International Symposium on Groundwater Sustainability (ISGWAS)

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Urban Hydrogeology in Developing Countries: A Foreseeable Crisis

Hirata, Ricardo 1; Stimson, Jesse 2 and Varnier Claudia 1,3

1Institute of Geosciences – University of São Paulo, Brazil2University of Waterloo, Waterloo, Ontario, Canada

3Geological Institute – Environment Secretariat of the State of São Paulo, Brazil

ABSTRACT

In many cities in the developing world, groundwater is an important source of public water supply. It isestimated that some 1.5 to 2.8 billion people worldwide are supplied by groundwater and twelve of 23megacities (cities with more than 10 million inhabitants) utilize groundwater as an important componentof their water supply system. The groundwater under many urban areas, which have high populationdensities, elevated groundwater exploitation, and many potential sources of contamination to thesubsurface, is extremely susceptible to overexploitation, subsidence and degradation. Peripheral, poorerareas of many cities, that do not receive water supply or sanitation services and use shallow wells andsprings as their only source of potable water, are also poorly-protected against contamination. Leaks fromwater supply distribution and sewer networks, along with discharge from septic systems and cesspools,can also lead to groundwater level increases, that flood and cause geotechnical damages to foundationsand underground structures. The sustainable utilization of groundwater requires that a sensiblemanagement program be conducted by a regulatory agency, that consults the affected communities.

1. INTRODUCTION

Urbanization has increasingly intensified in the last few decades and now some 48% of the world’spopulation lives in urban centres. By 2005, approximately 50% of the population in developing countrieswill live in cities (International Year of Freshwater, 2003), with a significant portion of these inhabitantsresiding in megacities (cities with populations of 10 million or more). The high population growth observedin developing countries has alerted multilateral organizations and governments that in the near futurethese inhabitants will require a much larger water supply and sanitation capacity, which will be difficult tomeet given the limited financial resource that developing countries possess. Given that groundwater is animportant source of water supply and susceptible to urban contamination, groundwater resources cannotbe ignored in water resource management of cities. By the year 2000, 1.5 to 2.8 billion people will beserved by groundwater as their source of potable water. Twelve of the 23 megacities in the world aredependent on groundwater for more than 25% of their water supply, including Mexico City (25.8 million),

Calcutta (16.5 million), Tehran (13.6 million), Shanghai (13.3 million), Buenos Aires (13.2 million), andJakarta (13.2 million). In China, 500 medium-to-large cities are supplied by aquifer pumping for more than60% of their water supply (Chéné, 1996) and a third of all major Russian cities are dependent ongroundwater (UNEP, 2003). Similar proportions of groundwater use are observed for cities in Latin Americaand Africa, although reliable data for water supply sources in Africa is lacking.

Even though the importance of groundwater is evident, little attention has been given to protecting thisimportant resource. Few groundwater protection and management programs are today in force indeveloping countries, and these programs often do not consider aquifer vulnerability to anthropogeniccontamination and the need to define wellhead protection areas. These programs often do not includemanagement of groundwater extraction, in order to avoid aquifer overexploitation, even when excessivegroundwater extraction has reduced baseflow to rivers and wetlands, caused subsidence, or inducedsaltwater intrusion. The lack of groundwater resource management has caused serious social problems forpoorer, periurban areas. In these peripheral zones, water supply and sanitation does not reach allinhabitants. These communities often use poorly-constructed excavated wells or springs from shallowaquifers for their water supply, which are located nearby and downgradient to cesspools and latrines(Limaye et al., 2000).

This paper examines the principal problems caused by improper or nonexistent groundwater managementto the sustainability of water supply for cities dependent on groundwater in developing countries. Specialattention is given to the inadequate control of groundwater exploitation, subsidence and salt waterintrusion, rising groundwater levels from urban water sources, and the potential contamination of theaquifer.

2. NEGATIVE IMPACTS OF ALTERING THE HYDROLOGIC CYCLE IN URBAN AREAS

Urbanization changes drastically the hydrologic cycle of natural areas. Buildings, roads, sidewalks, andparking lots make the land surface impermeable to the infiltration of precipitation, diminishing the naturalrecharge of the aquifer.

A residencial area of moderate population density over a sedimentary watertable aquifer in São Paulo,Brasil (Hirata et al., 2002) is estimated to cause an 82% reduction in aquifer recharge. Stephensen (1994)has shown that decreased infiltration results in increased runoff. In his study of two watersheds inJohannesburg, South Africa, he showed that runoff, as a percentage of total precipitation, is far moresignificant for an urbanized catchment (15%) than for an undeveloped catchment (4%).

Although natural infiltration decreases, artificial recharge from inadvertent leaks and discharges fromwater sources, such as water supply distribution and sewer systems, septic systems and cesspools, andirrigation of parks, augments aquifer recharge. In S„o Paulo, Brazil, water losses from impermeabilizationof the land surface are compensated by this artificial recharge. Total recharge to the aquifer has decreasedlittle from predevelopment conditions (~355 mm a-1) to today (308 mm a-1), and losses from the watersupply and sewer network is approximately 241 mm a-1. Similar figures have been determined for otherurban areas around the world (Figure 1). In semiarid climates, this effect is more noticeable; in Lima, Peru,

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Urban Hydrogeology in Developing Countries: A Foreseeable Crisis

loss from irrigation, canals and the water supply distribution system is 66% greater than from naturalrecharge (Lerner et al., 1982).

The overexploitation of groundwater is one of the most pressing problems for sustainability of urban waterresources. Many cases of uncontrolled exploitation, resulting in large drawdowns of the aquifer waterlevels, are reported throughout the world. Overexploitation can be defined as excessive drawdown which1.) significantly increases the cost of drilling and pumping wells, 2.) necessitates the deepening of exisitingwells, 3.) results in significant loss of saturated thickness (and production capactiy), 4.) results in poorquality groundwater being displaced to shallower depths of the aquifer, and 5.) results in significantreduction of baseflow to rivers, lakes and wetlands. These problems occur due to the imbalance between

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Hirata, Ricardo; Stimson, Jesse and Varnier, Claudia

Figure 1. Potential range of subsurface infiltration caused by urbanization (Foster et al.,1993).

recharge and extraction, and is accentuated by strong hydraulic interference that occurs between largewells operating in an area of dense water captation. This problem has been described for many cities,including Bangkok (Ramnarong, 1999), Calcutta (Choudhury et al., 1997), Jakarta (Schmidt et al., 1988),Xian, China (Lee et al., 1996), Agra, India (Mukherjee and Raj, 1997) and Salta, Argentina (Bundschuh,1998), to mention a few. In these localities, drawdown of water levels exceeding 30 m over a period of afew decades of intense exploitation. Another unfortunate outcome of overexploitation in coastal areas isthe inducement of saltwater intrusion. Unfortunately, to date, no integral method to evaluate the socialcosts of overexploitation exists.

The fact that groundwater levels are lowered due to pumping does not necessarily represent a case ofoverexploitation. If drawdown does not cause a loss in the value to the groundwater resource, negativeimpacts to riparian or wetland ecology, and subsidence, groundwater exploitation in of itself does notconstitute a resource crisis. In many urban areas of the world, the lack of proper conceptual models ofaquifer water balance, hydraulic interference between wells, and realistic costing of public waterextraction, results in the erroneous perception that aquifer overexploitation is occurring. The groundwaterresource manager ignores the fact that the very same aquifer exploitation leads to recharge through watersupply and sewage losses, which conteract water level declines.

A very serious outcome of excessive drawdown is the occurrence of subsidence of land surface, which hasvery serious and irreversible consequences (Holzer and Johnson, 1985). The dewatering of thickunconsolidated sedimentary basins, that are rich in clays, associated principally intermontaine valleys and,in some cases, large coastal alluvial formations and glacial deposits, have provoked subsidence thatreaches values greater than 10 m in some cases. The subsidence observed in Mexico City is one of the moststudied cases, with measured subsidence reaching more than 9 meters in some areas of the city.Subsidence has also been reported in Bangkok (Ramnarong, 1999), Calcutta (Choudhury et al., 1997), andJakarta (Schmidt et al., 1988).

Groundwater level rises have also been observed in urban areas. As cities stop groundwater extraction inurban centers, generally due to contamination in former industrial areas, the aquifer tends to recuperateor exceed its predevelopment water level, due to losses from water supply, sewage, cesspools andirrigation, since this additional water is often imported from outside of the watershed (Abu-Rizaiza, 1999).In the case of Riyadh, Saudia Arabia, where recharge is minimal, a considerable portion of water suppliedby desalinization plants is lost to the aquifer from leaks in the water supply distribution system, creatinghigh water levels of the aquifer (Foster et al., 1997).

This elevation of aquifer water levels causes serious geotechnical problems, such as flooding of basements,underground structures, drains, and sewers, the uneven movement of foundations (caused by varyingpressure changes as the unsaturated zone is saturated), and the contaminating of streams and soils.Another occasional consequence is the saturation of foundations with sulfate-rich groundwater, whichdeteriorates the structural capacity of cement (Shanin, 1988). Water level rise in urban areas has beenreported from cities all over the world, such as Tehran (Shamsi and Ardeshir, 1999) and Buenos Aires(Bianci and Leopardo, 2003).

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Urban Hydrogeology in Developing Countries: A Foreseeable Crisis

3. THE DEGRADATION OF GROUNDWATER QUALITY CAUSED BY URBAN LAND USE

Due to the large number of anthropogenic contamination sources in cities, and the failure of the soil andvadose zone to attenuate them, groundwater can easily be degraded. This phenomenon is especiallyserious in cities located over watertable aquifers, but also is present in urban areas that overliesemiconfined and confined aquifers. Most urban contamination is considered to be from dispersed sources,such as in situ sanitation and sewer, which leads to increases in salinity and nutrient and pathogengroundwater concentrations (Wakida and Lerner, 2005; Zubair and Rippey, 1999).

In San José, Costa Rica, nitrate concentrations have increased over the last two decades in the principalspring, Puente Mulas, which supplies potable water to the city. Reynolds et al. (in submission) hassuggested, using nitrogen isotopes and geochemistry, that the replacement of coffee plantations byresidencial areas with in situ sanitation is the probable cause of this increase. Poorly-constructed sanitarywell seals have been suggested as the cause of elevated pathogen contamination in the Patiño Aquifer inAsuncion, Paraguay, where 70% of drilled wells are contaminated with fecal coliforms. In the middle andupper class neighborhoods of São Paulo, Brasil, 60% of drilled wells have pathogenic contamination(Hirata et al., 2002). Mangore and Taigbenu (2004) found that 27% of wells in Bulawayo, Zimbabwe, werecontaminated with coliform bacteria, which was thought to be caused by leaking sewers.

Poor well design and the proximity of wells to in situ sanitation is one of the principal causes of the highincidence of child mortality from water-bourne diseases in poor countries. Foster et al. (1998) determinedthat 90% of water that is served to a household is discharged to septic systems or cesspools. The highdensity of cesspools, often located next to excavated wells in poorer neighborhood, does not permit thedegradation and dilution of nutrients and pathogens by the soil and aquifer matrix before reaching thedrinking water source.

In Urânia, Brasil, where a sewer system has been installed for the last 30 years, elevated nitrateconcentrations (up to 160 mg NO3 L-1) are found in unconfined, oxidized groundwaters to a depth of 60m, which illustrates that the existence of a public sewer system does not assure that other sources ofwastewater contaminate the aquifer (Cagnon and Hirata, 2004).

The presence of industries, gasoline stations, unregulated garbage dumps, and the improper handling ofdangerous products in cities, make these point sources important contributors to aquifer contamination.The presence of unlined landfills upgradient from the city’s only wellfields in Granada, Nicaragua, in anunconfined, permeable, volcanic aquifer, are threatening the water supply and may have already increasednitrate concentrations in the drinking water (Stimson and Espinoza, in submission).

Another pressing problem for city authorities is the management of urban areas that were contaminatedby previous activities, but have since been resold and rebuilt as residential or business districts. In many ofthese rezoned areas, there has not been a proper evaluation of abandoned contamination of soil andgroundwater, thus exposing the property owners and neighbors to environmental risks. In São Paulo, Brasil,there are more than 4 200 abandoned properties, of which approximately half have the potential tocontaminate groundwater (CETESB, 2005).

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Hirata, Ricardo; Stimson, Jesse and Varnier, Claudia

4. IMPROVING GROUNDWATER MANAGEMENT IN URBAN AREAS

Water resources management in cities is often conducted through national institututions or, in some cases,local agencies. However, the involvement and participation of the local communities in groundwatermanagement, which depends on many interlocking policy decisions about public and private water supply,sanitation, and zoning, is paramount. The framework for a groundwater management program shouldinclude the following components (Foster et al, 1998):

1.) the development of a clear definition of water rights (separate from property rights), by enforcinglicencing and fee payment for groundwater exploitation.

2.) the requirement that discharge of liquid effluents and deposition of solid wastes be allowed only witha permit.

3.) the creation of a national or local regulating agency, providing technical and financial support tosupervise environmental permitting programs, whose jurisdiction is delimited by hydrologicwatersheds, which permits an integrated approach to managing both surface water and groundwater.

Public water wells are often placed in close proximity to private wells. In many developing countries, thelack of a water well registration program results in the fact that water resource managers do not have dataon the extraction rates, location or even existence of most private wells. Even when an agency hasimplemented a registration program, this step is not sufficient to protect the groundwater resource, giventhat:

i.) there is a lack of technically-trained personnel and financial experts to run these registration programsfor extraction and contaminant discharge.

ii) the sustainable extraction potential and the vulnerability to contamination of the aquifer is not clearlyknown due to hydrogeological uncertainties.

One reason why the regulatory agency should canvass the affected communities, through publicconsultations and environmental education programs, is to convince the public of the importance ofadhering to these programs to protect groundwater. The public must not only be informed about thenegative impact of overexploitation and contamination to the sustainability of the resource, but that thelack of such management policies can result in real, immediate financial losses to the community due tomore expensive extraction regimes and water treatment programs. Avoiding groundwater contaminationand overexploitation will guarantee that the resource is preserved for future generations.

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