5.1 Chemical types of seismic observation wells
Using C.A. ЩукаЛев’s classification method, the water samples of 17 seismic observation wells in Shandong Province were classified into 7 types: Cl·SO4-Na·Ca, SO4-Na,Cl-Na, HCO3-Na·Ca, HCO3-Mg·Na·Ca, HCO3-Na and HCO3-Mg·Ca·Na (Fig.2, Table 2).
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 HCO3- 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 HCO3- 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 HCO3- up to 84%, is located at the northern end of LKFZ, and belongs to North China Plain seismotectonic zone. Controlled by faults, the groundwater flow 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 flow is developed. The main chemical components in above observation wells were HCO3−, Na+, Ca2+ and Mg2+,and the proportion of HCO3- 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 water samples of Lu no.01 Well(LC), Lu no.27 Well(HZ), Dongming Well(DM), Lu no.02 Well (CY), Lu no.32 Well(RC), Dashan Well(DS), Lu no.09 Well(SH), Lu no.04 Well(YC) were plotted in block B (Table1, 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). The chemical type of HZ(SO4-Na) should be a transitional type of continental salination. HZ well is located at the intersection of LKFZ and Heze fault, active in the late Quaternary, where 1937 Heze M7.0 earthquake and Heze 1983 Ms5.9 earthquake occurred. An Ordovician limestone aquifer at the depth of 1138-2000m in Heze well is the main observed layer. The DM, DS, SH and YC well are located in West Shandong plain and North Shandong plain controlled by LKFZ. Because of deep well depths, gentle terrains and slow groundwater alternation, Cl-Na type waters with high TDS are formed, with Na+ as the main cation and Cl- as the main anions. The TDS values of North Shandong plain, moreover, are higher than that of West Shandong plain, that is, the lower the terrains are, the higher the TDS values are. Located in the west side of Changyi-Dadian fault, CY well, is not affected by rainfall and surface water through seismic observation data for many years, with Na+ as the main cations and Cl- as the main anions. The RC well lies in low-lying areas in the east of Lidao-haixitou fault in Zhangjiakou-Bohai seismotectonic zone, with Na+ as the main cations and Cl- as the main anions. Long-term seismic observation shows that concentrated rainfall, permeating through loose Quaternary overburden, causes a short-term sudden rise in water level (Wang et al., 2019). The main chemical components of LC, HZ, DM, CY, RC, DS, SH, and YC well were SO42-, Cl- and Na+, and the proportion of Cl- and Na+ are nearly 50% , indicating that the chemical types of these wells(Cl·SO4-Na·Ca, SO4-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 Cl- and Na+, and the proportions of Cl- and 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.
5.2 Hydrogeochemical zonality
Affected by the factors of tectonic, topography, stratigraphy, hydrology and meteorology and others, hydrogeochemical characteristics of 17 seismic observation wells in Shandong Province show obvious horizontal and vertical hydrogeochemical zonality (Fig. 1, Fig. 2, Tables 1, 2, 3).
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 flow systems with rapid and short water flow were developed. Shallow buried or discharged in the form of spring, the groundwater here belongs to bicarbonate water with low TDS, with Na+, Mg2+ and Ca2+ as the main cations, HCO3- as the main anions, and the proportion of HCO3- as high as 88% (tables 1, 2). Transients sulfate groundwater are distributed in West Shandong plain, with Na+, and Ca2+ as the main cations, Cl- and SO42- as 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 flat terrains. The groundwater flow system with the slow and long flow is developed, and most of them are chloride waters with high TDS values, with Na+ as the main cation and Cl- as 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 seismic observation wells, under the control of the same fault and the similar deep groundwater flow systems, show the vertical hydrogeochemical zonality. For example, HZ well with a depth of 2000m and DM well with a depth of 4000m belong to LKFZ and be provided with similar hydrogeological conditions simultaneously. Due to the different well depths, Hz and DM well show obvious vertical hydrochemical zonality with medium TDS sulfate water gradually transited to high TDS chloride water from Hz to DM well.
5.3 Chemical characteristics of groundwater
From the Central-South Shandong mountains and the piedmont plains of Shandong Peninsula on both sides of YSFZ to the alluvial plain of middle and lower reaches of the Yellow River and coastal plains, the anion compositions mainly evolved from HCO3- to SO42- and Cl-, while the cations gradually evolved from Mg2+, Na+ and Ca2+ to Na+(Table 1,Table 2). The contents of HCO3- in observation wells are mostly relatively high, especially in mountains and piedmont plains of central-south Shandong on the west side of the YSFZ, where limestone, dolomite and others provide abundant HCO3-. Most observation wells in the West Shandong plain and the North Shandong plain are geothermal wells with high temperatures and large burial depths. The geothermal water reacts with karst fissure heat reservoirs, which provide abundant SO42-, HCO3-, Ca2+, Mg2+ plasma for geothermal wells. The high contents of Ca2+ and Mg2+ may be under the influence of deep formation water dominated by highly mineralized CaCl2 and MgCl2 water. A higher content of Cl- should 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). NO3- ions are mostly related to human activities (Jalali, 2006), which indicates that, except for well LC, MY, CY and DZ (no NO3-), 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 fissure underground fluids 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 Ca2+contents, and the Na+ contents in other seismic observation wells are much higher than the Ca2+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 Ca2+ 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). Br- was 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 F- contents 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 fluorine contents in wall rocks, slow water alternation, sufficient water-rock interactions and human pollution.
5.4 Genesis
5.4.1 Ion proportional coefficients
The content ratio coefficients 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 influence 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 coefficients 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×γSO4/γC1 indicating the degree of desulfurization, the smaller the desulfurization coefficient is, the more closed the formation is and the stronger the reduction environment is (Shen et al., 1999). The desulfurization coefficients of JN, CY and LL are 0, and the sealing degree is the best. DM and GR wells have low desulfurization coefficients, closed formation and strong reduction environment. DZ, MY, YS, YN and QX well have larger desulfurization coefficients, a higher aquifer opening degree and a faster groundwater circulation. Ca2+, Mg2+ in limestone and Na+, K+ in silicate are leached into observation wells, and the main hydrochemical type is bicarbonate type.
γMg/γCa coefficients 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 coefficients 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 (Table 1). The ratio coefficients of strong acid radicals and weak acid radicals are 4.10, 16.74 and 3.69, respectively. Anions in groundwater gradually evolved from SO42- to Cl-, and cations gradually evolved from Ca2+ to Na+. The values of γNa/γCl in groundwater of three wells are 2.40, 0.68 and 1.14, respectively, and the values of Cl/Br are 629.96, 4822.54 and 839.55, respectively, which are quite different from the values of γNa/γCl (0.85) and Cl/Br (300) of the seawater, indicating that LC, HZ and DM well are leached water not affected by the ocean. The γMg/γCa coefficients of LC, HZ and DM well are 0.20,0.35 and 2.29, which are all lower than those of seawater(5.5), indicating that the original sedimentary water has undergone considerable desalination. The concentrations of Cl- and Na+ and γC1/γHCO3+γCO3 coefficients are relatively high in GR and LL well, while contents of HCO3-, γNa/γC1 coefficients, and 100×γSO4/γC1 coefficients are relatively lower, indicating that GR and LL well are located in the groundwater discharge areas, the influence of atmospheric precipitation has been weakened, and the influence of deep formation water gradually has been strengthened.
5.4.2 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 that the variation trends of the main ions of HCO3- +CO32-, Cl-, Mg2+, and Na++K+ in sulfate-chloride observation wells are consistent, but SO42- and Ca2+ are different. Sulfate-chloride observation wells are located (DM, GR, DS, LL, SH, YC, CY) in West Shandong Plain and North Shandong Plain controlled by LKFZ and near-shore areas controlled by ZBFZ. The water-rock reactions and hydrolytic dissociation of sandstone in observed aquifers of carbonate rocks could be used as the recharge sources of HCO3-+CO32-, Cl-, Mg2+and Na+(table 2). The relative contents of SO42- and Ca2+ show obvious hydrogeochemical zoning. It can be preliminarily judged that the SO42- of LC and HZ have similar sources. The sources of Ca2+ ions of GR, LL and LC are similar, and the sources of Ca2+ ions of DS and SH are similar. It is inferred that, on the one hand, groundwater reacts with Cambrian and Ordovician limestone to generate Ca2+, Mg2+ and SO42-. On the other hand, it may be affected by CaCl2 water and MgCl2 water with high mineralized in deep formation. It can be seen from Figure 3 that the variation trends of the major elements of SO42-, HCO3- +CO32- and Cl- in the observation wells of bicarbonate are consistent, while the change trends of cations are different. The cations of QX well in hills of Shandong Peninsula, DZ well in North Shandong Plain, ZZ well in central-south Shandong mountains and hills and JN well in TLFZ have different trends, indicating that their different sources. The cation trends of MY, YN and YS on both sides of TLFZ are consistent, indicating similar sources.
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.
5.4.3 Hydrogen and oxygen stable isotopes
Hydrogen and oxygen stable isotope (2H(D),18O) ratio method is the most effective tracing method to study the origin and migration of water and other fluids in the crust. Many scholars have applied stable hydrogen and oxygen isotopes to groundwater research (Minisale,2004,2017; King et al., 2004; Yang et al., 2012; Zhang et al., 2013; Zhao et al., 2017).
It can be seen from figure 4 that δ18O values of 17 seismic observation wells in the study area range from -9.4 ‰ to - 4.3 ‰, and δD values are from -72.4 ‰ to -37.9 ‰. The δD-δ18O values are located near the global atmospheric water line (GMWL) and the local atmospheric water line (LMWL), indicating that observation wells are from the infiltration recharge of atmospheric prec ipitation. Because rocks are rich in oxygen and lack of hydrogen (Skelton A, et al., 2014), water rock reaction mainly causes the positive shifts of δ18O values in groundwater, while δD values are hardly affected. The greater the positive drifts of δ18O values are, the stronger the water-rock reactions are. Figure 4 shows that the δ18O values have different positive drifts, which indicate that water-rock reactions occurred to different extent due to the distance from the recharge areas and the length of the groundwater circulation time. The time of water-rock reactions of QX and LL well become less, and the degrees of 18O drifts are lower, which indicates that their recharge paths are shortest. The large18O drift of LC well is not only because its high water-temperature (52.1°C), but also due to the long recharge paths and sufficient water-rock reactions. The recharge area of LC is located in the Tailu uplift, that is, the central Shandong mountains (Research group of geochemical background field in north China, State Seismological Bureau,1989). Atmospheric precipitation is recharged by the long-distance and deep circulation (with a well depth of 2337.72m), and has strong water-rock interactions with carbonate rocks and other water bearing medium, which makes 18O extremely enriched. The seismic observation wells of SH, YN, JN, Hz, YS well with Ordovician limestone aquifers took place large 18O drifts. The observation aquifer of GR well with Tertiary siltstone, the 18O drift is the 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 18O 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 (Chen, 1996). 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 SH well. In addition, the δD values are clustered. The δD values of ZZ, YC, LC, GR, YC and DS well are similar, and the recharge depths are roughly the same, slightly shallower than DZ well. The δD values of YN, MY, JN, YS, QX, HZ, CY, DM well are similar, the recharge depths are roughly same, and groundwater recharge depths of RC and SH well are the shallowest.
d value (d=δD-8δ18O, Ma, 2005) indicates the isotope deviation degree of a water sample from modern atmospheric precipitation. d values of seismic observation wells in the study area are mostly between -6.7‰ -4.5‰, with an average value of -0.69‰(d<10‰), indicating that they are normal atmospheric precipitation under current climatic conditions.
5. 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 water-rock 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.
5.4.5 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 significant 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++Ca2+) are from 0.06 to 0.99, and anion contents ratios of Cl-/(Cl-+ HCO3-) 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 SO4·Cl-Na and Cl·SO4-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 finer the aquifer particles are, and the slower the groundwater flow is. Moreover, with the weakening of the influence of atmospheric precipitation, the influence 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 flow system, moreover, fresh water and salt water appear alternately in the same observation well, which may be affected by the seawater intrusion (CY well).