Présentations Colloques

Oral Presentation
Session 8.03: Geometrical structure and hydrogeological properties of Hard-Rock aquifers.
Hunter Williams Natalya
Characterisation of aquifer properties in data-scarce fractured rock aquifers- the Irish example
In the Republic of Ireland groundwater is an important resource, providing drinking water to approximately one third of the population via public and private water supplies, and sustaining rivers, lakes and groundwater dependent ecosystems. **The vast majority of groundwater resources in Ireland are found in bedrock aquifers, generally within sedimentary, metamorphic and igneous rocks of Upper Carboniferous age or older. Due to the age of the rocks, and their many cycles of burial, deformation and uplift, primary porosity is nearly always absent. Fissure porosity and permeability is dominant in the non-carbonate rocks. Carbonate rocks, depending on the proportion of clays, can also have solutionally-enhanced permeability, with the pure limestones being highly susceptible to karstification. Typically, fracture density and aperture, hence permeability and transmissivity, decrease with depth although flowing fractures and fracture zones can be encountered at depth. A ‘transition zone’ of highly weathered bedrock can occur at the top of all bedrock aquifers, with thickness varying rapidly from zero to a few metres. **The Geological Survey of Ireland (GSI) have classified the bedrock aquifers in a holistic manner by using lithology, structure, presence absence of large or small springs, borehole ‘productivity’, yield, karst features, baseflow signals and hydrographs. However, specific aquifer parameter information such as transmissivity is quite sparse. Representative aquifer hydraulic parameters for each of Ireland’s aquifers and rock unit groups have been established using a combination of high confidence data contained in the GSI database, and transmissivities derived from the more numerous, but lesser quality, specific capacity data in the GSI database using equilibrium approximation formula. Typical or representative values for a given rock unit group or aquifer are described using Best, Upper and Lower Estimate values, using a statistical characterisation based on summary descriptive statistics and graphical plots distributions. Characterisation of the transition zone focusses on capturing spatial variability statistics, together with hydraulic parameters. **