Method for determining dredging depth of shallow lake sediment based on resuspended nitrogen and phosphorus release

Abstract

This invention relates to a method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release, belonging to the field of water environmental protection technology. The method includes obtaining in-situ columnar sediment and overlying water in the target lake area; simulating dredging of the in-situ columnar sediment at different depths; loading the dredged in-situ columnar sediment into culture columns one by one; adding filtered in-situ overlying water without disturbance; and allowing the columns to settle for a preset time; after settling, mechanically disturbing and resuspending each culture column; taking water samples in batches at the middle depth of the overlying water at preset times after disturbance; analyzing and calculating the release rates of NH3-N, SRP, TN, and TP in different dredging treatment groups; and using the minimum dredging depth where the release rates of NH3-N, SRP, TN, and TP are all less than 0 as the final dredging depth. This provides a basis for determining the depth of environmentally friendly dredging of polluted sediments in shallow lakes and fills the gap in similar projects where dredging depth is difficult to determine and dredging data is insufficient.

Description

Technical Field

[0001] This invention relates to the field of water environment protection technology, specifically to a method for determining the dredging depth of shallow lake sediments based on the release of resuspended nitrogen and phosphorus. Background Technology

[0002] As sediment dredging is the most widely used engineering technology in my country's lake water environment management, it plays a vital role in lake water environment management and aquatic ecosystem restoration. For lake sediment dredging projects, dredging depth is a crucial technical parameter for ensuring dredging effectiveness, selecting dredging methods and tools, determining the amount of dredging waste, and controlling dredging project investment.

[0003] When the dredging depth determined in the engineering design is too shallow, incomplete dredging and cleaning will occur, significantly reducing the environmental benefits of the dredging project and failing to meet environmental improvement requirements. Conversely, when the dredging depth is too deep, over-dredging and excessive excavation will occur, leading to a substantial increase in dredging waste, a significant rise in dredging project investment, and wasted funds. Therefore, determining the dredging depth of lake sediment dredging projects rationally and accurately is of great significance. However, due to the lack of environmental quality standards for lake sediment and the complexity of sediment pollutant migration and transformation in the water and soil medium, the determination of lake sediment dredging depth has long relied on methods such as visual stratification, background value, empirical values, frequency, and standard deviation. These methods are often extensive, empirical, or imprecise, significantly impacting the dredging effect, the amount of dredging waste, and the investment in dredging projects. In recent years, more accurate methods for determining the depth of sediment dredging have been developed, such as the pollutant content inflection point method and the stratified release method. However, the former does not take into account the complex geochemical characteristics of the exchange of pollutants in lake sediments between water and soil, while the latter only considers the static hydrodynamic situation and simplifies the hydrodynamic conditions for the release of pollutants from sediments. Both of them deviate from the real situation in the lake water environment, resulting in a distortion of the depth of sediment dredging, which is particularly evident in shallow lakes.

[0004] A key characteristic of shallow lakes is the frequent disturbance and resuspension of bottom sediments caused by wind and waves. Without considering hydrodynamics, the migration of pollutants in lake sediments between the water and soil media is mainly influenced by the concentration gradient of pollutants in the two phases, water temperature, the redox state of the bottom water, and the physical properties of the sediment itself. However, in shallow lakes, frequent wind and waves and sediment resuspension strongly affect the migration of pollutants in the sediment between the water and soil media, significantly impacting pollutant release from the sediment. Studies have shown that a single strong wind and wave event can cause sediment resuspension that supplies nearly 30% of nitrogen and phosphorus pollutants to the overlying water, highlighting the crucial role of resuspension caused by wind and wave disturbance in the pollutant load of the overlying water. In shallow lakes, whether before or after dredging projects, the surface sediment is always subject to erosion by wind and waves, and the risk of pollution release caused by resuspension is always present and cannot be ignored. However, current shallow lake sediment dredging projects have not taken into account the risk of resuspension of sediment after dredging when determining the dredging depth, and there are obvious deficiencies and loopholes in the determination of sediment dredging depth. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the dredging depth of shallow lake sediments based on the release of resuspended nitrogen and phosphorus, in order to solve the problems mentioned in the background art.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a method for determining the dredging depth of shallow lake sediment based on resuspended nitrogen and phosphorus release, comprising the following steps: In-situ columnar sediment and overlying water were obtained from the target lake area. Simulated dredging of the in-situ columnar sediment at different depths was carried out, and qualitative filter paper filtration was performed on the in-situ overlying water. The dredged in-situ columnar sediment was loaded one by one into the culture column, and filtered in-situ top water was added without disturbance. The column was then allowed to stand for a preset time. After the settling period, each culture column was mechanically disturbed and then resuspended using a constant-speed stirrer as the mechanical disturbance source. After disturbance, take a 50 mL water sample from the middle depth of the water covering the culture column, and add the same volume of filtered in-situ water to ensure the stability of the culture system volume. A portion of the sampled water was filtered using a water-based filter membrane. The release rates of NH3-N and SRP in the filtered water sample were calculated. The release rates of TN and TP in the unfiltered water sample were also calculated. The final dredging depth is defined as the minimum dredging depth at which the release rates of NH3-N, SRPTN, and TP are all less than 0.

[0007] Preferably, simulated dredging is performed on the in-situ columnar bottom sediment at depths of 0 cm, 5 cm, 10 cm, 20 cm, and 30 cm, respectively, and an undredged control in-situ columnar bottom sediment is set up.

[0008] Preferably, the depth of the undisturbed sediment column inside the culture column is 20 cm.

[0009] Preferably, filtered in-situ top water is added to the culture column without disturbance, and the water depth is controlled at 30 cm.

[0010] Preferably, the depth of the mechanical disturbance is 5 cm below the water surface, the propeller speed is 200 rpm, and the disturbance culture lasts for 3 hours.

[0011] Preferably, water samples are taken from the middle depth of the water covering the culture column 1 hour before the start of the disturbance, 0 hours, 0.5 hours, 1 hour, 2 hours, 3 hours after the disturbance, and 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, and 9 hours after the end of the disturbance.

[0012] Preferably, the method of analyzing TN in water samples by alkaline potassium persulfate digestion ultraviolet spectrophotometry, the method of analyzing TP in water samples by alkaline potassium persulfate digestion molybdenum-antimony anti-spectrophotometry, the method of analyzing NH3-N in water samples by Nessler's reagent colorimetry, and the method of analyzing SRP in water samples by molybdenum-antimony anti-spectrophotometry.

[0013] Preferably, the release rates of NH3-N, SRPTN, and TP are calculated using the following formulas: )+ ] / A·t; Where: R is the release rate, and V is the volume of water covering the column. , The concentrations of TN, TP, NH3-N, or SRP at the nth, initial, and j-1th sampling times, respectively. The concentration of the substance added to the water sample. Let be the sampling volume of the (j-1)th time, A be the sediment-water interface contact area, and t be the release time; the release fluxes of TN, TP, NH3-N, or SRP are denoted as R-TN, R-TP, R-NH3-N, and R-SRP, respectively.

[0014] Beneficial effects: This invention obtains sediment in situ from lakes, simulates sediment dredging, and mechanically disturbs sediment for resuspension culture to assess the potential for the release of endogenous nitrogen and phosphorus from dredged sediment after resuspension, thereby determining the dredging depth. This invention will provide a basis for determining the depth of environmentally friendly dredging of polluted sediment in shallow lakes and fill the gap in similar projects where the dredging depth is difficult to determine and the dredging basis is insufficient. Attached Figure Description

[0015] Figure 1 This is a flowchart of the method for determining the dredging depth of shallow lake sediment based on the release of resuspended nitrogen and phosphorus, as per the present invention.

[0016] Figure 2 The figures represent the intrinsic TN release rates during the resuspension process of different dredged sediments in the embodiments of the present invention.

[0017] Figure 3 The figures represent the internal TP release rates during the resuspension process of different dredged sediments in the embodiments of the present invention.

[0018] Figure 4 The release rates of endogenous NH3-N during the resuspension process of dredged sediment in different embodiments of the present invention are shown.

[0019] Figure 5 The release rates of endogenous SRP during the resuspension process of dredged sediment in different embodiments of the present invention are shown. Detailed Implementation

[0020] To make the objectives and advantages of this invention clearer, the invention will be specifically described below with reference to embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of the invention and does not strictly limit the scope of protection specifically claimed by the invention.

[0021] Example: Reference Figure 1 As shown, the method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release includes the following steps: Step 1: Set up in-situ sediment column sampling points in the target lake area and collect multiple required in-situ sediment columns and overlying water. Step 2: Filter the in-situ overlying water obtained in Step 1 using rapid qualitative filter paper to remove large particles; Step 3: Simulate dredging at different depths for each in-situ columnar sediment sample collected in Step 1; In one specific embodiment, simulated dredging was carried out at dredging depths of 0 cm, 5 cm, 10 cm, 20 cm, and 30 cm to obtain columnar mud samples with different dredging treatments, such as no dredging control, dredging depths of 5 cm, 10 cm, 20 cm, and 30 cm.

[0022] Step 4: Load the original sediment column samples obtained in Step 3 for different dredging treatments into the culture column one by one, add filtered in-situ top water without disturbance, and let it stand for the preset time. In one specific embodiment, each of the 60 cm high acrylic culture columns is inserted, and the bottom sediment depth is controlled at 20 cm.

[0023] Step 5: Add the water sample obtained in Step 2 undisturbed to the plexiglass culture column containing the undisturbed sediment from different dredging treatments described in Step 4. In one specific embodiment, filtered in-situ topcoat water is added undisturbed to the culture column, with the water depth controlled at 30 cm.

[0024] In one specific embodiment, after water injection, the water was kept at room temperature and pressure (22±3) in the laboratory. o The mud-water culture system obtained in step 5 was allowed to stand for 8 hours at 1 standard atmosphere (C) to stabilize the system.

[0025] Step 6: After settling, heat at room temperature and pressure (22±3) in the laboratory. o Under the conditions of C (1 standard atmosphere), the above mud-water system was mechanically disturbed and resuspended for culture using a DC constant speed propeller electric mixer as the mechanical disturbance source. In one specific embodiment, the perturbation experiment controlled the propeller at a water depth of 5 cm, adjusted the propeller speed to 200 rpm, and the perturbation culture lasted for 3 h.

[0026] Step 7: After disturbance, take a 50 mL water sample from the middle depth of the water cover on the culture column, and at the same time add the same volume of filtered water in situ to ensure the stability of the culture system volume. In one specific embodiment, 60 mL of water samples are taken from the middle depth of the water covering the culture column 1 h before the start of the disturbance, 0 h, 0.5 h, 1 h, 2 h, 3 h after the disturbance, and 0.5 h, 1 h, 2 h, 3 h, 4 h, 6 h, and 9 h after the end of the disturbance. 50 mL of the water sample obtained in step 2 is added to the simulated column to compensate for the sampling loss.

[0027] Step 8: Filter a portion of the sampled water using a water-based filter membrane, and calculate the release rates of NH3-N and SRP in the filtered water sample; and calculate the release rates of TN and TP in the unfiltered water sample. In one specific embodiment, 30 mL of the water sample obtained in step 7 is filtered using a water-based filter membrane with a pore size of 0.45 μm and used to analyze NH3-N and SRP in the water sample. The remaining unfiltered water sample is used to analyze TN and TP in the water.

[0028] In one specific embodiment, the TN in the overlying water sample of step 8 is analyzed by ultraviolet wind spectrophotometry using alkaline potassium persulfate digestion; the TP in the overlying water sample of step 8 is analyzed by molybdenum antimony anti-spectrophotometry using alkaline potassium persulfate digestion; the NH3-N in the overlying water sample of step 8 is analyzed by Nessler's reagent colorimetric method; and the SRP in the overlying water sample of step 8 is analyzed by molybdenum antimony anti-spectrophotometry.

[0029] The release rates of TN, TP, NH3-N, and SRP during the resuspension process of sediment after different dredging treatments were calculated using the following formulas: )+ ] / A·t; In the formula: R is the release rate, and V is the volume of water covering the column. , The concentrations of TN, TP, NH3-N, or SRP at the nth, initial, and j-1th sampling times, respectively. The concentration of the substance added to the water sample. Let be the sampling volume of the (j-1)th time, A be the sediment-water interface contact area, and t be the release time; the release fluxes of TN, TP, NH3-N, or SRP are denoted as R-TN, R-TP, R-NH3-N, and R-SRP, respectively.

[0030] Step 9: Using the release rates R-TN, R-TP, R-NH3-N and R-SRP of TN, TP, NH3-N and SRP calculated in Step 8 as the evaluation criteria, select the minimum dredging depth treatment group where all four release rates are less than 0 as the ideal dredging treatment, and take this depth as the final dredging depth.

[0031] This application also provides a specific embodiment that comprehensively analyzes the effectiveness and superiority of the method provided above; the following settings are made: Taking Lihu Lake, a typical eutrophic shallow lake, as the research object, and the area with deeper soft silt accumulation in the eastern part of the lake as the study area, five 5 L samples of in-situ lake water were collected in this area using an acrylic water sampler and placed in clean polyethylene buckets rinsed with lake water. These samples were then transported back to the laboratory for subsequent simulated dredging and resuspension experiments.

[0032] In the same study area, five in-situ lake sediment core samples with a depth greater than 50 cm were collected using a gravity-fed sediment core sampler. The sampling process ensured that the sediment core samples were uncompressed, the interfaces undisturbed, and included in-situ overlying water. The samples were then sealed in plexiglass sampling columns, securely stored, and transported back to the laboratory in the dark for subsequent simulated dredging and resuspension experiments.

[0033] The collected in-situ lake water was filtered using rapid qualitative filter paper to remove large particles, then placed in a clean polypropylene storage tank and stored in the laboratory at room temperature and pressure (22±3 °C, 1 standard atmosphere). Experiments were conducted using the method of this invention, and the release rates of TN, TP, NH3-N, and SRP, R-TN, R-TP, R-NH3-N, and R-SRP, were calculated as follows: Figures 2-5As shown, compared with undredged sediment, the resuspension release rate of dredged sediment under mechanical disturbance conditions was significantly reduced. The release rates of TN, TP, NH3-N, and SRP generally showed a decreasing trend with increasing dredging depth. In summary, when the dredging depth reached 20 cm, the release rates of TN, NH3-N, and SRP all turned negative during resuspension, except for a slight positive release rate of TP. When the dredging depth reached 30 cm, the release rates of TN, TP, NH3-N, and SRP were all less than zero. Therefore, the dredging depth for the study area was set at 30 cm.

[0034] The embodiments of the present invention have been described in detail above with reference to the examples. However, the present invention is not limited to the above embodiments. For those skilled in the art, after learning the contents described in the present invention, several equivalent changes and substitutions can be made without departing from the principle of the present invention. These equivalent changes and substitutions should also be considered to fall within the protection scope of the present invention.

Claims

1. A method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release, characterized in that: Includes the following steps: In-situ columnar sediment and overlying water were obtained from the target lake area. Simulated dredging of the in-situ columnar sediment at different depths was carried out, and qualitative filter paper filtration was performed on the in-situ overlying water. The dredged in-situ columnar sediment was loaded one by one into the culture column, and filtered in-situ overlying water was added without disturbance. The column was then allowed to stand for a preset time. After the settling period, each culture column was mechanically disturbed and then resuspended using a constant speed stirrer as the mechanical disturbance source. After disturbance, take a 50 mL water sample from the middle depth of the water cover on the culture column, and at the same time add the same volume of filtered in-situ water cover to ensure the stability of the culture system volume. A portion of the sampled water was filtered using a water-based filter membrane. The release rates of NH3-N and SRP in the filtered water sample were calculated. The release rates of TN and TP in the unfiltered water sample were also calculated. The final dredging depth is defined as the minimum dredging depth at which the release rates of NH3-N, SRPTN, and TP are all less than 0.

2. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 1, characterized in that: Simulated dredging was carried out on the in-situ columnar bottom sediment at depths of 0 cm, 5 cm, 10 cm, 20 cm, and 30 cm, respectively, and an undredged control in-situ columnar bottom sediment was set up.

3. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 1, characterized in that: The undisturbed sediment column inside the culture column is 20 cm deep.

4. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 1, characterized in that: Add filtered in-situ topsoil water to the culture column without disturbing it, and control the water depth to 30 cm.

5. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 1, characterized in that: The mechanical disturbance was performed at a depth of 5 cm below the water surface, with the propeller speed set at 200 rpm, and the disturbance culture lasting for 3 hours.

6. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 1 or 5, characterized in that: Water samples were taken from the middle depth of the water covering the culture column 1 hour before the start of the disturbance, 0 hours, 0.5 hours, 1 hour, 2 hours, 3 hours after the disturbance, and 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 6 hours, and 9 hours after the end of the disturbance.

7. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 1, characterized in that: The following methods were used to analyze the total nitrogen (TN) in water samples: alkaline potassium persulfate digestion ultraviolet spectrophotometry; alkaline potassium persulfate digestion molybdenum-antimony anti-spectrophotometry; Nessler's reagent colorimetric method; and molybdenum-antimony anti-spectrophotometry.

8. The method for determining the dredging depth of shallow lake sediments based on resuspended nitrogen and phosphorus release according to claim 7, characterized in that: The formulas for calculating the release rates of NH3-N, SRPTN, and TP are as follows: )+ ] / A·t; Where: R is the release rate, and V is the volume of water covering the column. , The concentrations of TN, TP, NH3-N, or SRP at the nth, initial, and j-1th sampling times, respectively. The concentration of the substance added to the water sample. Let be the sampling volume of the (j-1)th time, A be the sediment-water interface contact area, and t be the release time; the release fluxes of TN, TP, NH3-N, or SRP are denoted as R-TN, R-TP, R-NH3-N, and R-SRP, respectively.

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