Derek F. Ball

An overview of groundwater in Scotland

Synopsis Groundwater is an important, but undervalued, national resource in Scotland. Groundwater is present both in the bedrock, where much of the flow is through fractures, and in the superficial deposits, where intergranular flow dominates. The most productive bedrock aquifers are the Permian sandstones and breccia in SW Scotland and the Devonian sandstones in Fife, Strathmore and Morayshire. Alluvium and fluvio-glacial sands and gravels can also form important aquifers and provide some of Scotland’s most highly yielding boreholes. Groundwater is generally weakly mineralized with total dissolved solids in the range 100 to 500 mg l−1 and is dominated by Ca and HCO3 type waters. There are in excess of 4000 boreholes in Scotland, and over 20 000 springs and wells used for private water supply. There are few available reliable data on the total volumes of groundwater abstracted; however, conservative estimates suggest that the total volume is approximately 330 megalitres per day for public water supply, industry, agriculture and private water supply. The volume of groundwater used in public water supply is growing annually; it is currently 7% of the daily 2400 megalitres per day supplied by Scottish Water. Scottish groundwater is under threat from agricultural activity, the mining legacy, septic tanks, localized high abstraction, and general land development. The implementation of the Water Framework Directive, and associated legislation, is an excellent opportunity to sustainably manage and develop groundwater in Scotland.

Mapping groundwater vulnerability in Scotland: a new approach for the Water Framework Directive

Synopsis A new methodology for groundwater vulnerability assessment has been devised for Scotland to meet the requirements of the Water Framework Directive. Using the methodology, a new GIS-based map of groundwater vulnerability has been produced, at a working scale of 1:100 000. The map is being used by the Scottish Environment Protection Agency (SEPA) to help characterize and assess risk to groundwater bodies. The methodology assesses the vulnerability of groundwater in the uppermost aquifer to the vertical downward movement of a non-specific contaminant from the ground surface. It considers the intrinsic properties of the pathway between the ground surface and the water table. The key difference from previous vulnerability maps in Scotland and the rest of the UK is that the new method assesses vulnerability in all aquifers regardless of resource potential. This reflects the diverse environmental objectives for groundwater bodies under the Water Framework Directive. This approach provides the flexibility to combine the groundwater vulnerability map with maps of pressures, groundwater resources or other groundwater-related receptors, as required.

Using transmissivity, specific capacity and borehole yield data to assess the productivity of Scottish aquifers

Abstract Aquifer properties data from more than 3000 groundwater sources across Scotland have been collated to form the Scottish Aquifer Properties Database, coordinated by the Scotland and Northern Ireland Forum for Environmental Research. The aim of the project was to better understand Scotlands aquifers, through the collation of a comprehensive set of quantitative data. Analysis of 157 transmissivity values, 307 specific capacity values and 1638 borehole yield values shows that Quaternary and Permo-Triassic age aquifers are the most productive, followed by those of Devonian and Carboniferous age. There is a strong correlation between specific capacity and transmissivity (r2 = 0.8), and the former may be used as a reliable indicator of aquifer productivity where no transmissivity data are available. The correlation between transmissivity and borehole yield data is significant (r2 = 0.57), although the quality of the yield data is lower overall than that of the specific capacity or transmissivity data. These data support recent categorization of bedrock aquifer productivity in Scotland, which until now has been validated only with limited quantitative datasets.

Agriculture and diffuse pollution: groundwater nitrate vulnerable zones in Scotland

Synopsis Nitrate derived from agricultural activity is a threat to Scotland’s surface water and groundwater environment. The pollution of groundwater by nitrate can impact drinking water in aquifers in addition to the quality of base-flow to many streams and rivers. European legislation (in the form of the Nitrates Directive) requires that waters with a nitrate concentration of greater than 50 mg-NO3 l−1 (and those at risk of exceeding this concentration) should be identified and the catchment areas designated as Nitrate Vulnerable Zones (NVZs). Groundwater NVZs for Scotland were developed using a risk-based methodology which combined several Scottish national scale datasets in a GIS. The risk of nitrate leaching was determined from soil, climate and land use data. Digital solid and superficial geology data (at 1:50 000 scale) were interpreted to give an indication of aquifer vulnerability. The final zones were then determined by identifying local surface water catchments associated with areas of greatest nitrate leaching and aquifer vulnerability. The results of the vulnerability/risk analysis were broadly consistent with the available nitrate data for much of Scotland. Groundwater nitrate data were collated from the monitoring network of the Scottish Environment Protection Agency (SEPA), water authority boreholes and private water supplies, with additional new data collected from targeted sites. Within the NVZs, 25% of approximately 1700 samples exceeded 50 mg-NO3 l−1 compared to the rest of Scotland, where less than 5% of approximately 700 samples exceeded this concentration. The nitrate vulnerable zones have now been designated by the Scottish Executive and action programmes established to reduce nitrate contamination in these areas.

Groundwater exploration in rural Scotland using geophysical techniques

Abstract Identifying the location, size, and characteristics of small aquifers is becoming increasingly important in Scotland with the growing demand for groundwater to supply isolated rural communities. These small, localized aquifers are found within superficial deposits, such as alluvium, blown sands and raised beach deposits, and in fracture zones in the underlying bedrock. Geophysical techniques offer a rapid and inexpensive method of characterizing these aquifers. Electromagnetic techniques using EM34 and EM31 instruments have proved useful in identifying variations in the thickness of superficial deposits and detecting buried channels, for example, at Palnure, SW Scotland. Ground penetrating radar has been used at several locations, including the island of Coll, to detect the boundary between bedrock and alluvium or blown sand. More detailed techniques, such as resistivity soundings, seismic refraction and resistivity tomography have been used to identify fractures in basement rocks and help calibrate the other methods, for example at Foyers, near Loch Ness. Magnetic profiling has also been used to locate dykes (acting as hydraulic barriers) within Permian aquifers in the west of Scotland.

The role of numerical modelling in understanding groundwater flow in Scottish alluvial aquifers

Abstract Groundwater in Scotland has been, until recently, an under-rated resource given the abundance of surface water resources. In the last decade, a number of new abstractions have been developed and existing ones enhanced. Implementing groundwater abstraction licensing through the Scottish Water Environment (Controlled Activities) Regulations (2005) has accelerated the need to understand such schemes. Simulating the groundwater systems, which are generally small in area, with an immature understanding and where subsurface data are often sparse, is a challenge. This challenge is amplified when groundwater abstraction is proposed from previously unexploited gravel valley deposits in close proximity to large rivers. Examples of recent work undertaken for Scottish Water illustrate the important role that groundwater models have in testing and refining conceptual understanding as well as convincing regulators of the suitability of the groundwater abstraction.

Towards understanding the Dumfries Basin aquifer, SW Scotland

Abstract The Dumfries Basin aquifer supports groundwater abstraction for public supply, agriculture and industry. Abstraction is concentrated in the western part of the basin, where falling groundwater levels and deteriorating water quality both reflect the effects of intense pumping. There are two bedrock units: a predominantly breccia-coarse sandstone sequence in the west, interfingering with a predominantly sandstone sequence in the NE and east. The basin is bounded by weakly permeable Lower Palaeozoic rocks, and is largely concealed by variable superficial deposits. Surface water flows onto the basin from the surrounding catchment via the Nith and the Lochar Water and their respective tributaries. Direct rainfall recharge occurs via superficial sands and gravels, especially in the north, and discharge is predominantly to the rivers in the central area rather than the sea. A picture is developing of two main aquifer types within the basin: the high-transmissivity western sector underlain by a fracture-flow system with younger water and active recharge and a high nitrate content, compared with the east where groundwater residence times are longer and the storage capacity is higher.