Spatial and physicochemical assessment of groundwater quality index in the urban coastal region of Sri Lanka

This study used the groundwater quality index (GWQI) and Geographic Information System (GIS) techniques to examine groundwater quality in the western coastal region of Sri Lanka. The spatial and temporal variation of 18 groundwater samples' physiochemical parameters [pH, electrical conductivity (EC), turbidity, total dissolved solids (TDS), sodium (Na + ), potassium (K + ), calcium (Ca 2+ ), magnesium (Mg 2+ ), chloride (Cl − ), and bicarbonate (HCO 3− )] were studied. According to the WHO and SLS, 11% of samples had EC levels that were above the acceptable range, and 22% had turbidity levels that were beyond the acceptable range. When considering, pH, TDS, other cations, and anions analyzed in the study were still below the standard permissible levels. The western coastline region, as well as several areas of the central study region, had signicant concentrations of physicochemical parameters. According to the GWQI, water was consumable in 77.78% of locations in the study region and unsatisfactory in 22.22%. Furthermore, due to severe coastal erosion, the quality of groundwater in the study region is deteriorating, therefore maintaining a comprehensive groundwater management strategy to promote sustainable water consumption is imperative.


Introduction
Groundwater pollution has a signi cant impact on the environment and human existence today, as it is the primary source of water (Farzaneh et  To address the issue of water scarcity and management, the United Nations (UN) has made priorities to achieve clean drinking water for everyone in their Millennium Development Goals and in the Agenda 30 by 2030 (WHO 2017). As a result, to combat ground water pollution, more consideration should be given, as many countries are currently experiencing a scarcity of fresh water resources (Pant 2011;UNEP 2018).
In several regions of Sri Lanka, speci cally in the Maha Oya river basin, there is a high amount of contamination. 74% of inhabitants in the western coastal region utilize their land for residential and industrial purposes (tourism, hotels, and restaurants), whereas 24% use their lands for agricultural production. Water contamination in the Western coastal region is induced by nutrient and toxic inputs from agricultural, as well as urban and industrial development. In the Maha Oya basin, untreated domestic water is usually released into rivers (Hayzoun et al. 2015). In light of these many issues, policymakers and decision-makers depend heavily on WQI to estimate the e ciency of a water source while also determining the usefulness of initiatives and activities aimed at improving groundwater quality. The Water Quality Index can be used to determine the quality of water (Jain et  From the Maha Oya River Mouth left bank, which is the southern boundary of Puttlam District, to the northwards, the study region encompasses a 24 km 2 area. The mouth of the Gin Ganga River is also included in the case study area. This region, which is part of the Wennappuwa Divisional Secretariat Division, comprises 28 Grama Niladhari Divisions. According to the 2012 census statistics from the Department of Census and Statistics, the population density distribution of this area is 1746km 2 . The alluvial deposits and ferruginous gravels, as well as the unconsolidated sands and spits of the coastal region, retain groundwater. The most prevalent groundwater abstraction technologies are dug wells, dugcum bore wells, and bore wells, and their yields are mostly governed by the recharge conditions in the region. The annual average rainfall in the district is 1174 mm, with November being the wettest month of the year. Puttalam, which is located on the seashore, is mostly at, though the land rises to approximately 60 meters inland. There are regions of reddish brown earth and low humic gley soils inland, and the soils are mostly red-yellow latosols.

Groundwater sample collection and chemical analysis
Groundwater samples were gathered at 18 locations between January 2019 and January 2020, from a drilled well that had previously been dug or a deep bore hole that had previously been drilled. The groundwater samples were taken according to APHA guidelines (APHA 2012) ( Table 1). The water samples were collected in pre-cleaned 1L high-density polythene sample vials. The samples were tagged and delivered to a chemical laboratory, where they were physicochemically evaluated under 4°C conditions (Jehan et al. 2020).
Using Arc GIS 10.2 software, the case study region will be partitioned into a 9«5 grid. A GPS position at the mid-point of selected spots for water quality monitoring method has been navigated in each cell of the grid in the study region. A handheld optical pH/EC/TDS meter (Hanna HI 9811-5) was used to determine physico-chemical parameters such as electrical conductivity (EC), hydrogen ion concentration (pH), and total dissolved solids (TDS) in the eld. The basic protocols of the American Public Health Association were used to examine other chemical parameters (APHA 2012). The AgNO 3 titration was used to determine the chloride (Cl − ) concentration. The magnesium (Mg 2+ ) was calculated using equations 1 and 2 (Adimalla & Taloor 2020).

Spatial distribution maps
The exact sampling locations were marked using a portable Global Positioning System (GPS, Garman

Groundwater quality index (GWQI)
The ground water quality index (GWQI) was developed using the collected data to assess the water's acceptability for drinking ( (1) has been assigned to parameters such as Na + and K + because they are less important in groundwater quality evaluation (Rajmohan 2021), and a maximum weight of ve (5) has been assigned to parameters which are more pertinent in groundwater quality evaluation such as TDS (Table 1).
According to WHO guidelines, each parameter's consistency rating scale (Qi) is calculated by multiplying its concentration in each water sample by its corresponding standard and then multiplying by 100 (Eq. 4).
Where ci is the concentration of each criterion of groundwater quality, Qi is the quality rating, and Si is the chemical parameter's recommended guideline value. Using equations 5 and 6, the Sub-index (SIi) and GWQI were determined.

Physicochemical parameters of the ground water
The signi cance of groundwater resource quality is critical since it is a fundamental factor in determining its suitability for potable use in the studied area. The descriptive information for the various physicochemical properties of groundwater samples is shown in Table 2. The data were also compared to WHO (2017) and SLS Guidelines to see if they were suitable for drinking in the study location. pH measures the acidity and alkalinity of groundwater. Despite the fact that pH has no direct impact on human health, it is one of the most important water quality parameters. According to WHO guidelines, a suitable pH range of 6.5 to 8.5 is recommended (WHO 2017). The groundwater samples were acidic to alkaline in nature, with a pH ranging from 6.21 to 7.68, with an average of 6.93, according to the study's ndings ( Table 2). The pH range for groundwater in Sri Lanka was 6.50-9.00, with a maximum permissible level of 9.00. However, no location was found to be above the maximum permitted limit in any of the groundwater tests (Table 2). According to Sampath et al. (2011), the pH range of ground water in Sri Lanka's Puttlam district was 6.30-8. 20. In addition, Young et al. (2011) detected a pH range of 5.76 to 8.70 in Sri Lanka's north-western province. The pH variation in ground water in Sri Lanka's western province was below the SLS permitted level (4.0 to 8.2) and was not hazardous for drinking (Premalal & Jayewardene 2015). Figure 2a depicts the geographic distribution of pH in the research area.

Electrical conductivity (EC)
According to (Kanga et al. 2020), the ionic concentration of groundwater is commonly measured by calculating the EC, which varies depending on the concentration, type of ions present in the water, and temperature. The EC in the research area's groundwater ranged from 430.10 to 99144.72 S/cm, with an average of 10709.76 S/cm ( Table 2). According to WHO drinking water recommendations, the maximum permitted EC concentration in water is 1500 S/cm (WHO 2017). The maximum allowed level of groundwater in Sri Lanka is 750 S/cm. Only 11% of groundwater samples were found to be above the permitted level (Table 2). Figure 2b depicts the geographic distribution of EC in the research area.
According to a study conducted by Sampath et al. (2011), EC levels were greater than the acceptable values of WHO drinking water quality guidelines in 76% of areas in the Puttlam area of Sri Lanka. Another study in Sri Lanka's Puttlam area found that the maximum permitted amount of EC has been exceeded by the WHO Table 2) The concentration of Mg 2+ in the sample region's groundwater was determined to be below the WHO (2017) or SLS-recommended maximum permissible level. The north and south-western regions of the sample area had higher concentrations (Fig. 3b). According to the study's ndings, the Ca 2+ concentration in groundwater in the research area ranged from 5 to 43 mg/L, with an average of 20 mg/L. (Fig. 3c). 3.1.5 Sodium (Na + ) and potassium (K + ) The geographic distributions of Na + and K + are depicted in Figures 4a and 4b, respectively. The research area's central and northern regions have higher Na + concentrations, while the northwestern and southeastern regions have higher K + concentrations. The content of Na+ in groundwater, on the other hand, varied from 22 to 173 mg/L, with a mean of 60 mg/L. ( Table 2). The ndings revealed that none of the groundwater samples tested ful lled the WHO and SLS criteria (WHO, 2006). The average Na + and K + concentrations in the north-western province, according to Young et al. (2011), were 79.77 mg/L and 6.12 mg/L, respectively. K + is the most important nutrient for humans (

Turbidity
Turbidity refers to the water's relative clarity, which inhibits light transmission. The turbidity of ground water in the research area ranged from 0.62 to 41.67 NTU (Figure 5c). The maximum turbidity level was surpassed in 22% of locations, according to the SLS and WHO drinking water quality guidelines. As a result, it's unlikely to be suitable for drinking. Several earlier investigations have found that the turbidity in the groundwater of Puttlam district was much higher than the WHO and SLS permitted values.According to Galhenage et al. (2021), turbidity ranged from 1.6 to 164 NTU, while Gunarathna et al. (2021) found that turbidity ranged from 44 to 723 NTU. Furthermore, the study found that the mean turbidity in ground water was much higher than in surface water (Gunarathna et al. 2021). Table 3 shows the GWQI values calculated for each groundwater sample. Table 3 shows that GWQI values in groundwater in the research region ranged from 45.12 to 2700.55, with an average of 337.78.

Conclusions
The groundwater condition in the western coastal region of Sri Lanka begins to depreciate largely due to seawater intrusion and therefore, the water consumption has become uncertain. Hence, investigating water quality in the groundwater was crucial in the western coastal region in Sri Lanka. This study was used GIS to create a spatial distribution map of various physicochemical parameters and locate suitable and unsuitable groundwater quality zones for drinking and evaluated groundwater quality by developing a groundwater quality index (GWQI).
The groundwater in the research area is neutral to slightly alkaline in the composition according to the study. The most abundant cations and anions were Na + and Cl − , respectively. This could be owing to the region's coastal erosion and seawater intrusion. Ca 2+ (5-43 mg/L), Na + (22-173mg/L), and K + (2-24.67mg/L) ions concentrations in the groundwater of the research region are within the maximum allowable limits when compared to WHO (Mg 2+ 300mg/L, Na + 200mg/L, K + Not recommended) drinking water quality guidelines and Sri Lankan water quality standards (Ca 2+ 100mg/L, Na + 200mg/L, K + Not recommended). Excessive quantities of Turbidity (22%) and EC (11%) have also been discovered at a few groundwater sample sites in the research area. According to the GWQI, the groundwater quality in the study region ranges from excellent to poor for drinking. Turbidity and electrical conductivity had the greatest impact on groundwater quality index. The groundwater samples that are acceptable for drinking comprise 77.78% of the total, while the samples that are not appropriate for drinking constitute 22.22%. The research area's poor and un t water for human consumption is mostly concentrated in the center and western coastal areas according to the spatial distribution maps. Therefore, to safeguard community well-being and safety, it is essential to make consistent monitoring valuation of seawater intrusion in the western coastal province of Sri Lanka.

Declarations
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Availability of data and materials -Not applicable  The selected case study region     Classi cation of groundwater quality index (GWQI) in the groundwater of the study area.