THE RELATION BETWEEN GEOCHEMICAL CHARACTERISTICS AND LANDSLIDE IN HUNGTSAIPING AREA, NANTOU, TAIWAN

Pei-Ying Lin1*, Louis Loung-Yei Tsai2

ABSTRACT Hungtsaiping is located at the south bank of the Yonglu stream (the upper stream of Jhangping Stream), Chungliao Village of Nantou County, central Taiwan. Occurring on September 20, 1999 UTC, Hungtsaiping landslide was triggered by the Chi-Chi earthquake (Mw=7.6) which occurred near the town of Chi-Chi in Nantou County, central Taiwan. Coping with the geological and geomorphologic investigation, this study attempts to find the relation between geochemical characteristics and landslide in Hungtsaiping area. Geochemical methods including isotope and hydrochemistry were used in this study. Water sampling was carried out from May 2008 to May 2009. Water samples were collected from springs, creeks, ponds, groundwater and the Yonglu stream once every month during the studying period. Oxygen and hydrogen stable isotopie, ionic concentrations, as well as electrical conductivities and pH values were analyzed. Analytic results indicate that calcium and magnesium bicarbonate-rich water were found on the top and the middle part of the slope. On the other hand, sodium bicarbonate-rich water as well as exceptionally high sulfate concentration were found on the foot of the slope, the sulfate content decreased with increasing elevations until the middle part of slope. Finally, a conceptual model of flow path and water origin in Hungtsaiping landslide was established by summarizing the features of hydrogeochemical analyses along two profiles in this study. Key Words: Landslide, Geochemical characteristics, Isotope, Hydrochemistry INTRODUCTION Landslides triggered by earthquakes are one of the most damaging calamities in Taiwan. The size of area affected by earthquake-induced landslides depends on the magnitude and the focal depth of the earthquake, the topography and geologic conditions near the fault, as well as the frequency and duration of ground shaking (Hays, 1981). There were many examples about the landslide triggered by earthquakes in the past such as 1964 March 28 03:36 UTC Magnitude 9.2 Alaska, and 2007 March 25 00:41 UTC Magnitude 6.7 Japan. The study area - Hungsaiping is located at the south bank of Yonglu stream (upper stream of Jhangping Stream), Chungliao village of Nantou county, central Taiwan (Fig. 1). According to the geological map of Taiwan - Puli sheet of Central Geological Survey, geological structures near Hungsaiping area include Tsukeng Anticline, Tahengpingshan Syncline, Shuilikeng Fault, Dingshuiku Fault, Kweipu Fault, and Shuangtung Fault. Moreover, all the formations belong to the Miocene age: Tanliaoti Shale (Tl), Shihmen Formation (Sm), Changhukeng 1 Graduate Student, Institute of Applied Geology, National Central University, Taoyuan County 320, Taiwan,

R.O.C. (*Corresponding Author; Tel.: +886-3-422-7151; Fax: +886-3-426-3127; Email: 986404001@cc.ncu.edu.tw)

2 Professor, Institute of Applied Geology, National Central University, Taoyuan County 320, Taiwan, R.O.C.

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Shale (Ch), and Shenkeng Sandstone (Sk) (Table 1). Occurring on September 20 , 1999 UTC , Hungtsaiping landslide was triggered by the Chi-Chi earthquake (Mw = 7.6) occurred fifteen kilometers away, near the town of Chi-Chi in Nantou County, central Taiwan, and it includes both east and west limbs of Tsukeng anticline. Furthermore, Hungtsaiping area involves at least two huge landslides events in history with different mechanisms, triggered by strong earthquakes in 1916 and 1999 Chi-Chi earthquake (Lin et al., 2008). Wei and Lee (2006) defined the boundary of Hungtsaiping landslide by using digital aerial photogrammetric techniques. Hungtsaiping Landslide was divided into four zones: A, B, C, and D zone and two types provided by Lo et al. (2008). Coping with the geological and geomorphologic investigation, this study attempts to find the relation between geochemical characteristics and landslide in Hungtsaiping area.

Hungtsaiping

Taiwan

Chungliao village ofNantou county Hungtsaiping

Taiwan

Chungliao village ofNantou county

Fig. 1 Location map of the study area

(Google Earth)3

Table 1 The stratigraphy of Hungtasiping area

Taichung

CPC exploration (1982)

Puli Huang et al. (2000)

Mio

cene

Middle

— Nanchuang Formation

Kuanyinshan Sandstone Shenkeng Sandstone

Talu Shale Changhukeng Shale

Early

Peliao Formation Shihmen Formation

Shihti Formation Taliao Formation Tanliaoti Shale

Mushan Formation Shihszeku Formation

In the past, the methods of landslide research comprehend interpretation of aerial photos and topographic maps, field investigation, geophysical exploration, borehole logging, laboratory experiments and on-site monitoring of water table by using piezometers in boreholes. However, for landslides with quick and large displacement, the monitoring instrument can easily be destroyed. Additionally, it is also dangerous for operators to monitor. According to the previous research studies, mass loading and slope gradient are two major factors affecting slope stability in the slide-prone area. Water also plays an important role for the mass movement in the slope region (Peng et al., 2007). Since 1990’s, a lot of researcher studied the characteristics of hydrogeochemistry and tried to relate them to landslide. The hydrogeochemical information can unravel the hydrological process in landslide triggering (Bogaard et al., 2007), many researchers also consider hydrogeochemistry is an effective tool for landslide study (Epstein and Mayeda, 1953; Guglielmi et al., 2000; Wang et al., 2001; Guglielmi et al., 2002; Cappa et al., 2004; Monety et al., 2007). Thus, the focus of landslide research had shifted over the past few decades from emphasizing on the morphology to hydrogeochemical characteristics of landslide. Both isotope and hydrochemistry can be used as characteristic indicators in landslide study. The purpose of this study is to apply information of hydrogeochemistry to landslide research. There are three aims in this study: (a) to use hydrogeochemical information to unravel the hydrological processes in triggering landslide; (b) to set up a conceptual model of flow path and water origin in Hungtsaiping landslide; and (c) to comprehend the relation of geochemical characteristics and landslide, particular parameters of landslide monitoring can thus be confirmed.

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SAMPLING AND ANALYTICAL METHODS Water sampling was carried out from May 2008 to May 2009. Evenly distributed sampling locations were designed to be as representative as possible in Hungtsaiping landslide area. 19 groundwater samples were collected from the boreholes which were drilled between 2005 and 2008. 15 to 18 surface water samples include creeks, ponds, springs and the Yonglu stream. Sampling sites are located by Global Positioning System - (GARMIN eTrex Vista Cx) as shown in Fig. 2. The experiment layout of this study is shown in Fig. 3. Electrical conductivities and pH values were determined. Otherwise, specific ion concentrations (Cu2+, Fe2+, Mg2+, Ca2+, Na+, K+, SO4

2-, and Cl-) were determined by atomic absorption spectrometry (AA) and high-pressure ionic chromatography (IC); bicarbonate concentration was determined by titration. The total relative uncertainties, including the device accuracy and the repeatability error, were all less than 5 per cent. Nevertheless, for samples with unexpectedly high ion concentration, dilution up to a factor of ten to twenty times is needed. The mean total uncertainty due to this dilution procedure is estimated at 10 to 15 per cent for the dominant species (Na+, Mg2+, Ca2+, SO4

2-, and Cl-) and less than 10 per cent for other compounds.

Fig. 2 The sampling locations of Hungtsaiping landslide.

C

B

A

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Fig. 3 The procedures of study. RESULTS AND DISCUSSION This study yielded useful information about the relation between geochemical characteristics and landslide. The oxygen and hydrogen isotopic compositions can display crucial information on climate conditions and hydro-geological processes in a specific area. Clearly, the findings indicate that precipitation has an effect on isotopic compositions in Hungtsaiping area. When precipitation was increased by typhoons, the stable isotopic compositions decreased conspicuously and last for several months (Fig. 4). δ18O versus δD of samples can provide some information about the variation in water types, season, months, and sampling locations (Fig. 5). To summarize, several findings are of interest: it is quite obvious that the change in surface water was greater than that of groundwater, the stable isotopic compositions for groundwater were heavier than surface water, and the time-delay effect was common in groundwater.

Fig. 4 Isotopic compositions of monthly mean and accumulated precipitation during May to December 2008 in Hungtsaiping area (S: surface water, G: groundwater).

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d18O-10 -8 -6 -4

-70

-60

-50

-40

-30

-20

LMWLGroundwater May-08Surface water May-08Groundwater June-08Surface water June-08Groundwater July-08Surface water July-08Groundwater Auguest-08Surface water Auguest-08Groundwater September-08 Surface water September-08Groundwater October-08Surface water October-08Groundwater November-08Surface water November-08Groundwater December-08Surface water December-08

δD=(7.8±0.1) δ18O+(8.2±0.6) (Peng et al., 2000) n=83

Fig. 5 The distribution of isotopic compositions for all samples, and the local meteoric water line (LMWL) represent the data of Peng et al. (2000). The data of hydrochemistry were analyzed by software Aqua Chem. v. 5.0, monthly water characteristics for each sampling location were shown on piper diagram, as the example presented graphically in Fig. 6. According to literature review, hydrogeochemistry had been taken notice of an effective feature to comprehend the triggered mechanism and to monitor the velocity in landslide area. For instance, a more detailed understanding of this relationship can be gained from Fig. 7. The concentration of sulfate for samples can be expected to detect the movement of landslide. So far, the data and the observation on field investigations appear to be highly related in a sense that the concentration of sulfate can be an indicative parameter in landslide studies.

Fig. 6 Hydrochemistry of Hungtsaiping landslide: piper diagram in November 2008.

Na-Cl

Na- HCO3-

Ca/Mg-HCO3-

Ca/Mg-SO42-

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Fig. 7 Isogram: the concentration of sulfate (ppm) in July 2008 in Hungtsaiping area. By integrating background information, hydrogeochemical characteristics and spatial distribution can be used to comprehend the hydrological processes in Hungtsaiping area (Fig. 8). Additionally as shown in Fig. 9, EH-9 and EH-1 corresponded to type III (Ca/Mg-SO4

2-) at different time and space. Since water types stand for the process of water, conspicuous geochemical reactions were expected nearby those locations, and the region were expected to be unstable. The extra recharge sources on specific location and duration probably contribute more to induce landslide. Furthermore, time-delay effect was more conspicuous, with longer pathway from origin of water bodies or with deeper recharge sources (Fig. 10). Finally, from conceptual flow model along two profiles, the geochemical characteristics can present the aquatic condition including recharge conditions, flow pathway, and water origin in the slope region (Fig. 11), and further display the role of water in triggering a landslide. Suggestions for landslide mitigation and prevention can thus be derived.

Fig. 8 Isotopic compositions of surface water and groundwater in Hungtsaiping area compared with the local meteoric water line established by Peng et al. (2000) (red region: more precipitation and recharge, blue region: strong evaporation).

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Fig. 9 The spatial and temporal variance on water types of groundwater in Hungtsaiping.

Fig. 10 Time-delay effects of one-month on BH-9B and monthly accumulated precipitation during studying periods in Hungtsaiping area.

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(m)(m)

(m)(m)

Fig. 11 The conceptual flow model on AC and BC-profile in Hungtsaiping landslide area. CONCLUSIONS The study provides some contributions to the development of using hydrogeochemistry on landslide studies, as well as it may lead to a better understanding of the relationship between geochemical characteristics and landslide. The implications are concluded as follows: 1. Hydrochemical data revealed that the water bodies can be classified into various groups in

space, and conspicuous geochemical reactions were expected nearby locations with type (III) Ca/Mg -SO4

2-. 2. The LMWL of Hungtsaiping area: δD = 8.4 × δ18O + 18.0 can be taken as the background

of isotope in this study. The variances of groundwater were smaller than those of surface water.

3. In Hungtsaiping, the degree of mineralization belongs to mineralized, much-mineralized, and highly mineralized on the unstable region which were recharged from deep and saturated zones of the slope near the Yonglu stream.

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4. Analytic results indicate that type (I) Ca/Mg-HCO3- was found on the top and the middle

part of the slope. On the other hand, type (II) Na-HCO3- as well as exceptionally high

sulfate concentration were found on the foot of the slope, the sulfate content decreased with increasing elevations until the middle part of slope.

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