*IST UL (University of Lisbon), Lisbon, Portugal. Corresponding author:

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1 Monitoring and simulation of saline water inflows to drainage systems in coastal areas. The Case of the Alcântara s Wastewaters System, in Lisbon, Portugal Catarina Pina Teixeira de Melo Amaro* *IST UL (University of Lisbon), Lisbon, Portugal. Corresponding author: catarina.p.teixeira@tecnico.ulisboa.pt Abstract: In Portugal, an important proportion of the wastewater drainage system is located in coastal areas, with a significant part of sewers installed below the sea water level. Due to broken pipes and overflows the saline waters enters the system. The inflow of seawater in drainage systems may result in undesirable effects, particularly hydraulic overload, corrosion of electromechanical equipment and electrical installations, as well as the increase of energy and chemical costs with wastewater treatment. It may also hamper the reuse of treated wastewater for agricultural purpose due to high chlorides concentration. Thus, the main purpose of this thesis is to contribute to the assessment of salt inflows in Lisbon s wastewaters, namely in the Alcântara s system. There were developed campaigns for monitoring the electric conductivity of the pumping station EE during June, July and August 6, within Alcântara s system. The conclusions were that for tide levels of,8 m, salt water starts to enter the system, but only for tide levels of,6 m the volume of inflow salt water is relevant. After an analysis of the results, carried out through mass balance it was obtained an empirical expression that relates the salt water inflows with tide levels for the Alcântara s system. The application of these studies allows to estimate annual values of such inflows, and evaluate the technical and economic impact on the performance of pumping infrastructure and treatment. It also allows the estimation of the salt-water inflows according to climate change effects. Keywords: Alcantara s system, conductivity, saline water inflow, saline wastewater, unwanted inflows, wastewater INTRODUCTION Urban drainage systems are absolutely essential for the economy, health and well-being of the population, but unwanted inflows might have adverse effects within these systems. While in elevated areas the infiltration contributions occur mainly in winter, by increasing the aquifers levels, in the case of low-lying coastal areas, the source of unwanted inflows also results from saline water inflows. This happens because a significant part of sewers are installed below the sea level and also because of a lack in the system tightness, particularly in sewers and manholes (Pimentel et al., 6). When the tide level reaches the sewers inlet level, the water begins to enter the system.

2 The inflow of saline water in drainage systems cause significant impact on system performance, resulting in hydraulic overload, corrosion and increase of energy costs of pumping and wastewater treatment. With the saline water inflows in the system there is a significant decrease in ph and an increase of chlorides concentration, having the effect of increasing the corrosion, accelerating the degradation of the concrete infrastructure and electromechanical equipment (David et al., 6). This inflow also causes significant impact on the performance of the WWTP (Waste Water Treatment Plant), both physical and chemical processes, such as the settling and the biological processes, which may lead to the inhibition or destruction of the microbial community and make the reuse of wastewater in agricultural irrigation not technically viable. This is particularly significant in high tide condition, when the levels in the estuary are higher. On the opposite scenario, in low tide, the coastal inflows are null or minimal (Pimentel et al., 6). According to A.Kumar et al.() the salinity ranges could be classified as it follows: Low salinity: to μs/cm Usually, the domestic wastewater conductivity values are in this range; Medium salinity: to μs/cm; High salinity: to 5 μs/cm. The main purpose of this paper is to contribute to the study of inflow saline waters in urban drainage systems, particularly from the point of strategies to estimate flow rates and impacts on these systems. This document focus on the lower drainage zone of Alcântara s basin, namely the trunk sewers of Algés - Alcântara, Cais do Sodré - Alcântara and Terreiro do Paço - Cais do Sodré - Alcântara, in Lisbon, Portugal. The experimental campaigns for measuring electrical conductivity took place in the pumping station EE, located in Alcântara downtown. The objectives of this study were: to deepen the study on unwanted coastal inflows in urban drainage, by continuously monitoring conductivity for different high tide values; to determine where and for which tide levels begin occurring saline inflows as for proposing an empirical equation to estimate the inflow volumes depending on the different tide levels. CASE STUDY The conductivity measurement campaigns were carried out continuously in the pumping station EE, which collect the wastewaters of three drainage trunk lines: Algés Alcântara, Cais do Sodré Alcântara and Terreiro do Paço - Cais do Sodré Alcântara, being from EE pumped to the WWTP of Alcântara through a pipe Ø mm wide. The three drainage lines are located in coastal zones, with sewers with installed below the sea water level and they include several outflows, some of them with tidal valves (Hidra, 6).

3 The Algés-Alcântara interceptor starts in Algés and includes two pumping stations in series, the pumping station (EE), near Centro Cultural de Belém and the pumping station (EE) in front of the Cordoaria building. The wastewater pumped by EE follows through a gravity interceptor to pumping station (EE). The Terreiro do Paço Cais do Sodré -Alcântara interceptor includes three pumping stations, Rocha Conde d Óbidos, Santos and Agências. Finally, Cais do Sodré- Alcantara interceptor includes also three pumping stations, EE, EE5 and EE6. Figure presents the three drainage lines with the respective pumping stations. EE EE5 EE6 EE EE EE EE9 EE EE AG Figure Three drainage lines with the respective pumping stations, in the Alcântara s system The Tagus River Estuary is located in the Portuguese west coast. According to values given from the Water Modelling System (MOHID), in Paço de Arcos, the average salinity is about %. Regarding the electrical conductivity, measurements taken at the Alcântara coastal area, indicate values between 5 and 6 µs/cm. In order to measure the tide level, there is in the estuary a tide gauge. Regarding to the tide height, it can be classified as high tide, when the sea water reaches its highest point in the tidal cycle, and low tide when the sea water reaches its lowest point in the tidal cycle. The tide height is referred to the hydrographic zero (HZ), which in Portugal has the value of,8 m (Antunes, 7). METHODOLOGY According to David et al. (6) and LNEC (6), in case of a subsystem as Alcântara with several drainage lines and pumping stations, it can be adopted the following methodology to study the saline water inflows: () System characterization under study and assessment of the effects or consequences magnitude () Collection of register information of the drainage system identifying the locations of discharge, overflows and tide tables () Identification of the causes and specific mechanisms () Verification of daily volume pumped; (5) Campaigns for measuring the conductivity along the drainage system (6) Visual inspection of drainage system upstream of selected pumping stations, with the support of information; (7) Increase access to structural funding to finance the necessary interventions and implement updating and management options.

4 In order to determine the salt water inflows to the pumping station EE, electrical conductivity measurement campaigns were carried out. Table shows the days of the three monitoring campaigns with respective high tide sea levels. These high tide levels are those shown in Tabela de Marés do Porto de Lisboa (6), and may not correspond to the real values. Table Campaigns days with respective high tide sea levels Day (6) 5/7 6/7 9/9 /9 /9 /9 Tide levels [m],6,,,, In addition to the measurement of electrical conductivity in EE, specific conductivity measurements were also taken on Tagus Estuary. Thus, it becomes possible to compare the conductivity values obtained in the estuary with the values measured at EE. Figure shows the monitoring campaigns at EE and Tagus Estuary. Figure Monitoring campaigns at EE and Tagus Estuary RESULTS AND DISCUSSION The aim of the monitoring campaigns was to find answers for three questions: from which tide level the saline waters start to enter the system, which are the areas with higher probability of inflows and what is the relation between the inflows and tide levels. The data obtained from monitoring campaigns are shown in Figure. For July 5 th and 6 th, to a tide level of,6 m, the maximum electric conductivity recorded was µs/cm and to a tide level of, m the maximum electric conductivity was 97 µs/cm. For September rd, th and 5 th, to a tide level of, m the maximum electric conductivity recorded was 857 µs/cm and to a tide level of, m the maximum electric conductivity was µs/cm. The average sewage conductivity without saline intrusion is about 5 µs/cm. For September 9 th and th, to a tide level of, m, the maximum electric conductivity recorded was 898 µs/cm and to a tide level of m the maximum electric conductivity was 5 µs/cm. The average sewage conductivity with saline intrusion is about µs/cm. The higher tide levels reach, the more saltwater enters the system.

5 9:6 :5 :5 : : : : :59 5:8 6:7 7:6 8:5 9: 9:5 : : : :9 :58 :7 :6 6: 6: 7:8 7: 8:6 8:5 9: 9:58 : :6 : : :8 : :56 : : : :8 :5 :8 5: 7: :6 5: 6:58 8: 9: :5 : :7 : :9 : : 5:6 6:5 8:8 9: :9 :55 : :7 5: 6:58 : :6 :8 : : 5: 6:6 7:8 8: 9: : :6 :8 : : : :6 :8 : 5: 6: 7: ,8,6 5/6 July,8, ,8 9/ Sept.,,,5,5,5,5,5 5/7 Date 6/7 Condutividade Conductivity in na EE EE Nivel Tide de level maré 9/9 Date /9 Condutividade Conductivity in na EE Nível Tide de level maré Sept., /5 Sept., Hour (h) Condutividade Conductivity in na EE EE Nível Tide de level maré /9 Date 5/9 Condutividade Conductivity in na EE EE Nível Tide de level maré Figure - Data obtained from monitoring campaigns of electric conductivity in EE In all cases there is a gap between the peak tide times curve and the peak of electric conductivity curve of about to 5 min. Based on this lag of both curves, it can be detected that in most cases the sea water begins to enter the system when the tide level corresponds to.8 m. Based on conductivity measurements in the Tagus Estuary, in this mass of water it was obtained an average value of conductivity of 55 µs/cm. The time gap is justified by the travel time that the residual water takes to reach EE. Knowing that the pumping station EE collects wastewater from three different drainage fronts, it should be noticed that salt water inflow may occur in all of them. In order to find out the likely areas of saline water inflows, it became necessary to calculate the travel times for each drainage fonts in Alcântara s system. It was estimated the travel times assuming that the average speed in gravity stretches is m/s, the average speed in the pumped stretches is, m/s and that the effluent stay in the pumping station around 5 min. 5

6 In Algés Alcântara interceptor there is an effluent delay of 6 min between pumping station EE and pumping station EE. In Cais do Sodré Alcântara interceptor there is an effluent delay of 85 min between pumping station EE6 and pumping station EE. Between Terreiro do Paço Cais do Sodré Alcântara, the effluent delay reaches 9 min between pumping station Agências and EE. EE EE5 EE6 EE EE EE EE9 EE EE AG Figure Sites with higher likely saline water inflows in Alcântas s subsystem Inside the red area there are sites with higher likely saline water inflows, it decreases until the yellow area, Figure. This prediction correlates with the study accomplished by Águas de Portugal e Vale do Tejo (6). It was made, then, an analysis among the pumped flowrate volume in the pumping station EE, the values of electric conductivity measured and the different tide levels. Table present the daily average volume, the average flowrate and the maximum conductivity in the pumping station EE for each day of campaign and the tide levels. Table Tide level, Daily average volume, Average flowrate and maximum conductivity for each campaign day. Day (6) Tide level [m] Daily average volume [m ] Average flowrate [m /h] Maximum conductivity [µs/cm] Sept /5th, Sept rd, July 6th, July 5th, Sept th Sept 9th,

7 : :55 :5 5: 7:5 9: :5 : 5:5 7: 9: :5 : :55 :5 :5 6: 8: :5 : :5 6: 8:5 : :55 :5 : :5 5: 7:5 9:5 : :5 5: 6:55 8:5 :5 :5 Volume [m ] : : : 5: 6:5 8:5 9:55 :5 :5 :55 6:5 8:5 : : : : : : 6: 7:5 9:5 :55 :5 :5 5:55 7:5 9:5 :55 :5 :5 :5 : 5: 6:5 8: Flowrate [m /h] Conductivity [µs/cm] The relation between the pumped flowrate in EE with values of measured conductivity, for the campaign of September rd, th and 5 th is shown in Figure 5. As shown at Table and Figure 5, the higher average flowrate reaches, the higher will reach the maximum conductivity and the tide levels. The flowrate tends to follow the conductivity rise. Flowrate [m /h] //5 Sept /9 Flowrate in EE Date /9Conductivity in EE 5/9 Caudal na EE Condutividade na EE Figure 5 Data of pumped flowrate and electric conductivity for the campaign of September rd, th and 5 th Figure 6 shows the relation between the pumped flowrates in EE and the tide levels on Tagus Estuary, for the same campaign. Volume [m ] //5 Sept ,,,,,,,,,5,,,5,5,5,5 /9 /9 5/9 Date Volume na in EE Nível Tide de level maré Figure 6 - Data of volume and tide level for the campaign of September rd, th and 5 th. At Figure 6 it is quite noticeable the relation between the daily average volume and the maximum tide level. The increasing flow, in times of high tide levels, seems to indicate the existence of saline waters inflow upstream the pumping station EE. 7

8 Salt water inflow volume [m ] As a final point, to determine the volume of saline water on the effluent that is pumped in EE and understand the impact of higher tide levels it was performed a mass balance with the data for each tide level. The results for inflow saline water volume are presented on Table and Figure 7. Table Salt water inflow volume by tide level Salt water Tide Level inflow, in EE [m] [m ], 8, 5, 5, , Salt water inflow volume by tide level [m ] V = 6 (N-,) / R =,98,,,,,5,6,7,8,9,,,, Volume Salt water de inflow entrada volume de água by tide salgada level por nível de maré Linha Potencial ajustada line (Salt (Volume water inflow de entrada volume de by água tide level) salgada por nível de maré) Figure 7 - Salt water inflow volume by tide level The obtained data was adjusted by a potential trend as a flow curve of a discharger (Q = C w b gh ). On equation, V represents the inflow saltwater volume [m ] and N the tide level [m]. The correlation coefficient obtained was,98. V = 6 (N,) / () As shown in Figure 7, as the tide level increase, the volume of inflows to EE increases exponentially. Through the equation, it is possible to estimate the inflow of salt water for different tide levels. Taking into account the obtained results it was concluded that for a tide level of,8 m, the waters starts entering the system, but only for tide levels of,6 m the volume of inflow salt water becomes relevant. Lastly, it was calculated the annual volume of saline water for 6 through equation. V saline inflow (%) = V saline inflow (m ) V total average (m ) V saline inflow (m ) () 8

9 Through empirical equation, it was concluded that the annual volume of saline water inflow is around 8% of the total annual volume pumping in EE. The volume of saline inflow is maximum when the tide level reaches the maximum value, which for the year 6 is, m relative to the hydrographic zero. Over the year there are two tides with, m which corresponds to about 5% of the volume pumped at EE. Knowing that the sea water is rising at a rate of mm/year (Antunes, 6), for the year 5 it will result in an increase of cm (corresponding to a tide level of,7 m) and an increase of cm for the year (corresponding to a tide level of 5, m). Applying equation, will result, to an exponential increase of saline inflow. CONCLUSIONS The inflow of saline waters to urban drainage systems is a problem in coastal zones, having a tendency to worsen due to climate change scenarios. According to the methodology presented by A.Kumar et al.() to the salinity ranges, it is now possible to classify obtained values. The pumped effluent in the pumping station EE for tide levels of, m,, m and, m is in the range of low salinity, as expected. The pumped effluent in the pumping station EE for tide levels higher than,6 m is in the range of high salinity with potential negative impacts for the system. Taking into account the obtained results it was concluded that for a tide level of,8 m, the waters starts entering the system, but only for tide levels of,6 m the volume of inflow salt water becomes relevant. Between the peak tide time and the peak of electric conductivity in EE, there is a time lag, justified by the wastewater travel by each of the three drainage lines. In order to connect the saline water inflow with the tide levels, an equation using values of electric conductivity, average daily volume and average flowrates resulted in a way that, for a tide level of, m the saline water inflow is 8 m. For a tide level of, m the saline water inflow is 5 m. For a tide level of, m the saline water inflow is 5 m. For a tide level of,6 m the saline water inflow is 779 m. For a tide level of m the saline water inflow is 6 m and for a tide level of, m the saline water inflow is 6978 m. The empirical expression obtained allows to make a saline water inflow estimation for any tide levels and estimate annual values of such inflows. For 6, it was estimate that the saline water inflows, through equation, volume corresponds to 8% of the total volume pumping in EE. The saline water inflows volume is maximum when the tide level reaches its maximum, which for the year 6 is, m relative to the hydrographic zero. Over the year there are two tides with, m which corresponds to about 5% of the volume pumped at EE. It is important to know the impact of sea level rising in the urban drainage systems. An increase in the average level of the seawater of cm by 5 and m in, will result, applying equation, to an exponential increase of saline inflow with environmental, social and economic 9

10 impacts. Naturally, through precipitation intensity and sea level rising, saline inflows will have many other significant impacts on system performance, especially in times of rain. After analyzing the results, it is necessary to give note to some uncertainties on this study, as the tide level referred is a tabulated value and may not correspond to the real value. And because of the high travel times along the interceptors, that lead to difficulties and complexity when analyzing the obtained results. REFERENCES A.Kumar, Bohra, C., & L.K.Singh. (). Environment, Pollution and Management. Águas de Lisboa e Vale do Tejo, D. (6). Entradas de maré no sistema intercetor de Alcântara. Antunes, C. (7). Previsão de Marés dos Portos Principais de Portugal. Obtido de Antunes, C. (6). Subida do nível médio do mar em Cascais, revisão da taxa anual. David, M., Barata, P., Pires, R., Pécurto, C., Álvaro, P., Coelho, P., & Martins, J. (6). Deteção e identificação de entradas de marés no subsisteama de Alcântara. EPAL, G. Á. (6). Qualidade da água para consumo humano na rede de distribuição de Lisboa. Hidra. (6). Estudos e Projectos para a frente de drenagem Algés-Alcântara, das infraestruturas desde o descarregador D até EE. LNEC. (6). IAFLUI. MARETEC. (). Sistema Operacional para o Estuário do Tejo. Obtido de MOHID: Pimentel, N., Duarte, F., Ferreira, F. S., & Matos, J. S. (6). Avaliação de Caudais na Frente de Drenagem Algés-Alcântara e Análise Preliminar de Afluência de Águas Salinas - º Relatório de Progresso. Pimentel, N., Duarte, F., Póvoa, P., Ferreira, F., & Matos, J. S. (6). Afluências Indevidas a Sistemas de Drenagem em zonas costeiras: O Interceptor Algés - Alcântara.