QSC graphs: an aid to classification of data-poor aquifers in Ireland

Abstract

Abstract The Geological Survey of Ireland’s aquifer classification system recognizes three main categories: Regionally Important, Locally Important and Poor Aquifers. This system is increasingly used to assist local authorities and state agencies to make decisions on planning applications and integrated pollution control licences, by prioritizing areas according to the value of their underlying groundwater resources. Most aquifers in Ireland are unconfined fractured hard rock aquifers, often of limited extent, which can exhibit a wide range of properties. Pumping test data are scarce, patchy and often of uncertain quality, and reliable transmissivity or permeability values are unavailable for many aquifers. Under these circumstances, the classification of a given geological formation in a given region can be difficult. The ‘QSC Graph’ compares the specific capacity (SC) for a borehole, determined by a pumping test, with the abstraction rate during the test (Q), and indicates a ‘borehole productivity index’, in five classes (I, II, III, IV and V from highest to lowest). From the relative frequency of productivity classes for a given aquifer, the appropriate aquifer category can be inferred. However, other types of information for the aquifer should also be considered. The current QSC data set comprises about 1100 boreholes, and for individual formations up to 150 boreholes. The minimum data set required for an aquifer, depending on the diversity or compactness of the data, is between 20 and 50. Examples are given of the application of the approach to a number of Irish aquifers.