Using Freshwater Heads to Analyze Flow Directions in Saline Aquifers of the Pingtung Plain, Taiwan

The hydraulic head is the most important parameter for the study of groundwater. However, a head measured from observation wells containing groundwater of variable density should be corrected to a reference density (e.g., a freshwater head). Some previous case studies have used unknown density hydraulic heads for calibrating flow models. Errors arising from the use of observed hydraulic head data of unknown density are, therefore, likely one of the most overlooked issues in flow simulations of seawater intrusion. Here, we present a case study that uses the freshwater head, instead of the observed hydraulic head, to analyze the flow paths of saline groundwater in the coastal region of the Pingtung Plain, Taiwan. Out of a total of 134 observation wells within the Pingtung Plain, 19 wells have been determined to be saline, with Electric Conductivity (EC) values higher than 1500 μS/cm during 2012. The misuse of observed hydraulic heads causes misinterpretation of the flow direction of saline groundwater. For such saline aquifers, the determination of a freshwater head requires density information obtained from an observation well. Instead of the purging and sampling method, we recommend EC logging using a month interval. Our research indicates that EC values within an observation well within saline aquifers vary not only vertically but also by season.

Application of Factor Analysis for Characterizing the Relationships between Groundwater Quality and Land Use in Taiwan’s Pingtung Plain

Although the average municipal water coverage in Taiwan is quite high, at 93.91%, only around half of the residents in the Pingtung Plain use tap water originating from the Taiwan Water Corporation to meet their needs. This means the exploitation of a substantial amount of groundwater as a source of water to meet drinking, agriculture, aquaculture, and industry requirements. Long-term groundwater quality surveys in Taiwan have revealed obvious contamination of the groundwater in several locations in the Pingtung Plain, with measured concentration levels of some groundwater quality parameters in excess of the permissible levels specified by the Taiwan Environmental Protection Administration. Clearly, establishing a sound plan for groundwater quality protection in this area is imperative for maximizing the protection of human health. The inappropriate use of hazardous chemicals and poor management of land use have allowed pollutants to permeate through unsaturated soil and ultimately reach the underlying shallow unconfined groundwater system. Thus, the quality of the water stored in shallow aquifers has been significantly affected by land use. This study is designed to characterize the relationship between groundwater quality and land use in the Pingtung Plain. This goal is achieved by the application of factor analysis to characterize the measured concentrations of 14 groundwater quality parameters sampled from 46 observation wells, the area percentages for nine land use categories in the neighborhood of these 46 observation wells, and the thicknesses of four unsaturated types of soil based on core samples obtained during the establishment of 46 observation wells. The results show that a four-factor model can explain 56% of the total variance. Factor 1 (seawater salinization), which includes the groundwater quality parameters of EC, SO42−, Cl−, Ca2+, Mg2+, Na+, and K+, shows a moderate correlation to land used for water conservation. Factor 2 (nitrate pollution), which includes the groundwater quality parameters of NO3−-N and HCO3−, shows a strong correlation to land used for fruit farming and a moderate correlation to the thickness of the gravel comprising unsaturated soil. Factor 3 (arsenic pollution), which is composed of groundwater quality parameters of total organic carbon (TOC) and As, is very weakly affected by land use. Factor 4 (reductive dissolution of Fe3+ and Mn2+), which involves Mn2+ and Fe3+, is weakly impacted by land use. Based on a geographic visualization of the scores for the four different factors and the patterns for land use, we can demarcate the areas where the groundwater in shallow unconfined aquifers is more vulnerable to being polluted by specific contaminants. We can then prioritize the areas where more intensive monitoring might be required, evaluate current land use practices, and adopt new measures to better prevent or control groundwater pollution.

Numerical Modeling of Surface Water and Groundwater Interactions Induced by Complex Fluvial Landforms and Human Activities in the Pingtung Plain Groundwater Basin, Taiwan

The landforms and human activities play important roles in quantifying surface water and groundwater interactions (SGIs) for water resources management. The study uses the groundwater and surface water flow (GSFLOW) model to quantify the dynamics of SGIs in the Pingtung Plain groundwater basin (PPGB) in southern Taiwan. Specifically, the study uses a physical-based numerical model to quantify the spatial and seasonal variations of water cycles influenced by complex fluvial landform conditions and human activities. Results of the model calibrations show good agreement with the data obtained from the available groundwater monitoring network and the selected stream stations. The basin-scale water budgets show highly nonuniform precipitation in the study area, and over 80% annual precipitation is from wet seasons in the PPGB. With high permeable surficial deposits in the PPGB, the year-averaged surface runoff and infiltration are approximately 57% and 40% of the total precipitation. The fluvial landforms with the high slope in the PPGB lead to 70% of annual surface runoff that becomes the streamflow, and the interflow dominates water interactions near streambeds. Results show that the interflow rate in the wet seasons is 200% more than that in the dry seasons. The net groundwater discharge to the streams is relatively small as compared to the interflow. Only 10% of the river flow is from the net groundwater discharge. In the PPGB, The pumping-induced variations of groundwater levels are insignificant as compared with the factor of the natural landforms. Because of the relatively small area of the proposed artificial lake, the contribution of the artificial lake on the local water budgets is insignificant, indicating the low impact of the artificial recharge lake on the surface water environment.

Modelling seawater intrusion in the Pingtung coastal aquifer in Taiwan, under the influence of sea-level rise and changing abstraction regime

A three-dimensional variable-density finite element model was developed to study the combined effects of overabstraction and seawater intrusion in the Pingtung Plain coastal aquifer system in Taiwan. The model was generated in different layers to represent the three aquifers and two aquitards. Twenty-five multilayer pumping wells were assigned to abstract the groundwater, in addition to 95 observation wells to monitor the groundwater level. The analysis was carried out for a period of 8 years (2008–2015 inclusive). Hydraulic head, soil permeability, and precipitation were assigned as input data together with the pumping records in different layers of the aquifer. The developed numerical model was calibrated against the observed head archives and the calibrated model was used to predict the inland encroachment of seawater in different layers of the aquifer. The effects of pumping rate, sea-level rise, and relocation of wells on seawater intrusion were examined. The results show that all layers of the aquifer system are affected by seawater intrusion; however, the lengths of inland encroachment in the top and bottom aquifers are greater compared with the middle layer. This is the first large-scale finite-element model of the Pingtung Plain, which can be used by decision-makers for sustainable management of groundwater resources and cognizance of seawater intrusion in coastal aquifers.

Land subsidence and managed aquifer recharge in Pingtung Plain, Taiwan

Taiwan is an oceanic nation with an area of approximately 36 000 km2. The Central Mountain Range was formed by the Eurasian and Philippine plates and stretches along the entire island from north to south, along the entire island, thus forming a natural line of demarcation for rivers on the eastern and western sides of the island. Pingtung Plain is formed by Quaternary alluvial fan material from the three main rivers. The aquifers comprise very coarse permeable sands and gravels under phreatic conditions in the north and less permeable sands under confined or even artesian conditions in the southern part of Pingtung Plain. The natural groundwater source is mainly from direct rainfall percolation and infiltration from the three main rivers, with their catchments lying partly outside the plain. The uncontrolled development of groundwater resources has led to undesirable effects, especially in the south, where aquaculture is concentrated. These effects are land subsidence, saline water intrusion, and lowering of water tables. It is thus one of the important key strategies in the solution of land subsidence, water resource development and flood control for sustainable development named Benefited Water Reuse from Storm Water in Pingtung, Taiwan. A serious of studies for this issue has been carried out. The feasibility study phase of the Artificial Recharge of Groundwater Project (ARGP) for Pingtung, Taiwan, was then implemented in 1997 using a MODFLOW simulation and an optimal model. Through the managed aquifer recharge model, the aquifer storage increases and the inundation scale decreases with land subsidence of the coastal area, as aimed at by the purpose of the controlled groundwater level. Infiltration mechanism simulation of artificial groundwater recharge, with the TOUGH2 model, was used to simulate the high infiltration behaviour in sequence in 2010. Both publications have been confirmed and approved by the Central Government and then approved by the environmental impact assessment. The ARGP was then implemented in May 2018 and operated for one wet season for the first phase with 50 ha and a total of 300 ha from May to October 2018. The sedimentation over the top of the recharge basin forms the clogging mechanism which has been analysed. The operation experience can further provide the mechanism process for research reference. The project, once implemented, can improve and mitigate land subsidence as well as formulate water banks for adapting and managing aquifer recharge.

Land subsidence due to groundwater extraction and tectonic activity in Pingtung Plain, Taiwan

Pingtung coastal plain, located at the active convergent boundary between Philippine Sea Plate and Eurasian Plate, is one of the most active areas regarding tectonic deformation in Taiwan. Groundwater over-pumping for aquaculture along the coast area of Pingtung plain induced a serious land subsidence problem for decades. How much land subsidence contributed by tectonic activity and groundwater pumping is a crucial issue for tectonic study and groundwater management in this area. This study collected the data in different fields and proposed a conceptual model to calculate the quantities of land subsidence caused by natural (tectonic) and human (pumping) factors. The data from the Global Positioning System (GPS) are used to illustrate the total subsidence concerning vertical displacement. A system called the multi-level compaction monitoring well (MCMW) is able to measure the vertical compaction in different depths from the earth surface to the depth of 200 m. Two GPS stations, named CLON and FALI, close to two MCMWs, named Jiadong and Fangliao, are adopted for analysis The data during 2007 and 2016 taken from MCMWs and groundwater observation wells indicate that the compaction in the shallow depth should be mainly caused by groundwater over-pumping due to their high correlation coefficients (from 0.58–0.95). The difference of the vertical deformation between GPS and MCMW indicates that there is deformation beyond the depth within 200 m. From the data and literature, the further vertical deformation should be due to tectonic activity associated with tectonic escape and extrusion of the Taiwan orogen with average vertical deformation from −3.0 to −4.4 mm. Therefore, the quantities of land subsidence contributed by local groundwater over-pumping and regional tectonic activities are successfully separated. The method and concept proposed in this study can be used in land subsidence quantification due to both tectonic activity and groundwater over-pumping.

Estimation of hydraulic parameter from geoelectrical measurements : A case study of the western Pingtung Plain in Taiwan

<p>In Taiwan, most of the rivers in the Pingtung Plain are ephemeral stream. In the dry season, the water source is unstable and cannot be used all the year round. Due to the uneven distribution of time and space, the supply and demand of water resources are often imbalanced. In order to provide a stable groundwater source strategy, it is necessary to understand the geologic characteristics of groundwater.</p><p>Electrical resistivity surveying methods have been widely used to determine the thickness and resistivity of layered media for the purpose of assessing groundwater potential and siting boreholes in fractured unconfined aquifers. In this study, we used CP configuration on the Electrical Resistivity Tomography (ERT) monitoring system at two study sites. One is the surface-borehole survey line at the Dashu, Kaohsiung City, Taiwan, and the other is the surface survey line at the Daliao, Kaohsiung City, Taiwan. Both of sites located on Quaternary alluvium of Pingtung Plain which composed of coarse sand and gravel.</p><p>The resistivity difference might be caused by the dynamic of the groundwater. We analyzed the change in the electrical properties of the gravel layer during the rainfall season at the Dashu site and analyzed the groundwater level change by ERT method during the pumping event at the Daliao site which is the pumping station to understand the groundwater replenishment situation. The ERT result can be calculated Relative Water Saturation (RWS) of the shallow formation fluid, and it reveal the permeability of the gravel layer and the hydrogeological characteristics of the sites. Furthermore, we assumed the different particle size and porosity to estimate the resistivity and the hydraulic conductivity coefficient theoretical trend line, compare the observation well data is used to estimate the actual porosity and the actual hydraulic conductivity range. Finally, for the groundwater conditions in the large area of the Pingtung Plain, we use the theoretical trend line to analyze the data of WRG’s 34 wells in western Pingtung Plain. The results show (i) the well logging resistivity is positively correlated with the hydraulic conductivity. (ii) the sandstone and mudstones with small variables have smaller well-measured resistivity and a lower hydraulic conductivity. (iii) affected by compaction, the porosity tends to decrease with increasing depth. (iv) on the west side of the Pingtung Plain, the particle sizes are relatively consistent, and the hydraulic conductivity is 10 <sup>-3</sup>~10 <sup>-4</sup> m/s.</p><p>This study is using ERT to provide hydrological data analysis on small areas and large areas in the Pingtung Plain, and also contributes to groundwater operations and management</p>