Strategy for groundwater management in developing countries: A case study in northern Costa Rica

Journal of Hydrology (2007) 334, 109– 124

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Strategy for groundwater management in developing countries: A case study in northern Costa Rica Andreas Mende

a,*

, Allan Astorga

a,1

, Detlev Neumann

b,2

a

Escuela Centroamericana de Geologı´a, Universidad de Costa Rica, Ciudada Universitaria, 2070 SanPedro, Montes de Oca, Costa Rica b Dr. Neumann & Busch Consulting, Hammerweg 2-4, D-52074 Aachen, Germany Received 22 June 2005; received in revised form 28 September 2006; accepted 3 October 2006

KEYWORDS Groundwater management; Vulnerability analysis; Geographic information systems; 3D modelling

Summary On the basis of a case study in northern Costa Rica, in an area of one of the country’s most important volcanic aquifers, we developed a GIS-based groundwater management system suitable for the limited financial, technical and data resources of developing countries. Input and processing of attribute data-like information about groundwater wells or geological outcrops are realized with the help of an ACCESS database. The GIS-platform ArcView is used for input and analysis of all types of spatial data, e.g. maps of geology, hydrogeology, land use, or locations of groundwater wells. A threedimensional model of the subsurface geology is constructed with the help of the interactive 3D-Modeller RHINOCEROS (R) – NURBS modelling for Windows. The GIS-platform ArcView is used for the final integrated spatial analysis of the three-dimensional model and the generated spatial and attribute databases. The basic concept for data analysis is a five-step numerical index as a simple but efficient matter of standardization in order to take into account a wide spectrum of input data of varying quality and precision. As an example of application, the assessment of the vulnerability of aquifers is presented, which can be calculated on the basis of the hydrogeological profile (sequence of different strata with varying permeability), the infiltration potential of the soil layer and the distance to fault zones for each location of the study area. ª 2006 Elsevier B.V. All rights reserved.

Introduction * Corresponding author. Tel.: +506 207 35 52. E-mail addresses: [email protected] (A. Mende), aastorga@ racsa.co.cr (A. Astorga), [email protected] (D. Neumann). 1 Tel.: +506 273 40 54. 2 Tel.: +49 241 405571.

Sustainable management of groundwater resources in underdeveloped regions is one of the essential objectives for the future, especially when the rising demand for clean drinking water by these fast growing communities is considered. Uncontrolled expansion of industries, agriculture and

0022-1694/$ - see front matter ª 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.10.016

110 settlements as well as insufficient management of waste disposal and sewage treatment have already contaminated huge amounts of surface and ground waters. The broad lack of basic information, e.g. systematic geological or hydrogeological maps as well as detailed information about well logs with geotechnical and hydrogeological parameters hardly allows the application of sophisticated groundwater management tools in these areas, because most of the necessary input data are not available. On the other hand, the risk for contamination of the groundwater recourses is extremely high because of poor infrastructure and insufficient regulations for land use within vulnerable areas. For this reason we developed a GIS-based groundwater management system of high practical use suitable for the limited financial, technical and data resources of underdeveloped regions. We tested the system through a study in northern Costa Rica, within an area of one of the country’s most important volcanic aquifers. Since the 1990s several methodologies for groundwater management and vulnerability assessment have been developed, which are increasingly being applied within GIS and data base environments. The vast majority of these projects have been carried out in first world countries, e.g. Europe, Australia and North America, where a wide range of information and sound technical and financial support by governmental or private institutions are available. A highly advanced approach for groundwater management and protection in Denmark has recently been presented by Thomsen et al. (2004). Their approach included spatially dense geophysical/hydrogeological mapping, dense perforation grids as well as numerical modelling, GIS and advanced database technology. Other advanced approaches include quantitative aquifer modelling for entire catchment areas using numerical groundwater flow modelling as a basis for sustainable aquifer exploitation and vulnerability assessment (e.g. Henriksen et al., 2003; Barthel et al., 2004; Sakiyan and Yazicigil, 2004; Vissers et al., 2004). However, in the case of reduced availability of the necessary input data, which is the typical case for developing countries, the results of those numerical modelling approaches tent to a growing uncertainty because a significant part of the basic input variables has to be estimated. Another focus of research activities regarding groundwater management is the assessment of groundwater vulnerability based upon varying numbers and types of input variables. The most widely used methodology is by far the DRASTIC approach using spatial data sets on depth to groundwater, recharge by rainfall, aquifer type, soil properties, topography, impact of the vadose zone and the hydraulic conductivity of the aquifer (Aller et al., 1987; Knox et al., 1993). In first world countries this model has proven to be useful for groundwater vulnerability assessment (e.g. Rundquist et al., 1991; Secunda et al., 1998; Fritch et al., 2000). When trying to apply the DRASTIC model in developing countries things are changing substantially because of the lack of necessary input data of sufficiently high quality and precision. The problem of missing input data exists in several studies from recent years applying the DRASTIC vulnerability assessment methodology in developing regions: in many cases several variables had to be excluded from the vulnerability assessment because of missing input data (e.g. Piscopo, 2001; Alemaw et al., 2004). In other cases

A. Mende et al. several input variables were roughly estimated, contributing to increasing uncertainty of the vulnerability-index categories (e.g. Al-Adamat et al., 2003). Our approach to groundwater management and vulnerability assessment focuses on the integration of advanced GIS, database and three-dimensional modelling technologies in order to take as much advantage as possible of the limited existing data. In addition, the extended use of remote sensing data, digital elevation data and simple geophysical surveys were applied to compensate for lacking basic information. A more flexible concept of data analysis based upon a simple five-step numerical index allows to adapt the applied methodology on the types of accessible input data. The aims of this paper are: (1) To present our methodology as an example for using GIS, database and three-dimensional modelling techniques as a practical decision support tool for government and private institutions in developing countries. (2) To discuss the application of the proposed methodology for groundwater management through the calculation of the contamination potential for one of Costa Rica’s most important volcanic aquifers.

Study area The study area is in northern Costa Rica and comprises a surface of about 750 km2 situated within the two topographic map sheets 1:50,000 ‘‘Monteverde-3147 IV’’ and ‘‘Carrillo Norte-3047 I’’, edited by the ‘‘Instituto Geogra ´fico Nacional’’ of Costa Rica (Fig. 1). This region is located within the volcanic plateau ‘‘Meseta Ignimbritica’’ (altitude range: 90–120 m asl) at the foot of the active volcanoes of the Cordillera de Guanacaste. The Meseta Ignimbritica is formed by the volcanic successions of the Pleistocene ‘‘Liberia Formation’’ which is dominated by pumice flow deposits and the Pliocene ‘‘Bagaces Formation’’ whose subunits 1–3 consist of ignimbrites, paleosoils, lava flows and volcaniclastic sediments (Fig. 2). The alluvial plain of the ‘‘Valle de Tempisque’’ is a depression of tectonic origin in the SW of the study area, filled by alluvial deposits of about 100 m in thickness. A well developed Pediment in the NE of the study area forms the transition zone to the chain of active volcanoes of the Cordillera de Guanacaste. More detailed information on the stratigraphic succession and the hydrogeological characteristics of the Meseta Ignimbritica are summarized in Fig. 3 and Table 1: the main aquifer of the study area comprises the fractured/columnar lava flows and autoclastic lava breccias of the Bagaces 2 unit forming one of the most important volcanic aquifers of Costa Rica. The pumice flow deposits of the uppermost Liberia Formation play an important role as a protection for the underlying aquifer systems because of their very low hydraulic conductivity caused by a very fine-grained ashy matrix. Nevertheless geochemical data prove that the Bagaces 2 aquifer is recharged to a minimal extent by surface waters descending through the Liberia Formation (Morera, 2000). For this reason the Bagaces 2 aquifer is characterized by semi-confined conditions. The climate exhibits semi-arid patterns with a well developed dry season from December to May reaching peak temperatures of more than 35 C and a rainy season from June to November where most of the annual rainfall of

Strategy for groundwater management in developing countries: A case study in northern Costa Rica

Figure 1

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Location of the study area in northern Costa Rica, marked by the black-edged rectangle; Digital Elevation Model (DEM).

Figure 2

Geological Map of the study area in northern Costa Rica.

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Basic concepts The GIS-based groundwater management system has been developed according to the following guidelines: (1) development of efficient tools for data collection, evaluation of

Figure 3 Stratigraphic column and hydrogeological profile of the Meseta Ignimbritica.

about 1600 mm is concentrated (ONU, 1975). Land use is dominated by agriculture production with low/high intensity cattle farming and fruit plantations within the Meseta Ignimbritica, while the alluvial plain of the Valle de Tempisque is dominated by rice and sugar cane cultivation. During the last years, industrial-style melon plantations for export production have become a new form of land use. The main urban centre, the city of Liberia with about 60,000 inhabitants, is situated within the Meseta Ignimbritica; the Pediment in the northeast of the study area still shows a nearnatural steppe and forest vegetation cover. Principal groundwater management problems arise from the intensive agricultural land use forms, especially the intensive cattle farming and related nitrate contamination or industrial-style melon plantations and sugar cane plantations using high amounts of pesticides and insecticides. In addition, a sharp increase of water demand has been caused by a steadily growing tourism industry along the nearby coast, especially related to the growing number of golf grounds and luxury hotels.

Table 1

Figure 4 Work flow diagram illustrating the different steps to establish the presented groundwater management system.

Stratigraphy and related hydrogeological characteristics of the volcanic plateau ‘‘Meseta Ignimbritica’’

Stratigraphy

Lithology

Hydrogeology

Formation Liberia (Pleistocene)

Pumice flow deposits: pumice clasts up to 10 cm in diameter in a fine grained white ash matrix including phenocrystals of biotite, quartz and plagioclase

Very poor hydraulic conductivity within a range from 0.1 to 2 (m/d) caused by the fine ash matrix, high importance as a protection for the underlying aquifers Bagaces 2/3

Formation Bagaces 3 (upper Pliocene)

Meter-bedded ryolitic to andesitic tuffs and ignimbrites with intercalated relicts of paleosoils up to 2 m in thickness (only partly preserved)

Moderate hydraulic conductivity within a range from 5 to 15 m/d.

Formation Bagaces 2 (upper Pliocene)

Predominantly andesitic lava flows with fractured or columnar textures, associated with autoclastic lava breccias free of primary matrix

Very high hydraulic conductivity up to 200 m/ d, high potential aquifer, used intensively for drinking water supply in NW-Costa Rica as well as agricultural consumption

Formation Bagaces 1 (middle Pliocene)

Volcaniclastic sediments, predominantly semilithified silt-/sandstones and conglomerates formed within fluvial/ lacustrine environments

Moderate hydraulic conductivity (10–40 m/ d), intermediate potential aquifer, locally importance for drinking water supply or agricultural consumption

Strategy for groundwater management in developing countries: A case study in northern Costa Rica data quality and analysis in order to take as much advantage as possible of the limited existing base data, (2) incorporation of remote sensing, digital elevation data and geophysical surveys to compensate lacking information on geology, hydrogeology, tectonics, geomorphology and land use, (3) integration of three-dimensional information on subsurface geology and hydrogeology to provide a well-founded base for evaluation of vulnerable areas with regard to groundwater contamination and (4) consequent numerical classification of all derived information in order to translate expert knowledge into thematic maps suitable to non-experts. The selection of the basic working scale determines the quantity and type of data which can be integrated into a GIS-database. Regarding our case study we selected a medium scale of 1:50,000 providing detailed information based on a broad spectrum of input data at reasonable costs. The work flow diagram of Fig. 4 represents the principal steps characterizing our GIS-based groundwater management system including (1) the collection of relevant input data, (2) the implementation of the collected geodata into a GIS-database, (3) the three-dimensional modelling of subsurface geology and hydrogeology and (4) the integrated spatial analysis of the produced information as a basis for groundwater management decisions.

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Input data collection Technical reports of about 100 drinking water wells provided information on subsurface geology as well as hydrogeological parameters, e.g. static/dynamic groundwater levels or hydraulic conductivity (K) of subsurface aquifers (Fig. 5). Of those 100 wells 51 reach the Bagaces 2 aquifer. In addition, several geological reports were available for basic information on the geological framework and the local stratigraphy (e.g. Civelli, 1990; Chiesa, 1991; Chiesa et al., 1994; Kempter, 1997; Bergoeing, 1998). The Ministry for Environment and Energy (MINAE) provided digital topographic maps at the scale of 1:25,000, elaborated in 1998 using photogrammetry. Because of its extended correction functionalities like iterative finite difference smoothing or automatic drainage enforcement (O’Callaghan and Mark, 1984; Hutchinson, 1989), the TOPOGRID module within the ArcInfo Program suite (ESRI) was applied to construct a hydrologically correct digital elevation model (DEM) based upon the provided topographic information. This DEM was used as the input grid layer for a subsequent topographic analysis. After import of the DEM grid into the ArcView GIS, the following derived raster maps have been calculated: (1) hillshades of

Figure 5 Distribution of the collected input data within the study area (1: outcrop information, 2: drinking water wells, 3: vertical electric soundings, 4: soil infiltration measurements).

114 Table 2 geodata

A. Mende et al. Remote sensing data used within the scope of the case study in northern Costa Rica and the respectively derived

Remote sensing method

Interpretation methods

Derived geodata

Resolution(cell size) (m)

Landsat TM

Spectral classification, filtering

Lineations, major faults, geology, geomorphology, land use

30

Spot PAN

Spectral classification, filtering, stereographic interpretation

Lineations, active faults, geology, geomorphology, land use, soil classification

10

ERS Georadar

Filtering (line reinforcement, contrast, brightness)

Major structural elements, geomorphology

12.5

Aerial/infrared photographs (1:20,000–1:80,000)

Stereographic interpretation, generation of orthophotos, filtering

Lineations, major faults, soil classification, geomorphology, geology, land use

0.2–0.8

DEM Hillshade

Filtering (line reinforcement, contrast, brightness)

Lineations, mayor faults, geology, geomorphology

10

DEM (illumination directions 90 and 270), (2) slope gradient, (3) slope aspect and (4) relative relief (maximum elevation difference/km2). Computer-based interpretation of remote sensing data (e.g. Landsat TM scenes, color/infrared aerial photographs, georadar images) has been used extensively to compensate lacking cartographic information on geology, tectonics, geomorphology and land use. Besides these remote sensing Table 3

data, the calculated hillshades of the DEM were successfully used for geodata generation because anthropogenic influences like roads or electricity wires and especially the dense vegetation have no influence on the data quality. Data analysis was done with the help of the raster-GIS software package ILWIS 3.0 (ITC, 2001) providing extended filtering tools such as line reinforcement, gradient filters, Laplace filters and spectral classification tools for satellite image interpre-

Guidelines for data quality assessment based on four data quality categories

Data quality

Qualitative guidelines

Examples

Implications for data analysis

I – High

Data achieved by standardized methods and/or laboratory analysis. Interpretative data based on high resolution remote sensing data and/or high quality outcrop data. High degree of coherence with neighbourhood data.

K-values of aquifers based on pumping tests. Soil infiltration measurements by means of standardized infiltrometers. Significant outcrop data (e.g. limits of geological units visible in an outcrop plane). Detailed geological well log data coherent with neighbourhood wells and outcrops. Geological faults mapped based on high resolution infrared photographs and significant field data.

Data of high credibility, results of data analysis should strictly reproduce all data within this data quality class.

II – Moderate

Data achieved by simple field tests. Moderate degree of coherence with neighbourhood data. Interpretative data based on moderate resolution remote sensing data and/or outcrop data in general.

Geological well log data moderately coherent with neighbourhood wells and outcrops. Hydrogeological data with little information about the applied methodology.

Data of moderate credibility, used where Class I data are not available, checked for incoherency to Level I data in the surroundings.

III – Low

Empirically achieved data. Low degree of coherence with neighbourhood data. Interpretative data based on low resolution remote sensing data.

Basic geological well log data less coherent with neighbourhood wells and outcrops. Hydrogeological data without information about the applied methodology.

Data of low credibility, used where no other data are available, always checked for incoherency to higher level data in the surroundings.

IV – Poor

Data with a low to very low degree of coherence with neighbourhood data of better quality. Logical/systematic errors in data acquisition.

Geological well log data with obvious logical errors. Groundwater level data taken within differing seasons of the year (e.g. dry/rainy season).

Data excluded from data analysis.

Strategy for groundwater management in developing countries: A case study in northern Costa Rica tation. Table 2 gives an overview of all remote sensing data used for the case study as well as the respectively derived information. Necessary fieldwork focused on geological/hydrogeological mapping based on appropriate checklists guaranteeing GIS-compatible and standardized data collection. Georeference of field data has been achieved with the help of a Global Positioning System (GPS), supplemented by printouts of the digital topographic maps at the scale of 1:25,000 for field orientation. Hydrogeological field data collection focused on measurements of the hydraulic conductivity K (m/s) of the upper soil layer using a tension disk infiltrometer and following the methodology proposed by Zhang (1997). A total number of 60 measurements were executed in the study area (Fig. 5). The drainage network was mapped to establish a complete drainage network map layer. Annual variations in run-off were estimated by comparing stream water levels in the dry and rainy season. Furthermore, natural springs were mapped and characterized by collecting information on (1) the estimated discharge, (2) the main cause of the spring (fault zone, limit of stratigraphic units, etc.) and (3) the physical properties of the spring water: pHvalue, temperature, conductivity, color and odor. Other hydrogeological information, such as the permeability of subsurface aquifers, can only be obtained by means of expensive well tests and laboratory core examination. Therefore, only existing data were used (see above). Additionally, to improve the data density on subsurface geology,

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a total number of 51 vertical electrical soundings (VES) with a maximum electrode spacing (1/2) of 300 m were carried out as a low-cost alternative for perforations (Fig. 5).

Assessment of data quality An important aspect of our methodology is the assessment of data quality. For this, we have established a four-step classification scheme using qualitative guidelines. Because of the different data types (e.g. numerical point data such as measurements of the infiltration potential, descriptive data such as geological well profiles or remote sensing data of varying resolution), a quantitative approach was not feasible. Table 3 resumes the most important guidelines for the four data quality categories as well as corresponding examples and implications for data analysis.

GIS database design The concept of the GIS-database is illustrated in Fig. 6 comprising a simplified version of the logical GIS-database model. Input and processing of attribute data, like information on known perforations, groundwater wells, geological outcrops and additional information, like meteorological data and possible sources of contamination, have been carried out with the relational ACCESS database. Second-degree normalization of all database attributes guarantees exten-

Figure 6 Logical model of the applied GIS-database (simplified version); comprising a relational ACCESS database for advanced attribute data management and the GIS-platform ArcView for graphical data input, processing and analysis.

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Figure 7 Principal elements of the three-dimensional model of the study area in northern Costa Rica, generated with help of the NURBS-modelling program RHINOCEROS.

sive data query and access possibilities. A central element of the attribute database is the submodel RECORDS comprising basic metadata such as data type, sheet number of topographic maps or date of entry and last change. Detailed administrative data have been collected within the submodel METADATA for subsequent data quality assessment. The submodel RECORDS served to connect the GEODATA submodels with the GEOMETRY submodel containing the database objects point, line and polygon. These database objects provide the connection with the GIS platform ArcView via the ODBC-interface (Open Data Base Connectivity). The graphical database has been realized within the GIS platform ArcView providing all necessary capabilities for input and analysis of all types of spatial data. It comprises the submodel BASEMAPS including all basic graphical information like outcrop/well locations or basic topographic map layers. Geodata from the attribute database, such as outcrop information or well log data are directly connected with the attribute tables of the corresponding geodata map layers (point, polyline, polygon themes) via the

ODBC-interface (Open Database Connectivity). This basic information is used to derive thematic input maps, e.g. bedrock/Quaternary geology or hydrogeological map layers. Integrated spatial analysis of the generated data sets, which also include three-dimensional information on subsurface geology and hydrogeology, results in normalized parameter maps constituting the base for integrated groundwater management.

Three-dimensional NURBS-modelling Groundwater management systems need to provide information on the three-dimensional extent of the hydrogeological units to guarantee a well-founded base for the evaluation of vulnerable areas regarding groundwater contamination. Conversely, rather irregularly distributed base data of highly varying quality and quantity, as typical for underdeveloped regions, do not allow the application of simple interpolation methods for the generation of threedimensional models. For our case study we applied the

Strategy for groundwater management in developing countries: A case study in northern Costa Rica interactive 3D-Modeller RHINOCEROS (http://www.rhino3d.com), which uses Non-Uniform B-Splines (NURBS) for the mathematical representation of the geometry. A detailed description of this method is given by Fisher and Wales (1992). The software provides an interface to the ‘‘ArcView Shape’’ data format allowing an easy interchange between the GIS-Platform ArcView and the 3D-Modeller RHINOCEROS. The NURBS Modeller RHINOCEROS provides several advantages: An interactive, step by step integration of all available input data (e.g. well log data, geological outcrops, constructed geological cross sections or geological limits of geological maps), the possibility to develop solid volumes based on bordering surfaces including the calculation of the enclosed volumes and the high accuracy of the generated surfaces. Preset data, e.g. perforation logs, borderlines or isoclines can be reproduced with reasonable accuracy within the three-dimensional model. Interpolated geometric objects (lines, surfaces) can subsequently be manipulated interactively by selection and subsequent shifting of a free number of control points with the mouse as well as by the entry of quantitative shift values. Surfaces can be trimmed in a geometrically exact way with lines or other surfaces, e.g. boundary surfaces of stratigraphic units with fault planes. By means of perspective views and cross sections all modifications can be controlled during the modelling process. Grid and vector data interchange between the GIS Platform ArcView and the 3D-Modeller RHINOCEROS was achieved through a RHINOCEROS plugin using the ArcView Shape interface.

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Fig. 7 represents some of the principal elements of the generated 3D-model including (1) the major fault planes trimmed with the digital elevation model (DEM), (2) the upper limit of the Quaternary deposits within the alluvial plane of the Valle de Tempisque, (3) the base of the high potential Aquifer Bagaces 2, (4) the base of the upward following Bagaces 3 unit, (5) the base of the Liberia Formation and (6) the actual topographic surface of the mountainous areas.

Results: integrated spatial analysis within the scope of groundwater management The GIS-platform ArcView has been applied for the final integrated spatial analysis of the three-dimensional model and the generated spatial and attribute database. The basic concept for data analysis is a five-step numerical index (1– 5) representing a simple but efficient standard which allows to take into account a wide spectrum of input data of varying quality and precision. This advantage is of special importance for developing countries where the variability of quality, precision and types of accessible data is very high in most of the cases.

Calculation of the contamination potential for the Bagaces 2 aquifer As an example of application, we present the calculation of the contamination potential for the high potential aquifer Bagaces 2, which is executed in two steps.

Figure 8 Representation of the ‘‘Formation Factor’’ of the study area, reflecting the protection of the regarded aquifer Bagaces 2 provided by overlaying strata with low hydraulic conductivity.

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Figure 9 Representation of the ‘‘Contamination Potential for the Bagaces 2 aquifer’’ based upon the calculated average of the three normalized parameter maps ‘‘Formation Factor’’, ‘‘Soil Infiltration’’ and ‘‘Fault Distance’’.

Calculation of the ‘‘Formation Factor’’ The so-called ‘‘Formation Factor’’ represents a numerical index from 1 to 5 reflecting an increasing degree of protection of the aquifer provided by overlying strata with low hydraulic conductivity. For the considered aquifer Bagaces 2 those overlying strata include the Bagaces 3 unit, characterized by moderate hydraulic conductivity (5–15 m/d), and the Liberia Formation with very low hydraulic conductivity values (0.1–2.0 m/d) (cf. Table 1 and Fig. 3). On the base of the three-dimensional model, the layer thickness of those two stratigraphic units were calculated and added to the ArcView-GIS as grid map layers. Because the hydraulic conductivity of the Liberia Formation (av. 1 m/ d) is about 10 times lower than for the case of the Bagaces 3 unit (av. 10 m/d) the layer thickness of the Bagaces 3 unit was divided by 10. Thus, the ‘‘Formation Factor’’, for the regarded aquifer Bagaces 2 is calculated as follows: outcrop areas of the aquifer Bagaces 2 do not provide any protection against contamination by overlying strata and are thus classified as 1, for areas covered by overlying strata the two grid layers ‘‘thickness of the Liberia Formation’’ and ‘‘thickness of the Bagaces 3 Formation · 0.1’’ were summed up and classified as follows: 1: 0–5 m, 2: 5–10 m, 3: 10–40 m, 4: 40–80 m, 5: >80 m (Fig. 8).

Calculation of the contamination potential for the aquifer Bagaces 2 In addition to the ‘‘Formation Factor’’, two more variables were considered for the final calculation of the contamina-

tion potential of the aquifer Bagaces 2 (Fig. 9). The variable ‘‘Fault Distance’’ refers to the significantly enhanced subsurface permeability in the vicinity of major faults (Richter, 1989). The resulting thematic input map layer ‘‘Distance to Major Faults’’, generated via the distance calculation algorithm of the ArcView GIS, was classified as follows: 1: 0– 10 m, 2: 10–50 m, 3: 50–200 m, 4: 200–400 m, 5: >400 m. The variable ‘‘Infiltration Potential’’ represents the hydraulic conductivity of the soil layer derived from field measurements with the tension disk infiltrometer (see Section ‘‘Input data collection’’). The infiltration data were interpreted using the classification scheme ‘‘Terrain Mapping Subunits’’. This scheme represents a three-step hierarchical terrain classification with subunits of approximately equal properties considering geomorphological processes, soil characteristics and subsurface geology. It was developed through a research project on sustainable land use management in southern Costa Rica (Mende and Astorga, in press). For each of the Terrain Mapping Subunits an average value of the infiltration potential was calculated based on the field data. The resulting thematic input map layer ‘‘Soil Infiltration’’ was classified as follows: 1: >30 m/d, 2: 30– 10 m/d, 3: 10–1 m/d, 4: 1–0.1 m/d, 5: <0.1 m/d. The contamination potential was then obtained by summing up and subsequently calculating the mean of the three variable values. Like this a five-step classifications I–V with a decreasing potential for groundwater contamination can be established. The resulting calculated grid map layer represents the final map layer of the ‘‘Contamination Potential of the Bagaces 2 aquifer’’ (Fig. 9).

Strategy for groundwater management in developing countries: A case study in northern Costa Rica

‘‘Index of Overuse’’: basis for recommendations of land use change A very useful application of the calculated map layer ‘‘Contamination Potential of the Bagaces 2 aquifer’’ is the comparison of the contamination potential categories of a given area with the suitability of the forms of actual land use regarding groundwater protection. The so called ‘‘Index of Overuse’’ was calculated based upon a cross operation of the map layer ‘‘Contamination Potential of the Bagaces 2 aquifer’’ with the map layer ‘‘Forms of actual Land Use’’. The crossing of the two map layers yielded information on the different kinds of actual land use for each of the five contamination potential classes. The suitability of the actual land use has been evaluated according to the three variable classes (1) ‘‘appropriate use’’, (2) ‘‘moderately overused’’ and (3) ‘‘highly overused’’ with the help of a rating matrix defining minimum values of the contamination potential classes for each land use type (Table 4). The qualification ‘‘moderately overused’’ is given to land use forms where the negative impact is of minor importance. Negative consequences can usually be limited by following better management practices. The qualification ‘‘highly overused’’ indicates serious terrain misuse and a land use change is necessary to avoid serious negative consequences. This classification was established based upon background information on the probable sources of groundwater contamination for each of the actual land use types. The resulting map layer ‘‘Index of Overuse’’ regarding the protection of the aquifer Bagaces 2 is represented in Fig. 10. Table 5 gives information on the distribution of the three overuse index levels as a function of the different land use forms expressed by the covered area (km2), the percentage of the distribution area of the Bagaces 2 aquifer and the percentage of the respective land use form. Of the total distribution area of the Bagaces 2 aquifer (546.8 km2),

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Table 4 Rating matrix for the ‘‘Index of Overuse’’ defining minimum values of the contamination potential classes for each land use type according to the three variable classes (1) appropriate use, (2) ‘‘moderately overused’’ and (3) ‘‘highly overused’’ Land use type

Appropriate use

Moderately overused

Highly overused

Sugar cane Rice Melon plantations Fruit-tree plantations Cattle farming – intensive use Cattle farming – low intensity use Agriculture Forests Steppe Settlements Waste disposals

5–3 5–3 5–4 5–3 5–3

2 2 3 2–1 2

1 1 2–1 – 1

5–3

2–1

–

5–3 5–1 5–1 5 5

2 – – 4–3 4

1 – – 2–1 3–1

ca. 83.4% or 456.04 km2 has an appropriate land use, while ca. 13.22% or 72.26 km2 is classified as moderately overused and ca. 3.38% or 18.5 km2 as highly overused. Highly overused terrains concentrate on high intensity cattle farming land within areas of high contamination potential (Index: 1). Those terrains count for 66.5% of all highly overused land. As several cases of nitrate contamination in drinking water wells within the Bagaces 2 aquifer have already been reported, it is highly recommended to initiate immediate changes within those areas to land use forms with a lower risk for aquifer contamination. Another problematic type of land use are industrial-style melon plantations using high

Figure 10 Calculated ‘‘Index of Overuse’’ regarding the protection of the aquifer Bagaces 2 for the study area in northern Costa Rica. The respective rating matrix for this Index is given in Table 4.

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Table 5 Distribution of the three index levels of overuse in function of the different land use forms expressed by the covered area (km2), the percentage of the distribution area of the Bagaces 2 aquifer and the percentage of the respective land use form Land use

Index of overuse

Area (km2)

Area (% of the area Overlaying Bagaces 2)

Area (% of the respective land use area)

Forest

Appropriate use Moderate High

259.56 0.00 0.00

47.74 0.00 0.00

100.0 0.00 0.00

Steppe vegetation

Appropriate use Moderate High

58.39 0.00 0.00

10.66 0.00 0.00

100.0 0.00 0.00

Cattle farming (extensive)

Appropriate use Moderate High

29.27 48.03 0.00

5.35 8.78 0.00

37.9 62.1 0.00

Cattle farming (intensive)

Appropriate use Moderate High

58.28 13.86 12.31

10.66 2.54 2.25

69.0 16.4 14.6

Agriculture

Appropriate use Moderate High

35.75 1.26 0.56

6.54 0.23 0.10

95.2 3.3 1.5

Melon plantations

Appropriate use Moderate High

0.05 0.45 4.66

0.01 0.08 0.85

0.9 8.8 90.3

Fruit tree plantations

Appropriate use Moderate High

1.02 0.00 0.00

0.19 0.00 0.00

100.0 0.00 0.00

Sugar cane

Appropriate use Moderate High

6.06 7.30 0.89

1.11 1.34 0.16

42.5 51.2 6.2

Settlements

Appropriate use Moderate High

7.66 1.36 0.01

1.40 0.25 0.00

84.8 15.1 0.1

Waste disposals (poorly managed)

Appropriate use Moderate High

0.00 0.00 0.07

0.00 0.00 0.01

0.00 0.00 100.0

amounts of agrochemicals. Within areas of high contamination potential those toxic substances can easily migrate into the Bagaces 2 aquifer resulting in serious risk for this important groundwater resource. For this reason these plantations should be moved as soon as possible to areas with an index of low contamination potential (4–5). Other cases of highly overused terrains include sugar cane plantations in areas of very high contamination potential (Index: 1) likewise implying a serious risk because of the intensive use of agrochemicals. A special case of highly overused terrain is the waste deposal site NE from the town of Liberia (cf. Fig. 10). The selected site exhibits acceptable characteristics for such a form of use (with an index value 4 of the contamination potential scale (‘‘moderately overused’’ , cf. Table 4). On the other hand this waste disposal site is very poorly managed and the process of waste disposal remains nearly uncontrolled, resulting in serious risks for contamination of ground and surface waters. In this case a substantial improvement of the waste deposal management is abso-

lutely necessary in order to avoid serious negative consequences for the environment as well as for public health. Moderately overused terrains likewise comprise a high portion of areas used for cattle farming: 66.5% of moderately overused land is related to low-intensity cattle farming in regions with the Contamination Potential Index situated between 1 and 2, while 19.2% counts for high-intensity cattle farming in areas where the Contamination Potential Index is 2. For those cases, improvement of management strategies and the general attempt not to use those areas more than necessary are recommended in order to avoid further groundwater contamination. Within sugar cane plantations in areas with a moderate contamination potential (making up ca. 6% of those areas) the use of agrochemicals should be avoided as far as possible. In addition, moderately overused terrains include urbanized areas within the town of Liberia close to major fault zones (ca. 2%) with a substantially increased hydraulic conductivity resulting in a certain risk for contamination of the Bagaces

Strategy for groundwater management in developing countries: A case study in northern Costa Rica 2 aquifer. In this case installations of potential risk, such as gasoline stations, industrial plants or the use of septic tanks in private houses, should be avoided as much as possible. About 82% of the distribution area of the Bagaces 2 aquifer are characterized by appropriate forms of land use without any serious risk for contamination of the Bagaces 2 aquifer. Land of this category was assessed with respect to its suitability for more intensive human land use, which may be necessary for future development as well as for the displacement of human activities in overused areas. This has been achieved by comparing the minimum values of the contamination potential index for a given land use type (rating matrix of Table 4) with the index value of the respective terrain: if the index value is higher than necessary it can be considered as ‘‘underused’’ and thus suitable to more intensive human use. For example, terrains with a very low contamination potential (Index: 5) are widespreadly used for cattle farming. Those terrains (21.8 km2) can provide sufficient space for future settlements as well as for intensive agricultural production or other economic activities.

Discussion and conclusions The presented case study in northern Costa Rica demonstrates how advanced GIS- and database technology, three-dimensional modelling as well as incorporation of remote sensing data and simple geophysical surveys can be applied as an useful tool in the context of groundwater management for developing countries. This approach offers new solutions for areas of limited financial, technical and data resources through improved data processing and analysis capabilities as well as cheep but efficient strategies for input data collection. The approach of three-dimensional modelling based on the NURBS-Modeller RHINOCEROS could also be very useful for groundwater management and vulnerability assessment in first world countries as an optimized strategy for data processing and analysis. In the context of quantitative numerical groundwater flow modelling it can provide a low cost but very exact methodology to provide a detailed volume-based three-dimensional model of the subsurface geology and hydrogeology which is a fundamental base for realistic groundwater flow modelling. Another highly promising approach would be the incorporation of three-dimensional information of the subsurface geology and hydrogeology into the DRASTIC groundwater vulnerability assessment model for the application in areas where sufficient input data are available (e.g. developed countries).

Uncertainty and reliability of the applied methodology As any methodology of groundwater vulnerability assessment our approach implies benefits as well as certain disadvantages which can have negative influence on uncertainty and reliability. In this section, the two most important aspects of this subject are discussed with more detail. Data selection The selection or rejection of data types has a decisive influence on the degree of uncertainty and reliability of the ap-

121

plied methodology, as it controls the definition of parameters used for data analysis. Details about principals of data selection based on data quality assessment have been illustrated in Table 3 (cf. Section ‘‘Assessment of data quality’’). For its high importance, this item is discussed with more detail based on the example of groundwater level data. The unsaturated zone, whose lower limit is defined by the groundwater level, plays an important role for aquifer protection, as it provides the highest capability for absorption and chemical/biological decomposition of many classes of water contaminants (e.g. pesticides). So indeed, not to include the thickness of the unsaturated zone within the calculation of the contamination potential of the Bagaces 2 aquifer is a source for higher uncertainty and lower reliability of the proposed methodology. The reason why this important aspect has been excluded from data analysis is the very poor data quality of the accessible data on groundwater level within the study area (category of data quality: IV – poor). In semiarid regions there is a high annual variation of the water table in function of the succession of dry and humid periods. Works of Collins (1999) and ONU (1975) found annual groundwater table variations of up to 10 m within single wells in and around the study area. On the other hand, available groundwater level data lack the date of the respective measurement. In many cases groundwater level data of nearby wells exhibit high differences without any chance to decide whether this is an effect of annual variations or indeed related to primary variations of the thickness of the unsaturated zone. Under such circumstances, in our opinion it is the better strategy to exclude parameters from data analysis, which cannot be estimated in a reasonable way because of the lack of sufficient input data. The strategy to estimate a parameter without sufficient background data finally has the potential to produce a lower degree of reliability, because in that way statements are made about a subject on which reliable estimations cannot be made. To exclude such parameters gives way to a more sincere way of vulnerability analysis because the classification of a given terrain is based upon a well-founded database and not on rough estimations. On the other hand, our strategy implies the disadvantage that the role of the unsaturated zone for aquifer protection is not accounted for. But finally, this fact is an important message for land management planning: there is not enough information available to provide a wellfounded estimation of that parameter. Thus, in the case of site evaluations for high impact activities, it is clear beforehand on which parameters data acquisition has to concentrate. To provide this information it is obligatory to include clear information about the used parameters and – even more important – excluded parameters on published vulnerability maps. It should be recognized that especially in the context of practical application it is questionable to provide vulnerability assessment based on parameters which cannot be estimated with sufficient accuracy. There is considerable risk that areas which are declared as little vulnerable indeed are highly vulnerable because of an error in estimating the groundwater level. Once published, it is hard work to convince public opinion of a change in vulnerability category, especially in the case of conflicts of interests.

122 Importance of the applied parameters In contrast to other vulnerability assessment methodologies like DRASTIC we decided to value the three selected parameters ‘‘Formation Factor’’, ‘‘Fault Distance’’ and ‘‘Infiltration Potential’’ equally without establishing weight factors. This decision was taken based on the following considerations: (1) The importance of each of the selected parameters for groundwater vulnerability is subject of decisive variations due to the type of human activity. Considering human activities on the topographic surface (such as cattle farming), the parameter ‘‘Infiltration Potential’’ is of high importance because it controls the process of penetration of potential water contaminants into the subsoil. Conversely, this parameter is without any relevance for any activity which directly interacts with the subsoil, such as septic tanks used for domestic sewage treatment throughout the study area. Thus the establishing of weight values would only be suitable for a specific type of human activities, whereas equal considering of the three established parameters allows a more general statement on groundwater vulnerability which indeed is the objective of the presented methodology. (2) The importance of each of the selected parameters for groundwater vulnerability is also influenced by interaction. For example, in the case of a very low infiltration potential, the Formation Factor is relatively less important regarding activities on the topographic surface, because only a very small portion of possible water contaminants will reach the subsoil. Likewise, close to major geological faults the Formation Factor is less important, because the subsoil permeability is significantly increased due to a higher density of joints and/or microfaults which decisively lower the provided aquifer protection. Under such circumstances it is questionable to define any kind of weight values for the three selected parameters because this would assume a fixed influence on groundwater vulnerability, which in fact is not the case. (3) From a more general point of view each of the three selected parameters represents a decisive controlling factor of aquifer protection. Therefore, it is considered as an acceptable policy to weight those parameters equally in the case of a general groundwater vulnerability assessment.

Critical remarks on the applied methodology Finally some critical remarks on the presented methodology shall avoid misuse of the methodology or over-interpretation of the presented data: (1) Obviously, the applied methodology was adapted to the given situation within the study area including the types and quality of available input data resources, the given geological, hydrogeological and

A. Mende et al. geomorphological framework as well as the present situation of human land use within the study area. Three-dimensional modelling of subsurface geology for example cannot be done if the quantity of accessible well log data is very low combined with a complex geological setting. Within highly urbanized areas, much more attention is required for mapping of probable sources of contamination like contaminated industrial sites or abandoned waste dumps. Nevertheless, the basic strategies to integrate advanced GIS- and database technology as well as remote sensing data and simple geophysical soundings provide a valuable framework for the execution of similar projects in developing regions with substantially different settings. (2) The applied data analysis approach based on a fivestep numerical index (1–5) is a simple but efficient standardization procedure, which allows the incorporation of data of highly varying quality and precision into the analysis. On the other hand, this classification method can be a source of errors, because an incorrect numerical classification of data potentially results in misinterpretation of terrain characteristics regarding the respective variables. Thus, all variable classes have to be defined carefully based upon background knowledge of the local geological–geomorphological conditions as well as the specific characteristics of the local forms of human land use. (3) The evaluation of the contamination potential of an aquifer system at the scale of 1:50,000, as presented here, serves as a basis of groundwater management and protection defining the general aquifer vulnerability of a given terrain. Especially high impact construction projects such as waste disposal sites, urbanization projects as well as high-impact agricultural projects like industrial-style melon plantations, require a more detailed analysis. Nevertheless, the generated database on groundwater vulnerability forms a sound basis for the preliminary selection of potentially appropriate sites for high impact activities. Those pre-selected sites with possibly appropriate conditions need then to be studied with more detail. For example, in the case of the selection of a new waste disposal site, it would be necessary to analyze in detail the depth of the groundwater level based on well measurements, the permeability of the subsurface strata (pumping tests) and the subsurface geology by means of additional well log data and/or geophysical surveys. In this connection, an intelligent preselection of potentially appropriate sites can prevent high investments for such cost-intensive surveys by excluding all those available sites from more detailed analysis that exhibit a moderate to high degree of contamination potential (cf. Fig. 9). Additional information for site pre-selection, which can be derived directly from the generated data base, would be for example the distance to wells used for drinking water extraction, the distance to the drainage network or the distance to natural springs.

Strategy for groundwater management in developing countries: A case study in northern Costa Rica

Outlook The presented groundwater management system will be used as a basis for the development of a groundwater protection concept for northern Costa Rica in cooperation with the local governmental institutions during the next year. Up to what point the established database can be incorporated into a valuable policy for the improvement of groundwater management and protection, remains to be seen and will be monitored in the future. In this context, it is essential to incorporate the local population in decision making by means of new approaches of participation like those proposed, e.g. by Sandoval (2004) or Chebaane et al. (2004). The participative process will initiate a process of mentality change towards a responsible way to use and protect the fundamentally important resource of groundwater.

Acknowledgements This study was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft – DFG) within the research projects ME 1882/1-3 and ME 1882/1-4. We would also like to express our gratitude to Dr. Jochen Bundschuh (Instituto Costariciense de Electricidad – ICE) and Dr. Hans J. Hartmann (Universite ´ de La Rochelle, France) for their careful review of this paper. Thanks are also due to the foundation Asociacio ´n para el Manejo de la Cuenca del Rı´o Tempisque (ASOTEM) and its director Ing. Mauren Ballestero who offered great support during the field work campaign.

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