With relatively high rainfall, upland areas suitable for the collection and storage of water and the presence of Lough Neagh, not surprisingly, surface water is the dominant source of water supply in Northern Ireland. Groundwater nevertheless is still an important source of water for public drinking water and for industrial, agricultural and domestic supply.

Thousands of boreholes and wells have been sunk for the supply of water and more new boreholes are drilled every year. The success of a borehole as a supply of water will depend on a number of factors including the local hydrogeological setting, the drilling and well construction technique, and the quantity and quality of water required. By law, anyone who intends to drill a borehole or sink a shaft or well more then 15 metres deep, anywhere in Northern Ireland, is required both to inform GSNI beforehand and keep a record of the borehole/shaft constructed. Further information regarding this can be obtained from Terry Johnston.

In Northern Ireland the most productive aquifer units are the Permo-Triassic sandstones within the Lagan and Enler Valleys and the numerous Quaternary sand and gravel deposits located mainly within the river valleys, such as those in the River Main valley around Clogh Mills and within the Oona catchment at Eglish. Modest and occasionally more significant supplies can be obtained from several other strata types including the Tertiary basalts and the Devonian sandstone and limestone sequences. Karstic conditions exist within parts of the Chalk (Ulster White Limestone) where it outcrops around the edge of the overlying basalt and within the Carboniferous limestones in Fermanagh.

Less useful areas for obtaining reliable supplies include the Dalradian rocks to the west of the country and the Ordovician/Silurian strata in counties Down and Armagh.