1. Introduction
Groundwater is an important water resource for domestic uses, drinking, agricultural irrigation, and industrialization, whereby 2.5 billion people globally depend on the groundwater sources [
1,
2]. In recent years, the issue of deterioration of groundwater quality has attracted worldwide attention due to the growth of agricultural areas, massive industrialization, and urbanization. The human population is predicted to have doubled from 3.9 billion to 7.0 billion over the last decades, but the need for water resources will be increased threefold. The swift population growth, industrialization, urbanization, and the expansion of the agricultural sector all inflict pressure on the existing water resources [
3]. The need for clean water sources for domestic and drinking purposes has doubled compared to the population growth worldwide [
4]. Approximately 1.8 billion people worldwide are anticipated to face water shortages and it is predicted two-thirds of the population will experience water stress by 2025 [
5].
Moreover, the depletion of surface water due to seasonal changes such as climate change and prolonged drought seasons affect the storage of surface water. Malaysia faced a prolonged dry season due to El Nino in 2019 and several states were affected, especially in Peninsular Malaysia. The utilization of groundwater is significantly important due to the water shortage in dry periods, the water demand due to population expansion and the deterioration of the surface water quality. The negative impact due to water shortages contributes to the main problems to the development of society, the urban development, and the basic life of the people [
6,
7]. Therefore, water shortage issues require alternative water resources to preserve water sustainability and for development of socioeconomic.
The quality of surface water is a sensitive issue where its emphasis on environmental sustainability, social welfare, and long-term economic development. The awareness of water pollution rises attention worldwide in recent years. Therefore, the sustainability of water quality needs to be implemented to achieve good management of water resources. Furthermore, a suitable index is required to assess the quality of water. The water quality index is one form of method or indicator in shortening complex water quality data and making it easier in communicating with general society. To date, Malaysia currently uses the existing Water Quality Index (WQI) (by the Department of Environment, DOE) and Drinking Water Quality Standard (by the Ministry of Health, MOH).
As an alternative of the clean water source, groundwater also needs the creation of an index to assess the level of quality. According to [
8], the lack of creation of a groundwater quality standard in Malaysia and the development of standardization for contamination of groundwater and land have become gaps in the study of the present status of groundwater in Malaysia. The assessment of hydrogeochemical properties of groundwater quality is crucial to sustaining the use of fresh groundwater aquifers for domestic, agricultural, and industrial utilization. Hence, the GWQI is a significant element in water resource management. By developing the index, the complex expressions of groundwater variables can be simplified [
9]. Furthermore, the GWQI is defined as a dimensionless number that combines the multiple variables of groundwater quality into a single number of standard values to the rating curves and simplifies the interpretation of the data monitoring [
10].
In the context of environmental indices, the first water quality index was proposed by Horton in 1965 [
11]. In 1970, Brown developed a water quality index which was later improved by Deininger [
12]. A novel kind of environmental index was introduced by Steinhart in 1982 to gather the trends and the status of technical information in the US/Canada ecosystem of the Great Lakes [
13]. Later, several types of water quality indexes were developed covering the aspects of drinking, river, marine, irrigation and recreation. Consequently, an index for determining the quality of groundwater is also needed in the environmental assessment to ensure its good quality.
Multivariate statistical techniques were used to determine the dominant parameters in the groundwater quality index. Principal component analysis (PCA) is the most prominently utilized method in hydrochemistry studies by normalizing the variables in the dataset and employing a correlation matrix [
14]. PCA is preferable in scientific studies because this analysis can reduce the dimensionality of a dataset while maintaining the characteristics of variables which contribute to the respective variation [
15,
16,
17].
The development of the GWQI needs to consider suitable parameters. The groundwater mainly consists of a series of major ions [
18]. According to the previous studies, the suitable parameters to determine the quality of groundwater consist of physical and chemical parameters. The physicochemical parameters include temperature, electrical conductivity, pH, salinity, turbidity, total dissolved solids (TDS) and dissolved oxygen (DO) [
19,
20]. Furthermore, seven important parameters which are responsible for 95% of groundwater analysis comprise major ions including magnesium (Mg), calcium (Ca), sodium (Na), potassium (K), sulfate (SO
42-), chloride (Cl
−) and bicarbonate (HCO
3−) [
21,
22,
23].
Hence, these parameters are considered as the basic qualities of groundwater and approximately more than 40% of the groundwater studies in Malaysia are associated with these parameters [
19,
24,
25,
26,
27]. Meanwhile, this study considered the parameters from the four important groups of parameters which were physicochemical characteristics, aggregate indicator, major ions, and trace elements. The suitable parameters to be assessed in the GWQI were selected from these groups of parameters. The potential of groundwater sources in agricultural areas needs to be assessed to ensure its suitability and safety for agricultural use. Hence, the aim of this study is to develop a suitable GWQI for domestic purposes, specifically in a tropical climate country such as Malaysia. The study areas focused mainly on the agricultural areas that uses groundwater wells as their alternative water sources.
4. Conclusions
The main contribution of this study is to develop a GWQI for agricultural areas to determine whether the groundwater is suitable to be an alternative clean water supply. The development of the GWQI was focused on the agricultural areas to identify the dominant parameters that could be the main indicators in the index to assess the quality of the groundwater. Referring to the results obtained, the application of fertilizers and manures from agricultural practices does not significantly show any degradation of the quality of groundwater.
Principal component analysis (PCA) was used to determine the dominant parameters in the groundwater quality index. Seven dominant parameters were selected as index indicators to assess the quality of groundwater in the agricultural area, which are magnesium, calcium, potassium, sodium, chloride, electrical conductivity, and chemical oxygen demand. Major ions were more dominant parameters that explained the groundwater source as these ions are generally abundant in groundwater sources. The abundance of major ions in the groundwater sources in Kuala Langat potentially contributes to the high electrical conductivity in the groundwater. Meanwhile, COD is an indicator of the capacity of water to consume oxygen during the decomposition and oxidation process of the major ions in the groundwater.
The GWQI used graphical features to describe the quality of the groundwater. The graphical index is user friendly and easier to comprehend. The GWQI comprises four categories named Class I, II, III, and IV in this study. Therefore, each class defines water as Excellent, Good, Fair, and Poor according to each class to determine the quality of groundwater. The GWQI has been verified for the purpose of testing whether the index is suitable to be used in different agricultural areas. Groundwater wells in Kampung Banggol, Kota Bharu, Kelantan were selected to verify the GWQI due to the fact this area is the prominent agricultural hub in Malaysia and is located near to the coastal areas. Besides, the groundwater sources have been used for many years ago in Kelantan for domestic and irrigation purposes on a big scale. The sensitivity of the GWQI index also has been analyzed and it shows high sensitivity to any changes in the pollutant parameters so the results from this index are reliable and significant.
The GWQI can be used to assess the quality of groundwater especially in locations which present similar characteristics like Kuala Langat, Selangor and Kota Bharu, Kelantan where this area located near the coastline and consists of agricultural areas. The development of GWQI is beneficial to the public, practitioners, and industries, and from another angle, this index can help to detect any form of pollution. Therefore, groundwater pollution can be controlled by detecting the source of the pollutant.
Regarding the groundwater sources in Kuala Langat, Selangor, and Kota Bharu, Kelantan, it can be concluded that the groundwater from both areas is very suitable for clean water and domestic uses such as washing and bathing. Due to a variety of unforeseen factors such as drought, water rationing, population expansion, and surface water pollution, the groundwater sources are suitable as an alternative to accommodate the water necessity. Therefore, the groundwater sources were evaluated in the current study wherein Kuala Langat, Selangor, and Kota Bharu, Kelantan potentially can be utilized as a source of clean water for domestic uses.
As an addition for the recommendation in the future study, the assessment of the geological analysis is necessary to determine the influences on different ion concentrations in groundwater. The soil types and characteristics play important roles to help identify the dominant elements in groundwater due to the thickness of hard rock, soil type, and the characteristics of rock and aquifer show the different accumulation of elements.
A detailed assessment regarding the land uses for agricultural areas is required to determine the substances potentially derived from agricultural practices that might permeate into groundwater. The fertilizer application from agricultural practices that affect the quality of groundwater is difficult to predicted unless a study on the soil is done. Therefore, a good agricultural practice system in agricultural areas can mitigate the contamination of groundwater sources.
Moreover, the clinical testing and health study details are also recommended for the purposes of determining if groundwater is a suitable alternative for drinking water. The use of groundwater for drinking purposes requires a detailed assessment in terms of evaluation of parameters to establish whether it is safe for human consumption as recommended by the Ministry of Health.