River nitrogen pollution source analysis method based on process simulation and isotopic tracing

Abstract

The invention discloses a river nitrogen pollution source analysis method based on process simulation and isotopic tracing, particularly relates to the technical field of river sewage treatment, and comprises hydrological simulation, nitrogen isotope analysis, nitrogen source analysis, model analysis and data processing and analysis. According to the method, the source and flux of nitrate nitrogen in a river system are simulated by utilizing a hydrological model, when long-term hydrogeological chemical data of a river are fully measured, a standard verification and simulation method is adopted, and for the river without measurement data, the emission amount of a field sample and nitrate nitrogen concentration data are used for helping to verify the hydrological model, so that the accuracy of the verification is improved. According to the method, a hydrological model is used for simulating river runoff and nitrate nitrogen flux so as to improve simulation accuracy, on the contrary, the hydrological model simulates river runoff and nitrate nitrogen flux so as to support calculation of isotope source flux, meanwhile, the hydrological model can also be used for verifying isotope-based results, and simulation results of the hydrological model and the isotope model are systematically integrated so as to realize reliable nitrate nitrogen space-time source tracking.

Description

Technical Field

[0001] The present invention relates to the technical field of river sewage treatment, and more specifically, to a method for analyzing river nitrogen pollution sources based on process simulation and isotope tracing. Background Art

[0002] As the most important component of nitrogen pollution in rivers, nitrate nitrogen has become the focus of river nitrogen pollution source tracing due to its strong mobility and wide distribution. Isotope mixing method and hydrological model are effective and practical tools for quantifying the source of nitrate nitrogen in rivers, but both methods have serious limitations. The isotope-based nitrate nitrogen pollution source tracing method mainly relies on the isotopic composition of different nitrogen pollution sources to identify the nitrate nitrogen source. Its accuracy depends on the spatiotemporal resolution of isotope and geochemical monitoring and well-constrained isotope inclusions. However, due to the overlapping characteristics of inclusions and the wide isotopic range, it is difficult to accurately constrain the inclusions. It is often challenging to identify the source of nitrate-nitrogen, resulting in great uncertainty in identifying the source of nitrate-nitrogen. When two isotopic contents are similar, it is difficult for the model to accurately quantify their contributions. Hydrological models, such as SWAT, HSPF, and SPARROW models, can simulate the generation, migration, and transformation of nitrogen in a watershed based on parameters such as land use type, climatic conditions, agricultural management measures, and soil type. The simulation results of hydrological models are of great significance in terms of temporal and spatial distribution, but are limited by factors such as lack of long-term data, subjective parameter settings, and omitted parameters (such as biological nitrate-nitrogen removal), and their simulation results are also uncertain.

[0003] Given that there is currently no single method, including isotope mixing models and hydrological models, that can accurately quantify the sources of nitrate-nitrogen in rivers, it is impossible to achieve robust and accurate nitrate-nitrogen traceability in river systems, thereby achieving a methodological breakthrough in the study of river nitrogen pollution. Summary of the invention

[0004] In order to overcome the above-mentioned defects of the prior art, an embodiment of the present invention provides a method for analyzing the source of nitrogen pollution in a river based on process simulation and isotope tracing. The method uses a hydrological model to simulate the source and flux of nitrate nitrogen in a river system. When the long-term hydrogeochemical data of the river have been fully measured, standard verification and simulation methods are used. For rivers without measurement data, the discharge and nitrate nitrogen concentration data of field samples are used to help verify the hydrological model, thereby improving the accuracy of the simulation. In turn, the hydrological model simulates river runoff and nitrate nitrogen flux to support the calculation of isotope source flux. At the same time, the hydrological model can also be used to verify isotope-based results. The simulation results of the hydrological model and the isotope model will be systematically integrated to achieve reliable spatiotemporal source tracking of nitrate nitrogen. The advantages of the isotope model and the hydrological model are integrated to solve the problems raised in the above-mentioned background technology.

[0005] To achieve the above object, the present invention provides the following technical solution: a method for analyzing the source of nitrogen pollution in a river based on process simulation and isotope tracing, comprising the following steps:

[0006] Hydrological simulation: Establish a river hydrological model to simulate the changes in water flow and nitrogen concentration in the river over time. The changes in water flow are predicted by precipitation and evaporation;

[0007] Nitrogen isotope analysis: by measuring the isotope ratio of nitrogen in water, the characteristics of different pollution sources can be identified and their contribution ratios can be calculated;

[0008] Nitrogen source analysis: Combined with the results of hydrological simulation and isotope analysis, a multi-source analysis model was used to calculate the contribution of each pollution source to nitrogen concentration;

[0009] Analytical model: Optimize the contribution ratio of each pollution source through optimization algorithm and accurately analyze the contribution of different nitrogen pollution sources;

[0010] Data processing and analysis: Combine hydrological data flow, precipitation and isotope data to pre-process, clean and denoise the data to generate accurate nitrogen pollution source analysis results.

[0011] In a preferred embodiment, the water flow calculation formula of the hydrological simulation is based on the dynamic changes of the hydrological process, taking into account the influence of precipitation and evaporation, and is obtained through the water conservation formula:

[0012]

[0013] Where Q(t) represents the water flow at time t; Q 0 represents the initial water flow; P(τ) represents the precipitation at time τ; E(τ) represents the evaporation at time τ; t is time; In addition, the hydrological model also includes the calculation of nitrogen concentration, assuming that the nitrogen concentration presents a time-varying relationship with the water flow and the input of pollution sources, and its expression formula is:

[0014]

[0015] in, represents the nitrogen concentration of the i-th pollution source; f i Indicates the contribution ratio threshold of the pollution source; Q iRepresenting the water flow of each source, through the watershed data collection, the data required for the establishment of the hydrological model are collected, mainly including land use data, soil data, daily meteorological data, hydrological and water quality site data, sampling point data, crop planting conditions, etc., to establish the model database, import each data into the model, perform parameterization processing, through the calibration and verification of the model, the collected field monitoring data including hydrological and water quality data are used to measure and verify the model, and perform accuracy analysis until the simulated data matches the measured data.

[0016] In a preferred embodiment, nitrogen isotope analysis identifies pollution sources and calculates their contribution ratios by measuring the isotope ratios of nitrogen in water, which is expressed as:

[0017]

[0018] Among them, δ 15 N represents the nitrogen isotope ratio; R sample Represents the isotope ratio of nitrogen-15 to nitrogen-14 in water samples; R standard Represents the nitrogen isotope ratio in the standard reference sample; according to the nitrogen isotope ratio of different pollution sources δ 15 N, establish a characteristic database of pollution sources, use the database to match and analyze the measured sample isotope ratios, and thus determine the contribution ratio of each pollution source. The specific isotope source analysis model uses an optimization method to solve the following problems, and its expression formula is:

[0019]

[0020] in, Indicates the optimization of the contribution coefficient f of each pollution source i ; that is, find a set of parameters that minimize the objective function δ 15 N measured (t) represents the actual isotope ratio of river nitrate nitrogen collected at time t, providing observation data and a basis for pollution source analysis. represents the isotope ratio of nitrate nitrogen of the i-th pollution source at time t, distinguishing the contribution of each pollution source to nitrogen in the river; f i Represents the contribution ratio of the i-th pollution source, and calculates the contribution of each pollution source to the total nitrogen pollution of the river; It means that the comprehensive isotopic contribution value of all pollution sources at time t is calculated by the model, and the nitrogen isotope value is simulated and estimated. The first part of the objective function is expressed as:

[0021]

[0022] The square of the difference between the observed value and the model calculated value is calculated by this formula, and the contribution ratio of the pollution source f is optimized by minimizing this difference. i , so that the model output value is close to the actual observed value, and the regularization term is introduced, and its expression formula is:

[0023]

[0024] Among them, λ is the regularization parameter, which controls the strength of the regularization term; Represents the summation expression; i represents 1, which is the starting index of the summation; n represents the ending index of the summation; nitrogen pollution in rivers usually comes from multiple pollution sources, such as agricultural runoff, domestic sewage, industrial emissions, etc., and isotope data provides a chemical composition-based labeling method to distinguish these pollution sources. Relying solely on hydrological models is not enough for pollution source analysis. By introducing isotope labeling, the specific contribution of each pollution source to nitrate nitrogen pollution can be more accurately estimated. In practical applications, this formula is part of the minimization problem, which is used to optimize the contribution ratio of each pollution source based on observational data and pollution source characteristic data. By introducing regularization terms, the robustness of the optimization process can be ensured and the model can be prevented from being too sensitive to pollution sources. The contributions of different pollution sources vary with time and location. The time parameter t in the formula allows independent estimation of each time point, which helps to establish a more dynamic and comprehensive pollution source distribution model.

[0025] In a preferred embodiment, nitrogen source analysis uses a multi-source analysis model to perform a weighted summation of the contribution ratios of multiple pollution sources, and the expression formula is:

[0026]

[0027] in, is the nitrogen concentration of the i-th pollution source; f i is the contribution ratio of the pollution source; n is the number of pollution sources. The contribution ratio of each pollution source is solved by the likelihood estimation method using the optimization algorithm and the sum of squared errors. i , thereby optimizing the nitrogen source analysis model, and its expression formula is:

[0028]

[0029] Among them, C measured (t) represents the measured nitrogen concentration; f i represents the contribution ratio of the i-th source; λ is the regularization coefficient.

[0030] In a preferred embodiment, the analytical model optimizes the contribution ratio of each pollution source through an optimization algorithm, adopts a nonlinear optimization algorithm based on the gradient descent method, and uses a time step for iterative update, and its expression formula is:

[0031]

[0032] in, represents the contribution ratio of the i-th pollution source in the t-th iteration; η represents the learning rate; L represents the loss function, Represents the loss function for f i After setting various parameters such as simulation step size and simulation time, the model is run to simulate the hydrological cycle and the migration and transformation of nitrogen in the basin. The output results of the model are statistically analyzed to obtain the contribution ratio and flux of nitrate nitrogen from different sources.

[0033] In a preferred embodiment, the data processing step includes multi-level preprocessing, denoising and standardization of hydrological data, flow, precipitation and nitrogen isotope data. The data preprocessing is performed by the following algorithm, which is expressed as follows:

[0034] D cleaned =D raw -Median(D raw )

[0035] Among them, D raw represents the original data set; D cleaned Represents the cleaned dataset, which ensures the quality of the model input data by removing outliers and noise.

[0036] In a preferred embodiment, the optimization algorithm optimizes the contribution ratio of each pollution source through an objective function, and the definition formula of the objective function is:

[0037]

[0038] Among them, δ 15 N measured (t) represents the measured isotope ratio; represents the nitrogen isotope ratio of the ith source; f i To express the contribution ratio of pollution sources, sampling points are set and different pollution sources in rivers and basins are sampled according to seasonal changes; the isotope ratio of nitrate nitrogen is analyzed by denitrification method, and the concentration of nitrate nitrogen in rivers is measured by ion chromatography to provide support for the calibration and verification of hydrological models, isotope calibration is performed in areas with significant denitrification, and data required for mixed model analysis are prepared. The isotope composition of nitrate nitrogen is analyzed by MCMC method to estimate the contribution ratio of different pollution sources in different seasons. The isotope composition of nitrate nitrogen is used to estimate the contribution ratio of each pollution source through the MCMC method. The expression formula is:

[0039]

[0040] Among them, δ 15 N measured (t) represents the measured nitrogen isotope ratio; represents the nitrogen isotope ratio of the i-th pollution source; f i represents the contribution ratio of the pollution source; λ is the regularization coefficient. The nitrogen flux of each pollution source is calculated based on the flow monitoring data of the sampling point. If there is no flow monitoring data at the sampling point, the runoff and total nitrate nitrogen flux simulated by the hydrological model are used for estimation. Based on the flow and nitrogen concentration, the nitrate nitrogen flux T from different sources is calculated. i , and its expression formula is:

[0041]

[0042] Where Q(t) is the flow rate, is the nitrogen concentration of the i-th pollution source; T i is the nitrogen flux of the ith source; for sampling points without flow monitoring, the runoff and total nitrate nitrogen flux simulated by the hydrological model are estimated, and the expression formula is:

[0043]

[0044] Among them, Q simulated (t) represents the simulated flow rate; the changes in flow rate and nitrogen concentration are calculated by integrating the data set and using numerical methods. The integral method is used for simulation calculation. The nitrogen isotope ratios of different pollution sources are calculated by the MCMC algorithm to obtain the optimal solution, thereby achieving accurate analysis of the pollution source. Sampling points are set up to conduct seasonal sampling of the river and collect samples of pollution sources in the basin. The denitrification method is used for isotope analysis. The ion chromatography method is used to analyze the nitrate nitrogen concentration in the river to determine the removal of nitrate nitrogen and provide support for the calibration and verification of the hydrological model. If there is significant denitrification, isotope calibration is performed to prepare for the MCMC mixed model. The MCMC mixed model is used to use the isotopic composition of nitrate nitrogen to estimate the contribution ratio of potential sources of nitrate nitrogen at each sampling point in four seasons, and the nitrate nitrogen flux from different sources is calculated. For sampling points with flow monitoring, the fluxes from different sources are calculated based on the monitoring data. If there is no monitoring site, the runoff and total nitrate nitrogen flux simulated by the hydrological model are used for calculation.

[0045] Technical effects and advantages of the present invention:

[0046] 1. Use isotope MCMC models to estimate the sources of nitrate nitrogen. At the same time, hydrological models are used to simulate the sources and fluxes of nitrate nitrogen in river systems. When the long-term hydrogeochemical data of the river have been fully measured, standard verification and simulation methods are used. For rivers without measurement data, discharge and nitrate nitrogen concentration data from field samples are used to help verify the hydrological model, thereby improving the accuracy of the simulation. In turn, the hydrological model simulates river runoff and nitrate nitrogen flux to support the calculation of isotope source flux. At the same time, the hydrological model can also be used to verify isotope-based results, especially when the isotope inclusions are not well constrained or overlapped. Ultimately, the simulation results of the hydrological model and the isotope model will be systematically integrated to achieve reliable spatiotemporal source tracking of nitrate nitrogen;

[0047] 2. Collect the data needed for the establishment of the hydrological model by collecting basin data, mainly including land use data, soil data, daily meteorological data, hydrological and water quality site data, sampling point data, crop planting conditions, etc. Second, establish a model database, import various data into the model, and perform parameterization processing. Third, calibrate and verify the model based on the collected field monitoring data (including hydrological and water quality data), and perform precision analysis until the simulated data matches the measured data. This step is the core of the hydrological simulation and determines the accuracy of the results. Fourth, run the model. After setting various parameters such as simulation step size and simulation time, run the model to simulate the hydrological cycle and nitrogen migration and transformation process in the basin, and perform statistics and analysis on the output results of the model to obtain the contribution ratio and flux of nitrate nitrogen from different sources;

[0048] 3. Set up sampling points, conduct seasonal sampling of rivers, and collect samples of pollution sources in the basin. Second, use the denitrification method to perform isotope analysis, and use ion chromatography to analyze the nitrate nitrogen concentration in the river to determine the removal of nitrate nitrogen and provide support for the calibration and verification of the hydrological model. Third, if there is significant denitrification, perform isotope calibration to prepare for the MCMC mixed model. Fourth, use the MCMC mixed model to use the isotopic composition of nitrate nitrogen to estimate the contribution ratio of potential sources of nitrate nitrogen at each sampling point in four seasons. Fifth, calculate the nitrate nitrogen flux from different sources. For sampling points with flow monitoring, calculate the flux from different sources based on the monitoring data. If there is no monitoring site, calculate it based on the runoff and total nitrate nitrogen flux simulated by the hydrological model. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a schematic diagram of the process of the present invention.

[0050] Figure 2 Schematic diagram of hydrological simulation and isotopic source analysis of river nitrogen pollutants. DETAILED DESCRIPTION

[0051] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] Refer to the instruction manual Figure 1 According to an embodiment of the present invention, a method for analyzing river nitrogen pollution sources based on process simulation and isotope tracing comprises the following steps:

[0053] Hydrological simulation: Establish a river hydrological model to simulate the changes in water flow and nitrogen concentration in the river over time. The changes in water flow are predicted by precipitation and evaporation;

[0054] Nitrogen isotope analysis: by measuring the isotope ratio of nitrogen in water, the characteristics of different pollution sources can be identified and their contribution ratios can be calculated;

[0055] Nitrogen source analysis: Combined with the results of hydrological simulation and isotope analysis, a multi-source analysis model was used to calculate the contribution of each pollution source to nitrogen concentration;

[0056] Analytical model: Optimize the contribution ratio of each pollution source through optimization algorithm and accurately analyze the contribution of different nitrogen pollution sources;

[0057] Data processing and analysis: Combine hydrological data flow, precipitation and isotope data to pre-process, clean and denoise the data to generate accurate nitrogen pollution source analysis results.

[0058] The water flow calculation formula for hydrological simulation is based on the dynamic changes of the hydrological process, taking into account the influence of precipitation and evaporation, and is obtained through the water conservation formula:

[0059]

[0060] Where Q(t) represents the water flow at time t; Q 0 represents the initial water flow; P(τ) represents the precipitation at time τ; E(τ) represents the evaporation at time τ; t is time; In addition, the hydrological model also includes the calculation of nitrogen concentration, assuming that the nitrogen concentration presents a time-varying relationship with the water flow and the input of pollution sources, and its expression formula is:

[0061]

[0062] in, represents the nitrogen concentration of the i-th pollution source; f i Indicates the contribution ratio threshold of the pollution source; Q iRepresenting the water flow of each source, through the watershed data collection, the data required for the establishment of the hydrological model are collected, mainly including land use data, soil data, daily meteorological data, hydrological and water quality site data, sampling point data, crop planting conditions, etc., to establish the model database, import each data into the model, perform parameterization processing, through the calibration and verification of the model, the collected field monitoring data including hydrological and water quality data are used to measure and verify the model, and perform accuracy analysis until the simulated data matches the measured data.

[0063] Nitrogen isotope analysis identifies pollution sources and calculates their contribution ratio by measuring the isotope ratio of nitrogen in water. The expression formula is:

[0064]

[0065] Among them, δ 15 N represents the nitrogen isotope ratio; R sample Represents the isotope ratio of nitrogen-15 to nitrogen-14 in water samples; R standard Represents the nitrogen isotope ratio in the standard reference sample; according to the nitrogen isotope ratio of different pollution sources δ 15 N, establish a characteristic database of pollution sources, use the database to match and analyze the measured sample isotope ratios, and thus determine the contribution ratio of each pollution source. The specific isotope source analysis model uses an optimization method to solve the following problems, and its expression formula is:

[0066]

[0067] in, Indicates the optimization of the contribution coefficient f of each pollution source i ; that is, find a set of parameters that minimize the objective function δ 15 N measured (t) represents the actual isotope ratio of river nitrate nitrogen collected at time t, providing observation data and a basis for pollution source analysis. (t) represents the isotope ratio of nitrate nitrogen of the i-th pollution source at time t, distinguishing the contribution of each pollution source to nitrogen in the river; f i Represents the contribution ratio of the i-th pollution source, and calculates the contribution of each pollution source to the total nitrogen pollution of the river; It means that the comprehensive isotopic contribution value of all pollution sources at time t is calculated by the model, and the nitrogen isotope value is simulated and estimated. The first part of the objective function is expressed as:

[0068]

[0069] The square of the difference between the observed value and the model calculated value is calculated by this formula, and the contribution ratio of the pollution source f is optimized by minimizing this difference. i , so that the model output value is close to the actual observed value, substituting into the regularization term, the expression formula is:

[0070]

[0071] Among them, λ is the regularization parameter, which controls the strength of the regularization term; Represents the summation expression; i represents 1, which is the starting index of the summation; n represents the ending index of the summation. Nitrogen pollution in rivers usually comes from multiple pollution sources, such as agricultural runoff, domestic sewage, industrial emissions, etc., and isotope data provides a chemical composition-based labeling method to distinguish these pollution sources. Relying solely on hydrological models is not enough for pollution source analysis. By introducing isotope labeling, the specific contribution of each pollution source to nitrate nitrogen pollution can be more accurately estimated. In practical applications, this formula is part of the minimization problem, which is used to optimize the contribution ratio of each pollution source based on observational data and pollution source characteristic data. By introducing regularization terms, the robustness of the optimization process can be ensured and the model can be prevented from being too sensitive to pollution sources. The contributions of different pollution sources vary with time and location. The time parameter t in the formula allows independent estimation of each time point, which helps to establish a more dynamic and comprehensive pollution source distribution model.

[0072] Nitrogen source analysis uses a multi-source analysis model to weight the contribution ratios of multiple pollution sources. The expression formula is:

[0073]

[0074] Among them, C i source (t) is the nitrogen concentration of the i-th pollution source; f i is the contribution ratio of the pollution source; n is the number of pollution sources. The contribution ratio of each pollution source is solved by the likelihood estimation method using the optimization algorithm and the sum of squared errors. i , thereby optimizing the nitrogen source analysis model, and its expression formula is:

[0075]

[0076] Among them, C measured (t) represents the measured nitrogen concentration; f i represents the contribution ratio of the i-th source; λ is the regularization coefficient.

[0077] The analytical model optimizes the contribution ratio of each pollution source through an optimization algorithm. It adopts a nonlinear optimization algorithm based on the gradient descent method and uses the time step for iterative update. The expression formula is:

[0078]

[0079] in, represents the contribution ratio of the i-th pollution source in the t-th iteration; η represents the learning rate; L represents the loss function, Represents the loss function for f i After setting various parameters such as simulation step size and simulation time, the model is run to simulate the hydrological cycle and the migration and transformation of nitrogen in the basin. The output results of the model are statistically analyzed to obtain the contribution ratio and flux of nitrate nitrogen from different sources.

[0080] The data processing steps include multi-level preprocessing, denoising and standardization of hydrological data, flow, precipitation and nitrogen isotope data. The data preprocessing is performed by the following algorithm, which is expressed as follows:

[0081] D cleaned =D raw -Median(D raw )

[0082] Among them, D raw represents the original data set; D cleaned Represents the cleaned dataset, which ensures the quality of the model input data by removing outliers and noise.

[0083] The optimization algorithm optimizes the contribution ratio of each pollution source through the objective function, and the definition formula of the objective function is:

[0084]

[0085] Among them, δ 15 N measured (t) represents the measured isotope ratio; represents the nitrogen isotope ratio of the ith source; f i To express the contribution ratio of pollution sources, sampling points are set and different pollution sources in rivers and basins are sampled according to seasonal changes; the isotope ratio of nitrate nitrogen is analyzed by denitrification method, and the concentration of nitrate nitrogen in rivers is measured by ion chromatography to provide support for the calibration and verification of hydrological models, isotope calibration is performed in areas with significant denitrification, and data required for mixed model analysis are prepared. The isotope composition of nitrate nitrogen is analyzed by MCMC method to estimate the contribution ratio of different pollution sources in different seasons. The isotope composition of nitrate nitrogen is used to estimate the contribution ratio of each pollution source through the MCMC method. The expression formula is:

[0086]

[0087] Among them, δ 15 Nmeasured (t) represents the measured nitrogen isotope ratio; represents the nitrogen isotope ratio of the i-th pollution source; f i represents the contribution ratio of the pollution source; λ is the regularization coefficient. The nitrogen flux of each pollution source is calculated based on the flow monitoring data of the sampling point. If there is no flow monitoring data at the sampling point, the runoff and total nitrate nitrogen flux simulated by the hydrological model are used for estimation. Based on the flow and nitrogen concentration, the nitrate nitrogen flux T from different sources is calculated. i , and its expression formula is:

[0088]

[0089] Where Q(t) is the flow rate, is the nitrogen concentration of the i-th pollution source; T i is the nitrogen flux of the ith source; for sampling points without flow monitoring, the runoff and total nitrate nitrogen flux simulated by the hydrological model are estimated, and the expression formula is:

[0090]

[0091] Among them, Q simulated(t) represents the simulated flow rate; the changes in flow rate and nitrogen concentration are calculated by integrating the data set and using numerical methods. The integral method is used for simulation calculation. The nitrogen isotope ratios of different pollution sources are calculated by the MCMC algorithm to obtain the optimal solution, thereby achieving accurate analysis of the pollution source. Sampling points are set up to conduct seasonal sampling of the river, and samples of pollution sources in the basin are collected. The denitrification method is used for isotope analysis. The ion chromatography method is used to analyze the nitrate nitrogen concentration in the river to determine the removal of nitrate nitrogen and provide support for the calibration and verification of the hydrological model. If there is significant denitrification, isotope calibration is performed to prepare for the MCMC mixed model. The MCMC mixed model is used to use the isotope composition of nitrate nitrogen to estimate the contribution ratio of potential sources of nitrate nitrogen in each sampling point in four seasons, and the nitrate nitrogen flux from different sources is calculated. For sampling points with flow monitoring, the monitoring data is used for The flux calculation of different sources is carried out. If there is no monitoring site, the calculation is carried out based on the runoff and total nitrate nitrogen flux simulated by the hydrological model. The field geochemical monitoring data of nitrate nitrogen in the isotope mixing method is also applied to the calibration or verification of the hydrological model, providing effective support for the simulation of nitrate nitrogen sources by the hydrological model, thereby solving the problem that the hydrological model cannot be carried out due to the lack of long-term monitoring data or the lack of required indicators in the monitoring data. The isotope mixing model relies on the field sampling data of the watershed, which can more truly reflect the actual situation of the watershed. This technology compares the field monitoring and source allocation results in the isotope study with the simulation results of the hydrological model, which helps to verify the accuracy of the hydrological model. This method solves the problem that the hydrological model may deviate from the actual situation due to subjective parameter setting and macro data simplification, but the verification is difficult, thereby improving the credibility of the model;

[0092] Using nitrate nitrogen isotopes to assess and even quantify the degree of nitrate nitrogen removal in a watershed can provide an important supplement to hydrological model simulation, thereby solving the problem of uncertainty caused by incomplete consideration of processes such as biological nitrate nitrogen removal in hydrological models. In order to reflect the nitrate nitrogen load in different ranges of the watershed, isotope mixing models need to be sampled at multiple sampling points. In the absence of monitoring sites, the instantaneous flow at the sample point outlet is usually used to represent the average flow for the entire month for calculation. This method may cause errors in flux results due to the variability of flow. This technology uses hydrological models and can be used in the absence of hydrological measurement data. In the case of insufficient data, the isotope mixing method can provide a reference for river discharge and nitrogen flux data, and verify the representativeness of field samples with the support of hydrological model simulation results, so as to solve the problem that the isotope mixing method is difficult to apply in long-term monitoring due to high economic costs such as manpower and material resources. The accuracy of the isotope mixing model depends on the spatiotemporal resolution of isotope and geochemical monitoring and well-constrained isotope inclusions. However, when two isotope inclusions are similar, it is difficult for the isotope method to accurately allocate their contributions, and during the sampling process, the samples spontaneously undergo biological transformation, which affects the results. This technology uses the simulation results of the hydrological model to distinguish nitrate nitrogen fluxes from different land use types, so it can provide a reliable nitrate nitrogen source for the isotope mixing method when the nitrate nitrogen isotope composition is very similar, thereby solving the problem that the isotope mixing method may generate uncertainty in the process of quantifying the source due to the overlapping characteristics of the inclusions and the wide isotope range, and the biologically driven nitrogen transformation may change the initial isotope value of the river.

[0093] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing, characterized in that: The following steps are involved: Hydrological simulation: Establish a river hydrological model to simulate the changes in water flow and nitrogen concentration in the river over time. The changes in water flow are predicted by precipitation and evaporation; Nitrogen isotope analysis: by measuring the isotope ratio of nitrogen in water, the characteristics of different pollution sources can be identified and their contribution ratios can be calculated; Nitrogen source analysis: Combined with the results of hydrological simulation and isotope analysis, a multi-source analysis model was used to calculate the contribution of each pollution source to nitrogen concentration; Analytical model: Optimize the contribution ratio of each pollution source through optimization algorithm and analyze the contribution of different nitrogen pollution sources; Data processing and analysis: Combine hydrological data flow, precipitation and isotope data to pre-process, clean and denoise the data to generate accurate nitrogen pollution source analysis results.

2. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing according to claim 1, characterized in that: The water flow calculation formula for hydrological simulation is based on the dynamic changes of the hydrological process, taking into account the influence of precipitation and evaporation, and is obtained through the water conservation formula: Among them, Q(t) represents the water flow at time t; Q0 represents the initial water flow, P(τ) represents the precipitation at time τ; E(τ) represents the evaporation at time τ; t is time; In addition, the hydrological model also includes the calculation of nitrogen concentration, assuming that the nitrogen concentration presents a time-varying relationship with the water flow and the input of pollution sources, and its expression formula is: in, represents the nitrogen concentration of the i-th pollution source; f i Indicates the contribution ratio threshold of the pollution source; Q i Represents the water flow rate at each source.

3. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing according to claim 2, characterized in that: Nitrogen isotope analysis identifies pollution sources and calculates their contribution ratio by measuring the isotope ratio of nitrogen in water. The expression formula is: Among them, δ 15 N represents the nitrogen isotope ratio; R sample Represents the isotope ratio of nitrogen-15 to nitrogen-14 in water samples; R standard Represents the nitrogen isotope ratio in the standard reference sample; according to the nitrogen isotope ratio of different pollution sources δ 15 N, establish a characteristic database of pollution sources, use the database to match and analyze the measured sample isotope ratios, and thus determine the contribution ratio of each pollution source. The specific isotope source analysis model uses an optimization method to solve the following problems, and its expression formula is: in, Indicates the optimization of the contribution coefficient f of each pollution source i ; δ 15 N measured (t) represents the actual isotope ratio of river nitrate nitrogen collected at time t, providing observation data and a basis for pollution source analysis; represents the isotope ratio of nitrate nitrogen of the i-th pollution source at time t, distinguishing the contribution of each pollution source to nitrogen in the river; f i Represents the contribution ratio of the i-th pollution source, and calculates the contribution of each pollution source to the total nitrogen pollution of the river; It means that the comprehensive isotopic contribution value of all pollution sources at time t is calculated by the model, and the nitrogen isotope value is simulated and estimated. The first part of the objective function is expressed as: The square of the difference between the observed value and the model calculated value is calculated by this formula, and the contribution ratio of the pollution source f is optimized by the difference. i , so that the model output value is close to the actual observed value, substituting into the regularization term, the expression formula is: Among them, λ is the regularization parameter, which controls the strength of the regularization term; Represents the summation expression; i indicates that 1 is the starting index of the summation; n indicates the ending index of the summation.

4. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing according to claim 3, characterized in that: Nitrogen source analysis uses a multi-source analysis model to weight the contribution ratios of multiple pollution sources. The expression formula is: in, is the nitrogen concentration of the i-th pollution source; f i is the contribution ratio of the pollution source; n is the number of pollution sources. The contribution ratio of each pollution source is solved by the likelihood estimation method using the optimization algorithm and the sum of squared errors. i , thereby optimizing the nitrogen source analysis model, and its expression formula is: Among them, C measured (t) represents the measured nitrogen concentration; f i represents the contribution ratio of the i-th source; λ is the regularization coefficient.

5. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing according to claim 4, characterized in that: The analytical model optimizes the contribution ratio of each pollution source through an optimization algorithm. It adopts a nonlinear optimization algorithm based on the gradient descent method and uses the time step for iterative update. The expression formula is: in, represents the contribution ratio of the i-th pollution source in the t-th iteration; η represents the learning rate; L represents the loss function, Represents the loss function for f i The partial derivative of .

6. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing according to claim 5, characterized in that: Data processing and analysis include multi-level preprocessing, denoising and standardization of hydrological data, flow, precipitation and nitrogen isotope data. Data preprocessing is performed using the following algorithm, which is expressed as: D cleaned =D raw -Median(D raw ) Among them, D raw represents the original data set; D cleaned Represents the cleaned dataset, which ensures the quality of the model input data by removing outliers and noise.

7. A method for river nitrogen pollution source analysis based on process simulation and isotope tracing according to claim 6, characterized in that: The optimization algorithm optimizes the contribution ratio of each pollution source through the objective function, and the definition formula of the objective function is: Among them, δ 15 N measured (t) represents the measured isotope ratio; represents the nitrogen isotope ratio of the ith source; f i Indicates the contribution ratio of pollution sources; by setting sampling points and sampling different pollution sources in rivers and basins according to seasonal changes, the isotope ratio of nitrate nitrogen is analyzed by denitrification method, and the concentration of nitrate nitrogen in rivers is measured by ion chromatography to provide support for the calibration and verification of hydrological models, isotope calibration is performed in areas with significant denitrification, and data required for mixed model analysis is prepared. The isotope composition of nitrate nitrogen is analyzed by MCMC method to estimate the contribution ratio of different pollution sources in different seasons. The isotope composition of nitrate nitrogen is used to estimate the contribution ratio of each pollution source through the MCMC method. The expression formula is: Among them, δ 15 N measured (t) represents the measured nitrogen isotope ratio; represents the nitrogen isotope ratio of the i-th pollution source; f i represents the contribution ratio of the pollution source; λ is the regularization coefficient; the nitrogen flux of each pollution source is calculated based on the flow monitoring data of the sampling point. If there is no flow monitoring data at the sampling point, the runoff and total nitrate nitrogen flux simulated by the hydrological model are used for estimation. Based on the flow and nitrogen concentration, the nitrate nitrogen flux T from different sources is calculated. i , and its expression formula is: Where, Q(t) is the flow rate; is the nitrogen concentration of the i-th pollution source; T i is the nitrogen flux of the ith source. For sampling points without flow monitoring, it is estimated based on the runoff and total nitrate nitrogen flux simulated by the hydrological model. The expression formula is: Among them, Q simulated (t) represents the simulated flow rate, and the changes in flow rate and nitrogen concentration are calculated by integrating the data set and numerical methods.

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