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HAZARDS INDUCED BY RECHARGE UNDER RAPID URBANISATION

GROWTH OF -supply provision value for unsewered without mains aximmainsum installation WATER IMPORTED M FROM DISTANT SOURCE Lima 1000

GROUNDWATER Merida FROM WASTE DISPOSAL WASTE WAST

WATER-SUPPLY AND WATER-SUPPLY Potential urban PEPERIURBAN recharger WELLFIELDS 500 (from mains leakage,le Santa urban drainagedrainag and GROUNDWATER Cruz FROM CITY NUCLEUSwastewater disposal)dis shallow wells deeper boreholes 200 INITIAL SETTLEMENT URBAN EXPANSION Hat Yai Water obtained Water obtained from Urban aquiferaq Water obtained from:- from shallow city city boreholes and largely abandonedab (i) peri-urban wellfield 100 WATER wells. wells. in favour of SUPPLY SUPPLY periurban wellfields. (ii) imported water Pr Natural (non-urban)se environmentob w abl ere e m - incipient contaminationd -i nurbanim waterw table - declines 50 city um value of urban wit h stabilisesstabilise but for outside city (cannot urbanmain aquiferaq meet increased water - possible urban well contaminatedcontamis drainage demand). yield reductions None due to water table - urban water-table Deep (mm/a) 20 decline. rises IMPACTS IMPACTS Recharge resulting - incipient contamination of from peri-urban wellfields 10 (Pre-urban recharge nil) Non-urban recharge- over exploitation - excess - over exploitation loading ICAL

ES - excess pollution - excess recharge 5 None loading (local to city centre) 2000 1000 500 200 100 50 20 10 HYDRO- CAUSES - excess pollution

GEOLOGICAL Rainfall (mm/a) loading

between: between: between: - urban groundwater - urban water supply - peri-urban water-users HUMIDNone usersSEMI-ARID (private vs public) vs wastewaterARID (municipal, , disposal rural supply) - public supply - water supply vs POTENTIAL CONFLICTS (peri-urban) and wastewater irrigation disposal/re-use

Q.4 - How do I assess the extent of urban groundwater use and the status of aquifer development?

Lawrence AR, Morris BL and Foster SSD 1998. Hazards Induced by Groundwater Under Rapid Urbanisation, in Maunds JG and Eddleston M (Eds) Geohazards in Engineering Geology. Geological Society, London, Engineering Geology Special Publications 15, 319-328. HAZARDS INDUCED BY UNDER RAPID URBANISATION

A R Lawrence, B L Morris & S S D Foster British Geological Survey, Wallingford OX10 8BB, UK

Abstract

Urban population growth across most of Latin America and Asia is relentless. Rapid urbanisation has profound impacts on the hydrological cycle, including major changes in groundwater recharge. Existing infiltration mechanisms are radically modified, and new ones introduced, with evidence of an overall increase in recharge rates. There is also widespread diffuse pollution of groundwater by nitrogen compounds (normally nitrate, but sometimes ammonium), (especially sodium chloride) and dissolved organic carbon. Groundwater contamination by petroleum and chlorinated hydrocarbons, and related synthetic organic compounds, and, on a more localised basis, pathogenic bacteria and viruses is also encountered. An analysis of the processes involved is made through citing detailed case histories.

INTRODUCTION

Urban Hydrogeological Cycle

The provision of water-supply, and drainage are key elements of the urbanisation process. Major 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. However, common factors to most urbanisation are impermeabilisation of a significant proportion of the surface and major importation of water from outside the urban limits.

Sanitation and drainage arrangements, which are fundamental to consideration of the hydrological cycle (Lindh 1983), generally evolve with time and vary widely with differing patterns of urban development. In most developing towns and installation of mains sewerage systems lags significantly behind population growth and water-supply provision.

Urbanisation causes radical changes in the frequency and rate of groundwater recharge, with a general tendency for volume to increase significantly and for quality to deteriorate substantially (Foster 1990). These changes cannot be measured directly and are thus difficult to quantify. The changes in recharge caused by urbanisation in turn influence groundwater levels and flow regimes in underlying . This can take considerable time, because the response constants of groundwater systems are normally the largest of all components of the urban hydrological cycle (Van de Ven 1990). It will usually be decades before aquifers reach equilibrium with the process of urbanisation.

Abstraction frequently masks changes in recharge regime, especially when groundwater levels are falling due to local . The most troublesome groundwater mounds are likely to develop under low-lying urban areas on relatively low-permeability unconfined aquifers, especially if they are not exploited for water-supply, as a result of poor . Rising groundwater levels can affect the stability, integrity and operation of subsurface engineering structures and installations. Thus, while the hydrogeologist tends to speak of 'the impact of urbanisation on groundwater' in terms of large- scale, long-term, temporal changes, the geotechnologist tends to think in terms of 'the impact of groundwater on urban structures' in a site-specific, steady-state sense.

1 ANALYSIS OF IMPACTS ON GROUNDWATER

A general summary of the principal urban processes affecting groundwater, and their relative impact, is given in Table 1. The impact of any given process will vary significantly with climatic regime, hydrogeological environment, and with the pattern of urban development itself.

Table 1: Summary of impact of urbanisation processes on groundwater Process Effect On Subsurface Infiltration Implications Principal Rates Area Time Base For Quality Contaminants MODIFICATIONS TO NATURAL SYSTEM Surface impermeabilisation & drainage - roofs, paved areas Reduction Extensive Permanent Minimal None - soakaways Increase Extensive Intermittent Marginally Cl, HC, CHC negative - mains drainage Reduction Extensive Continuous None None - canalisation Marginal Linear Variable None reduction Irrigation of amenity areas Increase Restricted Seasonal Variable N, Cl INTRODUCTION OF WATER- SERVICE NETWORK Water-supply system Increase Extensive Continuous Positive None Sanitation measures - unsewered Increase Extensive Continuous Negative N, FP - mains sewerage Marginal Extensive Continuous Marginally N, FP, DOC increase negatiive

Cl chloride and other major ions DOC dissolved organic carbon N nitrogen compounds (nitrate or ammonium) HC hydrocarbon fuels CHC chlorinated hydrocarbons FP faecal pathogens

Modifications to Natural Systems

Surface Impermeabilisation and Drainage

Urbanisation results in land-surface impermeabilisation, which reduces direct infiltration of excess rainfall, but also tends to lower and thus to increase, as well as accelerate, . Pluvial drainage arrangements determine whether there will be a net change in overall groundwater recharge rate, but anything from a major reduction to a modest increase is possible.

Surface impermeabilisation processes include the of roofs, and of paved areas, such as major highways, minor , parking lots, industrial patios, airport aprons, etc. While the proportion of the land area covered is a key factor, it should be noted that some types of urban pavement, such as tile, brick and porous asphalt, are, in fact, quite permeable and, conversely, that some unpaved surfaces become highly compacted with reduced infiltration capacity (Bouvier 1990).

If no pluvial drainage system is installed, runoff will either infiltrate at the edge of impermeable surfaces or via soakaways, enter channels, or accumulate in land-surface depressions. If pluvial (so-called storm water) drainage is installed, the mode of drainage-water disposal will exercise an important influence on urban groundwater recharge rates. The reduction in direct groundwater recharge, as a result of land-surface impermeabilisation, is quite commonly offset by increases in indirect recharge, when drainage is routed to soakaways or infiltration basins.

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Although this is excellent practice, where drainage from major highways and industrial patios is included, it will represent a significant increase in the risk of , due to spillages of hydrocarbon fuels and industrial chemicals in particular. Soakaways are also all too often used for the casual disposal of liquid waste from residential areas, such as used motor oils or for the illegal connection of septic-tank overflows.

Urbanisation also involves radical modification to the condition of surface water courses, which in turn can affect either the recharge and quality, or the discharge, of groundwater. The effects vary widely with hydrogeological environment and climatic type.

Irrigation of Amenity Areas

In where irrigation has to be continuously or intermittently practised, excessive application is commonplace in parks and , especially if water is applied by flooding from irrigation channels or hose pipes, as opposed to more efficient methods, such as sprinklers, which are more often used on sports fields.

In urban areas with permeable , over-irrigation can result in locally very high rates of groundwater recharge, although these are normally limited to a relatively small proportion of the total area urbanised. While irrigation return water is normally of relatively good quality, this is not the case where urban wastewater is used, which tends to overload the soil with nitrogen, and also in some situations, cause microbiological and/or organic groundwater contamination.

Introduction of Water-Service Network

Water-Supply System

Most urbanisation involves large water imports from beyond urban limits, except where local groundwater are adequate, in both quantity and quality, to provide the major contribution.

When expressed in hydrological terms the amount of water in circulation in distribution systems is very significant in relation to excess rainfall, even in relatively humid climates (Foster et al 1993). Since water mains for the most part are constantly pressurised, they are highly prone to leakage and in permeable soils most of this leakage occurs without surface manifestation as infiltration to the ground. This leakage becomes an important component of groundwater recharge.

Quantifying this recharge is, however, difficult as no direct measurements are feasible. Moreover, a significant proportion of subsurface leakage may in fact be intercepted by tree or enter sewers (or other subsurface ducts and trenches situated at greater depths than water mains), and ultimately form part of the overall surface runoff from the urban area.

However, the proportion of water put into distribution that apparently does not reach the consumer can vary from 20-50%. For one district of Lima, Perú, mains water leakage was estimated to be equivalent to 360mm/a (Geake et al, 1986).

While this source of recharge is often extremely important in the groundwater balance of unconfined urban aquifers, the existence of excessive leakage represents a major loss of revenue to water-supply undertakings. Even if most of the lost water is recuperated from local production boreholes, the additional costs for pumping are significant.

3 Sanitation Measures

In urban areas of developing nations in particular, the provision of mains sewerage lags greatly behind pollution growth and significantly behind the provision of mains water-supply. In many cases only small areas in the centres of cities are, in fact, sewered.

Unsewered (in-situ) sanitation units, such as latrines, and septic tanks, can provide an adequate service level for excreta disposal in urban areas under favourable site conditions. However, such on-site sanitation measures exert a major influence on groundwater recharge and are the predominant control on groundwater quality (Morris et al 1994).

Unsewered sanitation greatly increases the rate of urban groundwater recharge. Since consumptive use of water on domestic premises is usually taken to be 5-10%, more than 90% of the water-supply provided will end up as recharge to groundwater where sullage water is also disposed via sanitation units. Rates of recharge will generally be large and very significant in the urban groundwater balance, especially in areas of higher population density. If a significant proportion of the water-supply is derived from local groundwater, large-scale recycling will occur, whereas if a significant proportion is obtained from external sources, there will be a substantial increase in net groundwater recharge.

In some hydrogeological conditions, notably those with fractured bedrock close to surface and/or with a very shallow water-table, the standard design of in-situ sanitation unit results in high risk of penetration of pathogenic bacteria and viruses to nearby groundwater sources. This has been the proven vector of pathogen transmission in numerous disease outbreaks. It most often happens in densely-populated settlements, but can also occur in more prosperous urbanisations, where individual houses have both private shallow wells and septic tanks.

The deployment of in-situ sanitation measures to serve areas of moderate/high density population, will often result in an excessive load of nitrogen to the subsurface, and can cause widespread groundwater pollution problems by nitrate or, more rarely, by ammonium. The main factors determining the severity of such pollution are the non-consumptive per capita water use, the natural infiltration rate and the proportion of the gross nitrogen load that will be leached to groundwater as nitrate (Foster & Hirata 1988). The latter is very variable with type and operation of in-situ sanitation unit and local soil conditions, but in many documented cases exceeds 50%.

Sullage will also increase the risk of groundwater contamination, because of the widespread use of household products and community chemicals, which contain persistent halogenated synthetic organic compounds.

The installation of a mains sewerage system greatly reduces the rate of urban groundwater recharge, but does not eliminate the risk of groundwater contamination since there is increasing evidence of significant rates of sewer leakage.

Land Surface Storage & Disposal of Industrial Effluents

In many developing nations, extensive fringe urban areas remain without piped sewerage cover. Increasing numbers of industries, such as textiles, metal processing, vehicle maintenance, laundries, printing, tanneries and photo processing, tend to be located in such areas. Most of these industries generate liquid effluents, such as spent lubricants, solvents and disinfectants, which are often discharged directly to the soil and can also represent a serious long-term threat to groundwater quality.

Bigger industrial often use large volumes of process water and commonly utilise lagoons for the handling and concentration of liquid effluents. Moreover, urban wastewaters are increasingly being treated by retention in shallow oxidation lagoons, prior to discharge in , to the ground or for reuse

4 in irrigation. These lagoons are often unlined with high rates of seepage loss and have significant impact on local groundwater quality.

URBAN FIELD-SURVEY AREAS

The hydrogeological conditions and urbanisation situation in each of in the 4 survey areas are indicated in Table 2, and the most significant features are briefly summarised below.

It must be recognised that all hydrological data collection in urban areas is difficult and problems occur when estimating processes characterised by diffuse flowpaths for which direct measurement is impossible. A further complication arises in defining the survey limit and this affects results directly, since recharge rates are presented as averages over the entire survey area.

Various methods can be employed to make, and to corroborate, survey estimates, including statistical analysis of related parameters, hydrometeorological analysis, aquifer mathematical modelling and groundwater chemical indicators, but in most instances estimates will be subject to a significant degree of uncertainty.

Lima - Perú

The Lima survey area comprised a relatively-new, high-income, residential suburb (La Molina Alta) located on an intermontane, valley-fill, alluvial aquifer. It is of relatively small area with extremely arid , and prior to urbanisation was under irrigated with water being provided by a system of surface . These canals continue to operate delivering water for lagoons and amenity irrigation. This provides incidental recharge to an aquifer which is overexploited for public water- supply. The area has high coverage of mains water and sewerage systems, and extremely high per capita water usage, largely as a result of excessive irrigation of extensive private gardens.

Santa Cruz - Bolivia

The entire city within the outer ring comprised the survey area in this case. Santa Cruz is a low- rise, relatively low-density, city and one of the fastest growing in the Americas. It is unusual in that, up to the present, all of its water-supply is derived from wellfields within city limits, abstracting from a semi-confined, outwash-plain, alluvial aquifer. The city has relatively high coverage of mains water- supply, considering its very rapid growth, but only the older central area had mains sewerage at the date of survey. It has few parks, but all houses, including those close to the city centre, have substantial gardens and thus the proportion of the land surface impermeabilised is relatively low. Despite seasonal high-intensity rainfall, stormwater drainage to lined canals is only just being developed to overcome local flooding. For the most part surface run-off infiltrates around the margins of impermeable areas into the sandy subsoil.

Mérida - México

Mérida is similar in overall areal extension, population density and climatic conditions to Santa Cruz, but although a much higher proportion of the land area is paved, municipal parks and squares are more common. The city is situated on an extremely flat peninsula of highly- permeable karstic limestone, from which it derives all of its water-supply, although the greater part (almost 70%) is imported from wellfields outside urban limits. There is no mains sewerage nor stormwater drainage, all wastewater being returned to the ground via unsewered sanitation units and all surface drainage via soakaways.

Hat Yai - Thailand

Although much smaller in total population than Santa Cruz and Mérida, Hat Yai has higher overall

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URBANISATION Lima, Perú Santa Cruz - Bolivia Mérida - México Hat Yai - Thailand La Molina Alta Year of survey 1984 1989 1990 1990 Survey area (ha) 650a 14,500b 16,000b 1000b Population 21,000 650,000 525,000 140,000 Overall popl density (people/ha) 30 45 35 140 Maxm popl density (people/ha)c 50 120 175 210 Industrial zones none major major major Geological environment intermontane alluvium alluvialoutwash plain limestone Coastal alluvium Altitude (mASL) 180-320 390-420 8-10 5-10 Aquifer type phreatic semi-confined phreatic semi-confined Depth to groundwater (m) 25-45 1-6d 6-8 1-5d Mean rainfall (mm/a) 20 1200 1000 1900 Temporal distribution n/a seasonal seasonal seasonal mains water-supply coverage % 100 70 80 40 water supply (l/d/person)e 880 210 460 200 propn urban groundwater (%) 100 100 35 60 mains sewerage coverage (%) 70 20 0 0f stormwater drainage system none 10% cover, some to none, to soakaways extensive, to canals unsewered sanitation units septic tanks septic tanks, pit latrines septic tanks, cesspits septic tanks, direct to drains a outer high-income residential suburb d shallowest aquifer perched above regional water-table b entire city within outer ring road or municipal limit e gross value including distribution loses c as recorded in part of survey area f majority connected to open drain/ system, only small proportion direct to ground

Table 2: Summary of geological conditions and service arrangements in urban survey areas

6 population density and very high density (more than 200 people per hectare) in the city centre. As a result, a high proportion of the city area is impermeabilised. The water-supply situation is complex with most mains water-supply being imported from outside the urban limits and derived from surface sources. However, a minor proportion is obtained from groundwater within the urban area itself and local groundwater resources also provide industrial and commercial users. As a result groundwater abstraction represents perhaps as much as 60% of the total supply.

The city is situated on low-lying, coastal-alluvial, deposits and in consequence experiences problems with drainage. It is estimated that about 20% of wastewater disposal is directly to the ground via unsewered sanitation units, the remainder being connected to open drains which discharge into larger canals, which also receive stormwater runoff. However, as a result of overexploitation of groundwater and piezometric lowering in the semi-confined aquifer within the urban area, canal seepage now represents the single most important component of groundwater recharge.

INTEGRATED EFFECTS OF URBANISATION

Groundwater Recharge

An attempt has been made to assess the net effect of urbanisation on groundwater recharge (Figure 1).

r unsewered city without mains draina um value fo ge Maxim Lima 1000 Merida Potential urban recharge 500 (from mains leakage, Santa urban drainage and Cruz wastewater disposal) 200 Hat Yai 100 P Natural (non-urban)s environmentro ew bab er le ed minim 50 city um v wi alue for th ma ins drainage

Deep infiltration (mm/a) 20 Recharge resulting from urbanization 10 (Pre-urban recharge nil) Non-urban recharge 5 2000 1000 500 200 100 50 20 10 Rainfall (mm/a)

HUMID SEMI-ARID ARID

Figure 1: Potential range of increase of subsurface infiltration due to urbanisation

This diagram indicates very approximately the normal range of rainfall-infiltration relations for natural (non-urban) conditions and the potential recharge resulting from mains leakage, wastewater

7 and urban drainage, recognising that the former two will vary widely with population density and development level. Mains water leakage is the most consistent source of urban recharge, since it will normally comprise more than 20% of gross water production and will thus generally exceed 100mm/a. For largely unsewered cities it is possible for 90% of gross water production to find its way by various routes into the ground, since consumptive use (normally not exceeding 10%) will be the only loss. This is most likely to be the case in arid regions underlain by permeable strata, where irrigation of amenity areas is likely to be another important factor.

In the more humid regions, a larger proportion of urban drainage and wastewater will normally be directed to surface watercourses, because of the larger volumes of the former and reduced infiltration and storage capacity of the ground resulting from more frequent occurrence of strata and a shallow water-table. As a consequence the increase of deep infiltration due to urbanisation will be much less spectacular, but often still highly significant.

The shallow geology and hydrogeological regime is of crucial importance when trying to predict the effect of urbanisation on groundwater. This is well illustrated by the contrasting response in the cases of Mérida and Hat Yai. In the former case, the highly-permeable karstic limestone formation readily accepts all water discharged to the ground, albeit at the expense of groundwater quality. Deep infiltration to groundwater is estimated to have increased from 180 to 600 mm/a as a result. In the case of Hat Yai, shallow semi-confining beds are of insufficient permeability to accept urban drainage and wastewater and this effect is compounded by the existence of a shallow water-table. In consequence most is discharged to surface watercourses, but even so, groundwater recharge is estimated to increase from 170 to 370 mm/a as a result of urbanisation.

Groundwater Quality

In urban residential districts without, or with incomplete, coverage by mains sewerage, seepage from unsewered sanitation systems such as septic tanks, cesspits and latrines, probably represents the most widespread and serious diffuse pollution source. For groundwater, the immediate concern from on-site sanitation systems is a risk of direct migration of pathogenic bacteria and viruses to underlying aquifers and neighbouring groundwater sources (Lewis et al, 1982).

The karstic limestone aquifer beneath Mérida is especially vulnerable in this respect and widespread and gross bacteriological contamination of the urban groundwater was observed; faecal coliform counts in excess of 1000 per 100ml were not uncommon in samples obtained from shallow wells (Figure 2). In Mérida, the septic tanks discharge to soakaways whose bases are in fissured limestone and only 1-3m above the water-table, thus opportunities for pathogen attenuation within the unsaturated zone are limited.

Whilst bacterial contamination of shallow wells, in all types of geological formations, is believed to be widespread, it is likely, in the case of the unconsolidated deposits, to be more frequently a feature of improper design and construction of the well rather than of aquifer contamination. Migration of pathogens through unconsolidated strata to deep water-supply wells is extremely unlikely and any bacteriological contamination almost certainly reflects poor design and construction of the borehole.

The nitrogen compounds in excreta do not represent as immediate a hazard to groundwater but can cause much more widespread and persistent problems. The resulting concentrations of nitrate in groundwater are normally high, although quite variable. In Mérida, despite the exceptionally high percentage of nitrogen that is leached to the water-table (up to 100%, Morris et al, 1994), the mean groundwater nitrate concentration is only 4 mgN/l. This is largely as a result of the relatively low urban population density and the significant dilution afforded by both aquifer throughflow and high urban water use.

8 20000 500 ▲ SHALLOW KEY ▲ Spurious BOREHOLES ▲ WELLS value Maximum 400 ▲

▲ 25% Upper 15000 quartile ▲ 25% Median

300 25% Lower quartile Percentage of values 10000 ▲ ▲ ▲ 25% Minimum

▲ 200

▲ ▲ ▲ 5000 ▲ ▲ ▲ ▲ Faecal coliform count/100mls Faecal coliform count/100mls ▲ ▲ ▲ ▲ 100 ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ 0 0

7/91 9/91 1/92 4/92 7/92 9/92 1/93 4/93 7/91 9/91 1/92 4/92 7/92 9/92 1/93 4/93 (a)Sampling date (b) Sampling date

Figure 2: Bacteriological contamination of the karst limestone aquifer beneath Mérida: (a) shallow wells (<12m) (b) boreholes (>18m)

Conversely, in Santa Cruz probably only 10-20% of the nitrogen deposited is leached to the underlying alluvial aquifers; however, the greater population density and lower dilution (less throughflow and lower water consumption) ensures that average nitrate concentrations in the shallow aquifer are in the range 10-40 mgN/l. The lower percentage of deposited nitrogen that is oxidised and leached to groundwater beneath Santa Cruz as compared to Mérida reflects in part the lower dissolved oxygen status of the in the alluvial aquifers.

URBAN ▼ INFILTRATION SHALLOW GROUNDWATER (UP TO 50m) NO3N = 16 - 40+ Cl = 40 - 120+ SO4 = 30 - 90 dO2 = <5% sat HCO3 = 300 - 600 CLAY CLAY INTERMEDIATE GROUNDWATER (50 - 90m) NO3N = 5 - 25 Cl = 10 - 50 CLAY SO4 = 10 - 50 dO2 = <10% sat HCO3 = 250 - 350 Downward moving front DEEP of recharge containing GROUNDWATER urban pollutants (90 - 500m) NO3N = <10 Cl = <10 SO4 = <30 dO2 = <10% sat HCO3 = <300

Figure 3: Downward migration of contaminated water induced by pumping from deep aquifers beneath Santa Cruz, Bolivia.

9 (a)

Imported surface water from outside city Major groundwater abstraction Infiltration of some excess Discharge of Infiltration from canals rainfall wastewater + minor urban receiving urban drainage soakaway drainage & domestic waste water

Induced Shallow groundwater leakage contaminated by canals/soakaways Piezometric surface depressed by heavy pumping HAT YAI AQUIFER (sand and )

(b) RELATIVE POPULATION DENSITY OF URBAN AREA

High Moderate Low (N.B. High density area contains some 40 and 30 major abstraction wells respectively for hotels and factories)

Monitoring wells

10 GROUNDWATER QUALITY INDICATORS

5 NH4-N concentrations (mg/l) 2

03Kms Area with CI > 50 mg/l and K > 5 mg/l

Figure 4: Incipient effects of urban development on the Hat Yai coastal alluvial aquifer system of Thailand: (a) schematic and (b) general plan

10 Groundwater abstraction from the deep alluvial aquifers, beneath Santa Cruz, has induced the downward movement of shallow groundwaters, and elevated nitrate and chloride concentrations, are observed even at depths approaching 90m (Figure 3)

In Hat Yai, where the disposal of excreta to the ground by on-site sanitation systems is not always possible (because of surfacing of the water-table during the rainy season), human faeces and other wastes are discharged into surface water courses. As a consequence, they receive heavy loads of untreated effluent. Elevated groundwater nitrogen concentrations (mostly as ammonium) occur close to, and as a direct result of leakage from, these canals (Figure 4). The presence of ammonium (as opposed to nitrate) reflects the low dissolved oxygen status of the groundwaters and the absence of a significant unsaturated zone.

In addition to elevated nitrogen (as nitrate or ammonia) in urban groundwaters, increased concentrations of chloride (mostly from on-site sanitation systems) sulphate (from detergents and road runoff) and bicarbonate (derived from degradation of organic wastes) are frequently observed (Table 3).

The accidental spillage, leakage or improper disposal of industrial effluent can also cause serious pollution. In Merida, a survey of water-supply boreholes revealed widespread contamination by chlorinated solvents of the limestone aquifer. Although concentrations were generally low at less than 10ppb, it was considered that these values were underestimates of the true concentration due to the volatility of the solvents and the inherent difficulties in the collection and analysis of samples (Gooddy et al, 1993).

PRINCIPAL CONCLUSIONS

(1) Urbanisation generally results in significant increases in groundwater recharge, when cities are built upon unconfined or semi-unconfined aquifers, and the effect is particularly pronounced in more arid regions.

(2) The main processes responsible for this increase include water mains leakage, in-situ wastewater disposal, urban drainage soakaways and over-irrigation of amenity areas.

(3) The potential recharge contribution from these processes varies significantly with level of economic development, population density, climatic type and hydrogeological conditions, but almost always exceeds the reduction in direct rainfall infiltration due to surface impermeabilisation.

(4) The presence of less permeable strata at shallow depths beneath urban areas may result in mounding or perching of groundwater significantly above the regional water-table and cause serious negative impacts on subsurface engineering structures and installations. In most cases, however, it does not have this result, because aquifers are sufficiently permeable to prevent water-table mounding and/or because heavy abstraction of groundwater within the urban area depresses aquifer water-levels.

(5) This paper shows that urbanisation has a profound effect on groundwater recharge, not to mention quality. Considering the scale of implications for water management and engineering structures, it is important that more urban hydrogeological studies be undertaken. It could be argued that these should be an essential component of all city master plans, since the information they provide is at the heart of efficient engineering of the urban and environment.

11 Table 3: Urban impact on groundwater quality

CITY AQUIFER NITRATE (mgN/l) CHLORIDE (mg/l) BICARBONATE (mg/l) SULPHATE (mg/l) Back Urban Back Urban Back Urban Back Urban ground range ground range ground range ground range Mérida karst limestone (0- <5 5-301 <30 30-2003 <350 350-5504 <20 20-80 40m) Santa Cruz Alluvium outwash plain Shallow aquifer <10 10-401 <40 40->1201 <300 300-6005 <30 30-90 (0-45m) Deep aquifer <5 5-251 <10 10->501 <250 250-3505 <10 10-30 (45-200m) Hat Yai Shallow alluvium <2 2-222 5-10 10-802 0-20 20-905 0-10 10-40 (20-50m) includes NH4-N

Sources: 1 unsewered sanitation 2seepage from canals 3saline water at depth 4natural 5degradation of organic wastes 6source: detergents, road runoff

12 REFERENCES

Bouvier C, 1990. Concerning Experimental Measurements of Infiltration for Runoff Modelling of Urban Watersheds in Western Africa. IAHS Publn 198 : 43-49.

Foster S S D, 1990. Impacts of Urbanisation on Groundwater. IAHS Publn 198 : 187-207.

Foster S S D & Hirata R, 1988. Groundwater Pollution Risk Assessment: a Methodology Using Available Data. Pan American Centre for and Environmental Sciences (CEPIS), Lima, Peru.

Foster S S D, Morris B L & Lawrence A R, 1993. Effects of Urbanisation on Groundwater Recharge. In Groundwater Problems in Urban Areas, International Conference, June 1993, London.

Geake A K, Foster S S D, Nakamatsu N, Valenzuela C F & Valverde M L, 1986. Groundwater Recharge and Pollution Mechanisms in Urban Aquifers of Arid Regions. BGS Hydrogeology Research Report 86/11.

Gooddy D C, Morris B L, Vasquez J & Pacheco J, 1993. Organic Contamination of the Karstic Limestone Aquifer Underlying the City of Mérida, Yucatán, México. BGS Technical Report WD/93/8.

Lewis W J, Foster S S D & Drasar B, 1982. The Risk of Groundwater Pollution by On-site Sanitation in Developing Countries. WHO-IRCWD. Report 01-82, Duvendorf, Switzerland.

Lindh G, 1983. Water and the City. UNESCO (Paris, France)

Morris B L, Lawrence A R & Stuart M E, 1994. The Impact of Urbanisation on Groundwater Quality (Project Summary Report). BGS Technical Report WC/94/56.

Van de Ven, F H M, 1990. Water Balances of Urban Areas. IAHS Publn 198 : 21-32.

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