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1.INTRODUCTIONDark and foul-smelling water bodies are an unreasonable display of organic contamination in bodies of water, representing a severe state of pollution. The term “black odor” can be understood in two ways: through external visual cues and the underlying formation mechanisms. Visually, water bodies afflicted by black odor emit unpleasant smells or exhibit abnormal coloring, often causing discomfort, nausea, or repulsion in observers. Mechanistically, the phenomenon arises from a cascade of biological, physical and chemical reactions involving organic contaminator in water under anaerobic or anaerobic factor, with nitrogen and phosphorus playing key roles as odor-causing pollutants1,2. Typically, the phosphorus and nitrogen levels in both water bodies and deposits maintain a delicate equilibrium, which can easily be disrupted by external factors such as industrial and domestic sewage, serving as major origins of phosphorus and nitrogen contamination in water bodies3,4. Furthermore, the internal sediment also acts as a significant endogenous source of pollution, containing substantial quantities of heavy metals, phosphorus and nitrogen nutrients, organic matter, and various other substances that contribute to the shaping of foul-smelling in water bodies5,6. While domestic and foreign scholars have launched broadly study on the characteristics of phosphorus and nitrogen pollution in larger water bodies like major rivers, streams, lakes, and bays, smaller water bodies such as ditches, mountain streams, and ponds have received less attention in terms of monitoring and evaluation7-9. Wuhan is situated in the middle and lower regions of the Yangtze River Plain, as well as the eastern section of the Jianghan Plain, characterized by predominantly flat terrain with hills scattered throughout. It stands out as one of the world’s megacities endowed with abundant water resources, with the water area encompassing a significant 25% of the urban expanse, shaping Wuhan’s splendid and unique waterfront ecological landscape. The total area spans 8467 km2, with rural territories constituting 95% of the entirety. The national agenda in 2019 aimed to eradicate black and smelly water in rural parts of China by 203510, underscoring the escalating concern over this issue. Addressing such water pollution in rural regions holds the key to enhancing the quality of the living environment and overall ecological balance. This investigation examined approximately 380 water bodies surrounding primary population centers in the administrative villages under different districts and counties within Wuhan. Through sensory evaluation, water bodies exhibiting strong odors or unusual discoloration were identified, followed by a comprehensive analysis of water mass and phosphorus and nitrogen settlings contamination levels. The study aims to shed light on the pollution status and interrelationships, thus providing a foundational understanding of the characteristics of dark and foul-smelling water bodies in rural regions. This research serves as a pivotal stepping stone for subsequent pollution mitigation and prevention strategies. 2.MATERIALS AND METHODS2.1Sample screeningField investigations was conducted in the primary population centers of administrative villages in various districts and counties within Wuhan City. Methods such as on-site inspections, interviews, drone surveillance, etc., are employed to assess areas covering a radius of 1000 m, spanning over 2000 m2 in size or extending 500 m in length. Bodies of water identified, and then conducted on-site observation to detect any with unpleasant odors or unusual colors. A total of 120 polluted water sources in rural areas were identified from a sample of 380, including 54 channels and 66 ponds. 2.2Sample collection3 surveillance locations for every channel or trench (1 at the starting position, 1 at the concluding position, and 1 at the halfway point) is established. 2 monitoring sites for each reservoir (if there are water inputs and outputs, 1 spot near each is established; if only one is available, 1 spot is established near it and another at the opposite side) are created. In the absence of inputs or outputs, one spot is established at random and another at the opposite end. A sediment specimen from every water body is gathered, with the sampling spots matching those for water quality. Sampling processes conform to the guidelines specified in the “Technical Specifications for Surface Water and Sewage Monitoring” (HJT 91-2002). 2.3Analytical methodThe overall nitrogen concentration in water quality was evaluated through potassium persulfate digestion ultraviolet spectroscopy (HJ 636-2012), while the total phosphorus concentration was assessed by ammonium molybdate spectroscopy (GB 11893-1989). The determination of Organic Matter Content in Sediment was conducted using the potassium dichromate volumetric technique from “Water and Wastewater Monitoring and Analysis Methods” (Fourth Edition). For sediment analysis, the total nitrogen concentration was determined via the Kjeldahl method (HJ 717-2014), and the total phosphorus concentration was assessed using wavelength dispersive X-ray fluorescence spectroscopy (HJ780-2015). The data was statistically analyzed using SPSS 19.0. 3.RESULTS AND ANALYSIS3.1Monitoring resultsThe statistical results of the content characteristics of water quality monitoring indicators are shown in Table 1. Total phosphorus ranged from 0.03 to 3.24 mg/L, total nitrogen from 0.71 to 45.2 mg/L, and ammonia nitrogen from 0.034 to 32.52 mg/L. The variation coefficients of ammonia nitrogen, total nitrogen and total phosphorus were all greater than 100%, which belonged to strong variation, indicating that there were great differences in the water quality of the same index among individuals in black and odorous water bodies. Table 1.Content characteristics of each monitoring index of water quality.
The statistical results of the content characteristics of sediment monitoring indicators are shown in Table 2. Organic matter ranged from 0.3% to 14.8%, total nitrogen from 906 to 16810 mg/kg, and total phosphorus from 335 to 5709 mg/kg. The variation coefficients of organic matter, total phosphorus, and total nitrogen were all lower than 100%, which belonged to medium-strong variation, indicating that there were certain differences in the same index sediment among individuals in black and odorous water bodies. Table 2.Content characteristics of sediment monitoring indicators.
Furthermore, dark and foul-smelling channels and trenches are categorized as linear features, and an unpaired sample t-test is conducted with bodies of water. The results (Table 3) showed that the P values for total nitrogen and ammonia nitrogen in water quality were 0.002 (P<0.01) and 0.003 (P<0.05) respectively, indicating significant differences in the levels of total nitrogen and ammonia nitrogen in rivers, channels, and bodies of water. The average concentration of total nitrogen in channels and trenches was 10.18 mg/L, higher than the average concentration of dissolved oxygen in bodies of water, which was 4.49 mg/L. The concentration of ammonia nitrogen in gullies and channels was 5.69 mg/L, surpassing the mean level of ammonia nitrogen in bodies of water at 1.75 mg/L, suggesting more severe contamination of ammonia nitrogen in gullies and channels compared to bodies of water. There were no significant differences in total phosphorus in water quality, organic matter, total nitrogen, and total phosphorus in sediment among channels, trenches, and bodies of water. Table 3.T-test results of ditches, ditches and ponds.
3.2Correlation analysisPirsson correlation analysis is usually used to analyze the homology among different indicators, the higher the correlation of indicators, the more likely it is to have same contamination sources or movement characteristics11. The relevant analysis of water monitoring indicators was carried out, and the results (Table 4) There was important positive correlation (P<0.01) among ammonia nitrogen, total nitrogen and total phosphorus. The correlation coefficient between total nitrogen and ammonia nitrogen was 0.913 (R>0.8), which belonged to strong correlation, and the correlation coefficient between total phosphorus and total nitrogen was 0.679 (0.8>R>0.6), which belonged to strong correlation, indicating that total phosphorus, total nitrogen and ammonia nitrogen may have the same or similar pollution source types. Table 4.Correlation analysis of pollutants in water.
Note: ** indicates significant correlation at 0.01 level. The correlation analysis of sediment monitoring indicators shows that there is no significant positive correlation between total phosphorus, total nitrogen and organic matter, indicating that phosphorus and nitrogen in sediment are not mainly released by mineralization of organic matter in sediment (Table 5). Table 5.Correlation analysis of pollutants in sediment.
Sediment is the internal load of nutrients in water body, and the discharge of nitrogen and phosphorus in sediment is an important source of nitrogen and phosphorus in water body. Specifically in the case of reduced external sources, the internal load of nutrient-rich sediment still controls the nutrient level of water body for a certain period of time12. The correlation analysis (Table 6) of water quality and sediment monitoring indicators shows that although there is a distinctive positive correlation between total phosphorus in sediment and total phosphorus in water quality, the correlation coefficient is 0.332 (R<0.4), which belongs to low correlation. There was no significant correlation between total nitrogen in sediment and total nitrogen in water quality. It shows that the sediment is relatively stable in general, and has little impact on the overlying water body. Table 6.Correlation analysis between water quality and pollutants in sediment.
Note: ** indicates significant correlation at 0.01 level. 3.3Pollution assessmentThe all-round contamination indicator method of water mass is to make statistics on the relative pollution index of each monitoring contamination indicator, and then add each single pollution index and calculate the arithmetic average, so as to obtain the genneral pollution quota. The classification of contamination degree is shown in Table 7, and its calculation is as follows: equations (1) and (2)13. In the formula, Ci is the tested density value of the contaminant item, and C0 is the contaminantor item standard for the evaluation of surface water (Class V water quality standard for surface water is selected as the evaluation standard) Quasi), Pi is the contamination indicator of item, Pn is the genneral pollution of water mass index. Table 7.Evaluation criteria for comprehensive pollution index of water quality.
and The levels of total nitrogen, total phosphorus, and ammonia nitrogen in black and odorous water were assessed using the comprehensive pollution index method. Out of 120 black and odorous water bodies, 102 were found to be polluted to varying degrees, making up 85.0% of the total. Of these, 38 were classified as severely polluted, 21 as heavily polluted, 17 as moderately polluted, and 26 as lightly polluted, indicating varying levels of nitrogen and phosphorus pollution in the water bodies. Upon comparing the levels of total nitrogen, ammonia nitrogen, and total phosphorus with the thresholds set by the “Surface Water Environmental Quality Assessment Measures (Trial),” it was revealed that 50.0% of the 120 water bodies exceeded the limits for nitrogen and phosphorus simultaneously. Specifically, 77.7% of the total nitrogen levels (92 out of 120) exceeded Category V (>2.0 mg/L), 47.5% of the total phosphorus levels (57 out of 120) exceeded Category V (>0.4 mg/L), and 36.7% of the ammonia nitrogen levels (44 out of 120) exceeded Category V (>2.0 mg/L). The comprehensive sediment pollution assessment refers to the standards for environmental mass judgement system descripted by the Ministry of Environment and Energy of Ontario, Canada in 1992, and takes the minimum safe limit value in the environmental quality assessment of ecotoxic effects as the standard (total nitrogen 550 mg/kg, total phosphorus 600 mg/kg)9. The classification of pollution degree is shown in Table 8, and its calculation is as follows: (3) and (4).In the formula, Pi is the contamination exponent of contaminator in the sediment, Ci is the measured values of contaminator in the sediment, C0 is the assessment criteria of the component of pollutant, FF is the genneral pollution indicator, F is the mean of the total phosphorus and total nitrogen contamination quota. Fmax is the maximum single contamination indicator. Table 8.Evaluation criteria of sediment comprehensive pollution.
and The total phosphorus and total nitrogen content of black and odorous deposits were evaluated by overall contamination exponent method. Only 3 of 120 sediments were moderately polluted, and all the others were heavily polluted. From the perspective of individual pollution evaluation, the whole Nitrogen was evaluated as moderate pollution in 5 cases, others as severe pollution, total phosphorus was evaluated as light pollution in 24 cases, moderate pollution in 27 cases, and severe pollution in 69 cases. It shows that the total phosphorus and total nitrogen contamination in the settlings of dark and smelly water in the countryside is serious. Organic matter is an important carrier of phosphorus and nitrogen nutrients in bottom mud, and it is the main index reflecting the degree of organic nutrition. Meanwile, the level of organic pollution in sediment is measured by using it, and organic nitrogen is an important index to judge the degree of nitrogen pollution in sediment14. For the sake of making the evaluation outcomes more general, the organic pollution index way is used to further evaluate the current situation of sediment pollution. The evaluation classification standards are shown in Table 9, and their calculations are as follows: (5)-(7). Table 9.Evaluation of sediment organic index and grading criteria.
and and The method of organic index can be used to assess the sediments of dark and smelly waters. Types of organic pollutants in 120 sediments was 20, 29 and 71 respectively, indicating that 70.2% of the sediments existed Organic pollution, and 120 sediment organic nitrogen indices are all polluted. 4.CONCLUSIONThe quality of water in 84 rural bodies of water characterized by black and foul-smelling properties was assessed, with 102 of them showing varying levels of contamination by nitrogen and phosphorus, making up 85.0% of the total. Additionally, 50.0% of these bodies had nitrogen and phosphorus levels exceeding acceptable limits simultaneously. Nitrogen pollution was observed to be more severe in rivers and streams compared to ponds. In a study of 120 rural black and odorous water bodies, it was found that all sediment samples were tainted by total nitrogen and total phosphorus. Moreover, a significant proportion of sediments, amounting to 70.2%, displayed contamination due to organic matter, particularly organic nitrogen. Analysis revealed a strong positive relationship among total phosphorus, total nitrogen, and ammonia nitrogen in terms of water quality, suggesting a common source of pollution for these parameters, likely stemming from domestic wastewater discharges, agricultural runoff, and other external sources. Conversely, no notable correlation was noted between organic matter, total nitrogen, and total phosphorus in sediments, pointing towards nitrogen and phosphorus not being primarily sourced from organic material decomposition. It indicates that endogenous pollution is not the main contributor. Furthermore, though no notable connections were found between total phosphorus in sediment and water quality nor between total nitrogen in sediment and water quality, it implies that sediment, on the whole, remains relatively stable and has minimal impact on the overlying water bodies. In general, the rural water systems in Chongqing are in satisfactory condition. Only approximately 120 water bodies exhibited strong odors or visibly abnormal colors out of around 380 water bodies surveyed. By conducting monitoring and analysis, the nitrogen and phosphorus pollution characteristics on the surface were identified, along with an analysis of the pollution sources. Since black and odorous water bodies tend to display seasonality and recurrence, they are categorized by season and time period. It is important to carry out regular monitoring of water bodies to track the trend of water pollution changes and gather long-term monitoring data. Pollution sources for each water body should be investigated, determining the contribution rates of different types of pollution sources such as point sources, non-point sources, and endogenous sources, in order to support future efforts in controlling and preventing rural black and odorous water pollution ACKNOWLEDGEMENTSThis study was supported by Hubei Agricultural Science and Technology Innovation Center’s special fund “2020 key technology research and demonstration project for safe and efficient production of genuine medicinal materials” (N0:2020-620-000-002-04), The 2023 innovation system project of the Wuhan Academy of Agricultural Sciences (XKCX202307). REFERENCESLü, C. J., Gao, H. J., Li, X. J., Song, Y. H. and Shen, Q.,
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