Hydrochemistry of the Hot Springs in Western Sichuan Province Related to the Wenchuan M S 8.0 Earthquake

Hydrogeochemistry of 32 hot springs in the western Sichuan Province after the Wenchuan M S 8.0 earthquake was investigated by analyzing the concentrations of cation and anion and the isotopic compositions of hydrogen and oxygen. The water samples of the hot springs were collected four times from June 2008 to April 2010. Hydrogeochemical data indicated the water samples can be classified into 9 chemical types. Values of δD and δ 18O indicated that the spring waters were mainly derived from meteoric precipitation and affected by water-rock interaction and mixture of deep fluids. Concentrations of K+and SO4 − of the samples from the Kangding district exhibited evident increases before the Wenchuan earthquake, indicating more supplement of deep fluids under the increase of tectonic stress. The chemical and isotopic variations of the water samples from the area closer to the epicenter area can be attributed to variation of regional stress field when the aftershock activities became weak.

The observed geochemical anomalies related to earthquakes are usually attributed to the alteration of groundwater in the circulating system under the action of increasing crustal stress before and after earthquakes [6,7,13,14]. For understanding the hydrogeochemical anomalies related to earthquakes, some genetic mechanism models have been proposed, such as increased solubility of rocks under increased pressure and release of ions from rocks into water [15], pore collapse with fluids expulsion [8], the water-rock interactions at the enhanced reactive surfaces [16], aquifers rupture with mixing of different fluids [6] and expulsion of deep fluids by tectonic pumping [1].
In order to investigate the hydrogeochemical characteristics of the springs related to the seismic activities, the hydrogeochemical survey of 32 hot springs in the western Sichuan were performed four times from June 2008 to April 2010.

Geological Setting
The investigated area is located at the eastern margin of the Tibetan Plateau, in which there are four major fault zones  Figure 1) where earthquakes frequently occurred [26].Triassic littoral-neritic clastic rocks interbeded with carbonate and intrusive granite are exposed in the west of the LMSF and ANHF zones, but the strata from Late Paleozoic to Cenozoic are widespread exposed in the LMSF and ANHF zones and on the east of the fault zones, and granite is found in the intersection of the LMSF, XSHF and ANHF zones [27][28][29][30]. The fault zones act as the important passage for upward migration of thermal fluids from the deep earth, which is indicated by distribution of many hot springs in the MJF, LMSF, XSHF and ANHF zones ( Figure 1). Historically, a number of great earthquakes ( > 7.0) have occurred in the investigated area since 1800. For instance, the 1850 7.5 Xichang earthquake occurred in the ANHF zone, two 7.2 earthquakes of 16 and 23 August 1976 in the MJF zone, the 7.5 one of 25 August 1933 in the LMSF zone, and the 7.5 one of 1955 and the 7.9 one of 1973 in the XSHF zone [12]. The 8.0 Wenchuan earthquake resulted in a 285 km surface rupture zone along the pre-existing Yingxiu-Beichuan, Guanxian-Anxian and Qingchuan faults, with the maximum vertical surface offset of about 6.2 m [31].

Methods
Water samples were repeatedly collected four times (in June and October 2008, June 2009, and April 2010) at 32 sites of spas, wells, and springs ( Figure 1) in the MJF, LMSF, XSHF, and ANHF zones. The samples were sealed and stored in 500 mL glass bottles. The values of temperature were measured with a digital thermometer with an error of ±1% in the field. Isotopic compositions of D and 18 O were measured with a Picarro L1102 mass spectrometer in the Laboratory of Gas Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences in Lanzhou, and the errors are 0.5‰ for D and 0.1‰ for 18 O, respectively. The concentrations of cations (K + , Na + , Mg 2+ , and Ca 2+ ) and anions (F − , Cl − , Br − , NO 3 − , and SO 4 2− ) were determined with the Dionex ICS-900 ion chromatography (reproducibility within ±2%) that is installed in the Institute of Earthquake Science, China Earthquake Administration. The CO 3 2− and HCO 3 − concentrations were measured by the standard titration procedures with a ZDJ-100 potentiometric titrator (reproducibility within ±2%). The ion balance (ib) was calculated according to the following equation [11]:

Results
The physicochemical parameters of the 32 springs were listed in Table 1   The Scientific World Journal The Scientific World Journal The "n.d. " represents samples not analysed.

Discussion
The Scientific World Journal  (Table 1). The values of D and 18 O of the springs nos.10-25 from the higher mountain area were more negative, while those of the springs nos. 1-3 collected in the lower altitude region were less negative, which is constant with the previous results [32].

(2) Na(Ca)-HCO 3 (SO 4 ), Na(Mg)-HCO 3 (SO 4 ), and Na-Cl(HCO 3 ) Waters.
The water samples of the springs nos. 4-5, 10-23 and 25-32 were belong to these types ( Table 1). The springs nos. 4-5, 13-23 and 25 occur in granite, while the springs nos. 10-11 in Late Triassic clastic rock interbeded with volcanic rocks. The spring no. 12 is found in Late Permian volcanic tuff, while the spring no. 26 in Sinian pyroclastic rock. The spring nos. 27-29 and 31 are found in Cretaceous clastic rock, the spring no. 30 in Early Triassic sandstone, and the spring no. 32 in Paleozoic mixed layer (Figure 1). The springs nos. 4-5, 10-23 and 25-32 occur in the rocks enriched in feldspar resulting in similar higher concentrations of Na + and HCO 3 − because of rock-water interaction. In addition, the samples from the springs nos. 5, 19, 23, and 32 were characterized by the similar concentrations of Ca 2+ , Mg 2+ , and SO 4 2− (Table 1), which should be attributed to the water-rock interactions between groundwater and Devonian carbonate ( Figure 1). Cl − is known to be conservative and derive from the deep earth mainly [33,34]. The chemical type for the samples from the spring no. 16 was Na-Cl (HCO 3 ), with the Cl − concentration as 328.54 mg/l (Table 1), which suggested the input of deep fluid. Meanwhile, the high 3 He/ 4 He ratio (between 1.43 and 3.73RA, RA = 1.39 × 10 −6 ) [30] together with the high temperatures (between 80.0 ∘ C and 70.2 ∘ C) also are the evidences for the upwelling of the deep-earth fluids into the spring (Table 1).

(3) Ca(Na)-HCO 3 (SO 4 ), Ca(Mg)-HCO 3 , Mg(Ca)-HCO 3 , and Ca(Na)-SO 4 (HCO 3 ) Waters.
These included the samples from the springs nos.1, 6-9, and 24 ( Table 1). The spring no. 1 occurs in Jurassic carbonate interbedded with sandstone. The springs nos. 6 and 9 occur in Late Triassic clastic rock interbedded with carbonate. The springs nos. 7 and 8 occur in Middle Triassic clastic rock interbedded with carbonate, and the spring no. 24 is found in Carboniferous carbonate ( Figure 1). The springs nos. 1, 6-9, and 24 occur in strata enriched in carbonate, so that the main components of the waters were Ca 2+ , Mg 2+ and HCO 3 − because of the interaction between the groundwater and carbonate. The samples of the springs nos. 8 and 9 have the same chemical type of Mg (Ca)-HCO 3 , and similar Ca/Mg ratios (0.54 and 0.86, Table 1), which can be attributed to the dissolution of calcite combined with the low temperatures of the springs nos. 8 and 9 (8.8 ∘ C and 9.0 ∘ C respectively) ( Table 1).  (Table 1), indicating interaction between carbonate and groundwater enhanced by the sulfate from the oxidation of pyrite [35].  (Figure 3), which indicated gradual input decrease of deep-earth fluids that formed by water-rock reaction after the Wenchuan earthquake

Chemical Variations of the Spring Waters
The supplement of deep water characterized by higher mineralization and enriched in 18 O is considered as a main factor for controlling pre-and co-seismic geochemical variations of the groundwater and springs [2,6,7,11,13,14,16,36,37]. The Coulomb stress in the middle-north segment of the LMSF zone, the southeast segment of the XSHF zone, and the south segment of the MJF zone had been enhanced, which resulted in the Wenchuan 8.0 earthquake [38,39], and the geochemical variations of the samples from the springs nos. 1-5 and 20. Meanwhile, the input decrease of deepearth water during the sampling period in which seismic activity decreased gradually. 747 aftershocks with higher than 4.0 occurred in the three fault zones, and most of which (including all the 12 events with amplitude ranged from 6.0 to 6.4) occurred before September 10, 2008 (China Earthquake Network Center, Figure 1). As the aftershock activities became weak, the TDS values of the samples from the springs nos. 1-5 and 20 decreased gradually to the normal value (Figure 4), indicating the supplement decrease of the deep-earth fluid as the Coulomb stress being released after the events as indicated by the data of oxygen and hydrogen isotope compositions (Figure 3).
Notably, the concentrations of Na + , HCO 3 − , and TDS of the sample of the spring no. 15 increased by 53%, 38%, and 38% on April 17, 2010 (Figure 4), which was closely related to   Figure 1). Therefore, it can be infered that the hydrogeochemical variations of the spring no. 15 might be the short-term hydrogeochemical precursors for the 5.4 event. The geochemical parameters of the samples from the springs nos. 16-17 and 19 about 200 kilometers showed evident increase trend before the Wenchuan earthquake [20,24,25]. The K + concentrations of the samples of the springs nos. 16-17 and 19 increased by 19.3% to 54.4% before the main shock, and rapidly decreased to the normal values after the main shock. The concentrations of SO 4 2− for the samples from the springs nos. 16 and 19 increased by 32.0% and 59.6% respectively before the earthquake, and then dropped to the normal value after the main shock ( Figure 5). The springs nos. 16-17 and 19 occur in the intersection of the LMSF, XSHF and ANHF zones where the extrusion stress increased yearly from 2004 to 2007 before the Wenchuan earthquake [40,41]. Therefore, the increase of K + and SO 4 2− concentrations for the samples of the springs nos.16-17 and 19 could be attributed to the more supplement of the deep fluids with higher concentrations of K + and SO 4 2− [42], The 3 He/ 4 He ratios of the samples from the springs nos. 16-17 and 19 increased before the Wenchuan earthquake, but showed a remarkable drop after the shock, and then became normal indicating the more supplement of mantle-derived fluids before the main shock [30].

Spatial Variations.
The geochemical and isotopic compositions of the samples from the springs nos. 1-5 occur in the LMSF zone performed evident variation after the Wenchuan earthquake, with the highest amplitude as 95% for HCO 3 − of the sample from the spring no. 5. However, for the other samples from the other fault zones, just the samples from the springs nos. 15 and 20 presented obvious geochemical variations, with the highest amplitude as 53% for Na + of the sample from the spring no. 15 ( Figure 4). The Wenchuan earthquake happened in the LMSF zone, which enhanced the Coulomb stress of the zone [38,39], and with most of the aftershocks occurred there (Figure 1). For the springs nos. 1-5 occurred in the LMSF zone, the epicenter distances are between 50 and 110 kilometers, which are smaller than those of the hot springs nos. 6-32 (between 190 and 410 km). Therefore, the variation of the tectonic stress induced by the Wenchuan earthquake and the aftershocks may resulted in, 12 The Scientific World Journal the geochemical and isotopic variations for the springs nos. 1-5 after the Wenchuan earthquake.

Conclusions
120 water samples of the 32 hot springs in the western Sichuan have been analyzed after the Wenchuan earthquake. The following conclusions can be drawn.
(1) The waters of the 32 springs were mainly derived from meteoric precipitation. Because of the isotopic elevation effect, the D and 18 O of the samples from the higher mountain area were lower, but those of samples from the lower altitude region were higher. all of the analyzed water samples from the 32 springs can be classified into nine chemical types: Na ( (2) The concentrations of K + and SO 4 2− of the samples of the springs nos. 16-17 and 19 in the intersection of LMSF, XSHF, and ANHF varied evidently, with the amplitude ranging from 19.3% to 59.6% before and after the Wenchuan earthquake which may be attributed to the interfusion of the deep fluids with high K + and SO 4 2− induced by the increased tectonic stress.
(3) The hydrogeochemical variations of the springs closer to the epicenter performed more obviously after the Wenchuan earthquake. As the aftershock activities became weak, the geochemical parameters of the samples from the springs nos. 1-5, 15, and 20 located in the regions where the tectonic stress was enhanced before the Wenchuan 8.0 earthquake decreased by more than 20%, and with the 18 O the LMWL, which may be related to the change of tectonic stress when the aftershock activities became weak.