Chemical characteristics of inorganic water
The formation and evolution of groundwater hydrochemical composition is a very complicated process, which is closely related to lithology, structural conditions, climate conditions, hydrodynamic conditions and other factors, and comprehensively reflects the physical and chemical results and equilibrium conditions of water-rock interaction in the process of groundwater migration. Different water bodies and aquifers have different hydrochemical characteristics due to the different properties of surrounding rock, circulation conditions, REDOX conditions and mixing, which reflect the difference of recharge, runoff and discharge conditions. Through hydrogeochemical analysis of water samples, the water cycle conditions of groundwater can be determined and the water sources of different aquifers can be judged. One of the main laws of groundwater distribution is its zonation. The zoning of groundwater is mainly manifested in hydrogeological dynamic zoning and hydrogeochemical zoning.
(1) Basic characteristics of each aquifer
Surface water: The main surface water body in the study area is Nalin River water, with a low salinity (Fig. 3), a concentration of 303.00mg/L and a pH of 7.50, belonging to weakly alkaline water (Fig. 4). The main cation is Ca2+ with the concentration of 49.04mg/L, followed by Mg2+ and Na+; The main anions is HCO3-, with a concentration range of 218.14mg/L, followed by SO42- and Cl-. The main hydrochemical type is HCO3-Ca water (Fig. 5), which has the same water quality as meteoric precipitation.
Quaternary system: Because the terrain of Tahutu mine field is relatively flat, and the beach and sand dunes are distributed alternantly, the beach is the main one, and the sand dunes are widely distributed. The loose sand layer of the Quaternary system directly receives the infiltration of meteoric water, the infiltration coefficient of precipitation is 0.35~0.43, the groundwater runoff path is short, and the water cycle is positive. It is characterized by low salinity, weak alkalinity and calcium bicarbonate water. Its water quality characteristics are as follows: The pH value is 7.30 (Fig. 4), and the salinity is 363.72 mg/L (Fig.3); the main cation is Ca2+, and the concentration range is 61.66 mg/L, followed by Mg2+and Na+; the main anion is HCO3- with a concentration range of 232.36 mg/L, followed by SO42- and Cl-; the hydrochemical type of Quaternary water is mainly HCO3-Ca (Fig. 5).
Cretaceous system: The water chemical characteristics of Cretaceous Zhidan Group also show the characteristics of low salinity, weak alkalinity and calcium bicarbonate type water, and the pH value in the water sample is 7.90~7.94 (Fig. 4). The salinity in Zhidan Group aquifer is also low (277.00~301.87mg/L) (Fig. 3). The main reason is that there is no stable water barrier between Cretaceous strata and the upper Quaternary strata, so the hydraulic connection is very close and the water cycle is fast. The main cation is Ca2+ and Na+ with concentrations ranging from 28.90 ~ 30.96 mg/L and 29.21 ~ 39.72 mg/L, respectively, followed by Mg2+; the main anion is HCO3-, with concentrations ranging from 159.80 ~ 183.16 mg/L, followed by SO42- and Cl-; The hydrochemical types of the Cretaceous aquifer water are mainly HCO3-Ca·Na and HCO3-Na·Ca water (Fig. 5).
Straight Rom Group: Anding formation plays an important role in preventing the hydraulic connection between the upper and lower aquifers. With the increase of the depth of underground water, the pore and fissure development is poor, supply condition is poor, runoff condition is poor, circulation path of underground water is long, contact time with the surrounding rock is long, the space is relatively closed, and water quality renewal time is long. Under the action of long-term water and rock, water quality becomes worse and the salinity is higher. Groundwater of Zhiluo Formation is characterized by high salinity, weak alkalinity and sodium sulfate. The pH value of water sample is 7.40 (Fig. 5), and the salinity is 7447.44 mg/L (Fig. 4); the main cation was Na+ with the concentration of 1565.50 mg/L, followed by Mg2+ and Ca2+; SO42- is the main anion, with a concentration of 5001.84 mg/L, followed by HCO3- and Cl-. The water chemical type of Zhiluo Formation aquifer is SO4-Na water (Fig. 5).
Yan'an Group: The upper strata of Yanan Formation are unconformably in contact with the strata of Straight Rom Group, and can be directly replenished by the aquifer of Straight Rom Group. The lower part is the interbedded structure of multi-layer coal seam and sand and mudstone layer, which can effectively block the infiltration and overflow of groundwater in this section, so that the hydrochemical characteristics of aquifer water in this section are very close to those of Straight Rom Group. The pH value of groundwater in Yanan Formation is 7.30~8.36 (Fig. 4), and the salinity is 6621.00~8962.76mg/L (Fig. 5); The main cation is Na+ with the concentration of 1521.00~2601.00 mg/L, followed by Mg2+ and Ca2+; the main anion is SO42- with the concentration of 4409.97~5810.06mg/L, followed by HCO3- and Cl-. The hydrochemical type is SO4-Na water (Fig. 5). The concentration of Cl- ion in DCC-2-1~3 water samples is 64.50mg/L, which is significantly lower than that in other water samples (555.96~676.00mg/L), and the hydrochemical type is SO4-Na·Ca water, indicating that there were certain anomalies in DCC-2-1~3 water samples.
(2) Comprehensive analysis of aquifer characteristics
As can be seen from Piper trilinear chart (Fig. 5), surface water, Quaternary water and Cretaceous water samples are all located at the left end or middle left of the rhomboid. It directly shows the water quality characteristics of HCO3-Ca (Mg) type water, that is, the water quality type of low-salinity solution and filtration water. It represents that these aquifers (bodies) receive the replenishment of meteoric water, runoff condition is relatively good, and the cycle alternates positively. The water samples of Straight Rom Group and Yan’an Group are located at the right end of the rhomboid, which shows the water quality characteristics of SO4-Na water, namely high salinity, large amount of Na+ and SO42- ions dissolved into the water, showing obvious hydrochemical characteristics different from shallow aquifers.
Durov chart (Fig. 6) further shows that in surface water, Quaternary and Cretaceous groundwater, Ca2+ and HCO3- ion concentrations representing positive alternation of shallow water cycle are relatively high, while Cl-, SO42- and Na+ ions representing deep retained water quality are relatively low. On the contrary, the concentration percentages of Cl-, SO42- and Na+ in the groundwater of Straight Rom Group and Yan’an Group are significantly increased.
According to Schoeller’s chart (Fig. 7), the hydrochemical characteristic curves of surface water, Quaternary water and Cretaceous water have roughly the same shape trend, with only slight vertical movement. It shows that the chemical composition and specific gravity of each water sample are close, and the replenishment source is the same. The hydrochemical characteristic curves of Straight Rom Group and Yan’an Group are obviously different from those of shallow aquifers, indicating that there are different sources and genesis of shallow and deep groundwater.
In addition, combined with Scatter chart (Fig. 8), it can be further found that each aquifer increases with the burial depth. The Ca2+ ion concentration is relatively low in shallow aquifers and generally high in deep aquifers. The concentration of Na+, Cl- and SO42- also showed a similar pattern. However, the concentration of HCO3- ion has no direct relationship with the buried depth, indicating that with the buried depth of aquifer, groundwater migration and stagnant flow conditions, and long-term water-rock interaction results in a gradual increase in the concentration of most ions.
The above rules are also clearly reflected in Ludwig Langelier chart (Fig. 9), that is, the concentrations of Na++K+ and SO42-+Cl- conform to the vertical zoning of hydrogeochemistry. The concentrations of Na++K+ and SO42-+Cl- gradually increase with the increase of burial depth.
According to the hydrogeological data and groundwater runoff in Taohutu mine field, surface water and Quaternary aquifer are closely related to the hydraulic power of meteoric precipitation. Its runoff is controlled by topography, the groundwater runoff path is short, and the water cycle is active, so only a small amount of minerals are dissolved in the rainfall infiltration process, and the water contains different ion components, thus forming the bicarbonate fresh water with low salinity mainly caused by leaching and filtration, and providing abundant recharge water for underlying aquifer. There is no water-barrier between Cretaceous and Quaternary, which constitutes a unified aquifer group with close hydraulic connection. As a result, the groundwater of Cretaceous aquifer also belongs to the low salinity bicarbonate fresh water. Anding formation plays an important role in preventing the hydraulic connection between the upper and lower aquifers. With the increase of the depth of underground water, the pore and fissure development is poor, supply condition is poor, runoff condition is poor, circulation path of underground water is long, contact time with the surrounding rock is long, the space is relatively closed, and water quality renewal time is long. Under the action of long-term water and rock, water quality becomes worse and the salinity is higher ( Fig. 3 and Fig. 4). Combined with the analysis of hydrochemical characteristics, it can be seen that the change rule of total hydrochemical types from shallow aquifer to deep aquifer in Tahutu mine field is as follows: HCO3-Ca·Mg→HCO3-Na→SO4-Na.
Isotope hydrochemical characteristics
By using environmental isotope testing techniques to study of groundwater movement, it can quickly and effectively obtain important hydrogeological information which is difficult or impossible to obtain by other methods. Since environmental isotopes are used as natural tracers to “mark” the formation process of natural water and groundwater, it is possible to directly obtain the information of the formation and movement process of groundwater by studying their distribution in various water bodies. The approach is to reveal the origin, formation conditions, recharge mechanism and hydrodynamic relationship of groundwater by comparing the difference and variation rules of environmental isotopes between underground water and surface water through analysis of environmental isotopes. Therefore, this method has been widely used in the study of hydrogeology at home and abroad[18-20].
In order to study the source of groundwater in the Tahutu Mine Field, we gathered/collected and tested 14 groups of isotopic water samples from each aquifer (Table 3). As can be seen from the table, the water sample test results of DCC-1-2-3 (1) and DCC-1-3-3 (2) were abnormal, the following analysis was not included in the study.
Table 3 Environmental isotope test results
Water sample No.
|
Layer Position
|
δD
|
δ18O
|
Remark
|
Rain water-1
|
Rain water
|
-52.0
|
-7.9
|
|
18T-55-362
|
Surface water
|
-49
|
-6.1
|
CY-10 Ursue Haizi
|
DSG-1-3
|
Quaternary
|
-63.7
|
-8.5
|
Quaternary
|
#1-3
|
Quaternary
|
-59.7
|
-8
|
Civil well
|
ZDG-4-3
|
Zhidan group
|
-70.6
|
-9.4
|
Zhidan group
|
ZLG-1-3
|
Straight rom group
|
-69.1
|
-9.4
|
Straight rom group
|
DCC-1-2-3(1)
|
Yan'an Group
|
-56.3
|
-7.5
|
2 Coal roof aquifer ( abnormal )
|
DCC-1-2-3(2)
|
Yan'an Group
|
-90.8
|
-11.6
|
2 Coal-3 Coal aquifer
|
DCC-4-3
|
Yan'an Group
|
-90.9
|
-11.7
|
2 Coal
|
DCC-1-1-3
|
Yan'an Group |
-88.9 |
-11.3 |
2 Coal |
DCC-1-3-3(1)
|
Yan'an Group |
-89.3 |
-11.4 |
Aquifer between 3-1 coal and 4-1 coal |
DCC-1-1-3
|
Yan'an Group |
-91.1 |
-11.2 |
3 Coal |
DCC-2-3
|
Yan'an Group |
-91.8 |
-11.3 |
2 Coal roof aquifer |
DCC-1-3-3(2)
|
Yan'an Group |
-63.6 |
-8.6 |
Aquifer between 4-1 coal and 4-2 coal (anomaly ) |
Rainwater in this area has typical modern water (groundwater) characteristics (Fig. 10). The surface water collected from Wusuhaizi is enriched in heavy isotopes, which indicates that surface water is affected by strong evaporation and tends to sea water distribution area.
The degree to which the water sample isotopes deviate from the rainwater line can be indicated by a surplus of deuterium (d=δD-6.37δ18O). d=-3.69‰ when δ value falls on the rainwater line, and δ value falls on the lower right side of the rainwater line, d<-3.69‰, while the δ value falls to the upper left of the rainwater line, d<-3.69‰. The number of deviation from -3.69‰ of d value is equal to the distance of water sample along the direction parallel to the δD axis in Figure 11, which can often indicate the degree of evaporation in the groundwater source area and also reflect the degree of evaporation in recharge process. The δ18O value increases due to the exchange of 18O isotope between groundwater and rocks, resulting in 18O drift.
The environmental isotopes in the groundwater in the study area, as Quaternary → Cretaceous → Straight Rom Group → Yan’an Group, are gradually migrating to the deep groundwater (Figure 11). In particular, the deep Straight Rom Group and Yan’an Group have low values of δD and δ18O, and fall below the rainwater line of Ordos Basin, which are the groundwater with deep circulation depth and with good closed condition before mining. The lower values of δD and δ18O are due to the presence or mixing of Paleo-leaching and infiltration water formed under the conditions of palaeoclimate. Paleo-leaching and infiltration water refers to the groundwater formed by the infiltration of atmospheric precipitation under the paleoclimate conditions since the Quaternary. The δD and δ18O values of the paleo-groundwater formed during the glacial period are lower than those of modern water. High salinity water samples that deviate from the rainwater line and below the rainwater line are due to the occurrence of “oxygen isotope drift”, which results in the increase of δ18O value. Under the condition of high temperature, 18O in groundwater is enriched due to the isotope exchange between H216O and 18O in oxygen-bearing rocks (silicate rocks and carbonate rocks), while the δD value remains basically unchanged. On the whole, the values of environment isotopes D and 18O value objectively reflect that with the increase of the depth of underground water in Tahutu mine field, the formation age of water is longer.
Organic hydrochemical characteristics
On the basis of organic geochemistry and hydrogeology, this paper studies the quantity, composition and distribution of organic materials in groundwater and their roles in geological, geochemical and other processes by using qualitative and quantitative markers of various organic components in water[21-23]. Organic hydrochemical characteristics of each aquifer in Tahutu Mine can be established by testing the organic matter of each aquifer sample (Table 4).
Table 4: Basic information of organic matter test of water sample
Sample No.
|
Sampling point position
|
Layer position
|
Sample No.
|
Sampling point position
|
Layer position
|
Pond-2
|
Pond -2
|
Surface water
|
ZDG-3-2
|
Field 3 Zhidan group
|
Zhidan group
|
DSG-1-2
|
Field 1 Quaternary
|
Quaternary
|
ZLG-1-2
|
Field 1 Straight rom group
|
Straight rom group
|
#1-2
|
Quaternary
|
Quaternary
|
ZLG-4-1
|
Field 4 Straight rom group
|
Straight rom group
|
#2-2
|
Quaternary
|
Quaternary
|
ZLG-3-1
|
Field 3 Straight rom group
|
Straight rom group
|
#3-2
|
Quaternary
|
Quaternary
|
ZLG-2-1
|
Field 2 Straight rom group
|
Straight rom group
|
#4-2
|
Quaternary
|
Quaternary
|
DCC-1-2-1
|
Upper DCC-1-2, Field 1
2 Coal roof aquifer
|
Yan'an Group
|
#5-2
|
Quaternary
|
Quaternary
|
DCC-4-1
|
Upper DCC-1-2, Field 4、2 Coal
|
Yan'an Group
|
#6-2
|
Quaternary
|
Quaternary
|
DCC-1-1-1
|
Upper DCC-1-1, Field 1
2 Coal roof aquifer
|
Yan'an Group
|
#7-2
|
Quaternary
|
Quaternary
|
ZLG-1-2-1
|
Field 1 DCC-1-2 lower
Aquifer between 2 coal and 3 coal
|
Yan'an Group
|
#8-2
|
Quaternary
|
Quaternary
|
DCC-4-2
|
Field 4 lower DCC-4
2 Coal
|
Yan'an Group
|
#9-2
|
Quaternary
|
Quaternary
|
DCC-2-1
|
Upper DCC-2, Field 2
2 Coal roof aquifer
|
Yan'an Group
|
ZDG-1-2
|
Field 1 Zhidan group
|
Zhidan group
|
DCC-1-3-1
|
Upper DCC-1-3,Field 1
3 - 1 to 4 - 1 aquifers
|
Yan'an Group
|
ZDG-2-2
|
Field 2 Zhidan group
|
Zhidan group
|
DCC-2-2
|
Field 2 DCC-2 lower
2 Coal
|
Yan'an Group
|
ZDG-4-1
|
Field 4 Zhidan group
|
Zhidan group
|
DCC-1-1-2
|
Field 1 DCC-1-1 lower
3 Coal
|
Yan'an Group
|
Total Organic Carbon (TOC) was measured by multi N/C 2100 expert Total Organic Carbon/Total nitrogen analyzer. The water sample was filtered by 0.45μm filter membrane, and the filtrate was taken to detect the Total Organic Carbon content. The UV absorbance (UV254) was detected by Evolution 60 UV-Vis Photometer. The water sample was placed in a quartz dish of 1cm size to detect the UV absorption value (UV-254) at 254nm, and calibration with blank water sample.
According to the detection of TOC and UV254 in each aquifer (Fig. 12): in surface water, C(TOC)= 3.484 mg/L,C(UV254)= 0.064cm-1. The organic matter content in the water is low, which is similar to the situation in Nalin River No.2, Hongqing River, Balasu and other mines in this area, indicating that the nitrogen and organic content of industrial and agricultural pollution sources in this region is very low, and the surface vegetation is sparse. In Quaternary water, C(TOC)= 0.345~1.344mg/L,C(UV254)= 0.005~0.026cm-1, and Dissolved Organic Matter (DOM) concentration is low. The main reason is that the organic matter in surface water and aerated zone water enters Quaternary system and reacts further with DO, NO3 and other electron donors. The same characteristics are also shown in the Zhidan Group aquifer water, C(TOC)= 0.452~1.076mg/L, C(UV254)= 0.004~0.029cm-1, it is possible that Fe and Mn are involved in the reaction, and organic matter is the carbon source. In deep Zhiluo Formation aquifer water, C(TOC)= 0.5352~1.471mg/L, C(UV254)= 0.002~0.009cm-1. The organic matter concentration in the deep aquifer of Yanan Formation varies greatly, which may be affected by coal measure strata, C(TOC)= 0.329~ 3.943mg/L, C(UV254)= 0.0004~ 0.075cm-1.
In general, DOM concentration decreases with the increase of buried depth of aquifer (from surface water → Quaternary → Cretaceous →Straight Rom Group). The REDOX reaction of these substances with DO and NO3 in the process of migration with groundwater is the main factor leading to the decrease of content. TOC concentration in the water of Yanan Formation varies greatly, which may be influenced by the coal-bearing strata.
The characteristics of fluorescence spectrum distribution of dissolved organic matter vary with the types and contents of organic matter, and are corresponding to the characteristics of water samples, which are called “fluorescence fingerprints”[24-26]. Three-dimensional excitation/emission matrix (3DEEM) is the graph obtained by projecting the fluorescence intensity on the horizontal and vertical coordinates of the excitation wavelength and the emission wavelength. Its image is intuitive and contains rich information. It has the advantages of fast, high sensitivity, small sample size and no need for preprocessing and enrichment of samples, etc., and has been widely used in DOM composition and content analysis. According to the geological and hydrogeological conditions and aquifer distribution characteristics of Tahutu mine, the vertical distribution characteristics of groundwater chemistry in coal mine area were studied in this paper.
According to the classification method of natural organic matter in water, DOM three-dimensional fluorescence matrix of each aquifer in the study area mainly include (Fig. 8): I area (aromatic protein) - tyrosine, II area (aromatic protein and) - tryptophan, III area (like rich acid) - hydrophobic organic acids, IV area (dissolved microbial metabolites) - tryptophan-containing proteinlike, V area (like humic acid) - Marine humic acid. Because DOM concentration in water is generally low, the type and intensity of DOM fluorescence peak are relatively weak.
In field surface water samples, there are 5 fluorescence peaks in DOM fluorescence matrix (Fig. 13): I area (aromatic protein) - tyrosine, fluorescence intensity (FI), FI=433.2QSU; II area (aromatic protein) - tryptophan, FI=433.4QSU; III area (like - fulvic acid) - hydrophobic organic acid, FI=551.9QSU; IV area (soluble microbial metabolites) - proteinoid containing tryptophan, FI=340QSU; V area (like - humic acid) - Marine humic acid, FI=555.7QSU. Surface water directly receives dissolved organic matter from surface flora and fauna and human activities, resulting in higher organic matter concentration and fluorescence intensity in water bodies.
Quaternary aquifer is mainly fed by overlay surface water and meteoric water, and DOM fluorescence spectrum matrix fingerprint in the water is mainly similar to that in surface water, showing two characteristics in 10 water samples. In the nine water samples collected from the surrounding civil mines, fluorescence peaks in I area, II area and V area (Fig. 14), and I area (aromatic protein) - tyrosine (FI = 384.6-560.8 QSU) were found, which is very significant and can be used as the symbol of Quaternary water of civil mines. The fluorescence peaks in the III area and V area are significantly different. In Fig. 14 (a), the fluorescence peak intensities in I area and V area are 1334 QSU and 1378 QSU, respectively. In other water samples, the values are 70.52~509 QSU and 58.75~529 QSU respectively. In addition, DSG-1-2 is hydrological drilling water, and the fluorescence peaks in III area and V area mainly appear, FI =207.9 QSU and 202.6 QSU, indicating that I area (aromatic protein) substances are dissolved in the mine during application. In general, it can be seen from 3 DEEM finger pattern that the symbolic fluorescence peak in I area and the symbolic fluorescence peak between III area and V area appear in Quaternary water.
Because Zhidan Group is located in the lower part of Quaternary, groundwater recharge is relatively weak and water cycle time is longer, DOM fluorescence spectrum finger pattern in water samples is different from Quaternary water to some extent (Fig. 15), I area (aromatic protein) - tyrosine, FI=65.46~274.7QSU; II area (aromatic protein) -- tryptophan, FI=76.27~306.8 QSU; V area (like - humic acid) - Marine humic acid, FI=100~376.7 QSU. In general, the concentration of dissolved organic matter in the groundwater of Cretaceous Zhidan Group is less than that of Quaternary, and the landmark fluorescence peak appears in Ⅴ area (like - humic acid ).
In the deeper Straight rom group aquifer, the groundwater has entered the deep retained circulation system, and although the DOM from shallow source is almost exhausted, there are three fluorescence peaks (Fig. 16), especially in ZLG-3-1 water sample, because the Zhiluo Formation belongs to a relatively water-rich aquifer, V area (like - humic acid) - Marine humic acid, FI=1701 QSU. This preliminary indicates that DOM from other sources exists in Straight rom group aquifer; three fluorescence peaks in the other three water samples, I area (aromatic protein) - tyrosine, FI=151.7~173.1 QSU; II area (aromatic protein) - tryptophan, FI=99.59~169.1QSU; V area (like - humic acid) - Marine humic acid, FI=150.3~622.5 QSU. Especially, the fluorescence peak intensity in V area is high, which further proves that DOM from other sources exists in the groundwater of Zhiluo Formation.
The aquifer of Yan’an group may be affected by roof inflow and geological deposition, and there are similarities and differences between the three - dimensional fluorescence spectra of the two groups. There are some differences within the Yan’an group, which can be divided into upper Yanan Formation and lower Yan’an group.
There are mainly three fluorescence peaks in the upper part of Yan’an group (Fig. 17), I area (aromatic protein) - tyrosine, FI=121.8~253.1 QSU; II area (aromatic protein) -- tryptophan, FI=115.3~256.5QSU; V area (like - humic acid) - Marine humic acid, FI=157.9~611.1QSU. In addition, the fluorescence peak of III area in DCC-1-1-1 water sample, FI=894.6QSU. However, it does not appear in other water samples, which cannot be regarded as the organic hydrochemical characteristics of the upper aquifer of Yan’an group.
Three fluorescence peaks also appeared in the lower part of Yan’an group (Fig. 18), I area (aromatic protein) - tyrosine, FI=313.8~329.1 QSU; II area (aromatic protein) -- tryptophan, FI=376.8 QSU; V area (like - humic acid) - Marine humic acid, FI=317.1~2613 QSU. In particular, the fluorescence peak in V area of the water sample in Fig. 18 (a), FI=2613 QSU. It is suggested to continue to collect water samples from the lower part of Yan’an group in the later underground mining process to further determine the organic hydrochemical characteristics of the lower part of Yan’an group.
In general, each aquifer in the Taohutu mine field can be divided into several characteristics: Fluorescence peaks in I area and III area mainly appear in surface water and Quaternary water, and DOM sources in surface water are more abundant; fluorescence peak in I area also appears in the water of Zhidan Group, Straight Rom Group and Yan’an Group, and the fluorescence peak between I area and II area is a symbol. In addition, the fluorescence peak in V area is also a symbol; Cretaceous → Straight Rom Group → Yan’an Group. The fluorescence peak intensity in V area showed an increasing trend, indicating the existence of humus-like DOM from other sources in the deep aquifer.