Groundwater circulation patterns and isotope geochemistry in the Chalk of Northern Ireland

The Cretaceous Chalk or Ulster White Limestone Formation (UWLF) of Northern Ireland predominantly occurs as a fracture flow aquifer that exhibits both deep artesian conditions at subcrop, and shallow karstic behaviour at outcrop. The UWLF is underlain by impermeable Triassic/Jurassic mudstones and overlain by Tertiary basalt lavas. A conceptual regional groundwater circulation model has been developed using limited hydraulic head information. This has been tested using isotope geochemistry. Springs draining the outcrop section of the aquifer produce 'recent' waters with abundant modern carbon. Stable isotopes and hydrochemistry have demonstrated that surface runoff is the predominant source of recharge to the outcrop region (as opposed to basalt leakage recharge). UWLF spring water chemistry is often similar to that of the potential recharge waters. Deep regional flow in the aquifer can occur by seepage from the outcrop areas, but limited 'outlet' potential means that circulation is slow and distinct hydrochemical evolution occurs. Radiocarbon ages for this groundwater have been determined at between 10 000 years and 20 000 years. Isotope geochemistry in the UWLF of Northern Ireland thus provides a powerful method for differentiating groundwaters in terms of their interaction with rock, their circulation route and their age. These have major implications for resource modelling and vulnerability