Hydrogeomorphological mapping as a tool in groundwater exploration

Abstract

Hydrogeomorphological maps have a useful importance in exploration hydrogeology, engineering geosciences, geotechnical engineering and planning. The role of geomorphology is decisive to correctly evaluate groundwater resources. Hard-rock hydrogeological systems commonly exhibit complex geological and morphological features. This study highlights methodological guidelines for the preparation of hydrogeomorphological maps to support groundwater conceptual modelling, as well as for hydrogeological surveys and environmental sustainability issues. Cartographic techniques can provide an accurate way of improving the knowledge on groundwater and surface water circulation and the overall functioning of aquifer systems. A comprehensive evaluation of these subjects has been completed during the preparation of hydrogeomorphological maps for the Alardo groundwater system and Touca hydromineral system, both located on Gardunha mountain in Central Portugal. Thematic maps were prepared mainly from satellite imagery analysis, topographic, geological, geomorphological and hydrogeological field surveys. This information was presented to outline the recharge potential areas and infiltration rates. The paper also contributes to hydrogeomorphological mapping design and the conceptual model of groundwater in fractured hard-rock aquifer systems.

Keywords:

• GIS cartography
• applied geomorphology
• groundwater
• hydrogeomorphology
• engineering geosciences
• central Portugal

1. Introduction

Hard-rock aquifer systems are an important source of water for domestic, industrial, agricultural, and public supply purposes. Geology, geomorphology, climatology and hydrological properties control groundwater flow, occurrence and storage (Jaiswal, Mukherjee, Krishnamurthy, & Saxena, 2003; Surrette, Allen, & Journeay, 2008; Teixeira et al., 2010). Availability of groundwater that occurs in a given geological medium is totally dependent on recharge and discharge areas. Rainfall is the main source of recharge for existing discontinuous aquifers, while discharge depends on terrain slopes, water table gradients and also on ground hydrogeologic conditions (Freeze & Cherry, 1979; Sophocleous, 2002).

Hydrogeomorphology is an emerging interdisciplinary scientific field, which studies the relationships between geomorphology and hydrology (surface water/groundwater). In a general sense, it links together several fields related with geosciences, hydrology and physical geography, such as geology, hydrogeology, geomorphology, remote sensing, applied geophysics, soil and rock geotechnics, climatology and natural hazards (e.g. Babar, 2005; Bisson & Lehr, 2004; Dunne, 1994; Okunishi, 1991, 1994; Sidle & Onda, 2004; Teixeira et al., 2010; Tricart, 1961; Tricart, Cloots-Hirsch, & Griesbach, 1965). Hydrogeomorphology operates in an interdisciplinary framework focused on the relationship between hydrologic processes with Earth materials and the interaction of geomorphic processes relating surface water/groundwater flow regime (e.g. Babar, 2005; Cloots-Hirsch & Tricart, 1978; Gregory, 1979; Kudrna & Šindelářová, 2006; Leopold, 1982; Scheidegger, 1973; Sidle & Onda 2004; Teixeira, 2011; Teixeira et al., 2010; Tricart, 1958, 1961; Tricart et al., 1965).

In this paper, we present a methodological geographic information system (GIS)-based cartographic approach for the assessment of groundwater systems. Two examples are given: a normal groundwater site, and a hydromineral site. This procedure is based on techniques related to remote sensing and hydrogeomorphological mapping, combined with a hydrogeological inventory fieldwork and groundwater well features. Nevertheless, the interactions with the surface and subsurface hydrologic components were not neglected, as well as the influence of climate, geomorphology and geology. Hydrogeomorphological mapping and several derived thematic maps were created in order to outline the potential groundwater infiltration areas. This cartographic approach contributes to the development of hydrogeological conceptual models of fractured hard-rock terrains (see the main map).

2. Study area

The Alardo and Touca sites are located near the Gardunha mountain system, in central Portugal, within the Iberian Massif (Carvalho, 1996; Ribeiro et al., 2007; Figure 1). Both are well-known locations of hydrogeological and economic importance (Alardo is a mineral water bottling factory and Touca a thermal bath spa). The mountainous ridge divides the Covilhã and Fundão urban areas (Figure 2A and B). This ridge system trends NE-SW and is part of an extensive tardi- to post-Variscan granitic batholith (Oliveira, Pereira, Ramalho, Antunes, & Monteiro, 1992). The quartz, aplite and pegmatite veins outcrop in this area following the regional fault trends. The granitoids are surrounded by metasedimentary rocks, namely the schist-greywacke complex (Gama Pereira, 1976). The sedimentary deposits have a very low extension and thickness and are related to the water courses sediment dynamics (Figure 2C).

Figure 1. Hydrogeological background of the study areas: Alardo and Touca sites (adapted from Carvalho, 2006; Carvalho, Espinha Marques, Afonso, & Chaminé, 2007).

Figure 2. Regional framework of the study area (Alardo and Touca sites): (A) Morphotectonic general features from Northern Portugal (adapted from Brum Ferreira, 1991); (B) Satellite image (compiled from Landsat 7 ETM + data, 2000/01; all IR colour, bands 7-4-5 = RGB; adapted from Global Land Cover Facility) and main hydromineral springs (adapted from Carvalho et al., 2007); (C) Shaded relief and regional hydrogeology (adapted from Carvalho et al., 2007); (D) Slope of the region.

The Gardunha mountain system belongs to a wide range of ridges with a length of approximately 60 km, with a major trend NE-SW – the so-called ‘Cordilheira Central’ or Central Range (Brum Ferreira, 1980; Ribeiro, 1949; Thadeu, 1949). This mountain rises from a large flattened area (Figure 2B and D), at altitudes between 400 and 450 m, extending from its foothills to over 20 km south, near Idanha-a-Nova urban area. The North is bordered also by a flattened region, named ‘Cova da Beira’ (Ribeiro, 1939, 1949), which extends from Gardunha ridge, to the foothills of Estrela mountain (Vieira, Mora, & Ramos, 2003).

Locally, in the Alardo site the bedrock is mainly composed of moderate to slightly weathered (W ₃ – W _1-2) granitic core stones. The Schmidt Hammer hardness tests carried out in that granite have shown a uniaxial compressive strength around 35 MPa. The highly weathered granite (W _4-5) outcrops are observed in sloped areas. In contrast, the Touca hydromineral site is dominated by highly weathered granite (W _4-5) in a large flattened area comprising gentle slopes (‘Cova da Beira’). In addition, a granitic sandy-clayey regolith has been identified, and a slight to moderately weathered (W _1-2 – W₃) granite core stones also outcrop (Figure 3).

Figure 3. Field aspects of the studied sites: (a) View of Alardo site and ‘Cova da Beira’ plains in the background; (b) General view of Gardunha mountain ridge; (c) Hydrogeological inventory mapping: water mine excavated in granite; (d) Granitic core stones and weathered granite in Gardunha sloped area; (e) View of Touca plains, with Gardunha mountain system in the background; (f/g) Outcrop and weathered granite in Touca site.

3. Methods and results

Geomorphological cartography had a widespread application in the oil industry, hydraulic engineering, geotechnical engineering, engineering geosciences, environmental consultancy, and planning (Griffiths & Abraham, 2008; Tricart, 1958). A methodological approach for the preparation of applied geomorphological maps must be sufficiently flexible to meet the requirements of the end-user needs (Griffiths & Abraham, 2008). Geovisualisation is particularly relevant to hydrologic systems as it brings new approaches and challenges in map design (e.g. Cascelli, Crestaz, & Tatangelo, 2012; Dykes, Maceachren, & Kraak, 2005; Fisher, Dykes, & Wood, 1993).

Recent technological advances have brought GIS cartographic techniques to the forefront as tools for recommending water management methods (e.g. Ballukraya & Kalimuthu, 2010; Chowdary, Rao, & Sarma, 2003; Jaiswal et al., 2003; Jha, 2011; Jha, Chowdhury, Chowdary, & Peiffer, 2007; Yeh, Lee, Hsu, & Chang, 2009). The multicriteria approach, which encompasses several types of layers of information (e.g. topography, climatology, lithology, structure, slope, drainage, geomorphology, hydrogeology, land cover/land use) has been greatly enhanced by the versatile development of GIS hardware and software (Ettazarini, 2007).

The basic data collection techniques of geology, geomorphology and hydrogeology have been applied in this study (e.g. Assaad, Lamoreaux, Hughes, Wangfang, & Jordan, 2004; Brewer, 2005; Dykes et al., 2005; Fisher et al., 1993; Fookes, Lee, & Griffiths, 2007; Peterson, 2009; Smith, Paron, & Griffiths, 2011). The terminology and recommendations of the International Society for Rock Mechanics (ISRM, 2007), Geological Society Engineering Group (GSE, 1995) and the Committee on Fracture Characterisation and Fluid Flow (CFCFF, 1996) were followed. In this study, the topographic maps (Portuguese Army Geographic Institute), the geological maps (former Portuguese Geological Survey), aerial orthophotos and also LandSat ETM+ and SPOT5 images has been used to build a series of thematic maps.

A geodatabase was created to organise the input data for analysis. This database includes information on geological features (lithology, structure, and weathering grade), land cover, drainage, slope, and rainfall, as well as other relevant information for the site investigation (Figure 4, cf. Stage 1). Once all data were registered in the spatial database the data were weighted and overlay operations completed. This GIS analysis allowed the creation of several derivative map themes, in order to assess the spatial distribution and controls on groundwater infiltration. The identification of the explaining factors were compiled and revised from the selected bibliography (e.g. Babar, 2005; Jha, 2011; Jha et al., 2007; Krishnamurthy, Kumar, Jayaraman, & Manivel, 1996; Teixeira et al., 2010; Yeh et al., 2009 and references therein). The weight and score for each factor have been calculated using the ‘Analytical Hierarchy Process’ method (e.g. Kim, Ahn, & Marui, 2009; Saaty, 2008 and references therein). The intrinsic characteristics of each factor were largely accessed from data obtained during the field campaigns in both of the study sites, and some minor adjustments were made accordingly in the factors' weights and scores.

Figure 4. Conceptual flowchart of the hydrogeomorphological mapping methodology applied to the Alardo and Touca sites (Portugal).

The factors used in the calculation of the potential groundwater infiltration were grouped in to three main categories (see details in Teixeira, 2011; Teixeira et al., 2010): geological description of rock masses, geographical description, and hydromorphological features (Figure 4, cf. Stage 2). Each input map has its specific weight and score and was used to calculate groundwater infiltration potential areas. The grid data structure (a quadrangular block of cells with a pixel size of 1 × 1 m) was used, and each raster cell value is a result of the weighted sum of the input layers. The GIS overlay of these factors results in a map, which spatially reflects the index of infiltration potential, with values ranging from 0 to 100. This intermediate map was then overlaid with both the geomorphological map and the hydrogeological field-data inventory, to achieve an integrated hydrogeomorphological map (Figure 4, cf. Stage 3).

For each study site, three different scenarios were computed, and the relative weight of some factors was changed to account for local conditions. Scenario 1 corresponds to a more balanced setting between all the presented parameters. In this scenario, lithology and structure/weathering grade, tectonic lineament density and slope have a greater importance in explaining the groundwater infiltration potential of each study site. In Scenario 2, relative weights of lithology and structure/weathering grade, tectonic lineament density and slope were maintained. However, the weights for land use and slope were increased, and the importance of drainage network density was reduced. Given the low spatial variability and minimum change in average annual rainfall, this parameter becomes less important in this scenario and, even less in Scenario 3. In this last scenario, the density of drainage network assumes a very low weight, and higher relative weights are given to lithology and structure/weathering grade, tectonic lineament density and slope, and slightly lower than those, land use. The construction, analysis and comparison of these scenarios were completed to help the development of the conceptual model for each study site. This contributed to an improved knowledge and management of the groundwater resources.

The main parameters related to the groundwater recharge areas were identified, as being: (i) moderate-to-highly weathered granitic rock (including arenisation layers); (ii) moderate-to-close fracturing; (iii) low slope areas at the highest elevations; and, (iv) agricultural and forest areas (Table 1).

Table 1. Summary of hydrogeological ‘favourability’ for potential infiltration on fissured hard-rock, focused in to groundwater recharge/discharge for the studied sites (adapted from Teixeira et al., 2010).

Parameters	Recharge	Discharge
Lithological weathering grade
• W 1-2 to W 3	−	+
• W 4-5	+	+
Ground fracturing degree
• F 1-2	−	−
• F 3 to F 4-5	+	+
Tectonic lineaments	+	+
Topography/slope
• Flat top	+	−
• Bottom valley	−	+
• Steep convex/rectilinear	−	−
Drainage network
• Low-to-medium density	−	−
• High-density	+	+
Land cover
• Forest and agricultural areas	+	−
Rainfall	+	+
Ground fracture intercept (see ISRM, 2007): F 1-2: wide, F 3: moderate, F 4-5: close; Weathering grade (see ISRM, 2007): W 1-2: fresh to slight; W 3: moderate, W 4-5: high.

4. Conclusions and outlook

Hydrogeomorphological mapping combines geological, geomorphological and hydrogeological features. The location of groundwater recharge/discharge areas becomes more perceptible when using potential groundwater infiltration.

The cross-check of geomorphological and hydrogeological features in a GIS-based environment provided a simple and efficient tool to support groundwater exploration. The approach contributes to hydrogeological conceptual model building, which is an important tool in the decision-making process, regarding water resources management and territory planning (Jha, 2011; Kudrna & Šindelářová, 2006). This approach is indispensable for the management and protection of a fragile resource, considering its utilisation in the thermal spa (Touca) and bottling water industries (Alardo).

The conceptual ground models based in Earth systems have proven their value in geology, hydrogeology and engineering geosciences (e.g. Bisson & Lehr, 2004; Bredehoeft, 2005; Carvalho et al., 2005; Fookes, 1997; Griffiths & Stokes, 2008; LeGrand & Rosen, 2000; Shapiro, 2001; Teixeira et al., 2010). With regard to water resources management it is important to balance the water demands (quality, quantity and seasonality), as well as institutional and local logistics. Groundwater-related activities (e.g. hydrogeological site investigations, identification of potential contamination areas, definition of wellhead protection areas) can be significantly improved with the help of GIS-based analysis and hydrogeomorphological and hydrogeological mapping techniques, leading to a more efficient and sustainable management of water resources.

Software

ESRI ArcGIS 9.3 was used for map data visualisation, overlay analysis and layout creation. All overlay analysis was performed using a raster file format, with a pixel resolution of 1 × 1 m. The 3D hydrogeomorphological conceptual model was built using ESRI ArcScene 9.3 and OCAD for Cartography 9.

Acknowledgements

This work was completed under the framework of the Labcarga|ISEP re-equipment program (IPP-ISEP|PAD'2007/08) and Centre GeoBioTec|UA (PEst-C/CTE/UI4035/2011-2012). JT was supported by a doctoral scholarship from the Portuguese Foundation for Science and Technology (SFRH/BD/29762/2006). We acknowledge the anonymous reviewers for the constructive comments that helped to improve the clarity of the manuscript. The present paper is dedicated to the late Professor Jean Tricart (1920–2003) eminent French geomorphologist, who launches the basis of the hydrogeomorphological cartography applied to the environment, groundwater and engineering.