Donal Daly

Mapping groundwater vulnerability: the Irish perspective

Abstract The groundwater protection scheme used in the Republic of Ireland since the 1980s had not encompassed the vulnerability mapping concept. Yet internationally, vulnerability maps were becoming an essential part of groundwater protection schemes and a valuable tool in environmental management. Consequently, following a review of protection schemes world-wide, the scheme used in Ireland was updated and amended to include vulnerability maps as a crucial component of the scheme. The approach taken to vulnerability assessments and mapping in the Republic of Ireland has been dictated by the following fundamental questions: Vulnerability of what? Vulnerability to what? Which factors determine the degree of vulnerability? What is the appropriate scale for map production? How can limitations and uncertainties be taken into account? How can vulnerability assessments be integrated into environmental and resource management? The following decisions were made: (i) we should map the vulnerability of groundwater, not aquifers or wells/springs; (ii) the position in the groundwater system specified to be of interest is the water-table (i.e. first groundwater encountered) in either sand/gravel aquifers or in bedrock; (iii) we should map the vulnerability of groundwater to contaminants generated by human activities (natural impacts are a separate issue); (iv) as the main threat to groundwater in Ireland is posed by point sources, we should map the vulnerability of groundwater to contaminants released at 1–2 m below the ground surface; (v) the characteristics of individual contaminants should not be taken into account; (vi) the natural geological and hydrogeological factors that determine vulnerability are the sub-soils above the watertable, the recharge type (whether point or diffuse) and, in sand/gravels, the thickness of the unsaturated zone; (vii) based on these factors, four vulnerability categories are used (extreme, high, moderate and low); (viii) map scales of 1:50 000 and 1:10 000 are preferred; (ix) limitations and uncertainties are indicated by appropriate wording on the maps and a disclaimer; (x) vulnerability maps should be incorporated into groundwater protection schemes, which should be used in decision-making on the location and control of potentially polluting developments. Vulnerability maps have now been produced for a number of local authority areas. They are an important part of county groundwater protection schemes as they provide a measure of the likelihood of contamination, assist in ensuring that protection schemes are not unnecessarily restrictive of human economic activity, help in the choice of engineering preventative measures, and enable major developments, which have a significant potential to contaminate, to be located in areas of relatively low vulnerability and therefore of relatively low risk, from a groundwater perspective.

Assessing subsoil permeability for groundwater vulnerability

Groundwater vulnerability assessment is a key element of any groundwater protection scheme. In Ireland, groundwater vulnerability is determined mainly according to the thickness and permeability of the subsoils (glacial tills and other superficial deposits). The relative permeabilities of the subsoils are assessed qualitatively as high, moderate or low. To improve the robustness of the groundwater protection scheme, research was carried out into subsoil properties with the aims of refining the permeability ratings, and of improving the way in which subsoil permeability classes are assigned. This research focused on subsoils in the low and moderate permeability categories, mainly tills. Important issues investigated were the relationship between permeability and the grain size distribution of the subsoil, description of subsoils for permeability classification, correlation between permeability and indicators of aquifer recharge, and suitable field and laboratory methods for measuring subsoil permeability. A standard system for describing subsoils was selected, namely BS5930:1999, the choice being influenced by the familiarity of this system among the main users of the vulnerability maps. Analysis of subsoil field descriptions and grain size data indicate that those samples identified as ‘CLAY’ on the basis of BS 5930 correspond to the low permeability category, and tend to have more than 13% clay size particles. The permeability values obtained from each method are compared and indicate that the numerical boundary between moderate and low permeability lies in the region of 10−9 m/s. Differences between the results from laboratory and various field permeability test methods can be explained by differences in scale and by the presence of discontinuities. The research was successful in refining the permeability ratings and thereby in making the vulnerability maps more defensible against possible challenges. This research has improved the way permeability maps are produced in Ireland, and may prove useful in other countries where permeability data are scarce and mapping relies largely on field assessment of subsoils.

Development of a national groundwater recharge map for the Republic of Ireland

A groundwater recharge map has been created for the Republic of Ireland. This provides important baseline information for multiple purposes: aquifer water balance assessments using simple lumped models or distributed numerical groundwater models; assessment of the impact of groundwater abstractions as required under the European Water Framework Directive; and delineation of source protection zones. The groundwater recharge map is derived from existing hydrogeological and meteorological spatial datasets. The main hydrogeological controls on groundwater recharge include the permeability and thickness of superficial deposits (mainly tills in Ireland), the presence of saturated soils, and the ability of the underlying aquifer to accept percolating waters. Combinations of these factors are assessed, and a ‘recharge coefficient’ is established for different hydrogeological scenarios. To produce the groundwater recharge map, the geographic information system layer containing the recharge coefficients is multiplied by the effective rainfall. The recharge map is finally modified to take into account areas where the natural recharge capacity of the underlying aquifer is less than the estimated groundwater recharge potential. Originally drafted in 2005, the present map includes refined recharge coefficient estimates, improved representation of wet soils over gravel aquifers, and a full national coverage of full subsoil permeability and groundwater vulnerability mapping. There are a number of assumptions made in creating the groundwater recharge map (notably, only direct (diffuse) recharge is taken into account) and there are limitations of precision associated with the input datasets. For reasons of scale and generalization to a national coverage, the map is useful for giving regional estimates of recharge but these, in many cases, will need to be refined by site-specific studies.

Groundwater protection in a Celtic region: the Irish example

Abstract One of the key environmental objectives of the proposed EU Water Framework Directive is that Member States must prevent the deterioration of groundwater quality. A national groundwater protection scheme for Ireland has been published recently. This scheme shows certain broad similarities to the groundwater protection policy for England and Wales, incorporating the concepts of groundwater vulnerability, source protection zones and responses to potentially polluting activities. However, the Irish scheme is different in several important respects, reflecting the different hydrogeological conditions and pollution concerns in Ireland. Some of these hydrogeological conditions and pollution concerns are common to the other Celtic regions. A major feature of the Irish scheme is the importance given to subsoil permeability in defining groundwater vulnerability. At present, the subsoil permeability is classified in qualitative terms as high, moderate or low. For the protection scheme to be defensible, it is essential to adopt a systematic and consistent approach for assigning subsoil units to these permeability categories. In mapping groundwater vulnerability, it is also useful to take account of secondary indicators such as groundwater recharge potential, natural and artificial drainage density and vegetation characteristics. Another important issue in Ireland is the protection of groundwater in karst areas, since these areas are especially vulnerable to contamination.

Hydrogeological insights from groundwater level hydrographs in SE Ireland

Analysis of groundwater level records from Ireland’s South Eastern River Basin District (SERBD) allowed fundamental information about the nature of bedrock and gravel aquifers to be investigated. The hydrogeological setting of a monitoring point (with respect to, for example, recharge area, discharge area or proximity to a river) is the dominant factor influencing hydrograph character in bedrock aquifers, with aquifer type and subsoil properties producing secondary effects. Analysis of seasonal groundwater levels showed that the fractured bedrock aquifers recharge more quickly and typically have a longer recession period than gravel aquifers. The calculated recession periods for bedrock aquifers are longer than previous estimates for similar aquifers. Hydrograph analysis identified a number of notable phenomena including a gravel aquifer’s interaction with surface water and evidence of rejected recharge. Short-term groundwater level fluctuations caused by global seismic events, recorded via chart recorders, are discussed. Specific yield values were calculated, for a number of settings, from annual average groundwater level variations. The values supported estimates from previous research on similar aquifers. An analysis to investigate if any impacts of climate change were evident showed no consistent change in the timing of groundwater level minima or maxima.

Sources of nitrogen and phosphorus emissions to Irish rivers and coastal waters: Estimates from a nutrient load apportionment framework

More than half of surface water bodies in Europe are at less than good ecological status according to Water Framework Directive assessments, and diffuse pollution from agriculture remains a major, but not the only, cause of this poor performance. Agri-environmental policy and land management practices have, in many areas, reduced nutrient emissions to water. However, additional measures may be required in Ireland to further decouple the relationship between agricultural productivity and emissions to water, which is of vital importance given on-going agricultural intensification. The Source Load Apportionment Model (SLAM) framework characterises sources of phosphorus (P) and nitrogen (N) emissions to water at a range of scales from sub-catchment to national. The SLAM synthesises land use and physical characteristics to predict emissions from point (wastewater, industry discharges and septic tank systems) and diffuse sources (agriculture, forestry, etc.). The predicted annual nutrient emissions were assessed against monitoring data for 16 major river catchments covering 50% of the area of Ireland. At national scale, results indicate that total average annual emissions to surface water in Ireland are over 2700tyr-1 of P and 82,000tyr-1 of N. The proportional contributions from individual sources show that the main sources of P are from municipal wastewater treatment plants and agriculture, with wide variations across the country related to local anthropogenic pressures and the hydrogeological setting. Agriculture is the main source of N emissions to water across all regions of Ireland. These policy-relevant results synthesised large amounts of information in order to identify the dominant sources of nutrients at regional and local scales, contributing to the national nutrient risk assessment of Irish water bodies.

Fault control of groundwater flow and hydrochemistry in the aquifer system of the Castlecomer Plateau, Ireland

Summary The Westphalian strata of the Castlecomer Plateau contain two main sandstone aquifers that are in variable hydraulic continuity due to major fault displacements. The faulting separates the plateau into three effectively independent groundwater blocks. A hydrochemical study shows that carbonate dissolution and ion exchange are the two dominant chemical processes active in the aquifer and that the degree to which they occur is a function of the ability of the aquifer to transmit water which is controlled by fault displacement.

Nutrient Load Apportionment to Support the Identification of Appropriate Water Framework Directive Measures

A model for predicting the sources of nutrient loads (phosphorus and nitrogen) to water has been developed to support Water Framework Directive (WFD) implementation. The Source Load Apportionment Model (SLAM) framework described in this paper integrates catchment data and pressure information from point discharges and diffuse sources to enable characterisation of source pathway receptor relationships. Hydrogeological controls have a strong impact on nutrient fluxes, particularly in agricultural catchments, and have been incorporated into the diffuse agricultural model (the CCT). Results for the Suir catchment matched the measured loads of nitrogen and phosphorus well, and showed that pasture is the dominant source of nitrogen. The main sources of phosphorus in sub-catchments varied between diffuse agriculture, wastewater and industrial discharges. A relatively small proportion (13%) of the Suir catchment area requires a reduction in phosphorus emissions to achieve Good status. In these areas, model results can be used in conjunction with local knowledge gathered through the WFD characterisation process to identify significant pressures that contribute excessive nutrient loads. An example of assessing load reduction scenarios is presented to illustrate how modelling can support catchment scientists in identifying appropriate measures.