Using local groundwater for urban water supply: a case study in Lisbon, Portugal [1] Luís RIBEIRO, [2] Catarina SILVA [1] CVRM, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, [email protected][2] CeGUL, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1149-016 Lisboa, Portugal, [email protected]Abstract: Understanding groundwater flow and pollution processes in urban areas is a difficult task. Therefore there is a growing need to develop integrated methodologies to evaluate natural and artificial aquifer recharge and characterize the groundwater physic-chemical and bacteriological properties in order to quantify the degree of contamination of the aquifer formations. The area of Lumiar, located in the city of Lisbon, was selected because it is an area characterized by a large diversity of land uses, from urban to rural activities. Urbanisation was rapidly increased here in the last decades. Consequently the rate of impermeabilization is high as well the density of the subterranean infrastructures. To study groundwater flow in local aquifers, a monitoring network was implemented to measure piezometric levels and collect samples for water quality analysis. The magnitude of groundwater pollution was evaluated by comparing the ratios among some chemical species in the sampled points with typical values found in wastewaters originated from sewage systems through leaks. Keywords: Urbanization, recharge, hydrogeochemistry, water losses, groundwater pollution 1. Introduction Groundwater plays an essential role in urban water supply. Urban areas and groundwater resources interact in different periods of urban growth: from early stages, where the dependence for water supply is great, till last stages of urbanization where aquifer overexploitation and groundwater quality degradation may occur. If the development of the cities reduce the amount of urban aquifer recharge, because of the impermeabilisation of surfaces, it is also true that all the subterranean infrastructures for water supply, storm drainage and sewage generate significant amounts of recharge through leaks (Lerner, 2002). For these reasons recharge increase with urbanization, leading in some cases to the rise of groundwater levels, and therefore affecting underground infrastructures. Al Sefry, 2005 estimated that leakage from water mains and sewer networks in Jeddah led to a water table rise of 0,41 m between 1996 and 2000. Furthermore, as sewage network systems are not pressurized, wastewater flowing there is pushed by hydraulic gradient. For this reason, if sewers are located above the water table, wastewater leakage to the soil tends to occur. If water table is higher than the depth of flow in the sewer the inverse phenomena will also occur and groundwater will infiltrate in the drains. These in- or ex-filtration phenomena are closely related with other factors such the age of the pipelines, the type of the materials used in the construction processes and the level of conservation. Therefore, potential groundwater pollution can increase dramatically in some areas of the cities in addition to the potential leaching of pollutants originated from other sources (traffic roads, irrigation of gardens parks, gas deposit stations ruptures, cemeteries, etc..) In spite of these problems, urban groundwater resources may be used with or without treatment for other purposes than the human water supply (Ramos and Silva, 2005, Ramos et al., 2006, Silva et al., 2007, Lopes, 2007). 2. Location The study area is Lumiar, one of the outermost regions of the city Lisbon (Fig. 1). Last decades of XX century have assisted to a remarkable demographic exponential growth. This fast population increase led to an abrupt change in land use patterns, with a considerable urbanization specially in the old neighbourhood rural areas.
Transcript
Using local groundwater for urban water supply: a case study in Lisbon, Portugal
[1] Luís RIBEIRO, [2]Catarina SILVA
[1] CVRM, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, [email protected] [2] CeGUL, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1149-016 Lisboa, Portugal, [email protected]
Abstract: Understanding groundwater flow and pollution processes in urban areas is a difficult task. Therefore there is a growing need to develop integrated methodologies to evaluate natural and artificial aquifer recharge and characterize the groundwater physic-chemical and bacteriological properties in order to quantify the degree of contamination of the aquifer formations. The area of Lumiar, located in the city of Lisbon, was selected because it is an area characterized by a large diversity of land uses, from urban to rural activities. Urbanisation was rapidly increased here in the last decades. Consequently the rate of impermeabilization is high as well the density of the subterranean infrastructures. To study groundwater flow in local aquifers, a monitoring network was implemented to measure piezometric levels and collect samples for water quality analysis. The magnitude of groundwater pollution was evaluated by comparing the ratios among some chemical species in the sampled points with typical values found in wastewaters originated from sewage systems through leaks. Keywords: Urbanization, recharge, hydrogeochemistry, water losses, groundwater pollution 1. Introduction
Groundwater plays an essential role in urban water supply. Urban areas and groundwater resources interact in different periods of urban growth: from early stages, where the dependence for water supply is great, till last stages of urbanization where aquifer overexploitation and groundwater quality degradation may occur. If the development of the cities reduce the amount of urban aquifer recharge, because of the impermeabilisation of surfaces, it is also true that all the subterranean infrastructures for water supply, storm drainage and sewage generate significant amounts of recharge through leaks (Lerner, 2002). For these reasons recharge increase with urbanization, leading in some cases to the rise of groundwater levels, and therefore affecting underground infrastructures. Al Sefry, 2005 estimated that leakage from water mains and sewer networks in Jeddah led to a water table rise of 0,41 m between 1996 and 2000. Furthermore, as sewage network systems are not pressurized, wastewater flowing there is pushed by hydraulic gradient. For this reason, if sewers are located above the water table, wastewater leakage to the soil tends to occur. If water table is higher than the depth of flow in the sewer the inverse phenomena will also occur and groundwater will infiltrate in the drains. These in- or ex-filtration phenomena are closely related with other factors such the age of the pipelines, the type of the materials used in the construction processes and the level of conservation. Therefore, potential groundwater pollution can increase dramatically in some areas of the cities in addition to the potential leaching of pollutants originated from other sources (traffic roads, irrigation of gardens parks, gas deposit stations ruptures, cemeteries, etc..) In spite of these problems, urban groundwater resources may be used with or without treatment for other purposes than the human water supply (Ramos and Silva, 2005, Ramos et al., 2006, Silva et al., 2007, Lopes, 2007). 2. Location The study area is Lumiar, one of the outermost regions of the city Lisbon (Fig. 1). Last decades of XX century have assisted to a remarkable demographic exponential growth. This fast population increase led to an abrupt change in land use patterns, with a considerable urbanization specially in the old neighbourhood rural areas.
Fig.1. Location of the area in the city of Lisbon 3. Geological and hydrogeological setting The study area is situated in the northern part of Lisbon. Morphologically is a flat zone, with altitudes ranged between 80 and 100 m. Till the 50’s, Lumiar was a rural zone where some water courses occur. Runoff is from North to South, along the valley of the original “Ribeira do Lumiar”. Geological formations are from Holocene and Miocenic ages: Alluviums (a); “Calcários da Musgueira” (M2
va3); “Areias with Placuna miocenica” (M2va2); “Calcários do Casal Vistoso” (M2
va1); “Areias da Quinta do Bacalhau” (M2
IVb); “Argilas do Forno do Tijolo” (M2IVa); “Calcários de Entre-
Campos” (M2III); “Areolas da Estefânia” (M1
II) e “Argilas e Calcários dos Prazeres” (M1I) (Moitinho
de Almeida, 1986). There are heterogeneous formations composed by sandstone, sandy limestone, clayed sand and clay displaying various thickness from 5 to 35 m (Zbyszewski, 1963). Fractures or faults are rare, presenting different directions (Moitinho de Almeida, 1986). Hydrogeologically there are 2 main aquifers systems (Pinto, 2003): an alluvium system and a miocenic multilayer aquifer with 4 subsystems (Fig. 2): a) a limestone-sandstone unit, b) a clay unit named Argilas do Forno do Tijolo; c) a lower miocenic complex and d) a clay-limestone unit Argilas e Calcários dos Prazeres. The alluvium system, where well 13 is located, is considered to be a productive unit. The limestone-sandstone complex, corresponding to M2
Va3, M2Va2, M2
Va1 and M2IVb
formations, is an unconfined aquifer with high permeability and median and high productivities. Wells 11, 17, 21 and 22 were bored there. The hydrogeological complex Argilas do Forno do Tijolo presents low permeability, aquitard properties and an average thickness of 19 m. Wells 1, 8, 9 and 14 are located in this system. The lower miocenic complex is a semi-confined aquifer, corresponding to the lithological formations M2
III and M1II. Wells 2, 3, 4, 5, 6, 15, 16, 18, 19, 20 and 23 are located
there. The clay-limestone unit – Argilas e calcários dos Prazeres, with aquitard properties have low permeability corresponding to the lithological formation M1
I. There are no wells in this unit.
Fig. 2. Geological profile with indication of the main hydrogeological systems of Lumiar area 4. Urban recharge Evaluation of natural and artificial recharge in urban areas is a difficult task. In Lumiar area natural recharge from rainfall was estimated, considering previous works by Pinto, 2003. According to this author the water balance for the western part of the city (which can be representative of the area under study) has a surplus water of around 30 mm/yr. However, only part of this water is natural groundwater recharge, because of the extensive impervious areas. In Lumiar area these correspond to about 80 % of the total (see Fig. 3). Consequently the amount of water that naturally infiltrates the aquifer systems is estimated to be around 6mm/yr.
Fig.3. Land occupation in Lumiar zone Artificial recharge originated from the losses of the mains water distribution system can be estimated assuming a mean value of losses of 15%, corresponding to 18 millions m3/ yr per km of conduit (2005)
in the city of Lisbon. Taking into account the total length of the conduits in this area and the existing modern urban infrastructures (Fig. 4) a value of about 90 mm/yr was estimated which corresponds to more or less 5% of local mains water losses.
Fig. 4. Mains water supply system in Lumiar area, with the location of the sample data points In what concerns recharge from sewer systems ( Fig. 5) the estimation is much more difficult since we ignore the percentage of leakage and the actual conservation status of these infrastructures. Both infiltration and exfiltration rates can be calculated as well the amount of the losses of rainfall collect conduits using a combination of hydrochemical and isotopic techniques (Lenner, 2002). In this study we only identified some local areas where this problem can occur based on groundwater contamination indicators ( see sub-chapter 5.2).
Fig. 5. Sewer network system in Lumiar area with the location of sample data points 5. Groundwater monitoring data network A groundwater monitoring data network consisting of 20 points (16 shallow wells and 4 springs) were established in the area (Figs. 4 and 5). Depths of the wells ranged from 3 m (well 8) to 20 m (well 17). Some of these wells are currently used for local irrigation purposes. Three of the springs are located in the public garden Parque Monteiro-Mor displaying high productivities. A monitoring campaign was carried out during the months of May, June and July of 2006. Fig. 6 shows the main groundwater flow directions in the area. There are 2 main directions N-S in accordance to the hydrological network and SW-NE in the direction of the spring discharges of Parque Monteiro-Mor
Fig. 6. Groundwater flow directions in a sub-area of Lumiar
5.1. Hydrochemical facies
The collected samples present basic to neutral pH, ranging from 6.96 to 8.08. The electrical conductivity of the water varies between values of 325 (well 13) and 2230 µS/cm (well 23). The plot of the composition of water in a Piper diagram (Fig. 7) shows that all waters have a bicarbonate facies, with the exception of one analysis of well 22 which is Ca-Na sulphated type. Within the bicarbonate waters the majority is predominantly Ca (wells 5, 6, 9, 11, 13, 18, 19 and 20) or Ca-Na (wells 4, 14, 15, 16, 17, 21 and 23), with low percentages of waters with Na (wells 2, 3 and 8). Scatter plot Cl- vs SO4
2- (Fig. 8-a) shows a good positive correlation between those two dissolved anions. This correlation indicates that groundwater has a reasonable aquifer residence time which contributes to a more intensive water-rock interaction. Groundwater flow is faster in alluvium system and slower in limestone-sandstone complex.
A great percentage of groundwater samples show contamination by nitrates, positively correlated with the sulphate ion (Fig. 8-b). This aspect may have origin in fertilizers used in green spaces. In the sample collected in well 11, the increase of nitrates is more significant than the one observed in the sulphate. This fact indicates contamination by losses in sewage pipes. Samples of wells 2, 8, 13, 21, 22 and 23 showed no contamination by nitrates.
Fig 7 - Piper diagram
Cl vs SO4
0 58 115 175 226 300 400 500 600 653 700
SO4 (mg/l)
0
19
40
60
80
95
108
123
140
157
180
200
Cl (
mg/
l)
NO3 vs SO4
0 20 38 58 80 100 120 140 158 176 200 220 240 260
SO4 (mg/l)
0
20
40
60
80
NO
3 (m
g/l)
(a) (b)
Fig. 8. Scatterplots (a) Cl vs SO4 and (b) NO3 vs SO4
5.2. Groundwater pollution Microbiological analysis include the determination of micro-organisms, total coliforms, Escherichia coli; Enterococos, Clostridium perfringens. With the exception of sample 23 all the samples present a microbial contamination which implies that this groundwater can not be used for human consumption without a previous treatment (see Table 1)
Table 1 – Results of microbiological analysis of groundwater samples
In order to check if this contamination is originated from localized sewer conduits ruptures or from other origins, ratios among some chemical species in the sampled points with typical values found in wastewaters were compared. Typical wastewater values were derived from Pescod, 1992 and Howard et al. 2006. Table 2 show that, in some wells, the ratios between concentrations in the considered chemical species are very similar to those found in typical wastewaters. In sample points 1, 4, 6, 11, 13 and 20 are match to at least three of the ratios, which could be indicative of some degree of contamination with wastewater. The presence of E. coli in samples 6 and 13 and clostridium perfrigens in sample 1 validate this hypothesis (see table 1).
Table 2 – Ratios of several chemical species compared to usual values found in wastewater
(Symbol X indicates that there is similarity)
Well Cl-
/ SO42- K+ / Cl- Ca2
+/ Mg2+ NO3
- / Cl- Na+ / Cl- SO42/ NO3
-
1 X X X 2 3 X X 4 X X X 5 X 6 X X X X X 8 9 X X
11 X X X 13 X X X X 14 X 15 16 X 17 18 X 19 X 20 X X X X X 21 22 23
The ratio between chloride and bromide, (Vázquez-Suñé, 2003), may also contribute to analyse the origin of groundwater contamination. In this case values between 250-520 suggest that there is wastewater contamination.
According to figure 9, samples 6, 11, 17, 18, 20 and 23 have ratios comprised in this interval. Since the study area has a well developed sewer network, there is a high probability that local groundwater contamination is due to the wastewater from the ruptures of the sewer systems. Furthermore NO3 can also originated on exfiltration from sewer systems or a combination of leakage from sewers and fertilizers application 6. Conclusions Understanding groundwater flow and pollution processes in urban areas is not an easy task. Results of the application of different methods to an area of the city of Lisbon show a pattern of different groundwater pathways and travel times due to the high heterogeneity of the hydrogeological formations, the high rate of impermeabilization and the diversity of the underground infrastructures existing in the local. In spite of the magnitude of the pollution detected in some wells, urban groundwater resources can be used with or without treatment for other purposes than the human water supply such as gardens and public parks irrigation or washing public roads. Acknowledgements The authors gratefully acknowledge EPAL and EMARLIS for providing information of the subterranean network systems References
Al-Sefry A.S. Şen, Z. (2006) Groundwater Rise Problem and Risk Evaluation in Major Cities of Arid Lands – Jedddah Case in Kingdom of Saudi Arabia. Water Resources Management. Volume 20, Número 6: 91-108. Howard G. (2006) Human excreta and sanitation: Potential hazards and information needs. in Protecting Groundwater for Health: Managing the Quality of Drinking Water Sources. WHO. London. Lerner D. (2002) Identifying and quantifying urban recharge: a review. Hydrogeology Journal, 10: 143-152. Lopes L (2008) - Avaliação Quantitativa e Qualitativa das Águas Subterrâneas da Zona Urbana do Lumiar – MSc Thesis in Environmental Enginnering, IST, Lisbon, 107p. Moitinho de Almeida, F. (1986). Carta geológica do Concelho de Lisboa, escala 1:10000. Folha 1, Direcção Geral de Geologia e Minas. Serviços Geológicos de Portugal. Lisboa.
Pescod, M. B. (1992). Wastewater treatment and use in agriculture. FAO irrigation and drainage paper 47. Pinto C. (2003). Estudo dos Recursos Hídricos Subterrâneos do Concelho de Lisboa – Zona Ocidental. Relatório de Estágio Profissionalizante. Faculdade de Ciências da Universidade de Lisboa, Lisboa. Ramos A., Silva C. (2005) As Águas Subterrâneas de Alfama: Unidades Hidro-estratigráficas, Qualidade da Água e Modelo de Circulação´. Trabalho Final do Curso da Licenciatura em Engenharia do Ambiente, IST, 2005. Ramos A., Silva C, Ribeiro L., Silva C. (2006) - As águas subterrâneas de Alfama: tipos de recarga e qualidade da água in Actas do 8º Congresso da Água –’’Água - Sede de Sustentabilidade’’ ed. CDROM, APRH, Figueira da Foz . Rutsch, M. (2005). Quantification of sewer leakage - a review. 10th International Conference on Urban Drainage, Copenhagen/Denmark, 21-26 August 2005. Silva, C., Sanches F., Marques J., Latas, P., Cardoso, S., Carvalho, M.R. (2007) -Caracterização das águas subterrâneas da zona do Lumiar (Concelho de Lisboa).in Seminário sobre Águas Subterrâneas. Vazquéz-Suñé, E. (2003). Urban Groundwater. Barcelona City Case Study. Doctoral Thesis. Universitat Politècnica de Catalunya. 137 pp. Zbyszewski, G. (1963). Notícia Explicativa da Carta Geológica dos Arredores de Lisboa na escala 1:50000, Folha 4, Serviços Geológicos de Portugal, Lisboa.
