Characterisation of pollutants

For the purpose of considering groundwater management and as a prerequisite for describing fate and transport of pollutants it is recommended to characterise the properties of substances.

The spread of pollutants is influenced by their physical, chemical and biological properties.

Substance specific data should be assembled for the following:

o physical and chemical properties (e.g. water solubility, vapour pressure),

o environmental fate and partitioning (e.g. partitioning coefficients, abiotic degradation, biodegradation), and

o ecotoxicity data on dose-response-related effects regarding the aquatic environment.

As a basis for a thorough description of the behaviour and occurrence of inorganic and organic groundwater pollutants the BRIDGE WP 1 report (GRIFFIONEN et al, 2006; see www.wfd-bridge.net) can be taken. This report also provides an introduction on research results related to (eco)toxicological effects of potentially harmful substances on groundwater in chapter 8. For priority substances (see WFD, Annex X), which are 33 substances or groups of substances of highest concern in the field of water policy at Community level, substance specific data sheets recording data as described above have been assembled under the Common Implementation Strategy and are publicly available at the homepage of the European Commission

(http://forum.europa.eu.int/Public/irc/env/wfd/library?l=/framework_directive/i-priority_substances/supporting_background/substance_sheets&vm=detailed&sb=Title).

Furthermore it may be helpful to recognise categories of substances in accordance with how they occur and in particular to note that some substances have both natural and anthropogenic sources whilst others are completely synthetic.

• naturally occurring substances or ions which may also be introduced as pollutants by man.

Variability in the natural background levels of such substances in groundwater (both within and between bodies) may make it impractical to apply environmental thresholds without reference to the conditions in a particular groundwater body, for example ammonium.

• naturally occurring substances that are not normally found in elevated concentrations in groundwater, such that their presence alone would normally indicate either anthropogenic inputs or very unusual hydrogeochemical conditions, for example mercury.

• synthetic (man-made) substances not found in the natural environment, for example trichloroethylene.

• Parameters indicative of pollution, for example Conductivity.

These categories do not relate to chemistry or toxicity but are indicative of potential differences in monitoring, assessment and TV derivation and indeed in potential controls which may be used.

Deliverable 18 11 / 63 5.2 Characterisation of groundwater bodies

For the purpose of characterising groundwater bodies regarding pollutant behaviour, data are needed on

o the petrographic properties and the physical hydrogeology, o the hydrogeochemical natural background composition, o groundwater hydraulics and

o (bio)-geochemical characteristics.

The general chemical quality of groundwater is determined by a variety of factors, where the petrographic properties of the rocks of the vadose and groundwater saturated zone, and the regional hydrological and hydrodynamic conditions are the major natural factors. To facilitate and as a starting point for describing these elements BRIDGE has come up with a classification of hydrogeological units.

As outlined under 5.1, it is necessary to understand the natural background levels (NBL) of a substance in a groundwater body. NBL is defined as the concentration of a given element, species or chemical substance present in solution which is derived by natural processes from geological, biological or atmospheric sources. Substances need to be understood in the context of their geochemical setting. This may often be difficult where substances exhibit high NBL in relation to any presumed anthropogenic component.

Information on the NBL for a substance may be derived for a specific body from the monitoring carried out either already before the WFD or as part of the implementation of the WFD

provisions. However, where such data are not available or insufficient it would be useful to be able to refer to the composition of similar aquifers from an agreed European typology. Such a typology has been generated within BRIDGE and is described in chapter 6 of the report “Impact of hydrogeological conditions on pollutant behaviour in groundwater and related ecosystems”

(PAUWELS et al, 2006; see www.wfd-bridge.net) and may be of use for defining likely NBL in groundwater bodies.

There are, however, a variety of methods available for defining and determining NBL in practice. Some methods for determining NBL are also described within chapter 6 of the above mentioned report. Annex 1 of this report details a procedure that may be useful within the context of establishing natural background levels. The procedure integrates several

approaches providing flexibility according to data availability within a specific groundwater body.

Groundwater chemistry cannot be understood without an understanding of groundwater hydraulics. Therefore any classification of chemical status and consequently also a

methodology to derive environmental thresholds will often rely on data from any quantitative assessment. Such assessment would in principle have to establish the hydraulic connections between, and degree of impact on, flows and levels in dependent surface waters and terrestrial ecosystems.

In addition, the potential for reversals in flow direction that might result in risk to groundwater (e.g. from saline intrusion) would also have been established. An introduction and further information can be found in chapter 9 of the BRIDGE WP2 report (PAUWELS et al., 2006) Finally aquifers also exhibit variable potential for the attenuation of pollutants within them. This potential will depend on the geochemistry and biochemistry of the body and like the NBL above needs to be determined at the level of the individual body. A more detailed description of concepts for characterisation of aquifers regarding transport and attenuation of substances is provided in the chapters 4 and 5 of the above mentioned BRIDGE WP2 report. However, where data are either not available or insufficient, the typology developed for European aquifers can

Deliverable 18 12 / 63

help to provide information for similar circumstances. The summarising tables for sources and controls of naturally occurring contaminants can be considered when assessing status, particularly when undertaking the specific local investigation and assessment when threshold values are exceeded at single monitoring stations.

5.3 Characterisation of Receptors

The GWDD recognises that groundwater is a valuable resource which should be protected from pollution and that this is particularly so for groundwater dependent ecosystems and for drinking water resources.

Obviously, the WFD bodies (both surface and groundwater) have already been delineated and the reasons for “at risk” designations for individual bodies and the characterisation processes are set out in the WDF Article 5 reports. However, for determining Status further

characterisation is needed in order to have a better understanding of the relationship between the groundwater body and final receptor. When the final receptor is the groundwater body itself that may be relatively straightforward but for associated surface ecosystems it becomes more complex.

Hence, it is likely to be the relative mass flow of pollutant(s) exchanged between groundwater and surface water which is important in determining standards for protection of surface waters, rather than the concentration of pollutants in groundwater. This in turn will rely on knowledge of the groundwater contribution within the overall surface water load and may vary considerably between locations, and will also depend on the substance in question. In reality the variability of such relationships across Europe will be substantial, and is a strong argument for the derivation of environmental thresholds on a regional or local basis.

As groundwater dependent terrestrial ecosystems do not have status objectives in themselves, the relationship is somewhat different to surface waters. To date there is very little data on the chemical dependencies of such ecosystems on groundwater inputs. Even the extent of

groundwater support to many sites is unclear so the derivation and use of thresholds at the current stage of knowledge is likely to be unhelpful. Instead of establishing single chemical standards or thresholds a complementary approach using other ecological indicators could facilitate the procedure but probably would go beyond the scope of BRIDGE.

Groundwater is often an extremely important drinking water resource or freshwater resource for other uses (e.g. agriculture, industry) and at a regional scale it often underpins the sustainable development of society by means of social welfare and environmental quality. On the other hand there are geological situations and aquifers which are neither of ecological importance nor are able to support human demands due to their low transmissivity. In characterising

groundwater as the receptor it will be important to understand the reason why groundwater is the receptor and the risks to it. Characterisation may well need to include external factors such as the degree of existing treatment (for drinking water supplies) or the quality needed for other uses such as irrigation, washing or cooling. Therefore a ‘management choice’ can be made for any groundwater body taking into account regional water management considerations. The quality of regionally important groundwater resources needs to be protected to a high level by means of combining status assessment with the prevention and limitation of inputs to

groundwater and trend assessment.

5.4 Groundwater bodies and adoptions considering aquifer properties As BRIDGE has progressed, and through the information assembled within WP2 (see

PAUWELS et al., 2006) as well as by the experiences of the case studies performed under WP

Deliverable 18 13 / 63

4, it has become evident that significantly different approaches have been applied in the delineation of groundwater bodies and it seems that some groundwater bodies represent flow systems that may extend across different aquifer types. There is also an issue for aquifers of low velocity and groundwater bodies showing a long residence time (e.g. more than 50 years) in relation to the potential for anthropogenic inputs to affect status determination. Both aspects have implications for natural background conditions and the transport and attenuation processes that may act on pollutants since these will be dependent on flow characteristics, geochemical characteristics and changes of hydrogeochemical conditions along the flow path of groundwater.

As a result (and especially where groundwater is the receptor at risk) it will be difficult if not impossible to aggregate and average natural background levels of substances for some groundwater bodies that extend across different geochemical settings.

This shows the importance of developing a sound conceptual understanding of the body setting and the processes at work. For example, one simplified groundwater flow-system through surface waters and terrestrial ecosystems is shown in figure 3 with local, intermediary and regional type flow systems, which may show a huge variability in travel time between the recharge zone and discharge zones (from a few months to several thousand years).

local recharge local recharge

local recharge regional recharge

discharge

discharge

zone of influence of anthropogenic surface originated pollution regional geochemical background contamination

aquiclude

Groundwater body

local geochemical background contamination

Figure 3: Main elements of a subsurface flow system

Alternatively, there may be places where regional and local recharges are overlain; areas of mulit-layered aquifer systems where problems such as up-coning of chemically different groundwater occurs. Plus of course many aquifers show considerable variation in quality with depth.

Many of these issues will be addressed with an appropriate conceptual model, choice of appropriate and relevant monitoring points, and weighting of the monitoring results but it highlights the problem of defining a single threshold value for a pollutant across the entire

Deliverable 18 14 / 63

groundwater body. For example, if there is attenuation taking place in a groundwater body with a long travel time from the recharge zone to the surface water receptor, the tolerable

contaminant concentration close to the surface water will be less than in the recharge zone, as a back calculation would allow for attenuation. Expert judgement is needed as to whether it is appropriate (due to long travel times) to set a threshold value for the surface water receptor in this recharge zone. Nevertheless, any recharge zone is an important place to prevent and limit the input of pollutants to groundwater and if these requirements are more stringent, it will take priority over meeting threshold values. Thresholds must not be (mis)understood as an excuse to introduce further pollutants if the actual concentration levels are low (see chapter 2.5 and Annex VI for more detail on the relationship between Prevent/Limit and Status).

In summary, it is important, that Member States, appointed competent authorities and experts involved in developing River Basin Management Plans recognise the importance of controlling and revising the Conceptual Model and the characterisation of groundwater bodies. This is described for example within the Monitoring Guidance under the Common Implementation Strategy (GRATH et al., 2006). Continued improvement in understanding of the natural systems of the management unit needs to be taken account of when developing threshold values for groundwater bodies characterised as being at risk.

6 Procedure for status determination and threshold setting

This section provides the basic methodology proposed for threshold setting within Status determination. The method itself is supported by more detailed Annexes on particular topics.

The method relies initially on knowing the nature of the final receptor at risk and on knowing the natural quality of the groundwater concerned. A tiered approach is adopted which allows the targeted use of resources both in assessment and any remedial measures.

To assess groundwater quality at local scales it is quite common to make use of natural background levels and generic reference values according to possible receptors, which are in general ecosystems and human uses. As status assessment can be understood as being an integrated assessment at the large scale of groundwater bodies, both these criteria have to be included. Moreover status assessment has to take into account that the general chemical quality of groundwater as well as fate and transport of contaminants are determined by a variety of factors, where lithologic properties of rocks in the vadose and groundwater saturated zone, regional hydrological and hydrodynamic conditions and hydrogeochemical processes controlling the behaviour of natural and anthropogenic substances are of major importance. Thus status assessment needs to go beyond quality assessment and consider attenuation criteria like dilution, diffusion, retardation, and degradation. These criteria are specific to the properties of contaminants, hydrogeological units where specific hydrogeochemical processes govern, and the interaction with surface waters.

6.1 Assessing the Natural Background Level (NBL)

In the past there have been many local and regional studies of the ambient quality of

groundwater in terms of both the naturally–occurring and anthropogenic substances that may be present. More recently, cross border and EU-wide projects such as EU BaSeLiNe (Ref EVK-CT-1999-00006) have provided a good basis for understanding the natural background level of some substances in groundwater. The typology presented in the BRIDGE WP2 studies builds on this work and a procedure for determining the NBL in a groundwater body is

presented in Annex 1. The procedure allows NBL determination from monitoring data or by comparison with known similar aquifers in the typology.

Deliverable 18 15 / 63

The GWDD recognizes background level as a concentration or value of a substance or indicator in a groundwater body corresponding to no, or only very minor, anthropogenic alterations to undisturbed conditions. Given this definition it is understood that the future regulatory framework asks to determine background level as ‘natural background levels’ as they have been discussed within BRIDGE.

Depending on data availability Natural Background Levels (NBLs) can be defined following a hierarchy of possible options. To unify the starting point for groundwater status assessment BRIDGE has proposed a European aquifer typology, classifying 16 types (see PAUWELS et al, 2006; and Figure 4), and also has referenced NBLs from national studies accordingly. These NBLs might be used if no appropriate groundwater quality data are available but only the hydrogeological units of a specific groundwater body can be described.

Given a groundwater body where a limited set of data on the chemical composition of groundwater is available, a second option by a simplified and practical approach to determine NBLs based on a pre-selection method can be employed. As a prerequisite for applying a simplified pre-selection method common minimum requirements for groundwater quality data (e.g. deviation of the ion balance < 10 %) and appropriate pre-selection criteria to identify groundwater samples showing no significant anthropogenic impact (e.g. Nitrate < 10 mg/l) are to be defined. Finally given a groundwater body where a broad set of quality data is available, the third option to estimate NBLs is to apply scientifically sound methods (e.g. hydrochemical simulations, component separation by concentration separation analysis), which already have been established at national or international level.

Clearly, the procedure described only accounts for substances which occur naturally, whereas for substances that are purely synthetic with no natural sources (for example, TCE) then the NBL will be zero.

Figure 4: European aquifer typology for NBLs – map (Research Centre Jülich, draft November 2006)

Deliverable 18 16 / 63

6.2 Selection of the Reference Quality Standard

Generic reference values selected according to receptors which might be harmed by groundwater contaminants are to be used. Giving a focus to ecosystems and human uses means consequently that environmental quality standards (EQS) for surface waters or drinking water standards (DWS) are to be transferred and linked into groundwater status assessment.

The mentioned reference values might be defined at European level or at national level. With respect to the variety of possible substances contaminating groundwater it is also likely that for some substances no reference values are available at all.

As a hierarchy of options exist, it can be envisaged that unified European standards like e.g. the EQS for priority substances set out by the proposal of the European Commission (COM(2006)398 final) are preferable to, and therefore overrule national standards. If no agreed European standards exist, national reference values can be used. The EQS to be introduced in groundwater status assessment need to be expressed as annual average values (EQS-AA see COM(2006)398 final).

Finally for substances without established receptor-oriented reference values at European or national level a survey and evaluation of humantoxicity or ecotoxicity data will be necessary.

Depending on data availability the evaluation of these toxicity data should again be based on and refer to either agreed European procedures or national agreements.

The WFD and the GWDD are understood to outline the following receptors:

- aquatic ecosystems (surface water quality and ecology) - dependent terrestrial ecosystems (plants, vegetation) - drinking water supply

- other legitimate uses (e.g. irrigation or crop washing) - groundwater

These may be compared with the provisions of the GWDD and the appropriate reference quality standard selected (Table 1 and 2).

Table 1. Criteria for Good Chemical Status and suggested reference standards

Receptor Likely Substance Suggested reference standards Surface Water Ecological & Chemical Status

Aquatic ecosystem Any pollutant which causes

risk to a surface water body Surface water EQS (and ecotox data for aquatic organisms)

Dependent Terrestrial Ecosystems terrestrial ecosystems

(plants, vegetation) Any pollutant which causes significant damage to a groundwater dependent ecosystem

To be determined specifically (surface water EQS might be used as screening values)

In selecting the receptor it is important to remember that the Status determination will extend throughout the lifetime of the river basin management plan.

For cases where the human use of groundwater provides the ultimate receptor the choice of quality standard or reference value will depend on the needs of the use and the current

operational treatments being applied. Often, of course, there will not be any effective treatment in place. For example, in many places groundwater can be used directly for drinking water

Deliverable 18 17 / 63

without any treatment and so the most appropriate value will be from drinking water standards.

In other places substantial treatment of abstracted water is used to ensure that drinking water standards are met at the point of supply. In this case the value may stem from the existing

In other places substantial treatment of abstracted water is used to ensure that drinking water standards are met at the point of supply. In this case the value may stem from the existing