Groundwater flow in the Antrim Lava Group and Ulster White Limestone Formation, Northern Ireland

The Antrim Lava Group (the ‘basalts’) and the underlying Ulster White Limestone Formation (the ‘chalk’) occupy a third of the land area of Northern Ireland. Neither sequence offers any significant groundwater resource potential but the basalt yields a baseflow contribution of 2630 Ml day–1 to the Lough Neagh catchment. Large spring discharges occur from the eastern outcrop of the chalk along the Antrim coast, giving the chalk a misleading reputation as a significant aquifer. These springs represent karstic risings from streams flowing off the basalt towards the sea, mixed with a small proportion of groundwater that has seeped through the basalt into the chalk below, although the concealed chalk aquifer is, for the most part, confined. Groundwater flow in the basalt is almost entirely of shallow catchment scale following preferred pathways within fossil soil horizons and other sub-horizontal discontinuities within the basalt sequence. Towards the periphery of the basalt some deeper circulation penetrates the chalk to discharge at outcrop. The main subcrop of the chalk is relatively impermeable with tighter jointing that is not favourable for transport or for storage. Available evidence indicates that little groundwater circulation in the chalk aquifer derives from direct rainfall recharge to the overlying basalts and that groundwater circulation is limited away from outcrop.