Joao Paulo Lobo-Ferreira
Dr.-Ing., Head of the Groundwater Research Group
Laboratório Nacional de Engenharia Civil
Av. do Brasil 101
1700-066 LISBOA, Portugal
Tel. +351 21 844 3609, Fax +351 21 844 3016
E-mail: lferreira@lnec.pt
Web: www.dha.lnec.pt/nas/english/staff/lferreira.html
Introduction
The groundwater group of the European Community Commission (EEC) held a meeting on February 91, in the European Water Institute, in Brussels, with the purpose of establishing an international agreement on common methodologies for the elaboration of a groundwater resource inventory for all Member-States. Such an inventory has been made in the past, between 1979 and 1981, for all countries that were Community Members-States at the time. The existing inventory needed to be updated to new Members-States, e.g. Spain, Portugal and the new Länder of Germany.
It was decided in the meeting that it was necessary to uniformize the criteria and procedures used by each Member-State to evaluate, rank, and map groundwater pollution vulnerability. The advantages of this uniformization are that:
maps generated in one country are readily understandable by all;
In Section 2 below, we propose a definition of the term vulnerability in the context of groundwater pollution. In Section 3, we suggest a system of evaluation and ranking of groundwater pollution vulnerability as defined in 2, to be adopted by all EU Member-States in the elaboration of their new vulnerability maps. Conclusions of this paper and the Portuguese DRASTIC groundwater vulnerability map are presented in Sections 5.
A new concept for the definition of groundwater vulnerability to pollution
Before we can consider the evaluation of groundwater vulnerability to pollution, it is necessary to define the term vulnerability. The term vulnerability has been defined and used before in the area of water resources, but within the context of system performance evaluation, e.g. the definition given by Hashimoto et al. (1982).
These authors present an analysis of system performance, which focuses on system failure. They define three concepts that provide useful measures of system performance: (1) how likely the system is to fail is measured by its reliability, (2) how quickly the system returns to a satisfactory state once a failure has occurred is expressed by its resiliency, and (3) how severe the likely consequences of failure may be is measured by its vulnerability. This concept of vulnerability defined in the context of system performance may also be used in the context of groundwater pollution if we replace "system failure" by "pollutant loading". The severity of the consequences are measured in terms of water quality deterioration, regardless of its value as a resource (for example, regardless of whether or not the aquifer is being used for public supply or is given any use at all).
However, the concept of vulnerability has not yet been unambiguously defined in the context of groundwater pollution, and the term has been used to mean different things. Often, the term "vulnerability to pollution" is used with a composite meaning that would perhaps be better described by risk of pollution.
We believe that the most useful definition of vulnerability is one that refers to the intrinsic characteristics of the aquifer, which are relatively static and mostly beyond human control. We propose that groundwater vulnerability to pollution be defined, in agreement with the conclusions and recommendations of the international conference on "Vulnerability of Soil and Groundwater to Pollutants", held in 1987 in The Netherlands (Duijvenbooden and Waegeningh, 1987), as
The sensitivity of groundwater quality to an imposed contaminant load, which is determined by the intrinsic characteristics of the aquifer.
Thus defined, vulnerability is distinct from pollution risk. Pollution risk depends not only on vulnerability but also on the existence of significant pollutant loading entering the subsurface environment. It is possible to have high aquifer vulnerability but no risk of pollution, if there is no significant pollutant loading; and to have high pollution risk in spite of low vulnerability, if the pollutant loading is exceptional. It is important to make clear the distinction between vulnerability and risk. This because risk of pollution is determined not only by the intrinsic characteristics of the aquifer, which are relatively static and hardly changeable, but also on the existence of potentially polluting activities, which are dynamic factors which can in principle be changed and controlled.
Considerations on whether a groundwater pollution episode will result in serious threat to groundwater quality and thus to its (already developed, or designated) water supply are not included in the proposed definition of vulnerability. The seriousness of the impact on water use will depend not only on aquifer vulnerability to pollution but also on the magnitude of the pollution episode, and the value of the groundwater resource.
Suggested system of vulnerability evaluation and ranking of European Union
member-states
General Considerations
Given the definition of vulnerability proposed in Section 2, it is important to recognise that the vulnerability of an aquifer will be different for different pollutants. For example, groundwater quality may be highly vulnerable to the loading of nitrates at the surface, originated in agricultural practices, and yet be little vulnerable to the loading of pathogens.
In view of this reality, it is scientifically most sound to evaluate vulnerability to pollution in relation to a particular class of pollutant, such as nutrients, organics, heavy metals, pathogens, etc., i.e. to create specific vulnerability maps. This point of view has been expressed by other authors (e.g. Foster, 1987), and some work has been done in specific vulnerability mapping. An example is the work of Canter et al. (1987) for nitrate pollution of agricultural origin. Alternatively, vulnerability mapping could be performed in relation to groups of polluting activities, such as unsewered sanitation, agriculture, and particular groups of industries. This has been attempted for some activities. An example is the work of le Grand (1983) for waste disposal.
Although we recognise that specific vulnerability mapping is scientifically more sound, we must realise that there will generally be insufficient available data to perform specific vulnerability mapping. Therefore, it is necessary to adopt a mapping system that is simple enough to apply the data generally available, and yet is capable of making best use of those data in a technically valid and useful way. Various such systems of vulnerability evaluation and ranking have been developed and applied in the past. Examples are Albinet and Margat (1970), Haertle (1983), Aller et al. (1987), and Foster (1987).
Some of the systems for vulnerability evaluation and ranking include a vulnerability index which is computed from hydrogeological, morphological and other aquifer characteristics in some well-defined way. The adoption of an index has the advantage of, in principle, eliminating or minimising subjectivity in the ranking process. Given the multitude of authors and potential users of vulnerability maps in EEC countries, Lobo-Ferreira and Cabral (1991) suggested that a vulnerability index be used in the vulnerability ranking performed for Community maps.
Such a standardised index has been adopted in the U.S., Canada and South Africa, and is currently used in those countries: the index DRASTIC, developed by Aller et al. (1987) for the U.S. EPA. This index has the characteristics of simplicity and usefulness that we think necessary. The DRASTIC index method is briefly reviewed below.
3.2. The index of vulnerability DRASTIC
The index of vulnerability DRASTIC corresponds to the weighted average of 7 values corresponding to 7 hydrogeologic parameters. In the following table the DRASTIC parameters are presented together with the weights respectively for normal DRASTIC applications and for DRASTIC pesticide applications:
|
DRASTIC |
Pesticide DRASTIC |
Depth to the water table |
5 |
5 |
Net Recharge |
4 |
4 |
Aquifer material |
3 |
3 |
Soil type |
2 |
5 |
Topography |
1 |
3 |
Impact of the vadose zone |
5 |
4 |
Hydraulic Conductivity |
3 |
2 |
We attribute a value between 1 and 10 to each parameter, depending on local conditions. High values correspond to high vulnerability. The attributed values are obtained from tables, which give the correspondence between local hydrogeologic characteristics and the parameter value. Next, the local index of vulnerability is computed through multiplication of the value attributed to each parameter by its relative weight, and adding up all seven products. Thus, each parameter has a predetermined, fixed, relative weight that reflects its relative importance to vulnerability. The most significant factors have weights of 5; the least significant a weight of 1. A second weight has been assigned to reflect the agricultural usage of pesticides. The minimum value of the DRASTIC index is therefore 23 and the maximum value is 226. Such extreme values are very rare, the most common values being within the range 50 to 200.
Examples of the application of DRASTIC may be seen in:
Summary projects on the development of an inventory of the groundwater resources
of portugal including the vulnerability assessment of groundwater to pollution using the
drastic index method
In 1995, Laboratório Nacional de Engenharia Civil (LNEC) published a series of three volumes, Lobo-Ferreira et al. (1995a), containing a collection of 11 Reports developed by LNEC. The study was developed for the European Commission's General-Directorate on the Environment, Nuclear Safety and Civil Protection (DGXI), and for several Portuguese authorities, e.g. Direcção-Geral do Ambiente (DGA).
The main objective of the Project was the development of "An Inventory of the Groundwater Resources of Portugal", including:
The maps of the aquifer systems, hydrogeological parameters, aquifer's recharge and the final map of DRASTIC aquifer's vulnerability of Portugal, all in scale 1:500 000 were developed in ARC/INFO (the maps are presented in a 1:1 500 000 scale in Lobo-Ferreira et al. (1995a). In Figure 1 we present, developed by Lobo-Ferreira and Oliveira (1993), a one page map of the DRASTIC index vulnerability assessment of Portuguese groundwater (cf. www.dha.lnec.pt/nas/estudos/inventE.html).
Several other studies, that included DRASTIC groundwater vulnerability assessment, were developed in Portugal following the methodology presented Lobo-Ferreira and Oliveira (1993). Among those study we highlight the "Study for evaluation of the vulnerability of the reception capacity of coastal zone water resources in Portugal. The receiving water bodies: groundwater systems", Lobo-Ferreira et al. (1995b). In this study, the aquifers of the coastal areas of Portugal and the situation concerning groundwater exploitation were characterised. The main pollution problems affecting these areas, with a special attention to the salt water intrusion phenomenon, were described. In Fig. 2, an example of the DRASTIC vulnerability map for the Peniche area, in Portugal's central coastal zone is presented.
Figure 1. DRASTIC groundwater vulnerability mapping
of mainland Portugal developed by Lobo-Ferreira and Oliveira (1993)
Conclusions
It is our opinion that the vulnerability evaluation procedure should correspond to a well-defined computation of an index, in order to minimise subjectivity involved in the ranking. The system applied to the EEC countries in the seventies represented a pioneer effort, but regardless of its merit however allowed for subjectiveness in the evaluation process. This limitation was emphasised in the conference on "Vulnerability of Soil and Groundwater to Pollutants" held in The Netherlands in 1987. To overcome these limitations and in order to guarantee the compatibility and coherence of the various national databanks, we suggest that vulnerability ranking be made through a well-defined computation leading to a final index. Such an index should meet the requirements of being relatively simple, given the limitations of generally available data, while being technically sound and valid for vulnerability classification. One existing index that meets these requirements, is the index DRASTIC (by Aller et al., 1987), adopted by the U.S. EPA and used in Canada as well.
Lobo-Ferreira and Cabral (1991) recommend that the index DRASTIC be selected for the elaboration of the EEC groundwater vulnerability maps. The EU-DGXI sponsored the Portuguese as well as the Spanish Guadalquivir basin vulnerability assessment and mapping, both developed during the first half of the nineties. An important advantage offered by DRASTIC is the amount of existing experience on its application, in the U.S. and Canada.
Figure 5. Vulnerability map at the scale 1:100 000 of
Peniche coastal zone, in central Portugal, developed by Lobo-Ferreira et al. (1995b)
In our opinion that research based on the aquifer vulnerability concept and the corresponding data acquisition process, such as the one required by the DRASTIC index method, exemplified in this paper for Portuguese conditions, allows a sounder management of groundwater resources. DRASTIC contains an excellent conceptual basis for the application of mathematical groundwater flow and mass transport models, also for salt-water intrusion assessment in coastal zones.
References
Hashimoto, T., Stedinger, J.R. and Loucks, D.P. (1982) Reliability, Resiliency, and Vulnerability Criteria for Water Resource System Performance Evaluation, Water Resources Research, 18(1), p14-20.
Duijvenbooden, W. van and Waegeningh, H.G. van (1987) Vulnerability of Soil and Groundwater to Pollutants, Proceedings and Information No. 38 of the International Conference held in the Netherlands, in 1987, TNO Committee on Hydrological Research, Delft, The Netherlands.
Foster, S.S.D. (1987) Fundamental concepts in aquifer vulnerability, pollution risk and protection strategy, in W. van Duijvanbooden and H.G. van Waegeningh (eds.), Vulnerability of Soil and Groundwater to Pollution, Proceedings and Information No. 38 of the International Conference held in the Netherlands, in 1987, TNO Committee on Hydrological Research, Delft, The Netherlands.
Canter, L., Knox, R. and Fairchield, D. (1987) Ground Water Quality Protection, Lewis Publishers, Inc., Chelsea, Mi.
le Grand, H.E. (1983) A standardized system for evaluating waste disposal sites, NWWA, Worthington, Ohio.
Albinet, M. and Margat, J. (1970) Cartographie de la vulnérabilité a la pollution des nappes d'eau souterraine, Bull. BRGM 2me Series 3 (4).
Haertle, A. (1983) Method of working and employment of EDP during the preparation of groundwater vulnerability maps, IAHS Publ. 142(2).
Aller, L., Bennet, T., Lehr, J.H. and Petty, R.J. (1987) DRASTIC: a standardized system for evaluating groundwater pollution potential using hydrogeologic settings, U.S. EPA Report 600/2-85/018.
Lobo-Ferreira, J.P. and Cabral, M. (1991) Proposal for an Operational Definition of Vulnerability for the European Community's Atlas of Groundwater Resources, in Meeting of the European Institute for Water, Groundwater Work Group Brussels, Feb. 1991.
Navalur, K.C.S. and Engel, B.A. (1997) Predicting spatial distribution of vulnerability of Indiana State Aquifer system to nitrate leaching using a GIS, in www.ncgia.ucsb.edu/conf/SANTA_FE_CD-ROM/sf_papers/navulur_kumar/my_paper.html
Lynch, S.D., Reynders, A.G. and Schulz, R.R. (1993) Preparing input data for a national-scale groundwater vulnerability map of Southern Africa, in 6th National Hydrological Symposium, SANCIAHS, Sept. 1993 and Water S.A. (1994) 20, 239-246, Pietermaritzburg, South Africa.
Lobo-Ferreira, J.P., Oliveira, M. Mendes and Ciabatti, P.C. (1995) Desenvolvimento de um Inventário da Águas Subterrâneas de Portugal. Volume 1. Laboratório Nacional de Engenharia Civil, Lisboa.
Lobo-Ferreira, J.P. and Oliveira, M.M. (1993) Desenvolvimento de um Inventário das Águas Subterrâneas de Portugal. Caracterização dos recursos Hídricos Subterrâneos e Mapeamento DRASTIC da Vulnerabilidade dos Aquíferos de Portugal, Laboratório Nacional de Engenharia Civil, Relatório 179/93 - GIAS, Lisboa.
Lobo-Ferreira, J.P., Oliveira, M.M., Moinante, M.J., Theves, T. and Diamantino, C. (1995) Mapeamento das Águas Subterrâneas da Faixa Costeira Litoral e da Vulnerabilidade dos seus Aquíferos à Poluição. Relatório Específico R3.3, LNEC, Relatório 237/95 - GIAS, 585 pp., Laboratório Nacional de Engenharia Civil, Lisboa.
