Hydrogeology, Cainozoic of north-east Scotland
| Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul. |
Introduction
Across the whole of north-east Scotland groundwater occurs within a wide range of superficial deposits as well as in bedrock (Robins, 1990). Storage capacity and permeability are the main controlling factors determining both the volume of groundwater stored within the aquifers and the rate of flow.
Alluvial deposits
The deposits with the largest storage and highest permeability are the coarse alluvial gravels within the main river basins (P915257). The rivers Spey and Dee contain significant volumes of alluvial gravels below the water table; the Spey deposits are of particularly high permeability. Other rivers, notably the Deveron around Banff, and the Lossie, near Elgin, also have well developed alluvial gravel deposits and there are many other minor watercourses containing smaller amounts of water-saturated granular alluvium.
River gravels beneath valley floors typically contain large amounts of groundwater relative to the volume of the deposits. The gravels have greater lateral continuity than the more heterogeneous deposits, such as glaciofluvial sand and gravel, resulting in more predictable flow routes for groundwater in the alluvial sequences. They are also generally characterised by high rates of through flow.
The relatively homogeneous nature of alluvial deposits and the presence of shallow water tables under the river floodplains means that there is considerable potential for exploitation of this type resource for supply. The potential for recharge to alluvial aquifers is also high, because they are located in valleys and, as surface run-off moves towards the main watercourses, a significant proportion of it infiltrates into the ground. Alluvial deposits act as an important storage medium for groundwater prior to its discharge (sometimes after several years storage) as base flow to rivers.
Glaciofluvial deposits
Glaciofluvial sand and gravel deposits are much more widespread than alluvial sediments, particularly along the Moray coast, but they commonly have a lower overall permeability. The heterogeneous nature of many glaciofluvial deposits also results in complex groundwater flow paths, for example via gravel-filled channels, many of which are discontinuous.
Glaciofluvial sheet deposits commonly form terraces flanking the alluvium of the major river systems. They form higher ground on the valley sides and mostly lie above the water table. Glaciofluvial ice-contact sand and gravel forms extensive moundy deposits and much of this material also occurs above the local water table. The sandy nature of the soils developed on these terraced and moundy deposits means that recharge by rainfall to the water table is high where they occur, and both play an important role in the water cycle by providing base flow to rivers and wetland areas.
Blown sand
Blown sand deposits are very well sorted and have high intergranular storage capacity, but are generally of very limited thickness and lateral extent. Where underlain by peaty or clayey material above sea level, the basal parts of blown sand deposits are commonly saturated with groundwater and relatively high values for hydraulic conductivity are present. However, the deposits are restricted to coastal areas and the total volume of groundwater stored within them is generally quite small in comparison with that stored in other, less permeable material. The rate of recharge to these deposits is very high, with no surface runoff present, except where peaty interbeds form perched water tables.
Till
Till deposits are widespread across the district, most of them being relatively sandy. Owing to its compaction, higher clay content and heterogeneous nature, till contains less groundwater than the more porous sandy and gravelly deposits and, overall, its hydraulic conductivity is significantly lower. It is important, however, on a regional scale, for determining the movement of shallow groundwater. For example, many springs and seepage lines are present within this type of deposit, where sandier horizons and gravelly interbeds crop out. These can help sustain relatively high rates of groundwater flow. Springs and seepage lines are also commonly developed at the base of till units, particularly where they overlie less permeable fresh bedrock. Recharge to till deposits can be high where sandy horizons are present and more than 200 mm of rainfall per year can infiltrate to the water table.
Bedrock
Fresh pre-Palaeozoic igneous and metamorphic bedrock generally has a low permeability, but where shallow zones have been subjected to weathering, fractures may be enlarged allowing groundwater to move relatively freely. Flow and storage are dependent entirely on the presence of secondary voids such as joints and fractures within fault zones. This results in highly complex, unpredictable flowpaths. The Devonian and Permian sandstone aquifers near the Moray coast contain greater volumes of groundwater than the older rocks owing to their greater porosity and intergranular permeability. In spite of this, a large proportion of the total groundwater flow uses secondary voids to move through the aquifer.
Groundwater quality
Shallow groundwater quality in the district is variable, although many shallow aquifers contain potable groundwater. Chemical quality depends to a large extent on the nature of the surface deposits. For example, groundwater in the Spey gravels is normally of good quality, but where overlain by waterlogged layers of peat or silt, significant local concentrations of iron and manganese may occur. This is due to the groundwater having a relatively low content of dissolved oxygen and higher than normal acidity, causing the dissolution of these elements. Elsewhere, the effects of intensive agricultural practices have influenced the quality of the groundwater, with the presence of locally high concentrations of nitrate derived from fertilisers. This is particularly noticeable around the Ythan estuary. Groundwater is particularly vulnerable to pollution where the water table is less than 5 m below the ground surface, and where sand or gravel deposits are present below the soil layer. In general terms, groundwater is of very good quality in bedrock aquifers.
Groundwater exploitation
Groundwater from springs as well as shallow and deep sources is abstracted for many hundreds of private domestic supplies in north-east Scotland. In addition, public supply sources include two deep boreholes and many shallow bores into alluvial aquifers. The industrial usage of groundwater has, to date, been limited to maltings, distilleries and agriculture-related activities, and the absence of high-yielding aquifers in urban areas such as Aberdeen has hindered development. However, several water boreholes were sunk into Devonian sandstones in the Elgin area, between 1990 and 1998. This relatively unexploited source appears to have the potential for further development.
Historically, groundwater has provided a safe, reliable water supply to many communities and individual dwellings across the area. The absence of public supply networks across much of the more rural parts of the district has necessitated the exploitation of hundreds of springs in many different settings. Where natural occurrences of groundwater were absent, shallow wells were constructed, commonly to depths of 4 to 10 m. Most farms had private wells, many of which exploited gravelly interbeds in till or the more permeable zone at rockhead. Where high-yielding springs were present, communities would organise a limited water distribution network. The larger towns, such as Peterhead and Aberdeen, relied on groups of springs to supply large numbers of people. The Peterhead spring supply was still in use until after the Second World War and was based around a group of seven springs on the Hill of Longhaven, 5 km south-west of the town.
Beneath Aberdeen, superficial aquifers with more than 30 m of saturated granular material were exploited for factory supplies in the 19th century. The variable nature of the Quaternary deposits and indifferent water quality restricted usage, especially adjacent to the tidal sections of the rivers, where the shallow groundwater is brackish. The main supply for Aberdeen presently comes from intakes on the River Dee, including one at Cairnton (NO 665 965).
The nature of groundwater exploitation began to change towards the end of the 18th century when deep boreholes were first drilled into bedrock. Depths ranged from 10 m to greater than 100 m. However, the absence of widespread, highly permeable groundwater resources in the bedrock of the district has meant that relatively few deep boreholes have been drilled across north-east Scotland. Examples of public supply boreholes (P915257) include those at Turriff, where a Middle Devonian conglomerate aquifer is exploited, and at Burghead, where the local supply is sourced from Triassic, Permian and Devonian sandstones, but almost all the supply comes from the Permian strata. Deep boreholes in the Moray area produce low to moderate supplies for distilleries and maltings.
Owing to their favourable hydraulic characteristics, shallow superficial aquifers remain the most heavily exploited type of aquifer for public supply. The River Spey groundwater scheme, located immediately to the south of Fochabers, is the latest scheme to abstract groundwater from gravel deposits and was commissioned in 1996. The main well field lies just outside the district, but is a good example of large-scale groundwater development. A total of 40 boreholes, some up to 20 m deep, have been sunk within the river gravels on the eastern side of the Spey. Each is capable of an abstraction rate of 10 litres per second. Recharge to the aquifer is by leakage from the Spey into the gravel beds between the boreholes and the river, by lateral groundwater movement from the valley sides and by direct infiltration of rainfall on the floodplain. Another example of alluvial aquifer exploitation occurs in the valley of the River Deveron, south of Banff. This is a much smaller abstraction scheme than on the Spey at Fochabers. It uses a horizontal infiltration design to collect water from the gravel, rather than a vertical borehole.
The exploitation of natural springs for public supply is now rare within north-east Scotland, with most supplies having a surface origin. A total of 20 springs are maintained by the North of Scotland Water Authority including one in Moray, eleven in Gordon and eight in Banff and Buchan. Owing to continual improvements in supplies, many of these springs may be decommissioned, either to comply with European requirements on water quality, or because of the replacement of several previously separate supplies by a single regional network.