CORRELATION BETWEEN GROUNDWATER LEVEL AND ALTITUDE VARIATIONS IN

CORRELATION BETWEEN GROUNDWATER LEVEL AND ALTITUDE VARIATIONS IN LAND SUBSIDENCE AREA OF THE CHOSHUICHI ALLUVIAL FAN, TAIWAN Chieh-Hung Chen , Chung-Ho Wang, Ya-Ju Hsu, Shui-Beih Yu, Long-Chen Kuo 報告人: 蕭惠如 Institute of Earth Sciences, Academia Sinica, Taipei 115, Taiwan 1

Introduction Methodology Observations and discussion for groundwater level data Relationships of groundwater levels to GPS data Conclusions 2

In Taiwan, groundwater resources have been depleted in the western and southwestern regions in the past decades due to excessive extraction and caused extensive land subsidence along coastal areas. The most notorious land subsidence region is located at the Choshuichi Alluvial Fan of central Taiwan , with an active subsiding area of over 600 km 2 and a maximum subsiding rate up to 10 cm/yr. In short, the groundwater level dropped from a value close to sea-level down to − 30 m from 1974 to 2006 3

Fig. 1. Locations of the groundwater monitoring wells (open circles) and GPS sites (solid dots) in the Choshuichi Alluvial Fan of western Taiwan. The Choshui River flows through the middle of the fan and separates two sections: northern Changhua and southern Yunlin Counties. Groundwater flow directions are expressed in gray dashed arrows. The severe land subsidence is located in the southern area of the Yunlin County. Stations which are taken as examples and discussed in Figs. 5– 7 are marked with open squares. 4

Table 1. The locations and observation aquifers of Choshuichi alluvial fan used in this study County Changhua Changhua Changhua Changhua Changhua Changhua Chiayi Yunlin Yunlin Yunlin Station Chaochia Hsikang Chuanhsin Chutang Hsihu Hsichou Ershui Fangyuan Hanbao Hohsin Haoshiu Hsienhsi Huatang Kanyuan Kuoshen Lochin Hsiantien Tanchien Tungfang Tienwei Tienchung Wenchang Yuanlin Anho Sanho Tungjung Tungshi An-nan Paotze Chiungpu Fengjung Fangtsao Chiuchuang Chiahsin Hou-An Huwei Haifeng Code CC CG CH CT CU CZ ES FY HB HN HO HS HT JY KS LT ST TC TF TW TZ WC YL AH SH TR TS AN BT CP FG FT GC GH HA HE HF Longitude 120. 3872 120. 2813 120. 5043 120. 4202 120. 4708 120. 4931 120. 6098 120. 3123 120. 3442 120. 4500 120. 4501 120. 4595 120. 5352 120. 5255 120. 5610 120. 4220 120. 3689 120. 3394 120. 5078 120. 5192 120. 5787 120. 4114 120. 5666 120. 3045 120. 4798 120. 4274 120. 1464 120. 2407 120. 1434 120. 1992 120. 3028 120. 3659 120. 3925 120. 4514 120. 2267 120. 4242 120. 2178 Latitude 23. 9393 23. 8625 24. 1738 23. 8617 23. 9517 23. 8569 23. 8135 23. 9256 24. 0088 23. 8959 24. 0087 24. 1340 24. 0285 23. 8248 24. 0945 24. 0562 23. 8757 23. 8374 24. 0646 23. 8932 23. 8564 24. 0100 23. 9534 23. 5166 23. 6070 23. 5594 23. 4622 23. 7058 23. 6353 23. 5202 23. 7925 23. 7202 23. 6362 23. 6500 23. 7910 23. 7160 23. 7667 Aquifer 1 O O Aquifer 3 O O O O Aquifer 2 O O O O O O O O O O O O O O Aquifer 4 O O O O O O O 5

Because the Choshuichi Alluvial Fan can be divided into three aquifers for a depth of 250 m according to subsurface hydrogeology each station may have one to five screens situated in different wells for fully observing changes from shallow to deep aquifers. The groundwater levels of these aquifers indicate two major flow directions: northwest in Changhua county and southwest in Yunglin county 6

Fig. 2. Contours of groundwater level aquifer 1 from years of 1994– 2005 versus 2006 7

In this study, we (Chen et al. ) examine the correlation between the land subsidence (deduced from GPS data) and the groundwater level variations of monitoring wells in the period between 1994 and 2006. Our (Chen et al. ) aim is to quantitatively describe the relationship between vertical displacement on surface and groundwater level variation in identifying the distinctive effects among aquifers and derive the long-term trend for the land subsidence area. The behavior of aquifers is vital to the understanding of land subsidence process. The long-term trend is very valuable in developing an effective and appropriate remediation strategy for the land water resources management in a large scale. 8

Taiwan GPS Network was firstly established by the Institute of Earth Sciences, Academia Sinica in 1989. The number of continuous GPS sites had been rapidly increased to 320 after the 1999 Taiwan Chi-Chi earthquake. In this study, we use GPS vertical displacements from two continuous sites, PKGM (RGPSPKGM; 23. 5799°N, 120. 3055°E) and S 103(RGPSS 103; 23. 5644°N, 120. 4752°E) to analyze variations of vertical motions associated with groundwater levels from 1994 to 2006. 9

The groundwater-levels in the aquifers are recorded digitally every hour by piezometers. For a better and consistent comparison, records of the groundwater level and vertical displacement of GPS are both transferred as monthly data, RAaq. ST and RGPSST, where aq and ST denote the aqth aquifer and the station, respectively. Aaq. ST and GPSST, are respectively calculated as the yearly changes of the RAaq. ST and RGPSST with a step of one month. 10

The linear relationship between the Aaq. ST and GPSST can be written as: (1) where xaq and i denote the coefficients of the Aaq. ST and the sequence numbers of monitor aquifers, respectively 11

Because responses of land subsidence caused by excessive extraction in groundwater are generally not constant, for simplification in analysis, the unknown long term trend is expressed by a temporal function of 4 orders since 1974, and is added into Eq. (1). Thus, the linear relationship between GPS and groundwater level can be rewritten as: (2) where y is the observation year and xj is the coefficient of the long term subsidence 12

Since recording the temporal period of the Aaq. ST exceeds the unknown elements xaq and xj, the traditional least squares method is employed: (3) Here, A is the Aaq. ST in a particular year (y− 1974)j (y=1994 to 2006). B is the GPSST, and x represents the xaq and xj of Eq. (2). 13

When we solve the linear relationship, the synthetic surface variations (Sv) can be simultaneously given by A multiplied by x; and the obtained correlation coefficient (C. C. ) serves as an index which expresses the strength and direction of a linear relationship between the GPSST and Sv. In general, when the C. C. is larger than 0. 5, the relationship is mainly a positive correlation and the GPSST can be roughly estimated by the Sv. 14

aquifer 1 Fig. 2. Contours of groundwater level aquifer 1 from years of 1994– 2005 versus 2006 15

aquifer 2 Fig. 3. Contours of groundwater level aquifer 2 from years of 1994– 2005 versus 2006 16

aquifer 3 Fig. 4. Contours of groundwater level aquifer 3 from years of 1994– 2005 versus 2006 17

To explore the relationship between the land subsidence and the groundwater level changes, records of two GPS observations, PKGM in the severe land subsidence area and S 103 in a normal stable place, are compared with the groundwater variations of Peikang (PK) and Tungjung (TR) wells, 18

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Relations between groundwater level variations and GPSPKGMvertical changes at the Paikang (PK) site. (a) Time-series variations of the raw GPS, groundwater data of aquifers 2 and 3. (b) Time-series variations of the raw GPS (shadow line) and groundwater data of aquifers 2 and 3 without seasnal effect and the correlations between them. The synthetic vertical changes are expressed as lines of solid dots without long term subsidence accounted) and open triangles (with long term subsidence accounted), respectively. (c) The deduced long term trend of the land subsidence relative to 1974. The shadow zone represents study data covering the Chi-Chi earthquake. 20

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Relations between groundwater level variations and GPSs 103 vertical changes at the Tungjung (TR) site. (a) Time-series variations of the raw GPS s 103 and groundwater data of aquifers 1, 2 and 3. (b) Time-series variations of the raw GPS (shadow line) and groundwater data of aquifers 1, 2 and 3 without seasonal effect, and the correlation between them. Line with solid dots shows the synthetic vertical changes without long term subsidence estimation. The shadow zone represents study data covering the Chi-Chi earthquak 22

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Relations between groundwater level variations at the Tungkuang (TK) site and GPSPKGM vertical changes. (a) Time-series variations of the raw GPS (shadow line), groundwater data of aquifers 2 and 3 without seasonal effect and the correlations between them. The synthetic vertical changes are expressed as lines of solid dots (without long term subsidence accounted) and open triangles (with long term subsidence accounted), respectively. (b) The deduced long term trend of the land subsidence at the TK site. The shadow zone exhibits study data covering the Chi-Chi earthquake 24

Overdraft of groundwater in the Choshuichi Alluvial Fan has been the major mechanism for a negative impact of land subsidence. The elevation changes in the subsidence area are primarily affected by two factors: (1)the current groundwater level variations and (2)a long term trend caused by the past excessive extraction in aquifers. The two factors can be separated and estimated by a linear relationship and temporal functions. 25

In addition, the correlation coefficient between the synthetic and observed elevation changes can be served as an effective and quantitative indicator in differentiating the normal and/or subsidence area and weighting factor for various aquifers. The results of this study can provide a useful reference of remediation strategy for the land water resources management in active subsiding areas. 26