Hydrogeology. Kevin M. Hiscock

in sustaining agricultural productivity in this region of South Asia (MacDonald et al. 2016). Fifteen to twenty million water wells abstract an estimated 205 km³ a⁻¹, and this volume continues to increase at 2–5 km³ a⁻¹ as agricultural production intensifies (MacDonald et al. 2016). Although unevenly distributed, the IGB aquifer system also supplies drinking water for rural and urban populations across the whole area. The IGB aquifer system is complex and heterogeneous, with large spatial differences in permeability, storage, recharge and water chemistry that can also vary with depth (Bonsor et al. 2017). The IGB has the largest surface water irrigation system in the world, constructed during the nineteenth and early twentieth centuries to redistribute water from the Indus and Ganges through a canal network that is greater than 100 000 km long (MacDonald et al. 2016).

Fig. 1.16 The location of the IGB alluvial aquifer system (MacDonald et al. 2016).

      (Source: Adapted from MacDonald, A.M., Bonsor, H.C., Ahmed, K.M. et al. (2016) Groundwater quality and depletion in the Indo‐Gangetic Basin mapped from in situ observations. Nature Geoscience 9, 762–766.)

MacDonald et al. (2016) found that the water table within the IGB aquifer system is typically shallow (<5 m below ground level) and relatively stable since at least 2000 throughout much of the basin, with some important exceptions. In areas of high groundwater abstraction in north‐west India and the Punjab in Pakistan (Regions 2 and 4, Fig. 1.17), the water table can be >20 m bgl (below ground level) and in some locations is falling at rates of >1 m a⁻¹. In areas of equivalent high irrigation abstraction within Bangladesh, the average water table remains shallow (<5 m bgl) due to greater direct recharge and a high capacity for induced recharge. Groundwater levels are deep and falling beneath many urban areas, and particularly in large groundwater‐dependent cities such as Lahore, Dhaka and Delhi (Chatterjee et al. 2009). Shallow and rising water tables are found in the Lower Indus, parts of the lower Bengal Basin and in places throughout the IGB aquifer system, as a consequence of leakage from canals, rivers and irrigation (MacDonald et al. 2016).

Fig. 1.17 Groundwater level variations in metres below ground level (m bgl) across the IGB alluvial aquifer system. (a) Location of analysis regions (divided by aquifer and climate): (1) Sindh; (2) middle Indus; (3, 4) upper Indus; (5) drier Uttar Pradesh; (6) wetter Uttar Pradesh; and (7) Lower Ganges and Bengal Basin. (b) Data from 3429 monitoring points showing mean water‐table depths in individual wells for the period 2000–2012. Areas with high abstraction and lower rainfall show the deepest groundwater levels and a wide range in measured groundwater levels (MacDonald et al. 2016).

      (Source: Adapted from MacDonald, A.M., Bonsor, H.C., Ahmed, K.M. et al. (2016) Groundwater quality and depletion in the Indo‐Gangetic Basin mapped from in situ observations. Nature Geoscience 9, 762–766.)

From mapping specific yield from lithological and hydrogeological data, MacDonald et al. (2016) estimated that the total volume in the top 200 m of the IGB aquifer system is 30 000 ± 14 000 km³, equivalent to 20–30 times the combined mean annual flow in the rivers within the basin (1000–1500 km³ a⁻¹). Estimated trends in groundwater storage for the IGB aquifer system, derived from in situ measurements of water‐table variations and estimates of specific yield derived across the basin, indicate a net average annual groundwater depletion within the period 2000–2012 of 8.0 km³ a⁻¹ (range 4.7–11.0 km³ a⁻¹) with significant variation across the basin. The largest depletion occurred in areas of high abstraction and consumptive use in northern India and Pakistan, including Punjab, 2.6 ± 0.9 km³ a⁻¹; Haryana, 1.4 ± 0.5 km³ a⁻¹; Uttar Pradesh, 1.2 ± 0.5 km³ a⁻¹; and Punjab Region, Pakistan, 2.1 ± 0.8 km³ a⁻¹. In the Lower Indus, within the Sindh, groundwater is accumulating at a rate of 0.3 ± 0.15 km³ a⁻¹, which has led to increased waterlogging of land and significant reduction in the outflow of the River Indus (Basharat et al. 2014). Across the rest of the IGB aquifer system, changes in groundwater storage are generally modest (±10 mm a⁻¹) (MacDonald et al. 2016).

      From an analysis of satellite and well data, Asoka et al. (2017) found that groundwater storage in northern India has declined at a rate of 2 cm a⁻¹ between 2002 and 2013 and that groundwater storage variability in north‐western India is explained predominantly by variability in abstraction for irrigation, which itself is influenced by changes in precipitation. Asoka et al. (2017) suggested that declining precipitation in northern India is linked to Indian Ocean warming, in turn influencing groundwater storage either directly by changing recharge or indirectly by changing abstraction.

      Based on national surveys on water quality, MacDonald et al. (2016) found that groundwater quality is highly variable and often stratified with depth. The two main water quality concerns are salinity and arsenic. Elevated arsenic is primarily a concern for drinking water, while salinity affects irrigation and also the acceptability of groundwater for drinking. Other pollutants are present and most areas are vulnerable to contamination from nitrate and faecal pathogens. Of the estimated 30 000 km³ of groundwater storage in the basin, 7000 ± 3000 km³ (23%) is estimated as having salinity greater than 1000 mg L⁻¹. A further 11 000 ± 5000 km³ (37%) of groundwater storage is affected by arsenic at toxic concentrations.

      The origin of the saline groundwater is complex due to a variety of natural processes: saline intrusion, historic marine transgression, dissolution of evaporite layers and excessive evaporation of surface water or shallow groundwater. Natural salinity is exacerbated by the long‐term impact of irrigation and shallow water tables. Only the lower Bengal Basin has been subject to Quaternary marine influence, together with the Pakistan coast. The widespread salinity in the Indus Basin and drier parts of the Upper Ganges is terrestrial in origin, formed by a combination of natural and anthropogenic activities (MacDonald et al. 2016).

      Arsenic‐rich groundwater occurs in chemically reducing, grey‐coloured, Holocene sediments, mostly restricted to groundwater in the uppermost 100 m across the floodplains in the southern Bengal Basin, where arsenic is commonly present at >100 μg L⁻¹ (Fendorf et al. 2010). Less extreme arsenic concentrations, though still >10 μg L⁻¹ (the World Health Organization (1994) recommended limit), occur in other parts of the IGB aquifer system, including Assam; southern Nepal; the Sylhet trough in eastern Bangladesh; and within Holocene sediments along the course of the Ganges and Indus river systems (MacDonald et al. 2016). Intensive abstraction of shallow groundwater can flush aqueous arsenic from the aquifer (Shamsudduha et al. 2011), but there is concern that high‐capacity deep pumping may draw arsenic down to levels in the Bengal aquifer system which are otherwise of good quality, although retardation is expected to delay vertical migration by centuries (Radloff et al. 2011). Age‐depth profiles and hydrochemical data from monitoring wells in the coastal Bengal Basin aquifer system demonstrate the regional resilience of deep groundwater (>100 m) to the ingress of shallow, contaminated groundwater due to the high regional anisotropy of aquifer properties (Lapworth et al. 2018).

From their study, MacDonald et al. (2016) concluded that the complex and dynamic nature of the IGB aquifer system highlights the fundamental importance of regular and spatially distributed measurements of groundwater levels