Pingtung Plain Research Articles

A Simulation-Optimization Model for Seawater Intrusion Management at Pingtung Coastal Area, Taiwan

The coastal regions of Pingtung Plain in southern Taiwan rely on groundwater as their main source of fresh water for aquaculture, agriculture, domestic, and industrial sectors. The availability of fresh groundwater is threatened by unsustainable groundwater extraction and the over-pumpage leads to the serious problem of seawater intrusion. It is desired to find appropriate management strategies to control groundwater salinity and mitigate seawater intrusion. In this study, a simulation–optimization model has been presented to solve the problem of seawater intrusion along the coastal aquifers in Pingtung Plain and the objective is using injection well barriers and minimizing the total injection rate based on the pre-determined locations of injection barriers. The SEAWAT code is used to simulate the process of seawater intrusion and the surrogate model of artificial neural networks (ANNs) is used to approximate the seawater intrusion (SWI) numerical model to increase the computational efficiency during the optimization process. The heuristic optimization scheme of differential evolution (DE) algorithm is selected to identify the global optimal management solution. Two different management scenarios, one is the injection barriers located along the coast and the other is the injection barrier located at the inland, are considered and the optimized results show that the deployment of injection barriers at the inland is more effective to reduce total dissolved solids (TDS) concentrations and mitigate seawater intrusion than that along the coast. The computational time can be reduced by more than 98% when using ANNs to replace the numerical model and the DE algorithm has been confirmed as a robust optimization scheme to solve groundwater management problems. The proposed framework can identify the most reliable management strategies and provide a reference tool for decision making with regard to seawater intrusion remediation.

Conservation of groundwater from over-exploitation—Scientific analyses for groundwater resources management

Groundwater over-exploitation has produced many critical problems in the southern Taiwan. The accumulated stresses and demands make groundwater management a complex issue that needs innovative scientific analyses for deriving better water management strategies. In this study, we aimed to provide scientific analyses of the groundwater systems in the Pingtung Plain through soft-computing techniques to explore its spatial-temporal and hydro-geological characteristics for the elaboration of future groundwater management plans and in decision-making process. We conducted a study to assess the essential features of the groundwater systems based on the long-term large datasets of regional groundwater levels by using the principal component analysis (PCA), and the self-organizing map (SOM) with regression analysis. The PCA results demonstrated that two leading components could well present the spatial characteristics of the groundwater systems and classify the region into eastern, western and transition zones. The SOM results could visibly explore the behavior of regional groundwater variations in various aquifers and the multi-relations among climate and hydrogeological variables. Results revealed that the potential of groundwater recharge made by precipitation or river flow was higher in the eastern zone than in the western zone. Analysis results further showed an increase of the groundwater levels in the western zone after year 2006, while there were no obvious increases of the groundwater levels in the eastern or transition zones. Based on the investigated characteristics, we suggest that a sound groundwater management plan should consider zonal difference of the groundwater systems to achieve groundwater conservation.

Groundwater Simulations and Uncertainty Analysis Using MODFLOW and Geostatistical Approach with Conditioning Multi-Aquifer Spatial Covariance

This study presents an approach for obtaining limited sets of realizations of hydraulic conductivity (K) of multiple aquifers using simulated annealing (SA) simulation and spatial correlations among aquifers to simulate realizations of hydraulic heads and quantify their uncertainty in the Pingtung Plain, Taiwan. The proposed approach used the SA algorithm to generate large sets of natural logarithm hydraulic conductivity (ln(K)) realizations in each aquifer based on spatial correlations among aquifers. Moreover, small sets of ln(K) realizations were obtained from large sets of realizations by ranking the differences among cross-variograms derived from the measured ln(K) and the simulated ln(K) realizations between the aquifer pair Aquifer 1 and Aquifer 2 (hereafter referred to as Aquifers 1–2) and the aquifer pair Aquifer 2 and Aquifer 3 (hereafter referred to as Aquifers 2–3), respectively. Additionally, the small sets of realizations of the hydraulic conductivities honored the horizontal spatial variability and distributions of the hydraulic conductivities among aquifers to model groundwater precisely. The uncertainty analysis of the 100 combinations of simulated realizations of hydraulic conductivity was successfully conducted with generalized likelihood uncertainty estimation (GLUE). The GLUE results indicated that the proposed approach could minimize simulation iterations and uncertainty, successfully achieve behavioral simulations when reduced between calibration and evaluation runs, and could be effectively applied to evaluate uncertainty in hydrogeological properties and groundwater modeling, particularly in those cases which lack three-dimensional data sets yet have high heterogeneity in vertical hydraulic conductivities.

Spatial Analysis of Human Health Risk Due to Arsenic Exposure through Drinking Groundwater in Taiwan's Pingtung Plain.

Chronic arsenic (As) exposure continues to be a public health problem of major concern worldwide, affecting hundreds of millions of people. A long-term groundwater quality survey has revealed that 20% of the groundwater in southern Taiwan’s Pingtung Plain is clearly contaminated with a measured As concentration in excess of the maximum level of 10 µg/L recommended by the World Health Organization. The situation is further complicated by the fact that more than half of the inhabitants in this area continue to use groundwater for drinking. Efforts to assess the health risk associated with the ingestion of As from the contaminated drinking water are required in order to determine the priorities for health risk management. The conventional approach to conducting a human health risk assessment may be insufficient for this purpose, so this study adopts a geostatistical Kriging method to perform a spatial analysis of the health risk associated with ingesting As through drinking groundwater in the Pingtung Plain. The health risk is assessed based on the hazard quotient (HQ) and target cancer risk (TR) established by the U.S. Environmental Protection Agency. The results show that most areas where the HQ exceeds 1 are in the southwestern part of the study area. In addition, the high-population density townships of Daliao, Linyuan, Donggang, Linbian, Jiadong, and Fangliao presently have exceedingly high TR values that are two orders of magnitude higher than the acceptable standard. Thus, the use of groundwater for drinking in these townships should be strictly avoided. A map that delineates areas with high TR values and high population densities is provided. The findings broaden the scope of the spatial analysis of human health risk and provide a basis for improving the decision-making process.

Groundwater Vulnerability Assessment of the Pingtung Plain in Southern Taiwan.

In the Pingtung Plain of southern Taiwan, elevated levels of NO3−-N in groundwater have been reported. Therefore, efforts for assessing groundwater vulnerability are required as part of the critical steps to prevent and control groundwater pollution. This study makes a groundwater vulnerability assessment for the Pingtung Plain using an improved overlay and index-based DRASTIC model. The improvement of the DRASTIC model is achieved by reassigning the weighting coefficients of the factors in this model with the help of a discriminant analysis statistical method. The analytical results obtained from the improved DRASTIC model provide a reliable prediction for use in groundwater vulnerability assessment to nitrate pollution and can correctly identify the groundwater protection zones in the Pingtung Plain. Moreover, the results of the sensitivity analysis conducted for the seven parameters in the improved DRASTIC model demonstrate that the aquifer media (A) is the most sensitive factor when the nitrate-N concentration is below 2.5 mg/L. For the cases where the nitrate-N concentration is above 2.5 mg/L, the aquifer media (A) and net recharge (R) are the two most important factors.

Zonal management of multi-purposes groundwater utilization based on water quality and impact on the aquifer.

Groundwater is widely used for drinking, irrigation, and aquaculture in the Pingtung Plain, Southwestern Taiwan. The overexploitation and poor quality of groundwater in some areas of the Pingtung Plain pose great challenges for the safe use and sustainable management of groundwater resources. Thus, establishing an effective management plan for multi-purpose groundwater utilization in the Pingtung Plain is imperative. Considerations of the quality of the groundwater and potential impact on the aquifer of groundwater exploitation are paramount to multi-purpose groundwater utilization management. This study proposes a zonal management plan for the multi-purpose use of groundwater in the Pingtung Plain. The zonal management plan is developed by considering the spatial variability of the groundwater quality and the impact on the aquifer, which is defined as the ratio of the actual groundwater extraction rate to transmissivity. A geostatistical Kriging approach is used to spatially delineate the safe zones based on the water quality standards applied in the three groundwater utilization sectors. Suitable zones for the impact on the aquifer are then spatially determined. The evaluation results showing the safe water quality zones for the three types of utilization demands and suitable zones for the impact on aquifer are integrated to create a zonal management map for multi-purpose groundwater utilization which can help government administrators to establish a water resource management strategy for safe and sustainable use of groundwater to meet multi-purpose groundwater utilization requirements in the Pingtung Plain.

Health risk assessment of groundwater arsenic pollution in southern Taiwan

Residents of the Pingtung Plain, Taiwan, use groundwater for drinking. However, monitoring results showed that a considerable portion of groundwater has an As concentration higher than the safe drinking water regulation of 10μg/L. Considering residents of the Pingtung Plain continue to use groundwater for drinking, this study attempted to evaluate the exposure and health risk from drinking groundwater. The health risk from drinking groundwater was evaluated based on the hazard quotient (HQ) and target risk (TR) established by the US Environmental Protection Agency. The results showed that the 95th percentile of HQ exceeded 1 and TR was above the safe value of threshold value of 10-6. To illustrate significant variability of the drinking water consumption rate and body weight of each individual, health risk assessments were also performed using a spectrum of daily water intake rate and body weight to reasonably and conservatively assess the exposure and health risk for the specific subgroups of population of the Pingtung Plain. The assessment results showed that 0.01-7.50% of the population's HQ levels are higher than 1 and as much as 77.7-93.3% of the population being in high cancer risk category and having a TR value >10-6. The TR estimation results implied that groundwater use for drinking purpose places people at risk of As exposure. The government must make great efforts to provide safe drinking water for residents of the Pingtung Plain.

To Study Hydrological Variabilities by Using Surface and Groundwater Coupled Model – A Case Study of PingTung Plain, Taiwan

The main objective of this research is to study the hydrological variabilities by using surface and groundwater coupled model. On that Taiwan is labeled as mountainous watershed with accompanying heavy rains occurred frequently, it is important to understand better the water flow conditions after extreme event occurred. The particular geology and highly concentrated river system could provide more infiltration and percolation of rainwater from surface or streams through permeable soils into water-holding rocks. It may provide useful water supply to undergrounds. In order to study the possible water storage underground in PingTung Plain, we used WASH123D watershed model to study this issue. The WASH123D numerical model is a physical based computational model with dealing surface flow and groundwater interactions competently. We firstly used field wells data to construct modeling environment, and then applied observation data to calibrate simulated parameters. Modelling environment including rivers, overland and groundwater are constructed in our model. Three examples with including extreme typhoon events insides are studied. We designed the period at last one month to calibrate our model parameters. It is because the flow condition after extreme rainfall occurred is one of the major phenomena that we are interested. The reactions of pressure head on different layers after extreme event occurred have focused. And we examined this mode to evaluate the efficiency of an ongoing groundwater supply plan in this region.

Combining groundwater quality analysis and a numerical flow simulation for spatially establishing utilization strategies for groundwater and surface water in the Pingtung Plain

Summary Overexploitation of groundwater is a common problem in the Pingtung Plain area of Taiwan, resulting in substantial drawdown of groundwater levels as well as the occurrence of severe seawater intrusion and land subsidence. Measures need to be taken to preserve these valuable groundwater resources. This study seeks to spatially determine the most suitable locations for the use of surface water on this plain instead of extracting groundwater for drinking, irrigation, and aquaculture purposes based on information obtained by combining groundwater quality analysis and a numerical flow simulation assuming the planning of manmade lakes and reservoirs to the increase of water supply. The multivariate indicator kriging method is first used to estimate occurrence probabilities, and to rank townships as suitable or unsuitable for groundwater utilization according to water quality standards for drinking, irrigation, and aquaculture. A numerical model of groundwater flow (MODFLOW) is adopted to quantify the recovery of groundwater levels in townships after model calibration when groundwater for drinking and agricultural demands has been replaced by surface water. Finally, townships with poor groundwater quality and significant increases in groundwater levels in the Pingtung Plain are prioritized for the groundwater conservation planning based on the combined assessment of groundwater quality and quantity. The results of this study indicate that the integration of groundwater quality analysis and the numerical flow simulation is capable of establishing sound strategies for joint groundwater and surface water use. Six southeastern townships are found to be suitable locations for replacing groundwater with surface water from manmade lakes or reservoirs to meet drinking, irrigation, and aquaculture demands.

A Feasibility Analysis of Land-Based SINS/GNSS Gravimetry for Groundwater Resource Detection in Taiwan.

The integration of the Strapdown Inertial Navigation System and Global Navigation Satellite System (SINS/GNSS) has been implemented for land-based gravimetry and has been proven to perform well in estimating gravity. Based on the mGal-level gravimetry results, this research aims to construct and develop a land-based SINS/GNSS gravimetry device containing a navigation-grade Inertial Measurement Unit. This research also presents a feasibility analysis for groundwater resource detection. A preliminary comparison of the kinematic velocities and accelerations using multi-combination of GNSS data including Global Positioning System, Global Navigation Satellite System, and BeiDou Navigation Satellite System, indicates that three-system observations performed better than two-system data in the computation. A comparison of gravity derived from SINS/GNSS and measured using a relative gravimeter also shows that both agree reasonably well with a mean difference of 2.30 mGal. The mean difference between repeat measurements of gravity disturbance using SINS/GNSS is 2.46 mGal with a standard deviation of 1.32 mGal. The gravity variation because of the groundwater at Pingtung Plain, Taiwan could reach 2.72 mGal. Hence, the developed land-based SINS/GNSS gravimetry can sufficiently and effectively detect groundwater resources.

Analysis of space–time non-stationary patterns of rainfall–groundwater interactions by integrating empirical orthogonal function and cross wavelet transform methods

Summary Rainfall–groundwater interactions are complex and can depend upon a variety of hydro-geological and geographical conditions and can commonly present high nonlinearity. This study proposed an integration of cross wavelet transform and empirical orthogonal function (EOF) analysis to analyze the space–time nonlinear relationships between the precipitation and groundwater changes. The proposed method was applied to investigating the non-stationary and nonlinear precipitation–groundwater relationships in Pingtung Plain aquifer during 2005–2010. We analyzed daily groundwater and rainfall observations at 47 wells and 27 stations across the study area operated by Taiwan Water Resources Agency and Central Weather Bureau respectively. Results show that EOF method revealed three major space–time patterns of the groundwater levels. The cross wavelet transform further identified the lagged effects between precipitation and groundwater changes. The temporal lags can vary not only with respect to the temporal scales of groundwater processes, but also the different identified groundwater regions, which can associate with distinct physical mechanisms. In general, the lagged periods between high frequency signals of precipitation and groundwater can range from 0 to 18 days, showing how rainfall events perturbed the groundwater levels across space. The long-term lags for the three identified underlying groundwater processes were 3.71, 56.6, and 72.07 days, respectively, showing seasonal recharge patterns can depend upon the geographical and geological conditions. Our results distinguished the space–time recharge processes in different frequencies, which present nonlinear and non-stationary rainfall–recharge interactions of a groundwater system, which can be spatially and frequency dependent.

The assessment of high recharge areas using DO indicators and recharge potential analysis: a case study of Taiwan’s Pingtung plain

Identifying high groundwater recharge areas is important for the conservation of groundwater quality and quantity. A common practice used by previous studies is to estimate groundwater recharge potential (GRP) using recharge potential analysis (RPA) under different environments. These studies use the estimated GRP to identify the high potential groundwater recharge sites. However, the RPA parameters are subjectively defined for these previous studies. To remove the supposition, this study proposes a systematic approach that defines the RPA parameter values based on the theory of parameter identification. This study uses dissolved oxygen (DO) indicators to calibrate the RPA parameters. This calibration improves the correlation coefficient between the DO indicators and computed GRP values from 0.63 to 0.87. By comparing the initial values, these results indicate that the estimated RPA parameters better represent the field infiltration characteristic. This result also indicates that defining the RPA parameter values based on DO indicators is necessary and important for accuracy. These calibrated parameters are used to estimate the GRP distribution of Taiwan’s Pingtung Plain. The GRP values are delineated into five levels. High and excellent GRP areas are defined as high recharge areas, which compose about 26.74 % of the study area. Based on the proposed method, the estimated GRP distribution can accurately represent the study area’s field recharge characteristics. These study results can be a good reference for groundwater recharge analyses, specifically if well data is limited or difficult to obtain.

地下水模拟中识别参数结构的差分进化优化方法的应用

Parameter structure identification is formulated in terms of solving an inverse problem, which allows for a determination of an appropriate level of parameter structure complexity, and the identification of its pattern and the associated parameter values. With the increasing complexity of parameter structure identification in groundwater modeling, demand for robust, fast, and accurate optimizers is on the rise among researchers from groundwater hydrology fields. A novel global optimizer, differential evolution (DE), has been proposed to solve the parameter-structure-identification problem. The Voronoi tessellation is adopted for the automatic parameterization. The stepwise regression method and the error covariance matrix are used to determine the optimal structure complexity. Numerical experiments with a continuous hydraulic conductivity distribution are conducted to demonstrate the proposed methodology. The results indicate that the DE can identify the global optimum effectively and efficiently. A sensitivity analysis of the control parameters and mutation schemes implemented in the DE is employed to examine their influence on the objective function. The comparison between DE and genetic algorithm shows the advantage of DE in terms of robustness and efficiency. The proposed methodology is also applied to a real groundwater system, Pingtung Plain in Taiwan, and the properties of aquifers are successfully identified.

The sources and fluxes of dissolved chemistry in a semi-confined, sandy coastal aquifer: The Pingtung Plain, Taiwan

Groundwater chemical fluxes from the Pingtung Plain in SW Taiwan to the ocean were determined by analysing waters from 43 wells at varying depths through a 237m deep window across the Pingtung Plain, for major dissolved cations, anions, dissolved SiO2, and stable isotopic composition of O and H, and computing their subsurface water fluxes from measurements of hydraulic heads and formation permeabilities. The results show that between 1.5% (SO42-) and 12.3% (Ba2+) of the total chemical weathering flux discharged to the ocean (Kaoping River combined with groundwater) can be attributed to the groundwater. Estimated propagated errors at 1σ on subsurface fluxes are ±20%. Multi-year daily hydraulic head data give the direction of groundwater flow through the plain, and indicate that pumping has led to episodic reversals of flow, facilitating seawater intrusion in the near-coast aquifer. Tracing end-member proportions using mixing relationships shows that, in addition to seawater and meteoric water, hot-spring activity contributes to the dissolved chemistry of the groundwater. In addition to these three end-members, the weathering of carbonate and silicate minerals in the plain accounts for the remainder of the chemical budget. Hydrological connectivity exists throughout the drilled depth of the basin, but chemical gradients show that flow is stratified, with up to a twofold increase in silicate-derived Na+ seen in deeper horizons as compared to the near surface. For all ions except SO42-, the average concentrations of dissolved species in the coastal groundwaters exceed those of the river, ranging from a factor of 1.33 in the case of Li+ to 27.29 in the case of Cl−. The results suggest that submarine groundwater chemical fluxes through the drilled depth of the Pingtung Plain into the Taiwan Strait are modest in comparison to those related to surface runoff.

Integrating the spatial variability of water quality and quantity to probabilistically assess groundwater sustainability for use in aquaculture

Aquaculture industries generally extract large amounts of groundwater for the cultivation of fish. To reduce the adverse effects on fish cultivation and groundwater drawdown, this study develops a method that simultaneously considers the spatial variability of groundwater in terms of quality and quantity in order to determine sustainable groundwater use for aquaculture in the Pingtung Plain, Taiwan. Because of limited observation data with high prediction uncertainty, sequential indicator simulation (SIS) was adopted to reproduce the spatial distributions of the main pollutants in groundwater, manganese and ammonium-nitrogen. The corresponding distributions of the groundwater utilization ratio (GUR), which is defined as the ratio showing the volume of groundwater utilization compared to pond water, were determined based on the mass balance of the hydrochemical parameters in the groundwater, surface water, and pond water. Four categories in groundwater utilization—GUR < 0.1 (strictly limited), 0.1 ≦ GUR < 0.5 (minor), 0.5 ≦ GUR < 1 (major), and GUR = 1 (completely used)—were characterized based on estimated probabilities. A safe GUR was selected from a maximum estimation probability of the groundwater categories. SIS was also used to grade the transmissivity fields for gauging the pumping capacity of the aquifers. Finally, for different conditions indicating groundwater quality and quantity, the combinations were delineated spatially to provide decision makers with detailed information which will enable them to wisely select a reliable scheme for groundwater management of fishponds. The analytical results reveal that groundwater can be completely used for fish farming in 6.9 % of aquifers, but should serve as a minor water resource for fishponds in 8.6 % of aquifers.

Applying indicator-based geostatistical approaches to determine potential zones of groundwater recharge based on borehole data

Abstract Evaluating a potential zone of groundwater recharge is considerably critical for planning a scheme of quality protection and quantity management of groundwater. This study determined potential zones of groundwater recharge using indicator kriging (IK) based on borehole data in the Pingtung Plain, southern Taiwan. Data on subsoil textures were first classified into four categories — gravel (very rapid subsoil infiltration), coarse sand to medium sand (rapid subsoil infiltration), fine sand to very fine sand (slow subsoil infiltration), and silt, mud, and clay (very slow subsoil infiltration). IK was used to characterize the classifications of subsoil infiltration by selecting a maximum estimation probability. Then, contents of fast soil permeability in unsaturated aquifers (gravel to medium sand), which represent the capacity of unsaturated aquifer percolation, were estimated spatially and probabilistically using IK. Finally, combinations under different conditions of subsoil infiltration and unsaturated aquifer percolation were recommended to delineate potential zones of groundwater recharge. Owing to limited observed data, estimated probabilities of subsoil infiltration and unsaturated aquifer percolation obtained from IK can explore the uncertainty of estimated parameters. The analyzed results reveal that IK has good functionality for determining potential zones of groundwater recharge. The northeastern and southeastern regions are main groundwater recharge zones, approximately 19.6% of total area. Artificial ponds or wetlands can be established in the determined zones to enhance groundwater recharge.

Establishing an irrigation management plan of sustainable groundwater based on spatial variability of water quality and quantity

Summary Conditions of water quality and quantity are two key factors impacting groundwater utilization for irrigation. To obtain pristine groundwater and reduce pumping impacts on aquifers, this study establishes an irrigation management plan of sustainable groundwater in the Pingtung Plain in Taiwan based on the spatial variability of water quality and quantity parameters. According to an irrigation water quality standard, multivariate indicator kriging (IK) was adopted to estimate the integration of hydrochemical parameters in groundwater. IK was also used to grade transmissivity fields representing the pumping capacity of aquifers. Six risk categories of recommended groundwater used for irrigation were spatially delineated based on combinations of five levels of groundwater quality and quantity determined by estimated probabilities. The analyzed results indicate that 54.4% of aquifers are recommended to extract groundwater for irrigation. The aquifers belonging to excellent categories are the optimal locations, where groundwater can be used for irrigation without dilution. The aquifers belonging to ordinary categories are the regions, where groundwater mixed with surface water is appropriate for use in irrigation.

Recharge signal identification based on groundwater level observations

This study applied a method of the rotated empirical orthogonal functions to directly decompose the space-time groundwater level variations and determine the potential recharge zones by investigating the correlation between the identified groundwater signals and the observed local rainfall records. The approach is used to analyze the spatiotemporal process of piezometric heads estimated by Bayesian maximum entropy method from monthly observations of 45 wells in 1999-2007 located in the Pingtung Plain of Taiwan. From the results, the primary potential recharge area is located at the proximal fan areas where the recharge process accounts for 88% of the spatiotemporal variations of piezometric heads in the study area. The decomposition of groundwater levels associated with rainfall can provide information on the recharge process since rainfall is an important contributor to groundwater recharge in semi-arid regions. Correlation analysis shows that the identified recharge closely associates with the temporal variation of the local precipitation with a delay of 1-2 months in the study area.

Groundwater–seawater interactions off the coast of southern Taiwan: Evidence from environmental isotopes

We have conducted detailed hydrochemical surveys off the Pingtung Plain coast, SW Taiwan, using environmental isotope compositions (δ D, δ 18O, δ 13C DIC, tritium, and Δ 14C) as the main tracers to investigate groundwater–seawater interactions in the region. The δ 18O values of seawater in this area show significant variability with heavy isotope enrichment relatively to groundwater. The δ D and δ 18O data also confirmed that submarine groundwater discharge (SGD) originated from meteoric water and were mixed with seawater. Due to an overdraft of groundwater, seawater has intruded inwardly through the offshore outcrops for both the unconfined and confined aquifers at a depth of 300 m. However, δ 18O-depleted layer is found along deep parts of Kaoping Canyon. This result provides evidence for SGD in this region. This phenomenon is also reflected in depleted δ 13C DIC and lower Δ 14C values in deep canyon water than in adjacent seawater, and indicating that the discharge zone extends over a considerable distance to the offshore area (∼25 km). Based on these three parameters (δ 18O, δ 13C DIC and Δ 14C), we used two end-member mixing model to estimate that fresh submarine groundwater constitute. The submarine fresh groundwater contributed about 3%, and the estimated submarine fresh groundwater discharge was calculated as 4.5 × 10 9 m 3 per year. This result is comparable with groundwater annual loss in the Pingtung Plain. As documented by the isotopic composition of the collected samples, detailed results show that SGD in the Pingtung coastal zone raises a potential environmental concern and provides implication for the management of groundwater resource in the region.

Using differential SAR interferometry to map land subsidence: a case study in the Pingtung Plain of SW Taiwan

Synthetic aperture radar (SAR) interferometry (InSAR) is a geodetic tool widely applied in the studies of earth-surface deformation. This technique has the benefits of high spatial resolution and centimetre-scale accuracy. Differential SAR interferometry (DInSAR) is used to measure ground deformation with repeat-pass SAR images. This study applied DInSAR and persistent scatterers InSAR (PSInSAR) for detecting land subsidence in the Pingtung Plain, southern Taiwan, between 1995 and 2000. In recent years, serious land subsidence occurred along coastal regions of Taiwan as a consequence of over-pumping of underground water. Results of this study revealed that the critical subsidence region is located on the coast near the estuary of Linpien River. It is also found that subsidence was significantly higher during the dry season than the wet season. The maximum annual subsidence rate of the dry season is up to −11.51 cm/year in critical subsidence region and the vertical land movement rate is much slower during the wet season. The average subsidence rates in wet and dry seasons are −0.31 and −3.37 cm/year, respectively. As a result, the subsidence rate in dry seasons is about 3 cm larger than in wet seasons.