Abstract
The world's seventh-largest Indus Delta is gripped to extinction due to a continuous shortage of Indus River flows. The drastic fall in the ecology and coastal habitat due to the lack of freshwater flows, and the land degradation due to seawater intrusion is a simultaneous creeping hazard. The study aims to explore the potable water lens and their possible connection with seepage from freshwater bodies (rivers/ canals) to sustain them for drinking and agriculture use of 2 million populations. The study employed the electrical resistivity survey and 10 selected points along the Indus River at depths up to 300 m to baseline the rock type and groundwater quality; and drilling of bore logs at a maximum depth of 25 m. The result confirms the availability of two sandstone layers with marginal freshwater along the river and in some pockets; however brackish water was observed along the coast in a limestone formation. It is evident from the results that surface seepage from the Indus River and non-perennial ‘Pinyari’ canal has a progressive influence on the improvement of groundwater quality and confinement of seawater intrusion.
HIGHLIGHTS
Investigating the potable water lens and their possible connection with seepage from freshwater bodies (rivers/canals).
Investigation of baseline rock type and groundwater quality using electrical resistivity survey.
Research proposed the construction of Sindh Barrage for a sustainable delta and ecosystem.
INTRODUCTION
Ramsar protected Indus Delta is referred to as one of the largest deltas spread over 0.6 million hectares, consisting of the world's largest and unique arid climate mangroves (Saied et al. 2013). The Delta is now dying due to mismanagement, and a shortage of Indus River flows (Abro et al. 2020) which is now occasionally flowing from Khobar Creek only instead of 17 major and numerous minor creeks. Qamar (2009) concluded 67% reduction of mangroves forest from 1953 to 2001, at the alarming rate of 2.18% per annum, compared to 1.0% of Iran, 1.4% of Yemen, 0.02% of Bangladesh, 0.3% of India, and on an average 1.0% of Asia and 0.76% of the world.
The Indus River region is denoted as seriously affected by up to 40% by human settlements (Shah et al. 2021, 2023; Xu et al. 2020). The world's largest contiguous irrigation network, which expands both cropping area and cropping intensity, utilizes a significant portion of the available river water to cultivate food and fiber, this is in response to the growing demand from Pakistan's increasing population (Hussain et al. 2020; Singh et al. 2021; Sajid et al. 2022). The equitable distribution of available river water is a grave concern. To meet human needs, the construction of dams, barrages, and continuous alteration of the Indus River flows have undermined the ecological sustainability in the deltaic region (Salika et al. 2016; Shah et al. 2022). The lower riparians on the Indus River are demanding river water to sustain the Indus Delta, its ecosystem, and approximately 2 million people living in the coastal districts.
Potable groundwater is the main source of drinking and an additional source for household and small-scale agriculture and industrial usage. The increasing population and need to produce more food and facilities manifold the consumption of groundwater to meet the anthropogenic needs (Jehanzaib et al. 2020). Pakistan is ranked as the top five countries in the world, having a groundwater abstraction rate of around 80 BCM per annum (Wada et al. 2014). The increasing abstraction rate is more than the recharge rate, resulting in depletion and deterioration of groundwater level and quality all over Pakistan (Abro et al. 2018). Mahessar et al. (2015) conducted research in the coastal zone of Pakistan, categorizing it as a privileged and underserved area, they proposed alternative plans for groundwater recharge through rainwater harvesting and microfiltration treatment.
Per capita, water availability is declining in certain parts of the world, while the recharging of aquifers is getting slower due to a consistent change in land use and climate change impacts (Abro et al. 2018; Kumar et al. 2020, 2021). Developed countries have extensively researched methods to enhance water productivity, including smart irrigation, water reuse, and efficient water resource management (Bwambale et al. 2023). However, developing nations are still working to shift mindsets toward water conservation and maximizing benefits per unit of water. With the current rate of water consumption, Pakistan faces the looming threat of becoming one of the water-scarce countries by 2025. Rao (1993) concluded that the seepage from the canals is an important source of groundwater recharge along with rainfall and return flows (drainage) from the irrigated field.
Exploring fresh groundwater zones is an increasingly essential requirement to enhance the management of extraction and recharge processes, ensuring ample quantities for sustainable groundwater management (Ahmad & Al-Ghouti 2020). Along with the physical well drilling, there are geophysical investigations like ground penetrating radar (GPR) and vertical electrical sounding (VES) to identify the extent and quality of the aquifers. The VES has extensively utilized the Terrameter equipment to assess subsurface lithology and groundwater quality (Perpetual & Aladin 2023). This method implied the process of recording the resistance of materials directing ultrasound waves. Each material has its unique reflectance signature to differentiate it from other materials. Similarly, water quality can be assessed based on the presence of suspended salts. The higher the salt content, the lower the resistivity. The literature review indicates that <1 Ohm-meter (Ω-m) resistance shows the availability of saline water and resistivity <0.3 Ω-m designates the existence of seawater (Satriani et al. 2011; Saad et al. 2012; Costall et al. 2018; Gopinath et al. 2018). The VES interpretation can be conducted using software, such as IX1D, IPI2WIN, ZondIP1D, Earth-Imager 1D, etc.
VES has been extensively employed in exploring groundwater quality and quantity by various researchers around the world (Hassan et al. 2017; Hafeez et al. 2018; Shah et al. 2021). Apart from determination and quantifying groundwater VES has also been used for delineating seawater intrusion and behavior of seawater–freshwater interface to distinguish between different water qualities and quantify the volume of specific zones/aquifers (Zaidi & Kassem 2012; Soomro et al. 2019). The research conducted thus far using VES has primarily concentrated on identifying saline and freshwater zones. The current study, however, is centered on exploring lithology and groundwater quality, along with investigating the potential impact of seepage from water bodies. The findings from this research will contribute to formulating recommendations for maintaining a continuous flow of freshwater in the water conveyance system and rehabilitating lagoons and lakes during the flooding season. This is crucial for sustaining the Indus Delta, its ecosystem, and its habitat.
STUDY AREA AND DATA
Study area
Hydro-geology of the study area
The area is floodplain and formed through the alluvial deposits of the Indus River. The Indus River is termed the oldest and most documented river, formed by the collision of the Eurasian and Indian plates. The exact age of the Indus fan is, however, controversial among researchers, but the leading features were designated during the early Miocene time (Clift 2002). The Indus Basin forms 65% of the total country area, while the rest of the Indus Basin, spreads in Afghanistan, India, and China with other smaller rivers joining the main course. The river meanders over geological times, resulting in the delta shifts westward. Before late 1800, the river flowed with 17 major creeks into the Arabian Sea. During the British time, the river was blocked due to the construction of irrigation canals and flood levees. Currently, there is one functional channel (Khobar Creek) that flows rarely (Inam et al. 2007). The water and the once-heavy sediment flows have been abridged with the erection of large dams and water diversion structures. In the pre-damming situation, the Indus River transported 270+ million tons of sediment load per year to the delta; now the river carries little water or sediment as low as ∼13 Mt/year. The resulting erosion rate due to tidal movement averaged ∼69 Mt/year, and deposition averaged ∼22 Mt/year, providing a net loss of ∼47 Mt/year.
The notations on the geological map
S# . | Mark . | Symbol . | Description . | Epoch . | Age (Ma) . | Period . | Era . |
---|---|---|---|---|---|---|---|
1 | Qt | Tidal delta marsh deposits | Holocene | 0.01 | Quaternary | Cenozoic | |
2 | Qtf | ![]() | Tidal Mudflats deposits | ||||
3 | Qdx | ![]() | Deltaic floodplain deposits | ||||
4 | Qtx | ![]() | Tidal delta marsh deposits | ||||
5 | Qfx | ![]() | Flood plain deposits (lower terrace) | ||||
6 | Qmx | ![]() | Stream bed and meander-belt deposits | ||||
7 | Qb | ![]() | Beach sand and coastal sand bar deposits | ||||
8 | Qs | ![]() | Unconsolidated surficial deposits | ||||
9 | Ngg | ![]() | Gaj formation | Miocene | 24 | ||
10 | Pgn | ![]() | Nari formation | Oligocene | 34 | ||
11 | Pglk | ![]() | Laki formation | Eocene | 55 | Tertiary | |
12 | Pgpl | ![]() | Lakhra formation | Paleocene | 65 |
S# . | Mark . | Symbol . | Description . | Epoch . | Age (Ma) . | Period . | Era . |
---|---|---|---|---|---|---|---|
1 | Qt | Tidal delta marsh deposits | Holocene | 0.01 | Quaternary | Cenozoic | |
2 | Qtf | ![]() | Tidal Mudflats deposits | ||||
3 | Qdx | ![]() | Deltaic floodplain deposits | ||||
4 | Qtx | ![]() | Tidal delta marsh deposits | ||||
5 | Qfx | ![]() | Flood plain deposits (lower terrace) | ||||
6 | Qmx | ![]() | Stream bed and meander-belt deposits | ||||
7 | Qb | ![]() | Beach sand and coastal sand bar deposits | ||||
8 | Qs | ![]() | Unconsolidated surficial deposits | ||||
9 | Ngg | ![]() | Gaj formation | Miocene | 24 | ||
10 | Pgn | ![]() | Nari formation | Oligocene | 34 | ||
11 | Pglk | ![]() | Laki formation | Eocene | 55 | Tertiary | |
12 | Pgpl | ![]() | Lakhra formation | Paleocene | 65 |
Re-extracted from geological map of Sindh, Pakistan: Geological Survey of Pakistan.
Re-extracted from geological map of Sindh, Pakistan: Geological Survey of Pakistan.
METHODS
Vertical electrical sounding
Study area showing Indus River, and VES locations with respect to elevation.
RESULTS
Vertical electrical sounding
The spatial locations of 10 VES points have been marked to visualize their respective position and elevation. Figure 4 indicates that most of the study areas have a very slight difference in elevation with respect to the length of the area, and all VES points are located within the elevation range of 0–15 m from sea level. However, the north-west side has mountainous foothills which elevated up to the height of 250 + m. The sounding data were analyzed through two horizontal line profiles each consisting of five VES locations. Line profile 1, consisting of VES 1–5, starts from the delta, i.e., in the west, and stretches toward the east, crossing the Indus River. Line profile 2 (VES 6–10) starts from the north-west, foothills of Kohistan (Khirthar mountain range) near Keenjhar Lake and stretches toward the east crossing the river. Apart from the line profiles, the detailed analysis of each VES location has been discussed. The sounding analysis was guided based on bore-log information, and the geological foundation of the study area. The bore logs have been known as a prodigious source to determine the lithological formation. A total of 12 bore logs were drilled up to the depth of 25 m in the study area.
Geo-electric section of profile 01
Geo-electric sections of profile 1, (a) VES-01, (b) VES-02, (c) VES-03, and (d) VES-04.
Geo-electric sections of profile 1, (a) VES-01, (b) VES-02, (c) VES-03, and (d) VES-04.
VES-02 (Figure 6(b)) was conducted at latitude 24.1487 and longitude 67.4756. The area is located near Garho town of Thatta district which is situated 30 km away from the coast. Again, three different lithological layers were demarcated based on the aquifer. The results interpreted in the subsurface model show that the top layer has an apparent resistivity of 0.491 Ωm up to the depth of 19.7 m (19.7 m wide). The layer is interpreted as clay-bearing saturated lithology with a highly saline water aquifer, the middle layer with a resistivity of 4 Ω-m up to the depth of 55.5 m (33.8 m wide) is interpreted as a marginal freshwater aquifer, and the bottom layer with a resistivity of 1.07 Ω-m up to the depth of 300 m (247 m thick) is interpreted as saline water aquifer.
VES-03 m (Figure 6(c)) was conducted at latitude 24.1858N and longitude 67.5428. The area is near Ghorabari town which is situated on the side of the Indus River. The soil profile can be divided into four different aquifer-based lithological layers. The results interpreted in the subsurface model show that the top layer with an apparent resistivity of 2.18 Ω-m up to the depth of 4.23 m (4.23 m wide) is interpreted as an alluvium layer, the second layer with a resistivity of 15.8 Ω-m up to the depth of 15.5 m (11.3 m wide) is interpreted as freshwater aquifer present in sandstone lithology showing good porosity and permeability and has potential to hold fresh water, the third layer with a resistivity of 5 Ω-m up to the depth of 69.9 m (54.1 m thick) is interpreted as marginal freshwater (i.e. low saline) in clay-bearing lithology, while the bottom layer with a resistivity of 1.68 Ω-m up to the depth of 300 m (230 m thick) is interpreted as moderately saline water aquifer present in clay-bearing clay-bearing lithology and showing very low resistivity values and comparatively higher values. The results reflect the ground situation as VES-03 has been conducted near the riverbank. The first resistivity layer shows the alluvium soil due to river sediments, while the second layer has the presence of marginal fresh water due to the infiltration of fresh water from the Indus River in sand-bearing lithology. The third and fourth layers represent the general lithology of the entire area as saline.
VES-04 (Figure 6(d)) was conducted at latitude 24.2692 and longitude 67.9389. This VES location falls on the left-hand side of the Indus River in the geographical boundary of the Sujawal district. The soil profile can be divided into four different aquifer-based lithological layers. The results present in the interpreted subsurface model display the top layer with an apparent resistivity of 2.48 Ω-m up to the depth of 6.03 m (6.03 m thick) interpreted alluvium or weather layer. The second layer showing a resistivity of 0.345 Ω-m up to the depth of 13.45 m (7.42 m thick) is interpreted as a highly saline water aquifer zone present in clay-bearing lithology. The third layer with an apparent resistivity of 1.23 Ω-m up to the depth of 150 m (144.4 m thick) is interpreted as moderate saline. The fourth layer with a resistivity of 1.07 Ωm up to the depth of 300 m (150 m thick) is also interpreted as moderate saline water aquifer present in silt loam lithology. Apparently, the location of VES-04 is near coastal marshy land which deteriorates the groundwater quality due to tidal movement and stagnant seawater.
Geo-electric section of profile 02
Geo-electric sections of profile 2, (a) VES-06, (b) VES-07, (c) VES-08, and (d) VES-09.
Geo-electric sections of profile 2, (a) VES-06, (b) VES-07, (c) VES-08, and (d) VES-09.
VES-07 (Figure 7(b)) was conducted at latitude 24.7637 and longitude 67.8789 near Hadero Lake on the right-hand side of the Indus River (Thatta district). The sounding resulted in five different aquifer-based lithological layers at this location. The results present in the interpreted subsurface model show that the top layer with an apparent resistivity of 3.06 Ω-m up to the depth of 13.4 m (13.4 m thick), interpreted as an alluvium layer, the second layer with an apparent resistivity of 0.496 Ω-m up to the depth of 30 m (16.6 m thick) is interpreted as high saline water aquifer present in clay-bearing lithology, the third layer with an apparent resistivity of 2 Ω-m up to the depth of 67.1 m (37.1 m thick) is interpreted as moderate saline water aquifer present in clayey lithology, the fourth layer with an apparent resistivity of 0.43 Ω-m up to the depth of 150 m (82.9 m thick) is interpreted as high saline water aquifer present in sandstone lithology, and the fifth layer with a resistivity of 3.5 Ω-m up to the depth of 298 m (148 m thick) is interpreted as a brackish aquifer.
VES-08 (Figure 7(c)) was conducted at latitude 24.7779 and longitude 67.9940 near Keenjhar Lake, on the right-hand side of the Indus River (Thatta district). The sounding demarcated three different aquifer-based lithological layers. The results present in the interpreted subsurface model show that the top layer with apparent resistivity of 5.32 Ω-m up to the depth of 26.1 m (26.1 m thick) is interpreted as an alluvium layer having marginal freshwater aquifer present in sandstone lithology, the middle layer with an apparent resistivity of 1.97 Ω-m up to the depth of 141 m (121 m thick) is interpreted moderately saline water aquifer, the last layer with apparent resistivity of 0.937 Ω-m up to the depth of 300 m (155 m thick) is interpreted highly saline water aquifer present in clay-bearing lithology showing very low resistivity and very high salinity.
VES-09 (Figure 7(d)) was conducted at latitude 24.8530 and longitude 68.1150 in the geographical boundary of Sujawal District. The sounding results determined four different aquifer-based lithological layers. The results present in the interpreted subsurface model show that the top layer with apparent resistivity of 4.57 Ω-m up to the depth of 3.1 m (3.1 m thick) is interpreted as an alluvium layer, the second layer with apparent resistivity of 14.7 Ω-m up to the depth of 10.6 m (7.49 m thick) is interpreted as a marginal freshwater aquifer present in sandstone lithology which possesses good porosity and permeability, the third layer with apparent resistivity of 1.62 Ω-m up to the depth of 55.9 m (45.3 m thick) is interpreted as moderate saline water aquifer present in sandstone lithology, and the last layer with apparent resistivity of 0.706 Ω-m up to the depth of 300 m (247 m thick) is interpreted as high saline water aquifer present in clay-bearing lithology showing low resistivity and high salinity. Due to a close proximity to the Indus River, the second layer shows the marginal fresh groundwater to a depth of 10 m below the surface.
Pseudo-section and resistivity section of profile 1 (VES-01 to VES-05)
Pseudo and resistivity sections of profile 1 w.r.t. river/location (VES-01 to VES-05).
Pseudo and resistivity sections of profile 1 w.r.t. river/location (VES-01 to VES-05).
The resistivity cross-section profile from VES 01 to VES 05 shows the difference in resistivity with the subsurface. The topmost layer shows the alluvium layer, which is intermixed with saline material, the common trend of the resistivity section is producing the trend of resistivity downward. In general, three primary anomalous zones, characterized by high saline resistivity, moderate salinity, and a brackish water zone based on resistivity contrast, predominantly exhibit clay and sandstone lithology. This confirms the marginal freshwater aquifer along the Indus River through resistivity data in VES-03.
Pseudo-section and resistivity section of profile 2 (VES-06 to VES-10)
Pseudo and resistivity cross section of profile 02 w.r.t. river/location (VES-06 to 10).
Pseudo and resistivity cross section of profile 02 w.r.t. river/location (VES-06 to 10).
Bore-log of the area
The bore-log information of 12 locations in the study area is presented in Table 2 describing the lithology of different parts of the study area. The data display the dominant alluvial layer of sandy soil along with clay loam in the area. The study area is the delta consisting of different lithological formation formations as per anthropogenic activities. The bore logs are drilled up to 25 m in depth only to confirm the lithology and to collect the water samples for quality analysis.
DISCUSSION
The VES interpretation at various locations highlights the positive seepage influence of the Indus River and the earthen irrigation network. Periodic seepage from the irrigation network, along with sporadic seepage from the Indus River, contributes to the presence of a moderate freshwater layer in the top 10 m of depth. Notably, the ‘Pinyari canal,’ which operates seasonally during the ‘Kharif/summer’ season, supplying irrigation water only from April to September each year, remains closed during the rest of the year, preventing seepage.
Analysis of the VES data conclusively confirms that implementing a year-round freshwater supply through the irrigation system has the potential to fully reclaim the sand-dominant layer in the study area. This reclamation could significantly benefit approximately 2 million people by providing access to drinking water. Additionally, the initiative is expected to enhance vegetation, increase mangrove cover, and rejuvenate lagoons and shallow lakes. Such measures contribute to the sustainable preservation of the Indus Delta, as the consistent presence of freshwater acts as a deterrent to seawater intrusion inland.
Channeling flood and rainwater through gravity-driven irrigation networks, facilitated by gated weirs, presents a viable strategy. By harnessing the recurrent floods, this approach not only supplements the system with additional freshwater but also contributes to enhancing groundwater levels and replenishing aquifers. The cultivation of high delta crops further sustains groundwater levels through continuous seepage. Encouraging the cultivation of such crops in the region serves as a dual-purpose solution, revitalizing both agricultural productivity and the ecological sustainability of the area.
CONCLUSIONS
The overall resistivity profile of Thatta and Sujawal districts, respectively, on the right- and left-hand sides of the Indus River reveals three to five-layered soils with a top alluvial layer followed by clay, sandstone, limestone, and deep second clay layers.
VES-03 and VES-09 have been recorded adjacent to Indus River indicating freshwater. The lithology of both freshwater zones shows the presence of sandstone with good porosity and permeability.
VES-08 and 10 show moderate fresh to low saline groundwater, which has been taken in the area surrounded by an earthen irrigation network off-taking from ‘Pinyari’ canal.
The resistivity of VES-01 and VES-02 conducted near Keti Bander (Thatta District) at the southwest interpreted the high salinity below the alluvium layer. Both locations are adjacent to the marshy coastal area. The higher salinity observed in the upper layer of soil is due to the tidal impact of seawater.
The pre-damming scenario in the mid-1900s reduces the flow of the Indus River and sediments drastically which creates environmental and ecological cataclysm downstream of Kotri toward the Indus Delta. The provision of freshwater is the only solution to sustain the ecology, biodiversity, and habitat in the delta regions. Hence, it is recommended that the available non-perennial irrigation network in the Indus Delta must be converted to a perennial system by managing the water effectively introducing smart irrigation, conjunctive use of irrigation and marginal saline groundwater, and introducing rainwater harvesting in the area. The river water retention (dam) structure is proposed at downstream Kotri Barrage near to the delta to head up the flood/rainwater to maximize the seepage to augment fresh groundwater and deter seawater intrusion.
AUTHOR CONTRIBUTIONS
Z.A and S.A.S rendered support in formal analysis, data curation, and writing the original draft. Z.A., A. L. Q., and M. A. J. developed the methodology and validated the data. A. S. and S. A. A. R. visualized the data and investigated the work. A. L. Q. conceptualized the data and supervised the work. T.-W. K. and R. S. A. wrote the review and edited the article.
ACKNOWLEDGEMENTS
The research fund of Mehran University of Engineering and Technology Jamshoro supported the work. This work was also supported by the research fund of Hanyang University (HY-2022-1913). The authors also want to acknowledge the support provided by Researchers Supporting Project number (RSP2023R310), King Saud University, Riyadh, Saudi Arabia, the INTI International University, Malaysia & The Aror University of Art, Architecture, Design, and Heritage Sukkur, Pakistan for providing the research facilities.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.