Hydrogeological evaluation of Valongo (Porto), Paredes (Porto) and Arouca (Aveiro) areas, based on spring hydrograph analysis

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Hydrogeological evaluation of Valongo (Porto), Paredes (Porto) and Arouca (Aveiro) areas, based on spring hydrograph analysis Eduardo Gonçalves PhD student of Geosciences (concluded in 2013)

1 Hydrogeological evaluation of Valongo (Porto), Paredes (Porto) and Arouca (Aveiro) areas, based on spring hydrograph analysis Eduardo Gonçalves PhD student of Geosciences (concluded in 2013) Departamento de Geociências, Ambiente e Ordenamento do Território da Faculdade de Ciências da Universidade do Porto Abstract The present paper contains a brief quantitative hydrogeologic assessment, established by the spring hydrograph analysis of gravitational runoff for different water sources (water points), more precisely, three water mines and one natural spring. This assessment was developed though a standard hydrologic year (between October 2009 and September 2010). The analysis of the main recession periods allows the determination of different recession coefficients, which were the basis for the calculation of groundwater storage amount for the considered water points. Keywords: Runoff, recession coefficients, storage capcity. 1. Introduction One of the main activities related to the conception of a regional hydrogeologic research it is the hydrodynamic characterization (or hydrogeodynamic characterization) which can include a wide range of processes. These actions have the intent to assess the qualitative and quantitative hydrogeologic potential of an area or region. Through different lithologies of Valongo Anticline, the hydrogeologic research included an implementation of a quantitative method which involved spring and water mine hydrograph s analysis. These lithologies are essentially schists and greywackes of Montalto Unity (Precambrian y/or Cambrian), and usually slated schists of Valongo Formation (Middle Ordovician), both defined by Medeiros et al (1964, 1980). Effectively the assessment, based on spring hydrographs, it is a method only available to natural springs and water mines, this because that method make use of gravitational runoff, processed through a some period of time. In essence, this technic allows to understand if exists a quick answer (or not) for direct recharge (units of volume per time) generated by the rainfall, mainly at the extreme precipitation periods (Macedo & Lima 2007). Therefore, it will be necessary to consider the recession periods in the hydrographs for, following, to make the cross-checking between natural runoff and daily precipitation data in a hydrologic year, from local/regional climate stations. 2. Determination of recession rates and stored water volumes Like previously mentioned, 2009/2010 was the hydrologic year considered, between October 2009 and September However, as we will see, this period comprise slight variations, in matter of days, for each hydrograph (Fig. 2). For Valongo and Paredes area, were considered tree sources of water (water points), more precisely tree mines, implanted on distinct stratigraphical unities: two sources in the schists of Meddle Ordovician, and one third source in the schists of Precambrian y/or Cambrian. For the Arouca (Aveiro) area was only considered one water source, a natural spring localized in the schists and greywackes of Precambrian y/or Cambrian (Medeiros et al 1964, 1980). 1

Fig. 1. Geologic maps widh localization of studied water points (Adapted by Geologic chart of Portugal, sheets: 1 (1:200.000), 9-D and 13-B (1:50.000) Serviços Geológicos de Portugal.

2 Fig. 1. Geologic maps widh localization of studied water points (Adapted by Geologic chart of Portugal, sheets: 1 (1: ), 9-D and 13-B (1:50.000) Serviços Geológicos de Portugal. In all of hydrographs presented its possible to see relevant fluctuations in what concern of natural runoff, which are directly correlated to the variations in precipitation rates. The periods with highest rates of groundwater runoff reproduces a natural answer of the middle to episodes of more vertical recharge, derived by the precipitation. Reciprocally, the recession periods correspond to a quick reaction of lower (or absence) precipitation. It is in these times, of hydraulic head recession, which is possible determine the recession coefficients. According to Wanielista (1990) in Macedo & Lima (2007) the recession coefficients are rarely constant through time, insofar as it is influenced by the regional characteristics of saturated geological materials. Geological maps analysis (Fig. 1.) allows a good perception of lithological diversities present in Valongo Anticline, which reflect the strong hydrogeologic heterogeneity of geologic middle, were are present lateral variations in terms of hydraulic parameters like: hydraulic conductivity, trasmissivity and storage coefficient. However, these and the other hydraulic parameters are essentially controlled by tectonic fracturing and the weathering. Surface geologic surveys, realized on different areas of Valongo Anticline, allows remark different fracturing systems, with different degrees of opening, roughness and fil, but also different tectonic orientations. This situation can be seen in many places, even in the places nearly of the natural springs and water mines considered. This data suggests the presence of hydrogeological middles able to fast water runoff events through the open and permeable fractures, which eventually given origin to quick drops in terms of natural flow rates. On the other hand, the slow drops in water flow will be linked with the more closed fractures (with less permeability) of the middle, allowing smaller runoff. 2

3 Fig. 2. Spring hydrographs prepared for the studied water points. 3

4 The different water points were target of recharge events assessment, based precipitation data, collected in tow climatic local stations: Barragem de Castelo Burgães (Vale de Cambra Aveiro) and Pedras Rubras (Maia Porto). In represented hydrographs (Fig. 2) it can be seen a marked parallelism between the precipitation rates and increases of gravitational runoff. Hydrograph analysis also permits to outline the periods of hydraulic recession. According to Castany (1975), the hydraulic discharge can be explained by the using an exponential low that well-adjusted with Maillet equation (1905): Qt = Q 0 e -αt (1. 1.) were: - Qt = natural flow at any point of recession (m 3 /day); - Q 0 = natural flow at beginning of recession (m 3 /day); - α = recession coefficient (day -1 ); - t = recession interval (t 0 ) and the time (= t t 0 ), express in days; - e = 2, Developing the equation we have: log Qt = log Q 0 (αlog e)t (1. 2.) As the log of e is about 0, 4343, the expression to define the recession curves, based on the calculation of aquifer depletion coefficient (α), is: α = (log Q 0 log Qt)/0,4343t (1. 3.) This equation is suitable for the majority of the Valongo Anticline metassedimentary hydrogeological units, insofar as it is usually used in hydrological studies marked to low permeability of rocks. Fig. 3. Recession periods of spring hydrographs with respective recession coefficients. Maillet equation assumes the natural flow as a water function stored volume at the beginning of recession, the physical patterns (trasmissivity and storability) and geometry of aquifer (aquifer extension). According to Castany (1975), very low values of recession coefficient, indicates an hydrogeological middle with good storability. On the other hand, higher levels of depletion coefficient indicate reduced storability. 4

Achieving the recession constant (or coefficient), there are finely the conditions to infer the amount of stored water above base flow for any moment (Vt) by the using runoff flows, through following

5 Achieving the recession constant (or coefficient), there are finely the conditions to infer the amount of stored water above base flow for any moment (Vt) by the using runoff flows, through following expression: (1. 4.) In present study one the moments considered for the assessment of the stored water was the ends of recession periods. This option had the propose to obtain an estimate of a minimum (annual) stored water above base flow. However, the stored water volume in the aquifer, at the initial moment of each recession period, has particular importance, which can be determined by the following expression: (1. 5.) Indeed, the value V 0 represents the storage capacity of a watershed, which is equivalent to the regulatory reserves. According to Castany (1975), represent the volume of free water stored in an aquifer, throughout a period of natural recharge. A significant part of free water volume is represent the exploitable resources of an aquifer. 5

6 Final remarks A first final observation about this study highlights the fact that in all water points have high average values of recession coefficients. It was only the values registered in Aguiar de Sousa Mine were found values of recession coefficient adjustable to the rates usually found in fractured meddles. According to Costa (1997), recession coefficients up to 10-2 are usually recorded in karst environments. This situation suggest the presence of hydrogeologic middles with lower storage capacity for which, besides fracturing (at different scales), should to contribute factors like: primary porosity, the presence of cavities (derived from sulphides dissolution), weathering and evapotranspiration. In relation to the average volumes of stored water in aquifers, at the initial moments (V 0 ), it appears that the values recorded in the mines of Campo and Aguiar de Sousa approach the recorded value by Macedo & Lima (2007), at the fractured granite middles found in Vieira do Minho (Braga), more precisely 6,53x10 3 m 3. Though, if on the one hand, in Valongo mine has found a storage capacity considerably higher than that recorded at Vieira do Minho, by Macedo & Lima (2007), in Arouca that value was significantly lower. Bibliography CASTANY, G. (1975), Prospección y Explotación de las Aguas Subterrâneas. Ediciones Omega, S. A. Barcelona. COSTA, A. (1997), Sistema Aquífero Moura Ficalho. Parte 4 Quantificação das descargas do aquífero principal. pp MACEDO, L. F. V. & LIMA, A. S. (2007), Recursos Hídricos Subterrâneos em Aquíferos Graníticos Pouco Profundos: Vieira do Minho (NW Portugal). Actas do 8º Simpósio de Hidráulica e Recursos Hídricos dos Países de Língua Oficial Portuguesa. pp. 20. São Paulo. Brasil. MEDEIROS, A. C, (1964) Carta geológica de Portugal na escala de 1/ Noticia explicativa da folha 13-B (Castelo de Paiva). 58pp. Serviços Geológicos de Portugal. Lisboa. MEDEIROS, A. C, PEREIRA, E. & MOREIRA, A., Carta geológica de Portugal na escala de 1/ Notícia explicativa da folha 9-D (Penafiel). 46pp. Serviços Geológicos de Portugal. Lisboa. POEHLS, D. J. & SMITH, G. J. (2009), Encyclopedic Dictionary of Hydrogeology. First edition. Elsevier. 6

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