A GIS of aquifer productivity in Scotland: explanatory notes

GIS-based maps of aquifer productivity have been produced by the BGS for both bedrock and superficial deposits in Scotland as a tool to help characterise groundwater bodies for the Water Framework Directive. The maps are designed to be used at a scale of 1:100 000. The term “aquifer productivity” has been used to describe the potential of an aquifer (bedrock or superficial deposit) to sustain various levels of borehole supply. Properly sited boreholes (or for superficial deposits, a group of boreholes) in high and very high productivity aquifers have the potential to be considered a source for public supply or for industry. Low productivity formations are considered suitable for groundwater supplies to single or small groups of houses. The maps do not give any information on groundwater quality. The bedrock aquifer productivity map has five productivity classes, ranging from very high to very low. The rock formations and groups are also subdivided into three flow categories: dominantly intergranular flow, mixed fracture/intergranular flow and fracture flow. The classifications were made using BGS data and maps where available, and judgement from the lithological descriptions of the geology, where no hydrogeological data were available. All superficial deposits are assumed to have primarily intergranular groundwater flow (although fracture flow may be important in some tills). The superficial aquifer map is subdivided into three classes according to productivity (high, moderate and low), with the remainder classed as non-aquifer. The classifications were made using BGS data, geological descriptions and the Hydrology of Soil Types (HOST) classifications of the Macaulay Institute. The superficial deposits are further subdivided into two categories based on the likelihood of them being saturated and, therefore, containing a useable groundwater resource. In order to produce a guide to the likely location of superficial deposits that contain water, a model was created, based on a digital terrain model (DTM) and river elevation. The model was improved by including zones where certain soil classes indicate the presence of a shallow water table, using Hydrology of Soil Types (HOST) data. A model of superficial deposit thickness was then merged with the depth to water data to show zones where there is a high probability that the superficial deposit contains a usable groundwater resource. Outwith these zones, there is a lower probability of their being a usable groundwater resource within the superficial deposits. The maps are only intended as a guide to the aquifer conditions and are not a substitute for detailed site investigation. The maps can be improved as more information on drilling success and borehole yields is gathered throughout Scotland.