Method for assessing the possibility of continued use of deep groundwater such as hot spring water

Abstract

To provide a novel method for estimating possibility of exhaustion of deep part groundwater including exhaustion of hot spring, one of main causes of reduction of hot spring water jet amount, and a period until exhaustion thereof, and for evaluating the possibility of continuous use thereof.SOLUTION: A method of the present invention includes steps of: discriminating whether the deep part groundwater is fossil water or flow system water by estimating groundwater era of deep part groundwater of sampled groundwater body; discriminating whether new mix water is mixed to the groundwater body after sampling the deep part groundwater by acquiring vertical infiltration speed for new precipitation vertically infiltrating into the groundwater body; estimating total amount of the deep part groundwater from the three dimensional volume of a sedimentary layer with remaining deep part groundwater and an average effective porosity, when the deep part groundwater is fossil water and no new mixing water to the deep part ground water body; and estimating continuous use time of the deep part groundwater from the total amount of the deep part groundwater and a sampling amount per unit time.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the possibility of continued use of deep groundwater such as hot spring water. [Background technology]

[0002] Traditional hot springs can face business disruptions due to factors such as a decrease in the amount of water that gushes out. This can be caused by aging facilities and clogged hot spring wells, as well as the depletion of hot spring water deep underground (deep groundwater), which is a major problem.

[0003] These two issues are the main cause of the decline in hot spring output, but the deterioration of facilities and clogging of hot spring wells can be confirmed using borehole cameras, and methods for resolving these issues have already been established.

[0004] However, the depletion of hot spring water deep underground (deep groundwater) has been a problem that has remained unresolved, with no solution available to date. Much research has been conducted to explore the state of deep groundwater, with reports on sediments related to groundwater reserves and dating of groundwater, leading to the accumulation of basic data and technology (Non-Patent Documents 1-4). Hydrological environment maps have also been compiled for the purposes of effective use of local groundwater resources, such as groundwater and geothermal heat, and environmental protection of groundwater. These maps contain hierarchical data on topography, geology, groundwater levels, water quality, isotopic composition, and underground temperature, and are published electronically. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Marui, Atsuhisa (2017) Groundwater Survey Methods (Part 1) The Necessity and Basic Knowledge of Groundwater Hydrology - From the History of Water Science, Groundwater Technology, 58-3.4, 29-38pp. [Non-patent document 2] Marui, A., Yasuhara, M., Hayashi, T., and Higuchi, H. (2001) Deep groundwater in the Tokyo Bay area, Journal of the Japanese Society of Hydrological Sciences, 31-3, pp. 1-9. [Non-patent document 3] Koshigaya, M. and Marui, A. (2012) Three-dimensional model of geological boundaries and layer thickness of sediments used for estimating groundwater reserves in the Japanese archipelago, Geological Survey of Japan Research Materials Collection, no. 564 [Non-patent document 4] Hasegawa, T., Nakata, K., Kondo, H., Goshima, K., Muramoto, S., Tomioka, Y., Goto, K., and Kashiwatani, K. (2013) Evaluation of groundwater flow in coastal areas by dating, Journal of Earth Science, 122-1, pp. 116-138. Summary of the Invention [Problem to be solved by the invention]

[0006] However, even these studies and basic data are not enough to completely predict or understand the state of deep groundwater, and there is a need for a means to accurately predict the state of deep groundwater, particularly the possibility of hot spring water depletion, while taking into account the utilization of these studies and basic data.

[0007] If the continued availability of deep groundwater, such as hot springs, can be assessed, measures to ensure continued operation, such as drilling new hot spring wells, can be taken in preparation for future depletion of hot springs. It can also provide important information for the utilization of deep groundwater, such as for thermal use and waste disposal.

[0008] The present invention was made in consideration of the above circumstances, and aims to provide a new method for estimating the possibility of depletion of deep groundwater and the period until depletion, including the depletion of hot springs, which is one of the major causes of the decrease in hot spring discharge, and for evaluating the possibility of its continued use. [Means for solving the problem]

[0009] As a result of intensive research into solving the above problems, the inventors discovered a method for evaluating the feasibility of continued use of deep groundwater by making full use of chemical analysis methods for hot spring water and groundwater flow analysis techniques, and thus completed the present invention. That is, the method of the present invention for evaluating the possibility of continued use of deep groundwater is characterized by including the following steps (A) to (D). (A) a step of estimating the age of deep groundwater in a groundwater body being sampled and determining whether the deep groundwater is fossil water or flow-type groundwater; (B) determining the vertical infiltration rate of new precipitation that infiltrates vertically into the groundwater body and determining whether new water is being mixed into the groundwater body after the deep groundwater is collected; (C) When the deep groundwater is fossil water and there is no new water mixed into the groundwater body, estimating the total amount of the deep groundwater from the three-dimensional volume and average effective porosity of the sedimentary layer in which the deep groundwater is stored; (D) A process of estimating the continuous utilization time of the deep groundwater from the total amount of the deep groundwater and the amount extracted per unit time. [Effects of the Invention]

[0010] According to the present invention, it is possible to evaluate the continued availability of deep groundwater such as hot springs, and to take measures to continue business, such as drilling new hot spring wells, in preparation for the future drying up of hot springs. Furthermore, the present invention can provide important information for the utilization of deep groundwater, such as for the thermal utilization of groundwater and waste disposal. By using this method, it is possible to easily evaluate the possibility of continued use of deep groundwater such as hot spring water by using chemical analysis methods such as isotope analysis and groundwater flow analysis methods such as geographical methods. [Brief explanation of the drawings]

[0011] [Figure 1] This is a schematic diagram of deep fossil groundwater, such as hot spring water, being recharged by deep infiltration of groundwater originating from precipitation. [Figure 2] FIG. 10 is a diagram for explaining an example of calculating an estimated time until new water is mixed into the hot spring basin. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail. Figure 1 is a schematic diagram of how deep fossil groundwater, such as hot spring water, is recharged by deep infiltration of groundwater originating from precipitation, showing the general water cycle.

[0013] Groundwater exists as part of the Earth's water cycle, flowing from mountains to the sea. Generally, precipitation seeps through the surface layer, forming stagnant underground water, while groundwater from higher elevations, such as distant mountains, passes below. The water cycle on Earth is driven primarily by solar energy and gravity, and involves a series of processes: evaporation in the oceans, transport to land through the atmosphere, precipitation, formation of surface water and groundwater, and runoff into the oceans. Generally, the infiltration of water from precipitation, lake water, river water, reservoirs, and infiltration basins into the ground, i.e., becoming groundwater, is called "recharge." Groundwater recharge refers to the process by which precipitation and surface water permeate the ground and add to the groundwater flow system. Generally, most of the recharge comes from precipitation, but it also includes infiltration of river water and lake water, infiltration from rice paddies, infiltration from artificial recharge facilities (infiltration basins, recharge ponds, reduction wells, etc.), and leakage from water supply and sewerage systems.

[0014] These groundwaters are usually not completely mixed, but flow down in clumps along the topography. These are called free groundwater (unconfined groundwater) or confined groundwater, and are used as water sources (groundwater resources). Furthermore, below that, there is stagnant groundwater with low fluidity, which generally exists in denser strata such as Tertiary layers. Also, at great depths, there is groundwater with various characteristics, such as hot spring water and fossil water. Fossil water includes groundwater that is not involved in groundwater flow, and fossil seawater, which is groundwater with a salinity concentration similar to or higher than that of seawater. Deep groundwater refers to water that exceeds the range of normal use (for agriculture and general use).

[0015] There are three main sources of deep groundwater, such as hot springs. One is water supplied from deep fluids known as virgin water in volcanic regions, the other is fossil groundwater known as formation water, which was absorbed when the strata were formed, and the third is groundwater that originated from precipitation, seeped up from the surface, and heated by geothermal heat (Non-Patent Document 1).

[0016] Rainwater and other water seeps underground and becomes groundwater, but the length of time it stays there varies greatly. For example, on highly permeable volcanic slopes, the water flows down to the plains within a few years and most of it springs up to the surface, so the groundwater stays there for only a few years, which is extremely short. On the other hand, deep groundwater in coastal delta areas on plains is mostly confined in aquifers, and most of it has a residence time of several thousand to tens of thousands of years.

[0017] Groundwater flow on land is thought to be greatly influenced by geological structure. However, groundwater in coastal areas, where groundwater flow terminates, especially deep groundwater, exhibits a complex distribution pattern. This is because the geology of coastal volcanic plains generally consists of shallow-water sediments and alluvial deposits on top of deep-sea deposits. Because these areas were once filled with saltwater and were affected by large sea-level changes during the glacial period, saline and freshwater layers often overlap. In other words, beneath the highly fluid freshwater groundwater zone, saline zones where present-day seawater has invaded, non-fluid freshwater groundwater zones, and fossil saline zones are generally found in layers.

[0018] Process (A) In the method for evaluating the possibility of continued use of deep groundwater in this embodiment, in step (A), the groundwater age of the deep groundwater in the groundwater body being sampled is estimated, and it is determined whether the deep groundwater is fossil water or flow-type groundwater.

[0019] Groundwater age is the time since rain, snow, or other precipitation seeped into the ground and became groundwater, also known as residence time. Groundwater is created when precipitation seeps underground. The residence time is the time that has elapsed since the precipitation reached the groundwater table and was cut off from contact with the atmosphere.

[0020] Groundwater dating surveys have been conducted so far. Methods for measuring groundwater age include those that utilize the decay rate, accumulation rate, and non-equilibrium of radioactivity contained in groundwater from atmospheric origin or from the earth's crust, those that utilize the decomposition and transformation rate of chemical substances in groundwater, and those that utilize relationships with paleoclimate or geological events whose dates have already been determined. For example, there is a method that focuses on the change in concentration of radioactive materials dissolved in groundwater ( 3 H, 14 C. 36 Cl), and methods that focus on changes in the concentration of substances that accumulate in groundwater ( 4 Known methods include those that focus on changes in concentrations in groundwater due to anthropogenic influences and paleoclimates (e.g., chlorofluorocarbons, water isotopes, etc.).

[0021] Of these, the most direct method utilizes radioactivity in groundwater. Groundwater dating using radioactivity is based on the radioactive decay of radionuclides, where radioactivity decays to half of its original amount over a certain period of time (λ: decay constant, t: elapsed time). If the initial concentration C0 in the groundwater can be determined by some method, and if the groundwater system is closed and there is no supply of nuclides from the surrounding rock, and the groundwater flows in a single flow path as a piston flow without mixing or dispersion or diffusion, the groundwater age T can be estimated using the following equation, where T is the groundwater age, C0 is the initial radioactivity (T=0), C is the radioactivity at time T, and λ is the decay constant of the radionuclides. C=C0e - λ T

[0022] In other words, the concentration of radioactive materials dissolved in deep groundwater can be measured, and the age of the groundwater can be estimated based on their half-lives. Table 1 shows the representative radionuclides (isotopes) used in groundwater dating, their half-lives, and the reliability range of the measurements.

[0023] [Table 1]

[0024] In particular 14 The half-life of C is 5730 years (Table 1, Non-Patent Document 4), 14 The rate of radioactive decay of C can be used to efficiently determine the residence time of groundwater bodies at hot spring depths. CO2 in the soil originates from biological activities such as root respiration and decomposition of soil organic matter, and is absorbed by plants from the atmosphere. 14 Contains C. This biogenic 14 CO2 dissolves in seepage water from the surface and is supplied to groundwater, completing the carbon cycle. 14 If no new C is produced and if there is no new supply, it will decrease through beta decay with a half-life of 5730 years. 14 The age of groundwater can be determined from the C concentration, and many studies have been reported.

[0025] Conditions under which a hot spring may dry up include when the hot spring water is stored in a certain stagnant water zone, there is no new groundwater mixing into the groundwater body, and the volume is limited to a certain amount. In Japan, the average temperature near the ground surface (depending on the region and altitude) is roughly 10 to 20 degrees Celsius, and the geothermal gradient per 100 meters is about 2 to 3 degrees, so hot springs are found at depths of several hundred to 2,000 meters, and the water temperature is often around 30 to 70 degrees Celsius (the Hot Springs Act requires the water temperature to be above 25 degrees Celsius). According to Non-Patent Document 2, it is also known that the typical residence time ranges from several thousand to several tens of thousands of years.

[0026] Groundwater dating can indicate whether the groundwater flow is sufficiently slow. If the measured groundwater age is on a time scale significantly longer than the expected period of sampling, it is assumed that the groundwater has barely moved since deposition and is stored in a certain stagnant water zone. In other words, it is assumed that fossil water captured during deposition remains. If the groundwater age is young, it can suggest the possibility of groundwater flow. In other words, it can be determined whether the groundwater body in question is composed of fossil water or flowing groundwater.

[0027] Process (B) In the method for evaluating the possibility of continued use of deep groundwater in this embodiment, in step (B), the vertical infiltration rate is calculated for the incorporation of new precipitation that infiltrates vertically into the groundwater body, and it is determined whether new water has been introduced into the groundwater body after the deep groundwater has been collected.

[0028] Before new precipitation can mix with the groundwater body, it must penetrate deep through multiple aquifers and impermeable layers, as shown in Figure 1. Aquifers are geological layers with relatively good permeability and storage capacity, capable of continuously supplying groundwater as well intake or spring water. Typical geological layers include gravel and sand layers. Aquifers are generally divided into unconfined aquifers with a free groundwater table and confined aquifers, which are sandwiched between confined layers (i.e., layers above or below the aquifer) and layers with significantly lower permeability. Impermeable layers are composed of small-grained particles such as clay and silt, have very low permeability, and are difficult or impossible for groundwater to pass through. They are sometimes classified as impermeable or non-permeable layers. A typical example is a clay layer.

[0029] If we consider the strata in sedimentary rock areas in Japan to be generally horizontally deposited (because sedimentary rock areas are formed by uplift), and take into account the anisotropy of the flow in large aquifers, horizontal movement will be much faster than the flow rate of vertical movement, so we can consider vertical infiltration when considering the mixing of new precipitation.

[0030] The hydraulic conductivity of soil and earth structures, which is transported downward by gravity and serves as a component for recharging groundwater, is generally heterogeneous and anisotropic, meaning that the hydraulic conductivity changes depending on the direction. The horizontal hydraulic conductivity is greater than the vertical one. It is possible to calculate the equivalent hydraulic conductivity in directions parallel and perpendicular to the layers of layered soil.

[0031] The hydraulic conductivity is a value that indicates the permeability (ease of water passing through) of soil or rock saturated with water, and is widely used when dealing with the permeability of porous media. -7 ~10 -9 cm / sec, practically impermeable, fine sand, silt sand-silt-clay mixture is 10-3 ~10 -7 The sand and rubble have very low permeability at 10 cm / sec. 0 ~10 -3 cm / sec for medium and clean debris for 10 2 ~10 0 The apparent flow velocity of groundwater can be calculated by multiplying the hydraulic conductivity by the hydraulic gradient.

[0032] The apparent flow velocity of groundwater is the cross-sectional average flow velocity, calculated by dividing the flow rate in a porous medium by the cross-sectional area through which it passes. In groundwater, flow velocity generally refers to the apparent flow rate. Darcy's law is an empirical equation that describes the flow of water in porous materials, and is known as a basic law widely used in fields dealing with subsurface fluids. For example, the flow rate Q of water passing through a sand column is related to Q = KAΔh / L, where Δh is the head difference between the inlet and outlet, L is the length of the sample, and A is the cross-sectional area. K is a proportionality constant called the hydraulic conductivity.

[0033] The distribution of sedimentary layers in the sedimentary rock areas of the Japanese archipelago is known from previous research such as Non-Patent Document 3 and basic data. From this, it is possible to estimate the permeability coefficient of each layer and calculate the vertical infiltration rate, taking into account the physical properties of the sedimentary layers.

[0034] Figure 2 is a diagram illustrating an example of estimating the time until new water mixes into a hot spring-bearing layer. Consider a case where there are aquifers A1-A3, aquifer-impermeable layers C1-C3, and a hot spring-bearing layer directly below them. The infiltration time is determined by the hydraulic conductivity of each layer. If the hydraulic gradient is set to 1, the thinnest layer thickness is H, and the hydraulic conductivity of each layer is k, the time required for infiltration can be estimated as ΣH / k. The size of the hot spring-bearing layer is estimated from a sedimentary layer database, and the porosity is estimated from the sand-to-mud ratio. The time required for recharge is roughly determined by the time it takes to pass through the aquifer-impermeable layer.

[0035] In this way, the vertical infiltration rate can be calculated for the incorporation of new precipitation into the groundwater body by vertical infiltration, and it can be determined whether or not new water has been introduced into the groundwater body after the deep groundwater has been sampled. For example, this applies when the vertical infiltration rate is so slow that it is expected that no new water will be introduced into the groundwater body, or when the amount of new water introduced into the groundwater body estimated from the vertical infiltration rate is significantly smaller than the amount of fossil water sampled per unit time.

[0036] Process (C) In the method for evaluating the possibility of continued use of deep groundwater in this embodiment, in step (C), if the deep groundwater is fossil water and no new water is mixed into the groundwater body, the total amount of deep groundwater is estimated from the three-dimensional volume and average effective porosity of the sedimentary layer in which the deep groundwater is stagnating.

[0037] The sedimentary layer in which hot spring water is stored has a finite size, as described in Non-Patent Document 3. The three-dimensional volume of the sedimentary layer in which deep groundwater such as hot spring water is stored can be estimated based on previous research such as Non-Patent Document 3 and basic data.

[0038] The porosity of porous bodies such as rocks is expressed as the ratio of the volume of the voids (or gaps or pores), which are the non-solid parts of the porous body, to the total volume (volume of the voids + actual volume of the solid parts). For example, by excavating the strata using methods used for geological surveys and groundwater extraction for hot springs, etc., and collecting samples of the sedimentary layers, and by utilizing previous research and foundation data as needed, the average effective porosity of the sedimentary layers can be obtained. For example, porosity can also be estimated from the sand-to-mud ratio.

[0039] If we assume that the pores in the solid part of the sedimentary layer of a groundwater body are filled with water, then the total amount of deep groundwater, such as hot spring water, can be estimated from its three-dimensional volume and average effective porosity. The amount of groundwater estimated for each groundwater basin or aquifer is called groundwater potential, and it is known to be estimated from information such as the distribution of groundwater levels in the target area and the volume and porosity estimated from the shape of the aquifer.

[0040] Process (D) In the method for evaluating the possibility of continuous use of deep groundwater in this embodiment, in step (D), the continuous use time of the deep groundwater is estimated from the total amount of deep groundwater and the amount extracted per unit time.

[0041] When deep groundwater is fossil water, the pore spaces between rocks contain water when saturated, and gas (generally air) when unsaturated. When fossil water is extracted from hot springs or geothermal sources, unless new water is introduced into the groundwater body through vertical infiltration after extraction, the groundwater reserves decrease with the amount extracted. Once the amount of extracted fossil water reaches the total volume of deep groundwater, it is estimated that the water will be depleted. Therefore, the continuous use of deep groundwater can be estimated by averaging the amount of fossil water extracted over a year or month to determine the amount extracted per unit time, and then dividing the total amount of fossil water estimated from the three-dimensional volume of the sedimentary layer in which the fossil water is stored and the average effective porosity by the amount extracted per unit time. This allows the continuous use of deep groundwater to be estimated if the amount of water used per unit time is known from the number of business days and the amount extracted during operation, such as for hot springs, and the amount of water used per unit time is known. [Example]

[0042] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0043] The reference example is T Hot Spring in northwestern Hokkaido. Since the hot spring is located at a depth of about 1,200 m, it is estimated that the hot spring is drawing groundwater from the Lower Neo-Tertiary Formation, which has an average thickness of about 300 m and is located at a depth of 150 km. 2 Assuming an average porosity of 8%, the amount of hot spring water remaining is 3,600,000 m 3 This becomes:

[0044] From the integrated value of the vertical hydraulic conductivity, the hydraulic gradient is estimated to be the maximum, and the vertical infiltration time is estimated (the average hydraulic conductivity is 1 × 10 -5cm / sec), it would take 150,000 years for the precipitation to reach the hot spring reservoir.

[0045] The groundwater here is estimated to be about 30,000 years old, so no new water is expected to be recharged, and if it is used at a rate of about 100 tons per day, it will be depleted in 100 years. In reality, several thousand tons are used per day, and since it has been about 70 years since the company was founded, measures will need to be taken in the near future.

Claims

1. A step of estimating the age of deep groundwater in a groundwater body being sampled and determining whether the deep groundwater is fossil water or flowing groundwater, wherein the deep groundwater is stagnant groundwater with low fluidity that was taken in when a dense stratum below an aquifer and an aquifer-impermeable layer was formed and is located deep in a place where fossil water may exist; A step of determining whether or not new precipitation is mixed into the groundwater body by calculating the vertical infiltration rate and determining whether or not new precipitation is mixed into the groundwater body after sampling the deep groundwater; When the deep groundwater is fossil water and there is no mixing of newly mixed water into the groundwater body, estimating the total amount of the deep groundwater from the three-dimensional volume and average effective porosity of the sedimentary layer in which the deep groundwater is retained; A process of estimating the continuous use time of the deep groundwater from the total amount of the deep groundwater and the amount collected per unit time. A method for assessing the feasibility of continued use of deep groundwater, including:

2. The method of claim 1, wherein the concentration of radioactive materials dissolved in the deep groundwater is measured and the age of the groundwater is estimated based on the half-life of the radioactive materials.

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