Groundwater bodies and horizon assignment

1Background

1.1Water Framework Directive (WFD)

The Water Framework Directive (WFD, 2000/60/EC) considers a groundwater body (GWB) as a coherent management unit assigned to a River Basin District (RBD) which have to meet the environmental objectives (Article 4) .

The term “body of groundwater” should therefore be understood in the context of the hierarchy of relevant definitions provided under Article 2 of the WFD:

  • According to Article 2.2, “Groundwater” means all water, which is below the surface of the ground in the saturated zone and in direct contact with the ground or sub-soil;
  • According to Article 2.11, “Aquifer” means a subsurface layer or layers of rock or other geological strata of sufficient porosity and permeability to allow either a significant flow of groundwater or the abstraction of significant quantities of groundwater;
  • According to Article 2.12, “Groundwater body” means a distinct volume of groundwater within an aquifer or aquifers.

According to the definitions and the specifications laid down in the WFD, groundwater bodies are management units with the main purpose of enabling their quantitative and qualitative status to be accurately described and compared to the environmental objectives and of implementing the measures necessary for achieving these objectives. Groundwater management has to consider groundwater in relation to its usesand functions and its interactions with aquatic and terrestrial ecosystems and in relation to the natural conditions (geology, hydrogeology etc.) and anthropogenic influences (pressures).

1.2GWB delineation – horizontal dimension

The delineation of GWBs comprises both, the arealand the verticaldimension, consideringthe following features:

  • Groundwater flow divides, using surface water catchments and geological boundaries as proxies where information is limited.
  • Pressure variations, where these are significant at a river basin level and where they require different management.
  • Variations in natural background levels (NBL).
  • Coastline, unless there is specific evidence that groundwater beyond the coastline has a resource value in terms of legitimate uses.
  • Association to aquatic ecosystems or groundwater dependency of terrestrial ecosystems.
  • Boundary of a hydrographical entity that is already subject to a Local Management Plan.

1.3GWB delineation – vertical dimension

The verticalcharacterization of a GWB depends on:

  • whether the volume of the groundwater concerned is located within one or within several aquifers;
  • The risks according to the objectives of the WFD.

It is up to each Member State and the national groundwater management strategy how GWBs are delineated (in accordance with the definitions under WFD Article 2), whether GWBsaredefined separately for each individual stratum overlying each other or merging different strata.

The vertical heterogeneity/variability of ahydrogeological setting can lead to many different arrangements of differently delineated GWBs. If hydrogeology is not the only factor considered (which is probably the case), many additional ways of delineation and types of configuration are possible.

Figure 1 illustrates one example hydrogeological setting and four different (non-exhaustive) options of GWB delineation and arrangements which are in the following used to illustrate the challenge and proposed procedure of horizon assignment.

Figure 1: Four different options of GWBdelineation for one specific hydrogeological context

In addition to the areal variability, the verticalvariability makeshomogenization work at the pan-European scalevery complex, particularlyfor transboundary GWBs where the connected GWBs may be differently delineatedby the Member States because of different national approaches, focusesor management constraints (see chapter3).

1.4Horizon assignment

1.4.1Horizons: what for?

GWBs are three-dimensional entities; however the representation of the feature will be as 2-D polygons. Borders of polygons of GWBs are their projection on the surface. It is necessary that multiple overlapping groundwater bodies at different depths with non-identical boundaries are distinguished in different horizons (layers) (EU, 2009). The characterisation ofthe vertical position of GWBs by ‘horizons’should:

  • help toreflect the three-dimensionality of GWBs and their vertical (relative) position to other GWBs.
  • enable the stratified visualization of GWBs on maps and
  • help to identify and visualise those (parts of) GWBs which are probably most exposed to anthropogenic pressures on the surface – the uppermost horizon, the outcrops (like for geological maps).

1.4.2Concept – Guidance 2009

‘Horizon’ was a mandatory reporting element within WFD reporting to WISE. The concept of assignment of horizons is laid down in the ‘Guidance on the reporting of geographical data for the WFD’ (EU, 2009) and followed a simple numeration in the sense of the numerical position of groundwater body layer starting at the first GWB-horizon from the surface.

1.4.3Results from the first WISE reporting

The EU wide compilation and assessment of the provided geographical data (including the assignment to horizons) for GWBs showed that the data finally did not allow for achieving a non-ambiguous picture of the three-dimensionality of groundwater and GWBs which hampers the compilation of a GWB map for Europe. To date, a comparable vertical positioning and a clear mapping of GWBs is actually impossible at the European scale. The European map prepared for the 2nd Workshop on Groundwater Bodies (Duscher & Struckmeier, 2011) reveals several discrepancies.

The majority of 13,345 GWB polygonsreported in Europeto WISE are assigned to horizon 1 (10,871), followed by polygons allocated to horizon 2 (1,584). Polygons assigned to horizons 3 and more represent 3% of the total.

The main issues highlighted into the ETC/ICM report of 2013 are the following:

  • The majority of Member State reported GWBs up-to horizon 4;fewassigned their GWBs to more than 4 horizons (France, Italy, Estonia, Lithuania);
  • Some Member States did not assign their GWBs to horizons (Spain and UK (Northern Ireland));
  • Nine Member States did not consider subdivision and assigned all GWBs to horizon 1 and/or horizon 0;
  • Some Member States reported GWBswhich extend over several horizons;
  • Some GWBs overlay each other within the same horizon.

1.4.4Deficits of the current approach

The main structural deficits are:

  • The current definition only allows that each GWB as a whole is assigned to exactly one horizon. But for representing complex stratifications this simple approach is not appropriate. It turned out quite difficult to assignto a single horizon, for example, downgrade GWBs that outcrop somewhere and are overlain in other parts by other GWBs.
  • The definition of ‘horizons’ might not be explicit enough or probably not sufficiently genericleading to deviating interpretations or wrong assignments in the Member States.
  • The number of horizons is limited to four.
  • The parameter ’horizon‘ might not be sufficient to characterize the three-dimensionality of GWBs. In addition, other parameters such as “Average Depth”, “Average Thickness” and “Depth range” are helpful.
  • The current approach does not allow for a non-ambiguous join of transboundary GWBs (see chapter 3).

2Recommendationsfor horizon assignment – to enable homogeneous mapping at a pan-European scale

Within this chapter a revised approach for horizon assignment is proposed. This approach is illustrated by three different examples of GWB arrangements which are based on a hydrogeological example context.

2.1Amended proposal for horizon assignment

Considering the main problems identified in chapter 1.4.4, the existing methodology is proposed to be kept and extended by the following features:

  • The assignment of horizonsfollows a simple numeration in the sense of the numerical position of groundwater body layer starting at the first GWB-horizon from the surface (EU, 2009)
  • Groundwater bodiescan be split into sub-units for the purpose of assigning these sub-unitsindividually to corresponding horizons, depending on the overlap with other GWBs;
  • There is no limitation in the number of horizons;
  • Overlaying Groundwater bodies cannot be associated to the same horizon.

Horizon Code / Brief description
1 / (Part of)first GWBfrom the surface
2 / (Part of)second GWBfrom the surface
3 / (Part of)third GWBfrom the surface
4 / (Part of)fourth GWBfrom the surface
5 / (Part of)fifth GWBfrom the surface
…etc…. / …etc….

It has to be emphasized that the assignment of GWBs to horizons should not be mixed with (is separate from) the delineation of GWBs which is strictly subject to Member State’s decisions and methods. The purpose of horizon assignment is just a tool for harmonization to enable coherent visualisation of GWBs at the European level and to support transboundary coordination.

Except for the uppermost horizon (horizon 1) the assignment of a GWB to a certain horizon does not give any information about its absolute vertical position within the overall schema, just the relative position to overlying or underlying GWBs from the surface.

2.2Three Examples

The proposed procedure is illustrated in the following subchapters by three examples of GWB delineations which are based on an example hydrogeological setting.

It is important to consider, that the following examples are not intended to stipulate any discussion about the presented delineation and configuration of GWBs. All three examples are realistic. The only purpose of these examples is to demonstrate the ability of the proposed procedure to cover all types of GWB arrangements.

2.2.1Example 1

Hydrogeological context – Map view and sectional view
Delineated groundwater bodies
Example 1 – Map view / Example 1 – Sectional view
Horizon assignment – Map view
Example 1 – Horizon 1 / Example 1 – Horizon 3
Example 1 – Horizon 2
Horizon assignment – Vertical subsequential arrangement

2.2.2Example 2

Hydrogeological context– Map view and sectional view
Delineated groundwater bodies
Example 2 – Map view / Example 2 – Sectional view
Horizon assignment – Map view
Example 2 – Horizon 1 / Example 2 – Horizon 3
Example 2 – Horizon 2
Horizon assignment – Vertical subsequential arrangement
Horizon 1
Horizon 2
Horizon 3

2.2.3Example 3

Hydrogeological context– Map view and sectional view
Delineated groundwater bodies
Example 3 – Map view / Example 3 – Sectional view
Horizon assignment – Map view
Example 3 – Horizon 1 / Example 3 – Horizon 3
Example 3 – Horizon 2
Horizon assignment – Vertical subsequential arrangement

2.3Resumee

The proposed procedure covers both simple and complex hydrogeological configurations:

  • In areas with simple hydrogeology, or simpleGWBdelineation, the situation remains unchanged compared to the current procedure which means that additional efforts due to the changes are very limited.
  • For more complex situations, their complexity can fully be taken into account without the use of complementary parameters (such as “extra Horizon” proposed by Belgium in 2010).
  • It would be quite easy to prepare a map showing the (parts of) GWBs which are most exposed to pressures on the surface or in the uppermost layer (soil).

3References

Duscher, K., Struckmeier, W. (2011) – A common vision about groundwater entities in Europe, Presentation at the 2nd Workshop on Groundwater bodies held in Berlin 15th-16th December 2011.

ETC/ICM (2013) – Groundwater GIS reference layer. Submission/compilation status and evaluation. Version 3.

European Commission (2003) – Guidance Document No 2: Identification of Water Bodies. ISBN 92-894-5122-X. European Communities, Luxembourg.

European Commission (2009) –Guidance on reporting of spatial data for the WFD (RBMP). Tools and services for reporting under RBMP within WISE. Version 3.0. .

European Commission (2010) – Guidance Document No. 26. Guidance on Risk Assessment and the use of conceptual models for groundwater.

UK Technical Advisory Group on the Water Framework Directive (2012) – Defining & Reporting on Groundwater Bodies. Final version.

Ward, R. (2011) – 2nd Workshop on Groundwater Bodies held in Berlin 15/16 December 2011, Presentation at the 22nd Working Group C plenary meeting held in Brussels the 21st March 2012.