Method for identifying groundwater supply mode in thick vadose zone area through combination of multiple tracers

Abstract

The invention relates to the field of groundwater replenishment mode research, in particular to a method for identifying a groundwater replenishment mode in a thick vadose zone area through combination of multiple tracers. The method solves the problem that the groundwater supply process in a thick aeration zone-aquifer system is difficult to find. The method comprises the following specific steps: 1, observing the shape of tracers (Cl <->, NO3 <-> and 3H) in a farmland / grassland soil profile; 2, analyzing the change of underground water tracers (Cl <->, NO3 <->, delta D, delta 18O and the like) along with time and the response of the underground water tracers to rainfall; 3, determining the tritium content and 14C apparent age of underground water; and 4, comparing the similarity of the composition of rainfall, soil water and groundwater isotopes (deltaD, delta18O), and evaluating the evaporation effect. According to the method, a plurality of chemical tracers, stable isotope and radioactive isotope fingerprint technologies are combined, the groundwater supply mode is evaluated at the same time from the aspects of the aeration zone and the saturation zone, the accuracy of the identification result is verified through multiple technical means, and a comprehensive method framework is provided for solving the groundwater supply mechanism in the thick aeration zone area.

Description

Technical Field

[0001] This invention relates to the field of research on groundwater recharge mechanisms, and more particularly to a method for identifying groundwater recharge mechanisms in thick vadose zones using multiple tracers. Background Technology

[0002] Groundwater recharge mechanisms include two types: piston flow and preferential flow. Piston flow refers to the phenomenon where precipitation or surface water infiltrates uniformly, pushing the water body downwards as a whole; this typically occurs in homogeneous, porous media. Preferential flow refers to the phenomenon where water rapidly infiltrates through dominant pathways such as fissures or conduits in the soil or rock; this is common in heterogeneous media or areas with fissures. Accurately revealing groundwater recharge mechanisms is beneficial for characterizing the surface recharge process, reducing errors in groundwater recharge and storage estimation, and also helps assess groundwater renewal rates and pollutant migration times. This is of great significance for the sustainable use of regional water resources and the prevention and control of groundwater pollution.

[0003] Due to the high heterogeneity of the vadose zone medium in three-dimensional space, "how rainfall penetrates the thick vadose zone to reach the aquifer" has always been a difficult point in hydrogeological scientific and technological research. Previous inventions have rarely involved technical methods for assessing groundwater recharge patterns in deep vadose zones, resulting in groundwater recharge pattern identification being limited by inherent knowledge and traditional technical means, making it difficult to accurately reveal the recharge process and water resources in actual hydrogeological surveys. Summary of the Invention

[0004] To address the problems existing in the background technology, a method for jointly identifying groundwater recharge patterns in thick vadose zones using multiple tracers is proposed, including the following steps:

[0005] Step 1: Observe the shape of the tracer in the farmland soil profile;

[0006] Step 2: Analyze the changes of groundwater tracers over time and their response to rainfall;

[0007] Step 3, determine the tritium content of the groundwater and 14 C age;

[0008] Step 4: Compare the similarity of hydrogen and oxygen isotope compositions of precipitation, soil water, and groundwater to assess the evaporation effect;

[0009] Step 5: Based on the results of steps 1-4, comprehensively determine the groundwater recharge method.

[0010] Preferably, the tracer in the farmland soil profile is Cl. - NO3 - And tritium.

[0011] Preferably, step 1 further includes:

[0012] Step 1.1: Investigate the history of fertilizer application in local farmland to determine whether nitrogen fertilizer has been applied.

[0013] Step 1.2: Select typical farmland plots and collect soil samples in stratified layers;

[0014] Step 1.3: Extract soil water and test chemical indicators;

[0015] Step 1.4: Test the tritium content of the soil water.

[0016] Preferably, the groundwater tracer is Cl. - NO3 - δD and δ 18 O.

[0017] Preferably, step 2 further includes:

[0018] Step 2.1: Dynamically collect precipitation and groundwater data in the survey area within a hydrological year;

[0019] Step 2.2: Determine the stable hydrogen and oxygen isotopes δD and δ¹⁸O in precipitation and groundwater. 18 O.

[0020] Preferably, step 3 further includes:

[0021] Step 3.1: Concentrate the groundwater using a distillation and concentration device to meet the testing conditions of existing technology and equipment;

[0022] Step 3.2: Analyze the groundwater sampling depth in the aquifer. If the sampling is performed at a deeper depth in the aquifer or mixed in the well, the detection of tritium is generally considered to be due to the presence of piston flow recharge.

[0023] Step 3.3: Estimate the apparent density of groundwater using the following radioactive decay formula. 14 Age C:

[0024]

[0025] In the formula, 14 C DIC The isotopic composition (pMC) of dissolved inorganic carbon in groundwater. 14 C0 is the initial value. 14 C content (pMC) is generally considered to be 100 pMC. 14 The value of C0 should be between 80 and 100 pMC.

[0026] Preferably, step 4 further includes:

[0027] Step 4.1: Derive the local atmospheric precipitation line (LMWL) by regression analysis using literature data or measured precipitation hydrogen and oxygen isotope data. The general equation is δD = A * δ 18 O+B, where A and B are regression coefficients;

[0028] Step 4.2, the evaporation effect is assessed by lc-excess, and the lc-excess of soil water and groundwater is calculated by the following equation;

[0029] lc-excess=δ 2 Ha×δ 18 Ob

[0030] In the formula, a and b are the slope and intercept of LMWL, respectively;

[0031] Step 4.3: If lc-excess of soil water or groundwater is less than 0, it is considered that the evaporation signal has been transmitted from the surface to the aquifer; the stronger the evaporation, the more negative lc-excess will be.

[0032] Preferably, step 5 further includes:

[0033] Step 5.1: The relative contributions of the piston flow and the preferred flow can be estimated using a linear mixing model:

[0034] C gw =R d ·C sw +R pf ·C p

[0035] R d +R pf =1

[0036] In the formula, R d and R pf C represents the proportion (%) of piston flow and preferred flow, respectively. p C sw and C gw These represent the concentrations of tracers in precipitation, soil water, and groundwater, respectively.

[0037] Step 5.2: Calculate the contributions of piston and preferred flow supply using the MixSIAR model based on the Bayesian framework, implemented using the MixSIAR statistics package in R.

[0038] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention comprehensively applies chemical tracers, stable isotopes and radioactive dating isotopes to comprehensively determine the groundwater recharge mode from the perspectives of the vadose zone and the saturation zone. Multiple environmental tracers complement and verify each other, which greatly improves the efficiency and reliability of groundwater recharge mode identification and solves the problem of accurately identifying groundwater recharge mode in actual hydrogeological surveys in thick vadose zone areas. Attached Figure Description

[0039] Figure 1 Flowchart of a method for jointly identifying groundwater recharge patterns in thick vadose zones using multiple tracers;

[0040] Figure 2 This is a map showing the distribution of environmental tracer concentrations with soil depth in a soil profile.

[0041] Figure 3 A time-varying graph of hydrogen and oxygen isotopes in precipitation, soil water, and groundwater;

[0042] Figure 4 For precipitation, soil water and groundwater tritium and 14 C. Interannual variation of age;

[0043] Figure 5 This is a correlation diagram of hydrogen and oxygen dual isotopes in atmospheric precipitation, soil water, and groundwater. Detailed Implementation

[0044] To make the technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0045] To address the challenge of determining groundwater recharge patterns in thick vadose zones during actual hydrogeological surveys, such as... Figure 1 As shown, the present invention adopts the following technical solution:

[0046] A method for jointly identifying groundwater recharge patterns in thick vadose zones using multiple tracers includes the following steps:

[0047] Step 1: Observe the tracer (Cl) in the farmland soil profile. - NO3 - The shape of tritium (and tritium). For example... Figure 2 As shown, if the tracer concentration exhibits a single-peak characteristic with increasing soil depth, resembling a parabolic curve, it may support piston flow; if the profile has multiple peaks, it may support preferential flow.

[0048] Step 2, analyze the groundwater tracer (Cl) - NO3 - ,δD,δ18 The changes of O (etc.) over time and their response to rainfall. For example... Figure 3 As shown, if the tracer exhibits significant variation over time and shows a clear response to rainfall, it indicates that the preferred flow is dominant; otherwise, it supports a piston flow.

[0049] Step 3, determine the tritium content of the groundwater and 14 C. Age. (e.g., age) Figure 4 As shown, high tritium levels indicate that young, modern water is supplied by preferential flow, while low tritium levels and larger tritium levels... 14 The C-age indicates that the groundwater is ancient water primarily supplied by piston flow.

[0050] Step 4: Compare the similarity of hydrogen and oxygen isotope compositions in precipitation, soil water, and groundwater to assess the evaporation effect. For example... Figure 5 As shown, if groundwater isotopes are close to the local precipitation line (LMWL) and evaporation is not significant, preferential flow may be supported; if groundwater isotopes are close to deep soil water and carry a significant evaporation signal, piston flow may be supported.

[0051] Step 5: Based on the results of steps 1-4, comprehensively determine the groundwater recharge mode: piston flow, preferential flow, or both. If both recharge modes exist simultaneously, further quantify the relative contributions of piston flow and preferential flow using a linear mixture model or a Bayesian mixture model.

[0052] In a further embodiment, step 1 further includes:

[0053] Step 1.1: Investigate the history of fertilizer application in local farmland to determine whether nitrogen fertilizer has been applied.

[0054] Step 1.2: Select typical farmland plots and collect soil samples in layers. Generally, the soil profile depth should be more than 10m.

[0055] Step 1.3 involves soil water extraction and chemical index testing. Based on fertilizer usage history, a decision is made regarding whether to use a fertilizer source tracer (Cl). - NO3 - wait);

[0056] Step 1.4: Test the tritium content of soil water. Tritium is a ubiquitous environmental tracer applicable to any thick vadose zone.

[0057] In a further embodiment, step 2 further includes:

[0058] Step 2.1: Within a hydrological year, dynamically collect precipitation and groundwater data in the survey area, collecting precipitation data from every precipitation event as much as possible, and collecting groundwater data every half month or 10 days.

[0059] Step 2.2: Determine the stable hydrogen and oxygen isotopes δD and δ¹⁸O in precipitation and groundwater. 18 O. Water chemical indicators, such as Cl - NO3 - etc. are optional indicators.

[0060] In a further embodiment, step 3 further includes:

[0061] Step 3.1: Since tritium has a short half-life and the tritium content in groundwater is generally low, it is necessary to concentrate the groundwater using a distillation and concentration device to meet the testing conditions of existing technology and equipment.

[0062] Step 3.2, the criterion for judging high and low tritium is generally 2TU. Further consideration is needed regarding the sampling depth of groundwater in the aquifer. If the sampling is taken at a deeper depth in the aquifer or mixed in the well, the detection of tritium is generally considered to be due to the presence of piston flow recharge.

[0063] Step 3.3: Estimate the apparent density of groundwater using the following radioactive decay formula. 14 Age C:

[0064]

[0065] In the formula, 14 C DIC The isotopic composition (pMC) of dissolved inorganic carbon in groundwater. 14 C0 is the initial value. 14 The carbon content (pMC) is generally considered to be 100 pMC. Considering that this invention is applied to regions with thicker vadose zones, the thickness of the vadose zone will affect the initial carbon content. 14 The content of C, therefore 14 The value of C0 should be between 80 and 100 pMC.

[0066] In a further embodiment, step 4 further includes:

[0067] Step 4.1: Derive the local atmospheric precipitation line (LMWL) by regression analysis using literature data or measured precipitation hydrogen and oxygen isotope data. The general equation is δD = A * δ 18 O+B, where A and B are regression coefficients;

[0068] Step 4.2, the evaporation effect is assessed using lc-excess, calculated using the following equations for soil water and groundwater lc-excess. lc-excess is typically greater than δD and δ 18 O is more sensitive to isotopic differences among precipitation, soil water and groundwater, so lc-excess was used to observe the transmission of evaporation signals from the surface to the saturation zone.

[0069] lc-excess=δ2 Ha×δ 18 Ob

[0070] In the formula, a and b are the slope and intercept of LMWL, respectively.

[0071] Step 4.3: The lc-excess in precipitation is usually defined as zero. Deep soil water refers to soil water that is not directly affected by surface evaporation, water absorption by vegetation roots, or human activities, and its depth is generally below 5m. If the lc-excess of soil water or groundwater is <0, it is considered that the evaporation signal has been transmitted from the surface to the aquifer; the stronger the evaporation, the more negative the lc-excess.

[0072] In a further embodiment, step 5 further includes:

[0073] Step 5.1: The relative contributions of the piston flow and the preferred flow can be estimated using a linear mixing model:

[0074] C gw =R d ·C sw +R pf ·C p

[0075] R d +R pf =1

[0076] In the formula, R d and R pf C represents the proportion (%) of piston flow and preferred flow, respectively. p C sw and C gw These represent the concentrations of the tracer in precipitation, soil water, and groundwater, respectively. The tracer is generally a stable Cl-. - δD or δ 18 O etc.

[0077] Step 5.2 further utilizes the Bayesian framework-based MixSIAR model to calculate the contributions of the piston and preferential flow supply, implemented using the MixSIAR statistical package in R. Compared to a simple linear mixing model, this model exhibits lower uncertainty and simultaneously considers isotopic fractionation effects. The model can calculate the contribution rate by using two sets of tracers as inputs separately, with the average of the two sets of tracers serving as the final contribution rate, thus reducing uncertainty. The tracer is a combination of two sets of tracers, such as Cl... - and NO3 - δD and δ 18 O etc.

[0078] This invention comprehensively applies chemical tracers, stable isotopes, and radioactive dating isotopes to determine the groundwater recharge mode from two perspectives: the vadose zone and the saturation zone. Multiple environmental tracers complement and verify each other, greatly improving the efficiency and reliability of groundwater recharge mode identification. This invention solves the problem of accurately identifying groundwater recharge modes in actual hydrogeological surveys in thick vadose zone areas.

[0079] This patent was applied to investigate the groundwater recharge mechanisms in the Loess Plateau region, a typical geomorphological area. The soil profile Cl of farmland in this region... - NO3 - Tritium and groundwater Cl generally exhibit a unimodal characteristic. - NO3 - δD and δ 18 O and other tracers showed no significant changes over time and no obvious response to rainfall; most groundwater contained no tritium and 14 C ages are mostly from several thousand to tens of thousands of years, and groundwater δD and δ 18 O is close to deep soil water and carries evaporation signals. However, high tritium content was detected in some shallow groundwater samples from the loess plateau center. In summary, precipitation in the Loess Plateau region mainly replenishes groundwater through piston flow, with preferential flow existing only in a few areas such as the plateau center, waterlogged areas, loess caves, and depressions. For regional aquifers, piston flow accounts for 77%-89% of recharge, while preferential flow accounts for only 11%-23%.

[0080] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. Various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention.

Claims

1. A method for combined identification of groundwater recharge patterns in thick unsaturated zone areas using multiple tracers, characterized in that, The method comprises the following steps: Step 1, observing the shape of the tracer in the soil profile of the farmland; Step 2, analyzing the change of the tracer in the groundwater over time and its response to rainfall; Step 3, determining the tritium content of the groundwater and 14 C age; Step 4, comparing the similarity of the hydrogen and oxygen isotope compositions of the rainfall, soil water and groundwater to evaluate the evaporation effect; Step 5, comprehensively judging the recharge mode of the groundwater according to the results of steps 1-4.

2. The method for jointly identifying groundwater recharge mode in thick unsaturated zone region according to claim 1, characterized in that, The tracers in the soil profile of farmland are Cl - , NO3 - and tritium.

3. The method for jointly identifying groundwater recharge mode in thick unsaturated zone region according to claim 1, characterized in that, The step 1 further comprises: Step 1.1, investigating the history of fertilizer application in the local farmland to determine whether there is nitrogen input; Step 1.2, selecting a typical farmland sample site to collect soil samples in layers; Step 1.3, extracting soil water and testing chemical indicators; Step 1.4, testing the tritium content of the soil water.

4. The method for jointly identifying groundwater recharge mode in thick unsaturated zone regions according to claim 1, characterized in that, Groundwater tracer is Cl - , NO3 - , δD and δ 18 O.

5. The method for identifying groundwater recharge regime in thick unsaturated zone regions by combined use of multiple tracers according to claim 1, characterized in that, The step 2 further comprises: Step 2.1, dynamically collecting rainfall and groundwater in the study area within a hydrological year; Step 2.2, determination of hydrogen and oxygen stable isotopes δD, δ 18 O in precipitation and groundwater 6. The method for recognizing groundwater recharge mode in thick unsaturated zone regions by combined identification of tracers according to claim 1, characterized in that, The step 3 further comprises: Step 3.1, concentrating the groundwater by a distillation concentration device to meet the testing conditions of the existing technology equipment; Step 3.2, analyzing the groundwater at the sampling depth of the aquifer, if sampling at a deeper depth of the aquifer or mixed sampling in the wellbore, the detection of tritium is generally considered to exist piston flow recharge; Step 3.

3. The apparent age of the groundwater was estimated from the radioactive decay equation 14 C age: wherein 14 C DIC is the isotopic composition of the dissolved inorganic carbon (pMC) of the groundwater, 14 C0is the initial 14 C content (pMC), generally taken as 100 pMC, 14 C0should be in the range 80-100 pMC.

7. The method for recognizing groundwater recharge mode in thick unsaturated zone regions by combined identification of tracers according to claim 1, characterized in that, The step 4 further comprises: Step 4.1, Local Meteoric Water Line (LMWL) is derived from literature data or measured precipitation hydrogen and oxygen isotope data regression, equation is generally δD = A*δ 18 O + B, A, B are regression coefficients; Step 4.2, the evaluation of evaporation effect is carried out by lc-excess, the lc-excess of soil water and groundwater is calculated by the following equation; lc-excess = δ 2 H-a x δ 18 O-b In the formula, a and b are the slope and intercept of the LMWL respectively; Step 4.3, if the lc-excess of soil water or groundwater is <0, it is considered that the evaporation signal is transmitted from the surface to the aquifer; the stronger the evaporation, the more negative the lc-excess.

8. The method for recognizing groundwater recharge mode in thick unsaturated zone regions by combined identification of tracers according to claim 1, characterized in that, The step 5 further comprises: Step 5.1, the relative contribution of piston flow and preferential flow can be estimated by a linear mixing model: C gw = R d · C sw + R pf · C p R d +R pf =1 where R d and R pf represent the proportion (%) of piston flow and preferential flow, respectively, C p , C sw and C gw represent the concentration of the tracer in precipitation, soil water and groundwater, respectively. Step 5.2, the contribution of piston and preferential flow recharge is calculated using the MixSIAR model based on the Bayesian framework, which is realized by the MixSIAR statistical package in R language.

Images