Fate of Urban Groundwater in Shallow Confined Aquifers: Case Study of Baldia Town, Karachi, Pakistan

A study was carried out to assess the groundwater quality for drinking purpose in Baldia Town, Karachi. For this purpose, groundwater samples (n = 18) were randomly collected from various depths (> 100 feet) through boring wells after monsoon season. Data revealed that except two (BT-5, 8) which were turbid and smoky, rest of the samples were colorless, non-turbid and sweet in taste. Groundwater temperature fluctuates between 19-26  ̊C. The pH varies between slightly acidic to slightly basic (range: 6.8-7.3) where two third of total samples have pH < 7. All the samples have very high TDS content (range: 1240-16910 mg/L; mean: 6832 mg/L) which exceeded the national drinking water quality standard (1000 mg/L) set by PCRWR. Hardness values varied in the extreme range (1000-9500 mg/L; mean: 2366 mg/L). Relative abundance of major cations follows the order of Mg > Ca > Na> K while anions varied in the order of HCO3> SO4> NO3> Cl. Dissolved Fe+3 (mean: 0.01 mg/L) varies within WHO permissible limit (0.3 mg/L) while Mn showed concentration < 0.01 mg/L. Concentration of trace elements declined in the order of Ni > Zn > Cr > Co. It is concluded that groundwater of study area not fit for drinking purpose. It is strongly influenced by semi-arid climate and water rock interaction which is manifested by geochemical signatures of limestone (Ca, Zn) and clays (Ni, Co, Cr). Due to confined aquifer system the anthropogenic contamination is not significant.

It becomes more appealing if the limestone exposures intercept the pristine meteoric water first. Based on this hypothesis, Baldia town was taken as a precedence to delineate the repercussion of water-rock interaction where limestone is forming catchment area. Moreover, industrial effluent and municipal sewage were also assumed to be the sources of contamination in urban set up of Karachi city. If this would be the case, then health and life of the public is questioned as a result of drinking such groundwater in study area. Hence, there is a dire need of assessing the fate of groundwater in urban set up where water is tapped from shallow confined aquifers. Despite such grave concern, no study has been carried out so far in study area to screen the groundwater for its quality determination. Therefore, present study is aimed at assessing the groundwater quality of Baldia Town using hydrogeochemical approach. Other objective is to find out the sources of contaminants in study area.

Study Area
Baldia Town is located in the western part of Karachi, Sindh, Pakistan with latitude and longitude of 24°55′28″N -66°58′25″E and 24.92444°N -66.97361°E respectively (Figure 1). The area is bordered by SITE Town and Orangi Town to the east and by Kemari Town to the north and west, with most of the western boundary formed by part of RCD Highway. Many industries are located nearby study area as an integral part of SITE area on the east. The condition of sewerage system in Baldia Town is poor which is deteriorating day by day. Sewerage pipelines are broken sporadically in study area. When the drains are chocked, the sewerage water comes up and spills out in the study area. As a result, roads are severely damaged (cracked or pitted) or completely eroded away. Due to poor maintenance of the roads, most of the paved parts are washed out and unpaved surfaces are triggering infiltration of water in large amount. It becomes more pronounced during rainy season Geologically, Baldia town rest on the limestone member of Gaj Formation which belongs to Miocene age ( Figure 2). This limestone is interlayered by thin shale units which is served as ductile material during westward collision of Indian plate after Eocene time. This resulted in the formation of several asymmetrical folds followed by multiple normal faults in Karachi and its suburbs. Baldia town is located between western limb of Manghopir anticline and Lalji syncline ( Figure 3). The periphery of Lalji syncline is serving as catchment area for Baldia town basin where many streams are formed between dip and escarp slopes of limestone ridges. Groundwater table occurs at a depth of > 100 feet due to steep inclination of the rocks forming the confined aquifer system. The general groundwater flow direction is northwest to southeast.

Sample Collection
Groundwater samples (n = 18) were taken from various depths (> 100 feet) through electrically pumped boring wells. The samples were collected after running pump for approximately 3 minutes to remove the standing water in the well pipe. Plastic bottles of 1.5-liter capacity were used for sample collection. Bottles of 200 ml volume were also used for the analysis of nitrate in which boric acid was mixed in a small quantity. Before sampling, bottles were properly washed and rinsed with distilled water and well water respectively. Aesthetic characteristics of samples include color, taste, odor, temperature and turbidity. Locations of the wells were marked with the help of Global Positioning System (GPS).

Sample Analysis
The pH of collected groundwater samples was measured with pH meter (JENCO 6230N). Turbidity was visually observed where translucent samples were regarded turbid and transparent samples were declared clear/non-turbid. TDS and EC were measured with the help of EC meter (Eutech Cyber Scan CON 11). Sodium and potassium concentrations were determined by using flame photometer (Model No. Jenway PFP7). Ca was determined by EDTA titration method while Mg concentration was derived from standard formula using difference between Ca concentration and hardness content. Sulphate content was determined by gravimetric method and chloride was measured by standard titration method using silver nitrate. Similarly, HCO 3 was also estimated by standard titration method. On the other hand, minor (Fe and Mn) and trace (Ni, Cr, Co, Zn) elements were analyzed by atomic absorption spectrometer at flame mode (Model No. Analyst 400 Perkin Elmer).

Physical Characteristics
The taste, odor and color of samples were normal except for sample BT-5 and BT-8 which appear smoky while taken from wells (Table 1). Water quality is good in terms of aesthetic characters as large number of water samples were found safe. Color, cloudiness, particulate matter and visible organisms may create concerns about the quality of a drinking-water (WHO, 2006). Taste and odor in drinking-water may be indicators of some forms of pollution i.e. the presence of potentially harmful substances (WHO, 2017). All the samples were clear except BT-5 and BT-8 (Table 1). Turbidity in water is caused by suspended matter, such as clay, silt, finely divided organic and inorganic matter, soluble colored organic compounds, plankton and other microscopic organisms (Fink, 2005). Generally, it is transported to the aquifer depth through water infiltration from surface (Bitton, 1992). The pH of collected samples ranges between 6.78 -7.32 with a mean of 6.96. Thus, the pH of groundwater is within the WHO permissible range of 6.5 -8.5. The pH less than 6.5 or greater than 9.2 is not suitable for drinking-water purposes (WHO, 1985). About two third of total collected samples are slightly acidic (pH< 7). The lowering of pH is attributed to organic acids, by dissolution of sulphide minerals or decaying of vegetation (Davis and DeWiest, 1966). Organic acid generation can dissolve silicates more effectively as compared to inorganic acids leading to lower the pH groundwater (Zhang et al., 2009).
Although pH has no direct impact on human health but values above WHO guidelines (6.5-8.5) can impair the potability of drinking water (WHO, 1993). Toxicity of cyanides and sulphides increase with decrease in pH and ammonia becomes more toxic with pH increase (Williams, 2003).
The TDS content ranges between 1240 -16910 mg/l, with a mean of 6832.78 mg/l. Mean TDS content of groundwater in all collected samples is about 7 and 14 times higher than the Pakistan Environmental Protection Agency (1000 mg/L) and WHO (500 mg/L) guideline values for drinking water. TDS content is mainly comprising of inorganic salts of bicarbonates, chlorides and sulphates. Larger variation in TDS is mainly attributed to anthropogenic activities and to geochemical processes (Jeevanandam et al, 2007) prevailing in the region.  (Table 2). Calcium concentration varied from 130 to 2160 mg/l, with a mean of 513.67 mg/l ( Table 2). The prescribed limit of calcium in drinking water is 200 mg/l (WHO, 1976). About two third of the total wells (n = 18) showed objectionable concentration of Ca for drinking purpose. Likewise, a wide range of magnesium concentration (range: 230 -2298 mg/l; mean: 543.39 mg/l) is reported in the groundwater of Baldia Town. Prescribed limit of magnesium in drinking water is 100 mg/l (WHO, 1976) where all the samples exceeded the permissible guideline (Table 2). About 83% of the samples showed more than 5 times the recommended value by WHO. Potassium concentration ranges from 5.01 to 41.14 mg/l, with a mean of 22.72 mg/l. Maximum allowed limit of potassium in drinking water by WHO is 12 mg/l (WHO, 2009). Unfortunately, about two third of total samples showed concentration above 12 mg/l ( mg/l in drinking water (WHO, 2004). Except two, all the wells are reported to contain elevated content of bicarbonate (Table 2). About one third of total samples show HCO 3 content above 500 mg/L. Sulphate is second major anion and its concentration also shows large deviation (range: 22 -1174 mg/l; mean: 138.39 mg/l). Health based guideline for sulphate in drinking water is 500 mg/ liter (WHO, 2006). About two third of total collected samples have SO 4 content < 100 mg/. Only one sample (BT-13) has shown anomalous concentration (1174 mg/l) of sulphate where the calcium content is also very high (1360 mg/l). It suggests the gypsum dissolution from gypsiferrous shales. Similarly, very low concentration of chloride (range: 2-29 mg/l; mean: 12.8 mg/l). According of Pakistan standards, permissible limit of chloride in drinking-water is less than 250 mg/l. Nitrate has also shown variable ranges (5.05-76.22 mg/l) with mean of 28 mg/l. Guideline value of nitrate in drinking-water is 10 mg/l (WHO, 1993). About two third of total wells showed excessive nitrate content (NO 3 > 10 mg/l).

Minor and Trace Elements
Iron

Interrelationships Among Physicochemical Parameters
Weak correlation of pH is observed with all parameters except sodium where a moderately positive correlation (r = 0.4) is observed (Table 3). Weak correlation of pH with other parameters is consistent with the fact that very low range (6.8-7.3) of pH variation occurs in the groundwater of study area. TDS content revealed the strong correlation with Ca (r = 0.66), K (r = 0.65) and Cl (r = 0.75). This relationship suggests that these ions are responsible for the high TDS content in the groundwater of study area. Moreover, strong affinity of TDS with trace elements like Ni (r = 0.88) and Zn (r = 0.7) suggest that pH of groundwater has been disturbed by some chemical imbalances leading to lower the pH of groundwater. It is evident by the acidic pH in one third of total collected samples (Table 2). It is well established that acidification trigger the process of heavy metal desorption (Kjøller et. al, 2004). Hardness showed the strong correlations with Mg (r = 1.0), HCO 3 (r = 0.55) and Cr (0.8). Similarly, Mg showed strong correlation with HCO 3 (r = 0.56) and Cr (0.43). This relationship clearly explains the role of ion exchange to alter the hardness type from Ca to Mg. Generally, the Mg in groundwater comes from dolomitic limestone dissolution or from clays enriched with brucite Mg(OH) 2 . When the pH is lowered by chemical imbalance this brucite layer releases its OH into the groundwater to maintain the pH. As a result, magnesium is co-released with its conjugate acid into water. In study area clay units are dominantly composed of illite, montmorillonite and chlorite intermittently exposed with Talawa limestone of Gaj Formation (Figure 2). These clays are the main source of magnesium which is also evident by the strong correlation of hardness with Cr (Table 3). Upon subtle changes in pH of groundwater, the clays tend to release sorbed load of trace elements to maintain the pH equilibrium. Strong correlation of Ca with K (r 2 = 0.61), Cl (r 2 = 0.67) SO 4 (r 2 = 0.45) and Ni (r 2 = 0.78) suggest that the groundwater has been infiltrated by passing through gypsiferrous clay layers. Iron showed the moderate positive correlation with Ni (r 2 = 0.45) while Zn and Ni showed strong positive correlation with each other (r 2 = 0.52) indicating that a large part of Zn is released from the clays instead of limestone. Conversely, Mn expressed strong negative relationship with Co (r 2 = -0.53). This reverse relationship is manifested due to the fact that Mn is more mobile in aerobic environment while Co is more mobile in anaerobic aquatic environments than in aerobic freshwater environments (Mahara & Kudo, 1981). concentration (< 0.0001 mg/l) is an indicator of confined aquifer system. It is consistent with the fact that most trace elements are characterized by low mobility under alkaline and reducing conditions. Moreover, the concentrations of trace elements in confined aquifers are much smaller than the maximum permissible values for drinking water (Levins and Gosk, 2008). However, very high TDS content coupled with objectionable hardness is attributed to the semi-arid climate and low recharging characteristics of shallow aquifers in study area. A part of major solutes may be incorporated through salt leaching from roads and wetting of the engineered structures during rainy seasons.

Conclusion
Present study revealed that groundwater of Baldia Town is not suitable for drinking purpose due to its very high hardness (mean hardness: 2366 mg/L) and salt content (Mean TDS: 6832 mg/l), which may cause different diseases and disabilities upon long term use. High salt content is due to the excessive amount of major salts of Mg, Ca, Na and K while the high hardness is attributed to elevated concentration of bicarbonate and sulphate ions. The hardness of water is temporary (bicarbonate) which can be removed by boiling. Trace element geochemistry revealed that limestone dissolution (Ca, Zn) and ion exchange from shales (Ni, Co, Cr) are main natural processes to alter the chemistry of groundwater in study area. Despite the occurrence of industrial hub, anthropogenic contamination is not obvious which is due to the confined nature of shallow (depth < 200 ft) aquifers in study area. Further studies are required to understand the factors retarding the groundwater contamination from industrial and municipal waste in the proximity of study area.