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Isotope hydrochemical investigation of saline intrusion in the coastal aquifer of Karachi, Pakistan A. Mashiatullaha,* , R.M. Qureshia , M.A. Tasneema , T. Javeda , C.B. Gayeb , E. Ahmadc , N. Ahmadd a Pakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan b Isotope Hydrology Section, International Atomic Energy Agency, Vienna, Austria c World Wide Fund for Nature (WWF), Karachi, Pakistan d Postgraduate Centre for Earth Sciences, University of the Punjab, Lahore, Pakistan
Abstract Environmental stable isotope parameters δ 18 O and δ 2 H in water molecules, δ 13 C in Total Dissolved Inorganic Carbon (TDIC) and δ 34 S in SO4 , have been used in conjunction with physico-chemical tools to study the extent and origin of saline intrusion in the coastal aquifer system of Karachi. Physico-chemical data show that the shallow groundwater is moderately saline. Shallow wells in close proximity of Karachi coast have much higher values of electrical conductivity, salinity, contents of aqueous chloride and sulfate as compared to all other locations relatively far away from the coast. The mean stable isotope contents of 18 O and 2 H indicate that the shallow aquifer system is recharged by a mixture of fresh water of mainly Indus River origin and the polluted waters of the Layari and Malir Rivers and their tributaries, both under natural infiltration conditions and artificially induced infiltration conditions. Much depleted values of δ 13 C (less than −6h V-PDB) indicate the impact of pollution from the Layari and Malir Rivers into the shallow groundwater environment. Relatively deep groundwater is mostly saline and has high electrical conductivity and salinity as compared to shallow groundwater. Physico-chemical data of deep groundwater show that the deep wells have relatively higher values of electrical conductivity and salinity as compared to the shallow wells. The hydrochemical and stable isotope results indicate that the confined aquifer hosts a mixture of rainwater from the hinterlands and surrounding regions around coastal Karachi, as well as sea trapped water/seawater through intrusion under natural infiltration conditions or under induced recharge conditions. The present investigations prove seawater intrusion and existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifer in coastal Karachi. Keywords: Stable isotopes, δ 2 H, δ 13 C, δ 18 O, δ 34 S, Surface water, Groundwater, Deep well, Shallow well, Karachi Sea, Pakistan
1. Introduction Karachi metropolis is located on the northern boundary of the Arabian Sea and hosts a coastline extending up to ∼80 km. It is by far the most populous (∼12 million inhabitants) and * Corresponding author. Address: Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan; fax: (+92) 51 9290275; e-mail: [email protected], [email protected]
RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08031-9
© 2006 Elsevier Ltd. All rights reserved.
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the largest industrial base (∼1000 large industrial units) of Pakistan. The drainage system in Karachi mainly comprises the Layari River and the Malir River systems. In general, there are five sources of recharge to groundwater reserves in and around coastal Karachi. These include: (i) rainfall, (ii) Indus River water supply system, (iii) Hub River/Hub Lake water supply system, (iv) Layari/Malir Rivers and their contributory channels that drain domestic, industrial and agricultural wastewater, and (v) seawater. Contribution to groundwater recharge by local precipitation seems very small due to very poor frequency of rainfall events in coastal Karachi. The long term (15 years annual record) mean monthly average precipitation for Karachi is between 0–15 mm during the months of January to June, 23–91 mm during the months of July to September and 0–7 mm during the months of October to December (IAEA, 1992). As the rainfall intensity in the study area is very low (around 75–100 mm per year), the city of Karachi suffers from a deficit of precipitation, and it appears that the contribution to shallow groundwater storage from rainfall in the Karachi metropolis is very little except for the very shallow thin sandy lenses. Nevertheless, rainfall in the hinterlands and other areas surrounding Karachi may significantly contribute to the confined groundwater flow system. It appears that the remaining four sources can play a significant role in recharge to shallow and deep groundwater system in coastal Karachi. The pollution inventories in the water courses are quite alarming as a number of shallow to deep pumping wells (called hydrants) and mechanized hand-pumps are installed along these rivers causing a threat of artificially induced recharge to the local groundwater system. In addition, the long term pumping of energized tube-wells installed in the immediate vicinity of seashore can also cause intrusion of seawater into the coastal groundwater system. Some small-scale sporadic groundwater quality surveys involving classical hydrochemical have been made in the past to estimate groundwater pollution status in coastal Karachi. There is a vast literature on application of classical hydrochemical techniques and isotope hydrogeochemical techniques for evaluation of saline intrusion in coastal aquifers (Freeze and Cherry, 1997; Goswani, 1969; Klein and Ratzlaff, 1989; Yurtsever and Payne, 1978; Kulkarni et al., 1979). In the present investigation, the stable isotope contents of 18 O, 2 H (in water molecules), 13 C in the Total Dissolved Inorganic Carbon (TDIC) and the related hydrochemical data (major cation analysis) of all surface water samples and the coastal groundwater samples collected are statistically evaluated to postulate the origin of groundwater and associated salinity in the shallow and deep aquifer system in coastal Karachi. This paper documents results of a first ever study on the evaluation of groundwater recharge characteristics and origin of salinity in coastal Karachi using environmental isotope techniques.
2. Methods Standard field sample collection/preservation methods were used for subsequent chemical, and stable isotopic analysis in the laboratory. 2.1. Field sampling Surface water samples were collected from various locations along polluted streams/rivers namely: Layari River, Malir River and local sea (shallow seawater off Karachi coast). Shallow
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Fig. 1. Map of Karachi showing sampling points.
groundwater and deep groundwater were collected from different localities of Karachi city. Number of samples collected are as follows: Indus River (1), Layari River (3), Malir River (2), Karachi Sea (5), shallow groundwater (12) and deep groundwater (16). Figure 1 shows the location of sampling points. Geo-location of each sampling point was determined with a standard GPS (M/S Garmin). Shallow groundwater samples (12) were collected from hand-pumps and dug wells. All water samples were collected in leak-tight/lined cap plastic bottles or glass bottles. 2.2. Field in-situ analyses In the field, the water samples were immediately analyzed for specific physiochemical parameters such as pH, temperature, dissolved oxygen, turbidity, electrical conductivity and
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salinity. Turbidity was measured with a portable turbidity meter (Model 6035, JENWAY). Electrical conductivity and temperature were measured with a portable conductivity meter (Model HI 8633, M/S HANNA Instruments). Dissolved oxygen was measured with a portable DO Meter (Model 9070, JENWAY). Salinity was measured with a portable Salinometer (refractometer). 2.3. Laboratory analyses In the laboratory, the collected water samples were analyzed for stable isotope content of 18 O and 2 H in the water molecule; 13 CTDIC in TDIC, 34 S from sulfate of water and major ions −1 such as Cl− , SO−2 4 , HCO3 (Clark and Fritz, 1997; Qureshi et al., 2001). Chloride contents were determined by ion selective electrodes with an Orion Microprocessor Ion Analyzer/901. Carbonates and bicarbonates were measured by titration. Sulfate concentrations were determined by turbidimetric/spectrophotometric method (Hitachi 220-A Double Beam Spectrophotometer). The environmental stable isotope analyses were performed using a modified Varian Mat GD-150 Mass Spectrometer. All stable isotope data are expressed in conventional δ (h) notation and referred to the standards namely: SMOW (Standard Mean Ocean Water) for 18 O and 2 H analyses, PDB (Pee-Dee Belemnite) for 13 C analysis of TDIC, and CDT (Canyon Diablo Troilite) for 34 S. The overall analytical uncertainties are ±0.1h for 18 O and 13 C 2 TDIC and ±1h for δ H measurements.
3. Results and discussion 3.1. Surface water sources 3.1.1. Local precipitation No significant rainfall events occurred in coastal Karachi during the sampling period. However, stable isotope data on precipitation for the period from 1961 to 1975 are available from the IAEA Precipitation Network for Karachi Station (IAEA Precipitation Network Code: 41780000, Lat. 24◦ 90 N, Long. 67◦ 13 E, Altitude: 23 meters above mean sea level). The following stable isotope indices of precipitation in Karachi as quoted by IAEA were used for interpretation purposes: Long Term Weighted Mean δ 18 O (water): −3.93 ± 1.94h V-SMOW. Long Term Weighted Mean δ 2 H (water): −23.5 ± 18.1h V-SMOW. Long Term Monthly Correlation between δ 18 O and δ 2 H: δ 2 H = 7.56δ 18 O ± 0.34 + [3.41 ± 1.50]. 3.1.2. Indus River The Indus River (IR) water sample was collected from the River course near Thatta city whereby the Indus River water is partly diverted to Karachi for irrigation and drinking water purposes. Typical chemical indices and stable isotope indices of oxygen and hydrogen in the Indus River water molecules as used in evaluation of coastal groundwater recharge characteristics are given in Table 1.
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Table 1 Physico-chemical and isotopic ranges of Indus River, Layari River, Malir River and Karachi Sea in shallow and deep groundwater samples Indus River
Layari River
Malir River
Karachi Sea
Shallow groundwater
Deep groundwater
PH E.C. (mS/cm) Salinity Turbidity (NTU) HCO−1 3 (ppm) Cl−1 (ppm) SO−2 4 (ppm) −1 SO−2 4 /Cl −1 Cl/HCO3 δ 18 O (h V-SMOW) δ 2 H (h V-SMOW) δ 34 SSO4 (h CDT) δ 13 C (h PDB)
7.56 0.48 1 36 108 14 13 0.69 0.22 −5.9 −48.12 6.28 −1.66
7.46–8.40 1.5–9.02 1–5.0 54–76 502–660 431–1300 33–195 0.06–0.11 1.48–3.39 −5.4 to −2.7 −44.4 to −31 7.2–8.9 −7.2 to −0.2
7.7–7.9 3.2–3.4 3.5–5 97–98 490 2020 271 0.10 6.52 −0.95 to −0.66 −27.5 to −5.91 11.6–14.2 −4.5 to −2.4
7.7–8.5 49.3–53.7 31.0–39.0 52.6–195.5 145–196 21580–25230 2080–2320 0.064–0.06 190–300 0.27–1.1 4.9–8.6 17.6–19.5 −3.9 to 0.8
6.75–8.3 0.9–12.5 1–5.1 13.9–95 246–520 58–580 26–220 0.10–0.79 0.33–6.35 −6.74 to −4.41 −53.89 to −33.06 2.69–9.7 −11.23 to −1.72
6.94–7.94 1.9–32.5 1.3–7.4 2.7–59.6 102–760 500–12800 61–2220 0.05–0.36 1.8–215 −6.5 to −3.04 −72.59 to −26.7 −17.97 to −2.7 −12.04 to −2.47
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Parameters (units)
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3.1.3. Local polluted rivers The Layari River and the Malir River represent the local rivers in the study area as these facilitate the main drainage of domestic and industrial waste generated by the entire Karachi metropolis to the Karachi Sea. Thus, the two rivers get polluted downstream. The Layari River (LR) was monitored at three locations along its flow from North Karachi (upstream region) to Sher-Shah Bridge (downstream region) near Keamari/Karachi Harbour. The range of variation (along the flow direction) in typical chemical and stable isotope contents of oxygen and hydrogen in the water molecules, δ 34 S of aqueous sulfate of water and carbon isotope in TDIC of LR water as used in evaluation of coastal groundwater recharge characteristics are shown in Table 1. There is a good correspondence between electrical conductivity (EC) and salinity along the river. Generally, the EC and salinity values tend to decrease downstream. Maximum values (9.02 mS/cm) of EC were observed at the origin of the Layari stream near Yousuf Goth area. In this zone, the Layari stream receives minor spring water, domestic wastewater from small isolated dwellings and wastewater from industries (pharmaceutical industry, electronic industry, etc.) which host tube-wells with quite high salinity values. Turbidity levels in the river water fluctuate depending upon the concentration of inputs from industrial and domestic sector. High concentrations of Cl− (1300 ppm) coupled with mildly alkaline pH values are found in the upstream regions of the river. Significantly, high values of Cl− in the upstream region indicate that the source of water in the river is the saline water discharged from deep tube-wells installed in the nearby industrial complexes. The δ 13 CTDIC , δ 18 O (water) and δ 34 S values are also quite enriched in this zone of LR as compared to local shallow groundwater, and are in fact relatively closer to sea values. The Malir River (MR) was monitored at two locations along its flow from Karachi East to Qayyum Abad Bridge where the MR joins Ghizri Creek on the north-west side of Karachi coast. The range of variation (along the flow direction) in typical chemical and stable isotope contents of oxygen and hydrogen in the water molecules and of carbon in the TDIC and 34 S of aqueous sulfate of water in MR water, used in evaluation of coastal groundwater recharge characteristics, are shown in Table 1. Like LR, there is good correspondence between EC and salinity along the flow in MR. The reducing conditions of the river become adverse as it receives more and more industrial effluents and sewage from Korangi Industrial Trading Estate (KITE) zone and Qayyum Abad/Ghizri Creek area along its course towards the sea. 3.1.4. The Karachi Sea The coast of Karachi is about 40 km long. Shallow seawater samples were collected during the high tide period from 5 locations in the inter-tidal zone along Karachi coast. The range of variation in chemical and stable isotope contents of oxygen and hydrogen in the water molecules, and of carbon in the TDIC in Karachi seawater, used in evaluation of coastal groundwater recharge characteristics are shown in Table 1. The mildly alkaline pH values of ∼8 for open seawater off Karachi coast generally conform to those for normal ocean waters. EC values for Karachi seawater range between 49.3 and 53.7 mS/cm, while the salinity values are ∼38. The EC values higher than 50 mS/cm correspond to relatively non-polluted open seawaters. The seawater temperature off Karachi coast is fairly constant about 25.5◦ C. Slightly higher temperature is observed near Ghizri coast, which is attributed to input of relatively warmer wastewaters of industrial and domestic origin. Turbidity values of open seawater are higher than the on-shore surface water sources. This is attributed to much higher contents of partic-
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ulate matter in seawater as compared to on-shore surface water sources. The lowest turbidity values are observed along Clifton coast, as this coast is relatively free of pollution along southeast side of the Karachi coast. Cl− contents of seawater off Karachi coast range from 21,500 to 25,230 ppm, while the SO−2 4 concentrations are in the range of 2080–2210 ppm. The stable 13 carbon isotope contents (δ CTDIC ) of TDIC vary in the range of −3.9 to +0.8h in different zones off Karachi coast. This is indicative of different levels and sources of dissolved inorganic carbon in seawater due to input of domestic and industrial wastewater into the sea from key industrial trading estates via polluted drains. The highest δ 13 CTDIC value of +0.8h and δ 34 S values of 19.1h corresponds to relatively non-polluted seawater along the northwest coast of Karachi (Buleji Coast). The lowest δ 13 CTDIC value of −3.9h corresponds to highly polluted seawater in Korangi Creek that receives industrial and domestic waste drains from KITE. 3.2. Groundwater in the coastal aquifer Shallow groundwater samples (n = 12) were obtained from hand pumps (n = 4), dug wells (n = 1) and shallow mini-bores with centrifugal pumps (n = 7), installed at depths less than 45 m in the coastal aquifer of Karachi. Relatively deep groundwater was obtained from pumping wells (n = 16) installed at depths between 50 and 100 m in the coastal aquifer of Karachi. These cased wells also tap various proportions of shallow groundwater in addition to deep groundwater. Table 1 presents the ranges of physico-chemical and environmental stable isotope data of shallow groundwater samples and shallow mixed deep groundwater samples collected from the municipal jurisdiction of Karachi. 3.2.1. Shallow groundwater Physico-chemical data show that shallow groundwater is moderately saline. The mean chemi18 2 cal concentrations of Cl− , HCO− 3 and SO4 and the mean isotope content of O, H in shallow 34 13 groundwater, S of aqueous sulfate of water and C in TDIC are shown in Table 2. The mean stable isotope contents of 18 O and 2 H indicate that the shallow aquifer system is recharged by a mixture of fresh waters of mainly IR and polluted waters of LR and MR and their tributaries, both under natural and artificially induced infiltration conditions. Much depleted values of δ 13 CS.G. (less than −6h) indicate the impact of pollution from LR and Table 2 Results of chemical and isotopic analysis of shallow groundwater samples Groundwater
Concentration
Mean Cl− (shallow groundwater) Mean HCO− 3 (shallow groundwater) Mean SO− 4 (shallow groundwater) Mean δ 18 O (shallow groundwater)
280 ± 380 ppm (n = 12) 360 ± 110 ppm (n = 12) 74 ± 55 ppm (n = 12) −5.98 ± 0.66h V-SMOW (n = 12) −47.62 ± 5.41h V-SMOW (n = 12) −7.52 ± 2.9h PDB (n = 12) 6.21 ± 2.1h CDT (n = 12)
Mean δ 2 H (shallow groundwater) Mean δ 13 C (TDIC-shallow groundwater) Mean δ 34 S (sulfate-shallow groundwater)
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MR into the shallow groundwater environment. It is noteworthy that polluted river waters are mixtures of public supply water of local aquifer origin, Hub River (draining spring water and flood water) origin and IR origin. Majority of shallow groundwater samples have Cl/HCO−1 3 ratios higher than 0.66 coupled with a trend towards increase in the Cl− content. Generally, most of coastal shallow groundwater samples have Cl− /HCO−1 3 ratios between 0.6 and 6.36 −1 − (n = 6) (Cl /HCO3 ratios of <0.66 are attributed to groundwater of freshwater origin (rain and river recharge). Higher Cl− /HCO−1 3 ratios in shallow coastal groundwater samples 18 coupled with a trend towards δ O values of local seawater indicate diffusion/intrusion of seawater. These samples indicate that the zone of diffusion of seawater into groundwater has extended to a considerable lateral extent into the coastal aquifer. 3.2.2. Deep groundwater Physico-chemical data of deep groundwater show that the deep wells have relatively higher values of EC and salinity as compared to the shallow wells. Further, the deep groundwater is quite saline. The mean chemical concentrations of Cl− , HCO−1 3 and SO4 and the mean isotope content of 18 O, 2 H and 34 S in shallow mixed deep groundwater, and 13 C in TDIC in shallow mixed deep groundwater are shown in Table 3. The hydrochemical and stable isotope results indicate that the confined aquifer hosts a mixture of rainwater from hinterlands and surrounding regions around coastal Karachi, as well as sea trapped water/seawater through intrusion under natural infiltration conditions, or under induced recharge conditions. 3.3. Groundwater recharge characteristics and seawater intrusion 3.3.1. Origin of coastal groundwater recharge The analysis of natural variations of the heavy isotope contents of oxygen (18 O) and hydrogen (2 H) represents one of the classical applications of isotope hydrology in studying the origin and dynamics of groundwater. Due to their different contents observed in groundwater and seawater, 18 O and 2 H have been used to assess or confirm seawater intrusion as the main mechanism of salinization. Figure 2 shows the δ 18 O versus δ 2 H plot of groundwater in coastal Karachi vis-à-vis isotopic indices of local precipitation (rain), Indus River water and local Table 3 Results of chemical and isotopic analysis of deep groundwater samples Groundwater
Concentration
Mean Cl− (shallow mixed deep groundwater) Mean HCO− 3 (shallow mixed deep groundwater) Mean SO4 (shallow mixed deep groundwater) Mean δ 18 O (shallow mixed deep groundwater) Mean δ 2 H (shallow mixed deep groundwater) Mean δ 13 CTDIC (shallow mixed deep groundwater) Mean δ 34 SSO4 (shallow mixed deep groundwater)
3900 ± 400 ppm (n = 16) 340 ± 160 ppm (n = 16) 520 ± 530 ppm (n = 15) −5.0 ± 1.0h V-SMOW (n = 16) −43.0 ± 11.0h V-SMOW (n = 16) −8.47 ± 3.0h PDB (n = 16) 6.85 ± 4.9h CDT (n = 16)
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Fig. 2. δ 18 O versus δ 2 H plot for groundwater, coastal Karachi (GMWL = Global Meteoric Water Line).
seawater. The long term mean stable isotopic indices of 18 O and 2 H in precipitation as well as the Indus River fall just below the Global Meteoric Water Line (GMWL), while the surface water samples (Hub Lake water and the river water, local seawater) and the groundwater samples fall well below the GMWL. There is a large scatter in the stable isotope plot of oxygen vs. hydrogen for shallow groundwater and deep groundwater. The δ 18 O vs. δ 2 H values of shallow groundwater and deep groundwater mainly range between the mean δ 18 O vs. δ 2 H values of the Indus River (δ 18 O = −5.9h and δ 2 H = −48.12h) and mean δ 18 O vs. δ 2 H values of precipitation (δ 18 O = −3.9 ± 1.94h and δ 2 H = −23.5 ± 18.1h), as well as shallow seawater along Karachi coast (δ 18 O = +0.76h and δ 2 H = 6.85h). The shallow groundwater samples cluster around the mean δ 18 O value of −5.98 ± 0.65h. The deep groundwater samples cluster around the mean δ 18 O value of −5.0 ± 1.05h. Isotopically, some of these deep groundwater samples partly overlap the shallow groundwater. Nevertheless, the large shift in δ 18 O values of shallow and less deep groundwater towards right of Local Meteoric Water Line (LMWL) is attributed to mixing of various proportions of recharge of local precipitation and the mixed waters from polluted rivers and seawater. The possibilities of major contribution to groundwater recharge of shallow/phreatic aquifer directly by the local rainfall seems very small due to extremely poor frequency of rainfall events, the rainfall intensities in Karachi and high evaporation rates. The long term (15 yr annual record) mean monthly average precipitation for Karachi is between 0–15 mm during the months of January to June, 23–91 mm during the months of July to September and 0–7 mm during the months of October to December (IAEA, 1992). Due to the current drought conditions in the area, the direct recharge from precipitation is negligible. This leaves seawater as the 2nd major source of recharge to coastal groundwater system. Under the present surface water supply practices, drought conditions and significant withdrawal of groundwater, the stable isotope index of 18 O and 2 H in shallow and less deep groundwater may shift towards the isotopic index of rainwater/seawater in association with higher groundwater salinities.
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3.3.2. Origin of coastal groundwater salinity Gonfiantini and Araguás (1988) compiled a number of case studies on seawater intrusion carried out under different hydrogeological settings. The examples described in their paper illustrate both simple and complex approaches used to characterize coastal aquifers and to shed light on the nature of salinization. Yurtsever and Payne (1978) presented a typical case of interactions between three components or water types, in which most of the observed increase in salinity was due to the upward leakage of deep groundwater, and only in few cases a direct seawater intrusion was active. In other cases, these tools were used to show a complex geochemical behavior of dissolved salts in groundwater systems, due to water rock-interaction (adsorption, cation-exchange), long residence time of groundwater in the aquifer or the presence of deep brines (connate water). From the Ghijben–Herzberg relationship, the freshwater–saltwater equilibrium requires that the water table or the piezometric surface lies above the sea level and slopes downward towards the ocean. Freeze and Cherry (1997) described a simple and useful scheme for recognizing groundwater salinity, based on the contents of Total Dissolved Solids (TDS). Seawater intrusion into coastal aquifers is usually assumed when an increase in (TDS) or EC of the extracted groundwater is observed. Seawater encroachment is the most common mechanism operating in well fields located at short distance from the coastline, mainly due to intensive pumping in selected sites. The TDS of the seawater is approximately 35000 ppm. Surface water or groundwater containing more than 2000–3000 ppm of TDS is generally too salty to drink. Cl−1 usually plays a minor role in groundwater, but it is a dominant ion of seawater. In contrast, HCO−1 3 is usually the most abundant negative ion in groundwater. There is such a large difference between the proportions of Cl− and HCO−1 3 in groundwater and in sea− water, that the ratio between these two ions (i.e. the ratio: Cl /HCO−1 3 ) is a useful index of the presence of seawater in groundwater. Hence, an increase of chloride content in groundwater is the most reliable indicator of the first stage of salt-water intrusion in groundwater. Goswani (1969) presented field studies at Digha (India) and, making use of Cl−1 content of water samples, he delineated the groundwater body on isochlor of 500 ppm (TDS = 100 ppm) and Cl− /HCO−1 3 ratio of 0.66. The boundary zone of the 300–500 isochlors demarcates the diffusion zone of seawater (Freeze and Cherry, 1997). To verify possible seawater intrusion in shallow groundwater and mixed deep groundwater and/or existence of trapped seawater in deep groundwater, the concentrations of Cl− , as −2 18 well as Cl− /HCO−1 3 ratios, are plotted against δ O values, and the concentrations of SO4 −2 − are plotted against SO4 /Cl ratios for shallow groundwater samples in Figs. 3, 4 and 5, respectively. It may be realized that the extrapolated or forecast trends for shallow groundwater samples do not fall on the data points for local seawater (or other tropical seawaters). However, the extrapolated or forecast trends for shallow groundwater samples, excluding the near shore coastal groundwater sample, project towards higher salinities. Lastly, the extrapolated − or forecast trends for shallow mixed deep groundwater samples (with high SO−2 4 and Cl 18 contents, Cl− /HCO−1 3 ratios greater than 0.66 and relatively enriched δ O values) project towards very high salinities, and tend to pass through the data points for local seawater (or other tropical seawater). The present investigations, therefore, prove seawater intrusion/existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifer in coastal Karachi.
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Fig. 3. Chloride versus 18 O trend line for shallow and deep aquifers, coastal Karachi.
18 Fig. 4. Trend line of Cl− /HCO−1 3 ratios versus O shallow and deep groundwater in coastal Karachi.
−2 − Fig. 5. Trend line of SO−2 4 versus SO4 /Cl ratios for shallow and deep groundwater in coastal Karachi.
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4. Conclusion The studies carried out with conjunctive use of stable isotope techniques and conventional non-nuclear chemical and biological techniques have successfully facilitated recognition of seawater intrusion in the shallow groundwater and deep groundwater systems in coastal Karachi. The present investigations prove seawater intrusion/existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifers in coastal Karachi.
Acknowledgements These studies have been performed with financial assistance provided by the International Atomic Energy Agency and Pakistan Institute of Nuclear Science and Technology/Pakistan Atomic Energy Commission under IAEA-Research Contract PAK-11322. Special thanks are due to Dr. E. Ahmed for in-kind provision of base camp laboratory facilities and partial transport for fieldwork, and to Dr. M. Ahmed and Mr. M.R. Sheikh for facilitating chemical analysis.
References Clark, I.D., Fritz, P. (1997). Environmental Isotopes in Hydrology. Lewis Publishers, New York, 328 pp. (Chapters 1 and 10). Freeze, R.A., Cherry, J.A. (1997). Groundwater. Prentice Hall, Englewood Cliffs, NJ, 604 pp. Gonfiantini, R., Araguás, L. (1988). Los isótopos ambientales en el estudio de la intrusión marina. In: LopezCamacho Camacho, B. (Ed.), Tecnología de la Intrusión Marina en acuíferos costeros. IGTE, Almuñecar, Spain, pp. 135–190. Goswani, A.B. (1969). Study of salt-water encroachment in the coastal aquifer at Digha, Midnapore district west Bengal. Bulletin of International Assocociation of Hydrological Sciences, India 13, 77. International Atomic Energy Agency (1992). Statistical Treatment of Data on Environmental Isotopes in Precipitation. STI/DOC/10/331. IAEA Technical Reports Series No. 331. IAEA, Vienna, Austria, ISBN92-0-100892-9, 781 pp. Klein, H., Ratzlaff, K.W. (1989). Changes in saltwater intrusion in the Biscayne aquifer, Hialeah–Miami springs area, Dade County, Florida. U.S. Geological Survey Water-Resources Investigations Report No. 87-4249. Kulkarni, K.M., Navada, S.V., Nair, A.R., Rao, S.M., Shivanna, K., Sinha, U.K., Sharma, S. (1979). Drinking water salinity problem in Coastal Orissa–India – Identification of past transgression of sea by isotope investigation. IAEA-SM-349/18. In: Proceedings of the International Symposium on Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and Atmosphere. Vienna, Austria. Qureshi, R.M., Mashiatullah, A., Rizvi, S.H.N., Khan, S.H., Javed, T., Tasneem, M.A. (2001). Marine pollution studies in Pakistan by nuclear techniques. The Nucleus 38, 41–52. Yurtsever, Y., Payne, B.R. (1978). Application of environmental isotopes to groundwater investigations in Qatar. IAEA-SM-228/24. In: Proceedings of the International Symposium on Isotope Hydrology. Neuherberg, Germany.
Isotope hydrochemical investigation of saline intrusion in the coastal aquifer of Karachi, Pakistan A. Mashiatullaha,* , R.M. Qureshia , M.A. Tasneema , T. Javeda , C.B. Gayeb , E. Ahmadc , N. Ahmadd a Pakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan b Isotope Hydrology Section, International Atomic Energy Agency, Vienna, Austria c World Wide Fund for Nature (WWF), Karachi, Pakistan d Postgraduate Centre for Earth Sciences, University of the Punjab, Lahore, Pakistan
Abstract Environmental stable isotope parameters δ 18 O and δ 2 H in water molecules, δ 13 C in Total Dissolved Inorganic Carbon (TDIC) and δ 34 S in SO4 , have been used in conjunction with physico-chemical tools to study the extent and origin of saline intrusion in the coastal aquifer system of Karachi. Physico-chemical data show that the shallow groundwater is moderately saline. Shallow wells in close proximity of Karachi coast have much higher values of electrical conductivity, salinity, contents of aqueous chloride and sulfate as compared to all other locations relatively far away from the coast. The mean stable isotope contents of 18 O and 2 H indicate that the shallow aquifer system is recharged by a mixture of fresh water of mainly Indus River origin and the polluted waters of the Layari and Malir Rivers and their tributaries, both under natural infiltration conditions and artificially induced infiltration conditions. Much depleted values of δ 13 C (less than −6h V-PDB) indicate the impact of pollution from the Layari and Malir Rivers into the shallow groundwater environment. Relatively deep groundwater is mostly saline and has high electrical conductivity and salinity as compared to shallow groundwater. Physico-chemical data of deep groundwater show that the deep wells have relatively higher values of electrical conductivity and salinity as compared to the shallow wells. The hydrochemical and stable isotope results indicate that the confined aquifer hosts a mixture of rainwater from the hinterlands and surrounding regions around coastal Karachi, as well as sea trapped water/seawater through intrusion under natural infiltration conditions or under induced recharge conditions. The present investigations prove seawater intrusion and existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifer in coastal Karachi. Keywords: Stable isotopes, δ 2 H, δ 13 C, δ 18 O, δ 34 S, Surface water, Groundwater, Deep well, Shallow well, Karachi Sea, Pakistan
1. Introduction Karachi metropolis is located on the northern boundary of the Arabian Sea and hosts a coastline extending up to ∼80 km. It is by far the most populous (∼12 million inhabitants) and * Corresponding author. Address: Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan; fax: (+92) 51 9290275; e-mail: [email protected], [email protected]
RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08031-9
© 2006 Elsevier Ltd. All rights reserved.
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the largest industrial base (∼1000 large industrial units) of Pakistan. The drainage system in Karachi mainly comprises the Layari River and the Malir River systems. In general, there are five sources of recharge to groundwater reserves in and around coastal Karachi. These include: (i) rainfall, (ii) Indus River water supply system, (iii) Hub River/Hub Lake water supply system, (iv) Layari/Malir Rivers and their contributory channels that drain domestic, industrial and agricultural wastewater, and (v) seawater. Contribution to groundwater recharge by local precipitation seems very small due to very poor frequency of rainfall events in coastal Karachi. The long term (15 years annual record) mean monthly average precipitation for Karachi is between 0–15 mm during the months of January to June, 23–91 mm during the months of July to September and 0–7 mm during the months of October to December (IAEA, 1992). As the rainfall intensity in the study area is very low (around 75–100 mm per year), the city of Karachi suffers from a deficit of precipitation, and it appears that the contribution to shallow groundwater storage from rainfall in the Karachi metropolis is very little except for the very shallow thin sandy lenses. Nevertheless, rainfall in the hinterlands and other areas surrounding Karachi may significantly contribute to the confined groundwater flow system. It appears that the remaining four sources can play a significant role in recharge to shallow and deep groundwater system in coastal Karachi. The pollution inventories in the water courses are quite alarming as a number of shallow to deep pumping wells (called hydrants) and mechanized hand-pumps are installed along these rivers causing a threat of artificially induced recharge to the local groundwater system. In addition, the long term pumping of energized tube-wells installed in the immediate vicinity of seashore can also cause intrusion of seawater into the coastal groundwater system. Some small-scale sporadic groundwater quality surveys involving classical hydrochemical have been made in the past to estimate groundwater pollution status in coastal Karachi. There is a vast literature on application of classical hydrochemical techniques and isotope hydrogeochemical techniques for evaluation of saline intrusion in coastal aquifers (Freeze and Cherry, 1997; Goswani, 1969; Klein and Ratzlaff, 1989; Yurtsever and Payne, 1978; Kulkarni et al., 1979). In the present investigation, the stable isotope contents of 18 O, 2 H (in water molecules), 13 C in the Total Dissolved Inorganic Carbon (TDIC) and the related hydrochemical data (major cation analysis) of all surface water samples and the coastal groundwater samples collected are statistically evaluated to postulate the origin of groundwater and associated salinity in the shallow and deep aquifer system in coastal Karachi. This paper documents results of a first ever study on the evaluation of groundwater recharge characteristics and origin of salinity in coastal Karachi using environmental isotope techniques.
2. Methods Standard field sample collection/preservation methods were used for subsequent chemical, and stable isotopic analysis in the laboratory. 2.1. Field sampling Surface water samples were collected from various locations along polluted streams/rivers namely: Layari River, Malir River and local sea (shallow seawater off Karachi coast). Shallow
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Fig. 1. Map of Karachi showing sampling points.
groundwater and deep groundwater were collected from different localities of Karachi city. Number of samples collected are as follows: Indus River (1), Layari River (3), Malir River (2), Karachi Sea (5), shallow groundwater (12) and deep groundwater (16). Figure 1 shows the location of sampling points. Geo-location of each sampling point was determined with a standard GPS (M/S Garmin). Shallow groundwater samples (12) were collected from hand-pumps and dug wells. All water samples were collected in leak-tight/lined cap plastic bottles or glass bottles. 2.2. Field in-situ analyses In the field, the water samples were immediately analyzed for specific physiochemical parameters such as pH, temperature, dissolved oxygen, turbidity, electrical conductivity and
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salinity. Turbidity was measured with a portable turbidity meter (Model 6035, JENWAY). Electrical conductivity and temperature were measured with a portable conductivity meter (Model HI 8633, M/S HANNA Instruments). Dissolved oxygen was measured with a portable DO Meter (Model 9070, JENWAY). Salinity was measured with a portable Salinometer (refractometer). 2.3. Laboratory analyses In the laboratory, the collected water samples were analyzed for stable isotope content of 18 O and 2 H in the water molecule; 13 CTDIC in TDIC, 34 S from sulfate of water and major ions −1 such as Cl− , SO−2 4 , HCO3 (Clark and Fritz, 1997; Qureshi et al., 2001). Chloride contents were determined by ion selective electrodes with an Orion Microprocessor Ion Analyzer/901. Carbonates and bicarbonates were measured by titration. Sulfate concentrations were determined by turbidimetric/spectrophotometric method (Hitachi 220-A Double Beam Spectrophotometer). The environmental stable isotope analyses were performed using a modified Varian Mat GD-150 Mass Spectrometer. All stable isotope data are expressed in conventional δ (h) notation and referred to the standards namely: SMOW (Standard Mean Ocean Water) for 18 O and 2 H analyses, PDB (Pee-Dee Belemnite) for 13 C analysis of TDIC, and CDT (Canyon Diablo Troilite) for 34 S. The overall analytical uncertainties are ±0.1h for 18 O and 13 C 2 TDIC and ±1h for δ H measurements.
3. Results and discussion 3.1. Surface water sources 3.1.1. Local precipitation No significant rainfall events occurred in coastal Karachi during the sampling period. However, stable isotope data on precipitation for the period from 1961 to 1975 are available from the IAEA Precipitation Network for Karachi Station (IAEA Precipitation Network Code: 41780000, Lat. 24◦ 90 N, Long. 67◦ 13 E, Altitude: 23 meters above mean sea level). The following stable isotope indices of precipitation in Karachi as quoted by IAEA were used for interpretation purposes: Long Term Weighted Mean δ 18 O (water): −3.93 ± 1.94h V-SMOW. Long Term Weighted Mean δ 2 H (water): −23.5 ± 18.1h V-SMOW. Long Term Monthly Correlation between δ 18 O and δ 2 H: δ 2 H = 7.56δ 18 O ± 0.34 + [3.41 ± 1.50]. 3.1.2. Indus River The Indus River (IR) water sample was collected from the River course near Thatta city whereby the Indus River water is partly diverted to Karachi for irrigation and drinking water purposes. Typical chemical indices and stable isotope indices of oxygen and hydrogen in the Indus River water molecules as used in evaluation of coastal groundwater recharge characteristics are given in Table 1.
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Table 1 Physico-chemical and isotopic ranges of Indus River, Layari River, Malir River and Karachi Sea in shallow and deep groundwater samples Indus River
Layari River
Malir River
Karachi Sea
Shallow groundwater
Deep groundwater
PH E.C. (mS/cm) Salinity Turbidity (NTU) HCO−1 3 (ppm) Cl−1 (ppm) SO−2 4 (ppm) −1 SO−2 4 /Cl −1 Cl/HCO3 δ 18 O (h V-SMOW) δ 2 H (h V-SMOW) δ 34 SSO4 (h CDT) δ 13 C (h PDB)
7.56 0.48 1 36 108 14 13 0.69 0.22 −5.9 −48.12 6.28 −1.66
7.46–8.40 1.5–9.02 1–5.0 54–76 502–660 431–1300 33–195 0.06–0.11 1.48–3.39 −5.4 to −2.7 −44.4 to −31 7.2–8.9 −7.2 to −0.2
7.7–7.9 3.2–3.4 3.5–5 97–98 490 2020 271 0.10 6.52 −0.95 to −0.66 −27.5 to −5.91 11.6–14.2 −4.5 to −2.4
7.7–8.5 49.3–53.7 31.0–39.0 52.6–195.5 145–196 21580–25230 2080–2320 0.064–0.06 190–300 0.27–1.1 4.9–8.6 17.6–19.5 −3.9 to 0.8
6.75–8.3 0.9–12.5 1–5.1 13.9–95 246–520 58–580 26–220 0.10–0.79 0.33–6.35 −6.74 to −4.41 −53.89 to −33.06 2.69–9.7 −11.23 to −1.72
6.94–7.94 1.9–32.5 1.3–7.4 2.7–59.6 102–760 500–12800 61–2220 0.05–0.36 1.8–215 −6.5 to −3.04 −72.59 to −26.7 −17.97 to −2.7 −12.04 to −2.47
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Parameters (units)
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3.1.3. Local polluted rivers The Layari River and the Malir River represent the local rivers in the study area as these facilitate the main drainage of domestic and industrial waste generated by the entire Karachi metropolis to the Karachi Sea. Thus, the two rivers get polluted downstream. The Layari River (LR) was monitored at three locations along its flow from North Karachi (upstream region) to Sher-Shah Bridge (downstream region) near Keamari/Karachi Harbour. The range of variation (along the flow direction) in typical chemical and stable isotope contents of oxygen and hydrogen in the water molecules, δ 34 S of aqueous sulfate of water and carbon isotope in TDIC of LR water as used in evaluation of coastal groundwater recharge characteristics are shown in Table 1. There is a good correspondence between electrical conductivity (EC) and salinity along the river. Generally, the EC and salinity values tend to decrease downstream. Maximum values (9.02 mS/cm) of EC were observed at the origin of the Layari stream near Yousuf Goth area. In this zone, the Layari stream receives minor spring water, domestic wastewater from small isolated dwellings and wastewater from industries (pharmaceutical industry, electronic industry, etc.) which host tube-wells with quite high salinity values. Turbidity levels in the river water fluctuate depending upon the concentration of inputs from industrial and domestic sector. High concentrations of Cl− (1300 ppm) coupled with mildly alkaline pH values are found in the upstream regions of the river. Significantly, high values of Cl− in the upstream region indicate that the source of water in the river is the saline water discharged from deep tube-wells installed in the nearby industrial complexes. The δ 13 CTDIC , δ 18 O (water) and δ 34 S values are also quite enriched in this zone of LR as compared to local shallow groundwater, and are in fact relatively closer to sea values. The Malir River (MR) was monitored at two locations along its flow from Karachi East to Qayyum Abad Bridge where the MR joins Ghizri Creek on the north-west side of Karachi coast. The range of variation (along the flow direction) in typical chemical and stable isotope contents of oxygen and hydrogen in the water molecules and of carbon in the TDIC and 34 S of aqueous sulfate of water in MR water, used in evaluation of coastal groundwater recharge characteristics, are shown in Table 1. Like LR, there is good correspondence between EC and salinity along the flow in MR. The reducing conditions of the river become adverse as it receives more and more industrial effluents and sewage from Korangi Industrial Trading Estate (KITE) zone and Qayyum Abad/Ghizri Creek area along its course towards the sea. 3.1.4. The Karachi Sea The coast of Karachi is about 40 km long. Shallow seawater samples were collected during the high tide period from 5 locations in the inter-tidal zone along Karachi coast. The range of variation in chemical and stable isotope contents of oxygen and hydrogen in the water molecules, and of carbon in the TDIC in Karachi seawater, used in evaluation of coastal groundwater recharge characteristics are shown in Table 1. The mildly alkaline pH values of ∼8 for open seawater off Karachi coast generally conform to those for normal ocean waters. EC values for Karachi seawater range between 49.3 and 53.7 mS/cm, while the salinity values are ∼38. The EC values higher than 50 mS/cm correspond to relatively non-polluted open seawaters. The seawater temperature off Karachi coast is fairly constant about 25.5◦ C. Slightly higher temperature is observed near Ghizri coast, which is attributed to input of relatively warmer wastewaters of industrial and domestic origin. Turbidity values of open seawater are higher than the on-shore surface water sources. This is attributed to much higher contents of partic-
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ulate matter in seawater as compared to on-shore surface water sources. The lowest turbidity values are observed along Clifton coast, as this coast is relatively free of pollution along southeast side of the Karachi coast. Cl− contents of seawater off Karachi coast range from 21,500 to 25,230 ppm, while the SO−2 4 concentrations are in the range of 2080–2210 ppm. The stable 13 carbon isotope contents (δ CTDIC ) of TDIC vary in the range of −3.9 to +0.8h in different zones off Karachi coast. This is indicative of different levels and sources of dissolved inorganic carbon in seawater due to input of domestic and industrial wastewater into the sea from key industrial trading estates via polluted drains. The highest δ 13 CTDIC value of +0.8h and δ 34 S values of 19.1h corresponds to relatively non-polluted seawater along the northwest coast of Karachi (Buleji Coast). The lowest δ 13 CTDIC value of −3.9h corresponds to highly polluted seawater in Korangi Creek that receives industrial and domestic waste drains from KITE. 3.2. Groundwater in the coastal aquifer Shallow groundwater samples (n = 12) were obtained from hand pumps (n = 4), dug wells (n = 1) and shallow mini-bores with centrifugal pumps (n = 7), installed at depths less than 45 m in the coastal aquifer of Karachi. Relatively deep groundwater was obtained from pumping wells (n = 16) installed at depths between 50 and 100 m in the coastal aquifer of Karachi. These cased wells also tap various proportions of shallow groundwater in addition to deep groundwater. Table 1 presents the ranges of physico-chemical and environmental stable isotope data of shallow groundwater samples and shallow mixed deep groundwater samples collected from the municipal jurisdiction of Karachi. 3.2.1. Shallow groundwater Physico-chemical data show that shallow groundwater is moderately saline. The mean chemi18 2 cal concentrations of Cl− , HCO− 3 and SO4 and the mean isotope content of O, H in shallow 34 13 groundwater, S of aqueous sulfate of water and C in TDIC are shown in Table 2. The mean stable isotope contents of 18 O and 2 H indicate that the shallow aquifer system is recharged by a mixture of fresh waters of mainly IR and polluted waters of LR and MR and their tributaries, both under natural and artificially induced infiltration conditions. Much depleted values of δ 13 CS.G. (less than −6h) indicate the impact of pollution from LR and Table 2 Results of chemical and isotopic analysis of shallow groundwater samples Groundwater
Concentration
Mean Cl− (shallow groundwater) Mean HCO− 3 (shallow groundwater) Mean SO− 4 (shallow groundwater) Mean δ 18 O (shallow groundwater)
280 ± 380 ppm (n = 12) 360 ± 110 ppm (n = 12) 74 ± 55 ppm (n = 12) −5.98 ± 0.66h V-SMOW (n = 12) −47.62 ± 5.41h V-SMOW (n = 12) −7.52 ± 2.9h PDB (n = 12) 6.21 ± 2.1h CDT (n = 12)
Mean δ 2 H (shallow groundwater) Mean δ 13 C (TDIC-shallow groundwater) Mean δ 34 S (sulfate-shallow groundwater)
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MR into the shallow groundwater environment. It is noteworthy that polluted river waters are mixtures of public supply water of local aquifer origin, Hub River (draining spring water and flood water) origin and IR origin. Majority of shallow groundwater samples have Cl/HCO−1 3 ratios higher than 0.66 coupled with a trend towards increase in the Cl− content. Generally, most of coastal shallow groundwater samples have Cl− /HCO−1 3 ratios between 0.6 and 6.36 −1 − (n = 6) (Cl /HCO3 ratios of <0.66 are attributed to groundwater of freshwater origin (rain and river recharge). Higher Cl− /HCO−1 3 ratios in shallow coastal groundwater samples 18 coupled with a trend towards δ O values of local seawater indicate diffusion/intrusion of seawater. These samples indicate that the zone of diffusion of seawater into groundwater has extended to a considerable lateral extent into the coastal aquifer. 3.2.2. Deep groundwater Physico-chemical data of deep groundwater show that the deep wells have relatively higher values of EC and salinity as compared to the shallow wells. Further, the deep groundwater is quite saline. The mean chemical concentrations of Cl− , HCO−1 3 and SO4 and the mean isotope content of 18 O, 2 H and 34 S in shallow mixed deep groundwater, and 13 C in TDIC in shallow mixed deep groundwater are shown in Table 3. The hydrochemical and stable isotope results indicate that the confined aquifer hosts a mixture of rainwater from hinterlands and surrounding regions around coastal Karachi, as well as sea trapped water/seawater through intrusion under natural infiltration conditions, or under induced recharge conditions. 3.3. Groundwater recharge characteristics and seawater intrusion 3.3.1. Origin of coastal groundwater recharge The analysis of natural variations of the heavy isotope contents of oxygen (18 O) and hydrogen (2 H) represents one of the classical applications of isotope hydrology in studying the origin and dynamics of groundwater. Due to their different contents observed in groundwater and seawater, 18 O and 2 H have been used to assess or confirm seawater intrusion as the main mechanism of salinization. Figure 2 shows the δ 18 O versus δ 2 H plot of groundwater in coastal Karachi vis-à-vis isotopic indices of local precipitation (rain), Indus River water and local Table 3 Results of chemical and isotopic analysis of deep groundwater samples Groundwater
Concentration
Mean Cl− (shallow mixed deep groundwater) Mean HCO− 3 (shallow mixed deep groundwater) Mean SO4 (shallow mixed deep groundwater) Mean δ 18 O (shallow mixed deep groundwater) Mean δ 2 H (shallow mixed deep groundwater) Mean δ 13 CTDIC (shallow mixed deep groundwater) Mean δ 34 SSO4 (shallow mixed deep groundwater)
3900 ± 400 ppm (n = 16) 340 ± 160 ppm (n = 16) 520 ± 530 ppm (n = 15) −5.0 ± 1.0h V-SMOW (n = 16) −43.0 ± 11.0h V-SMOW (n = 16) −8.47 ± 3.0h PDB (n = 16) 6.85 ± 4.9h CDT (n = 16)
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Fig. 2. δ 18 O versus δ 2 H plot for groundwater, coastal Karachi (GMWL = Global Meteoric Water Line).
seawater. The long term mean stable isotopic indices of 18 O and 2 H in precipitation as well as the Indus River fall just below the Global Meteoric Water Line (GMWL), while the surface water samples (Hub Lake water and the river water, local seawater) and the groundwater samples fall well below the GMWL. There is a large scatter in the stable isotope plot of oxygen vs. hydrogen for shallow groundwater and deep groundwater. The δ 18 O vs. δ 2 H values of shallow groundwater and deep groundwater mainly range between the mean δ 18 O vs. δ 2 H values of the Indus River (δ 18 O = −5.9h and δ 2 H = −48.12h) and mean δ 18 O vs. δ 2 H values of precipitation (δ 18 O = −3.9 ± 1.94h and δ 2 H = −23.5 ± 18.1h), as well as shallow seawater along Karachi coast (δ 18 O = +0.76h and δ 2 H = 6.85h). The shallow groundwater samples cluster around the mean δ 18 O value of −5.98 ± 0.65h. The deep groundwater samples cluster around the mean δ 18 O value of −5.0 ± 1.05h. Isotopically, some of these deep groundwater samples partly overlap the shallow groundwater. Nevertheless, the large shift in δ 18 O values of shallow and less deep groundwater towards right of Local Meteoric Water Line (LMWL) is attributed to mixing of various proportions of recharge of local precipitation and the mixed waters from polluted rivers and seawater. The possibilities of major contribution to groundwater recharge of shallow/phreatic aquifer directly by the local rainfall seems very small due to extremely poor frequency of rainfall events, the rainfall intensities in Karachi and high evaporation rates. The long term (15 yr annual record) mean monthly average precipitation for Karachi is between 0–15 mm during the months of January to June, 23–91 mm during the months of July to September and 0–7 mm during the months of October to December (IAEA, 1992). Due to the current drought conditions in the area, the direct recharge from precipitation is negligible. This leaves seawater as the 2nd major source of recharge to coastal groundwater system. Under the present surface water supply practices, drought conditions and significant withdrawal of groundwater, the stable isotope index of 18 O and 2 H in shallow and less deep groundwater may shift towards the isotopic index of rainwater/seawater in association with higher groundwater salinities.
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3.3.2. Origin of coastal groundwater salinity Gonfiantini and Araguás (1988) compiled a number of case studies on seawater intrusion carried out under different hydrogeological settings. The examples described in their paper illustrate both simple and complex approaches used to characterize coastal aquifers and to shed light on the nature of salinization. Yurtsever and Payne (1978) presented a typical case of interactions between three components or water types, in which most of the observed increase in salinity was due to the upward leakage of deep groundwater, and only in few cases a direct seawater intrusion was active. In other cases, these tools were used to show a complex geochemical behavior of dissolved salts in groundwater systems, due to water rock-interaction (adsorption, cation-exchange), long residence time of groundwater in the aquifer or the presence of deep brines (connate water). From the Ghijben–Herzberg relationship, the freshwater–saltwater equilibrium requires that the water table or the piezometric surface lies above the sea level and slopes downward towards the ocean. Freeze and Cherry (1997) described a simple and useful scheme for recognizing groundwater salinity, based on the contents of Total Dissolved Solids (TDS). Seawater intrusion into coastal aquifers is usually assumed when an increase in (TDS) or EC of the extracted groundwater is observed. Seawater encroachment is the most common mechanism operating in well fields located at short distance from the coastline, mainly due to intensive pumping in selected sites. The TDS of the seawater is approximately 35000 ppm. Surface water or groundwater containing more than 2000–3000 ppm of TDS is generally too salty to drink. Cl−1 usually plays a minor role in groundwater, but it is a dominant ion of seawater. In contrast, HCO−1 3 is usually the most abundant negative ion in groundwater. There is such a large difference between the proportions of Cl− and HCO−1 3 in groundwater and in sea− water, that the ratio between these two ions (i.e. the ratio: Cl /HCO−1 3 ) is a useful index of the presence of seawater in groundwater. Hence, an increase of chloride content in groundwater is the most reliable indicator of the first stage of salt-water intrusion in groundwater. Goswani (1969) presented field studies at Digha (India) and, making use of Cl−1 content of water samples, he delineated the groundwater body on isochlor of 500 ppm (TDS = 100 ppm) and Cl− /HCO−1 3 ratio of 0.66. The boundary zone of the 300–500 isochlors demarcates the diffusion zone of seawater (Freeze and Cherry, 1997). To verify possible seawater intrusion in shallow groundwater and mixed deep groundwater and/or existence of trapped seawater in deep groundwater, the concentrations of Cl− , as −2 18 well as Cl− /HCO−1 3 ratios, are plotted against δ O values, and the concentrations of SO4 −2 − are plotted against SO4 /Cl ratios for shallow groundwater samples in Figs. 3, 4 and 5, respectively. It may be realized that the extrapolated or forecast trends for shallow groundwater samples do not fall on the data points for local seawater (or other tropical seawaters). However, the extrapolated or forecast trends for shallow groundwater samples, excluding the near shore coastal groundwater sample, project towards higher salinities. Lastly, the extrapolated − or forecast trends for shallow mixed deep groundwater samples (with high SO−2 4 and Cl 18 contents, Cl− /HCO−1 3 ratios greater than 0.66 and relatively enriched δ O values) project towards very high salinities, and tend to pass through the data points for local seawater (or other tropical seawater). The present investigations, therefore, prove seawater intrusion/existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifer in coastal Karachi.
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Fig. 3. Chloride versus 18 O trend line for shallow and deep aquifers, coastal Karachi.
18 Fig. 4. Trend line of Cl− /HCO−1 3 ratios versus O shallow and deep groundwater in coastal Karachi.
−2 − Fig. 5. Trend line of SO−2 4 versus SO4 /Cl ratios for shallow and deep groundwater in coastal Karachi.
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4. Conclusion The studies carried out with conjunctive use of stable isotope techniques and conventional non-nuclear chemical and biological techniques have successfully facilitated recognition of seawater intrusion in the shallow groundwater and deep groundwater systems in coastal Karachi. The present investigations prove seawater intrusion/existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifers in coastal Karachi.
Acknowledgements These studies have been performed with financial assistance provided by the International Atomic Energy Agency and Pakistan Institute of Nuclear Science and Technology/Pakistan Atomic Energy Commission under IAEA-Research Contract PAK-11322. Special thanks are due to Dr. E. Ahmed for in-kind provision of base camp laboratory facilities and partial transport for fieldwork, and to Dr. M. Ahmed and Mr. M.R. Sheikh for facilitating chemical analysis.
References Clark, I.D., Fritz, P. (1997). Environmental Isotopes in Hydrology. Lewis Publishers, New York, 328 pp. (Chapters 1 and 10). Freeze, R.A., Cherry, J.A. (1997). Groundwater. Prentice Hall, Englewood Cliffs, NJ, 604 pp. Gonfiantini, R., Araguás, L. (1988). Los isótopos ambientales en el estudio de la intrusión marina. In: LopezCamacho Camacho, B. (Ed.), Tecnología de la Intrusión Marina en acuíferos costeros. IGTE, Almuñecar, Spain, pp. 135–190. Goswani, A.B. (1969). Study of salt-water encroachment in the coastal aquifer at Digha, Midnapore district west Bengal. Bulletin of International Assocociation of Hydrological Sciences, India 13, 77. International Atomic Energy Agency (1992). Statistical Treatment of Data on Environmental Isotopes in Precipitation. STI/DOC/10/331. IAEA Technical Reports Series No. 331. IAEA, Vienna, Austria, ISBN92-0-100892-9, 781 pp. Klein, H., Ratzlaff, K.W. (1989). Changes in saltwater intrusion in the Biscayne aquifer, Hialeah–Miami springs area, Dade County, Florida. U.S. Geological Survey Water-Resources Investigations Report No. 87-4249. Kulkarni, K.M., Navada, S.V., Nair, A.R., Rao, S.M., Shivanna, K., Sinha, U.K., Sharma, S. (1979). Drinking water salinity problem in Coastal Orissa–India – Identification of past transgression of sea by isotope investigation. IAEA-SM-349/18. In: Proceedings of the International Symposium on Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and Atmosphere. Vienna, Austria. Qureshi, R.M., Mashiatullah, A., Rizvi, S.H.N., Khan, S.H., Javed, T., Tasneem, M.A. (2001). Marine pollution studies in Pakistan by nuclear techniques. The Nucleus 38, 41–52. Yurtsever, Y., Payne, B.R. (1978). Application of environmental isotopes to groundwater investigations in Qatar. IAEA-SM-228/24. In: Proceedings of the International Symposium on Isotope Hydrology. Neuherberg, Germany.