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The impacts of climate change on Irish groundwater resources
Groundwater resources in Ireland
As the largest store of available fresh water, groundwater is a priceless resource that needs to be protected. The importance of this resource in terms of water supply is clear; in Ireland around 25 % of the water supplies come from groundwater, with this proportion rising in rural areas, where houses are not connected to the public mains supplies but have private wells instead. In addition, groundwater also plays a vital role in less visible ways such as maintaining the river base flow, or providing water to ecosystems like fens or turloughs.
Accurate estimations of groundwater recharge are necessary to quantify the groundwater resources available. Furthermore, recharge estimates have other applications, for example the issuing of abstraction licences or the preservation of the ecology of a certain area. Recharge calculations can be carried out in a large range of spatial and temporal scales, depending on the purpose of the investigation. In the case of water resources management, recharge is typically calculated at catchment scale since catchments are the normal unit of water management.
Climate Change and projections for Ireland
It is known that climate has been changing over the earth’s history causing cold periods, such as glaciations, and warmer periods. During the last 150 years global temperatures have experienced a significant increase, with rises being more acute in the last half century. Historically, the climate changes experienced by the Earth were linked to tectonic processes, volcanic activity or changes in the incoming solar radiation. In contrast, the changes that have occurred in the last century and a half are partly due to anthropogenic activities such as changes in land use and the increase in emissions of greenhouse gases and aerosols (IPCC 2013). To summarize, there are two drivers of the current changing climate: the natural forcing and the anthropogenic activities. The long-term changes in climate patterns due to the combination of these two drivers are known as global climate change (Figure 1).
A research project coordinated by Met Eireann generated updated simulations for Ireland, using the results of the fifth assessment report from the International Panel on Climate Change as a baseline. The results show that mean temperatures are likely to increase by about 1.5°C by mid-century with respect to the reference period (1981-2000). Moreover, winters are expected to be wetter and summers drier. An increase in the frequency and intensity of rainfall events is also predicted (Gleeson et al. 2013).
Implications of changes in recharge rates
Groundwater recharge mainly occurs by infiltration of the rainfall or by transfer from another water body. The main discharges occur naturally to rivers and ecosystems, and as seepage along the coast. Artificial discharges include abstractions by pumping wells or drainage works. As a consequence, any changes in groundwater recharge could affect both natural outflows and water supply abstractions. Regarding rivers, a reduction in baseflow would lower the river stage during dry periods and could lead to a deterioration in river water quality due to a higher concentration of pollutants. Groundwater dependent ecosystems could face similar problems. Furthermore, these ecosystems are highly sensitive to changes in water levels so a variation of a few centimetres can potentially affect the flora and fauna inhabiting them.
Natural springs occur when the groundwater table intersects with the land surface. Therefore, a relatively small depletion of the groundwater level could lead to a reduction of springs flows or even dry them up completely.
In another vein, source protection areas are delineated around springs and wells to protect their zone of contribution against possible polluting activities. In a hypothetical scenario, with lower recharge rates the area of the contribution zone to a given water source would increase in order to maintain the same abstraction rate (Hunter Williams and Lee, 2000). Therefore, the size of protection area would need to increase correspondingly.
Groundwater quality could also deteriorate if there is a reduction in groundwater recharge. Moreover, since changes in rainfall patterns and intensity are also anticipated, it could alter the mobilization of pollutants in the soil. For example the flushing of contaminants accumulated during dry periods in the soil could be enhanced by higher rainfall intensities.
Finally, in coastal aquifers there is a natural equilibrium between the saline water and the discharging groundwater. This equilibrium prevents coastal aquifers from becoming salinized, but can be easily disturbed by overexploitation of the aquifers or a depletion of groundwater levels. This issue is not particularly important in Ireland under the current climatic and abstraction conditions, but it could change in the future if recharge decreases and exploitation of coastal aquifers is intensified.
Assessing the impacts: from catchment to national scale
Most of the existing impact studies in groundwater are at catchment scale, since these are natural divisions of larger river basins. This helps to simplify the groundwater estimations as catchments can be treated as independent systems with defined properties. The smaller variation in some characteristics such as land use, soil and subsoil type, and lithology and aquifer type simplifies the understanding of the different processes occurring. It can then be used in other locations with similar characteristics or in a wider area.
Some relevant properties such as permeability, porosity or the presence of fractures, depend on some of the catchments characteristics mentioned above (e.g: subsoil type, lithology (rock type), etc.). These features have an important limiting effect on the amount of water that can reach the water table and be stored within the aquifers. Because of that, the response of the aquifers towards possible climate alterations depend on the local settings and so, the magnitude of the impact of climate change depends on the characteristics of each catchment.
This is especially relevant in Ireland where the range in subsoils and aquifer properties is particularly large. For example, glacial deposits cover a significant part of the Island, and they are characterised by having low permeability (Figure 2). As a consequence, the infiltration down to the underlying groundwater bodies will be lower than in areas covered by more permeable materials such as alluvial sands and gravels (Misstear et al., 2008).
Similarly, the range of lithologies across the island leads to diverse aquifer types with contrasting properties. Nevertheless, two thirds of the country are underlain by aquifers regarded as “Poorly Productive” because of their restricted capacity to store and transmit water.
The combination subsoil and aquifer type control the recharge rates and the groundwater storage. For this reason, it is expected that climate change will have an unequal impact across the country, depending on local settings.
The aim of this four year research project, funded through iCRAG (Irish Centre for Research in Applied Geosciences) and based at Trinity College Dublin, is to determine how these properties control recharge to be able to assess the possible impacts of climate change. To do so, three study catchments with contrasting properties have been selected so far: The Mattock (Co. Louth), Nuenna (Co. Kilkenny) and Dripsey (Co. Cork) (Figure 3). In these areas, groundwater recharge under current climatic conditions has been calculated and the sensitivity of the results tested by varying the assumed catchment properties mentioned previously. Other variables such as rainfall amount, its intensity or seasonal variation are also modified to see how groundwater could be affected under these hypothetical climatic conditions.
Acknowledgements
We want to acknowledge the Geological Survey of Ireland and the Environmental Protection Agency for providing respectively GIS datasets of the subsoils of Ireland and catchments contours used in this article to generate Figures 2 and 3.
Èlia Cantoni, Irish Centre for Research in Applied Geosciences, Laurence Gill, Trinity College Dublin and Bruce Misstear, Trinity College Dublin
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