Vulnerability assessment using DRASTIC (Aller et al., 1987) and COP methods

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

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.

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.

Vulnerability assessment output map using COP method

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.

This is a groundwater vulnerability map of the Rockingham Catchment, based on the DRASTIC groundwater vulnerability method. The Rockingham Catchment, is a hydrogeological catchment to Rockingham Spring source and has an area of 16 km2, located in Western 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.

The aquifer media layer is created by direct assignment of soil layer to S index values from 1-10. The majority of the pilot area (80 %) is covered by subsoils > 5 m thick, with further 10 % subsoils are > 3 m thick. The majority of the catchment is given the lower ‘S’-ratings due to soil and subsoil cover, with areas of little or no soil given a value of 10.

This factor takes into account the amount and frequency of precipitation in the study area. Annual average precipitation is fairly uniform across the pilot area and was assigned a value of 0.2. The average number of rainy days for this pilot is 225, which results in a low intensity value for for precipitation and an overall low reduction of protection ‘P’ score overall

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