Conceptual and Numerical Flow Model of the Sines Aquifer System (Alentejo, South Portugal) J. P. MONTEIRO Algarve University Geo Systems Centre UALG/ CVRM, Marine and Environmental Sciences Faculty, Campus de Gambelas 8005-139 Faro, Portugal e-mail: [email protected]

A. CHAMBEL Geophysics Centre of Évora, Department of Geosciences, University of Évora, 7002-554 Évora, Portugal

J. MARTINS Algarve University Geo Systems Centre UALG/ CVRM, Marine and Environmental Sciences Faculty, Campus de Gambelas 8005-139 Faro, Portugal

Abstract Two separate finite-element flow models are presented as synthetic representations of semi-independent conceptual hydrogeologic models of the 250km2 multilayer Sines Aquifer System. The regional flow pattern of the top detritic aquifer is analysed, considering the influence of hydraulic connection with the stream network, and the variation of the regional values of transmissivity. The deep confined carbonate aquifer is hydraulically connected with the ocean. The spatial distribution of transmissivity was characterized by inverse modelling, thus allowing a reliable simulation of the hydraulic functioning of this artesian aquifer. In addition to the analysis of parameters and boundary conditions controlling the regional flow pattern, the implemented flow models are valuable tools for water management of the Sines aquifer system. Key words: multilayer karstic and detritic aquifers, superficial-groundwater interactions, inverse modelling

INTRODUCTION The Sines Aquifer, with an approximate area of 250km2, is located about 80km south of Lisbon. The socio-economic relevance of this multilayer Aquifer System is linked to the support of the urban water supply of the Sines, Santiago do Cacém and Grândola municipalities. Despite the importance of the abstraction volumes in these municipal wells, most of the detected 200 boreholes in the area are used for farming (agriculture and cattle breeding). Water abstractions are also important for the industrial area of Sines municipality, including a refinery and other chemical industries. Agriculture (cattle and irrigation) and the chemical industry are also activities threatening water quality. In addition to economic interests, groundwater management in this area is also of great relevance, due to the ecological aspects, of the river aquifer interaction, and consequently the role of the top phreatic aquifer in the control of the freshwater balance of the Melides, Santo André and Sancha lagoons in the coastal area of the aquifer. These rich fauna and flora coastal lagoons must therefore be considered as groundwater dependent ecosystems. The identification of the Sines Aquifer as a multilayer aquifer system was reported in several technical documents in the 80’s and 90’s, and described the regional

hydrogeologic conditions (Horta da Silva & Almeida, 1982; Rodrigues, 1985, Costa, 2004). Several studies describing the hydrochemistry of the Sines Aquifer system were done resulting in the characterisation of hydrochemical facies and water quality. (Rodrigues, 1985; Lavaredas & Silva, 1998, 1999; Fernandes et al., 2006a, 2006b). The typical mineralization of the water in the top phreatic aquifer is lower than that in the deep carbonate aquifer, as shown by the typical values of electrical conductivity (EC) around of 200 - 500 μS/cm in the top aquifer, increasing to values in the order of the 600 - 1000 μS/cm in the deep aquifer. Intermediate compositions between the hydrochemical facies identified in the deep confined carbonate aquifer and in the top detritic phreatic aquifer can be explained as a mixture of these end-members as stated by Fernandes et al. (2006b).

GEOLOGIC SETTING AND HYDROSTRATIGRAPHIC UNITS The sedimentary infill of the Meso-Cenozoic intracratonic Sines basin, supporting the multilayer Sines Aquifer, reaches a thickness of more than 1000m. The basin is in contact with low permeability Carboniferous flysch turbidites in the south and the east and with the subvolcanic Sines massif in the southwest. The Grândola Fault defines the northern boundary of the basin. The oldest sediments in the Sines Basin consist mainly of red continental sandstones and mudstones (Silves Formation). Above this, Triassic siliciclastic rocks, with an average thickness from 80 to 120m, a sequence of Hettangian-Rhaetian evaporites clay and marls (80m) and dolomites (15-40m) were deposited. Sinemurian-Hettangian deposits (130m) are dominated by marls, clays, dolostones and calcareous rocks intruded by basaltic and doleritic dikes and are followed by the Jurassic carbonate rocks. These support the most important and deeper regional aquifer of the basin, which is artesian in an important part of its area. The sequence of these carbonate rocks, starts with Early Jurassic (Toarcian-Sinemurian) limestones, dolostones and marls (100m). It is followed by Middle Jurassic (CallovianBathonian) limestones (250m) and ends with 600m of Late Jurassic limestones, marls and conglomerates (Oxfordian-Kimmeridgian). Due to the absence of Cretaceous rocks (except for the off-shore part of the basin where sediments of this age were identified) the Miocene deposits follow the Jurassic rocks in the sedimentary sequence (Inverno et al., 1993). Outcrops of Miocene lithologies are scarce in the Sines Basin and are represented by sands, silts sandstones and biocalcarenites. The thickness of the Miocene deposits is in the order of 30m, and was identified in boreholes under PlioPleistocene and Holocene deposits, at a depth varying between 8m and 40m (Oliveira et al., 1984). The Plio-Pleistocene silt and clay sands, together with beach and dune sands are the most important deposits covering the Jurassic rocks. These detritic rocks are present in the western coastal plain, which is limited by the ocean in the west and by the Santiago do Cacém Heights (with an altitude between 50 and 200masl) in the east. These heights, built by early and middle Jurassic carbonate rocks outcrops, mark the limit of the Mesozoic formations of the Sines Basin and constitute the main recharge area of the deep carbonate aquifer. The area with outcrops of late Jurassic carbonate rocks is very limited. These rocks occur in the coastal plain, covered in most of its area by the Miocene, Plio-Pleistocene and Holocene sediments. The Holocene deposits, associated with lowlands at the Melides (0.4km2), Santo André (2.5km2) and Sancha (0.2km2) lagoons, were deposited in the terminal reaches of the Melides, Ponte, Badoca and Sancha creeks. These deposits consist essentially of detritic, minerogenic and organic sediments arranged in several units and attain a

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maximal thickness of about 40 m (Freitas et al. 2002). These deposits limit the hydraulic connection between the lagoons and the shallow detritic aquifer, due to the fine fraction of the sediments in the lagoons. However, as the rivers associated with the lagoons aquifer are effluent in the terminal reaches, where sediments are coarser than in the bottom of the lagoons, the water balance of the lagoons is controlled by the hydraulic connection of the stream network with the shallow aquifer.

TRANSLATION OF THE CONCEPTUAL FLOW MODEL IN NUMERICAL FLOW MODELS The external boundaries used for the generation of the finite element mesh are the limits of the Sines Aquifer System defined by Almeida et al. (2000). The proposed conceptual flow model was defined considering the existence of two permeable layers, according to the geology, structure and geometry of the hydrostratigraphic units

s ntainha Ribª Fo

a

e Melid Ribª

Lagoa de Melides

Ribª Ponte

Lagoa de Stº André Ri

b

s

B bª

Rib ª

a oc ad

Lagoa da Sancha

Sa nch a

Ribª Moinhos 0m

3500m 7000m

Fig. 1 Geometry of the flow model, finite element mesh with 13966 nodes and 2713 triangular linear elements. Areas of recharge considered for the top phreatic detritic aquifer (a) and deep carbonate artesian aquifer (b).

described in the previous section. Part of the aquifer (about 155 km2 - shaded area a in Fig. 1) corresponds to a deep confined artesian aquifer, without hydraulic connection with the top detritic aquifer. In the remaining area (95 km2 - shaded area b in Fig. 1) the two main layers of the aquifer are hydraulically connected, allowing the recharge

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of the deeper carbonate aquifer directly in outcrops of the carbonate rocks and across the detritic rocks of the top aquifer. Thus, in this area, the two aquifers are hydraulically connected. The boundary between the two shaded areas in Fig. 1 was defined according to two factors: The northern part of this limit corresponds to the geometry of the Santo André Fault, with a direction NNE-SSW. The inflection of this line to the W does not match with a well defined geologic structure and was defined in order to delimit the deep aquifer in such a way that the defined boundary defines the limit of the occurrence of flowing artesian wells abstracting in the deep carbonate aquifer. The lithologies supporting the artesian aquifer (Jurassic carbonate rocks) are present in outcrops in the East border of the system, and constitute the area with the highest recharge rates. In the area where the occurrence of artesian wells is common, near the coastal strip, the confining unity of the deep artesian aquifer underlies a superficial detritic phreatic aquifer in hydraulic connection with the stream network. It must be noted that the hydraulic connection between the two main hydrostratigraphic units of the aquifer system can take place only in the shaded area b, corresponding to the recharge area of the deep confined aquifer (Fig. 1). According to the assumed recharge areas, the water balance in the simulations comes to 32×106m3⋅year-1, and corresponds to 12×106m3⋅year-1 in the 95km2, of the recharge area of the deep aquifer and 20×106m3⋅year-1 in the remaining 155km2, corresponding to the recharge area of the top detritic aquifer. These values correspond to the recharge rates calculated by Rodrigues (1985). The physical principles in the basis of the simulation of the hydraulic behaviour of the aquifer system are expressed by the equation (1): S

(

)

∂h + div − [T ] grad h = Q ∂t

(1)

Where: T is transmissivity [L 2 T -1], h is the hydraulic head [L], Q is a volumetric flux per volume unit [L 3T -1L -3], representing sources and/or sinks and S is the storage coefficient [-]. The solution of equation 1 was implemented using a standard finiteelement model based in the Galerkin method of weighted residuals.

Sensitivity Analysis of transmissivity for the top detritic aquifer Numerical modelling was used as a tool for the analysis of the influence of transmissivity in the balance of water transferences between each of the rivers of the stream network and the top detritic aquifer. The total balance of these transferences for each river, as well as the dimension and position of the influent and effluent reaches in each water course, depends on the regional transmissivity values. A batch procedure was implemented to study the sensibility of these changes. To do this, 24 simulations were performed varying the values of transmissivity T between 50 [m 2 d -1] and 1200 [m 2 d -1] at an interval of 50 [m 2 d -1]. The rivers were represented in the model as imposed heads, corresponding to the river levels in the corresponding nodes of the finite element mesh. These transferences were studied for each of the previously mentioned Melides, Badoca and Sancha, rivers, and are characterized by the presence of the coastal lagoons of Melides, Santo André and Sancha, installed in their respective terminal areas. The same calculations were performed for the Fontaínhas and Moinhos rivers, located respectively in the north and south terminal areas of the Sines Basin, at

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higher altitudes than the rivers with associated coastal lagoons in the central area of the basin. The results of these simulations are represented in Fig. 2 and Fig. 3. The obtained results show unequivocally that the regional values of transmissivity control

0m

5000m

10000m

Fig. 2 Piezometric maps of the top detritic aquifer for values of transmissivity of 100 [m 2 d -1] (left) and 1000 [m 2 d -1] (right). 0.5

Ribª das Fontainhas

0.4 0.3 0.2

Q (m3.s-1)

0.1

Ribª de Moinhos

0 -0.1 -0.2

Ribª De Melides (Lagoa de Melides)

-0.3 -0.4 -0.5

Ribª Sancha (Lagoa Sancha)

-0.6 -0.7 -0.8

Ribª Badoca (Lagoa de Stº André)

0

200

400

600

800

1000

1200

Transmissivity (m2.day-1)

Fig. 3 Representation of the water balance of the rivers in hydraulic connection with the top detritic aquifer. Negative values represent transferences from the aquifer to the river and vice-versa.

the balance of transferences between the top detritic aquifer and the stream network in the Sines Basin. It can be seen that the minimum values of transmissivity compatible with a realistic balance of transferences, matching the observed behaviour of the real system, are in the order of T ≈ 1000 [m 2 d -1]. For lower values, the Fontaínhas and Moinhos rivers tend to show a global effluent balance (when in reality they present mainly an

80

influent behaviour) and the Melides, Badoca and Sancha lagoons present very slight effluent balances. These are not compatible with their main effluent behaviour and are clearly demonstrated by the presence of wetlands in their terminal reaches, where the phreatic level intercepts the topographic surface. The results of these simulations are very useful to interpret the piezometric maps drawn with data of wells implanted in the top aquifer, and which often reveal apparently different “erratic local flow directions”. This can easily be explained as being a result of a groundwater flow pattern mainly controlled by the hydraulic connection with the stream network.

Inverse calibration of transmissivity for the deep carbonate aquifer The deep carbonate confined aquifer system is characterised by the occurrence of flowing artesian wells, with natural discharges attaining maximum values in the order of 100 l/s in the shaded area a represented in Fig. 1. In the deep confined aquifer, the regional flow is essentially from east to west, in the direction of the continental platform adjacent to the coastal plain. The detailed analysis of the available piezometric data, resulting from the more detailed dataset of water level measures in the deep carbonate aquifer, shows a spatial distribution of hydraulic head more complex than the simple decrease in the direction of the sea. In fact, a large number of observation points present fluctuations around this regional trend, showing that there are sectors of the Jurassic carbonate rocks with different degrees of connectivity with the regional discharge area of the deep aquifer in the offshore. The model for the deep aquifer was implemented with the same finite element mesh shown in Fig. 1. However, in this case, the reliability of the simulation of the spatial distribution of hydraulic head is of special importance, due to the need to perform simulations allowing the support of decisions for aquifer management, including the definition of wellhead protection areas of water wells (Chambel & Monteiro, 2007; Monteiro & Chambel, submitted). In this case, the model again used equation (1) in the definition of adequate parameters and boundary conditions. However, in this case, the calibration of transmissivity was done by inverse modelling. The Gauss-Marquardt-Levenberg was used and implemented with the nonlinear parameter estimation software PEST (Doherty, 2002). The zonation defined for the estimation of transmissivity is shown in Fig. 4. Individual values of transmissivity (T) were defined for 31 zones where the behaviour of piezometers allows a reasonable fitting of field data using a single value for this parameter. The optimisation of the results was based in about thousand model runs performed by inverse modelling. Several variants of the model were tested to search for the best possible reproduction of the aquifer equipotential surface, accommodating values of water balance. An additional aspect of the flow domain parameterization consisted in the definition of restricted zones in the vicinity of individual wells of special importance abstracting in the deep aquifer. In these cases, the hydraulic behaviour of these wells can be analysed for scenarios where the considered hydraulic parameters were approached by the use of values determined at the local scale. This was done in order to “force the model” to present local responses, similar to the ones obtained by analytical models for the analysis of the flow domain at the well scale. The spatial distribution of the observation points, available for the analysis piezometric levels is not representative of the entire flow domain. However, as shown in Fig. 4, the implemented and calibrated model can be considered as a reliable synthetic representation of the real aquifer system. The average error in the 68

81

piezometers, used for the calibration of the model is 7.4m. This value can be

200

Simulated hydraulic head, in meters about sea level

180 160

a

140

c

120 100 80 60 40 20 0 0

20 40 60 80 100 120 140 160 180 200

Hydraulic head computed from measurements, in meters above sea level

6 7

b 9 8 16

17 5 4 32 1

15 14 13 12 11 10 28 27 26 25

24 23 22 31 21

19

18

20

29

0m

5000m

10000m

Fig. 4 Observed versus calculated hydraulic head (a); zonation of the flow domain used for inverse calibration of transmissivity (b) and equipotential lines drawn from field data (black) and from values obtained using the regional finite element model (red) after the inverse calibration (c). Blue circles are observation wells. Field data sets collected during field work in August 2005 and February 2006.

considered as reasonable, as the amplitude of the values registered in datasets is in the order of 190m. CONCLUSIONS

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The analysis of simulations with different values of transmissivity for the top aquifer allowed the identification of the minimal values of transmissivity compatible with the occurrence of effluent watercourses in the coastal area in the central area of the basin. Therefore, the regional values of transmissivity control the fluvial input, and thus the water balance of freshwater of the coastal lagoons of Sancha, Melides and Santo André lagoons, allowing the classifications of these lagoons as groundwater dependent ecosystems. On the other hand, the inverse calibration of transmissivity, based on piezometric data obtained in deep observation points, allowed a reliable simulation of the hydraulic behaviour of the deep carbonated artesian aquifer under different stress conditions. The models presented in this paper are thus a valuable tool for the management and protection of water resources in this area.

Acknowledgements This work was partly funded by the Portuguese Foundation for the Science and Technology (Project POCTI/AMB/57432/2004) and by the Águas de Santo André SA.

REFERENCES Chambel, A. & Monteiro, J.P. (2007) Sistema Aquífero de Sines – Estudo Hidrogeológico do Sistema Aquífero e Definição dos Perímetros de Protecção das Captações de Água Subterrânea das Águas de Santo André SA, com recurso a Modelação Matemática [Hydrogeologic Study and Definition of the Wellhead Protection Zones of the Águas de Santo André SA using a Mathematical Model]. Águas de Santo André SA., Relatório Técnico [Technical Report], 83 pp. Costa, Q. (1994) Execução de um Furo de Pesquisa e Eventual Captação de Água Subterrânea Destinado ao Abastecimento das Águas de Santo André. Furo ADSA2 – Judia. [Drilling of a Prospecting Borehole and Water Well for Water Supply of the Águas de Santo André. Well ADSA2 – Judia]. Gabinete Técnico de Stº André – INAG Relatório Técnico [Technical Report], 11 pp. Doherty, J. (2002) PEST, Model-Independent Parameter Estimation. 4th Edition, Watermark Numerical Computing, Australia, 279 pp. Fernandes, M. R. C. (1984) Execução e Ensaio da Sondagem de Captação JKC10 (Empreitada DPSB-84/83) [Drilling and Execution Well Tests of the Borehole JKC 10]. Gabinete da Área de Sines. Relatório Técnico [Technical Report], 17 pp. Fernandes, P. G., Carreira, P. & Silva, M. O. (2006a) Identification of Anthropogenic Features Through Application of Principal Component Analysis to Hydrochemical Data from the Sines Coastal Aquifer, SW Portugal. Mathematical Geology. Volume 38, Number 6, 765-780. Fernandes, P. G., Carreira, P. & Silva, M.O. (2006b) Geochemical Modeling Through Groundwater Mineralization Appraisal: Sines Coastal Aquifer, SW Portugal. In: International symposium – Aquifers Systems Management, June 2006, Dijon, France, 8 pp. Freitas, M. C., Andrade, C., Cruces, A., Amorim, A., Cearreta, A. & Ramalho, M. J. (2002) Coastal Environmental Changes at Different Time-scales: the Case of the Melides Barrier-Lagoon System (SW Portugal). In: Littoral 2002, The Changing Coast. EUROCOAST / EUCC, Porto, 397-402. Horta da Silva, J. A. & Almeida F. (1982) Condições Hidrogeológicas na Área de Sines [Hydrogeologic Conditions in the Area of Sines]. Gabinete da Área de Sines, Laboratório de Geotecnia e Materiais de Construção. Technical Report nº 22/82, Vol. I and II. Inverno, C. M. C., Manuppella, G., Zbyszewski, G., Pais, J. & Ribeiro, M. L. (1993) Carta Geológica de Portugal na Escala 1/50 000 e Notícia Explicativa da Folha 42-C SANTIAGO DO CACÉM [Geologic map 42-C, Santiago do Cacém, at the scale 1:50,000 and Explanatory Text]. Serviços Geológicos de Portugal, Lisboa, 75 pp. Lavaredas, J. M. & Silva, M. O. (1998) Contribuição para o Conhecimento Hidrogeológico do Sistema Aquífero de Sines [Contribution to the Hydrogeologic Know-how of the Sines Aquifer System]. In: Proceedings do 4º Congresso da Água “A Água como Recurso Estruturante do Desenvolvimento”, APRH, Lisboa, 17 pp. Lavaredas, J. & Silva, M. (1999) Sines's aquifer system - Some Contribuitions for it’s Knowledge. International Association of Hydrogeologists. In: Proceedings of XXIX IAH Congress. Bratislava, CD-ROM, 861-865. Monteiro, J. P. & Chambel, A. (submitted) Contribution of a regional numerical flow model for the definition of wellhead protection zones in the Sines Basin. In: XXXVI Congress of AIH. Toyama. Japan. Oliveira, J. T., Andrade, A. S., Antunes, M. T., Carvalho, D., Coelho V. P., Feio, M., Gonçalves, F., Manuppela, G., Marques, B., Monteiro, J. H., Munhá, J., Ramalho, M., Rey, J., Ribeiro, A., Rocha, R. & Zbyszewsky, G. (1984) Carta Geológica de Portugal na Escala 1/200 000 e Notícia Explicativa da Folha 7 [Geological Map of Portugal Number 7 at the scale 1: 200,000 and Explanatory Text]. Serviços Geológicos de Portugal, 77 pp.

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Rodrigues, J. D. (1985) Estudo da Qualidade das Águas subterrâneas da Bacia de Santo André [Study of the Groundwater Quality of the Santo André Basin]. Laboratório Nacional de Engenharia Civil – LNEC, Relatório Técnico 177/ 85 – NP [Technical Report], Lisbon, 26 pp.

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Conceptual and Numerical Flow Model of the Sines ...

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