Dry or drowning?
asks whether it is within the capability
of humankind to ensure sustainable
urban water supplies
By the year 2000, some 23 cities - 18 of them in the developing world - will have populations exceeding 10 million. On a global scale, half of the world's people will live in urban areas. Many of these urban dwellers will depend on groundwater for their water supply. This year we urgently need to consider whether groundwater resources in cities are sustainable.
Cities used to be centres of plague and illness. During
the past 150 years urban sanitary engineering and medical epidemiology have promoted rapid improvements to human health in the cities of the industrial world. A celebrated example was the pioneering work of Dr. John Snow who, in the mid-19th century, traced the source of the London cholera epidemic to a public water pump in Broad Street.
Most cities evolved from small settlements, and the availability of a suitable water supply was often the primary factor in their location. Often, though, these original water sources quickly became inadequate in quality or quantity, and sometimes are now completely forgotten. New sources and larger quantities of water were required. Groundwater may have had to be drawn from deep aquifers, even from beyond the city boundaries. Today, groundwater plays a critical but complex (and often largely unrecognized) role
in the urban environment.
Underground aquifers are major sources of municipal and industrial water supply. Some of our largest cities (Beijing, Buenos Aires, Dhaka, Lima, Mexico City) depend heavily on them. Some of the fastest-growing cities are completely dependent on groundwater. Groundwater from aquifers beneath or close to Mexico City, for example, provide it with more than 3.2 billion litres per day.
But, as groundwater pumping increases to meet water demand, it can exceed the aquifers' rates of replenishment, and in many urban aquifers water levels show long-term decline. With excessive extraction comes a variety of other undesirable effects:
Severe depletion of groundwater resources is often compounded by a serious deterioration in its quality.
- Increased pumping costs.
- Changes in hydraulic pressure and underground flow directions (in coastal areas this can result in intrusion
- Saline water drawn up from deeper geological formations.
- Poor-quality water from polluted shallow aquifers leaking downwards.
Problems of land subsidence can also follow rapid depletion of aquifers. This can affect both upland and lowland cities. Mexico City (about 2,000 metres above sea level) and Bangkok (at sea level), for example, are both suffering severe groundwater-induced subsidence, resulting in millions of dollars worth of damage.
Land subsidence not only damages individual buildings and roads, but also underground piped services, further increasing water depletion and contamination. Leaks from water mains and sewers, and ruptured oil pipelines and underground tanks can add to shortages and soil and groundwater pollution.
Where a town or city's water demand has increased rapidly and aquifers have become depleted or polluted, groundwater must be brought from further away, often from peri-urban areas. The development of water sources from the areas around cities and towns can cause different problems by creating competition for resources between the water-hungry municipality and the surrounding farming areas. The latter may be long-established and highly productive agricultural zones which supply the urban centre with food. Both user communities may have strong claims and may be powerful and influential in pressing them. Such conflicts can be difficult to resolve.
Another direct impact of urban activities on groundwater quality arises from waste disposal. The development of waterborne sewerage systems has enabled domestic and industrial effluents to be more easily collected, and
sewerage has become an efficient and unobtrusive feature of urban infrastructure in most developed cities. But elsewhere, in parts of major cities in the developing world for example, urban wastewater is sometimes discharged untreated, or only partially treated, into surface water courses. These water courses become little more than conduits for taking effluents away from the centres. Meanwhile, downstream, particularly in arid/semi-arid settings, these wastewater-dominated surface flows may be used for irrigation.
In many cities, substantial areas remain without water-borne sewerage, and domestic and industrial effluents may be directly disposed of into the ground, through septic tank systems or disposal wells. The resulting deterioration in
the groundwater quality, in terms of pathogens, nutrients, industrial chemicals and salinity, may pose severe risks to health, or even render the water unusable as a potable supply.
Large-scale urban development can greatly modify the hydrological processes and pathways by which water reaches underlying aquifers. Even though covering the land surface with buildings, roads, car parks and other facilities would intuitively seem to reduce the amount of rain infiltration, the collected runoff from roofs and gutters can, for example, still find its way into the aquifer either directly via on-site soakaways or indirectly via storm-drains discharging into unlined canals or river beds. But it does not solve the problem of contamination. Water infiltration from paved surfaces can carry pollutants associated with cars, de-icing chemicals, pesticides and bacteria.
All water distribution systems leak to some extent; water losses ranging from 25 to 40 per cent are not uncommon. Much of it leaks down to underlying groundwater aquifers, and the volumes can be substantial. As an extreme example, in Peru water lost from Lima's distribution system in one district provides up to an equivalent of 360 millimetres per year of aquifer recharge water, compared to the natural level of 20 millimetres per year or less characteristic of an arid region. Although this water would generally be good quality, where sewer systems are also leaking, the recharge water may be of poor chemical and bacteriological quality.
Rising water tables
The sources and processes of aquifer recharge in urban areas, however, can be greatly modified and the resulting extra infiltration can help to offset the effects of increased abstraction. In an extreme case in the Arabian Gulf region, recharge from imported water is so much greater than the negligible natural replenishment that it exceeds the capacity of the aquifer. As a result, water levels have risen to the surface, causing flooding of basements.
In some developed world cities, the combination of increased recharge and reduced groundwater withdrawal (because of declining or relocated industries) has resulted in a local recovery of groundwater levels. Aquifers that had been de-watered over decades are now fully recharged. In fact, such cities as Birmingham, London, and Tokyo are starting to encounter problems of rising water tables affecting road tunnels, foundations, car parks and metro systems constructed before the ground became saturated.
Urban development has a significant impact on groundwater, affecting quantity, flow and quality. Changes in groundwater levels, both reductions and increases, can also pose engineering problems for infrastructure and facilities. In fast-growing cities urban groundwater can be managed sustainably, but only if the sometimes conflicting needs and priorities of water supply, waste disposal and underground engineering can be reconciled. This goal seems within human capabilities.
John Chilton, Hydrogeology Group, British Geological Society, London, United Kingdom.