Hydrogeochemical characteristics and genesis of Seismic observation wells in Shandong Province

Hydrogeochemical characteristics, controlling factors, and recharge sources of the seismic observation wells in Shandong Province were investigated by analyzing cation and anion concentrations, hydrogen and oxygen isotopes in well water. A total of 17 water samples in seismic observation wells were collected on April 25-29, 2018. The results show that temperatures of seismic observation wells were in the range of 14.8 to 52.1°C, and the values of δD and δ 18 O ranged from -72.4 ‰ to -37.9 ‰ and from -9.4 ‰ to -4.3 ‰ , respectively. Using C.A. ЩукаЛев’s classication method, the water samples of 17 seismic observation wells were classi ﬁ ed into 7 types: Cl·SO 4 -Na·Ca, SO 4 -Na, Cl-Na, HCO 3 -Na·Ca, HCO 3 -Mg·Na·Ca, HCO 3 -Na and HCO 3 -Mg·Ca·Na. The results indicate that the hydrogeochemical characteristics of 17 seismic observation wells, with a certain spatial distribution pattern, are affected by several factors, such as the tectonic, topography, stratigraphy, hydrology and meteorology. The analyses of ratio coecients, Schooller diagram, hydrogen and oxygen isotopes compositions, Giggenbach and Gibbs diagram suggest that the atmospheric precipitation is the main recharge source of 17 observation wells. The recharge sources of deep lateral runoff and sedimentation water, moreover, play a signicant role in some seismic observation wells. Combined with the amount of precipitation, the distance from recharge areas, the closure degree of observation wells and the stage of water-rock reactions, the development directions of faulting and topography control the directions of groundwater recharge, runoff and discharge, which make hydrogeochemical characteristics represent complex spatial distribution rules. run off, and discharge in the study area. Due to the amount of precipitation, the distance from recharge area, the closure degree of observation wells, and the stage of water-rock reaction, the spatial differences of hydrogeochemical characteristics are presented.


Introduction
The study of seismogenic mechanism shows that the material migration, energy release and stress evolution in the deep earth are accompanied by the abnormal variations of physical and chemical parameters of groundwater and the concentrations of escaping gas. Anomalies of underground uids responded sensitively to the medium changes in seismogenic and peripheral areas , being closely related to the regional tectonic activities (Che et al.,1997(Che et al., ,1999Liu et al.,1999;Zarrocaet al.,2012;Sano et al.,2017;Chen et al., 2015Chen et al., ,2018Kumar et al.,2020). The underground uid, bringing abundant information about the physical and chemical evolution in the earth's interior, re ecting the development process of earthquakes, and providing reliable earthquake precursors,is an effective and promising research medium for earthquake monitoring and prediction (Igarashi et al.,1995;Koizumi et al.,2004;Gresse et al.,2016;Menzies et al., 2016).It is hydrophysical observations of the water level, water temperature and ow rate of seismic observation wells that provide valuable basic data for quantitative research on the coupling relationship and spatio-temporal variation rules among the stress eld, energy eld and seepage eld (Wang et al., 1999;Montgomery et al., 2003;Ren et al.,2004). A large deal of useful information on the source, migration and lithosphere of ground uids could be recorded by hydrochemical compositions and environmental isotopes in seismic observed wells, providing the basis for the determination of geochemical characteristics, such as the types, genesis and water-rock reaction degrees of groundwater (Du et al.,2003;Song et al.,2006;Woith et al.,2013).
Not all anomalies of the underground uids are associated with earthquakes and tectonic activities, some factors, such as precipitation, recharge and discharge of the surface water, human activities, seismological observation conditions, and others, may also cause anomalies of the underground uids (Huang et al.,2005;Wang et al.,2010;Che et al.,2011;Zhao et al.,2014aZhao et al., , 2014a. Eliminating those interference anomalies scienti cally and effectively is crucial to identify earthquake precursors successfully and grasp earthquake situations correctly. It could improve the accuracy and reliability of the hydrodynamic analysis of well-aquifer systems to combination the hydrophysical and hydrochemical information (Zhang et al., 2014).
Although there are a good many hydrophysical background data of seismic observation wells in the study area, hydrogeochemical data are relatively scarce.
In this paper, 17 seismic observation wells, observed for decades, are selected. Based on data of the deep hydrogeological surveys and test results of ion components and hydrogen and oxygen isotopes, the hydrogeochemical types, ion composition characteristics, local controlling factors, and recharge sources of the well water were determined. It is of great practical signi cance to improve hydrogeochemical background information of the study area, let hydrogeochemistry play a more important role on the veri cation and tracking of earthquake precursors, and strengthen the effect of hydrogeochemistry in earthquake prevention and disaster reduction. The traditional research objects of groundwater, moreover, are mostly focus on shallow-circulation groundwater ow systems, while the research objects in this study largely pay close attention to deep-circulation wells with a maximum depth of 4000m, distributed along fault zones with frequent seismic activities, and sensitive to deep tectonic movement. The water volume, water level, water temperatures, hydrogeochemical characteristics and isotopes in deep wells contain a great deal of signi cant information from the deep underground, providing a window of the deep earth phenomena. Therefore, this study can reveal the deep tectonic environments and water-rock reactions, enrich the understanding of deep-circulating groundwater ow systems near the tectonic fault zones. It is of great theoretical and reference signi cance for comprehensive study of deep groundwater ow systems.

meteorology and hydrology
The study area is located in the east of Taihang Mountains in North China, with the Bohai Sea to the north and the Yellow Sea to the north and east(34°22 '-38°23'N,114°19' -122°43'E) (Fig.1). It has a monsoon climate of medium latitudes with an average annual precipitation of 550-950mm, an average annual temperature of about 12.5°C, and the same period of rain and heat. Under the in uence of land-sea location, topography, climate and other factors, the spatiotemporal variation of precipitation is signi cant. The precipitation in summer and autumn is concentrated, increasing from January to July and decreasing from July to December, with the maximum value in July and the minimum value in January. Strong convective weather occurs frequently in the central-south Shandong mountains. Surrounded by the sea on three sides and traversed by mountains from east to west, north part of Shandong Peninsula often affected by cold currents and heavy snows. The local landforms in study area have a great impact on the precipitations with medium-small scales (Xu and Gao,1962;Wang et al.,2018). Shandong Province lies in the Lower Reaches of the Yellow River and the eastern part of the North China Plain, where rivers are all rain-source types in monsoon area. Besides the Yellow River traversing from the east to the west and the Beijing-Hangzhou Canal going across from the north to the south, other small-medium-sized rivers cover the whole study area. According to topographic conditions, the rivers in study area can be classi ed into mountain-stream type and plain-slope-water type, with Mountain-stream rivers mainly distributed in mountains and hills of central-south Shandong and Shandong Peninsula, while plain-slope rivers mainly scattered in North-West Shandong plain (Feng et al.,2018).In terms of geomorphologic units, the groundwater in study area can be divided into three types: limestone karst water dispersed in the south of Yellow River and the east of Nansihu lake, piedmont inclined plain groundwater distributed in Shandong Peninsula and the Central Shandong inclined plains, and alluvial plain groundwater scattered in the alluvial plain of middle and lower reaches of the Yellow River to the west of Nansi lake and the north of Yellow River (Qu, et al.,2002).

Geological and seismic setting
From west to east, there lie Liaokao fault zone(LKFZ), Yishu fault zone(YSFZ) and Zhangjiakou-Bohai fault zone(ZBFZ) (Fig.1). LKFZ, a favorable place for earthquakes, is the boundary fault between the West Shandong fault block uplift and North China fault block depression, with a huge tectonic difference between the east and the west sides of it. YSFZ, consisted of Changyi-Dadian fault, Anqiu-Juxian fault, Yishui-Tangtou fault and Tangwu-Gegou fault, is a segment in Shandong Province of TanLu fault zone (TLFZ), with a structural pattern of "one horst sandwiched by two grabens". Tancheng 1668 Ms8.5 earthquake occurred on YSFZ. ZBFZ is a large-scale and NW trending active tectonic zone in North China seismotectonic region, which starts from the west of Zhangjiakou, goes southeast through Huailai, Nankou, Shunyi, Sanhe and Tianjin, passes through Bohai Sea to the southeast, and extends to Penglai and the North Yellow Sea in Yantai, Shandong province. The intersection of the TLFZ and ZBFZ experienced a series of strong and medium-small earthquakes (Su,2000;Wang et al,2000). Since historical earthquakes were recorded, 17 earthquakes with MS ≥ 6.0 have occurred in Shandong and its adjacent areas. Among them, 4 earthquakes occurred in LKFZ with the largest one being Heze 1937 Ms7.0 earthquake, 5 earthquakes occurred in YSFZ with the largest one being Tancheng 1668 Ms8.5 earthquake, and 5 earthquakes occurred in northern sea areas of Shandong Peninsula with the largest one Bohai 1969 Ms7.4 earthquake (Liu et al.,2019)

Seismic observation wells
In China, numerous subsurface, surface water well and spring parameters are being monitored through a large network of stations distributed in China sponsored by China Earthquake Administration (CEA). In this paper, 17 seismic observation wells in Shandong Province, mainly along LKFZ, YSFZ and ZBFZ, were selected, which data are shown in Fig.1

Methods
A total of 17 water samples were collected from 17 seismic observation wells respectively in Shandong Province in April 2018. Water temperatures, as unstable parameters, were measured in the field with a BENETECH-GM321 hand-held infrared thermometer that were calibrated prior to sampling with an error of ±1.5%. The water samples were stored in 50ml high density polyethylene bottles .The sample bottles were washed with tap water before sampling, soaked in ultrapure water for 24 hours, and then ultrasonically cleaned and dried. Before eld collection, use a syringe with a 0.45μm microporous membrane to lter 3 times to eliminate the in uence of microorganisms and impurities, and use observation wells water in the sampling eld to repeatedly rinse the water sample bottles more than 3 times. Sealed and protected from light, ensure that there were no bubbles in sampling bottles and the samples are sent to the laboratory within one week (Chen et al., 2014). For each water sample, at least 3 bottles were collected as parallel samples. For cation analysis, reagent-grade HNO 3 with molar concentrations up to 14M was added to the sample collected at each well to bring the pH level to below 1 to 2. The pH, electrical conductivity and ion concentration in the water samples of seismic observation wells were all measured at Key Laboratory of Earthquake Forecasting, China Earthquake Administration. The pH and electrical conductivity were measured by a PH-200 pH meter with an error of ±2% and a COM-100 Conductivity Meter with an error of ±2%, respectively. The concentrations of cations (K + , Na + , Mg 2+ and Ca 2+ ) and anions (F -,Br -,Cl -, NO 3 -,and SO 4 2-) in the water samples were measured with a Dionex ICS-900 ion chromatography system (detection limit 0.1mgL -1 ). The CO 3 2and HCO 3 concentrations were measured by standard titration procedures with a ZDJ-100 potentiometric titrator. For calibrating the chromatography, standard samples were measured before and after measuring each batch of water samples, with the deviation of the measurements within ±2% (Chen et al., 2015). The data were evaluated by the ion balance (Woith et al. 2013
17 water samples were plotted in blocks A, B and C ( Figure 2). The water samples of Lu no.15Well (ZZ),Lu no.33 Well (MY),Lu no.14 Well (JN),Yinan Well (YN),Yishui Well (YS),Lu no.07 Well (QX) and Dezhou Well (DZ) were plotted in block A (Table1, Fig.2). ZZ, MY, JN, YN and YS Well, with the proportion of HCO 3 ranging from 38% to 80%, are located in the mountains and piedmont inclined plains on both sides of YSFZ, where limestone, dolomite, sandstone and others are widely distributed, and tectonic activities have been strong since the late Pleistocene (Geng et al., 2003;Zhou et al.,2019). QX well, with the proportion of HCO 3 being 43%, is located in the Mountain stream zone of Shandong Peninsula to the east of YSFZ, where granite, quartzite and metamorphic rocks are scattered. DZ well, with the proportion of HCO 3 up to 84%, is located at the northern end of LKFZ, and belongs to North China Plain seismotectonic zone. Controlled by faults, the groundwater ow systems are well open in the place with well-developed fractures around above observation wells. Meanwhile, affected by the oceanic monsoon,a local underground water system with fast and short ow is developed. The main chemical components in above observation wells were HCO 3 − , Na + , Ca 2+ and Mg 2+ ,and the proportion of HCO 3 was even higher than 80%,indicating that chemical types of above observation wells (HCO3-Na,HCO3-Na·Ca,HCO3-Mg·Na·Ca,HCO3-Mg·Ca·Na) should mainly be attributed to groundwater-rock interactions ( Table 2).
The  Figure 2). The observation data (chemical titration method) in the past 20 years show that the ion contents of LC well (Cl·SO4-Na·Ca) are relatively high. Located in the alluvial plain of the lower Yellow River on east side of northern section of LKFZ, LC Well is a thermal reservoir, and the observed layer, 828-928m thick, is an Ordovician limestone dissolution zone ( Table 1). As a deep and large fault in the upper mantle, LKFZ developed many regional concealed faults with a good water conductivity. They not only communicate the upwelling of deep heat sources, but also serve as the main channels for deep circulation of the groundwater (Wang et al., 2008;Sun et al.,2013 , Cland Na + , and the proportion of Cland Na + are nearly 50% , indicating that the chemical types of these wells(Cl·SO 4 -Na·Ca, SO 4 -Na, Cl-Na) should mainly be attributed to groundwater-rock interactions between the underlying limestone, granite and sandstone and the groundwater. At the same time, it was related to the cation exchange adsorption in the silt deposit. The chemical type of Cl-Na may be related to near-shore submarine groundwater and marine deposits, or continental salination groundwater.
The waters of Lu no.03 Well (GR) and Lu no.26 Well (LL), with siltstone aquifers, were plotted in block C(Table1, Fi.2), which main chemical components were Cland Na + , and the proportions of Cland Na + are nearly 50%, indicating that the chemical types of GR and LL(Cl-Na) should mainly be attributed to continental groundwater salination, which should be speculated from the higher TDS values of two data points.
The horizontal hydrogeochemical zonality is mainly manifested in the following aspects: Affected by the southeast marine monsoon and local high-altitude topographies, observation wells (ZZ, MY,JN, YN, YS, QX) , along both sides of YSFZ, in central and southern mountains, piedmont inclined plains of Shandong Province and hills of Shandong Peninsula, are characterized by plentiful precipitations. In addition, because of the strong incised topographies, the well-open geological tectonics, and the steep slopes, local groundwater ow systems with rapid and short water ow were developed. Shallow buried or discharged in the form of spring, the groundwater here belongs to bicarbonate water with low TDS, with Na + , Mg 2+ and Ca 2+ as the main cations, HCO 3 as the main anions, and the proportion of HCO 3 as high as 88% (tables 1, 2). Transients sulfate groundwater are distributed in West Shandong plain, with Na + , and Ca 2+ as the main cations, Cland SO 4 2as the main anions (HZ, LC). The seismic observation wells (DM, GR, DS, LL, SH, YC, CY) in the Bohai depression and the alluvial plain of middle and lower reaches of the Yellow River in the west of Nansihu lake and the north of the Yellow River are weakly affected by the southeast marine monsoon, with less precipitations, far away from the recharge areas, and low and at terrains. The groundwater ow system with the slow and long ow is developed, and most of them are chloride waters with high TDS values, with Na + as the main cation and Clas the main anion. In addition, the lower the altitudes in the lower Yellow River are, the smaller the hydraulic differences would be and the slower the groundwater circulation may be, that is, the TDS values of North Shandong plain in the north are higher than those of the West Shandong Plain in the west . the in uence of deep formation water dominated by highly mineralized CaCl 2 and MgCl 2 water. A higher content of Clshould be attributed to the greater solubility of chloride salts, which are not easily absorbed by the surface of the formation and could be enriched in groundwater (Hu et al., 2015). NO 3 ions are mostly related to human activities (Jalali, 2006), which indicates that, except for well LC, MY, CY and DZ (no NO 3 -), other observation wells are disturbed by human activities to a certain extent. Except for MY, YN, and YS well, Na + contents of other observation wells were very high, while K + contents were greatly low (Table 2), which may be due to the sealing conditions of some observation wells were poorly, and NaCl in the overlying argillaceous strips was leached into well water, while Na + were not easy to be crystallized out and could be stored in water for a long time (QX and RC).K + , moreover, easily enters the lattice of secondary minerals that are insoluble in water, and K + is more easily absorbed by soil colloids than Na + , so the concentrations of K + in observed wells and ssure underground uids become lower (Qian and Ma, 2012). The Na + contents in MY, YN, YS, LC, ZZ and QX well were lower than or equivalent to the Ca 2+ contents, and the Na + contents in other seismic observation wells are much higher than the Ca 2+ contents, which may be due to the water-rock reactions of groundwater in igneous and carbonate rocks, and the hydrolyses of sandstone could be used as the recharge sources of Na + , moreover, Na + contents may be increased by cation exchange between Ca 2+ in water and Na + in rocks (Table 1).
Among 17 seismic observation wells, 11 wells contain Li + (LC, HZ, DM, JN, CY, QX, RC, GR, LL, SH and YC)( Table 2),which may be attributed to volcanic activities and magmatic rocks (Shen et al., 1999). The Li + contents in GR and LL well are as high as 1.92 mg/L and 1.44 mg/L, which may be due to the fact that two observation wells are located in the groundwater discharge areas where Li + is gradually enriched. Br is a halogen element, mainly enriched in shale (4mgL -1 ) and limestone (6.2mgL -1 ) of marine deposits (Shen et al., 1999). Brwas found in LC, HZ, DM, QX, RC, SH and YC well, the contents of RC and SH well were as high as 2.81 mgL -1 , followed by DM and YC well, and the contents of QX well was only 0.09 mgL -1 ,which should be related to the reactions between groundwater and limestone, marine sedimentary and igneous rocks. The Fcontents of GR and LL well are 254.78 mgL -1 and 106.76 mgL -1 , and that of JN well is 3.39 mgL -1 , which may be attributed to high uorine contents in wall rocks, slow water alternation, su cient water-rock interactions and human pollution.

Ion proportional coe cients
The content ratio coe cients of various components can not only judge the genesis , source and formation process of chemical compositions (Shen et al., 1999), but also effectively eliminate the in uence of environmental changes, and extract seismic hydrochemical anomalies more intuitively (Sun et al., 2016).
γNa/γC1 is an important indicator of formation sealing degree, metamorphism of formation water and groundwater activity (Shen et al., 1999). The γNa/γC1 coe cients of RC and GR observation wells are 0.83 and 0.87, respectively, and there may be seawater mixing (about 0.85). The γNa/γC1 values of LC, YN, YS, CY, DS, LL, and SH well are less than 0.85, and there may be marine sediment water that has undergone cation exchange. HZ, DM, ZZ, MY, JN, QX, DZ, YC well have a value of γNa/γC1 greater than 1.0, which should be a general leaching groundwater.
100×γSO 4 /γC1 indicating the degree of desulfurization, the smaller the desulfurization coe cient is, the more closed the formation is and the stronger the reduction environment is (Shen et al., 1999). The desulfurization coe cients of JN, CY and LL are 0, and the sealing degree is the best. DM and GR wells have low desulfurization coe cients, closed formation and strong reduction environment. DZ, MY, YS, YN and QX well have larger desulfurization coe cients, a higher aquifer opening degree and a faster groundwater circulation. Ca 2+ , Mg 2+ in limestone and Na + , K + in silicate are leached into observation wells, and the main hydrochemical type is bicarbonate type.
γMg/γCa coe cients can be used to judge whether groundwater comes from limestone or dolomite (Shen et al., 1999). In the wells with limestone aquifer, the γMg/γCa coe cients of LC, Hz and JN well are less than 1, indicating that the groundwater of these three wells comes from limestone aquifer.
HZ, LC and DM wells are all hot spring wells, located on LKFZ ( has undergone considerable desalination. The concentrations of Cland Na + and γC1/γHCO 3 +γCO 3 coe cients are relatively high in GR and LL well, while contents of HCO 3 -, γNa/γC1 coe cients, and 100×γSO4/γC1 coe cients are relatively lower, indicating that GR and LL well are located in the groundwater discharge areas, the in uence of atmospheric precipitation has been weakened, and the in uence of deep formation water gradually has been strengthened.

Schoeller Diagram
It is obviously inconsistent for the variations of relative contents of main ions in seismic observation wells. The relative content changes of main ions are shown in Fig.3.
It can be seen from Fig.3  . On the other hand, it may be affected by CaCl 2 water and MgCl 2 water with high mineralized in deep formation. It can be seen from Figure  Combined with ion milligram equivalent ratios, it is believed that the development direction and geomorphology of the fault zone control the direction of groundwater recharge, run off, and discharge in the study area. Due to the amount of precipitation, the distance from recharge area, the closure degree of observation wells, and the stage of water-rock reaction, the spatial differences of hydrogeochemical characteristics are presented.

Hydrogen and oxygen stable isotopes
Hydrogen and oxygen stable isotope ( 2 H(D), 18 O) ratio method is the most effective tracing method to study the origin and migration of water and other uids in the crust. Many scholars have applied stable hydrogen and oxygen isotopes to groundwater research (Minisale,2004(Minisale, ,2017King et al., 2004;Yang et al., 2012;Zhang et al., 2013;Zhao et al., 2017).
It can be seen from gure 4 that δ largest. It shows that whether it is carbonate or siltstone aquifers, as long as the groundwater is far away from recharge areas and buried deeply, the 18 O drifts would be greatly obvious and even siltstone may be more obvious than carbonate .
The value of δD will decrease with the increase of groundwater recharge depth . Excluding seasonal effects and continental effects, it is speculated that, among the 17 seismic observation wells in the study area, the deepest source of groundwater recharge is DZ well and the shallowest source is

4. 4 Giggenbach diagram
Generally, Na-K-Mg triangle diagram (Giggenbach,1988) is used to judge whether the water-rock reaction is in equilibrium. The three-terminal elements of the triangle are Na/1000,K/100 and √Mg(mgL -1 ).The water-rock balance of seismic observation wells in study area is shown in Fig.5. It can be seen from Fig.5 that the waters in ZZ, QX, YS and YN well plot close to the Mg-comer. The water in MY well is somewhat far from the Mg-corner, indicating the time of waterrock interactions become longer. The waters in HZ,RC and LC well also fall into immature waters area, but close to partialy equilibrium waters area, indicating the recharge paths and circulation time are longer. Above data points may then be taken to correspond to those of "immature" waters generally, indicating removal of some of the Na and K, with Mg remaining unaffected by the deposition of secondary minerals. A number of additional processes such as admixture of immature waters with their generally high Mg-contents will also lead to deviations from the full equilibrium curve and close to the Mg-corner (Giggenbach,1988).These waters are still in the primary stage of water-rock reactions, that is, the dissolution is still in progress, and the groundwater circulation is relatively fast. The waters in DS,DM,JN,SH,DZ,YC,GR and LL well fall into partially equilibrated area, indicating recharge sources are from not only the atmospheric precipitation, but from the deep formation water. These wells may be in the stage of partially water-rock equilibrium in stagnant reservoirs of formation waters with low water-circulation, especially the waters in GR and LL well, water-rock reactions nearly reach the fully equilibrium curve, indicating they are mainly recharged by deep formation water.

Gibbs diagram
A series of hydrochemical interactions may occur between groundwater and surrounding rocks during the process of groundwater movement, such as leaching, concentration, and others, which will lead to various changes in chemical compositions and TDS values of groundwater (Zhang Renli et al., 2011;Fetter et al., 2011). Gibbs diagram is a signi cant method to analyze the main factors controlling the evolution process of groundwater, such as the evaporation and concentration, rock weathering and precipitation. TDS values of well waters in the study area range from 102mg·L -1 to 19750mg·L -1 , cation contents ratios of Na + /(Na + +Ca 2+ ) are from 0.06 to 0.99, and anion contents ratios of Cl -/(Cl -+ HCO 3 -) are from 0.02 to 0.99. It can be seen from Fig.6 that main control factors in the study area present obvious spatial distribution rules. The control types of rock weathering include ZZ, MY, JN, YN, YS, QX and DZ well located in mountains, uplifts and piedmont slopes controlled by fault zones and in the recharge-runoff areas of groundwater, with bicarbonate water and low TDS contents. The data points controlled by evaporation-concentration include HZ, DM, LC, GR, DS, LL, SH, YC, CY and RC Well, located in the depression areas controlled by concealed faults and in the evaporation-discharge areas. The hydrochemical types gradually transit from SO 4 ·Cl-Na and Cl·SO 4 -Na·Ca types to Cl-Na type with high TDS values.
To sum up, in the mountainous hills and piedmont inclined plains, the chemical components in groundwater mainly come from the dissolution of weathering minerals by leaching, with fast and shallow groundwater circulations, low TDS values, and bicarbonate waters. During the process of the runoff, with hydraulic differences smaller, the groundwater gradually transited from recharge-runoff areas to discharge areas. The closer the discharge area is, the ner the aquifer particles are, and the slower the groundwater ow is. Moreover, with the weakening of the in uence of atmospheric precipitation, the in uence of deep runoff and sedimentary water increases, and the TDS values increase. At this time, the hydrochemical type gradually changes from sulfate water to chloride water. In the deep groundwater ow system, moreover, fresh water and salt water appear alternately in the same observation well, which may be affected by the seawater intrusion (CY well).
(2) The seismic observation wells, bicarbonate water with low salinity, lie in South-central Shandong mountains controlled by YSFZ, local uplifts of the alluvial plains in middle and lower reaches of the Yellow River and the East Shandong hills. The data points, chloride water with high salinity, are located in West Shandong plain controlled by LKFZ. In data points located in North Shandong plain and coastal plain, Na + is the main cations, while Clis the main anions in seismic observation wells.
(3) The results of ratio coe cients, Schooller diagram, hydrogen and oxygen isotopes compositions, Giggenbach and Gibbs diagram suggest that atmospheric precipitation is the main recharge source. In addition, there are recharge sources of the deep lateral runoff and sedimentation water. It is fault development directions and topographies that control the directions of groundwater recharge, runoff and discharge in the study area. It shows hydrogeochemistry spatial differences because of the amount of precipitations, the distance from recharge areas, the closure degree of observation wells and the stages of water-rock reactions. Whether it is carbonate or sandstone groundwater, as long as it is far away from the recharge areas and buried deeply, the 18 O drift is very obvious, even to the extent that, the 18 O drift in sandstone rocks may be more obvious than that in carbonate rocks.
(4) The following understandings of the deep circulation of the groundwater ow systems and water rock interaction processes near earthquake fault zones are obtained: In mountainous hills and piedmont inclined plains, the groundwater is shallowly buried or discharged in terms of spring, with leaching as the leading factor. The chemical components of groundwater are mainly from the dissolution of minerals in water-rock reactions, and the water circulations are relatively fast. The main chemical types are bicarbonate with low TDS values. During the process of the runoff, groundwater gradually transitions from recharge-runoff areas to the discharge areas controlled by tectonic depressions. The closer the discharge area is, the ner the aquifer particles are, and the slower the groundwater ow is. Moreover, with the weakening of the in uence of atmospheric precipitation, the in uence of deep runoff and sedimentary water increases, and the TDS values increase. At this time, the chemical type gradually changes from sulfate water to chloride water.

Declarations
Availability of data and materials Not applicable A total of 17 water samples were collected from 17 seismic observation wells respectively in Shandong Province in April 2018. Water temperatures, as unstable parameters, were measured in the field with a BENETECH-GM321 hand-held infrared thermometer that were calibrated prior to sampling with an error of ±1.5%. The water samples were stored in 50ml high density polyethylene bottles .The sample bottles were washed with tap water before sampling, soaked in ultrapure water for 24 hours, and then ultrasonically cleaned and dried. Before eld collection, use a syringe with a 0.45μm microporous membrane to lter and ion concentration in the water samples of seismic observation wells were all measured at Key Laboratory of Earthquake Forecasting, China Earthquake Administration. The pH and electrical conductivity were measured by a PH-200 pH meter with an error of ±2% and a COM-100 Conductivity Meter with an error of ±2%, respectively. The concentrations of cations (K+, Na+, Mg2+ and Ca2+) and anions  in the water samples were measured with a Dionex ICS-900 ion chromatography system (detection limit 0.1mgL-1). The CO32-and HCO3-concentrations were measured by standard titration procedures with a ZDJ-100 potentiometric titrator. For calibrating the chromatography, standard samples were measured before and after measuring each batch of water samples, with the deviation of the measurements within ±2% (Chen et al., 2015). The data were evaluated by the ion balance (Woith et al. 2013). The hydrogen and oxygen isotopes of the well water samples were measured with a Mat-253 gas isotope mass spectrometer in Analytical Laboratory Beijing Research Institute of Uranium Geology (ALBRIUG). δD values were determined by zinc reduction of hydrogen isotopes in water (DZ/T 0184. ,and δ18O values were measured by carbon dioxide-water balance method of oxygen isotope in natural water (DZ/T 0184.21-1997). The values of 18O and δD used V-SMOW as a standard with the accuracy of ± 0.1 ‰ and ± 0.5 ‰, respectively.

Competing interests Not applicable
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper Data are cited from Geng et al., 2003;Wang et al., 2019).