Saline intrusion : a screening tool for the assessment of risk to coastal aquifers in Scotland

A simple methodology has been devised for use by SEPA non-technical staff in Scotland for use in the initial assessment of the risk to aquifers of seawater intrusion caused by the construction of a new abstraction borehole close to the coast. The methodology has been translated into a GIS-based format. The methodology is not intended to provide all the answers as to whether there is a risk from seawater intrusion. Rather, it provides a screening tool to omit all new sources that will not be at risk, whilst allowing those that are perceived to be at risk to be assessed individually by SEPA hydrogeologists using other, more detailed, datasets. The methodology assumes that no intrusion will take place at distances greater than 4 km from the coast. All bedrock aquifers are assumed to depend on fracture-flow and to have an equal risk potential. Where low volume coastal abstraction takes place fractured bedrock is at greater risk of pulling in sea water than are intergranular media such as Quaternary sands. This is due to the focussed conduit flow in fractures compared to the diffuse transport in porous media. The variables taken into consideration in the risk tables are: distance from the coast, dynamic gradient and proposed abstraction rate from new boreholes. It must be emphasised that this methodology is not intended to be the complete assessment for risk of seawater intrusion. It is to be used as an initial screening by administrative staff when processing applications for new water abstraction boreholes and utilises only very basic level information. It is to distinguish between, for example, single cottage supplies located 3.5 km from the coast from larger abstractions intended for locations closer to the sea. For the latter, more detailed investigations will be carried out using detailed data within a more complex methodology.