# Hydrogeochemical Characteristics of the Haiyuan-Liupanshan Seismic Belt at the Northeastern Edge of the Tibet Plateau

Authors: Xinyan Li; Xiaopeng Liu; Jianguo Du; Yueju Cui; Fengxia Sun
Journal: Geofluids
 (2022)

---

## Abstract

The hydrogeochemical characteristics and groundwater recharge environments along the deep-cut Haiyuan-Liupanshan fault zone (HLFZ) were investigated by major ion and isotopic compositions (δ18O and δD) of the water samples collected at 14 sites from 2016 to 2017. The Schukalev classification method, fuzzy membership function, the proportion coefficients of major ions and principal component analysis (PCA) were adopted to analysis the hydrogeochemistry, water quality, origins and controlling factors of groundwater. In terms of chemical types, the sites along HLFZ can be divided into two groups. Specifically, the Group 1 is in the north and west part of the HLFZ and the water samples are characterized by higher salinity and enriched in Na+ and SO42-, reflecting the control of evaporate dissolution in cretaceous and tertiary aquifers, reverse ionic exchange, stronger water-rock reaction and deep fluids. The higher concentrations of Ca2+ + Mg2+ and HCO3- were found in Group 2, which is located in the southeast of the HLFZ and may be attributed to carbonate weathering and the recharge of karst fissure water from ordovician dolomite and limestone. Contamination of groundwater in HLFZ is severe, with 78.6% of the sites classified as grade V, which means they cannot be used for drinking water or agriculture. The primary contaminants found in Group 1 are TDS, Na+, Cl- and SO42- and the exceedance in NO3-and pH value affect water quality in the Group 2. The 18O and δD values of groundwater in HLFZ indicated that meteoric water contributed dominantly to the groundwater and it was influenced by the water cycle and water-rock reaction. Furthermore, tectonic movement, seismicity, and the level of deformation along different segments of HLFZ are important factors in the migration of deep to shallow groundwater, and water-rock reactions during groundwater migration, which results in differences in chemical composition of the groundwater between Group 1 and Group 2. Most of the sites with higher TDS in HLFZ fall on the a higher stress state region (represented by a low b value<0.75), which reflected a possible evidence of seismic activity affected the hydrogeochemistry of groundwater.


---

## Body

## 1. Introduction

The hydrogeochemical environment is influenced by several factors, including hydrogeologic activity, tectonic movement, physical conditions and human activities, all of which influence each other and are reflected by the local hydrogeochemical characteristics [1–4]. Specifically, the hydrological cycling conditions were affected by landform, which was a determinant of surface runoff and drainage conditions for groundwater [5, 6]. The soluble salts in the groundwater are transported through the processes of evaporation, precipitation and weathering [7]. Additionally, the mineral components and petrochemical properties of rocks also affect the permeability of aquifers and the concentration of dissolved ions in groundwater [8]. Active fault systems, which have been active for the last hundred thousand years, have significantly affected the occurrence and migration of groundwater [9]. Chemical types and origins of groundwater are typically affected by mechanical properties, lithology and fluids of fracture zones [10]. Fluid within a fault zone can originate from various sources [11], except meteoric precipitation, mineral dehydration, progressive metamorphism, and shear heating may also be a source of fluid in shallow active fault zones [12]. Furthermore, the fluid derived from the deep magma or mantle may also contribute [13]. During the course of migration and circulation along the fault zone, the deep fluids continuously interact with the surrounding rock, resulting in the physical and chemical characteristics changes to the fluids. As a consequence, hydrological characteristics along the deep-cut fault zones can provide valuable insight into fault activity and recharge sources of local groundwater.Consequently, most of the water samples were collected from the fault zone, and major ions and isotopes were analyzed in order to learn more about hydrogeochemistry and controlling factors [2, 4, 14]. Hydrogeochemistry was also investigated in carbonate-evaporate aquifers controlled by deep-seated faults, providing evidence that thermal waters with higher TDS recharged with precipitation interact with the deep carbonates with embedded evaporitic minerals to form thermal water that overflows along the fault [4]. Water samples collected from arid tectonically active regions revealed that endogenic fluids ascend along faults and increase salinity, together with metal ions and overall degradation of water quality [15]. Moreover, major ion chemistry was identified in the basement fault of Mahanadi delta, and there was distinct saline contamination across the terrain and different regimes of water types [16]. These studies focused on the control effect of fault structure and geographical conditions on groundwater with high salinity content. Few studies have examined the influence of environmental factors, deep fluids, and seismic activity on fault zone contamination.The seismic belt extends along the active Haiyuan-Liupanshan fault zone (HLFZ) that marks the northeastern boundary of the Tibet Plateau, northwest China. Numerous investigations of the region’s seismic dynamics [17, 18], geological structure [19], and paleoearthquakes [20, 21] were conducted in the past hundred years, revealing that many large earthquakes have occurred along the Haiyuan fault zone (HFZ), A magnitude 8.5 earthquake struck Haiyuan on December 16, 1920, resulting in a 240 km-long rupture zone [21]. Owing to the arid and semi-arid conditions in this region of western China, groundwater resources are scarce and salinization is widespread [22, 23]. However, only a few hydrological investigations have been performed for the formation and migration of groundwater in some small watersheds, and hydrogeochemical variations along local fracture zone have been discussed [22, 23]. Nonetheless, the genesis of hydrogeochemical characteristics in different segments along HLFZ remains uncertain.The hydrogeochemical methods has been widely used to investigate the origin and hydrogeochemical characteristics of groundwater on a regional scale. Typically, proportion coefficients calculated by major ion concentrations are used to efficiently reveal the formation of groundwater distribution and water-rock interactions [24–26]. A fuzzy membership function can be used to evaluate groundwater pollution and determine the quality of water [27, 28]. The δ18O and δD values can indicate the origins of water and the mixture of different waters [29–31]. In this study, the chemical types, origins, contamination of groundwater in HLFZ were investigated using major ion concentrations as well as isotope compositions of hydrogen and oxygen, and multiple methods were utilized to examine the hydrogeochemical characteristics and genesis of water samples. We expect the results to provide background data for seismic monitoring in fault zones, as well as theoretical support for local water pollution control and sustainable development of groundwater resources.


## 2. Materials and Methods

### 2.1. Study Area

The study area is located near the northeastern edge of the Tibet Plateau, next to the stable Ordos block to the east (Figure1). In history, dozens of earthquakes of M >5, including ten of M >7, have occurred in this region. It is believed that there are four main active fault zones, HFZ, Liupanshan fault zone (LFZ), Yunwushan fault zone (YFZ) and Xiaoguanshan fault zone (XFZ) (Figure 1).Figure 1
Topography map of the study region showing the main faults, epicentres of earthquakes and sampling locations (solid triangle).HFZ is a left-lateral strike-slip fracture zone with a complex geometric structure including secondary faults and strike-slip basins [21]. Several small volumes of basalt and pre-basaltic metamorphic rocks occur along the western segment, while Tertiary sandy mudstones and sandy conglomerates are widely exposed to the middle and eastern segments. LFZ connecting to the southeastern end of HFZ is characterized by thrust and left-lateral strike-slip movement. The widespread Early Cretaceous strata along LFZ are dominated by mudstone and sandstone. No earthquakes of M >6 have occurred on LFZ in the last 100 years, suggesting that the fault has a significant seismic potential to produce a large earthquake [32].The climate in HFZ is characterized by temperate intracontinental semi-aridity, limited precipitation, strong winds and higher evaporation. The average precipitation for the HFZ is between 800 to 1200 millimeters per year, of which most falls during the rainy season from May to September, and less falls during the dry season from October to next April. An average annual temperature of 7°C is recorded with a minimum temperature of -6.7°C (in January) and a maximum temperature of 19.7°C (in August). Furthermore, the HFZ is located in the upper reaches of the Yellow River and in a hilly loess region and the loess is generally water-scarce. In valley plain depressions, there is abundant quaternary pore water.In contrast, the climate in LFZ is characterized by a continental and oceanic monsoon marginal climate with more precipitation. The Jinghe River rises at the eastern end of the LFZ and water resources are relatively abundant than HFZ. Groundwater comprises weathered fissure water and cretaceous interlayer water; the aquifer has great joint fissures and is composed of early cretaceous conglomerate, sandstone, and siltstone.


### 2.2. Samples and Analysis

The field investigations for the springs and deep wells (150–250 m) along HLFZ were conducted in 2016 to 2017 (Table1). Water samples at 14 sites were collected in March 2016, August 2016, November 2016, July 2017 and November 2017. The container of water samples was 250-mL polyethylene bottles. Nine of the sites were springs with natural flow conditions all year round, and five sampling sites were confined wells in the seismic monitoring stations. Spring temperatures and pH were measured with a portable measuring device (OHAUS Starter300), and well temperatures were recorded using a digital thermometer (SZW-II). The major ions (K+, Na+, Ca2+, Mg2+, SO42-, NO3-, Cl-) were determined using a DionexICS-900 ion chromatograph at the Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting (China Earthquake Administration) within one week of the sampling. HCO3- and CO32- were analyzed using the titration method with an error of <10%. Meanwhile, the ion balance (ib) is less than 10% [33]. The stable isotope ratios (δ18O, δD) were measured with a liquid water isotope mass spectrometer (Picarro L1102-i) in the Laboratory of Water Isotope and Water Rock Reaction (Chinese Academy of Sciences) with an error of ±0.1‰ and ±0.5‰ for δ18O and δD, respectively. The average of major element compositions were calculated using five samplings. According to the tectonic location of sampling sites, the study area was divided into two tectonic zones for the purposes of analyzing the hydrogeochemical characteristics, the sample sites of H1 ~ H8 in HFZ and those of L1 ~ L6 in LFZ.Table 1
The sampling location and lithology of each site.
SiteLon.(°)Lat.(°)Altitude (m)Well depth (m)Fault zoneLithologyH1105.3036.651974306.73HFZQh (sandy soil, fine-clastic rock)H2105.6036.532143SpringHFZQp3 (loess, medium–coarse grained sandstone)H3105.9636.451544618.39HFZE-N (sandstone)H4105.6336.252052416.5HFZK1 (pebbly sandstone, medium–coarse sandstone)H7106.2536.591406SpringYFZE (packsand, calcareous mudstone)H8106.0836.011759SpringHFZE (packsand, calcareous mudstone)H5106.0436.131819SpringLFZK (mudstone and marls)L6105.7335.811739351.2LFZE2s (shaly sandstone)H6105.8936.081945SpringHFZE (calcareous siltstone, silty mudstone)L1106.435.951793SpringXFZK1 (sandy mudstone)L2106.1835.742494SpringLFZK1 (sandy mudstone)L3106.2335.682175SpringLFZK1 (sandy mudstone)L4106.2735.61996239.5LFZE2 (sandstone, conglomerate)L5106.435.451724SpringLFZE2 (sandstone, conglomerate)


### 2.3. Data and Analysis

To identify the hydrochemical types, the anions and cations containing more than 25% of the six ions (Na+, Ca2+, Mg2+, SO42-, Cl-, HCO3-) were combined to define the primary types using the Schukalev classification method. And then a fuzzy membership function was used to assess the water quality using the method described by Zhang [34]. There were six factors evaluated in this study, including TDS, pH, Na+, Cl-, SO42-, NO3- were selected for evaluation in the study and, their limiting value of each grade is listed in Table 2. A standard for evaluating groundwater quality was adopted from the Chinese groundwater quality standard (GB/T1484-2017).Table 2
The limiting value of each grade І from Groundwater Quality Standard (mg/L).
pHTDSNaClSO4NO3UtilizabilityLimiting value of grade І8.50300.00100.0050.0050.002.00Drinking waterLimiting value of gradeII8.50500.00150.00150.00150.005.00Drinking waterLimiting value of grade III8.501000.00200.00250.00250.0020.00Drinking waterLimiting value of grade IV9.002000.00400.00350.00350.0030.00Agricultural and industrial waterLimiting value of grade V9.002000.00400.00350.00350.0030.00Other waterPrincipal component analysis (PCA) was aslso employed to convert inter-correlated hydrochemical indicators into uncorrelated variables [35]. Eight indicators, including Na+, K+, Mg+, Ca+, Cl-, SO42-, NO3-, and HCO3– were performed to PCA to determine the contribution of chemical composition on water quality through a load matrix of initial factors. More specifically, variance associated with high loading can be used to evaluate and interpret the groundwater chemistry. In this study, primary factors were selected based on eigenvalues greater than 1. In addition, the proportion coefficients of major ions were used to identify different origins and water-rock interreaction of groundwater.


## 2.1. Study Area

The study area is located near the northeastern edge of the Tibet Plateau, next to the stable Ordos block to the east (Figure1). In history, dozens of earthquakes of M >5, including ten of M >7, have occurred in this region. It is believed that there are four main active fault zones, HFZ, Liupanshan fault zone (LFZ), Yunwushan fault zone (YFZ) and Xiaoguanshan fault zone (XFZ) (Figure 1).Figure 1
Topography map of the study region showing the main faults, epicentres of earthquakes and sampling locations (solid triangle).HFZ is a left-lateral strike-slip fracture zone with a complex geometric structure including secondary faults and strike-slip basins [21]. Several small volumes of basalt and pre-basaltic metamorphic rocks occur along the western segment, while Tertiary sandy mudstones and sandy conglomerates are widely exposed to the middle and eastern segments. LFZ connecting to the southeastern end of HFZ is characterized by thrust and left-lateral strike-slip movement. The widespread Early Cretaceous strata along LFZ are dominated by mudstone and sandstone. No earthquakes of M >6 have occurred on LFZ in the last 100 years, suggesting that the fault has a significant seismic potential to produce a large earthquake [32].The climate in HFZ is characterized by temperate intracontinental semi-aridity, limited precipitation, strong winds and higher evaporation. The average precipitation for the HFZ is between 800 to 1200 millimeters per year, of which most falls during the rainy season from May to September, and less falls during the dry season from October to next April. An average annual temperature of 7°C is recorded with a minimum temperature of -6.7°C (in January) and a maximum temperature of 19.7°C (in August). Furthermore, the HFZ is located in the upper reaches of the Yellow River and in a hilly loess region and the loess is generally water-scarce. In valley plain depressions, there is abundant quaternary pore water.In contrast, the climate in LFZ is characterized by a continental and oceanic monsoon marginal climate with more precipitation. The Jinghe River rises at the eastern end of the LFZ and water resources are relatively abundant than HFZ. Groundwater comprises weathered fissure water and cretaceous interlayer water; the aquifer has great joint fissures and is composed of early cretaceous conglomerate, sandstone, and siltstone.


## 2.2. Samples and Analysis

The field investigations for the springs and deep wells (150–250 m) along HLFZ were conducted in 2016 to 2017 (Table1). Water samples at 14 sites were collected in March 2016, August 2016, November 2016, July 2017 and November 2017. The container of water samples was 250-mL polyethylene bottles. Nine of the sites were springs with natural flow conditions all year round, and five sampling sites were confined wells in the seismic monitoring stations. Spring temperatures and pH were measured with a portable measuring device (OHAUS Starter300), and well temperatures were recorded using a digital thermometer (SZW-II). The major ions (K+, Na+, Ca2+, Mg2+, SO42-, NO3-, Cl-) were determined using a DionexICS-900 ion chromatograph at the Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting (China Earthquake Administration) within one week of the sampling. HCO3- and CO32- were analyzed using the titration method with an error of <10%. Meanwhile, the ion balance (ib) is less than 10% [33]. The stable isotope ratios (δ18O, δD) were measured with a liquid water isotope mass spectrometer (Picarro L1102-i) in the Laboratory of Water Isotope and Water Rock Reaction (Chinese Academy of Sciences) with an error of ±0.1‰ and ±0.5‰ for δ18O and δD, respectively. The average of major element compositions were calculated using five samplings. According to the tectonic location of sampling sites, the study area was divided into two tectonic zones for the purposes of analyzing the hydrogeochemical characteristics, the sample sites of H1 ~ H8 in HFZ and those of L1 ~ L6 in LFZ.Table 1
The sampling location and lithology of each site.
SiteLon.(°)Lat.(°)Altitude (m)Well depth (m)Fault zoneLithologyH1105.3036.651974306.73HFZQh (sandy soil, fine-clastic rock)H2105.6036.532143SpringHFZQp3 (loess, medium–coarse grained sandstone)H3105.9636.451544618.39HFZE-N (sandstone)H4105.6336.252052416.5HFZK1 (pebbly sandstone, medium–coarse sandstone)H7106.2536.591406SpringYFZE (packsand, calcareous mudstone)H8106.0836.011759SpringHFZE (packsand, calcareous mudstone)H5106.0436.131819SpringLFZK (mudstone and marls)L6105.7335.811739351.2LFZE2s (shaly sandstone)H6105.8936.081945SpringHFZE (calcareous siltstone, silty mudstone)L1106.435.951793SpringXFZK1 (sandy mudstone)L2106.1835.742494SpringLFZK1 (sandy mudstone)L3106.2335.682175SpringLFZK1 (sandy mudstone)L4106.2735.61996239.5LFZE2 (sandstone, conglomerate)L5106.435.451724SpringLFZE2 (sandstone, conglomerate)


## 2.3. Data and Analysis

To identify the hydrochemical types, the anions and cations containing more than 25% of the six ions (Na+, Ca2+, Mg2+, SO42-, Cl-, HCO3-) were combined to define the primary types using the Schukalev classification method. And then a fuzzy membership function was used to assess the water quality using the method described by Zhang [34]. There were six factors evaluated in this study, including TDS, pH, Na+, Cl-, SO42-, NO3- were selected for evaluation in the study and, their limiting value of each grade is listed in Table 2. A standard for evaluating groundwater quality was adopted from the Chinese groundwater quality standard (GB/T1484-2017).Table 2
The limiting value of each grade І from Groundwater Quality Standard (mg/L).
pHTDSNaClSO4NO3UtilizabilityLimiting value of grade І8.50300.00100.0050.0050.002.00Drinking waterLimiting value of gradeII8.50500.00150.00150.00150.005.00Drinking waterLimiting value of grade III8.501000.00200.00250.00250.0020.00Drinking waterLimiting value of grade IV9.002000.00400.00350.00350.0030.00Agricultural and industrial waterLimiting value of grade V9.002000.00400.00350.00350.0030.00Other waterPrincipal component analysis (PCA) was aslso employed to convert inter-correlated hydrochemical indicators into uncorrelated variables [35]. Eight indicators, including Na+, K+, Mg+, Ca+, Cl-, SO42-, NO3-, and HCO3– were performed to PCA to determine the contribution of chemical composition on water quality through a load matrix of initial factors. More specifically, variance associated with high loading can be used to evaluate and interpret the groundwater chemistry. In this study, primary factors were selected based on eigenvalues greater than 1. In addition, the proportion coefficients of major ions were used to identify different origins and water-rock interreaction of groundwater.


## 3. Results and Discussion

The physical and chemical parameters of the water samples are shown in Table3. There was a small fluctuation in the temperature of the well water, but the temperatures of the spring water varied a more extensive range of 8.5°C ~25.1°C, samples in H7 maintains a temperature of 20°C. Approximately 31.64~100486.2 mg/L of TDS is present in the spring water. Expect for L4, the TDS value in well samples was greater than 2000 mg/L, which was considered to be saltwater. Especially, spring samples in H8 with average TDS>83000 mg/L, were considered to be brine. Besides H7, H8 and L1, TDS value of other springs in LFZ were less than 1000 mg/L. The pH values ranged from 7.35 to 11.1. The values of δ18O and δ D ranged from −13.29% to −8.5% and from −95.58% to −58.26%, respectively.Table 3
Physical and chemical composition of water samples.
Site no.DateT(°C)pHTDS(mg/L)Na(mg/L)K(mg/L)Mg(mg/L)Ca(mg/L)Cl(mg/L)SO4(mg/L)CO3(mg/L)HCO3(mg/L)NO3(mg/L)δ18O(‰)δD(‰)Chemical typeH1Mar 201613.308.632431.85636.027.02101.1512.47711.75855.517.44200.340.32-12.62-90.90Na-cl•SO4Aug 201613.309.242320.97629.716.99105.5312.05691.35788.9524.19124.420.00——Na-cl•SO4Nov 201613.308.562380.26611.246.65106.9311.67726.41817.1739.01121.000.70——Na-cl•SO4July 201713.309.812420.34612.066.30104.8510.67730.51860.1446.2299.210.00-13.18-89.95Na-cl•SO4Nov 201713.308.582291.13615.726.56107.8610.76678.63745.8455.80139.930.00——Na-cl•SO4H2Mar 20169.108.20321.4214.213.2730.6459.388.5073.640.00257.043.31-10.91-72.07Mg.Ca-SO4.HCO3Aug 201611.208.62322.3812.474.9430.7756.827.3073.450.00266.113.58——Mg.Ca-SO4.HCO3Nov 201610.108.14247.1914.2315.2032.1334.676.2665.520.00168.065.16——Mg.Ca-SO4.HCO3July 201713.108.88327.7015.7184.3432.6259.816.7969.130.00273.092.75-10.61-71.28Mg.Ca-HCO3Nov 20179.508.09345.5513.4964.6434.7962.456.6161.870.00313.914.74——Mg.Ca-HCO3H3Mar 201616.207.784113.28949.637.75208.08291.861794.20727.390.00257.045.82-12.55-91.65Na-clAug 201616.208.236056.19932.125.70207.58289.462757.801791.400.00103.6820.38——Na-cl•SO4Nov 201616.207.914571.44859.315.99197.46263.031873.701354.200.0026.224.66——Na-cl•SO4July 201716.208.634771.52901.405.60204.33279.691918.401420.900.0075.713.32-12.28-90.99Na-cl•SO4Nov 201716.207.824666.65880.495.14193.28289.701868.301358.40.00136.153.22——Na-cl•SO4H4Mar 201610.507.812664.09306.833.9288.631512.27117.6325910.0086.940.39-13.11-95.58Na.Ca-SO4Aug 201610.508.253687.30318.823.4393.156539.58148.292548.70.0069.120.72——Na.Ca-SO4July 201710.508.572937.75295.206.4890.04520.19112.221878.70.0058.485.64-13.29-95.43Na.Ca-SO4Nov 201710.508.012726.79282.273.8598.29495.66104.7116910.00102.110.00——Na.Ca-SO4H5July 201717.108.66810.3877.796.2170.83122.5214.21284.240456.896.13——Mg•ca-SO4•HCO3Nov 201712.208.621216.1494.408.47102.60225.240.3288.714.88900.120.00-11.31-72.23Mg•ca-SO4•HCO3H6July 201712.908.73717.72108.472.3953.1572.4013.83297.80.00330.014.67——Na•mg•ca-SO4•HCO3Nov 201710.108.54702.9596.9873.1351.2165.3011.65255.050.00416.0211.60-9.78-69.6Na•mg•ca-SO4•HCO3H7Mar 201620.307.3723156.858476.4235.196.50666.37318.405920.000.00888.30.00-9.44-88.49Na-cl•SO4Aug 201622.507.5233367.949002.0019890.72349.0113944.009370.700.00801.7912.68——Na-cl•SO4July 201725.107.3724246.628943.70235.994.20336.608235.306175.000.00331.0560.40-9.45-87.19Na-cl•SO4Nov 201721.107.3532272.5210179.00211.4115.50402.312419.008633.000.00624.030.00——Na-cl•SO4H8Mar 201614.509.1190869.6930068.00108.8028.4045.9013420.0046329.00580.3578.340.00-11.38-80.92Na-cl•SO4Aug 201617.409.10100775.929055.00123.1058.90174.2016245.0054459.00283.4753.410.00——Na-cl•SO4July 201715.809.0173736.0224463.00156.5090.70220.1011019.0037318.00155.1472.0378.30-11.29-81Na-cl•SO4Nov 201713.309.0969725.3525063.00105.6059.80408.4010967.0032317.00491627.810.00——Na-cl•SO4L6Mar 201617.209.453365.851003.507.652.233.53490.481719.4029.76223.020.00-12.23-89.17Na-cl•SO4July 201717.2010.52884.151020.909.223.2218.33357.561313.20100.2112.795.19-12.59-88.67Na-SO4Nov 201717.209.252620.56968.067.366.1312.94321.161151.3089.28128.590.00——Na-SO4L1Aug 201618.709.851934.53657.093.382.008.0436.64985.4679.49324.860.00——Na-SO4•HCO3Nov 201613.809.321903.11620.682.231.252.4225.00971.1887.94379.802.50——Na-SO4•HCO3July 201722.409.881950.10618.872.110.750.0031.731002.4087.83411.980.41-11.97-83.02Na-SO4•HCO3Nov 201714.109.081860.54617.922.241.152.9622.54875.77145.10385.760.00——Na-SO4•HCO3L2Aug 201619.309.26357.6225.121.1132.0860.652.17106.310.00252.294.03——Mg.Ca-SO4•HCO3Nov 201610.009.03285.4127.051.2431.2129.862.4990.190.00198.304.21——Mg.Ca-SO4•HCO3July 201720.009.33255.5223.731.3124.5338.832.3750.430.00219.314.68-8.50-58.26Mg.Ca-HCO3Nov 201710.008.99330.0320.681.1730.3651.3981.9464.6174.4162.634.17——Mg.Ca-HCO3L3Aug 201611.59.23355.4119.9611.88731.9058.475.55121.010.00221.186.04——Mg•ca-SO4•HCO3Nov 201610.59.01302.2721.6852.21932.9833.425.15111.690.00178.815.72——Mg•ca-SO4•HCO3July 201711.79.24376.5122.4042.20933.5158.246.70110.990.00271.006.96-11.00-71.10Mg•ca-SO4•HCO3Nov 201710.29.13342.4119.5311.81934.4852.194.2886.3896.7279.4227.30——Mg•ca-SO4•HCO3L4Mar 201611.19.99615.95276.913.580.662.0621.152.30174.80268.380.25-11.9082.70Na- HCO3Aug 201611.110.60676.52272.222.220.532.3133.376.85186.60344.800.00Na- HCO3Nov 201611.19.96587.65265.332.240.693.3325.864.87112.40342.831.51Na- HCO3July 201711.111.3611.85262.61.910.672.38323.414.46142.30347.240.53-12.70-81.9Na- HCO3Nov 201711.110.2648.65268.742.421.004.0921.170.00256.70189.100.00Na- HCO3L5Mar 20169.28.1734.6558.772.2348.443119.2617.06284.020.00362.8823.43-9.55-66.6Mg•ca-SO4•HCO3Aug 201610.48.2815.3159.612.4851.598103.8530.328344.310.00359.4243.42——Mg•ca-SO4•HCO3Nov 20168.58.08598.6757.632.7551.60348.95321.645267.010.00242.0028.08——Na•mg•ca-SO4•HCO3July 201710.48.18677.6253.192.4450.04998.30421.195257.220.00336.7926.82-9.56-65.00Mg•ca-SO4•HCO3Nov 20168.88.12765.4052.582.3056.388119.0322.221241.820.00476.5332.80——Mg•ca-SO4•HCO3


### 3.1. Hydrogeochemistry

According to the Schukalev classification method, all water samples were classified in eight chemical types: Na-Cl•SO4, Na•Ca-SO4, Na-CO3•HCO3, Mg•Ca-HCO3, Na-SO4•HCO3, Mg•Ca-SO4•HCO3, Na-SO4, Na•Mg•Ca-SO4•HCO3 and Na-Cl (Table 2). Concentrations of SO42-, HCO3- and Cl in H2, H3, L2 and L6 water samples show seasonal fluctuation. The content of HCO3- was affected by soil CO2 dissolving in the precipitation [36], the concentration of HCO3- at H2 and L2 was increased with rainfall in 2017. Additionally, the concentration of SO42- decreased with HCO3- increased. As Cl- concentration is directly correlated with TDS in arid areas, the rainy season led to a decrease in Cl- concentration as a result of lower salinity water at L6. From samples H5, H6, H2, L3, L5, and L2, the proportions of Ca2+ were greater than 40%, but those of Cl- were less than 40%. The concentrations of HCO3-, CO32- and Na+ are significantly higher in the water sample from site L4 than in other well samples. There are large seasonal fluctuations of concentrations of SO42- + Cl-, Mg2++Ca2+ and HCO32- + CO32- in the water samples from the sites H5, H6, L3 and L2, which can be attributed to more precipitation recharge. Ion concentrations in water samples collected from the sites H7, H8, L6, L1 and L5 varied little between different seasons, indicating that those sites are supposed to be the perfect locations for seismic monitoring.In light of chemical types, the sites can be divided into two groups, H1, H3, H4, H7, H8, L1 and L6 with higher TDS belong to the group 1 and H2, H5, H6, L2, L3 L4 and L5 with lower TDS are geared to group 2. The spatial variation in hydrochemical types along HLFZ (Figure2) showed a significant difference in group1 and group 2. The dominant cation and anion in group 1 are Na+ and SO42-, implying the strong leaching effect in the recharge process of groundwater and deeper circulation, longer residence time and higher water-rock equilibrium. In group 2, Mg2+, Ca2+ and HCO3- are the major iron, it represents the first reaction process between meteoric waters and rocks, reflecting shallower and younger hydrological cycle characteristics in this area [37]. In addition, the different chemical compositions in the two areas may also be caused by the different aquifer minerals.Figure 2
Voronoi diagram of hydrochemical types in HLFZ(Earthquake with M >3 since 1970 were selected).


### 3.2. Water Quality

According to the results of groundwater assessed by Fuzzy membership (Table4, Figure 3), groundwater quality in HLFZ was grouped into three grades. Water samples in group 1 were classified into grade V, which is only suitable for use in monitoring earthquakes or other industrial purposes. The primary contaminants found in group 1 are TDS, Na+, Cl- and SO42- which are indicative of serious salinization processes of groundwater. The water quality of H5 and H6 with a medium level of chemical components is of grade III, and can be used as drinking water or irrigation water. The water in H2 was excellent and was classified as grade I. Expect for H2, H5 and H6, all other samples in group 2 were classified as grade V, due to excessive NO3- or pH. Contamination of groundwater with NO3- was a result of anthropogenic impacts, in particular chemical fertilizers used in local agriculture. Water samples collected from the LFZ were alkaline water with a pH value >9, which is associated with a higher concentration of HCO3-. In general, the groundwater contamination in HLFZ is serious, with 78.6% of the sites that is not suitable for drinking and agricultural water.Table 4
The groundwater quality of each site.
The grade of water qualityThe main contaminantsH1VpH, TDS, Na+, cl-, SO42-Group 1H2INO3-Group 2H3VTDS, Na+, cl-, SO42-,NO3-Group 1H4VTDS, cl-, SO42-Group 1H5IIIpH,TDS, cl-, SO42-, NO3-Group 2H6IIIpH, TDS, cl-, SO42-, NO3-Group 2H7VTDS, Na+, cl-, SO42-, NO3-Group 1H8VpH,TDS, Na+, cl-, SO42-, NO3-Group 1L1VpH, Na, SO42-Group 1L2VpH, NO3-Group 2L3VpH, NO3-Group 2L4VpH, Na+Group 2L5VNO3-Group 2L6VpH, TDS, Na+, cl-, SO42-Group 1Figure 3
Voronoi diagram of water quality grade in HLFZ.


### 3.3. Water-Rock Interreaction

The mCa2+/Na+(molar ratio) vs. mHCO3-/Na+ can be used to determine reactions between groundwater and different rock types, the molar ratio of carbonates controlling end is close to 50 and 120, and the molar ratio of silicates controlling end is close to 0.35 ± 0.15 and 2 ± 1, respectively(Figure 4, [1, 24]). The result was shown in Figure 4. In H7 and L4, samples were primarily controlled by evaporate dissolution, while samples from Group 2 were controlled by carbonate weathering and silicate weathering. Other samples in Group 1, such as H1, H3, H4, H8, L6 and L1 were not controlled by the weathering of the three rocks, which implied that other factors contributed to their sources.Figure 4
Molar ratio bivariate plots of Na+-normalized Ca2+ vs. Na+-normalized HCO3- for the water samples were collected from HFZ and LFZ.It is thought that Na+ and K+ primarily formed through the weathering of evaporates or silicates; Ca2+ and Mg2+ are strongly correlated with evaporate dissolution, silicate weathering and carbonate weathering; Cl- and SO42- are associated with the dissolution of evaporates; and HCO3- is predominantly originated from carbonates [38]. In terms of the proportion coefficients of the major ions, most water samples from the Group 1 plot below the 1 : 1 line of Cl- + SO42- vs. HCO3 (Figure 5(a)) and deviate from the 1 : 1 line of Ca2++Mg2+ vs. HCO3-(Figure 5(b)); this demonstrates that evaporate dissolution dominates over carbonate weathering. As the dissolution of evaporate plays an essential role in the chemical composition of water, the ratio of Na++K+ vs. Cl- is near the 1 : 1 line. As shown in Figure 5(c)), samples in H1, H3 and H8 from the Group 1 are near the 1 : 1 line, indicating that evaporates (e.g., NaCl and KCl) are the primary sources of the Na++K+ and Cl− concentrations. The samples in L1, L4 and L6 are above the 1 : 1 line of Ca2++Mg2+ vs. HCO3- (Figure 5(b)), suggesting a strong dissolution of carbonate in these sites. Moreover, samples L1, L4, L6, H7 and H8 also above the 1 : 1 line of Ca2++Mg2+ vs. HCO3-(Figure 5(g)), which shows significant cation exchange between Ca2+ or Mg2+ and Na+ in these samples. Two chloro-alkaline indices CAI 1 and CAI 2 were defined by Schoeller [39] to determine the effect of cation exchange. When both indices are positive, reverse ionic exchange (Na-Ca) will occur in the groundwater, i.e., the Na2+ content in groundwater will alternately adhere to surface of rock particles with Ca2+. In Figure 5(h), expect H7, CAI 1 and CAI 2 were positive in high-salinity groundwater with TDS>2000 mg/L and negative in low-salinity groundwater, indicating forward ionic exchange (Ca-Na) usually decreased the hardness (Ca2+ content) of the groundwater, resulting in lower salinity water. The majority of the Group 1 are near the 1 : 1 line of Na+ vs. SO42- (Figure 5(e)), but deviate from the 1 : 1 line of Na+ vs. HCO3− (Figure 5(d)), indicating that the dissolution of sulfate minerals is not an primary source of Na+ in this region. Ca2+ and Mg2+ in most sites are weakly corrected with SO42-(Figure 5(f)) in the Group 1 also illustrates that the dissolution of sulfate minerals is not the major source of Ca2+ and Mg2+.Figure 5
Proportion coefficients of major ions in groundwater from the HFZ and LFZ zones for samples collected in the dry and rainy seasons.
(a)(b)(c)(d)(e)(f)(g)(h)This study examined some water samples from the Group 2 plot near the 1 : 1 line of Cl- + SO42− vs. HCO3- (Figure 5(a)) and Ca2++Mg2+ vs. HCO3- (Figure 5(b)), suggesting the dissolution of carbonates and evaporate control the composition of groundwater in this area. The samples in Group 2 fall below the 1 : 1 line of Na++K+ vs. Cl-(Figure 5(c)), demonstrating that the precipitation and dissolution of evaporate are not the primary sources of Na+ and K+. Most samples from this area plot near the 1 : 1 line of Na+ vs. HCO3- and Na+ vs. SO42- (Figure 5(d), Figure 5(e)), reflecting that the dissolution of sulfate minerals is a significant source of Na+ in this area. In the major of water samples from the Group 2, a strong correlation was found between Ca2++Mg2+ and SO42- and HCO3-, suggesting that the main origins of Ca2+, Mg2+, HCO3-, and SO42− in the groundwater are the dissolution of calcite (CaCO3), dolomite [CaMg (HCO3)2], and gypsum (CaSO4). In contrast to other well samples, the chloro-alkaline indices in L4 is negative, indicating that forward ionic exchange is an important factor of the sodium water (Na - HCO3) with low–salinity.In HFZ, the H1 well is located in the meizoseismal areas of the 1920 Haiyuan earthquake, the underlying aquifer is proluvial gravelly sand with good permeability and a deep burying depth (water table deeper than 19 m), and waters flows along the fault zone. Although the groundwater in H2 also originated from Quaternary fissure water, but there is lower TDS than H1, which indicates that the water-rock reaction of the surface rupture zone is relatively stronger. The aquifer of the H3 well has multiple layerswith a lithology of tertiary silty sand and fine sand. It is recharged by weathering infiltrating along the piedmont fissures and pores caused by atmospheric precipitation. The cretaceous sandstone contacts the fault zone at the H4 site, forming an artesian well where the higher concentrations of SO42- and Ca2+ in H4 are associated with gypsum. H7 and H8 exposed in the Cretaceous packsand and calcareous mudstone with abundant mirabilite are ascending springs, with white sediment of Na2SO4 around the spring, resulting in the Na+- rich water, and the TDS reached 22268~32566 mg/L and 73284~ 100486 mg/L, respectively. The H5 and H6 are ascending springs, and the spring waters is primarily stored in weathering fracture water of cretaceous layered rocks.In LFZ, L1 spring is exposed in the Cretaceous sandstone near the XGSF zone, owing to its dense joint distribution and higher rupture degree of rocks, the groundwater in this site is connected with the deep fluids of the fault zone along joint fissures, which results in a higher temperature and salinity than any other spring in LFZ. The spring water in L2, L3 and L5 is stored in the pores and fissures of the clastic rocks of the Lower Cretaceous system and is fed by the lateral inflow through the bedrock mountain system. L4 is exposed in the clastic rocks and minor carbonate intercalations of the lower Cretaceous near the XFZ zone, which has resulted in a higher concentration of HCO3- and CO32- than other samples. L6 is exposed in the Xiji graben basin in the western LFZ, and the aquifer is a pore confined aquifer of tertiary argillaceous, sandy conglomerates. The poor recharge in the basin, coupled with sulfate, chloride and Na+-rich sediments in the aquifer, has resulted in brackish water with higher TDS concentrations in L6.In general, the distribution of high sodium water in the HFZ area was also related to the dissolution of soluble salts in cretaceous and tertiary red bed aquifers, such as gypsum, mirabilite and magnesite. The higher Ca2+ contents of samples from the LFZ area reflect karst fissure water from Ordovician dolomite and limestone.


### 3.4. Contributions of Different Sources to Groundwater

A correlation coefficient between different groundwater components in HLFZ shows that there is a strong correlation between Na-, Cl-, and SO42- (Table 5), indicating the sources of these constituents are similar. The result of PCA was shown that a total of 75.79% of the variance was accounted for by two main factors (Table 6). The first factor shows high loading of chemical composition Na+, K+, Cl-, SO42-, NO3- and HCO3- with a maximum contribution of up to 55.98% of the total variance, which indicates mineral dissolution, cation exchange, deep fluid input and agricultural contamination. The HFZ is characterized by calcareous mudstone with a high CaCO3 content, which contains a considerable amount of HCO3- formed during dissolution. Additionally, CO2 is discharged from the fault zone and dissolves in water to form HCO3-, especially in H7 and H8. The Ningxia Geophysical Network reported that the concentration of CO2 was 0.12% and 3.62%, respectively. The variables Na+, K+ and Cl- have strong positive loadings (greater than 0.9), which may be caused by the dissolution of halite (e.g., KCl and NaCl) in the surrounding rock. Furthermore, reverse ionic exchange (Na-Ca) also contributes to the enrichment of Na+ in the groundwater in Group 1(Figure 6(h)). The concentration of Cl- in H7 and H8 exceeded 10 g/L, demonstrating brine input from the fault zone. Most samples contain higher amounts of SO42-, which is related to sulfate minerals, such as mirabilite and gypsum. While higher loading of NO3- suggested an anthropogenic source of agricultural chemical fertilizes.Table 5
Correlation coefficient of chemical composition of groundwater.
TDSNa+K+Mg2+Ca2+Cl-SO42-NO3-HCO3-pHTDS1.001.000.690.100.310.920.990.470.57-0.49Na+1.000.700.070.290.920.990.470.58-0.07K+1.000.190.540.910.570.500.59-0.43Mg2+1.000.610.230.050.15-0.15-0.67Ca2+1.000.470.240.200.09-0.69Cl-1.000.850.530.60-0.30SO42-1.000.430.53-0.01NO3-1.000.41-0.07HCO3-1.00-0.50pH1.00Table 6
Component matrix.
VariablePonent matrixFactor 1Factor 2Na+0.925-0.193K+0.880.10Mg2+0.290.87Ca2+0.50.75Cl-0.980.02SO42-0.86-0.222NO3-0.620.03HCO3-0.68-0.41Eigenvalue4.481.58%Total variance55.9819.81%Cumulativ evariance55.9875.79Bold values indicates high loadings.Figure 6
Plot ofδ18O and δD in groundwater from the HFZ and LFZ fault zones for samples collected in the dry and rainy seasons. GMWL, Global Meteoric Water Line [40]; LMWL, Northwestern China Local Meteoric Water Line.The second factor explained 19.81% of the total variance with high loading of the variable Ca2+ and Mg2+, which represents rock weathering. The correlation coefficient indicated that Ca2+ and Mg2+ were positively correlated with each other, but they were negatively correlated with pH value, demonstrating that Ca2+ and Mg2+ originated from the dissolution of carbonate rocks and that the solubility of carbonate rocks decreases with the increase of pH value. Furthermore, there is a correlation between pH and TDS, which suggests the dissolution of carbonate in HLFZ is not uncommon. As a result of possible silicate weathering to HCO3-, the correlation between HCO3- and Ca2+ and Mg2+ is poor.


### 3.5. Groundwater Recharge

Except for H7, most of the water samples plot near the Global Meteoric Water Line (GMWL) defined by Craig [40] and the Northwestern China Local Meteoric Water Line (LMWL: δD = 7.4 δ18O+ 7.2 [41]; Figure 6). It follows that local meteoric water is a significant factor in recharging groundwater along the HLFZ zone. For H7, there is a positive shift in δ18O, which reflects strong water-rock interaction related to high temperature. Well samples always have lower δ18O and δD values than spring water samples owing to slower circulation, less input from precipitation, and stronger evaporation. Some spring samples (H5, L2, and H2) plot above the LMWL and have shallow groundwater circulation depths along with rapid circulation speeds. Spring samples from the LFZ area have higher δ18O and δD values than those from the HFZ area, reflecting a greatly rainfall recharge. In contrast with LFZ, most samples from HFZ deviated from the local meteoric line, indicating intense water-rock interaction in this region. The dry season (Mar 2016) isotopic compositions, which are relatively depleted in heavy isotopes, are evidence of colder temperatures and limited evaporation during rainfall; this is consistent with other studies, which have demonstrated the meteoric origins of groundwater in arid and semi-arid regions of China [42].


### 3.6. The Geological and Seismic Factors

Along with stratum, lithology, geography and climatic factors, tectonic movement plays an influential role in the formation, movement and enrichment of groundwater. According to geological and microgeomorphological evidence, 5 major earthquakes with a magnitude of about 8.5 have occurred in HFZ since Holocene; The 1920 Haiyuan earthquake is the latest, and the active period of the quaternary in the western segment occurred earlier than in the eastern segment [18]. The occurrence of fault activities has controlled the depositional environment of the aquifer and has led to differences in the structure of the aquifer, as well as the conditions of water runoff and discharge in each segment. It is believed that these factors have indirectly affected the salinity of groundwater in different segments of HLFZ and have resulted in the current distribution pattern of hydrochemical compositions. It was found that the continuum deformation field derived from GPS velocities showed that the shear strain of the north and western of the HLFZ (About 25 nanostrain/yr) is normally larger than that of the southeast in HLFZ(about 0 ~ 8 nanostrain/yr, [43]). This larger deformation lead to a higher rupture degree of rocks in the north and western of the HLFZ, which also contributes to the exchange of groundwater between the deep with higher salinity and the shallow. Furthermore, As a strike-slip active fault, HFZ provides provides more opportunities for deep fluids to migrate towards the surface and to mix with shallow groundwater than a thrust fault [44]. Therefore, the groundwater with a higher salinity was widespread within the HFZ. Comparatively, as a large-scale thrust fault, the LFZ zone has experienced locking and strain accumulation for the last 100 years [4, 45]. The low level of fault activity and lower shear strain prevent the interchange of groundwater between the deep and the shallow [46].Earthquakes have usually occurred in fault zones that are highly stressed [47]. Rock fracture experiments in the laboratory [48] and studies on seismicity related to fluid extraction [49] have shown a correlation between stress and Gutenberg-Richter magnitude (M) - frequency (N) [50] that can be expressed as equation (1).
(1)lgN=a−bMwhere The lower the b value the higher the stress state. Thus, b value is one of the main parameters to estimate the relative stress level of a fault zone. Using the seismic data from the Ningxia Network, a distribution of the b value was calculated mapped by earthquakes in HLFZ with the magnitude >2.5 for the period from 1970 to 2017, illustrating 83.3% of all the sites with TDS>2000 mg/L (red marks) in the north and western HLFZ fall on the area of the b value <0.75 (Figure 7, 0.75 is a threshold in the region [51]); for the LFZ, all the sites are in the area of the higher b value. Additionally, small and moderate earthquakes have also been experienced more frequently in low b value areas since 1970. Due to Frequent earthquakes and higher stress, micro-fractures have developed the development that make it possible for meteoric water to infiltrate into the subsurface, where it interacts with deep fluids. Consequently, hot springs exposed in this area with higher temperatures and salinity and abundant gas-effusion (such as H7, H8, and L1) are direct indicators of fault activity [45, 52–55]. Furthermore, deep fluids also contribute to energy for seismic activity [56]. Although further research and exploration using long-term observations is required to obtain more evidence concerning the effect of seismic activity on hydrogeochemical characteristics in fault zones.Figure 7
The spatial distribution of b value in the HLFZ.


## 3.1. Hydrogeochemistry

According to the Schukalev classification method, all water samples were classified in eight chemical types: Na-Cl•SO4, Na•Ca-SO4, Na-CO3•HCO3, Mg•Ca-HCO3, Na-SO4•HCO3, Mg•Ca-SO4•HCO3, Na-SO4, Na•Mg•Ca-SO4•HCO3 and Na-Cl (Table 2). Concentrations of SO42-, HCO3- and Cl in H2, H3, L2 and L6 water samples show seasonal fluctuation. The content of HCO3- was affected by soil CO2 dissolving in the precipitation [36], the concentration of HCO3- at H2 and L2 was increased with rainfall in 2017. Additionally, the concentration of SO42- decreased with HCO3- increased. As Cl- concentration is directly correlated with TDS in arid areas, the rainy season led to a decrease in Cl- concentration as a result of lower salinity water at L6. From samples H5, H6, H2, L3, L5, and L2, the proportions of Ca2+ were greater than 40%, but those of Cl- were less than 40%. The concentrations of HCO3-, CO32- and Na+ are significantly higher in the water sample from site L4 than in other well samples. There are large seasonal fluctuations of concentrations of SO42- + Cl-, Mg2++Ca2+ and HCO32- + CO32- in the water samples from the sites H5, H6, L3 and L2, which can be attributed to more precipitation recharge. Ion concentrations in water samples collected from the sites H7, H8, L6, L1 and L5 varied little between different seasons, indicating that those sites are supposed to be the perfect locations for seismic monitoring.In light of chemical types, the sites can be divided into two groups, H1, H3, H4, H7, H8, L1 and L6 with higher TDS belong to the group 1 and H2, H5, H6, L2, L3 L4 and L5 with lower TDS are geared to group 2. The spatial variation in hydrochemical types along HLFZ (Figure2) showed a significant difference in group1 and group 2. The dominant cation and anion in group 1 are Na+ and SO42-, implying the strong leaching effect in the recharge process of groundwater and deeper circulation, longer residence time and higher water-rock equilibrium. In group 2, Mg2+, Ca2+ and HCO3- are the major iron, it represents the first reaction process between meteoric waters and rocks, reflecting shallower and younger hydrological cycle characteristics in this area [37]. In addition, the different chemical compositions in the two areas may also be caused by the different aquifer minerals.Figure 2
Voronoi diagram of hydrochemical types in HLFZ(Earthquake with M >3 since 1970 were selected).


## 3.2. Water Quality

According to the results of groundwater assessed by Fuzzy membership (Table4, Figure 3), groundwater quality in HLFZ was grouped into three grades. Water samples in group 1 were classified into grade V, which is only suitable for use in monitoring earthquakes or other industrial purposes. The primary contaminants found in group 1 are TDS, Na+, Cl- and SO42- which are indicative of serious salinization processes of groundwater. The water quality of H5 and H6 with a medium level of chemical components is of grade III, and can be used as drinking water or irrigation water. The water in H2 was excellent and was classified as grade I. Expect for H2, H5 and H6, all other samples in group 2 were classified as grade V, due to excessive NO3- or pH. Contamination of groundwater with NO3- was a result of anthropogenic impacts, in particular chemical fertilizers used in local agriculture. Water samples collected from the LFZ were alkaline water with a pH value >9, which is associated with a higher concentration of HCO3-. In general, the groundwater contamination in HLFZ is serious, with 78.6% of the sites that is not suitable for drinking and agricultural water.Table 4
The groundwater quality of each site.
The grade of water qualityThe main contaminantsH1VpH, TDS, Na+, cl-, SO42-Group 1H2INO3-Group 2H3VTDS, Na+, cl-, SO42-,NO3-Group 1H4VTDS, cl-, SO42-Group 1H5IIIpH,TDS, cl-, SO42-, NO3-Group 2H6IIIpH, TDS, cl-, SO42-, NO3-Group 2H7VTDS, Na+, cl-, SO42-, NO3-Group 1H8VpH,TDS, Na+, cl-, SO42-, NO3-Group 1L1VpH, Na, SO42-Group 1L2VpH, NO3-Group 2L3VpH, NO3-Group 2L4VpH, Na+Group 2L5VNO3-Group 2L6VpH, TDS, Na+, cl-, SO42-Group 1Figure 3
Voronoi diagram of water quality grade in HLFZ.


## 3.3. Water-Rock Interreaction

The mCa2+/Na+(molar ratio) vs. mHCO3-/Na+ can be used to determine reactions between groundwater and different rock types, the molar ratio of carbonates controlling end is close to 50 and 120, and the molar ratio of silicates controlling end is close to 0.35 ± 0.15 and 2 ± 1, respectively(Figure 4, [1, 24]). The result was shown in Figure 4. In H7 and L4, samples were primarily controlled by evaporate dissolution, while samples from Group 2 were controlled by carbonate weathering and silicate weathering. Other samples in Group 1, such as H1, H3, H4, H8, L6 and L1 were not controlled by the weathering of the three rocks, which implied that other factors contributed to their sources.Figure 4
Molar ratio bivariate plots of Na+-normalized Ca2+ vs. Na+-normalized HCO3- for the water samples were collected from HFZ and LFZ.It is thought that Na+ and K+ primarily formed through the weathering of evaporates or silicates; Ca2+ and Mg2+ are strongly correlated with evaporate dissolution, silicate weathering and carbonate weathering; Cl- and SO42- are associated with the dissolution of evaporates; and HCO3- is predominantly originated from carbonates [38]. In terms of the proportion coefficients of the major ions, most water samples from the Group 1 plot below the 1 : 1 line of Cl- + SO42- vs. HCO3 (Figure 5(a)) and deviate from the 1 : 1 line of Ca2++Mg2+ vs. HCO3-(Figure 5(b)); this demonstrates that evaporate dissolution dominates over carbonate weathering. As the dissolution of evaporate plays an essential role in the chemical composition of water, the ratio of Na++K+ vs. Cl- is near the 1 : 1 line. As shown in Figure 5(c)), samples in H1, H3 and H8 from the Group 1 are near the 1 : 1 line, indicating that evaporates (e.g., NaCl and KCl) are the primary sources of the Na++K+ and Cl− concentrations. The samples in L1, L4 and L6 are above the 1 : 1 line of Ca2++Mg2+ vs. HCO3- (Figure 5(b)), suggesting a strong dissolution of carbonate in these sites. Moreover, samples L1, L4, L6, H7 and H8 also above the 1 : 1 line of Ca2++Mg2+ vs. HCO3-(Figure 5(g)), which shows significant cation exchange between Ca2+ or Mg2+ and Na+ in these samples. Two chloro-alkaline indices CAI 1 and CAI 2 were defined by Schoeller [39] to determine the effect of cation exchange. When both indices are positive, reverse ionic exchange (Na-Ca) will occur in the groundwater, i.e., the Na2+ content in groundwater will alternately adhere to surface of rock particles with Ca2+. In Figure 5(h), expect H7, CAI 1 and CAI 2 were positive in high-salinity groundwater with TDS>2000 mg/L and negative in low-salinity groundwater, indicating forward ionic exchange (Ca-Na) usually decreased the hardness (Ca2+ content) of the groundwater, resulting in lower salinity water. The majority of the Group 1 are near the 1 : 1 line of Na+ vs. SO42- (Figure 5(e)), but deviate from the 1 : 1 line of Na+ vs. HCO3− (Figure 5(d)), indicating that the dissolution of sulfate minerals is not an primary source of Na+ in this region. Ca2+ and Mg2+ in most sites are weakly corrected with SO42-(Figure 5(f)) in the Group 1 also illustrates that the dissolution of sulfate minerals is not the major source of Ca2+ and Mg2+.Figure 5
Proportion coefficients of major ions in groundwater from the HFZ and LFZ zones for samples collected in the dry and rainy seasons.
(a)(b)(c)(d)(e)(f)(g)(h)This study examined some water samples from the Group 2 plot near the 1 : 1 line of Cl- + SO42− vs. HCO3- (Figure 5(a)) and Ca2++Mg2+ vs. HCO3- (Figure 5(b)), suggesting the dissolution of carbonates and evaporate control the composition of groundwater in this area. The samples in Group 2 fall below the 1 : 1 line of Na++K+ vs. Cl-(Figure 5(c)), demonstrating that the precipitation and dissolution of evaporate are not the primary sources of Na+ and K+. Most samples from this area plot near the 1 : 1 line of Na+ vs. HCO3- and Na+ vs. SO42- (Figure 5(d), Figure 5(e)), reflecting that the dissolution of sulfate minerals is a significant source of Na+ in this area. In the major of water samples from the Group 2, a strong correlation was found between Ca2++Mg2+ and SO42- and HCO3-, suggesting that the main origins of Ca2+, Mg2+, HCO3-, and SO42− in the groundwater are the dissolution of calcite (CaCO3), dolomite [CaMg (HCO3)2], and gypsum (CaSO4). In contrast to other well samples, the chloro-alkaline indices in L4 is negative, indicating that forward ionic exchange is an important factor of the sodium water (Na - HCO3) with low–salinity.In HFZ, the H1 well is located in the meizoseismal areas of the 1920 Haiyuan earthquake, the underlying aquifer is proluvial gravelly sand with good permeability and a deep burying depth (water table deeper than 19 m), and waters flows along the fault zone. Although the groundwater in H2 also originated from Quaternary fissure water, but there is lower TDS than H1, which indicates that the water-rock reaction of the surface rupture zone is relatively stronger. The aquifer of the H3 well has multiple layerswith a lithology of tertiary silty sand and fine sand. It is recharged by weathering infiltrating along the piedmont fissures and pores caused by atmospheric precipitation. The cretaceous sandstone contacts the fault zone at the H4 site, forming an artesian well where the higher concentrations of SO42- and Ca2+ in H4 are associated with gypsum. H7 and H8 exposed in the Cretaceous packsand and calcareous mudstone with abundant mirabilite are ascending springs, with white sediment of Na2SO4 around the spring, resulting in the Na+- rich water, and the TDS reached 22268~32566 mg/L and 73284~ 100486 mg/L, respectively. The H5 and H6 are ascending springs, and the spring waters is primarily stored in weathering fracture water of cretaceous layered rocks.In LFZ, L1 spring is exposed in the Cretaceous sandstone near the XGSF zone, owing to its dense joint distribution and higher rupture degree of rocks, the groundwater in this site is connected with the deep fluids of the fault zone along joint fissures, which results in a higher temperature and salinity than any other spring in LFZ. The spring water in L2, L3 and L5 is stored in the pores and fissures of the clastic rocks of the Lower Cretaceous system and is fed by the lateral inflow through the bedrock mountain system. L4 is exposed in the clastic rocks and minor carbonate intercalations of the lower Cretaceous near the XFZ zone, which has resulted in a higher concentration of HCO3- and CO32- than other samples. L6 is exposed in the Xiji graben basin in the western LFZ, and the aquifer is a pore confined aquifer of tertiary argillaceous, sandy conglomerates. The poor recharge in the basin, coupled with sulfate, chloride and Na+-rich sediments in the aquifer, has resulted in brackish water with higher TDS concentrations in L6.In general, the distribution of high sodium water in the HFZ area was also related to the dissolution of soluble salts in cretaceous and tertiary red bed aquifers, such as gypsum, mirabilite and magnesite. The higher Ca2+ contents of samples from the LFZ area reflect karst fissure water from Ordovician dolomite and limestone.


## 3.4. Contributions of Different Sources to Groundwater

A correlation coefficient between different groundwater components in HLFZ shows that there is a strong correlation between Na-, Cl-, and SO42- (Table 5), indicating the sources of these constituents are similar. The result of PCA was shown that a total of 75.79% of the variance was accounted for by two main factors (Table 6). The first factor shows high loading of chemical composition Na+, K+, Cl-, SO42-, NO3- and HCO3- with a maximum contribution of up to 55.98% of the total variance, which indicates mineral dissolution, cation exchange, deep fluid input and agricultural contamination. The HFZ is characterized by calcareous mudstone with a high CaCO3 content, which contains a considerable amount of HCO3- formed during dissolution. Additionally, CO2 is discharged from the fault zone and dissolves in water to form HCO3-, especially in H7 and H8. The Ningxia Geophysical Network reported that the concentration of CO2 was 0.12% and 3.62%, respectively. The variables Na+, K+ and Cl- have strong positive loadings (greater than 0.9), which may be caused by the dissolution of halite (e.g., KCl and NaCl) in the surrounding rock. Furthermore, reverse ionic exchange (Na-Ca) also contributes to the enrichment of Na+ in the groundwater in Group 1(Figure 6(h)). The concentration of Cl- in H7 and H8 exceeded 10 g/L, demonstrating brine input from the fault zone. Most samples contain higher amounts of SO42-, which is related to sulfate minerals, such as mirabilite and gypsum. While higher loading of NO3- suggested an anthropogenic source of agricultural chemical fertilizes.Table 5
Correlation coefficient of chemical composition of groundwater.
TDSNa+K+Mg2+Ca2+Cl-SO42-NO3-HCO3-pHTDS1.001.000.690.100.310.920.990.470.57-0.49Na+1.000.700.070.290.920.990.470.58-0.07K+1.000.190.540.910.570.500.59-0.43Mg2+1.000.610.230.050.15-0.15-0.67Ca2+1.000.470.240.200.09-0.69Cl-1.000.850.530.60-0.30SO42-1.000.430.53-0.01NO3-1.000.41-0.07HCO3-1.00-0.50pH1.00Table 6
Component matrix.
VariablePonent matrixFactor 1Factor 2Na+0.925-0.193K+0.880.10Mg2+0.290.87Ca2+0.50.75Cl-0.980.02SO42-0.86-0.222NO3-0.620.03HCO3-0.68-0.41Eigenvalue4.481.58%Total variance55.9819.81%Cumulativ evariance55.9875.79Bold values indicates high loadings.Figure 6
Plot ofδ18O and δD in groundwater from the HFZ and LFZ fault zones for samples collected in the dry and rainy seasons. GMWL, Global Meteoric Water Line [40]; LMWL, Northwestern China Local Meteoric Water Line.The second factor explained 19.81% of the total variance with high loading of the variable Ca2+ and Mg2+, which represents rock weathering. The correlation coefficient indicated that Ca2+ and Mg2+ were positively correlated with each other, but they were negatively correlated with pH value, demonstrating that Ca2+ and Mg2+ originated from the dissolution of carbonate rocks and that the solubility of carbonate rocks decreases with the increase of pH value. Furthermore, there is a correlation between pH and TDS, which suggests the dissolution of carbonate in HLFZ is not uncommon. As a result of possible silicate weathering to HCO3-, the correlation between HCO3- and Ca2+ and Mg2+ is poor.


## 3.5. Groundwater Recharge

Except for H7, most of the water samples plot near the Global Meteoric Water Line (GMWL) defined by Craig [40] and the Northwestern China Local Meteoric Water Line (LMWL: δD = 7.4 δ18O+ 7.2 [41]; Figure 6). It follows that local meteoric water is a significant factor in recharging groundwater along the HLFZ zone. For H7, there is a positive shift in δ18O, which reflects strong water-rock interaction related to high temperature. Well samples always have lower δ18O and δD values than spring water samples owing to slower circulation, less input from precipitation, and stronger evaporation. Some spring samples (H5, L2, and H2) plot above the LMWL and have shallow groundwater circulation depths along with rapid circulation speeds. Spring samples from the LFZ area have higher δ18O and δD values than those from the HFZ area, reflecting a greatly rainfall recharge. In contrast with LFZ, most samples from HFZ deviated from the local meteoric line, indicating intense water-rock interaction in this region. The dry season (Mar 2016) isotopic compositions, which are relatively depleted in heavy isotopes, are evidence of colder temperatures and limited evaporation during rainfall; this is consistent with other studies, which have demonstrated the meteoric origins of groundwater in arid and semi-arid regions of China [42].


## 3.6. The Geological and Seismic Factors

Along with stratum, lithology, geography and climatic factors, tectonic movement plays an influential role in the formation, movement and enrichment of groundwater. According to geological and microgeomorphological evidence, 5 major earthquakes with a magnitude of about 8.5 have occurred in HFZ since Holocene; The 1920 Haiyuan earthquake is the latest, and the active period of the quaternary in the western segment occurred earlier than in the eastern segment [18]. The occurrence of fault activities has controlled the depositional environment of the aquifer and has led to differences in the structure of the aquifer, as well as the conditions of water runoff and discharge in each segment. It is believed that these factors have indirectly affected the salinity of groundwater in different segments of HLFZ and have resulted in the current distribution pattern of hydrochemical compositions. It was found that the continuum deformation field derived from GPS velocities showed that the shear strain of the north and western of the HLFZ (About 25 nanostrain/yr) is normally larger than that of the southeast in HLFZ(about 0 ~ 8 nanostrain/yr, [43]). This larger deformation lead to a higher rupture degree of rocks in the north and western of the HLFZ, which also contributes to the exchange of groundwater between the deep with higher salinity and the shallow. Furthermore, As a strike-slip active fault, HFZ provides provides more opportunities for deep fluids to migrate towards the surface and to mix with shallow groundwater than a thrust fault [44]. Therefore, the groundwater with a higher salinity was widespread within the HFZ. Comparatively, as a large-scale thrust fault, the LFZ zone has experienced locking and strain accumulation for the last 100 years [4, 45]. The low level of fault activity and lower shear strain prevent the interchange of groundwater between the deep and the shallow [46].Earthquakes have usually occurred in fault zones that are highly stressed [47]. Rock fracture experiments in the laboratory [48] and studies on seismicity related to fluid extraction [49] have shown a correlation between stress and Gutenberg-Richter magnitude (M) - frequency (N) [50] that can be expressed as equation (1).
(1)lgN=a−bMwhere The lower the b value the higher the stress state. Thus, b value is one of the main parameters to estimate the relative stress level of a fault zone. Using the seismic data from the Ningxia Network, a distribution of the b value was calculated mapped by earthquakes in HLFZ with the magnitude >2.5 for the period from 1970 to 2017, illustrating 83.3% of all the sites with TDS>2000 mg/L (red marks) in the north and western HLFZ fall on the area of the b value <0.75 (Figure 7, 0.75 is a threshold in the region [51]); for the LFZ, all the sites are in the area of the higher b value. Additionally, small and moderate earthquakes have also been experienced more frequently in low b value areas since 1970. Due to Frequent earthquakes and higher stress, micro-fractures have developed the development that make it possible for meteoric water to infiltrate into the subsurface, where it interacts with deep fluids. Consequently, hot springs exposed in this area with higher temperatures and salinity and abundant gas-effusion (such as H7, H8, and L1) are direct indicators of fault activity [45, 52–55]. Furthermore, deep fluids also contribute to energy for seismic activity [56]. Although further research and exploration using long-term observations is required to obtain more evidence concerning the effect of seismic activity on hydrogeochemical characteristics in fault zones.Figure 7
The spatial distribution of b value in the HLFZ.


## 4. Conclusions

The purpose of this study was to investigate the hydrogeochemical groundwater environment along the HLFZ seismic belt, using observations of hydrochemical data from fourteen observation sites. Major ion and isotope (δ18O and δD) compositions were employed to reveal groundwater’s hydrochemical characteristics, water quality, origins and controlling factors in the HLFZ. Based on the chemical types, all sites can be classified into two groups: H1, H3, H4, H7, H8, L1 and L6 with higher TDS in the group 1 and H2, H5, H6, L2, L3, L4 and L5 with lower TDS in the group 2. There is a dominant Na+ and SO42- type water with in group 1, controlled by evaporate dissolution, slow dissolution rates of soluble minerals, reverse ionic exchange, stronger water-rock reaction and deep fluids. In contrast, the Ca2+ and Mg2+ types of water are dominant in the group 2, controlled by carbonate weathering, good discharge conditions and aquifer lithology. The water quality in the group 1 is poorly related to higher salinity, and the exceedance in NO3-and pH value affects inferior water quality in the group 2. The groundwater contamination in HLFZ is serious, with 78.6% of the sites classified as grade V, which means they are not suitable for drinking or agricultural water.The18O and δD compositions of groundwater from the HLFZ area indicate that local meteoric water contributes significantly to recharge of the groundwater. However, a positive shift in δ18O for sample H7 suggests strong water-rock interaction at this site.Consequently, due to the effect of tectonic movement, frequent activity, large deformation and a higher stress state (represented by a low b value) in the north and western of the HLFZ have a better condition for deep groundwater to migrate to the shallow than the part of southeast in HLFZ, which result in significant differences in Group 1 and Group 2.



---

*Source: 6894229-2022-05-17.xml*