in the mainstream and tributaries with spatial locations. The results revealed that sewage and manure had major impacts on riverine nitrate in the upper reaches of both the Luohe River and Yihe River, where forest vegetation was widely distributed. However, the contributions from soil organic nitrogen and chemical fertilizer were higher in the upper reaches than in downstream ones. The contributions of sewage and manure still increased in the downstream reaches. Our results confirmed the primary impacts of point sources, e.g., sewage and manure, on riverine nitrate in the studied area, and the contributions of nonpoint sources, e.g., chemical fertilizer, had not increased as the agricultural activities elevated the downstream. Therefore, more attention should be paid to point source pollution treatment, and the high-quality development of ecological civilization in the Yellow River Basin should be maintained.
In order to determine the occurrence of mercury (Hg) in the dewatered sewage sludge (SS) from municipal wastewater treatment plants (MWTPs) in China, 315 SS samples were collected from 40 MWTPs. The total Hg (THg) contents of the sludge samples were analyzed using a DMA-80 Hg analyzer. It was found that THg in the samples ranged from 0.45-15.42 mg ·kg-1. The THg data followed a log-normal distribution with a geometric mean of (2.19±3.16) mg ·kg-1. THg contents in all the sludge samples meet the criteria set for disposal (by co-landfilling) of MWTPs sludge (GB/T 23485-2009). In terms of THg contents, 97.8% of the sludge was suitable for land application in neutral and alkaline soils, while 86.7% of the sludge was suitable for land application in acidic soil. THg in SS varied greatly, not only among cities (variation coefficient of 105%), but also in the same MWTP (variation coefficient of 0.6%-53.6% over seven days). Mercury contents in SS of China showed a descending trend of North > Northeast > Northwest > Southwest > East > Central > South China. Using the corresponding urban soil background values of THg as references, Hg pollution levels of SS were evaluated using the geological cumulative index method. It was found that SS from more than 60% of the cities sampled were heavily polluted by Hg. Mercury contents in the SS of China showed a temporal trend of increasing then decreasing, with 2000-2009 being the peak period. Results of the present study provide significant data support for the prevention and control of mercury pollution in sewage and SS in China.
Urban domestic sewage is one of the important nitrate (NO-3) sources for surface water; however, their NO-3 concentrations and nitrogen and oxygen isotope values (δ15N-NO-3 and δ18O-NO-3) remain unclear, and the factors affecting NO-3 concentrations and δ15N-NO-3 and δ18O-NO-3 values of effluents in the waste water treatment plant (WWTP) are still unknown. Water samples in the Jiaozuo WWTP were collected to illustrate this question. Influents, clarified water in the secondary sedimentation tank (SST), and effluents of the WWTP were sampled every 8 h. The ammonia (NH+4) concentrations, NO-3 concentrations, and δ15N-NO-3 and δ18O-NO-3 values were analyzed to elucidate the nitrogen transfers through different treatment sections and illustrate the factors affecting the effluent NO-3 concentrations and isotope ratios. The results indicated that ① the mean NH+4 concentration was (22.86±2.16) mg·L-1 in the influent and decreased to (3.78±1.98) mg·L-1 in the SST and continuously reduced to (2.70±1.98) mg·L-1 in the effluent of the WWTP. The median NO-3 concentration was 0.62 mg·L-1 in the influent, and the average NO-3 concentration increased to (33.48±3.10) mg·L-1 in the SST and gradually increased to (37.20±4.34) mg·L-1 in the effluent of the WWTP. ② The mean values of δ15N-NO-3 and δ18O-NO-3 were (17.1±10.7)‰ and (19.2±2.2)‰ in the influent of the WWTP, the median values of δ15N-NO-3 and δ18O-NO-3 were 11.9‰ and 6.4‰ in the SST, and the average values were (12.6±1.9)‰ and (5.7±0.8)‰ in the effluent of the WWTP. ③ The NH+4 concentrations of influent had significant differences compared to those in the SST and the effluent (P<0.05). The reduction of NH+4 concentrations in the SST was due to the above nitrification during the aerobic treatment process, which transferred NH+4 to NO-3. The NH+4 concentrations in the SST had no significant differences with that in the effluent of the WWTP (P>0.05). ④ The NO-3 concentrations in the influent had significant differences with those in the SST and the effluent (P<0.05), and minor NO-3 concentrations but relatively high δ15N-NO-3 and δ18O-NO-3 values in the influent were probably due to denitrification during the pipe sewage transportation. The obviously increased NO-3 concentrations (P<0.05) but decreased δ18O-NO-3 values (P<0.05) in the SST and the effluent resulted from water oxygen incorporation during the nitrification. The above results confirmed the impacts of aerobic and anaerobic treatment processes on NO-3 concentrations and isotope ratios of effluent from the WWTP and provided scientific basis for the identification of sewage contributions to surface water nitrate via average δ15N-NO-3 and δ18O-NO-3 values.
Agricultural fertilizers (AFs) have provided vegetation with necessary nutrients, but unabsorbed constituents have been retarded in soil, potentially affecting the quality of adjacent surface water and groundwater. AFs element contents and stable isotope compositions have often been utilized to assess and calculate AFs pollution to nitrate and sulfate in surface water and groundwater; however, due to various AFs applied, the dissolved ion concentrations and isotope ratios are still unknown. This study collected commercial AF widely utilized in Henan province, China, to constrain their ion concentrations and isotope values. The dissolved ions (1 g AFs dissolved in 1 L ultrapure water), sulfate sulfur, and oxygen isotope values(δ34S and δ18O) were analyzed, and total nitrogen (TN) contents coupled with nitrogen isotope values(δ15N) in solid AFs were determined to elucidate their elemental and isotopic compositions. These characteristics provided a scientific basis for further assessing their contributions to surface water and groundwater contaminations. The results indicated that pH values in the AFs solutions varied from 3.6 to 10.2, with a mean value of 6.7±1.5 (n=30, 1σ). Sulfate (SO42-) and nitrate (NO3-) concentrations ranged from 4.38 mg·L-1 to 827.29 mg·L-1 and from 1.34 mg·L-1 to 208.90 mg·L-1, with median values of 192.80 mg·L-1 and 13.51 mg·L-1 and average values of (256.19±239.83) mg·L-1 (n=30) and (37.07±53.21) mg·L-1 (n=29), respectively. Dissolved sulfate δ34S and δ18O values in AFs varied from -3.5‰ to 19.0‰ and from 6.7‰ to 18.5‰, with median values of 4.1‰ and 10.1‰ and mean values of (5.8±5.5)‰ (n=22, 1σ) and (10.7±2.7)‰ (n=22, 1σ), respectively. TN and δ15N values in AFs ranged from 0.5% to 38.9% and from -2.7‰ to 3.4‰, with median values of 13.3% and 0.0‰ and average values of (14.8±9.3)% (n=25) and 0.0±1.5‰ (n=24, 1σ), respectively. The lower averaged δ34S values and positive averaged δ18O values potentially resulted from sulfuric acids added as raw materials, giving rise to a negative relationship between pH values and SO42- concentrations (P<0.05). The δ15N values of AFs were close to that of air N2, corresponding to the fact that NO3--N and NH4+-N were synthesized via air N2. Our results revealed the dissolved ion concentrations of SO42-, NO3-, and NH4+ and their δ34S, δ18O, and δ15N values of typically applied AFs in Henan province, which provided the scientific basis for studying the AFs contributions to SO42- and NO3- pollutions in surface water and groundwater surroundings.
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