Groundwater Quality Mapping using Geographic Information System: A Case Study of District Thatta, Sindh

Access to safe and affordable drinking water for all is an important goal of SDGs (Sustainable Development Goals). Degradation of water quality of coastal aquifers is a major concern throughout the world including the Indus River delta. Looking at the present changing climate scenario, the study was conducted to assess and map the spatial variation in the groundwater quality of district Thatta using GIS (Geographic Information System). The groundwater samples from hundred (100) randomly selected hand pumps of the district were collected such that all union councils of the district were sampled. The water samples were analyzed for different physicochemical parameters, i.e. taste, color, odor, pH, turbidity, EC (Electrical Conductivity), calcium, magnesium, total hardness, chloride, total dissolved solids, and arsenic using standard laboratory techniques. The results of water analysis revealed that 85% of the groundwater samples had TDS (Total Dissolved Solids) concentration beyond the permissible limit described by WHO (World Health Organization). Whereas, all the groundwater samples had chloride concentration beyond permissible limit of 250 mg/l. Analysis for arsenic revealed that only 20% of groundwater samples had a concentration higher than the safe limit of 10 ppb. The study indicated that in most of the areas, the groundwater quality was not as per drinking standards prescribed by WHO, hence was not suitable for drinking purpose. The GIS maps of groundwater quality parameters were prepared using spatial interpolation Kriging tool. These maps provide the visual analysis and interpretation of spatial variability of different groundwater quality parameters, hence are supportive in monitoring and managing the vulnerability of groundwater contamination.


INTRODUCTION
G roundwater is considered as one of the most vital renewable and extensively circulated resources of the earth which are also an important source of water supply throughout the globe [1]. It is used for domestic, agriculture, industrial and many other human needs. Human health isdirectly associated with groundwater; therefore, its quality is of a prime importance for human health [2]. In areas where potable water resources are limited, an accurate and reliable estimateof sustainable use or yield is critical [3].
Specifically, optimal or sustainable groundwater resource userequires setting upper limits on water withdrawal (or sustainable yield) to avoid compromising the source [4].
Estimates reveal that in Pakistan 30% of all diseases and 40% of all deaths occur due to use of contaminated water while every fifth person experiences illness because of contaminated water [5]. It is also reported that more than three million people encounter the water-borne diseases every year while 0.1 million die [6].
In coastal areas, up to one billion people utilize groundwater as the major source of drinking throughout the globe [7]. In such areas, seawater intrusion is the main environmental issue for contamination of groundwater [8][9][10]. The groundwater becomes unsuitable for domestic, irrigation as well as industrial purposes when mixed with salty water even in very small quantities [11]. In coastal aquifers, increase in seawater intrusion is an interrelated and dynamic phenomenon which depends on different parameters such as recharge, groundwater extraction from coastal aquifers, and the topography of such aquifers [12][13].
The unplanned and unsystematic groundwater withdrawal from aquifers is the primary reason of groundwater salinization in coastal areas [14]. Because of environmental change, sea level is rising that is also one of the fundamental cause of seawater intrusion and contamination of the coastal aquifers [15][16]. The deteriorating quality of water in developing countries has caused diarrhea [17], hepatitis E [18], dental caries and oral hygiene [19], anemia in children [20], reducing intelligence in children, and brain functioning and development [21].
In Pakistan, more than 60% of the population utilizes groundwater for drinking purpose [22] and the degradation of groundwater is one of the significant concerns caused by overexploitation of the groundwater resources and discharge of untreated polluted wastes into the water bodies. Sixty-Eight percent (68%) rural population of Pakistan is drinking the water of poor quality [23]. One hundred million cases of diarrheal diseases are being registered in hospitals of Pakistan within 1 year [24]. It is also reported that in Pakistan, around 0.25 million children die due to drinking of contaminated water [22]. Polluted water, purity, and hygiene practices are the fundamental cause of most diarrheal cases, the primary cause of around 0.25 million children die under the age of five years annually [22].
According to a study led by UNICEF, around 20-40% of the hospital beds in Pakistan were occupied by patients experiencing waterborne diseases,such as, typhoid, dysentery, cholera, and hepatitis, which are the reasons of 33% of all deaths [25]. The accessibility of safe and affordable drinking water significantly affects the waterborne diseases.
The southern Sindh province of Pakistan adjoins the Arabian Sea coast where drinking water quality is deteriorating due to the dumping of industrial and urban waste and use of agrochemicals and yet has limited freshwater resources [26]. The groundwater quality is badly affected by salinity, arsenic, fluoride and microbial pollution, which further deteriorates in low-lying, deltaic and floodplains of Sindh [27]. In these areas, people mostly depend on the groundwater for drinking and irrigation purpose because of low precipitation and reduced flow of Indus River [28]. In the coastal areas, seawater intrusion further degrades the quality of groundwater [2]. The contamination in groundwater in most of the areas of the Thatta district is due to seawater intrusion, as seawater contains many trace metals [29]. Therefore, monitoring of quality of drinking water is a key factor for control of waterborne diseases in such areas.
Initial studies on arsenic distribution in the groundwater show that high arsenic concentrations (10-600), exceeding WHO [30] permissible limit of 10 ppb, occur in the groundwater of Tando Mohammad Khan, Thatta, Mitiari, Khairpur and other parts of Sindh province of Pakistan [30][31][32][33]. Memon et. al. [26] conducted a study in the southern districts i.e. Thatta, GIS is an effective tool widely used for monitoring and mapping of the water quality, evaluating the spatial variability of water quality and detecting the environmental change [34]. Considering the concerns of civil society about the quality of groundwater of Thatta district, the present study was carried out. Distinctive physicochemical parameters of groundwater were analyzed and compared the standard allowable limits described by WHO to assess the quality of groundwater for drinking purpose. The study accordingly planned to assess the groundwater quality and develop a spatial distribution database using GIS interpolation techniques, based on the results of physicochemical analysis of groundwater of district Thatta. Theresults obtained from the study will be useful for government, policy makers as well as the public to be aware of existing groundwater contamination and will besupportive for the monitoring and managing the vulnerability of groundwater contamination in the district. The temperature ranges between 23.8-28.7 0 C [35][36]. The agriculture in the area depends on both the surface water (Indus River canals) and groundwater. It is a part of Lower Indus alluvial and a deltaic plain covered by thick alluvial deltaic sediments which host the aquifers in the area [37].

The Study Area
Its soil constitutes fine-grained sediments, rich in organic matter containing a high amount of arsenic, which is supposed to become part of aquifers by various geochemical processes [38][39]. Most of the residents of the district use groundwater extracted from the handpumps drilled into shallow aquifers down to 5-12 m for their drinking and other daily water use.

Analysis of the Groundwater Samples
The groundwater samples were collected from randomly selected 100 hand pumps installed at public places i.e. schools, bus stops, restaurants as well as at the residential places. These hand pumps were

RESULTS AND DISCUSSION
The water quality maps are useful for assessing the suitability of water for drinking purpose [1]. These maps provide the visual interpretation of spatial variability of different groundwater quality parameters, hence are helpful in monitoring and managing the vulnerability of groundwater contamination.

Taste, Color, Odor, and pH
The results of the study revealed that about 55% of the groundwater samples of the district Thatta had bitter and salty taste, while 16% samples had color values greater than the permissible limit of 15 TCU. However, odor and pH in most of the water samples were within the permissible limits set by WHO for human consumption. Spatial distribution of these parameters is shown in Fig. 3(a-d).

Turbidity
The groundwater quality analysis revealed that the turbidity in the groundwater of the study area ranged from 0.5-34.1 NTU with a mean value of 5.4 NTU while its permissible limit for drinking water is 5 NTU [30]. The suspended particles in water strengthen the connection of overwhelming metals and other dangerous minerals and pesticides, making the water cloudy and opaque, thus create human health issues [29]. The spatial distribution of turbidity in the study area is shown in Fig. 4. The Fig. 4

Electrical Conductivity
EC is the most significant parameter used as the primary index to decide the suitability of water for drinking as well as for irrigation purpose [40][41]. For drinking purpose, its permissible limit is 0.7 dS/m [30].

Calcium
The calcium is one of the fundamental parameters available in different types of rocks. Its maximum permissible limit in water for drinking purpose is 75 mg/l [30]. In the study area, the concentration of calcium ranged from 34-883 mg/l with an average value of 163 mg/l. The spatial distribution of calcium concentration in the groundwater of the study area is shown in Fig. 6. The spatial distribution indicated that calcium concentration was very high in the study area, which may be due to seawater intrusion in coastal aquifers of the study area.

Magnesium
Magnesium is another fundamental parameter available in different types of rocks. Its maximum permissible limit in water for drinking purpose is 50 mg/l [30]. However, in the study area, its concentration ranged from 24-156 mg/ l with an average value of 56 mg/l. The spatial distribution of magnesium in the groundwater of the study areais shown in Fig. 7. The spatial distribution shows that there was a high concentration of magnesium in many places of the study area which may be due to the geological formation of aquifers.

Total Hardness
Hardness is an important parameter of water for its use in a domestic sector [2]. The calcium and magnesium are key parameters for total hardness. In the case of hard water, the excess use of soap is needed to achieve cleaning.
Hardness affects the toxicity of many substances in the water [27]. As per WHO standards, the most desirable limit of total hardness for drinking purpose is 100 mg/l and the maximum acceptable limit is 500 mg/l [1].
According to Freeze and Cherry [42], the most desirable limit of TH for drinking water is 80-100 mg/l. In the study area, the total hardness values ranged from 57-883 mg/l with a mean value of 216 mg/l. In this case, just 11 samples out of 100 exceeded the maximum allowable limit of 500 mg/l. But according to Sawyer and McCarty [43], groundwater exceeding the cutoff limit of 300 mg/l is considered very hard. Such classification based on TH is given in Table 2.
The Table 2 shows that most the groundwater samples of the district fall in a hard water category. Around 60 samples out of 100 were considered as hard and need treatment before use for drinking purpose. The spatial distribution of TH in the groundwater of the study area is shown in Fig. 8. The spatial distribution shows that there was a high concentration of hardness especially in those places of the study area which are very close to the Arabian Sea.

Chloride
For potable water, maximum allowable limit of chloride concentration is 250 mg/l. In the present study, groundwater samples were contaminated by chloride concentration from 372-6275 mg/l with a mean value of 1505 mg/l. All the groundwater samples had chloride concentration beyond the allowable limit of 250 mg/l.
Chloride concentration in rainwater is usually under 10 mg/l, however, it might be high in coastal areas and in desert tracts [44]. The presence of chloride concentration beyond allowable limits in drinking water has a potential health impact, causes stomach discomfort, nose/eye irritation, and increases corrosive nature of water [29].

Total Dissolved Solids
The concentration of TDS in water is a general sign used to decide the suitability of water for drinking as well as including seawater intrusion [45]. The higher concentration of TDS causes undesirable taste, gastrointestinal irritation, corrosion or incrustation [27].
The spatial distribution of dissolved solids in the groundwater of district Thattais shown in Fig. 10. Spatial distribution map shows the highest values of TDS in many places of the study area which are probably close to the Arabian Sea.

Arsenic
Analysis of groundwater samples revealed that about

Overall Groundwater Quality with Respect to WHO's Permissible Limits
Overall summary of groundwater analysis with different water quality parameters lying beyondWHO's permissible limits is given in Table 3.