Understanding groundwater sources and movement using water chemistry and tracers in a low matrix permeability terrain: the Cretaceous (Chalk) Ulster White Limestone Formation, Northern Ireland

Abstract

The Cretaceous Chalk [Ulster White Limestone Formation (UWLF)] in Northern Ireland has been diagenetically altered to a hard, low porosity, low matrix-permeability rock. It subcrops extensive beneath thick horizontal Tertiary Basalt lavas, but has a restricted strip of outcrop (approximately 80 km 2) around the periphery of the overlying igneous rock. Despite its nature and location, numerous springs issue from the Chalk and are used for public and local supply, although little is known about the origin of the groundwater or the type of flow within the Chalk. We have addressed these unknowns using hydrogeochemical data from both rock types and surface stream waters, and fluorescent dye tracing. We have demonstrated that leakage recharge from the basalt to the Chalk accounts for less than 20% of the total water issuing from the Chalk springs using geochemical mass balance of conservative species and discharge data. The majority of the discharge derives from streams coming off the basalt and entering sink holes in the Chalk aquifer. Field and water tracing evidence shows that groundwater flow in the Chalk is dominated by conduit and fissure flow near to the outcrop areas. Karstification has been widespread near outcrop but has not ocurred in the sub-basalt region. Beneath the basalts, the Chalk exhibits different hydraulic properties due to the reduced recharge and dissolution potential that has prevented extensive development of fissure permeability in the UWLF. The Chalk aquifer thus appears to be strongly zoned in terms of the source of water, flow regime and recharge rate.