CN108614088B - Method for tracing soil loss in karst region by using rare earth elements - Google Patents
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Abstract
The invention discloses a method for tracing soil loss in a karst region by using rare earth elements, which is characterized by comprising the following steps of: selecting an observation area according to research contents, collecting and testing background values of various rare earth elements in the selected observation area, selecting tracer rare earth elements and calculating release amount, preparing a tracer soil sample, laying the tracer soil sample by a strip method and a section method, marking the surface soil position by marking, collecting and testing the content of the selected rare earth elements after the observation period, and calculating and analyzing the underground leakage amount of the soil. The method utilizes the rare earth elements to trace the soil loss/leakage of the karst region, can quantitatively define the contribution of the soil surface loss and the underground leakage of the karst rock desertification region, and has universal applicability, high accuracy and simple operation in the karst region.
Description
Technical Field
The invention belongs to the field of soil erosion, water and soil conservation and environmental science, and particularly relates to a method for tracing soil leakage in a karst region by using rare earth elements.
Background
The karst landform has a multi-stage and multi-position ground surface and underground double-layer structure which is integrated with water, soil, rocks and organisms into a whole due to long-term karst action, so that the water circulation process and the soil erosion process in the region have particularity and complexity, and a series of special environmental geological problems such as water and soil loss, stony desertification, ground settlement and collapse are generated. Water and soil loss in the karst region is an important factor for degradation of the ecological environment, agricultural sustainable development in the karst region is severely restricted, and the key for controlling the development of stony desertification is to fully reveal the occurrence of water and soil loss and migration mechanism of the karst region sloping field.
Although the research on the special soil loss types in karst regions is concerned in recent years, including surface loss, underground loss and the like, the surface loss research is based on a series of methods such as field observation, indoor simulation, isotope tracing and the like, and the erosion process, the mechanism and the influence factors are systematic; however, the karst underground runoff sediment field direct observation difficulty coefficient is large, and no feasible research method and means exist at present, so that the research on the special underground leakage mechanism based on the karst region is less, the karst underground runoff sediment field direct observation difficulty coefficient is only in the stages of qualitative description and indoor simulation exploration at present, and the leakage mechanism of the karst underground runoff sediment field direct observation difficulty coefficient cannot be disclosed in the existing research.
Rare earth elements have recently been identified as desirable trace elements for the study of soil erosion. The method is characterized in that a stable rare earth element tracing technology is utilized to research soil erosion distribution, the basic principle is that a tracing element oxide and soil are uniformly mixed and then distributed at different terrain parts of a research area, the tracing element oxide and the soil migrate along with runoff sediment in the whole rainfall erosion process, the eroded sediment sample is collected, the content of the tracing element is measured by methods such as neutron activation analysis and the like, and therefore the erosion sediment source and the soil erosion difference characteristics of different terrain parts are researched, and the purposes of distinguishing the erosion sediment source and obtaining the soil loss amount of different terrain parts are achieved. The method is mainly suitable for the non-karst region and the slope with the runoff plot, and for the karst region with a special erosion type, underground leakage hardly receives leaked water and soil directly, so that the process of the crack soil leakage can be reflected only by tracing the distribution of carriers in cracks.
Disclosure of Invention
The invention aims to provide the rare earth element tracing method for the underground soil leakage in the karst region, which has the advantages of high determination precision, simple operation and environmental friendliness.
The purpose of the invention is mainly realized by the following technical scheme: a method for tracing soil leakage in karst regions by using rare earth elements comprises the steps of selecting an observation region according to research contents, collecting and testing background values of various rare earth elements in the selected observation region, selecting tracing rare earth elements and calculating release amount, preparing a tracing soil sample and laying the tracing soil sample by a 'strip method + section method', marking the surface soil position by marking lines, collecting and testing the content of the selected rare earth elements after an observation period, and calculating and analyzing soil underground leakage amount;
the method specifically comprises the following steps:
(1) according to research contents, selecting a plurality of typical observation areas capable of representing the soil in a research area to be subjected to surface loss or underground leakage, wherein the selected observation areas need to comprise part of rock interfaces in consideration of the particularity of soil loss or leakage of rock-soil interfaces;
(2) collecting a soil sample of a rare earth element background value in an observation area, namely collecting the soil sample in a layering manner with a layering thickness of 2cm at the central position of a grid by using the size of a 20-50cm grid in the observation area, taking the soil in a range of 2cm close to a rock interface as a soil sample of the rock-soil interface, collecting the soil sample of the rock-soil interface with the same layering thickness, and collecting the soil sample to a depth range of 8-20 cm;
(3) selecting various rare earth elements for carrying out a test, wherein the release concentration and the release amount are respectively calculated according to the following formula:
Ci＝BCix a (formula 1),
wherein i is the ith trace rare earth element, i is 1, 2, 3, …, n; ciThe concentration of the ith trace rare earth element is applied, namely mg/kg; BCiIs the background value of the ith trace rare earth element, mg/kg; miThe release amount of the i & ltth & gt trace rare earth element oxide is g; siThe weight of soil required by the i & ltth & gt tracing rare earth element is kg; a is a concentration expansion coefficient of 20-100; b is a concentration correction coefficient, namely the purity of the rare earth oxide, which is generally 0.9999; c is an oxide correction coefficient, and the relative molecular mass of the rare earth element accounts for the proportion of the relative molecular mass of the oxide;
said SiCalculated as follows:
wherein, in the formula, WL is the sample area size of the observation area, cm2；HiLaying depth, cm, for the ith trace rare earth element; r is the volume weight of the soil with the distribution depth, g/cm3(ii) a 1000 is the unit transformation coefficient;
(4) preparing a tracer soil sample, and mixing the rare earth oxide with the calculated release amount with soil; adopting a dilution method in the mixing process of the trace rare earth oxide and the soil, and gradually diluting to approach the discharge concentration;
(5) the configured tracer soil samples are distributed by adopting a strip method and a section method, the configured tracer soil samples are filled into strip pits of an observation area in a layered mode and tamped to the same volume weight as the original slope surface, and meanwhile, the surface soil position is marked at a rock-soil interface;
(6) during the study period: and (3) after rainfall or a month or a season or a year, carrying out the soil sample according to the step (2), measuring the content of the rare earth element of the sample, and carrying out the following two conditions in a collection mode:
taking a rock-soil interface mark as a datum point, accurately collecting soil samples according to the layering thickness of 2cm, and recording the thickness of each collected layer of soil samples;
secondly, taking the rock-soil interface mark as a datum point, measuring the downward movement displacement of the soil layer by using a standard ruler, and accurately collecting a soil sample at the surface layer of the downward movement soil layer by the layering thickness of 2 cm;
(7) the soil loss or leakage analysis and calculation includes the following two cases:
calculating the thickness of the soil sample which is not collected below the reference point, namely the soil loss/leakage thickness, and calculating the soil loss/leakage amount according to a formula 3; comparing the concentration change of the rare earth element of the soil after the research period with the concentration change of the background value, if the difference between the concentration change and the background value is not large, the soil is proved to be mainly subjected to overall creep loss, and if the difference between the concentration change and the background value is large, the soil is analyzed according to the data condition;
calculating the average value of the soil loss/leakage thickness measured by the ruler, and calculating the soil loss/leakage amount according to the formula 3; comparing the concentration of the rare earth elements in the soil with the concentration change of the background value after the research period, whether the creeping loss of the soil is caused by the loss of the rock-soil interface can be judged.
The size of the observation area in the step (1) is determined by the actual situation outside the field, and 0.5m multiplied by 0.5 m-2 m multiplied by 2m is selected.
The background value of the rare earth element of the soil sample in the step (2) is determined by an INNA (InNA) method of instrument neutron activation analysis and an inductively coupled plasma mass spectrometer ICP-MS (inductively coupled plasma mass spectrometer); after the background value soil sample is collected, all the soil within the depth range of 8-20cm is collected and dried indoors for later use.
The rare earth elements used for tracing the soil leakage in the step (3) are well combined with the soil, are insoluble in water, are not easily absorbed by plants and are harmless to the soil environment, and the rare earth tracing elements have the characteristics of low soil background value, small application amount and easiness in detection; the optional rare earth elements comprise dysprosium Dy, europium Eu, samarium Sm, cerium Ce, lanthanum La and neodymium Nd, and the corresponding rare earth oxides are Dy2O3、Eu2O3、Sm2O3、CeO2、La2O3、Nd2O3。
The soil in the step (4) is air-dried and sieved soil; and (4) the operation process of gradually diluting and approaching the concentration application is that a small amount of test dry soil with the particle size smaller than 0.075mm is fully mixed with the rare earth oxide with the required weight to dilute to form a high-concentration soil sample, then the required soil is added to mix with the squeezer to form a secondary high-concentration soil sample, and finally the air-dried soil which can be filled with the tracing depth is added to fully mix.
The strip method in the step (5) of the strip method and section method layout is a strip with limited width of the selected observation area, the soil loss or soil leakage is equal to or close to the soil loss/leakage amount of the research area, and the rare earth elements are laid in the strip; the section method is that rare earth tracer elements and traced soil are all mixed and distributed on the section, and different tracer elements are distributed on different soil layers.
The layout principle of the strip method and the section method in the step (5) is as follows:
firstly, the structure of the lower soil layer is not damaged as much as possible, and the thickness of the laid soil layer is controlled within a certain range;
secondly, the difference between the leakage and the leakage is ensured to be obvious, and the arrangement of the soil layer is combined with the sampling depth and can be controlled to be 2 cm;
thirdly, the surface layer is laid to be deep enough to ensure that the surface loss is met, and the thickness is 4 cm;
fourthly, in order to ensure that the leakage can be seen in the 4cm layer, the sampling depth is small enough; meanwhile, the method can be detected under the condition of small leakage, and the sampling depth is controlled to be 2cm or below;
fifthly, under the condition of ensuring accuracy, the principle of economy, labor and high efficiency is emphasized.
And (5) marking the surface soil layer position of the rock-soil interface by adopting paint.
And (6) measuring the rare earth element content of the soil sample after the observation period by using an INNA (InNA) method of instrument neutron activation analysis and an inductively coupled plasma mass spectrometer ICP-MS (inductively coupled plasma mass spectrometer).
And (6) if the soil samples need to be collected at different time points, dividing the grid number equally, dividing the rock-soil interface equally according to the same batch, collecting the divided soil samples each time, and backfilling the surrounding soil after collection to ensure that the next soil loss/leakage process is not influenced.
The method can quantitatively define the contribution of soil surface loss and underground leakage in the karst stony desertification area, and has universal applicability, high accuracy and simple operation in the karst area.
Drawings
FIG. 1 is a flow chart of the present invention for underground soil loss in karst regions using rare earth tracing;
FIG. 2 is a schematic diagram of the arrangement of rare earth trace elements lost in the soil underground in the karst region by using rare earth trace according to the present invention;
FIG. 3 is a graph of fracture soil loss/leakage displacement calculated by a rare earth element tracing method according to the present invention;
FIG. 4 is a RE concentration graph which is calculated by a rare earth element tracing method and changes from a background value after soil loss of a fractured soil layer;
FIG. 5 is a RE concentration diagram of soil change of a soil layer after soil loss of a fractured rock-soil interface is calculated by a rare earth element tracing method.
Detailed Description
The invention is further described below with reference to examples and figures.
A method for tracing soil leakage in karst regions by using rare earth elements comprises the steps of selecting an observation region according to research contents, collecting and testing background values of various rare earth elements in the selected observation region, selecting tracing rare earth elements and calculating release amount, preparing a tracing soil sample and laying the tracing soil sample by a 'strip method + section method', marking the surface soil position by marking lines, collecting and testing the content of the selected rare earth elements after an observation period, and calculating and analyzing soil underground leakage amount;
the method specifically comprises the following steps:
(1) according to research contents, selecting a plurality of typical observation areas capable of representing the soil in a research area to be subjected to surface loss or underground leakage, wherein the selected observation areas need to comprise part of rock interfaces in consideration of the particularity of soil loss or leakage of rock-soil interfaces;
(2) collecting a soil sample of a rare earth element background value in an observation area, namely collecting the soil sample in a layering manner with a layering thickness of 2cm at the central position of a grid by using the size of a 20-50cm grid in the observation area, taking the soil in a range of 2cm close to a rock interface as a soil sample of the rock-soil interface, collecting the soil sample of the rock-soil interface with the same layering thickness, and collecting the soil sample to a depth range of 8-20 cm;
(3) selecting various rare earth elements for carrying out a test, wherein the release concentration and the release amount are respectively calculated according to the following formula:
Ci＝BCix a (formula 1),
wherein i is the ith trace rare earth element, i is 1, 2, 3, …, n; ciThe concentration of the ith trace rare earth element is applied, namely mg/kg; BCiIs the background value of the ith trace rare earth element, mg/kg; miThe release amount of the i & ltth & gt trace rare earth element oxide is g; siThe weight of soil required by the i & ltth & gt tracing rare earth element is kg; a is a concentration expansion coefficient of 20-100; b is a concentration correction coefficient, namely the purity of the rare earth oxide, which is generally 0.9999; c is an oxide correction coefficient, and the relative molecular mass of the rare earth element accounts for the proportion of the relative molecular mass of the oxide;
said SiCalculated as follows:
wherein, in the formula, WL is the sample area size of the observation area, cm2(ii) a Hi is the laying depth of the ith trace rare earth element, cm; r is the volume weight of the soil with the distribution depth, g/cm3(ii) a 1000 is the unit transformation coefficient;
(4) preparing a tracer soil sample, and mixing the rare earth oxide with the calculated release amount with soil; adopting a dilution method in the mixing process of the trace rare earth oxide and the soil, and gradually diluting to approach the discharge concentration;
(5) the configured tracer soil samples are distributed by adopting a strip method and a section method, the configured tracer soil samples are filled into strip pits of an observation area in a layered mode and tamped to the same volume weight as the original slope surface, and meanwhile, the surface soil position is marked at a rock-soil interface;
(6) during the study period: and (3) after rainfall or a month or a season or a year, carrying out the soil sample according to the step (2), measuring the content of the rare earth element of the sample, and carrying out the following two conditions in a collection mode:
taking a rock-soil interface mark as a datum point, accurately collecting soil samples according to the layering thickness of 2cm, and recording the thickness of each collected layer of soil samples;
secondly, taking the rock-soil interface mark as a datum point, measuring the downward movement displacement of the soil layer by using a standard ruler, and accurately collecting a soil sample at the surface layer of the downward movement soil layer by the layering thickness of 2 cm;
(7) the soil loss or leakage analysis and calculation includes the following two cases:
calculating the thickness of the soil sample which is not collected below the reference point, namely the soil loss/leakage thickness, and calculating the soil loss/leakage amount according to a formula 3; comparing the concentration change of the rare earth element of the soil after the research period with the concentration change of the background value, if the difference between the concentration change and the background value is not large, the soil is proved to be mainly subjected to overall creep loss, and if the difference between the concentration change and the background value is large, the soil is analyzed according to the data condition;
calculating the average value of the soil loss/leakage thickness measured by the ruler, and calculating the soil loss/leakage amount according to the formula 3; comparing the concentration of the rare earth elements in the soil with the concentration change of the background value after the research period, whether the creeping loss of the soil is caused by the loss of the rock-soil interface can be judged.
The size of the observation area in the step (1) is determined by the actual situation outside the field, and 0.5m multiplied by 0.5 m-2 m multiplied by 2m is selected.
The background value of the rare earth element of the soil sample in the step (2) is determined by an INNA (InNA) method of instrument neutron activation analysis and an inductively coupled plasma mass spectrometer ICP-MS (inductively coupled plasma mass spectrometer); after the background value soil sample is collected, all the soil within the depth range of 8-20cm is collected and dried indoors for later use.
The rare earth elements used for tracing the soil leakage in the step (3) are well combined with the soil, are insoluble in water, are not easily absorbed by plants and are harmless to the soil environment, and the rare earth tracing elements have the characteristics of low soil background value, small application amount and easiness in detection; the optional rare earth elements comprise dysprosium Dy, europium Eu, samarium Sm, cerium Ce, lanthanum La and neodymium Nd, and the corresponding rare earth oxides are Dy2O3、Eu2O3、Sm2O3、CeO2、La2O3、Nd2O3。
The soil in the step (4) is air-dried and sieved soil; and (4) the operation process of gradually diluting and approaching the concentration application is that a small amount of test dry soil with the particle size smaller than 0.075mm is fully mixed with the rare earth oxide with the required weight to dilute to form a high-concentration soil sample, then the required soil is added to mix with the squeezer to form a secondary high-concentration soil sample, and finally the air-dried soil which can be filled with the tracing depth is added to fully mix.
The strip method in the step (5) of the strip method and section method layout is a strip with limited width of the selected observation area, the soil loss or soil leakage is equal to or close to the soil loss/leakage amount of the research area, and the rare earth elements are laid in the strip; the section method is that rare earth tracer elements and traced soil are all mixed and distributed on the section, and different tracer elements are distributed on different soil layers.
The layout principle of the strip method and the section method in the step (5) is as follows:
firstly, the structure of the lower soil layer is not damaged as much as possible, and the thickness of the laid soil layer is controlled within a certain range;
secondly, the difference between the leakage and the leakage is ensured to be obvious, and the arrangement of the soil layer is combined with the sampling depth and can be controlled to be 2 cm;
thirdly, the surface layer is laid to be deep enough to ensure that the surface loss is met, and the thickness is 4 cm;
fourthly, in order to ensure that the leakage can be seen in the 4cm layer, the sampling depth is small enough; meanwhile, the method can be detected under the condition of small leakage, and the sampling depth is controlled to be 2cm or below;
fifthly, under the condition of ensuring accuracy, the principle of economy, labor and high efficiency is emphasized.
And (5) marking the surface soil layer position of the rock-soil interface by adopting paint.
And (6) measuring the rare earth element content of the soil sample after the observation period by using an INNA (InNA) method of instrument neutron activation analysis and an inductively coupled plasma mass spectrometer ICP-MS (inductively coupled plasma mass spectrometer).
And (6) if the soil samples need to be collected at different time points, dividing the grid number equally, dividing the rock-soil interface equally according to the same batch, collecting the divided soil samples each time, and backfilling the surrounding soil after collection to ensure that the next soil loss/leakage process is not influenced.
Example (b): referring to fig. 1-4, a method for tracing soil loss in karst regions by using rare earth elements comprises the following steps:
(1) by using the method, in a rocky desertification sloping field with carbonate rock as dolomite in the brook area Qing Yan city of Guiyang, Guizhou province, 6 observation areas (namely, stone grooves or cracks) with the size of 0.5m multiplied by 0.5m are selected, and the basic conditions are recorded as shown in Table 1.
TABLE 1 fundamental conditions of observation point fissures
(2) According to the method, in the selected observation area, the soil sample with the layering thickness of 2cm and the rock-soil interface soil sample are collected to the depth of 16cm by a 33cm multiplied by 33cm grid method.
(3) By adopting the method, ICP-MS is adopted to measure the content of the rare earth elements in the soil sample with the background value, and the rare earth elements used for tracing are Ce, La and Sm.
(4) The method of the invention is used for calculating the release concentration and the release amount of the trace rare earth element, wherein the release concentration expansion coefficient is 20, and the release concentration and the release amount of each observation area are shown in the table 2:
TABLE 2 rare earth trace oxide concentration and application
Taking observation area S1 as an example, where the actual measured width of the crack is 25cm, the length is 50cm, and the trace depths are designed for different rare earth elements: ce is 4cm, La is 2cm, Sm is 2cm, soil
The volume weight is actually measured to be 1.2g/cm3The Ce element background value of the observation area is measured to be 6.20 mm/kg. Calculating the Ce application concentration according to equation 1, i.e.
CCe＝BCCe×a＝6.20mg/kg×20＝124.0mg/kg，
Calculating the soil weight required by the Ce laying layer according to the formula 3, namely
SCe＝WLHiR/1000＝25cm×cm×4cm×1.2g/cm3/1000＝6kg，
The amount of released Ce oxide was calculated according to equation 2, where the purity of purchased Ce oxide was 99.99%, and the oxide correction factor was 0.81, i.e.
MCe＝CCe×SCe/1000bc＝124.0mg/kg×6kg/(1000×0.9999×0.81)＝0.92g
(5) According to the method, a small amount of test dry soil with a small particle size (0.075mm) is fully mixed with the rare earth oxide with the required weight, diluted to form a high-concentration soil sample, then the required soil is added to mix with a squeezer to form a secondary high-concentration soil sample, and finally, air-dried soil which can be filled with the tracing depth is fully mixed.
(6) The method of the invention adopts a strip method and a section method, fills the prepared tracer soil sample into a broken noodle pit, tamps the broken noodle pit to the same volume weight as the original slope surface, and marks the surface soil position at the rock-soil interface, in this example, the broken noodle pit is marked by red paint.
(7) After rainfall of 104.5mm, 151.2mm and 332.7mm, measuring the downward displacement of the soil layer by using a standard ruler, accurately collecting a section soil sample and a soil sample at a rock-soil interface at the layered thickness of 2cm at the surface layer of the downward soil layer, bringing the section soil sample and the soil sample back to the room for air drying, and measuring the content of rare earth by using ICP-MS.
(8) Soil loss/leakage displacement characteristics in observation area
The soil creep displacement of each observation point under the rainfall of 332.7mm is 1.7cm in S1 crack, 2.0cm in S2 crack, 2.0cm in S3 crack, 2.8cm in S4 crack, 2.4cm in S5 crack and 1.6cm in S6 crack in sequence. The soil creep displacement corresponding to the first observation period (the rainfall is 104.5mm) of each observation point is 1.3, 1.8, 1.7, 2.5, 2.0 and 1.0cm in sequence according to the number sequence of the observation points, and the creep displacement proportion of the soil in the whole observation period is 76.5%, 90.0%, 85.0%, 89.3%, 83.3% and 62.5% in sequence; the soil creep displacement corresponding to the second observation period (the rainfall is 46.7mm) is 0.2, 0, 0.2 and 0.4cm in sequence according to the number sequence of observation points, and the creep displacement proportion of the soil in the whole observation period is 11.8%, 0, 8.3% and 25.0% in sequence; the soil creep displacement corresponding to the third observation period (the rainfall is 181.5mm) is 0.2, 0.3, 0.2 and 0.2cm in the order of the number of the observation points, and the creep displacement proportion accounts for 11.8%, 10.0%, 15.0%, 10.7%, 8.3% and 12.5% in the whole observation period in the order. Obviously, the soil creep displacement generated by the rainfall of 104.5mm is between 1.0 and 2.5cm, the creep displacement proportion of 332.7mm under the rainfall is more than 62.5 percent, the soil creep displacement generated by the larger rainfall of 181.5mm is only between 0.2 and 0.3cm, and the creep displacement proportion of 332.7mm is less than 15.0 percent. As shown in fig. 3:
(9) content change characteristic of rare earth elements in observation area
S1 shows that the concentration of Ce is highest in soil layers with the thickness of 0-4 cm under different rainfall levels (104.5mm, 151.2mm and 332.7mm), which is consistent with the trend of Ce laying, and the crack soil surface loss is shown. Compared with the background value, at the rainfall level of 104.5mm, the Ce concentration is obviously reduced in soil layers of 0-4 cm, and is in a weak reduction trend in soil layers of 6-10 cm, while in other soil layers, the Ce concentration is obviously higher than the background value, which shows that Ce elements distributed on the surface layer of 0-4 cm enter soil layers with the soil grains leaking into cracks of 4cm below, including soil layers of 4-6 cm and soil layers of 10-16 cm, and Ce elements in the soil layers of 6-10 cm enter soil layers with the soil grains leaking into soil layers with the soil grains below 10 cm. The soil particles are obviously leaked at rainfall levels of 151.2mm and 332.7mm, the Ce concentration of soil layers of 0-4 cm is obviously lower than the background value, the Ce concentration of the soil layers of less than 4cm is higher than the background value, the Ce element of the soil layers of 0-4 cm on the surface layer enters soil layers of 4-16 cm in the cracks along with soil particles, and the Ce element is accumulated in each soil layer of 2 cm. Comparing the Ce concentration changes at different rainfall levels, the Ce concentration and background value of soil layers of 0-2 and 2-4 cm are reduced along with the increase of rainfall, which shows that the soil particles on the surface of the crack have the tendency of moving downwards to the soil layer at the lower part of the crack along with the rainfall, and the Ce concentration and background value of the soil layers of 4-16 cm are increased along with the increase of the rainfall, which proves that the Ce accumulation of the soil layers of 4-16 cm is caused by the soil particle leakage of the surface layer of 0-4 cm. Similarly, the La concentration of the soil layer of 4-6 cm is the largest difference with the background value according to the La concentration change of different rainfall and different soil layer depths, which is consistent with the trend of La arrangement, and the La arrangement accords with the expected target, except for the soil layer of 2-4 cm under the second rainfall of 151.2mm (the La concentration is obviously higher than the background value, which is probably caused by creep deformation of the fractured soil layer); under different rainfall levels, the La concentrations of soil layers of 0-2 cm and 4-10 cm are negative overall, namely the concentrations are lower than background values, which shows that the La concentrations of certain grain-sized particles of the soil layers tend to move towards the lower layer of the soil layers, but the La concentrations of the soil layers except the La distribution layer are smaller in overall comparison with Ce, which shows that the soil particles of the soil layers of 4-6 cm in the distribution layer move downwards and accumulate in the lower soil layers (6-16 cm) in a small amount. From the graph showing that the concentration of Sm in different soil layers is changed from the background value, the difference between the concentration of Sm in the Sm layer (6-8 cm) and the background value is the largest, the expected design is met, and the concentration value of Sm below the Sm layer is a positive value, namely higher than the background value, which shows that the Sm-carrying soil particles moving downwards from the soil layer are accumulated in the soil layer below the Sm layer, and the accumulation amount of Sm-carrying soil particles generally increases along with the increase of rainfall, as shown in figure 4.
The concentration of Ce in the soil at the rock-soil interface was significantly different from the background at different rainfall levels (104.5mm, 151.2mm and 332.7 mm). Compared with the background value, the Ce concentration is obviously reduced in the laying layer (0-4 cm) under different rainfall, the reduction degree is reduced along with the increase of the rainfall, and on the contrary, the Ce concentration of the soil in each layer below the laying layer is increased along with the increase of the rainfall (except for 8-10 cm layers), which shows that the content of the soil particles carrying Ce by the rock-soil interface laying layer moves down and is accumulated in different soil layers is increased along with the increase of the rainfall. The concentration of Ce in 4-8 cm layers of fissure rock-soil interfaces under a 104.5mm rainfall event and 8-10 cm layers of soil under an 332.7mm rainfall event is lower than a soil background value, the reason mainly comprises two aspects, soil particles which do not carry Ce on the upper layer of the fissure rock-soil interfaces move downwards to the soil layers, soil particles which carry Ce on the soil layers move downwards to other soil layers, but in general, the concentration of Ce under a Ce laying layer at the S1 fissure rock-soil interfaces is generally increased, and the concentration of Ce carried by the laying layer mainly moves downwards. Under different rainfall, the La concentration is obviously lower than the background value in the distribution layer (4-6 cm), and because only the distribution layer is the REE high concentration (see the experimental design), the concentration of the La distribution layer is mainly reduced by moving La particles carried by the La distribution layer downwards to other depth layers of a rock-soil interface, and the La accumulation layer is mainly 10-16 cm of the rock-soil interface. The Sm concentration in the distribution layer (6-8 cm) is also lower than the background value under different rainfall, and the La concentration below the distribution layer is higher than the concentration above the distribution layer, as shown in FIG. 5.
In terms of Ce, La and Sm distribution layers, the Ce concentration of a Ce distribution layer (0-4 cm) is obviously reduced and is lower than a background value, so that the possibility that soil particles carrying Ce on the layer move downwards is shown; the change of the La and Sm concentrations in the layer from the background value shows that the La and Sm concentrations in the layer under the rainfall event of 104.5mm are also lower than the background value, similar to the Ce concentration change, which indicates that the soil particles in the layer actually move downwards under the rainfall event, and the La and Sm concentrations in the layer under the rainfall events of 151.2mm and 332.7mm are higher than the background value, which only indicates that the soil particles carrying Ce and not carrying La and Sm move downwards. The La concentration of the La distribution layer (4-6 cm) is obviously reduced and is also lower than the background value, the concentrations of Ce and Sm of the La distribution layer are both increased and are higher than the background value, the difference between the Ce concentration and the Sm concentration is obvious, and the reason for the difference is that certain size fraction particles of the La distribution layer randomly move downwards; it can be seen that the soil particles moving down in the Ce distribution layer in the 104.5mm rainfall event are only shown to be accumulated in the Sm concentration change, and are not shown to be accumulated in the Ce and La concentration changes, which indicates that the soil particles moving down at the fractured rock-soil interface do not fixedly move down to the next layer, and may move down to a deeper layer, and the change is the same as the actual situation in the field, and when the soil is dry and contracted, fine cracks may appear at the rock-soil interface, and when the rainfall occurs, some soil particles in the upper layer may move down to a deeper layer along with the rock-soil interface. The concentration of Sm of the Sm distribution layer (6-8 cm) is also lower than the background value, the concentration of Ce of the Sm distribution layer is higher than the background value (except 104.5mm rainfall), the concentration of La of the Sm distribution layer is lower than the background value, the variation trends of Ce and La are opposite, and the variation of the concentration of REE of the La layer above the Sm distribution layer is different, so that the condition that soil particles move downwards cannot be judged, as shown in figure 5.
The invention utilizes rare earth elements to trace the soil loss/leakage of the karst region to reveal the process and mechanism of the soil underground leakage of the region, which can play an important role in promoting the researches on the soil underground leakage of the karst region, the stony desertification control and the like.
It should be noted that the above embodiments are only for further illustration and description of the technical solution of the present invention, and are not for further limitation of the technical solution of the present invention.
Claims (10)
1. A method for tracing soil loss in karst regions by using rare earth elements is characterized by comprising the following steps: selecting an observation area according to research contents, collecting and testing background values of various rare earth elements in the selected observation area, selecting tracer rare earth elements and calculating release amount, preparing a tracer soil sample, laying the tracer soil sample by a 'strip method + section method', marking the surface soil position by drawing lines, collecting and testing the content of the rare earth elements in the selected observation area after a research period, and calculating and analyzing underground soil leakage amount;
the method specifically comprises the following steps:
(1) according to research contents, selecting a plurality of typical observation areas capable of representing the soil in a research area to be subjected to surface loss or underground leakage, wherein the selected observation areas need to comprise part of rock interfaces in consideration of the particularity of soil loss or leakage of rock-soil interfaces;
(2) collecting a soil sample of a rare earth element background value in an observation area, namely collecting the soil sample in a layering manner with a layering thickness of 2cm at the central position of a grid by using the size of a 20-50cm grid in the observation area, taking the soil in a range of 2cm close to a rock interface as a soil sample of the rock-soil interface, collecting the soil sample of the rock-soil interface with the same layering thickness, and collecting the soil sample to a depth range of 8-20 cm;
(3) selecting a plurality of rare earth elements for carrying out a test, wherein the release concentration and the release amount of the rare earth elements are respectively calculated according to the following formula:
Ci＝BCix a (formula 1),
wherein i is the ith trace rare earth element, i is 1, 2, 3, …, n; ciThe concentration of the ith trace rare earth element is applied, namely mg/kg; BCiIs the background value of the ith trace rare earth element, mg/kg; miThe release amount of the i & ltth & gt trace rare earth element oxide is g; siThe weight of soil required by the i & ltth & gt tracing rare earth element is kg; a is a concentration expansion coefficient of 20-100; b is a concentration correction coefficient, namely the purity of the rare earth oxide, which is generally 0.9999; c is an oxide correction coefficient, and the relative molecular mass of the rare earth element accounts for the proportion of the relative molecular mass of the oxide;
said SiCalculated as follows:
wherein, in the formula, WL is the sample area size of the observation area, cm2；HiLaying depth, cm, for the ith trace rare earth element; r is the volume weight of the soil with the distribution depth, g/cm3(ii) a 1000 is the unit transformation coefficient;
(4) preparing a tracer soil sample, and mixing the rare earth oxide with the calculated release amount with soil; adopting a dilution method in the mixing process of the trace rare earth oxide and the soil, and gradually diluting to approach the discharge concentration;
(5) the configured tracer soil samples are distributed by adopting a strip method and a section method, the configured tracer soil samples are filled into strip pits of an observation area in a layered mode and tamped to the same volume weight as the original slope surface, and meanwhile, the surface soil position is marked at a rock-soil interface;
(6) the research period is as follows: and (3) after rainfall or next month or season or next year, carrying out soil sample according to the step (2), and measuring the content of the rare earth elements in the sample, wherein the collection modes are divided into the following two conditions:
taking a rock-soil interface mark as a datum point, accurately collecting soil samples according to the layering thickness of 2cm, and recording the thickness of each collected layer of soil samples;
secondly, taking the rock-soil interface mark as a datum point, measuring the downward movement displacement of the soil layer by using a standard ruler, and accurately collecting a soil sample at the position of the surface layer of the downward movement soil layer by the layering thickness of 2 cm;
(7) the soil loss or leakage analysis and calculation includes the following two cases:
calculating the thickness of the soil sample which is not collected below the reference point, namely the soil loss/leakage thickness, and calculating the soil loss/leakage amount according to a formula 3; comparing the concentration change of the rare earth element of the soil before and after the research period with the concentration change of the background value, if the difference between the concentration change and the background value is not large, the soil is proved to be mainly subjected to overall creep loss, and if the difference between the concentration change and the background value is large, the soil is analyzed according to the data condition;
calculating the average value of the soil loss/leakage thickness measured by the ruler, and calculating the soil loss/leakage amount according to the formula 3; comparing the concentration of the rare earth element of the soil with the concentration change of the background value before and after the research period, whether the creeping loss of the soil is caused by the loss of the rock-soil interface can be judged.
2. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: the size of the observation area in the step (1) is determined by the actual situation outside the field, and 0.5m multiplied by 0.5 m-2 m multiplied by 2m is selected.
3. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: the background value of the rare earth element of the soil sample in the step (2) is determined by an INNA (InNA) method of instrument neutron activation analysis and an inductively coupled plasma mass spectrometer ICP-MS (inductively coupled plasma mass spectrometer); after the background value soil sample is collected, all the soil within the depth range of 8-20cm is collected and dried indoors for later use.
4. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: the rare earth elements used for tracing the soil leakage in the step (3) are well combined with the soil, are insoluble in water, are not easily absorbed by plants and are harmless to the soil environment, and the rare earth tracing elements have the characteristics of low soil background value, small application amount and easiness in detection; the optional rare earth elements comprise dysprosium Dy, europium Eu, samarium Sm, cerium Ce, lanthanum La and neodymium Nd, and the corresponding rare earth oxides are Dy2O3、Eu2O3、Sm2O3、CeO2、La2O3、Nd2O3。
5. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: the soil in the step (4) is air-dried and sieved soil; and (4) the operation process of gradually diluting and approaching the concentration application is that a small amount of test dry soil with the particle size smaller than 0.075mm is fully mixed with the rare earth oxide with the required weight to dilute to form a high-concentration soil sample, then the required soil is added to mix with the squeezer to form a secondary high-concentration soil sample, and finally the air-dried soil which can be filled with the tracing depth is added to fully mix.
6. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: the strip method in the step (5) of the strip method and section method layout is a strip with limited width of the selected observation area, the soil loss or soil leakage is equal to or close to the soil loss/leakage amount of the research area, and the rare earth elements are laid in the strip; the section method is that rare earth tracer elements and traced soil are all mixed and distributed on the section, and different tracer elements are distributed on different soil layers.
7. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: the layout principle of the strip method and the section method in the step (5) is as follows:
firstly, the structure of the lower soil layer is not damaged as much as possible, and the thickness of the laid soil layer is controlled within a certain range;
secondly, the difference between the leakage and the leakage is ensured to be obvious, and the arrangement of the soil layer is combined with the sampling depth and can be controlled to be 2 cm;
thirdly, the surface layer is laid to be deep enough to ensure that the surface loss is met, and the thickness is 4 cm;
fourthly, in order to ensure that the leakage can be seen in the 4cm layer, the sampling depth is small enough; meanwhile, the method can be detected under the condition of small leakage, and the sampling depth is controlled to be 2cm or below;
fifthly, under the condition of ensuring accuracy, the principle of economy, labor and high efficiency is emphasized.
8. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: and (5) marking the surface soil layer position of the rock-soil interface by adopting paint.
9. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: and (6) measuring the rare earth element content of the soil sample after the observation period by using an INNA (InNA) method of instrument neutron activation analysis and an inductively coupled plasma mass spectrometer ICP-MS (inductively coupled plasma mass spectrometer).
10. The method for tracing soil loss in karst regions by using rare earth elements as claimed in claim 1, wherein: and (6) if the soil samples need to be collected at different time points, dividing the grid number equally, dividing the rock-soil interface equally according to the same batch, collecting the divided soil samples each time, and backfilling the surrounding soil after collection to ensure that the next soil loss/leakage process is not influenced.
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