The Hydraulic conductivity layer is taken as a direct assignment of the hydraulic conductivities from the aquifer parameter database to the C index values according to DRASTIC, ranging from 1 to 10.

The aquifer media layer is created by direct assignment of aquifers to A index values from 1-10. The metamorphic and igneous rocks were given the lowest ‘A’-value of 3 and the karstified limestone is given the highest value of 10. The majority of the catchment can be grouped into bedded sedimentary rocks and given a value of 6.

This layer considers the nature and thickness of soils, subsoils, unsaturated zone confinement. The lowest values correspond to areas of karst aquifer with no overlying subsoil and the water table is at or near the surface. The highest values are for non karst rocks overlain by thick impermeable subsoils with a deep water table. The biggest influencing factor here was the presence and thickness of clay subsoil

The topography layer is created by calculation of the slope value from the national topographic data. S values were classified into DRASTIC S values from 1-10. Due the generally flat topography, the majority of the catchment is assigned the higher ‘T’-values.

I layer is calculated by direct assignment of vadose zone material to I index values according to DRASTIC methodology, raning from 1 to 10. In areas covered by subsoil deposits, the vadose zone media classification (the ‘I’-rating) was based on soil and subsoil type. In areas of exposed bedrock and subcrop or areas with < 3 m of subsoil coverage, the characteristics of the vadose zone was taken from the aquifer characteristics.

I layer is calculated by direct assignment of vadose zone material to I index values according to DRASTIC methodology, raning from 1 to 10. In areas covered by subsoil deposits, the vadose zone media classification (the ‘I’-rating) was based on soil and subsoil type. In areas of exposed bedrock and subcrop or areas with < 3 m of subsoil coverage, the characteristics of the vadose zone was taken from the aquifer characteristics.

The D layer is the depth to the water table from the surface. The Boyne study area is based on 401 historical water level data points along with spring data and data from groundwater surface water interactions. Spatial interpolation using kriging was used and then the values were classified into the D index values. The highest D values correspond to where the water table is shallowest, generally found in the lower areas in the catchment, with the lowest D values meaning greater depth to the watertable, found in the upland areas

Vulnerability assessment using DRASTIC method (Aller et al., 1987)

R values are classified from 1 to 10. Groundwater recharge (GSI National Groundwater Recharge Map) varies between 14 mm/a and 573 mm/a across the Boyne catchment. Areas underlain by peat have the lowest groundwater recharge and lowest R values. The highest groundwater recharge occurs in areas where rock is at near the surface or the area is underlain by well drained soils and sand and gravels. These are assigned the highest ‘R’-values.

This is a groundwater vulnerability map of the Boyne Catchment, based on the DRASTIC groundwater vulnerability method. The Boyne Catchment, as defined by WFD 2016, has an area of 2693 km2, located in Eastern Ireland. The 7 DRASTIC input layers are Depth to water table, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone and hydraulic Conductivity. These input values are weighted and added together to give an overall DRASTIC score.