Assessment of Groundwater Vulnerability

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Matthias Hinderer

Darmstadt University of Technology,Institute of Applied Geosciences,Schnittspahnstr.9,64287 Darmstadt,Germany

1　General aspects

The protection of water resources,under quantitative and qualitative aspects,is one of the most urgent problems in the world-wide.Groundwater is sensitive against pollution by various chemicals from human activities via seepage through the soil and unsaturated zone.However,not all groundwater is endangered to the same degree,e.g.the rocks of an aquifer may directly reach the earth surface or they may be covered by more or less impermeable layers.The concept of groundwater vulnerability tries to classify these differences.

To protect groundwater and water supply systems against pollution a survey of the nature and status of the groundwater system as well as recommendations to decision-makers for environmental and infrastructure planning,agricultural and instrial management practices,potential danger of water resources to be contaminated,remediation measures,and scenarios for the future must be undertaken.The survey of a groundwater system with respect to its pollution involve:（i）Assessment of susceptibility against pollution,（ii）Behaviour of pollutants in the subsurface,（iii）Screening and long-term monitoring of groundwater quality,and（iv）Identification of sources of potential pollution.Decisions have to be made with respect to（i）Land use planning,（ii）Agricultural management practice,（iii）Remediation techniques,and（iv）Future scenarios.

The principle of sustainability aims to primarily protect groundwater against pollution be-fore it may reach wells and drinking water extraction areas.Therefore,assessment of the susceptibility of groundwater against pollution,i.e.the danger to be contaminated is the major task of this concept.This principle is not completely new in the sense that drinking water wells in instrialised countries have been protected by various concepts of protection zones since several decades.Protection zones are defined by estimated travel times of contaminants and restrictions to land use increase with decreasing distance to the extraction zone（see contribution Prof.Balke“Water protection zones”）.The concept of groundwater vulnerability is more universal and not connected to groundwater exploitation.Instead,a large number of environmental information is used to assess groundwater vulnerability for large areas（more and more entire river basins and states）and on a long-term perspective.Recently,this comprehensive principle of water protection also has become part of the EU framework directive of water and is increasingly applied in developing countries.

2　Definition and application

Indivial environmental parameters and processes can enhance or rece the negative impact of pollutants on groundwater.Just as man-made filtering device can be overloaded,so can the natural filtering capacity of soil and groundwater.The concept to define the susceptibility and filtering capacity of the groundwater system is called vulnerability.Physical,chemical,and biological processes in the soil,unsaturated zone,and saturated zone control the mobility or decay of dissolved pollutants（details see e.g.“Mobility of heavy metals in soils,groundwater,and surface water”）.

One can distinguish between：

（1）Intrinsic vulnerability,which only considers geogenic and natural process-es.Parameters under consideration are geological-lithological（infiltration and percolation zones）,morphological（surface forms）,geochemical（binding conditions of soil,composition of pollutants）,physical（adhesive and cohesive forces in soils）,and hydrogeological（surface and subterranean flow,flow direction and speed）.

（2）Specific vulnerability,which also consider the potential impact of human activities by adding temporal and/or economic aspects.Hereby,thresholds of an acceptable level of contamination（e.g.limiting values for drinking water quality,critical loads）may be defined.

The concept of specific vulnerability remains ambiguous because it mix up intrinsic characteristics with pollution risk.It is more useful to distinct between both aspects of groundwater pollution.In this sense the term vulnerability is better used in its restricted sense of intrinsic vulnerability.On the International Conference on“Vulnerability of soil and groundwater to pollutants”held in 1987 the following definition has been given in this sense（Duijvenbooden and Waegeningh,1987）:Groundwater vulnerability is 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.Considerations on whether a groundwater pollution episode will result in serious threat to groundwater quality and thus to its water supply are not included in the proposed definition of vulnerability（Fig.1）.

Applications of the concept of vulnerability are:

（1）Water protection zones for drinking water wells，

（2）Regulation of agricultural soil treatment（e.g.fertilizers,pesticides,herbicides），

（3）Optimisation of groundwater monitoring，

（4）Risk assessment in city planning and new instrial developing areas，

（5）Risk assessment of contaminated sites，

Fig.1　Pollution path and assessment of pollution risk,intrinsic vulnerability,and specific vulnerability for groundwater

（5）Set-up of scenarios of groundwater pollution.

3 Methods

3.1　Hydrogeological system analysis

Until the late seventies,classified analogue maps have been evaluated to qualitatively assess groundwater vulnerability on a large scale.Several European countries published overview maps of groundwater vulnerability based on this concept.Geological maps and soil maps are reinterpreted in terms of permeability,thickness of unsaturated zone,and groundwater recharge rates.Nowadays,GIS techniques allow for a much more complex and resolved analysis of environmental data.

The qualitative analysis of the hydrogeological system is still useful to identify most relevant parameters and processes for further analysis.Especially for karst aquifers specific concepts have been developed,e.g.the COST action No.65“Hydrogeological aspects of groundwater protection in karst areas”of the European Union（COST-Action 65,1995）.Recently,a specific definition of protection zones in karst areas is used in Germany by the national surveys（Fig.2）.

Fig.2　Important components of the hydrogeological system analysis（after Magiera,2000）

3.2　Index methods and analogue relations

In the 1980ies,index methods and analogue relations have been developed to determine the decay behaviour of pesticides in soils.Based on a simplified transport equation using physical,chemical and pesticide-specific data the Attenuation Factor（AF）have been introced by Rao et al.（1985）.In contrast to other indices it has widely used in case studies.A more general index,which is not related to a specific pollutant is the Aquifer Vulnerability Index（AVI）（van Stempvoort et al.1993）.It is based on the thickness and permeability of each sediment layer above the groundwater surface（“hydraulic resistance”）.Index methods are still in use for screening of agricultural areas but have been rarely verified（Fig.3）.

Fig.3　Important components of index methods and analogue relations（after Magiera,2000）

3.3 Point rating and matrix systems

Point rating and matrix systems have been developed for medium to large-scale areas.They are completely empirical and use a“general”pollutant.The parameters are selected according to their relevance for groundwater protection.A classification and ranking of these parameters lead to a final assessment of vulnerability.

Most parameters are related to the properties of the cover and groundwater.Because these methods only need a limited number of data they are often used for less surveyed areas.Among others the DRASTIC concept of the Environmental Protection Agency of the United States is widely applied（Aller et al.1985）.Several official vulnerability maps of US states and also in the European Union（e.g.Portugal）use this concept.It is based on seven parameters:Depth to water,Recharge,Aquifer media,Soil media,Topography（slope）,Impact of vadose zone（hydraulic）,Conctivity of the aquifer.All these parameters are evaluated and ranked and four classes of vulnerability are formed.

In Germany the national surveys use point rating systems to evaluate the protection capacity of the cover（Holting et al.,1995）.Parameters under consideration are restricted to soil and rock proper-ties:thickness of covering layers（M）,rock type（G）,field capacity of the soil（B）,groundwater recharge（W）and presence of artesian（D）or perched（Q）groundwater.The parameters are evaluated by point rating and summed up.Groundwater recharge acts as a weighting factor.Five vulnerability classes from very low to very high are distinguished（Fig.4）.

Fig.4　Important components of point rating and matrix systems（after Magiera,2000）

3.4　Mathematical models

Nowadays,numerous mathematical models are available to simulate groundwater flow and contaminant transport.They have become a standard tool for small-scale as well as large-scale groundwater management.Deterministic modelling of contaminant transport,however,needs a large data base,a careful field survey,and often also lab experiments（e.g.leaching experiments）.Thus,applications of sophisticated mathematical modelling of contaminant transport is usually restricted to intensively studied sites and dangerous point sources of contamination,e.g.contaminated instrial sites,waste deposits,oil spills.Some authors also consider diffuse groundwater pollution by combining groundwater models with models of contaminant leaching,unsaturated flow（e.g.nitrate,pesticides）or by coupling with point rating and matrix systems（e.g.DRASTIC）（Fig.5）.

Fig.5　Important components of mathematical models（after Magiera,2000）

3.5　Statistical methods

Statistical methods are increasingly used.They overcome the problem of selecting key parameters and allow for a much better documentation of uncertainties of data and methods.In order to do this,a complete digital data base is necessary,e.g.soil parameters,thickness of cover,permeabilities,hydrochemistry,environmental isotopes.Instead of defining arbitrarily artificial classes of groundwater vulnerability,probabilities and spatial correlations are calculated.Correlation of regionalized groundwater pollution data or environmental isotope data with data on soils,cover thickness and aquifer allow to verify the assessment and to extract control-ling parameters and temporal evolution.Several authors estimate groundwater vulnerability only from groundwater data,e.g.presence of tritium,nitrate concentrations,and specific groups of pollutants.Prerequisite is a dense monitoring network.Usually,data on soil and the geology of surface strata are easier to evaluate especially in developing countries（Fig.6）.

4　Examples of vulnerability maps

Nowadays,vulnerability maps are developed from a digital data base,which is managed in a GIS.The data necessary to create maps of intrinsic vulnerability include:

（1）Point information:wells and springs（lithology,permeability,hydrochemistry,isotopes etc.）

（2）Linear information:rivers,irrigation and drainage channels,etc.

Fig.6　Important components of statistical methods（after Magiera,2000）

（3）Areal information:geology,groundwater level,groundwater recharge,topography,soil,land use.

Point information might be regionalized and become areal data,e.g.permeability maps.（Fig.7）

Fig.7　Groundwater vulnerability map for the center of Berlin

5 Conclusions

The assessment of groundwater vulnerability is fundamental to any environmental planning and sustainable water management.Methods differ with respect to scale,selected parameters and processes,verification,pollutants,and data availability.Most definitions and methods follow the instrinsic properties of a groundwater system,which is mostly beyond human control,and do not specify the pollutant.In spite of the usefulness to evaluate vulnerability to pollution in relation to a particular class of pollutants,such as nutrients,organics,heavy metals,pathogens,etc.,generally there will be insufficient data available to perform specific vulnerability mapping.

In instrial countries numerous overview maps on various scales exist.They are particular useful for infrastructural,water resources and land use management and provide an important decision-making tool.In particular,recommendations or regulations for the use of fertilizers and pesticides in agriculture,for the selection of sites for waste deposits,for the handling of dangerous substances,and for mining activities can be given by such maps.The vulnerability concept is part of the EU water directive,which is based on the principle of a comprehensive protection of all groundwater against any kind and intensity of contamination.For this purpose index methods,analogue relations,point rating,and matrix systems are preferentially used which focus on the soil and unsaturated zone.

For point sources of contamination a pollutant and time-specific assessment of groundwater vulnerability is more appropriate.Here,mathematical and statistical models are preferentially applied which focus on the saturated zone.They are mostly verified and scenarios on pollution transport can be calculated by considering the residence time of contaminants,and natural attenuation processes.

References

[1]Aller L,Bennet T.,Lehr J.H.,and Petty,R.J.DRASTIC:a standardized system for evaluating groundwater pollution potential using hydrogeological settings,U.S.EPA Report 600/2-85/018.（1987）.

[2]COST-Action 65.Karstgroundwater protection.Final report-European Commission,Report EUR 16547 EN,（1995）,Brussels,Luxembourg.246 p.

[3]Duijvenbooden W.van and Waegeningh H.G.van.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.1987.

[4]Hölting B.,Haertle T.,Hohberger K.H.,Nachtigall K.H.,Villinger E.,Weinzierl W.,and Wrobel J.P.Konzept zur Ermittlung der Schutzfunktion der Grundwasserüberdeckung.Geologisches Jahrbuch Reihe C,Heft（1995）,63:5～24.

[5]Magiera P.Methoden zur Abschätzng der Verschmutzungsempfindlichkeit des Grundwassers.-Grundwasser,2000,5:103～114.

[6]Rao P.,Homsby A.and Jessur R.Inices for ranking the potential for pesticide contamination of groundwater.Proc.Soil Crop Sci.Soc.Florida,1985,44:1～8.

[7]Stempvoort D.van,Ewert L.,Wassenaar L.Aquifer vulnerability index:A GIS-compatible method for groundwater vulnerability mapping.Canad.Water Res.J.,1993,18:25～37.