Science of the Total Environment 485-486 (2014) 769–775
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Risk-based prioritization of ground water threatening point sources at catchment and regional scales Niels Døssing Overheu a,⁎, Nina Tuxen a, John Flyvbjerg b, Jens Aabling c, Jens Asger Andersen a,1, Jørn K. Pedersen d, Tina Thyregod d, Philip J. Binning e, Poul L. Bjerg e a
Orbicon, Denmark Capital Region of Denmark, Denmark Danish EPA, Denmark d Region of Southern Denmark, Denmark e Technical University of Denmark, Denmark b c
H I G H L I G H T S • • • • •
A Danish EPA handbook supports systematic large scale risk-based prioritisation. A flexible tool box guides the users through the necessary steps. Communication is supported by standardised GIS-themes, graphs and tables. Two case studies with very different challenges and needs are presented. The concepts are general and can be applied where similar challenges are faced.
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Article history: Received 26 June 2013 Received in revised form 18 March 2014 Accepted 18 March 2014 Available online 13 April 2014 Editor: D. Barcelo Keywords: Contaminated soils Ground water Risk assessment Prioritization Management Catchment scale
a b s t r a c t Contaminated sites threaten ground water resources all over the world. The available resources for investigation and remediation are limited compared to the scope of the problem, so prioritization is crucial to ensure that resources are allocated to the sites posing the greatest risk. A flexible framework has been developed to enable a systematic and transparent risk assessment and prioritization of contaminant point sources, considering the local, catchment, or regional scales (Danish EPA, 2011, 2012). The framework has been tested in several catchments in Denmark with different challenges and needs, and two of these are presented. Based on the lessons learned, the Danish EPA has prepared a handbook to guide the user through the steps in a risk-based prioritization (Danish EPA, 2012). It provides guidance on prioritization both in an administratively defined area such as a Danish Region, and within the bounds of a specified ground water catchment. The handbook presents several approaches in order to prevent the prioritization from foundering because of a lack of data or an inappropriate level of complexity. The developed prioritization tools, possible graphical presentation and use of the results are presented using the case studies as examples. The methodology was developed by a broad industry group including the Danish EPA, the Danish Regions, the Danish Nature Agency, the Technical University of Denmark, and consultants — and the framework has been widely accepted by the professional community in Denmark. The concepts are quite general and can be applied in other countries facing similar challenges. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Contaminated sites threaten ground water resources all over the world. In the European Environment Agency's most recent estimate, ⁎ Corresponding author. E-mail address:
[email protected] (N.D. Overheu). 1 Formerly with Danish Nature Agency.
http://dx.doi.org/10.1016/j.scitotenv.2014.03.083 0048-9697/© 2014 Elsevier B.V. All rights reserved.
there may be as many as 3 million contaminated sites in the EU, of which about 250000 sites require clean up (EEA, 2007). These numbers are increasing and despite the scope of the problem, the available resources for remediation are very limited. For these reasons prioritization is crucial to ensure that resources are allocated to the sites posing the greatest risk. The key questions are: Which actions should be taken at each site and in which order? To answer these questions, risk assessment and prioritization of contaminated sites are required at the site,
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catchment and regional scale, and must be developed on the basis of data of varying quality — ranging from very detailed to very poor information. The motivation for initiating clean-up is often determined by the measured impact, or estimated future impact on receiving waters or water supply wells. To address this need it has been proposed that risk assessments should be conducted at the catchment scale, where the risk of a contaminant point source to supply wells in the catchment is evaluated (e.g. Einarson and Mackay, 2001; Frind et al., 2006; Tait et al., 2004; Troldborg et al. 2008). The advantage of such a catchment-scale approach is that the risk of different sites is assessed for the same receptor and so can be compared, which is essential for prioritization. A different approach is to compare the risks of all contaminated sites in an administrative region instead of a water catchment. Both approaches are discussed here. Until recently, risk assessment in Denmark has been conducted only at the site itself, while catchment and regional impacts have not been treated systematically. In order to develop systematic catchment scale approaches, different risk assessment and prioritization methods have been tested by Danish authorities and a flexible framework for transparent risk assessment and prioritization has been developed. This paper presents the new approaches for regional and catchment scale risk assessment employed in Denmark. 2. Background: the legal and administrative framework Ground water resources are often threatened by many different sites and so it is crucial to obtain an overview of both the most hazardous sites, and how the cumulative impact of minor sites contributes to the overall impact on the quality of the ground water resource. If such a comprehensive overview is not obtained, there is a risk that investments in expensive remediation projects will not produce the desired results. The Danish Regions are responsible for the publicly financed efforts to locate, investigate and remediate contaminated sites which: 1. Pose a risk to groundwater resources suitable for use as drinking waterand/or
2. Pose a human health hazard — either because of an impact on indoor climate or through contact with contaminated soil. The Regions are the Danish public authority with the task of preventing, removing or limiting damaging effects of soil contamination on groundwater, human health and ecosystems. If a risk assessment indicates a threat to a receptor (reflected in exceeded threshold values), remediation is needed. The administrative workflow for determining which sites require cleanup is divided into a number of phases and is shown in Fig. 1. Public remediation efforts must be organized and prioritized within the constraints of the available economic resources. Classical risk assessment does not handle the question of how to prioritize between different sites within a larger area when the available economic resources are insufficient to assess all of them within a limited time frame. Other branches of the governmental also benefit from a better overview of the risks of contaminated sites. The Water Framework Directive and water action plans require coordinated efforts between different public authorities in order to ensure the best possible ground water protection. The increased need for interaction between authorities means that local scale risk assessment must be supplemented by broader consideration of contaminant impacts on catchment and regional scales. In Denmark, the Water Framework Directive is being implemented by the Danish Nature Agency who manages government policy on the natural environment. The Nature Agency is responsible for mapping the threats posed by pollution from agriculture and point sources to the most vulnerable ground water resources. A major challenge for the Nature Agency is the need to view groundwater and surface water as a single connected resource, and this increases the demand for risk assessment methods which examine more than local impacts. Since the Regions have the task of managing contaminated sites, while the Nature Agency focuses on groundwater resources, a collective overview of the groundwater threats is necessary to coordinate the public efforts — and do so cost-effectively.
Fig. 1. Phases in the Danish administrative workflow to locate, investigate and remediate contaminated sites. The figure shows the number of sites typically proceeding to each phase. Out of 100 potentially contaminated sites, 1–4 will typically need to be remediated after a number of investigation steps. Risk assessment is needed to decide which sites should proceed to the next phase, and prioritization is needed to determine the order in which sites should be addressed. Data from the Danish Regions annual report to the EPA (Danish EPA, 2010). “Oper.” means Operation and maintenance.
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3. A handbook for risk-based prioritization A review conducted for the Danish EPA concluded that the largest barriers to risk-based prioritization were lack of guidelines on how to carry out the task and a lack of systematically available data (Danish EPA, 2011). It was concluded that if the new guidelines were to be useful in administrative practice, they had to be flexible so they can be adapted to suit many types of problems and work with many types of available data. To address these needs, a handbook was prepared and published by the Danish EPA in 2012 (Danish EPA, 2012). The purpose of the handbook is to guide the user through the steps in a risk-based prioritization of groundwater threatening contaminated sites. The handbook presents several approaches in order to prevent the prioritization from foundering because of a lack of data or an inappropriate level of complexity. Thus, the handbook “tool box” contains a range of methods, from simple assessments to complex calculations on large data sets. The handbook provides guidance on how to select the appropriate prioritization methods.
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addresses their specific needs and challenges. Generally prioritization at the regional scale is more suitable for areas with a low density of contaminated sites and/or no critically threatened waterworks, while prioritization at catchment scale is the better approach when one or more waterworks are threatened by multiple (e.g. more than 10) contaminated sites. 3.2. Prioritization principles A number of different impact measures have been used to systematize prioritization in the past (Table 1). In recent years they are increasingly being supplemented by mass discharge and uncertainty estimates as these measures are well suited to compare the impacts of different sites. The Danish Regions have typically used different combinations and weighting of the parameters in Table 1 in their prioritization efforts. One of the intentions of the handbook was to systematize and document how to approach the problem and to introduce manageable ways to include mass discharge and uncertainty estimates.
3.1. Two approaches to prioritization 3.3. Methodology The handbook is a short and specialized guide. The prioritization needs and strategies of regulators vary, and the handbook introduces two general approaches to risk-based prioritization: Prioritization at a regional scale where the purpose is to prioritize sites at a given phase of the administrative workflow, within a larger geographical area that is not necessarily based on groundwater catchment boundaries (such as an entire Danish Region). The approach is shown in Fig. 1 and addresses the question: “How can a risk assessment framework be used to prioritize further efforts on a group of sites at a particular phase of the administrative workflow?” Prioritization within a groundwater catchment based on an overview of the cumulative risk posed by a group of contaminated sites within a given groundwater catchment. The purpose of this approach is to answer: “In which order should we target one or more contaminated sites in a catchment, if good quality groundwater is to be available in the catchment in future?” The two approaches are illustrated in Fig. 2. A regulatory authority can choose the approach (or combination of approaches) that best
A prioritization based on risk requires a systematic assessment of the potential risk of each site and compares the potential contaminant impacts of the sites. To describe these impacts it is useful to determine both contaminant concentrations and the mass discharges leaching from each site. 3.3.1. Local scale mass discharge calculation Two mass discharge calculation methods of different levels of complexity are included in the handbook. A simple method calculates a steady state mass discharge given by Eq. (1) below. MD ¼ C A N
ð1Þ
where MD is the mean contaminant mass discharge [g/year], C is the mean contaminant concentration in the source area [mg/L], A is the source area [m2], and N is the net infiltration through the source area [m/year]. A more advanced method employs analytical leaching models and measured data at each site to calculate a time-dependent mass discharge and concentrations leaching to the saturated zone (Troldborg et al., 2008; Chambon et al. 2011). The leaching models employ a simple
Fig. 2. Overview of the difference between prioritization at the regional scale and groundwater catchment scale. Dark circles indicate contaminated sites. These can be prioritized by comparing sites at the same stage of the administrative workflow (grouped in rows by solid lines) or to other sites within a groundwater catchment (grouped in columns by dotted lines).
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Table 1 General prioritization parameters. Parameters
Relevance
Traditionally used parameters
Location within administrative areas Distance to extraction wells Aquifer vulnerability
Recently included parameters
Contaminant type Contaminant concentration Contaminant mass discharge Uncertainty in the extent of contamination
High priority to sites in groundwater protected areas High priority to sites close to extractions wells, due to higher risk and shorter timeframe for preventive actions High priority in areas with thin or discontinuous clay layers and/or with a high net infiltration, due to higher risk and shorter timeframe for preventive actions High priority to toxic and/or mobile contaminants High priority to sites with high concentrations High priority to sites that cause a high contaminant load (kg/year) An uncertain source strength can have high or low priority, depending on the strategy and overall workflow in Fig. 1. Some authorities prioritize field work at uncertain sites highly to gain the best possible site knowledge before deciding with sites should be remediated. Others prioritize well-investigated sites highly to quickly pass them on to the remediation phase.
approach to link mass discharge to the remaining source mass and can be adapted to suit the available site knowledge at each site. 3.3.2. Local scale uncertainty estimation The uncertainty of the mass discharge estimates for each site can either be qualitatively assigned to one of three categories (low, medium or high uncertainty), or more systematically assessed using a scoring system. The scoring system is a qualitative/quantitative approach based on a series of questionnaires, where both the historical data and the site investigation data are evaluated (Overheu et al. 2010). An aggregated uncertainty score between 0 and 1 is evaluated which allows for intuitive visualization of the results. Local scale mass discharge and uncertainty estimates can be shown on GIS maps, which provide a good overview of the threats in a given area (Fig. 3). 3.3.3. Catchment scale aggregation For catchment scale prioritization, the individual and combined impacts on down gradient extraction wells or other receiving waters can be used for to prioritize risk-reducing efforts. Fate and transport modeling can be used to evaluate time-dependent or stationary impacts. Several calculation methods are available depending on the data at hand, and approaches range from simple catchment water balance analyses (Appelo and Postma, 2005) to 3D numerical modeling (Troldborg et al. 2008). Typically, the individual contaminant mass discharges are assigned to calculated or estimated pathways and the resulting time-dependent impacts on the water supply are determined by dividing the mass discharge by the extracted amount of water, to obtain the concentration of the contaminant at the well. Predicted contaminant breakthrough curves for the individual sites can then be plotted and compared with measured data and with the water quality criteria. An example of aggregated impacts is shown in Fig. 4. 3.3.4. Scoring system Prioritization can be supported by a flexible scoring system adapted from the widely used DRASTIC method (Aller et al., 1987), in which a number of relevant risk-parameters from Table 1 are weighted against each other as shown in Table 2. The final prioritization can be performed on the basis of these scores, overview maps such as Fig. 3 and breakthrough curves such as those shown in Fig. 4. The scoring system is particularly useful in regional scale prioritization where breakthrough curves at waterworks are not available for decision support. 3.3.5. Systematized data collection One of the intentions of the handbook is to utilize existing data in new ways and avoid unnecessary extra work when performing large scale prioritization. Usually, investment in data collection and
conceptual model development is required for local scale risk assessment. It is recommended that data collection is systematized so that data is readily available for local scale risk assessment and large scale prioritization. One way to do this is by using the short spread sheet based check list included in the handbook, describing the necessary information that needs to be gathered (and maintained) for each site. 4. Case studies The handbook framework has been tested in several catchments in Denmark, where the approach has been adapted to suit the requirements of the area. The framework is illustrated for two case studies with very different conditions and aims. Case study 1 covers a densely populated area of about 110 km2 in Greater Copenhagen with a large ground water extraction and a heavy contaminant load from several hundred potentially hazardous sites. Case study 2 covers a rural area of about 40 km2 with a low density of potentially hazardous sites, but with a highly prioritized ground water resource. Case study 1 is an example of prioritization within a number of groundwater catchments, while case study 2 is regional scale prioritization with some catchment-scale elements, cf. Fig. 2. 4.1. Case study 1: Greater Copenhagen The case study was carried out in 2011–2012 as part of the Danish Nature Agency's efforts to map the quality and quantity of groundwater in a densely populated and industry heavy area north of central Copenhagen (Danish Nature Agency, 2012). The area includes groundwater catchments for 12 public water supplies (waterworks) and 13 pumpand-treat facilities. The density of contaminated sites is high, and a large number of pump-and-treat facilities are in operation to preserve the drinking water resource in the area. Chlorinated solvents pose the most important threat to the groundwater. The purpose of the case study was to gain an overview of which drinking water extractions were threatened, and to determine where and in which order it would be expedient to conduct risk-reducing initiatives. The total number of potentially groundwater threatening sites was estimated to be 750, of which 120 were selected using a number of screening criteria for further analysis. The primary screening criterion was the presence of chlorinated solvents at the sites. For each of the 12 waterworks a comprehensive risk assessment was carried out on the basis of local scale mass discharge calculations, uncertainty estimates, and catchment scale fate and transport modeling was conducted using particle tracking in a ground water model. This was supplemented by catchment analysis and mass balance calculations for each of the pump-and-treat facilities. Figs. 3 and 4 are examples of
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Fig. 3. Overview of the calculated maximum mass discharge and its uncertainty for chlorinated solvent contaminated sites in the Søborg Waterworks catchment considered in case study 1. The light shaded area is the catchment area of the waterworks, while the darker area is the catchment area for two large pump-and-treat facilities within the bounds of the waterworks catchment.
the overview maps and breakthrough curve analyses created in the case study. The overview obtained made it possible to determine the most beneficial remedial actions in order to reduce the cumulative risk, and which data was needed to improve knowledge of the overall threat. The analysis showed that a number of waterworks were threatened by chlorinated solvents, and recommendations included prioritized actions at both the local (e.g. site surveys or remediation) and catchment
scales (e.g. monitoring or pumping strategies). Among the recommendations for the waterworks were: • Monitoring programs for sub-regions to provide early warning of possible contaminant plumes • Additional site investigations or revision of monitoring programs for critical sites • Maintenance or cessation of extraction at pump-and-treat facilities
Fig. 4. Calculation of the cumulative impact of chlorinated solvents in the extraction wells supplying the Søborg Waterworks in case study 1. Individual site impacts are not shown. The dotted curve shows the cumulative impact of all sites within the catchment area, while the solid curve omits impacts from sites which are hydraulically controlled by pump and treat, cf. Fig. 3. The breakthrough curves are compared with measured concentrations in the extraction wells and the Danish EPA threshold value for chlorinated solvents. The case study showed that current pump-and-treat operations reduce the potential contamination level at the waterworks by an order of magnitude. Efforts to further reduce the future impacts can be prioritized by looking at the individual site impact curves (not shown) and assessing which sites should be remediated to reduce the cumulative impacts to the desired level.
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Table 2 Example of a prioritization of a number of contaminated sites using several of the risk-parameters from Table 1. Normalized mass discharge is the mass discharge (e.g. g/year) normalized by the contaminant threshold value (e.g. μg/l). The normalized mass discharge represents the amount of potentially contaminated ground water per year, and allows for comparison between contaminant types with different threshold values. OSD areas in Denmark are highly valued groundwater resources (special drinking water interests), while OD areas have a lower priority. The scores and weights shown are not mandatory values, since local conditions may support other values or entirely different prioritization categories. Site address
Phase in admin, flow
Industrivej 20-22 Ronnebasrvej 2a Pilehaven 16 Industrivej 17-19 0stergade 9
Ext. survey Ext. survey Ext. survey Ext. survey Ext. survey
Contaminant risk
Normalized mass discharge
Groundwater protection
Infiltration
Category
Points
Weight
Category
Points
Weight
Category
Points
Weight
Category
Points
Weight
Chi. solvents MTBE Chi. solvents Oil BTEX
10 7.4 10 5.1 8.6
4 4 4 4 4
b10.000 m3/yr b10.000 m3/yr b100 m3/yr b1.000 m3/yr b100.000 m3/yr
5 5 1 3 7
4 4 4 4 4
OSD OSD OSD OSD OD
10 10 10 10 3
5 5 5 5 5
110 120 120 140 100
4 4 4 5 3
2 2 2 2 2
• Investigation of possible changes to the impacts of critical sites if extraction from a waterworks is changed • Site investigation of potentially critical sites where there is no data available • Tracing to find unidentified source(s) within a catchment zone • Recommendation to not take any actions because the threat level was assessed to be low This case study is an example of how the methodology can be used as a platform for cooperation between different authorities. • The Capital Region helped the Danish Nature Agency by providing data and information on the status of all of the contaminated sites, and the Region was responsible for assessment of the sites where it has conducted remediation and monitoring efforts. • The Danish Nature Agency helped the Region with information about hydrogeological conditions, mass discharge calculations and catchment scale risk assessment as an input to their further prioritization of efforts. • The results helped local municipalities and water supply companies by providing an overview of the threats towards water wells and by determining where it is expedient to intervene. The overview could readily be used in their own action plans. • Finally, the project worked as a platform for dialog between the Danish
Aggregated score 118 108 102 92 83
Nature Agency, the Region and several of the involved municipalities — for the benefit of all parties. For example, the project ensured that all the newest data was used and the parties were mutually aware of each other's needs for data, risk assessment and prioritization. 4.2. Case study 2: Triangle Region A valuable groundwater resource is located in the rural area between the cities of Vejle, Fredericia and Kolding in southern Denmark. The area contains relatively few contaminated sites and the knowledge level of these sites is relatively low. The Region of Southern Denmark wished to assess whether there is a potential threat towards the overall groundwater resource. If so, this would lead to prioritization of the critical sites, and if not, it would be documented that public efforts on the contaminated sites in the area have been sufficient (Region of Southern Denmark, 2011). The total number of contaminated sites potentially affecting groundwater in the area was 86, of which 13 were chosen for further analysis, primarily based on contaminant types at the sites. The risk of exceeding contaminant threshold values at the waterworks in the area now and in the future was assessed to be very small, and generally the risk to the groundwater resource was assessed to be negligible, cf. Fig. 5. The case study showed that under present
Fig. 5. Calculated maximum future impacts on waterworks for the sites considered in case study 2, compared to threshold values. The figure shows the sensitivity of the impacts for different degradation scenarios.
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Fig. 6. Calculated maximum future impacts on local groundwater resource for the sites in case study 2, compared to threshold values. The figure shows the sensitivity of the impacts for different degradation and recharge scenarios.
conditions no further action was needed to secure the ground water quality in the area. The local groundwater resource near (b100 m) two of the sites was calculated to be potentially threatened, cf. Fig. 6. However, the two site assessments were very uncertain, and the impacts were estimated conservatively, honoring the precautionary principle. If further assessment is needed in the future (e.g. if extraction wells are to be moved near the two sites), a small supplementary investigation may be needed to improve the reliability of the impact estimate of the two sites. Such a site investigation will be inexpensive and is likely to lead to the conclusion that there is no threat to the entire groundwater resource. As indicated, prioritization efforts in this case study were kept simple (i.e. no scoring sheets), and the outcome showed that only a few sites have a potential, but uncertain impact. 5. Perspectives A prioritization effort using the handbook guidelines is intended to provide the best possible overview of existing data by structuring, aggregating and visualizing the available data for the included sites and the area in which they are located. The overview can reveal whether investigation is needed before completing the mapping of an area, or if there are certain sub-areas that need to be prioritized to maintain groundwater quality. The methodology can be used as a dynamic tool which is updated with new knowledge at site or catchment scale. Sites with low priority will often be “parked” for many years before they are prioritized for renewed consideration. Authorities are often asked “why is nothing being done at this site?” The overview and documentation can assist regulatory authorities in communicating their justification for particular actions. The methodology was developed by a broad industry group including the Danish EPA, the Danish Regions, the Danish Nature Agency, the Technical University of Denmark, and consultants — and the framework has been widely accepted by the professional community in Denmark. The concepts are quite general and can be applied in other countries facing similar challenges. Risk-based prioritization and large-scale risk analyses are active research areas, and there is no European consensus on how to conduct such analyses. The methodology presented in the handbook has been
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