OR/15/047 Typologies

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MacDonald A M, Bonsor H C, Taylor R, Shamsudduha M, Burgess W G, Ahmed K M, Mukherjee A, Zahid A, Lapworth D, Gopal K, Rao M S, Moench M, Bricker S H, Yadav S K, Satyal Y, Smith L, Dixit A, Bell R, van Steenbergen F, Basharat M, Gohar M S, Tucker J, Calow R C and Maurice L. 2015. Groundwater resources in the Indo‐Gangetic Basin: resilience to climate change and abstraction. British Geological Survey Internal Report, OR/15/047.

As described in Chapter 3, the aquifer properties, water chemistry and groundwater recharge vary throughout the Indo Gangetic Basin groundwater system. Much of the spatial variation at the basin scale is predictable and relates to the geological and climatic history of the area, and more recently to some of the irrigation practices. These systematic changes in the groundwater resource across the basin are described by seven major typologies, which present the significant differences in the groundwater resources in the different areas.

Three minor typologies at the margin of the basin accompany these large over‐arching major typologies. Each typology is summarised below with a block diagram and the extent of the typology shown in Figure 11. Table 1 summarises the main differences and characteristics of the typologies.

Figure 11 The main groundwater typologies of the Indo‐Gangetic basin.

Typology 1 The Piedmont

The piedmont typology lies along the northern margin of the basin at the edge of Himalayas and comprises a narrow strip 10s of km wide including the Bhabhar zone, Terai plain and intermountain valleys (Lovelock and Murti 1972[1]; GDC 1986). The aquifer is heterogeneous, made up of stacked alluvial fans comprised predominantly of gravels and coarse sands eroded from the foothills of the Himalayas, and ranges from boulder to silt sized sediment (Day 1971[2]; Singhal et al. 2010[3]). The typology is variable in thickness, but often not more than 100 m. The permeability of the aquifer is variable, 1–50 m/d, and specific yield high 20–30% (Lovelock and Murti 1972[1]; GeoConsult 2012[4]). There is little evidence for regional separation between shallow and deep, although shallow and deeper groundwater may be locally disconnected.

Rainfall in the typology is high, often >1000 mm and the aquifer is recharged through rainfall (Narula and Gosain 2013[5]). Saline groundwater is not a major issue, but concentrations of arsenic vary considerably across the typology, related to the source of the sediment and redox conditions and concentrations can be greater than 0.05 mg/L (Shrestha et al. 2004[6]; Gurung et al. 2005[7], Pokhre et al. 2009[8]). Much of the abstraction occurs from shallow tube wells between 0 and 50 m, where yields can be 5–15 l/s; higher yields are common in deeper tubewells (50–80 m) where yields of up to 40 L/s are reported (GeoConsult 2012[4]). On valley sides, and in higher altitude intermontane settings, there remains widespread reliance on diffuse hillslope springs — see Box 2.

Typology 2 The Upper Indus and Upper and Mid Ganges

This major typology comprises an extensive highly permeable aquifer with good quality groundwater that runs from the Upper Indus, through the Upper Ganges and Mid Ganges Basin (Singh 1985; Sinha et al. 2005a[9], 2005b[10]). The sediments are thick (possibly >200 m), generally medium to coarse grained with a high permeability — typically 30–50 m/d but up to 50–70 m/d locally, and high specific yield 10–25% (Chaudri 1966[11]; Bennett 1969[12]; Niwas and Singhal 1985[13]; Shukla et al. 2001[14]; CGWB 2007[15]). Low permeability layers can stratify the aquifer, but these are rarely continuous over more than a few kilometres (Singh et al. 1999[16]; Samadder et al. 2011[17]; Srivastava et al. 2003[18]). Overall anisotropy (Kv/Kh) varies from approximately <25 in the Upper Indus to 50–100 in the Middle Ganges; recent deposits next to the major rivers are less anisotropic (Malmberg 1975[19]; Grey et al. 1979[20]; Srivastava et al. 2003[18]). The aquifer is highly exploited with many shallow tube wells (<100 m), hand dug wells and a growing number of deeper tube wells (100–150 m) (CGWB 2007[15]).

There are many canals throughout this typology and much of the area is irrigated from both surface water and groundwater (Dhiman 2012[21]). Rainfall is high, generally >750 mm and groundwater recharge occurs through both rainfall recharge and canal seepage (CGWB 2007[15]). The presence of saline groundwater is not a major problem, but it may occur at shallow levels as a consequence of water logging, or in pockets at depth associated with evaporite sequences deposits under previous climates (Goodbred and Kuehl 2000[22]; Basharat 2012[23]). Natural elevated arsenic concentrations can occur in various localities, usually associated with younger Holocene deposits, and the groundwater can be contaminated due to the intensive agriculture and urbanisation (Lawrence 1985[24]; CGWB 2007[15]; Saha et al. 2011[25]; Sinha 2011[26]; Basharat 2012[23]).

Typology 3 The Lower Ganges and Mid Brahmaputra

This typology comprises highly permeable alluvium sediments, in a series of stacked channel and interchannel deposits — similar to typology 2 (Singh 1996[27]; Sarma 2005[28]; DPHE 2007[29]; Holly and Voss 2009). The aquifer tends to be highly permeable throughout, with permeability typically 40–80 m/d, and specific yield generally in the range 10–20% (Bennett 1969[30]; Kinniburgh and Smedley 2000[31]). The aquifer is often greater than 200 m thick. Since the aquifer runs along the front of the Himalayas, it still receives sediment input along it length (apart from on the elevated Pleistocene Tract regions) and anisotropy is likely to be similar to typology 2, in the range 25–100 (Michael and Voss 2009[32]). Low permeability units are unlikely to be extensive, extending to several kilometres (Kinniburgh and Smedley 2000[31]). A series of mega fans comprising coarse highly permeable form the upper sequence of parts of this typology, improving its aquifer properties (Shukla et al. 2001[33]).

Rainfall across this typology is high with annual rainfall >1000 mm and canal irrigation is limited to a relatively small area fed from the Rivers Kosi and Ghandak (CGWB 2010[34]). Groundwater recharge is therefore dominated by rainfall recharge and also seasonal flooding from the rivers. Potential recharge is high, and there is evidence that actual recharge is limited by the availability of space in the aquifer to receive it (Shamsudduha et al. 2011[35]). Groundwater levels can be shallow or even at ground surface, causing long periods of flooding. Groundwater salinity is not a widespread issue within this typology, but elevated arsenic concentrations are common in localised areas associated with the Holocene deposits (Harvey et al. 2006[36]; Shah 2008[37]; Kumar et al. 2010[38]; Bhattacharya et al. 2011[39]). Groundwater is widely used, particularly within Bangladesh.

Typology 4 The fluvial influenced deltaic area

The fluvial influenced deltaic area is dominated by extremely high concentrations of arsenic in shallow (generally <100 m deep) groundwater. The sediments comprise alluvium sediments deposited in fluvial to deltaic and tidally influenced setting, and therefore have a greater proportion of silts and fine sands then typologies further upstream (Jones 1985[40]; Allison et al 2003[41]; DPHE 2007[29]; Holly and Voss 2009). The geological setting gives a complex highly heterogeneous aquifer. Permeability can be low — typically 10–25 m/d and specific yield is <10% (Mott MacDonald and Partners 1982[42], 1986[43]; Kinniburgh and Smedley 2000[31]; Mukherjee et al. 2007[44]). Low permeability units are continuous over 10s of kilometres and as a consequence the aquifer is highly anisotropic, with mean horizontal permeability 10 000 times greater than vertical permeability (Michael and Voss 2009[32]).

Rainfall across the typology is high, greater than 2000 mm and increasing from west to east — potential recharge is therefore dominated by rainfall recharge and actual recharge is limited by the space in the aquifer to receive the water and also by the presence of low permeability soils in some places (CGWB 2007[15]; Shamsudduha et al. 2009). Elevated arsenic concentrations in shallow groundwater are widespread, with very high concentrations (>200 µg/L) common (Kinniburgh and Smedley 2000[31]; Acharyya 2005[45]; Harvey 2006[36]; Mukherjee et al. 2011[46]). At depth (>150 m) groundwater can have lower arsenic concentrations, due to the complex history of deposition, historic flushing, redox conditions and the presence of the pervasive low permeability layers which limit the downward movement of the shallow groundwater. The deeper groundwater, however, receives little modern groundwater recharge (Kinniburgh and Smedley 2000[31]; Shah 2008[37]; Hoque and Burgess 2009[47]; Fendorf et al. 2010[48]; Burgess et al 2010[49]).

Typology 5 Middle Indus and Upper Ganges

The Middle Indus and Upper Ganges typology comprises a highly permeable aquifer stretching across the drier area of the middle Indus basin and into the Upper Ganges basin. The aquifer comprises a thick sequence of stacked channel and interchannel alluvial sediments (Singh 1996[27]; ISWARI 2005[50]). The permeability of the aquifer is generally high, often 30–50 m/d, locally up to 50–60 m/d, with high specific yield, 10–20% and regional anisotropy, 25–100, although much lower in the recent deposits next to modern river channels (Bennett 1969[30]; Mott MacDonald and Partners 1982[51]). Evaporite deposits are common within the alluvial stratigraphy at depth leading to areas with saline groundwater.

Rainfall across this typology is highly seasonal with often less than 25 wet days within a year and average annual rainfall is less than 500 mm. Therefore, although rainfall recharge can occur, it does not dominate. Historically, the aquifer was recharged from the rivers, and large, thick (>100 m) fresh water lenses occur close to the rivers (Mott MacDonald and Partners 1986[52]). At the present day, the aquifer is recharged both from the rivers, and the extensive canal network (Basharat 2012[23]). River flow has diminished due to the high volume diverted to the canal network. In general groundwater salinity is < 1000 mg/L close to the rivers, and >2500 mg/L away from the influence of the rivers (ISWARI 2005[50]). Recharge from seepage from the canals can lead to a partial flushing of the shallow groundwater, but also to waterlogging and increased salinization in some areas. Elevated natural fluoride and arsenic concentrations and nitrate from agricultural practices are also common (Gupta et al. 2005[53]).

Typology 6 The Lower Indus

The Lower Indus Basin, found within the Sindh in Pakistan is dominated by the presence of saline groundwater (IWASRI 2005[54]). Salinity is especially widespread at depth, and there is a greater probability of finding good quality groundwater at shallower depths. The aquifer comprises alluvial sediment with a high proportion of fine sands and silts (Mott MacDonald and Partners 1986[52]; Schroder 1993[55]). However, despite this, the average permeability remains generally in the range 1–20 m/d, and specific yield 5–15% (Bennett 1969[30]; Mott MacDonald and Partners 1990). The increased presence of laterally extensive silt layers does decrease the regional vertical permeability and anisotropy is in the region of 100–500. Evaporite sequences in the sediment are common.

Rainfall in the Lower Indus is low, <250 mm, and evapotranspiration is high and, therefore, groundwater recharge from rainfall is negligible (IWASRI 2005[50]). Historically, groundwater was recharged from flow from the Indus River and this led to thick lenses >50 km wide of fresh water around modern and paleo river channels (Basharat et al. 2014[56]). However, river flow in the Lower Indus has significantly reduced in the last 40 years due to irrigation and diversions, and recharge from the river is restricted to a smaller area next to the main Indus channel. Groundwater is recharged from the canal network leading to exensive water logging (Mott MacDonald and Partners 1994). This has lead to the development of thin freshwater lenses in some locations, but also increased phreatic salinisation where the water table is very shallow. Groundwater salinity is mostly > 2500 mg/L. However, next to the Indus, and in localised area, freshwate lenses can exist and groundwater can be <1000 mg/L (Mott MacDonald and Partners 1986[52]; IWASRI 2005[50]).

Typology 7 The marine influenced deltaic areas

A marine groundwater typology exists within the coastal margins of both the Indus and Ganges‐ Brahmaputra river systems. Permeability of these aquifers tends to be low <10 m/d, specific yield <5% and anisotopy very high, 20 000, as a result of the aquifer being composed of highly stratified silt and clay sediments which were deposited in deltaic or marine‐influenced settings (Mott MacDonald and Partners 1986[52]; Kinniburgh and Smedley 2000[31]; Michael and Voss 2009[32]). Shallow groundwater is not used in the coastal regions; here, deep groundwater, below the occurrence of excessive salinity, is a vital resource, especially in the large coastal towns (Taylor et al. 2014[57]).

In Bangladesh, rainfall is high and there is much river water, allowing for recharge in the shallow groundwater both from rainfall and river infiltration (Shamsudduha et al. 2009). Deeper groundwater in this typology receives little modern recharge due to the low vertical permeability (Michael and Voss 2009[32]). In Pakistan, rainfall is negligible and river flow significantly diminished leading to a rapid decline in the availability of freshwater, with a corresponding imapct on the mangrove ecosystems (Basharat et al. 2014[56]). Therefore in Pakistan the groundwater is extensively saline in this typology, both from the influence of sea water intrusion through the creeks and also from terrigenous impact described in Typology 6. The saline water in the coastal areas of Bangladesh is more complex (Allison et al. 2003[41]; Kinniburgh and Smedley 2000[31]). Shallow groundwater can be saline far inland from the impact of storm surges, and deeper groundwater (>100 m depth) can have much lower salinity due to its partially isolation from the modern influence of the sea due to the presence of clay and silt and the high anisotropy.

Minor Typologies

There are several minor typologies identified across the IGB alluvial aquifer system, including the southern marginal alluvium in the Ganges basin, the western Indus basin piedmont, and the Sylhet trough in Bangladesh and deeper groundwater within the Bengal Basin.

Deeper groundwater in the Bengal Basin
Within the Bengal Basin deep groundwater (from 150–350 m) is an important resource. Across the rest of the IGB alluvial aquifer system groundwater is rarely used below 150 m (hence the restriction of much of this report to discussing the shallowest 200 m). However, due the widespread contamination of shallow groundwater with arsenic and the evidence of low arsenic concentrations in groundwater > 150 m (DPHE/BGS 2001[58]) in much of the Bengal Basin, deeper groundwater is routinely exploited to find groundwater of good quality. The likely extent of this deeper groundwater is shown in Figure 11 (DPHE 2006, Michael and Voss 2008[59], Burgess et al. 2010[49]). The hydraulic properties of these older sediments are variable, again dependent on intersecting sand and gravel layers within the more permeable silts. Many deeper tubewells intersect sufficiently permeable strata to sustain yields of >10 L/s and the groundwater system has been successfully modelled with an effective horizontal hydraulic conductivity of 40 m/d (Michael and Voss 2008[59]).

The deeper groundwater is not subject to recent recharge (Hoque and Burgess 2012[60]) and therefore effective monitoring of abstraction is required to manage abstraction, and to ensure substantial groundwater abstraction from the deep aquifer is sustainable for decades or centuries to come (Ravenscroft et al. 2013[61]). Careful monitoring is also required to ensure the quality of the deeper groundwater is sustainable in the long term — as abstraction alters the hydraulic gradients within the aquifer and may locally draw down younger groundwater into the aquifer. This is examined as a case study within this current project — see Box 3.

The Southern Marginal Alluvium
Located south of the Yamuna River along the southern edge of the upper and central Ganges basin, the marginal alluvium represents a genetically distinct typology composed of sediment sourced from Precambrian and Basaltic trap rocks south of the IGB (Heroy et al. 2003[62]). The effective thickness of the aquifer typology is also substantially less — ranging from over 200 m in the north, to 100 m or less at the southern edge (Singh 1996[27]). The permeability and specific yield of the marginal aquifer is, however, comparable to that within the upper and central Ganges basin (typology 2), as a result of similar processes of fluvial deposition, comparable sediment coarseness, and proportion of sand‐dominated to silt and mud‐dominated deposits (Saha et al. 2010[63]). The groundwater quality of the typology is generally good, with large freshwater potential. Shallow groundwater is locally saline >1000 mg/l at the western limit of the typology as a consequence of water logging, or in pockets at depth associated with evaporite sequences deposits under previous climates.

The Sylhet Basin
The Sylhet Basin occupies a distinctive region in east Bangladesh of tectonic subsidence, and forms a discrete typology in the IGB aquifer system of significantly lower aquifer permeability <10 m/d and specific yield <5%. The typology is composed of a high proportion of silts, muds and clays (>60%) deposited in low energy fluvial and wetland settings in the basin (Johnson and Alam 1991[64]). Channel deposits are often separated by significant thicknesses (tens of metres) of muds, and individual channel deposits have to be targeted in groundwater development (Johnson and Alam 1991[64]). Depth to groundwater is very shallow (<3 m bgl), with water logging characteristic of the typology. Lower aquifer units are semi‐confined or confined and typically have a piezometric head which is above the water‐level in the upper aquifer units (Kinniburgh and Smedley 2000[31]).

Groundwater abstraction is limited within the typology, probably as a result of the abundant rainfall. Actual groundwater recharge can be low, due to the lack of spare capacity within the aquifer to receive the recharge. There are locally elevated concentrations of arsenic within shallow groundwater and methane concentrations can also be elevated (Kinniburgh and Smedley 2000[31]).

Western Indus Basin piedmont
The Western Indus piedmont forms a relatively narrow band of coarse high permeability deposits along the western margin of the Indus basin comparable in many respects to the Himalaya piedmont along the northern margin of the IGB. The typology is composed of poorly sorted gravels and coarse sands, with a minor component of silts. The typology is variable in thickness, but often not more than 100 m. In contrast to the Himalaya piedmont, average annual rainfall is less than 500 mm in the western Indus, and rainfall recharge is much more limited. Groundwater is generally saline, but with local exceptions. Localised recharge occurs from rivers and spate irrigation systems along the piedmont. Depth to groundwater is variable, and locally deep (>50 m).

Box 2 — Case study: Groundwater resilience in the Middle Hills of Nepal

Groundwater resources in the Middle Hills of the Himalaya perform a major role in supplying domestic and irrigation water and also in regulating flows in the major rivers in the basin (Andermann et al 2012[65]). However, the significance of groundwater in this region is often unrecognised, and hydrological research has largely focussed on the glaciers with negligible field study of the groundwater. In this case study we undertake fieldwork within the Middle Hills of Nepal to investigate groundwater occurrence and use and also monitor its sensitivity to seasonal variability.

Case study objectives:
The groundwater resources in two contrasting catchments in the Middle Hills were investigated: Ramche at an elevation of 2000–3000 m, with subsistence terraced farming; and Madanpokhara which is largely below 1000 m, with expanding commercial agriculture. Springs, tubewells and streams within the two catchments were investigated using a combination of water use surveys, flow measurements, and sampling for inorganic chemistry, stable isotopes (δ18O and δ2H), groundwater residence time indicators (CFC and SF6) and noble gases. Groundwater sampling was conducted both pre‐ and post‐monsoon. Several springs were monitored weekly for 1 year for flow and variations in stable isotopes. Preliminary results from the study are given in Bricker et al (2014)[66].

Key results:
There is widespread dependence on springs for water supply in the Middle Hills, particularly at higher altitudes. Diffuse high altitude springs showed a wide variability in flow throughout the year, but discrete, geologically controlled springs had more stable flow. Groundwater residence time indicators (CFC and SF6) indicate a mean residence time of 10‐20 years for baseflow in the springs, and stable isotopes (δ18O and δ2H) suggest this water is recharged locally. This evidence of decadal ages for groundwater suggests some in‐built resilience to annual changes in precipitation.

Increased use of shallow groundwater resources in Madanpokhara in the last 5–10 years through the installation of hand‐drilled tubewells in floodplain deposits, has reduced reliance on spring flows, and increased the resilience of communities to climate change. There has been increased agricultural development and inward population migration, but the resource is potentially vulnerable to over‐ exploitation. Concentrations of iron and manganese in excess of the Nepal Drinking Water Quality Standards were measured at some tubewells.

A)Typical terrace farming on hillslopes in the middle hills; B) shallow tubewell with handpump in Madanpokhara ©NERC 2014

Box 3 — Case study: Deep groundwater in Bangladesh and West Bengal

Deep groundwater (beyond 150 m depth) provides a strategic water supply for tens of millions of people in the Ganges‐Brahmaputra‐Meghna (GBM) delta region of Bangladesh and West Bengal as an alternative to shallow As contaminated groundwater. This case study generated new evidence of aquifer hydraulics in the GBM delta and the influences of both intensive deep groundwater abstraction (>150 m) and climate change.

Case study objectives: The study instrumented a series of nested piezometers to monitor high frequency variations in groundwater head at different depths both close to the coast and further inland. Repeated groundwater sampling was undertaken in both pumping and non‐pumped tubewells (see Fig 1) for a suite of environmental tracers — including groundwater age tracers CFCs and SF6, stable isotopes and noble gases. Preliminary results from the study are given in Taylor et al (2014)[57].

Key results:
Variations in deep groundwater head in the delta, remote from intensive abstraction, are dominated by the elastic response of the aquifer sediments to surface water loading. In coastal sites this is dominated by the rhythmic effect of tides, and further inland the response is dominated by monsoon flooding.

The chemical tracers provide evidence that prolonged, intensive deep groundwater pumping modifies groundwater flow locally to that tubewell altering the sources of recharge and inducing a proportion of modern water to the deep abstraction tubewell.

Depth profiles for groundwater residence time tracer CFC‐12 (left) and dissolved arsenic (right), with plot of δ18O v δ2H annotated to illustrate depth and regime of pumping at Khulna; regression line for shallow sites (<100 mbgl) shown as a solid line, regression line for deeper sites (>100 mbgl) shown as a dashed line

Location of study sites in GMB delta region

Table 1 Summary of the main characteristics of the different groundwater typologies.
Hydrogeological properties Hydrochemistry characteristics Recharge Development status
Piedmont margin Variable spatial extent and aquifer thickness, often ≤100 m. K variable (1–50 m/d); Sy high (20–30 %) Salinity not significant, arsenic variable, and can be >0.05 mg/L Predominantly rainfall recharge (average annual rainfall >1000mm) Low‐moderate abstraction ‐ (yield 5‐15 l/s) Reliance on springs in hillslopes
Upper Indus and Upper Mid Ganges Extensive, highly permeable aquifer, at least 200 m thick

High K (30–50 m/d, up to 70 m/d locally); Sy high (10–25%)
Locally saline groundwater and local elevated arsenic. Contamination from intensive agriculture and urbanisation Recharge occurs through both canal leakage and rainfall

Extensive canals within typology, and average annual rainfall >750 mm
Highly exploited — many shallow tubewells (<100 m) and hand dug wells. Growing number of deeper tube wells
Lower Ganges and Mid-Brahmaputra Extensive, highly permeable aquifer, at least 200 m thick. High K (40–80 m/d); high Sy (10–20%) Water levels often very shallow, or at ground surface causing flooding Salinity not significant. Local elevated arsenic concentrations in shallow groundwater Rainfall recharge, and seasonal flooding from rivers

High annual rainfall >1000 mm; canal irrigation limited
Groundwater is exploited but variable — many shallow tube wells (<100 m) and hand dug wells. Water‐levels often shallow however.
The fluvial influenced deltaic area of the Bengal basin Aquifer contains higher proportion of fine sands and laterally extensive low permeability silts. K and Sy can be moderate (10–25 m/d and <10% respectively Widespread high arsenic in shallow groundwater (>200 µg/L). Lower arsenic in deeper groundwater (>100 m) Predominantly rainfall recharge (average annual rainfall high >2000mm) Moderate to high groundwater abstraction

Increasing focus on using deeper groundwater (>150 m).
Middle Indus and Upper Ganges Extensive permeable aquifer within low rainfall region

High K (30–50 m/d); and high Sy (10–20%)
Extensive salinisation of groundwater (>2500 mg/L) away from rivers Locally elevated arsenic, fluoride, and nitrate. Aquifer recharge predominantly from rivers and canal leakage Limited rainfall recharge with low, seasonal annual rainfall (<500 mm, <25 wet days). High groundwater abstraction, falling water — tables and increasing salinity.
The Lower Indus Moderately permeable aquifer within arid region. High proportion of fine sands and low permeability silts in aquifer; K (1–20 m/d), Sy (5–15%) Extensive salinisation of groundwater (>2500 mg/L) away from rivers. Some shallow thin freshwater lenses exist. Locally elevated natural arsenic, fluoride, and nitrate Recharge almost entirely from canal leakage and irrigation returns. Annual rainfall <250 mm; and high evapotranspiration. Limited groundwater abstraction for productive purposes but some for drainage.
The marine influenced deltaic areas Low permeability aquifers in coastal regions composed of highly stratified silts and clays

Moderate — low K (<10 m/d) and Sy (<5%)
Pakistan coast: groundwater extensively saline

Bangladesh coast: shallow groundwater saline deeper groundwater can have much lower salinity
Pakistan coast: limited recharge from rivers and canals; negligible rainfall recharge

Bangladesh coast: high rainfall recharge potential and river flow infiltration
Limited or no shallow groundwater abstraction; deeper non‐saline groundwater is a vital resource to coastal towns in Bangladesh


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