Research papers
Predictions of groundwater vulnerability and sustainability by an integrated index-overlay method and physical-based numerical model

https://doi.org/10.1016/j.jhydrol.2021.126082Get rights and content

Highlights

  • An integrated physical and index-overlay method helps predict groundwater vulnerability.

  • The numerical model improves the accuracy of the index-overlay method.

  • Land use plays an essential role in regional variations of groundwater sustainability.

  • The groundwater system in the Pingtung plain is in a critical condition.

Abstract

This study presents a new concept that integrates the index-overlay method and a physical-based numerical model for predicting groundwater sustainability under various climate conditions and anthropogenic activities. The index-overlay DRASTIC method was modified with an analytical hierarchy process theory and employed to create groundwater vulnerability maps for the Pingtung plain groundwater basin in southern Taiwan. The physical-based MODFLOW model was used for predicting the dynamics of a basin-scale groundwater system. Solutions and calibrated hydrogeological parameters in the MODFLOW model provide feedback to the factors in the modified DRASTIC method which enables predicting groundwater vulnerability. In this study, different climate conditions were considered in the numerical model to obtain the changes in depth of water and net recharge for predicting future groundwater vulnerability and for evaluating the current state of the sustainability indicators. Results show that the depth of water and net recharge obtained from the groundwater model improve the accuracy of the groundwater vulnerability prediction. The variations of future climate conditions have less influence on the variations of groundwater vulnerability because of the dense river network that controls the shallow groundwater levels in the Pingtung plain groundwater basin. Therefore, the influence of climate conditions on the risk of groundwater contamination is also relatively low. Based on the analysis of the sustainability indicators, we found that the groundwater resource system in the Pingtung plain groundwater basin is in a critical condition of high vulnerability.

Introduction

Groundwater (GW) is a vital supplement for domestic use and essential for agriculture, industry, and related ecosystems in semi-arid regions where surface water is scarce (Aller et al., 1987). However, population growth, agricultural activities, urbanization, and climate change are threatening the groundwater quantity and quality in many groundwater basins (Rahman, 2008). The effective management and development of policies are of great importance for ensuring the sustainable use of groundwater. Over the past few decades, the integration of various tools or methods has been proposed to protect and manage groundwater deterioration and degradation (Chang et al., 2017, Jang et al., 2016, Lin et al., 2017), whereby providing scientifically plausible instruction for policymakers and governors.

Groundwater vulnerability (GV) is an effective technique for practical groundwater pollution prevention and remediation tasks (Jang et al., 2017). Existing methods for assessing groundwater vulnerability can be classified into three categories (Nobre et al., 2007): (1) index and overlay methods, (2) process-based methods, and (3) statistical methods. One of the popular index-overlay methods for quantifying groundwater vulnerability is the DRASTIC method, which was initially developed by the US EPA for ease of use, minimal data requirements, and a clear interpretation of GV (Aller et al., 1987). Furthermore, associated with GIS tools, the DRASTIC model can be applied for large-scale regions with low application costs. However, it has some limitations because the ratings and weights assigned in the model depend on the site-specific conditions and judgment of experts (Busico et al., 2020). Also, in recent years, many studies have modified the DRASTIC method by adding parameters or ignoring the existing parameters according to site-specific settings (Huan et al., 2012, Kazakis and Voudouris, 2015, Sadat-Noori and Ebrahimi, 2016, Sener et al., 2009). The conventional DRASTIC model could be calibrated and verified by using the analytic hierarchy process (AHP), geo-statistics, artificial neural networks (ANN), or artificial intelligence (AI) to optimize the weights and to fit the observations (Baghapour et al., 2016, Dixon, 2005, Goudarzi et al., 2017, Saida et al., 2017). Previous studies proved that the DRASTIC model coupled with AHP could produce high accuracy and reliable GV maps, which can be validated by field measurements of pollutants (Neshat et al., 2014, Sener and Davraz, 2012, Thirumalaivasan et al., 2003). In addition, land use is also one of the critical components for improving the reliability of GV because of its influence directly on GV. The integration of land-use factors and the conventional DRASTIC model was proposed by many researchers to predict the impacts of human activities on groundwater quality (e.g., Huang et al., 2009, Li and Merchant, 2013, Lima et al., 2011).

Previous studies have conducted investigations of the GV approach, which focused on the prediction of GV under the impact of climate change. Climate change can potentially affect the variation of GV, which is heavily dependent on factors such as depth to water (DTW), net recharge, or land-use conditions. Therefore, focusing on GV that may be affected by both climate and land-use changes is an excellent approach because it provides governments with valuable information on groundwater resources management. Several studies have quantified GV under different scenarios of climate and land-use; for example, Huang et al. (2009) predicted future GV under a combination of climate change scenarios and urbanization. They concluded that the whole Hunan province faces a high risk of groundwater pollution in the future. Li and Merchant (2013) determined how and where changes in depth to water, net recharge, and land-use caused by different scenarios of climate change affected GV. The results showed that the north and northwest regions of Eastern North Dakota would continuously have high GV under different scenarios of climate conditions. Nevertheless, it can be seen that in these studies, groundwater recharge and DTW were estimated based on the empirical equations or hydrological methods without considering the geological or hydrogeological condition and other parameters, as well as lack of calibration and validation.

Physical-based numerical models play an essential role in the planning and management of GW resources and in forecasting the effects of management measures. The models have been used as assessment tools for evaluating recharge, discharge, and aquifer storage processes and as prediction tools for forecasting future conditions or impacts of human activities (Zhou, 2009). With the rapid improvement of computing power and the widespread use of Geographic Information System (GIS), significant progress has been made in the use of 3D groundwater flow models such as MODFLOW. MODFLOW is a US Geological Survey modular finite-difference flow model with a flexible modular structure, complete coverage of hydrogeological processes, and is available in the public domain. Although applications of the groundwater flow model (MODFLOW) for various purposes have been conducted in many studies (e.g., Singh, 2013, Mahmoudpour et al., 2016, Mi et al., 2016, Huang and Chiu, 2018, Lin et al., 2017, Li et al., 2016, Su et al., 2020), investigations on the combination of a physical-based model and an index-overlay method to quantify the variation of groundwater vulnerability have not yet been reported. Physically meaningful parameters such as the water pumped from the aquifers or the precipitation influenced by climate change may interact with other parameters in the DRASTIC method. Users adjust the DRASTIC settings and weightings to reflect changes in hydrogeological conditions, however the characteristics of the DRASTIC parameters cannot solely rely on changing the weightings without considering the physical mechanisms. A potential solution to such a situation is to use a physical-based model to quantify changes in hydrogeological conditions. Based on the fixed parameters and weightings, the variation of vulnerability in the DRASTIC method can then be obtained by applying the variations of parameters obtained from a physical model. Moreover, GV is one of the key components to evaluate the degree of GW sustainability, which supports the policymakers in establishing comprehensive protection and quality conservation policies for highly vulnerable areas. GV informs the public about the potential of GW contamination and the necessary solutions for management and protection. The GV indicator is a practical economic and social tool for protecting the GW system through land-use planning (Vrba and Lipponen, 2007).

In addition, to reach the goal of sustainable and reasonable GW utilization and management, it is necessary to develop the policies and to implement effective groundwater management options in practice that need managerially important information on the quality and quantity of groundwater systems. The relevant information can be quantified through the indicators, which act as an essential assessment tool for policymakers, planners, and the public (Vrba and Lipponen, 2007). The sustainability indicators (SIs) of GW management provide information about the current status and trends in groundwater systems. They also help analyze the impacts of anthropogenic activities and the extent of natural processes on the groundwater system and promote the awareness of the public in the utilization and management of GW sustainably (Vrba and Lipponen, 2007). However, they also have drawbacks because of their dependency on social-economic development, geographical conditions, and hydrogeological structures (Russo et al., 2014). Due to distinct study areas, the characteristics of an aquifer system will vary dramatically, which means that the fundamental analysis of each aquifer system is unique (Juwana et al., 2012). Vrba and Lipponen (2007) developed a meaningful set of 10 indicators from more than one hundred conceptual water-related indices as guiding principles. These indicators have been employed to assess groundwater sustainability in several countries at the national level (Hirata et al., 2007, Lavapuro et al., 2008, Perez et al., 2015). None of them considered a physical-based numerical model as a tool for providing information on aquifer characteristics to calculate the sustainability indicators. Water balance, the dynamics of groundwater levels, or estimation of net recharge would be extracted from the calibrated GW flow model to support the assessment of GW sustainability. This paper is a first attempt to integrate a physical-based groundwater flow model and proposed sustainability indicators for delineating a complete representation of groundwater system sustainability.

In this study, we propose a concept of integrated procedures that is capable of quantifying groundwater vulnerability changes to site-specific problems under changing climate conditions while evaluating the sustainability of groundwater resources in the Pingtung plain groundwater basin in southern Taiwan. Generally, the MODFLOW model was used for simulating the physical responses of the groundwater system. The aquifer parameters calibrated in the numerical model provide feedback for the key parameters such as the depth of water, net recharge, and hydraulic conductivity used in the DRASTIC method. The DRASTIC method incorporated with land-use and AHP theory was then employed to quantify the groundwater vulnerability, which was validated by the field data from nitrate measurements. This integration of the MODFLOW and DRASTIC system acts as an assessment process to calculate the vulnerability changes induced by climate conditions. Different scenarios of climate conditions are applied to the numerical model to obtain changes in water level and net recharge in the aquifer systems. Additionally, the sustainability of groundwater resources in the study area has been evaluated based on the water budget from the MODFLOW model and the results of GV in the current year of 2017. The results of this study are expected to provide useful information about the status of the groundwater resources system that supports policymakers in planning and adopting strategies of sustainable groundwater management.

Section snippets

The conceptual framework

The objective of this study is to propose a new concept that integrates a physical-based numerical model with an index-overlay method and sustainability indicators to quantify changes in groundwater vulnerability affected by climate conditions, as well as to assess the GW resource sustainability. To integrate the index-overly method with the physical-based model, the parameters in the index-overlay method interact with the parameters and/or site-specific conditions in the calibrated

Data preparation and processing of GV mapping

Data preparation and processing of the modified-AHP DRASTIC approach is described in detail by Vu et al. (2019). In this study, we only briefly present the critical information of GV mapping. Parameter maps for GV mapping were created by different types of data using the ArcGIS environment. The study area was discretized using a grid of 1000x1000m, which has a projection of TWD_1997_TM_Taiwan. The Ordinary Kriging interpolation method was used to present the spatial distribution of all DRASTIC

Conceptual model

The steady-state GW flow model of the Pingtung plain was developed using MODFLOW-2000, as presented briefly in Table 2. The model has a domain area of 1355 km2, which is oriented north–south and discretized into 90 rows and 40 columns with a uniform grid size of 1000 × 1000 m. The aquifer system can be schematically subdivided into three hydrogeological layers that correspond to the well-field inventory by the defined depths of 10–30 m, 30–60 m, and below 60 m (Ting, 1992). The topmost layer is

Physical-based GW model

In this study, the GW flow system in the Pingtung plain was modeled under steady-state conditions. The output of the GW flow model constitutes the GW balance, the physical response of the GW system under different constraints, and the interactions of GW and surface-water, as well as flow patterns. The model must first be calibrated to estimate model parameters and conditions that are best matched between measured and simulated heads. The model calibration attempts to simulate the GW flow

Conclusions

This study has presented a new concept that integrates the MODFLOW model and the DRASTIC method for predicting GV and groundwater sustainability in the Pingtung plain groundwater basin. The assessments focus on various climate conditions that are simulated by the MODFLOW model. The predictions of future GV and groundwater sustainability then rely on the results obtained from the calibrated GW flow model. The calibrated depth of water, net recharge, and hydraulic conductivity generated from the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was partially supported by the Ministry of Science and Technology, Republic of China under grants MOST 107-2116-M-008 -003 -MY2, 108-2625-M-008 -007, 108-2638-E-008-001-MY2 and 109-2621-M-008 -003.

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