Global state and trends of freshwater

9.4.2 Water withdrawals

Box 9.2: Water quality impacts of mining

Modern mining generates large volumes of tailings (finely ground rock remaining after extracting ore) and waste rock (non-mineralized rock; low-grade ore), often containing iron sulphide minerals (e.g. pyrite). Exposed to the surface environment, these can react with water and oxygen to form sulphuric acid, producing acid metalliferous drainage (AMD). AMD can degrade water quality and impact aquatic biodiversity. Recent tailings dam failures (e.g. Mount Polley, Canada; Samarco, Brazil) demonstrate that mine wastes escaping into the environment can also significantly impact aquatic ecosystems and biodiversity, with tailings particles smothering riverbeds, reducing light penetration and oxygen levels, and affecting river geomorphology (Mudd et al. 2013).

Human and environmental water demands vary spatially and culturally across rural and urban areas. While an average of 70 per cent of water withdrawals worldwide are for the agricultural sector, this varies widely across regions and countries (Hoekstra and Mekonnen 2012, p. 3232; Food and Agriculture Organization of the United Nations [FAO] 2016; UN-Water 2017). South-East Asia uses more than 80 per cent of its available freshwater for agriculture (FAO 2016).

The North American region has the highest per capita freshwater use (Hoekstra and Mekonnen 2012, p. 3232; UNEP 2016a, p. 71), although increased water-use efficiency is helping to lower demand, despite population and economic growth (UNEP 2016a, p. 71). Water withdrawals by all sectors in the United States of America (Figure 9.3) illustrate high water usage for cooling in electricity production.

Groundwater is increasingly important globally, representing estimated withdrawals of about 982 km3 (Margat and van der Gun 2013), equivalent to nearly 33 per cent of total water withdrawals (Seibert et al. 2010, p. 1863; Famiglietti 2014, p. 945). Since conventional groundwater withdrawal technology is easily accessible to landowners, extraction is highly decentralized. Groundwater in confined artesian basins (Bundesanstalt für Geowissenschaften und Rohstoffe [BGR] 2008) can be accessed at depths of up to 2 km, and often provides a strategic water resource, especially during droughts (e.g. Great Artesian Basin, Australia [GABCC] 2016); Table Mountain Group, South Africa) (Hay and Hartnady et al. 2001; Weaver et al. 2002; Blake et al. 2010).

Figure 9.3: United States water withdrawals from all sources (1950-2010)
Note: 1 billion gallons = 3.8 million m3. Source: Maupin et al. (2014, p. 46).

Industries that abstract from aquifers include industrial agriculture, mining, geothermal energy and ground-source heat pumps, disposal and/or storage of hazardous wastes (e.g. landfills, nuclear waste), fluid injection (e.g. oil and gas extraction through hydraulic fracturing or ‘fracking’ and associated wastewater reinjection), and underground construction activities. Such pressures are leading inexorably to stronger competition/interactions between the different industries, with sometimes unforeseen consequences.

Groundwater use has plateaued in some regions but is increasing elsewhere (Figure 9.5), such as in Asia and the Pacific and West Asia (e.g. about two-thirds of freshwater utilized in West Asia). About 75 per cent of European Union (EU) inhabitants rely on groundwater for drinking (European Commission 2008, p. 7), and groundwater use, compared with surface water, has increased substantially to 1.3 trillion m3 per year across North America (Famiglietti and Rodell 2013, p. 1301). Groundwater accounts for 30 per cent of water withdrawals in Latin America (Campuzano et al. 2014, p. 38) and an estimated 75 per cent of the African population depends on it (Altchenko and Villholth 2013, p. 1498). It must be noted, however, that estimates of groundwater withdrawals and use vary widely, constituting a critical data gap.

Figure 9.5: Global trends in increasing groundwater use
Source: Shah (2014, p. 12).

Increased agricultural groundwater use has led to rising depletion rates in major aquifers in arid and semi-arid zones (UNEP 2012b). Pumping rates that for decades have exceeded long-term natural recharge have resulted in some larger aquifers being ‘mined’ unsustainably (Famiglietti 2014, p. 946). Five of the world’s seven largest aquifers are in Asia and the Pacific, and are overstressed (UNEP 2016b, p. 84).

Excessive groundwater abstraction has caused land subsidence in some coastal cities (e.g. Bangkok; Ho Chi Minh City; Jakarta; Manila) (UNEP 2016b, p. 87). Overexploitation of an aquifer can also impact wetland ecosystems. Hydraulic fracturing (fracking) for oil and gas extraction merits concern for its groundwater impacts (see Box 9.2). Groundwater is often underexplored on some islands due to surface-water availability, while other islands can be wholly reliant on it. Climate change impacts may lead to a greater reliance on and pose a threat to ground water because of sea level rise. Further studies are needed since islands are experiencing increasing freshwater shortages (Famiglietti 2014, p. 946).

Figure 9.6: Examples of surface streams affected by acid and metalliferous drainage (AMD) and/or tailings discharges: (left) Urban stream severely affected by AMD in western Witwatersrand Basin, Johannesburg, South Africa; (right) Tailings sediment from Samarco Dam