Groundwater recharge in urban areas

Atmospheric Environment Vol. 24B, No. 1, pp. 29-33, 1990. Printed in Great Britain.

0957-1272/90 $3.1)0+0.00 © 1989 Pergamon Press plc

G R O U N D W A T E R RECHARGE IN URBAN AREAS DAVID N. LERNER Hydrogeology Research Group, School of Earth Sciences, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, U.K. (First received 1 August 1988 and in final form 1 April 1989)

Abstract--The two interlinked networks of hydrological pathways in urban areas are described with particular reference to the links with groundwater. As well as reducing direct recharge, urbanization creates new pathways and sources of water for recharge, including leaking water mains, sewers, septic tanks and soakaways. The net effect is often to increase recharge to pre-urbanization rates, or higher in dry climates and cities with high densities and large imported water supplies. Key word index: Groundwater, recharge, urban, leaking pipes, sewers, irrigation, septic tanks, soakaways, water resources.

l. I N T R O D U C T I O N

2. U R B A N H Y D R O L O G I C A L

Much of the surface of urban areas is rendered impermeable by buildings, roads and surface coverings. Because of this covering, the classical view of the effect of urbanization on groundwater is that recharge is reduced. For example, " . . . groundwater o u t f l o w . . , decreases with urbanization, with direct r u n o f f . . , increasing." (Douglas, 1983), and similarly from Lindh (1983) "Infiltration to the groundwater is markedly reduced . . . . with less water reaching the aquifer, wells may have to be deepened." In fact urbanization alters all parts of the hydrological cycle so much that no simple analysis of the effects on groundwater is possil~le. Many of the alterations will increase recharge where there is permeable ground below the city. This has been recognized in principle by some writers, for example Gray and Foster (1972) and Monition (1977), but the increases have rarely been quantified. It has not been realized that urban recharge is often as high, or higher, than pre-urbanization rates (Lerner, 1986). This is due in part to the lack of appropriate data. Those concerned with water in urban areas (i.e. urban hydrologists, water supply engineers, sewage engineers, etc.) usually have little interest in urban groundwater except as a nuisance, and do not quantify the interactions of their systems with it. This paper has two purposes. Firstly it is a review of the few data available on urban recharge, in order to show that it is not a single, simple process. Secondly it is a plea for urban recharge to be recognized as an area for research, and especially for the collection of suitable data. 29

PATHWAYS

Water does not often follow a cycle in the urban environment because it enters and leaves across the urban boundary. Rather it follows a network of pathways. There are two such networks of pathways in urban areas which are interlinked at many points. These are the (heavily modified) natural pathways and the water supply-sewage pathways. The principal pathways of the natural network are precipitation, evapotranspiration, runoff, infiltration, recharge and groundwater flow. The principal paths of the water supply network are shown in Fig. 1, which particularly emphasizes the interconnections with groundwater. Boreholes bring groundwater into the network; a variety of paths including leakage from pipes, over-irrigation and septic tanks carry water down to the groundwater system. The water supply network carries large flows. Table 1 compares, for a number of major cities, precipitation-usually the main input to the natural network--with imports of water and local groundwater abstraction, which are the inputs to the water supply network. In highly urbanized areas, e.g. Hong Kong, and in arid or semi-arid climates, e.g. Doha and Lima, flows in the water supply network exceed those in the natural system. Even in temperate, moderate density cities (Vancouver, Birmingham) flows in the two systems are comparable in size.

3. R E C H A R G E ~ N

URBAN AREAS

3.1. Recharge from the natural network Large urban areas often have a meso-climate which may alter rainfall and evapotranspiration rates (e.g. Shaw, 1988). These are probably second order effects on recharge when compared to the changes caused by

30

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Table 1. Relative sizes of inputs to the urban hydrological networks City Urban Sweden 2 Mexico City 2 Hang Kong 24 Hang Kong 56 Sydney 2 Vancouver 2 Lima s Doha, Qatar ~° Birmingham 11

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Imports

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i Many of the areas are for a supply zone and include rural and semi-rural land. 2 Grimmond and Oke, 1986. 3nd--No data given in source document. 4 Whole city. 5 Lerner, 1986; Geotechnical Control Office, 1982. 6 Thirteen districts on Hang Kong Island. v Assuming minimum night flows are 40% of supply. s Lerner et al., 1982; Wild and Ruiz, 1987. 9 Includes agricultural use. lo Pencol, John Taylor and Sons, 1985. ~Written communications, Severn Trent Water Authority. 2 Local groundwater only used by industry.

surface coverings which generally reduce infiltration, a n d increase a n d accelerate runoff. This runoff is often carried in s t o r m sewers, drains, or o t h e r artificial waterways. T h u s it is p r o b a b l e t h a t direct recharge is reduced in u r b a n areas. It should be n o t e d in passing t h a t permeable p a v e m e n t s are sometimes used to reduce runoff, a n d these will increase recharge by allowing infiltration while reducing vegetation cover a n d so reducing e v a p o t r a n s p i r a t i o n . This was d e m o n -

strated by van D a m a n d van der Ven (1984). There is potential for recharge from s t o r m sewers a n d drains, even w h e n these are designed to carry water out of the city. Sewers are well k n o w n to leak a n d Lerner (1986) presents evidence t h a t such leakage recharges g r o u n d water in H a n g Kong. In a water resources study of Liverpool, the University of B i r m i n g h a m (1984) concluded t h a t significant a m o u n t s of s t o r m water leaked from sewers into the g r o u n d w a t e r system, more t h a n

Groundwater recharge in urban areas replacing the reduction in direct recharge due to impermeable cover. Storm water is often deliberately recharged. In the U.K. for example, soakaways are commonly used to dispose of runoff from domestic roofs, and for road runoff from some motorways and in some cities. As no evapotranspiration takes place from such water, the amount of recharge is probably higher than the direct recharge that would have occurred without urbanization. Recharge basins for storm water are used, for example on Long Island where they are thought to bring recharge up to pre-urbanization rates (Seaburn and Aronson, 1974). In arid climates there is often no provision for storm runoff, and the (rare) increased runoff from impermeable surfaces will infiltrate into the permeable surroundings.

attributed it to leaking mains. Lerner (1986) concludes that recharge rates of 100-300 mm a - ~ are common. Sewers leak, as shown by the many examples of groundwater pollution below cities by sewage. Many urban areas do not have sewers, relying on septic tanks and soakaways to dispose of effluents--this water must recharge groundwater. For example in Bermuda, with an average rainfall of 1460 mm a - t , water supply is from roof catchments and sewage is disposed of to septic tanks. These, and soakaways for storm drainage, increase recharge from 365 mm a - ~ in rural areas to 575 mm a - t in the urban area (Thomson and Foster, 1986). The third main source of recharge from the water supply network is over-irrigation of parks and gardens. These are irrigated for aesthetic rather than commercial reasons. Application is often by unskilled labour. The amount of water applied rarely depends on plant water needs, but on the affluence of consumers, pricing policies for water supplies and, in the case of municipal parks, on bureaucratic procedures. For these reasons over-irrigation is normal with many multiples of the potential evapotranspiration being applied. Excess water percolates deep to recharge groundwater. An example is Doha in Qatar where over-irrigation has raised groundwater levels to the surface in many low lying areas (La Dell, 1986).

3.2. Recharge from the water supply network All water supply networks leak. Few authorities claim to be able to reduce leakage below 10% of supply, and rates of 50% have been reported. This source alone can generate a potential recharge of up to 3000 mm a ~, and Lerner (1986) showed that some of this leakage did in fact recharge groundwater. He gave examples from Hong Kong and Lima, Peru, with several lines of evidence. Groundwater balances and flow modelling showed that additional sources of recharge were needed to sustain groundwater levels. There was hydrochemical evidence of mains water in groundwater, and in the Hong Kong study there was piezometric evidence under completely paved areas. More recently Lerner (1989a) presents informal evidence of the downward percolation of leaking mains water from a large underground Byzantine cistern. Rivett et al. (1989) report localized contamination of groundwater in Birmingham by trihalomethanes, and

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31

3.3. Net effect of urbanization Figure 2 summarizes the changes in recharge that urbanization can cause. With so many changes, and with different conditions in every city, it is difficult to generalize about the net effect. However it is clear that there will always be man-made sources of recharge. In drier climates, or with large imported supplies, or with

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32

DAVIDN. LERNER

poor maintainance of piped systems, recharge in urban areas is likely to exceed that in rural areas. Table 2 gives some examples of water balances for urbanized aquifers.

infiltration, soil moisture or pressure, or tracer concentrations require knowledge of the leak location; more importantly they need a method to move from point measurement to a more regional scale.

3.4. Urban groundwater quality

4.2. Water resource and other engineering studies

There are many potential threats to groundwater quality in urban areas, including:

The multiplicity of recharge sources and complexity of landuse and surface cover, make it extremely difficult to estimate recharge in urban areas for engineering studies. For example, consider the possible sources of recharge from the water supply network:

- - w a s t e disposal activities, - - industrial premises, - - c h e m i c a l stockpiles and storage tanks, - - s p i l l a g e s during transport,

- - sewers, and --surface drainage, including roads. Many of these threats have been realized, and there are numerous cases of contaminated groundwater beneath cities, of which just a few need be cited here. The Triassic sandstone aquifer beneath Birmingham is widely polluted by chlorinated hydrocarbon solvents (Rivett et al., 1989) as is the alluvial aquifer beneath Milan, Italy (Cavallaro et al., 1986). Examples of microbiologicaUy polluted groundwater are given by Galbraith et al. (1987), while Eisen and Anderson (1980) showed that Milwaukee's groundwater is contaminated by C1- and SO ] - . Bermuda's limestone aquifer is contaminated with NO3 and some microbiological organisms (Thomson and Foster, 1986).

4. ESTIMATING R E C H A R G E IN URBAN AREAS

4.1. Scientific studies This review has shown that there are few accurate data on the effects of urbanization on recharge, either for individual components or overall, but that such data are needed because there are significant effects to quantify. Those familiar with urban environments will realize that collecting accurate data on such episodic (in time and space) phenomena will be difficult. As just one example, consider the problem of leaking storm sewers. Water balances will be of especially low accuracy during storms. Methods based on measuring

Table 2. Example water balances of urbanized aquifers (10 3 m3 day- 1) City Area (km2) Recharge from: precipitation rivers agricultural irrigation park irrigation leaking mains sewers and septic tanks soakaways

Lima* Dohat Bermudas 400 0 280 ~ 390 ~ 340 0? 0

294 11.5 0 0 37.6 25.3 17.6 -

* Lerner et al., 1982; Wild and Ruiz, 1987. t La Dell, 1986. $ Thomson and Foster, 1986.

6.3 4.83 0 0 0 0 3.13 -

recharge = leakage from water mains +external losses from consumers' properties +leakage from sewers + flow to septic tanks + deep percolation from domestic irrigation +deep percolation from municipal irrigation.

(1)

Considering all these components individually, with their associated errors, can lead to a large accumulated error in the recharge estimate. To reduce errors, it is preferable to consider the overall balance of the network and estimate net recharge as follows: net recharge = imports of water + local abstractions of groundwater -consumptive use - e f f l u e n t leaving area. (2) Methods of using both equations are discussed by Lerner (1989b). Estimating recharge from the natural network presents even greater problems, the most difficult of which is losses from storm sewers. All studies of leaking sewers have been of those which collect water, not lose it, because this is an important factor in their design. Except in research studies, water balance over individual storms are unlikely to be accurate enough because of the uncertainties in both measurement of precipitation and runoff, and in estimates of detention and evaporation. Field tests in combined sewers at times of low flow, or injecting and tracking known flows or tracers, or analysis of groundwater responses may provide more accurate estimates for small parts of a city, but are impractical for the whole system. The best approach will be to set upper and lower bounds on recharge by water balance methods. These estimates can then be refined by calibration of a groundwater flow model against groundwater responses and outflows. 5. C O N C L U S I O N S

This review argues that there are more sources and pathways of groundwater recharge in urban than rural areas. The large imports of water to cities give rise to large recharges from leaking pipes, and storm water is often partly diverted to groundwater, whether in-

Groundwater recharge in urban areas tentionally or otherwise. Despite the increased impermeable area, the net result is t h a t total u r b a n recharge is often at least as large as would occur w i t h o u t the city. G r o u n d w a t e r quality is often p o o r b e n e a t h cities where there is n o geological (low permeability) protection. Few reliable d a t a are available on u r b a n recharge, either for the individual c o m p o n e n t s or the total, a n d further scientific a n d engineering studies are needed. Acknowledgements--An earlier version of this paper was presented to UNESCO's International Symposium on Hydrological Processes and Water Management in Urban Areas held in Duisburg, F.R.G., during April 1988.

REFERENCES

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33

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