Seasonal river-groundwater volume exchange rate estimation method

Abstract

The invention relates to a seasonal river-groundwater volume exchange rate estimation method. According to the method, the RnMBM and the riverbed permeability coefficient are creatively fused, estimation of the river-underground water volume exchange rate under the seasonal river complex hydrogeological condition is completed, the RnMBM uses radon as a natural tracer, the concentration difference of the radon in the river and the underground water reaches 1-2 orders of magnitude, the chemical property is stable, the exchange flux of surface water and the underground water is accurately inverted, and the calculation result is accurate. And the exchange flux and a riverbed permeability coefficient are compared and analyzed, and the accuracy of an RnMBM result is further verified. The method is simple to operate, can adapt to complex hydrogeological conditions, dynamically responds to seasonal changes, reduces the field monitoring cost and manpower investment, has relatively high generalization performance, and is beneficial to improving the overall hydrologic monitoring level.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of water resource monitoring, in particular to a seasonal river-groundwater water exchange rate estimation method. BACKGROUND

[0002] The interaction between surface water and groundwater is an important process of hydrological cycle in nature, and plays an important role in maintaining the water environment quality and ecological system stability of a basin. In recent years, with the increasing development of industry and agriculture and the increasing demand for water in production and life, the exploitation and utilization of surface water and groundwater have increased dramatically, and the interaction between surface water and groundwater has become more frequent. These factors have led to significant changes in river runoff and groundwater recharge conditions, causing ecological and environmental problems such as reduction of river base flow, decline of groundwater level, variation of water resource structure, water pollution, and shrinkage of lakes. According to statistics, nearly 60% of global groundwater resources are used in the agricultural production sector, and the continuous and intense exploitation of groundwater has caused the depletion of major aquifers worldwide, resulting in dramatic changes in the interaction between surface water and groundwater and the flux, and significant impacts on groundwater resources, river base flow, and ecological environment evolution. Therefore, in-depth study on the interaction between surface water and groundwater and the quantification of the interface flux is crucial for promoting efficient use of water resources and maintaining a coordinated ecological system.

[0003] At present, there are many studies on the interaction between surface water and groundwater, but there are few studies on the estimation of surface water-groundwater water exchange rate, and most of the studies focus on perennial rivers.

[0004] Chinese patent CN108332816A discloses a device for measuring the exchange water volume between surface water and groundwater in a river channel and a measuring method thereof. The patent manufactures different surface water and groundwater exchange water volumes in the collection cylinder, then measures the water head difference between the riverbed sediment and the river channel by using a pressure gauge, measures the corresponding flow, deduces the vertical permeability coefficient of the local riverbed sediment based on the Darcy's law, and finally deduces the actual surface water and groundwater exchange water volume based on the vertical permeability coefficient and the water head difference of the sediment under natural conditions. The invention can measure continuously for a short time and has a long service life. Although the patent can measure the exchange water volume between surface water and groundwater, it ignores the fact that some seasonal rivers may be dry in the dry season, and it is difficult to achieve the distance requirement (10-15 cm) between the lower end of the collection cylinder and the riverbed surface in actual operation. At the same time, the patent adjusts the flow to calibrate the relationship between the water head difference and the flow, which may significantly change the original water conditions of the riverbed, especially in a heterogeneous riverbed, and this disturbance may cause local flow pattern disorder, affecting the accuracy of the data. Finally, the patent calculates the permeability coefficient and the exchange water volume based on the Darcy's law, but the actual riverbed sediment generally has heterogeneity, which is too idealized.

[0005] Chinese invention patent CN119223356A discloses an in-situ monitoring device for the surface water-groundwater conversion relationship and flux. The device mainly includes a control platform and probes. The control platform, located on the riverbank, includes a power supply module, a data processing module, and a flow measurement module. The probes, positioned below the surface water surface, are used to collect data; one end of the probe is sharply inserted into the riverbed, while the other end is exposed in the surface water. The probes are connected to the control platform. This invention allows for adjusting the number and position of probes embedded in the riverbed according to different monitoring needs, enabling real-time and continuous monitoring of the surface water-groundwater conversion relationship and flux at different depths on the slope. It has low installation requirements, and while automating data acquisition, it also features self-checking to prevent equipment damage. While this patent can monitor the surface water-groundwater conversion relationship and flux in real time and considers selecting different particle sizes of gravel based on different riverbed conditions, it does not account for changes in water sediment content, such as seasonal river floods. High sediment content in the water flow could cause probe blockage, significantly reducing measurement accuracy. Meanwhile, compressible sediments such as clay may deform under the action of water flow, leading to dynamic changes in porosity and permeability coefficient. However, the device relies on a fixed parameter model and cannot be revised in real time. Finally, this patented equipment is relatively complex, with high costs and maintenance expenses. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention aims to provide a method for estimating the seasonal river-groundwater exchange rate. This innovative method integrates radon-based molecular weight molecular weight microscopy (RnMBM) and riverbed permeability coefficient to estimate the river-groundwater exchange rate under complex hydrogeological conditions. RnMBM utilizes radon as a natural tracer, whose concentration difference between river and groundwater reaches 1-2 orders of magnitude. It exhibits stable chemical properties and accurately inverts the exchange flux between surface water and groundwater. The exchange flux is then compared with the riverbed permeability coefficient to further verify the accuracy of the RnMBM results. This method is simple to operate, adaptable to complex hydrogeological conditions, dynamically responds to seasonal changes, reduces field monitoring costs and manpower input, and has strong scalability, contributing to the improvement of overall hydrological monitoring capabilities.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for estimating the seasonal river-groundwater exchange rate, characterized by comprising the following steps:

[0009] Step 1: Determine the upstream and downstream flow rates of the river section to be tested. 222 Rn concentration data were measured in groundwater near the river section to be tested. 222 Rn concentration data were measured;

[0010] Step 2, based on the upstream and downstream flow rates of the river section to be measured obtained in Step 1, 222 Rn concentration data and groundwater 222 Rn concentration data were used to calculate the river-groundwater exchange rate.

[0011] Based on the above scheme, the specific steps for calculating river-groundwater exchange data in step 2 are as follows:

[0012] Upstream of the river section to be tested 222 Both Rn concentration and flow rate were lower than those downstream of the tested river section. 222 Given the concentration and flow rate of Rn, the groundwater flow rate q per unit length is calculated using the following formula. g :

[0013]

[0014] Upstream of the river section to be tested 222 Both Rn concentration and flow rate were higher than those downstream of the tested river section. 222 Given Rn concentration and flow rate, the infiltration rate q per unit length of river is calculated using the following formula. r :

[0015]

[0016] Upstream of the river section to be tested 222 Rn concentration and flow rate and downstream of the river section under test 222 When the differences between Rn concentration and flow rate are opposite, the groundwater flow rate q per unit length is calculated according to the following two formulas. g and the river infiltration rate per unit length q r (q g and q r All data pertain to river-groundwater exchange.

[0017]

[0018] In the above equations, C u C d The upstream and downstream sections of the river to be tested are respectively 222 The concentration of Rn; Q u Q d C represents the flow rates upstream and downstream of the river section to be measured; g In groundwater 222 The concentration of Rn; L is the distance between upstream and downstream sampling points; α is the total loss coefficient, which is the sum of the radioactive decay coefficient β and the total degassing loss coefficient γ, and β and γ are calculated by the following formulas:

[0019]

[0020] In the above equations, λ Rn =2.08*10-6 s -1 , is the radioactive decay coefficient of radon; v is the average flow velocity of the river section; D is the diffusion coefficient of gas molecules; h is the average depth of the river, T air is the air temperature.

[0021] The seasonal river-groundwater water exchange rate estimation method has the beneficial effects that:

[0022] The seasonal river-groundwater water exchange rate estimation method under complex hydrogeological conditions of the seasonal river is provided by innovatively combining the RnMBM and the riverbed permeability coefficient, the method uses radon as a natural tracer, can accurately invert the exchange flux of surface water and groundwater, is simple in method operation, can adapt to complex hydrogeological conditions, dynamically responds to seasonal changes, reduces the field monitoring cost and the labor input, and has strong popularization, and is helpful to improving the overall hydrological monitoring level. BRIEF DESCRIPTION OF DRAWINGS

[0023] The seasonal river-groundwater water exchange rate estimation method has the beneficial effects that:

[0024] Figure 1 The vertical pipe method for determining the riverbed permeability coefficient is a schematic diagram. DETAILED DESCRIPTION

[0025] In order to realize the seasonal river-groundwater water exchange rate estimation, the Rn mass balance model (RnMBM) is combined with the riverbed permeability coefficient in the application, different formulas are selected according to the different upstream and downstream flow rates and radon concentrations, the river-groundwater water exchange rate is calculated, and the accuracy of the result is further verified in combination with the test result of the permeability coefficient. 222 Rn mass balance model (RnMBM) 222 RnMBM) and the riverbed permeability coefficient, different formulas are selected according to the different upstream and downstream flow rates and radon concentrations, the river-groundwater water exchange rate is calculated, and the accuracy of the result is further verified in combination with the test result of the permeability coefficient.

[0026] The seasonal river-groundwater water exchange rate estimation method includes the following steps:

[0027] Firstly, a suitable river section is selected, the river width, river depth, flow rate, water temperature, the distance between the upstream and downstream of the river section (the upstream and downstream flow rates of the river section to be measured are calculated through these parameters), and the upstream and downstream Rn concentrations and the riverbed permeability coefficient of the river section are measured. 222 Rn concentrations and the riverbed permeability coefficient of the river section are measured.

[0028] Secondly, a domestic well close to the river in the river basin is selected as a groundwater sampling point, and the Rn concentration in the well is measured. 222 Rn concentration in the well is measured.

[0029] Thirdly, the upstream and downstream flow rates and the Rn concentrations of the river section are combined to calculate the river-groundwater water exchange rate. 222Based on the concentration of Rn, select an appropriate formula (formulas (5)-(7b) below) to calculate the river-groundwater exchange rate, and at the same time calculate the riverbed permeability coefficient according to formula (10) below.

[0030] The fourth step is to compare and analyze the results obtained from the formula with the calculated riverbed permeability coefficient to verify the accuracy of the results.

[0031] The invention will be further described below with reference to the formulas and figures. To estimate the rate of river-groundwater exchange flux, the formula shown in (1) is used. 222 Rn mass balance model:

[0032] Q d C d =Q u C u +△M (1)

[0033] In the formula C u and C d These refer to the upstream and downstream river water respectively. 222 Rn concentration (Bq / m³) 3 );Q u and Q d These refer to the flow rates of the upstream and downstream rivers (m³). 3 / s); △M refers to the distance between river sampling points. 222 Variations in Rn content can be caused by factors including atmospheric escape, radioactive decay, groundwater discharge, river recharge, tributary recharge, and others. 226 The supply of Ra decay.

[0034] Because the interaction between surface water and groundwater is complex, the following assumptions are made: Tributaries in the study basin are not included and their influence is ignored in the actual calculations. Secondly, sampling was conducted during a season with low rainfall and temperature, and there was no rainfall before or after sampling; therefore, the effects of rainfall and evaporation can be neglected. 226 Regarding Ra decay replenishment, due to the presence of [resources] in river water and riverbed sediments... 226 The radon content in Ra decay is very low and can be ignored. Furthermore, the model assumes that radon in the river water can mix instantaneously, and that its concentration is uniformly distributed horizontally and vertically, meaning that a water sample collected at any point along a cross-section can represent the average concentration of that section.

[0035] Based on the above assumptions and the law of conservation of mass, the following mass conservation equation is established:

[0036] Q d C d =Q u C u e (-αL) +∫0 L qg C g e (-αL) dx C u <C d Q u d (2)

[0037] Q d C d =Q u C u e (-αL) -∫0 L q r C u e -α(L-x) dx C u >C d Q u Q d (3)

[0038]

[0039] Q d =Q u +(q g -q r )LC u >C d Q u d or C u <C d Q u Q d (4b)

[0040] In the formula, q g and q r These refer to groundwater flow rate per unit length and river seepage rate (m). 3 / (s·m)), C g groundwater 222 Concentration of Rn (Bq / m³) 3 L refers to the distance between upstream and downstream sampling points (m), x refers to the distance (m) from the point where the river interacts with groundwater (i.e., the point where surface water and groundwater interact along the river channel) to the downstream sampling point, and α is the total loss coefficient (m). -1 ).

[0041] According to upstream and downstream 222 Different formulas are selected based on the differences in Rn concentration and flow rate.

[0042] When upstream sampling point 222 When both Rn concentration and river flow are lower than those at the downstream sampling point, as shown in (2), it indicates that there is groundwater replenishing the river.

[0043] ​​When the upstream sampling point 222 Rn concentration and river flow are both higher than those of the downstream sampling point, i.e. as shown in (3), it indicates that there is river water infiltration to recharge groundwater;

[0044] When the upstream and downstream sampling points 222 When the difference of Rn concentration and river flow is opposite, i.e. as shown in (4a) and (4b), it indicates that river water infiltration and groundwater discharge jointly act on the river section, and it is assumed that both of them are uniformly distributed along the river section.

[0045] According to equations (2)-(4b), corresponding solutions can be obtained, as shown below:

[0046]

[0047] In formula (2), α refers to a total loss coefficient, which includes a radioactive decay coefficient (β) and a total loss coefficient (γ) caused by degassing:

[0048] α = β + γ (8a)

[0049]

[0050] Here, β is a decay coefficient related to the average flow rate of the river (m -1 ), λ Rn refers to the radioactive decay coefficient of radon, λ Rn = 2.08*10 -6 s -1 , v refers to the average flow rate of the river section, and γ refers to the loss coefficient (m -1 ) caused by degassing.

[0051] It is assumed that the research river section is mainly turbulent flow, and a surface renewal model is used to describe the loss of radon activity caused by gas exchange, and the degassing loss coefficient can be represented by the following formula:

[0052]

[0053] Here, D refers to the diffusion coefficient of gas molecules (m 2 / s), v refers to the average flow rate (m / s), h refers to the average depth of the river (m), and Tair refers to the air temperature (℃).

[0054] The present application combines 222 the results calculated by the RnMBM formula, and adopts a vertical pipe method (single pipe method, Figure 1) to verify the accuracy of the formula calculation results. The device mainly collects a certain thickness of undisturbed soil of the riverbed through a transparent acrylic pipe (1) with a scale, then fixes it in a bucket filled with water (2), and finally fills water into the acrylic pipe, allowing it to naturally infiltrate, records the water level change at different times, and calculates through formula (10):

[0055]

[0056] where: K v is the vertical permeability coefficient (m·d -1 ); L is the thickness of the sediment in the pipe (m); H1 and H2 are the water head of the water level in the pipe to the reference surface (water surface) at t1 and t2, respectively (m).

[0057] Take Xiliugou, one of the ten large pore outlets in Ordos, as an example. As a representative river of the ten large pore outlets, it is a typical seasonal river. In October 2024, we set up surface water and groundwater sampling points in the Xiliugou basin and completed the collection and determination of related samples (water quality indicators, river parameters, radon concentration, vertical permeability coefficient). Among them, 17 sampling points were set up for surface water, and 15 sampling points were set up for groundwater (G represents the river).

[0058] According to the differences in upstream and downstream flow and radon concentration, select the appropriate formula to calculate the discharge flux of surface-groundwater interaction, the results are as follows: From Table 1, it can be seen that except for the G6-G7 river section, which is mainly supplied by surface water to groundwater, the rest of the river sections are basically supplied by groundwater to surface water, and the discharge flux of G10-G11 and G13-G14 river sections is much higher than that of other river sections. The riverbed permeability coefficient of some river sections is also shown in Table 2, from which it can be seen that the measured riverbed vertical permeability coefficient from G10 to G14 is larger, which is consistent with the calculation results of the radon mass balance model, further proving the accuracy of the model calculation results.

[0059] Table 1 RnMBM calculation results

[0060]

[0061] Table 2 Vertical permeability coefficient of some river sections

[0062]

[0063] The contents not described in detail in the specification belong to the prior art known to those skilled in the art.

Claims

1. A method for estimating the rate of seasonal river-groundwater exchange, characterized in that, Includes the following steps: Step 1: Determine the upstream and downstream flow rates of the river section to be tested. 222 Rn concentration data were measured in groundwater near the river section to be tested. 222 Rn concentration data were measured; Step 2, based on the upstream and downstream flow rates of the river section to be tested obtained in Step 1, 222 Rn concentration data and groundwater 222 Rn concentration data were used to calculate the river-groundwater exchange rate.

2. The method for estimating the seasonal river-groundwater exchange rate as described in claim 1, characterized in that, The specific steps for calculating the river-groundwater exchange data in step 2 are as follows: Upstream of the river section to be tested 222 Both Rn concentration and flow rate were lower than those downstream of the tested river section. 222 Given the concentration and flow rate of Rn, the groundwater flow rate q per unit length is calculated using the following formula. g : Upstream of the river section to be tested 222 Both Rn concentration and flow rate were higher than those downstream of the tested river section. 222 Given Rn concentration and flow rate, the infiltration rate q per unit length of river is calculated using the following formula. r : Upstream of the river section to be tested 222 Rn concentration and flow rate and downstream of the river section under test 222 When the differences between Rn concentration and flow rate are opposite, the groundwater flow rate q per unit length is calculated according to the following two formulas. g and the river infiltration rate per unit length q r : In the above equations, C u C d The upstream and downstream sections of the river to be tested are respectively 222 The concentration of Rn; Q u Q d The flow rates upstream and downstream of the river section to be measured are respectively; C g In groundwater 222 The concentration of Rn; L is the distance between upstream and downstream sampling points; α is the total loss coefficient, which is the sum of the radioactive decay coefficient β and the total degassing loss coefficient γ, and β and γ are calculated by the following formulas: In the above equations, λ Rn =2.08*10 -6 s -1 , where is the radioactive decay coefficient of radon; v is the average flow velocity of the river section; D is the diffusion coefficient of gas molecules; h is the average depth of the river; T air This refers to the air temperature.

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