Risk-Based Assessment of Soil and Groundwater Quality in the Netherlands: Standards and Remediation Urgency

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  • Risk Analysis, Vol. 19, No. 6, 1999

    Risk-Based Assessment of Soil and Groundwater Quality inthe Netherlands: Standards and Remediation Urgency

    Frank A. Swartjes1

    To assess soil and groundwater quality two generic (i.e. multifunctional) risk-based standards,Target and Intervention Value, have been developed, in the framework of the Dutch SoilProtection Act. These standards allow soil and groundwater to be classified as clean, slightlycontaminated orseriously contaminated.The Target Value is basedon potential risks to ecosys-tems, while the Intervention Value is based on potential risks to humans and ecosystems. Inthe case of serious soil contamination the site has, in principle, to be remediated, making itnecessary to determine the remediation urgency on the basis of actual (i.e. site-specific) risksto humans and ecosystems and, besides, actual risks due to contaminant migration.

    KEY WORDS: Soil contamination; remediation urgency; standards; human exposure; ecotoxicologicalrisks; risk due to contaminant migration.


    1.1. History

    In 1983 the Dutch government published theInterim Soil Remediation Act and in support to thisAct the Soil Remediation Guideline.(1) This Guidelineoutlines how to take action on soil contaminationand included three soil and groundwater quality stan-dards:

    c the A-Value or Reference Value, based on thebackground concentration in the Nether-lands: where the average soil concentrationexceeded the A-Value soil contaminationwas present;

    c the C-Value, derived using expert judgement:where the average soil concentration exceededthe C-Value there was a need for remediation;

    c the B-Value, the average between A- and C-

    1 National Institute of Public Health and the Environment, Soiland Groundwater Research Laboratory, P.O. box 1, 3720 BA,Bilthoven, The Netherlands.

    12350272-4332/99/1200-1235$16.00/1 1999 Society for Risk Analysis

    Values: where the soil concentration exceededthe B-Value there was a need to perform afurther site investigation.

    The main purpose of the Soil Protection Act,introduced in 1987, is to establish the accountabilityof individuals in contributing to soil pollution, andincludes the question of financial responsibility forthe consequences of soil contamination. An evalua-tion of the Soil Remediation Guideline of the InterimSoil Remediation Act was also started in 1987, re-sulting in a number of studies, which were to lead toa major revision of parts of this Guideline. The maingoals of the revision were:

    c to provide scientifically based criteria for de-ciding on whether to undertake remediation(this criterion has been redefined as serioussoil contamination).

    c to link up with the general philosophy on risk-based standards in the Netherlands(2,3) and touse risk assessment and toxicological informa-tion to evaluate and if necessary adjust theformer A-, B-, and C-Values. The C-Values

  • 1236 Swartjes

    were renamed Intervention Values in accor-dance with the Framework of Definitions forEnvironmental Policy.(3)

    Considering the interest on integrated soil qual-ity standards and uniformity of definitions, it has beendecided to develop Intervention Values applicableto soils and sediments. In 1994 the Intervention Val-ues and the methodology to determine the urgency ofremediation were formalized and the Soil ProtectionGuideline (formerly called the Soil RemediationGuideline) was incorporated into the Dutch Soil Pro-tection Act; the Interim Soil Remediation Act wasdisposed of. In 1997 the Soil Protection Guideline wasextended by incorporating standards for the second(4)

    and the third series(5) of contaminants and in 1999by incorporating standards for the fourth series ofcontaminants(6) via Ministerial Circulars.(7,8)

    In 19911992 the Ministry of Housing, SpatialPlanning and the Environment (Ministry of VROM)initiated several working groups to evaluate the soilquality assessment procedures. Other relevant minis-tries participated in these groups, as well as represen-tatives from the provinces, municipalities, researchinstitutes, consultants and industry. The workinggroups, still operational, aim at evaluating and im-proving the procedures; they work in anticipation ofrapid developments in environmental policy.

    This paper describes the procedure for derivingthe risk-based Target and Intervention Values andpresents the procedures to determine the remedia-tion urgency for contaminated sites. A differentiationis made between potential and actual risk assessment.Potential risk is the risk that would occur under stan-dardized conditions and is independent of site-spe-cific characteristics like land-use. Actual risk, on thecontrary, is based on site-specific risks, e.g. site-spe-cific human exposure and site-specific ecological ef-fects. Actual risk is a function of land-use, humanbehavior, soil characteristics, et cetera.

    1.2. Soil and Groundwater Quality Assessment

    In the Netherlands site investigation starts withan Orientating Investigation, in which the first stepis an extended historical survey of the site and sur-rounding area, possibly leading to a hypothesis onthe spatial variability of the contaminant. Based onthis information a distinction is made between a ho-mogeneous and a heterogeneous contamination inthe protocol.(9) The heterogeneous contamination

    cases are divided into those for which the contamina-tion core is known and those with an unknown core.The answer to the following three questions form theresult of the Orientating Investigation:

    c Is the soil contaminated?c Was the hypothesis on spatial variability right?c Does the degree of contamination require fur-

    ther investigation?

    Site investigation related to this last question is tobe continued by the Further Investigation.(10) Thepurpose of this investigation is to quantify the typeand extent of the contamination. Both investigationshave to be carried out according to standardized pro-cedures for sampling strategy.

    Two soil quality standards have been derived toassess soil and groundwater quality: i.e. the TargetValue and Intervention Value (Fig. 1). Both stan-dards are based on potential risks, i.e. the risk thatwould occur under standardized conditions: theTarget Values on potential risks to ecosystems andthe Intervention Values on potential risks to humansand ecosystems. Note that a different ecotoxicologi-cal risk level is used for derivation of Target vs. Inter-vention Values. A further criterion used is the non-risk-based Intermediate Value, which is simply theaverage of Target and Intervention Values. Target,Intermediate and Intervention Values are employedindependent of soil use, e.g. for a residential or indus-trial site, nature reserve, et cetera. These are generic(i.e., multifunctional) criteria.

    From the site investigation the following implica-tions can result:

    c Concentration , Target Value (clean soil)means no restrictions.

    c Concentration . Target Value and , Interme-diate Value (slightly contaminated soil) meansno Further Investigation; (Minor) restrictionscan be imposed on soil use.

    c Concentration . Intermediate Value and ,

    Fig. 1. Sol and groundwater quality standards and assessment.

  • Standards and Remediation Urgency 1237

    Intervention Value means starting with theFurther Investigation. If this still results in soilquality , Intervention Value, restrictions canbe imposed on soil use. These are mainly basedon other instruments than the Soil ProtectionAct (e.g., no growth of sensitive food crops, nodirect use of groundwater as drinking water).

    c An average soil volume concentration of atleast 25 m3 (for soil quality assessment) or anaverage concentration in the porewater of awater-saturated soil volume of at least 100 m3

    (for groundwater quality assessment) . Inter-vention Value (seriously contaminated soil)means that in principle remediation will benecessary; the urgency of remediation has tobe determined.

    The derivation of Target and Intervention Values isextensively described in Chapter 3.

    The purpose of determining the urgency of re-mediation is distinguish between two urgency classes:urgent and non-urgent cases of serious soil contami-nation. Non-urgent cases are taken up in the provin-cial soil remediation program without a defined timefor starting the remediation. For urgent cases remedi-ation has to be initiated within one generation (circa20 years). The determination of remediation urgencyis based on actual (i.e., site-specific) risks to humans,ecosystems and risk due to contaminant migration.The procedure to determine the remediation urgencyis extensively described in Chapter 4.

    Because risk can show a large variation withinone type of soil use, soil use-specific standards havenot been derived. This means that the actual riskson the site have to be determined for each situationas a function of soil, soil use, building characteristicsand infrastructure and, ultimately, human behavior.Urgent cases of remediation are categorized intothree groups for which remediation has to be initiatedwithin 4 years (category I), between 4 and 10 years(category II) and between 10 and 20 years (categoryIII). This categorization is also based on risk scoresderived from the actual risks to humans, ecosystemsand risk due to contaminant migration. Finally, prio-ritization within one category is based on economic-financial, social and/or other environmental cri-teria.(11)


    The risk tools needed to derive soil and ground-water quality standards (Chapter 3) and to determine

    the remediation urgency (Chapter 4) are describedin this chapter.

    2.1. Human Toxicological Risk Assessment

    To assess the risk for humans, exposure and tol-erable exposure will have to be determined. The tol-erable exposure is based on human effects.

    2.1.1. Human Exposure

    Two human exposure models have been devel-oped: CSOIL for exposure to contaminated terres-trial soils and SEDISOIL for exposure to contami-nated sediments. Three elements are recognized inthese models:

    c contaminant distribution over the mobilephases of the soil and of the sediment;

    c contaminant transfer from (the different mo-bile phases of) the soil and sediment into con-tact media;

    c direct and indirect exposure to humans.

    To enable assessment of exposure to contaminantsin terrestrial soils the CSOIL calculation(12) uses asstarting-point the total soil concentration as repre-sentative soil content (Fig. 2).The distribution over the mobile soil phases (porewater and soil gas) is calculated according to thefugacity theory of Mackay and Paterson.(13) Formulaefor the following exposure routes have been includedin the model:

    c soil ingestion;c crop consumption;c drinking-water intake;c inhalation of air;c inhalation of soil particles;c inhalation of air during showering;c dermal uptake via soil;c dermal uptake during showering.

    In the Netherlands there are many cases ofgroundwater contamination with volatile contami-nants. The VOLASOIL model has been devel-oped(14) because the processes that determine theindoor air concentration are difficult to quantify,and the spatial and temporal variability of theindoor air concentration hampers accurate measur-ing. This model enables one to assess an indicationof the site-specific indoor air concentration via a

  • 1238 Swartjes

    Fig. 2. CSOIL model to quantify exposure to contaminated terrestrial soils.

    crawl space as a function of type and positioningof the contaminants, building and soil characteris-tics, and groundwater depth.

    To enable assessment of exposure to sediments,the SEDISOIL calculation(15) uses as starting-pointthe total sediment concentration as representativesediment content (Fig. 3).The distribution over the surface water and the sus-pended particles is calculated using average partitioncoefficients. Formulae for the following exposureroutes have been included in the model:

    c sediment ingestion;c surface-water ingestion;c suspended particles ingestion (together with

    surface water);c fish consumption;c dermal uptake via sediments;c dermal uptake via surface water.

    Fig. 3. SEDISOIL model to quantify exposure to contaminatedsediments.

  • Standards and Remediation Urgency 1239

    2.1.2. Tolerable Exposure

    A distinction has been made between non-threshold contaminants (genotoxic carcinogens) andthreshold contaminants (non-carcinogens and non-genotoxic carcinogens).(16) A tolerable exposure canbe derived for the threshold contaminants for whichno adverse effects for humans are likely to occur incases where this exposure is not exceeded. For thenon-threshold contaminants even the lowest expo-sure rate results in an increased chance of adverseeffects for humans.

    For non-genotoxic carcinogens and non-carcino-genic contaminants, the toxicological Tolerable DailyIntake (TDI), as derived by the WHO, is taken as theMaximum Permissible Risk for intake (MPRhuman). Ifno TDI is available, an Acceptable Daily Intake(ADI) is derived using the same procedure as thatused to derive a TDI. A TDI or ADI is the thresholdexposure of a contaminant to which humans can beorally exposed to daily on the basis of body weightwithout experiencing adverse effects on health.

    For genotoxic carcinogens, the MPRhuman is de-fined as the dose of a contaminant (based on bodyweight for oral intake or air volume for inhalatorintake) which forms a risk of one additional case oflethal tumor in 10,000 lifelong exposed individuals;this definition is based on a political decision.(2) Thetoxicity data have been profiled for all selected con-taminants of the first series,(17,18) the second series,(4)

    the third series(19) and the fourth series.(20) The valuesfor MPRhuman are given in Appendix A.

    2.2. Ecotoxicological Risk Assessment

    Two relationships for each contaminant havebeen derived to quantify the ecotoxicological effectson ecosystems, i.e.:

    c the relationship between soil concentrationand irreparable damage to terrestrial speciescomposition.(21,22)

    c the relationship between soil concentrationand adverse effects on microbial and enzy-matic processes.(23)

    The respective relationships are represented bythe HCp-terrestrial species and HCp-processes (Haz-ardous Concentration functions, where p representsthe threatened percentage of the ecosystem). Therelationships are derived on an empirical basis bystatistical interpretation of observed NOECs (No

    Observed Effect Concentrations) and LOECs (Low-est Observed Effect Concentrations),(24) assumingthat the sensitivity of species in an ecosystem can bedescribed by a statistical frequency distribution. IfNOECs are insufficiently available, L(E)Cs (LethalEffect Concentrations) are used. In this case theL(E)Cs are divided by a factor of 10 to account foruncertainty. The ecotoxicological data are selectedaccording to predefined criteria(23) and normalized forthe influence of soil characteristics on the bioavail-ability, using the organic matter and clay contentaccording to empirically derived formulae.(25) If notenough data on terrestrial species and microbial pro-cesses are available to derive a reliable rela...


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