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close this bookManaging Water Resources for Large Cities and Towns (HABITAT, 1996, 398 p.)
close this folderIII. CASE STUDIES
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View the documentGroundwater Resources Beneath Rapidly Urbanizing Cities - Implications and Priorities for Water Supply Management

Groundwater Resources Beneath Rapidly Urbanizing Cities - Implications and Priorities for Water Supply Management

S.S.D. Foster, A.R. LAWRENCE & B.L. Morris, British Geological Survey (Groundwater & Geotechnical Surveys Division), Nottingham & Wallingford, UK

Abstract

Urban population growth is occurring on an unprecedented scale, such that close to half of the 80 % of the world's population who live in developing countries will be urban dwellers by the year 2000. Many of the cities are sited on unconfined or semi-confined alluvial aquifers which possess abundant, but fragile, groundwater resources. It has become increasingly evident that inadequately-controlled groundwater exploitation and indiscriminate liquid effluent and solid waste disposal to the ground widely result in significant groundwater degradation, both within the urban area itself and downstream. This degradation is a contributory cause of escalating water-supply cost, increasing water resource scarcity and growing health hazard. The importance of urban and periurban groundwater resources to the successful development of many cities is such that proactive, rather than passive, management of groundwater resources, based on systematically-identified priorities and simple pragmatic criteria is needed to avoid premature loss of major investment in groundwater development.

Impacts of Urbanization on Groundwater

Changes in the Hydrological Cycle as a result of urbanization process

The three key services of water-supply, sanitation and drainage are crucial to the urbanization process. Substantial differences in development sequence exist between higher-income areas, where the process is normally planned in advance, and lower-income areas, where informal settlements are progressively consolidated into urban areas and may lack adequate provision of one or more of these three services. However, common factors include impermeabilization of a significant proportion of the land surface and major importation of water from outside the urban limits. Sanitation and drainage arrangements also have a fundamental effect on the urban hydrological cycle. They generally evolve with time and vary with differing patterns of urban development, but mains sewerage construction generally lags considerably behind population growth and water-supply provision.

Urbanization causes radical changes in the frequency and rate of subsurface infiltration with a general tendency to significantly increase volume and for quality to deteriorate substantially [1]. These changes cannot be measured directly, and are thus difficult to quantify, but in turn influence groundwater levels and flow regimes in underlying aquifers. Subsequently groundwater quality degradation occurs, both within the urban area itself and in downstream alluvial aquifers.

Groundwater Quality Deterioration within the City

Some urbanization processes cause radical changes in the quality of subsurface infiltration. This is widely the cause of marked, but essentially diffuse, pollution of groundwater by nitrogen compounds, increasing salinity and elevated dissolved organic carbon concentrations. The oxidation of the high organic load can lead to enhanced mobilisation of iron and/or manganese as reducing conditions develop. In very shallow aquifers, especially those where flow through fissures is important, contamination by faecal pathogens can occur. The intensity of impact varies widely with the pollution vulnerability of underlying aquifers and with the type and stage of urban development. In alluvial formations the uppermost unit is vulnerable to pollution from human activities at the land surface, given its shallow water-table, and this may have an effect on deeper (less vulnerable) aquifers, the pumping of which provides the hydraulic head differences which can induce downward leakage of pollutants across intervening lower permeability layers.

Rapid urbanization and industrialization with indiscriminate use of the ground for liquid effluent and solid waste disposal present a complex array of activities which have the potential to pollute groundwater. In many districts without mains sewerage, a heavy subsoil contaminant load originates from in-situ sanitation and the disposal of sullage waters increases the risk of shallow groundwater contamination, because of the presence of various household chemicals. In addition to elevated nitrogen concentrations, increased concentrations of chloride (partly from excreta), sulphate, and bicarbonate (from oxidation of organic matter) are frequently observed. A further, increasingly-frequent, cause of shallow groundwater contamination in residential areas of developing cities is hydrocarbon fuel leakage from underground storage tanks at service stations.

In many rapidly-developing cities, burgeoning industries (such as textile mills, tanneries, metal processing, vehicle maintenance, laundry and dry cleaning establishments, printing and photoprocessing, etc) are located in extensive fringe urban areas which lack mains sewerage. Most of these industries generate liquid effluents, such as spent lubricants, solvents and disinfectants, which are often discharged directly to the ground and whose slow rates of degradation can represent a long-term threat to groundwater quality.

Santa Cruz, Bolivia, is a low-rise, relatively low-density, fast-growing city, whose municipal water-supply is derived entirely from wellfields within the city limits, extracting from a semi-unconfined, outwash-plain, alluvial aquifer. The city has a relatively high coverage of mains water-supply, but until recently only the older central area had mains sewerage, and most domestic/industrial effluents and stormwater drainage were disposed to the ground. The uppermost aquifer unit thus shows substantial deterioration in groundwater quality (Fig 1) down to depths of about 40 m [2]. Groundwater abstraction from the deep alluvial aquifer has induced downward movement from the shallow horizons and a component of contaminated water is now observed at depths in places approaching 90 m. The heavy development of the shallow aquifer for private water-supplies, however, effectively provides a degree of protection for deeper municipal wellfields by intercepting, abstracting and recycling part of the polluted water, which is fortuitously a good management practice provided that none of the supplies provided by these wells are destined for sensitive use.

In some cities located on low-lying coastal alluvium, direct disposal of wastewater to the ground via on-site sanitation is not possible and effluents are discharged into rivers and canals, which can become influent to aquifers as a result of groundwater abstraction. Hat Yai, Thailand, is an example of this condition [2]. Most of its limited mains water-supply is imported from external surface water sources, but as private sector abstraction is also important, about 60% of the overall supply is provided from local groundwater resources. The disposal of domestic and industrial effluents to the ground by on-site sanitation systems is not always possible because of the shallow water-table and such wastes are discharged into rivers and canals. The heavy groundwater abstraction results in induced leakage to the shallow aquifer. This is detectable most readily by high ammonium concentrations (Fig 2), reflecting the low redox potential of the aquifer system. With increasingly heavy groundwater development it is believed that induced canal seepage is now a major component of groundwater recharge in the central part of the city.


FIGURE 1: Shallow groundwater pollution caused by rapid urbanization with induced downward leakage to deep aquifers in Santa Cruz, Bolivia

Downstream Riparian Effects

Although the provision of mains sewerage lags considerably behind population growth and water-supply provision, sewage effluent (termed here wastewater) is generated in large volumes by the majority of, but not all, rapidly-developing cities. This wastewater is normally discharged to surface watercourses after minimal treatment from where, especially in more arid climates, it is used on an uncontrolled basis for agricultural irrigation in downstream riparian areas. Such areas may be underlain by important alluvial aquifers [3] and examples of this situation include many cities in northern and central Mexico and northeastern China.

The city of Leon in Guanajuato State, Mexico is situated in a wide intermontane semi-arid valley with a complex partially-confined aquifer, heavily exploited for municipal water-supply by several wellfields. It is among the fastest growing cities in Mexico, and one of the most prominent leather processing and shoe manufacturing centres in Latin America. Leon is extensively, although not comprehensively sewered, and produces some 250 Ml/d of sewage effluent, which is used for agricultural irrigation of an area immediately southwest of the city. The continuous infiltration resulting from low-efficiency agricultural irrigation with wastewater is sufficient to cause the formation of a major groundwater mound above the piezometric surface of the regional aquifer, which is depressed elsewhere some 40-90 mbgl as a result of heavy pumping.


FIGURE 2: Impact of urban development on the Hat Yai, Thailand, coastal alluvial aquifer

The impact on groundwater quality is marked [4], with deep municipal boreholes in the wastewater irrigation area being threatened by increasing salinity due to the downward movement of a chloride front (Fig 3). Wastewater in the main sewerage collectors from industrial areas contains 500-600 mg/l chloride, 50-70 mg/l nitrogen, 15-40 mg/l chromium and a very heavy organic load, but relatively little nitrogen is oxidised and leached to shallow groundwater (< 12 mg as nitrate) and almost all chromium (which is not deposited in streambed or irrigation reservoir sediments) accumulates in the soil with concentrations in the top 30 cm commonly in the range 50-250 mg/kg.

Many of the large number of cities of the alluvial plains of northeastern China are highly dependent on groundwater for their municipal water-supply, and downstream riparian wellfields are very commonly developed. The city of Shenyang obtains almost 1000 Ml/d (70% of its total supply) from downstream riparian wellfields along the Hunhe River. Various boreholes are said to have experienced quality degradation due to rising ammonium or nitrate concentrations and traces of soluble oils and phenols. This is believed to be a consequence of infiltration of river water heavily polluted by urban wastewater, either directly through induced streambed recharge or indirectly by heavy rates of irrigation of agricultural land from river water.


FIGURE 3: Impact of wastewater irrigation downstream of Leon (Guanajuato), Mexico (vertical exaggeration × 30 approx)

Consequences of Uncontrolled Aquifer Exploitation

Groundwater quality issues cannot be divorced from those of resource development. The most common quality impact of inadequately-controlled aquifer exploitation, particularly in coastal situations, is the intrusion of saline water. For thin alluvial aquifers this takes the classical wedge-shaped form, but in thicker multiaquifer sequences salinity inversions often occur with intrusion of modern seawater (or retention of palaeo-saline water), due to pumping of near-surface aquifer horizons, and with fresh groundwater below.

Contamination of deeper (semi-confined) aquifers, where they underlie a shallow poor-quality phreatic aquifer affected by anthropogenic pollution and/or saline intrusion, is a frequent consequence of uncontrolled exploitation. This occurs as a result of inadequate well construction (leading to direct leakage down wells which accidentally link one or more aquifers and act as vertical conduits) and/or pump-induced vertical leakage. Such a mechanism can allow penetration of more mobile and persistent contaminant species (Fig 4). Evidence has been accumulating since the 1980s of widespread drawdown of the piezometric surface by 20-50 m or more in various Asian megacities, as a result of heavy exploitation of alluvial aquifers, and both of the aforementioned side-effects are quite widely observed.


FIGURE 4: Evolution of groundwater quality problems in a typical coastal alluvial aquifer system of the humid tropics following rapid urbanization

A recent Asian Development Bank technical cooperation programme on water resources management in megacities included case histories for 4 Asian cities in the humid tropics, each possessing major alluvial groundwater resources. The results of these studies have been reviewed, and amplified by further direct data collection and other references [5][6][7][8], with the aim of drawing generic conclusions [9]. Among the cities surveyed, groundwater remains the major component of municipal (public) water-supply only in Dhaka, Bangladesh (Table 2), having been substituted in other cases by long-distance imports of surface water. This was often due to quality deterioration through saline intrusion and/or anthropogenic pollution, but sometimes the result of reduction of individual borehole yields, from falling water-levels or poor well construction and maintenance.

The situation is not as simple as it might at first appear, however, since in the other cases (Bangkok, Jakarta and Manila) resultant shortage and increasing cost of water-supplies imported from outside the city led to a major growth in private well drilling (Table 2). As a result, the overall exploitation of groundwater increased, despite attempts to initiate control, as fears increased about further saline intrusion and/or land subsidence. There is little point in controlling municipal abstraction if private groundwater exploitation is not similarly managed. In effect, what has occurred in Bangkok, Jakarta and Manila is the replacement of a moderate number of municipal groundwater supplies, which were at least capable of being systematically controlled, monitored, protected and treated, by a very large number of shallower, largely uncontrolled, unmonitored and untreated sources.

Tapping groundwater at location of demand makes sense for many industrial users and for amenity irrigation, since both uses may demand large volumes and unit supply cost is important. However, it is questionable in densely-populated areas, both on economic and on public health grounds, for domestic supplies and for sensitive

TABLE 2: Recent water-supply statistics for selected Asian megacities located on coastal alluvial aquifers


POPLN (M)

POPLN DENSITY (cap/ha)

WATER-SUPPLY (Ml/d)

GROUNDWATER ABSTRACTION (Ml/d)

CITY



public
(propn served)

total

public (propn)

private

Bangkok (Thailand)

5.9

40

3220 (70%)

3840

190 (6%)

620

Dhaka (Bangladesh)

6.1

380

670 (60%)

900

670 (100%)

230

Jakarta (Indonesia)

8.2

130

1200 (45%)

1820

60 (5%)

620

Manila (Philippines)

8.3

130

2280 (75%)

3120

90 (3%)

840

Industries (such as food and beverage preparation). An added concern is illegal connection of private wells to the mains water-supply by users in order to even out fluctuations in supply, usually without measures to prevent "back syphoning" at times of reduced mains pressure. Contamination of "down-system" supplies can result.

Implications for Groundwater Management

Rapid urbanization has been shown to have a profound effect on groundwater recharge, flow and quality. The scale of implications for the security and safety of developing city water-supplies is considerable. The paper has selected examples from tropical alluvial aquifers in view of their importance internationally, and brevity prevented presentation of a wider spectrum of hydrogeological conditions.

While, in many instances, institutional and regulatory arrangements will need strengthening to implement management strategies, such strategies must also be soundly based on hydrogeological criteria if they are to achieve more sustainable exploitation of groundwater resources. Some ways in which hydrogeological considerations can be meshed into a groundwater resource management programmes are suggested:

Aquifer Pollution Control

Given typical socioeconomic and hydrogeological conditions, it is not realistic to attempt to protect shallow alluvial aquifers from some quality deterioration during the urbanization process. However, it will be prudent to control those activities which most threaten groundwater quality overall, especially that in deeper (less vulnerable) aquifers.

This will be best achieved through the following strategy:

(a) undertake a rapid survey of subsoil contaminant loading [10] to identify those activities likely to pose the greatest threat to groundwater through mode or intensity of discharge and presence of persistent and/or toxic chemicals;

(b) establish the degree of existing deterioration of the uppermost aquifer unit from anthropogenic pollution by a sampling survey of representative shallow wells, with analysis for appropriate indicator determinants;

(c) introduce selective controls on subsoil contaminant loading (where demonstrated necessary) through extension of mains sewerage and treatment, incentives for improved handling/control of industrial chemical effluents, or more efficient use and reuse of process chemicals/effluent.

Resource Management Criteria

The use of urban groundwater also needs to be directed and rationalized, taking account of the quality distributions and trends identified. A better balance needs to be achieved between shallow and deep groundwater abstraction, for example by directing non-sensitive water users towards exploitation of groundwater of inferior quality so as to minimise the downward migration of high concentrations of anthropogenic contaminants. As non-sensitive uses are generally of lower economic cost, there may be hidden financial advantages in such regulation, as the extraction will normally be from shallower aquifers usually with lower associated pumping costs. Municipal groundwater development strategies need to be harmonized with private groundwater use patterns and with wastewater disposal and/or reuse strategies.

The hydrogeological complexity of tropical alluvial aquifers means that groundwater recharge evaluation is always subject to considerable uncertainty, and rarely forms a sound basis for resource management. Other simple criteria are needed and combinations of the following approaches, appropriately adapted to local hydrogeological conditions, are likely to prove more useful:

(a) determine the extent of vertical layering of the aquifer system, and associated hydraulic head and groundwater quality variations, by selective field data collection,

(b) appraise the susceptibility of the system to saline intrusion and/or land subsidence following major depression of the piezometric surface, in a qualitative sense on simple geohydrological criteria [II],

(c) use target piezometric levels as the resource management criterion, since this is more likely to maximise groundwater production while minimising irreversible side-effects which threaten sustainability,

(d) spread public water-supply abstraction geographically to avoid large cones of piezometric depression in the deeper semi-confined aquifers, especially in areas susceptible to saline intrusion and/or land subsidence,

(e) in situations where significant encroachment (or intrusion) of saline water has already occurred, avoid abandonment of pumping from salinized wells and try to encourage continued abstraction at reduced rates for appropriate uses to reduce landward hydraulic gradients; assess risk of upcoming using hydrogeological techniques and if necessary reduce unit abstraction.

(f) avoid generation of large downward hydraulic gradients by balancing the abstraction between shallow and deep aquifers, by encouraging non-sensitive users (industrial process and cooling water, amenity irrigation, etc) to drill shallow wells and by reserving deeper aquifers for potable supplies and sensitive industrial uses; this may be achieved by direct licensing controls or differential abstraction tariffs.

Acknowledgements

This paper is published by permission of the Director of the British Geological Survey (BGS). Its aim is to highlight key messages for the sustainable development of groundwater resources arising from recent surveys of a number of developing cities, which were funded by the (British) Overseas Development Administration, the World Health Organisation, the United Nations Environment Programme and the European Community. The major contribution of numerous national groundwater professionals and certain BGS colleagues, especially John Chilton, to these surveys is fully appreciated and gratefully acknowledged.

References

1. Foster S S D, Morris B L, Lawrence A R (1994) Effects of urbanization on groundwater recharge. Proc ICE Intl Conf "Groundwater Problems in Urban Areas" (London - June 1993): 43-63.

2. Lawrence A R, Morris B L & Foster S S D (1996) Groundwater recharge - changes imposed by rapid urbanization. Quart J Eng Geol: in press.

3. Foster S S D, Gale I N & Hespanhol I (1994) Impacts of wastewater use and disposal on groundwater. BGS Report WD/94/55.

4. British Geological Survey, National Water Commission of Mexico, Autonomous University of Chihuahua, Municipal Water Authority of Le1996. Effects of Wastewater Reuse on Urban Groundwater Resources of LeMexico. Final Report-Feb 1996. BGS Technical Report WD/95/64 Keyworth.

5. Ahmed K M, Woobaidullah A S M & Hasan M A (1995) Hydrogeology of the Dupi Tila Aquifer of Dhaka City, Bangladesh. Acta Univ Carolina Geol 39: 113-121.

6. Munasinghe M (1990) Managing water resources to avoid environmental degradation: policy analysis and application. World Bank Environ Working Paper 41.

7. Ramnarong V & Buapeng S (1991) Mitigation of groundwater crisis and land subsidence in Bangkok. J Thai Geosci 2: 125-137.

8. Schmidt G, Soefner B & Soekardi P (1990) Possibilities for groundwater development for the city of Jakarta, Indonesia. IAHS Publn 198: 233-242.

9. Foster S S D & Lawrence A R (1996) Groundwater quality in Asia: an overview of trends and concerns. UN-ESCAP Water Res J: in press.

10. Foster S S D & Hirata R C A (1988/1991) Groundwater pollution risk assessment: a methodology using available data (also in Spanish & Portuguese). WHO-PAHO-CEPIS Publication: 79 pp.

11. Foster S S D (1992) Unsustainable development and irrational exploitation of groundwater resources in developing nations - an overview. IAH Hydrogeology Selected paper 3: 321-336.