Hydrogeology and water supply of the Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
High rainfall, leading to an abundant supply of surface water, coupled with a relatively sparse population over much of the region, allows most of the public water supply to be drawn from reservoirs, springs and shallow wells or boreholes. Deeper aquifers are only exploited on a substantial scale in the Dumfries area, which is one of the few areas where high productivity aquifers are found (see below). Elsewhere, the deep valleys and low population density have enabled the construction of several very large, dammed water supply reservoirs, mainly to supply towns and cities in adjacent areas of the Midland Valley. The most extensive of these is the combined Talla-Fruid and Megget schemes, some 20 km north-east of Moffat, which are the principal contributors to Edinburgh’s water supply. The Loch Bradan scheme, in the western part of the region, supplies water to domestic and industrial consumers in central Ayrshire, while similar schemes at Daer and Camps, near Crawford, send water to the large towns of South Lanarkshire. The chemistry of the surface waters is closely related to the regional geology as exemplified by the variation in pH (P912380) of stream water, which has a negative correlation with the available Ca, Mg and total carbonate content of bedrock. However, the very low pH (< 6) over the high ground in the west of the Southern Uplands has also been influenced by other factors such as impeded surface drainage and the presence of a thick peat cover. Afforestation has been extensive since the 1960s and has also contributed to surface water acidity. Moreover, the pH of surface waters in this part of the region has been significantly modified by ‘acid rain’, the deposition of atmospheric acids derived from the combustion of fossil fuels and introduced by the prevailing westerly winds. Away from these influences, there is acidic drainage (pH ~ 6) over the Criffel and Cheviot granite outcrops, broadly neutral conditions (pH ~7) over that part of the Southern Uplands underlain by Ordovician wacketype sandstones or by those of the Silurian Gala Group, and mildly alkaline surface waters (pH ~8) over the carbonate-rich, Silurian sandstones of the Hawick and Riccarton groups. Similarly high pH levels prevail across the Carboniferous, limestone-bearing succession of the Berwickshire Merse, whilst the low pH in the south and across the border into the Kielder Forest area of northern England can be ascribed to the broadly acidic lithofacies within the Border Group, enhanced by the effects of afforestation.
Lower Palaeozoic rocks
Ordovician and Silurian rocks, comprising mainly wacke-type sandstone and mudstone, have by far the largest outcrop area across the south of Scotland. Together with the granitic plutons intruded into them, they form low productivity aquifers with very low intergranular permeability, though that may be enhanced in any well-developed weathered zone present at rockhead. Groundwater storage and flow is almost entirely via fractures, and groundwater flow paths are likely to be relatively shallow, short and localised. Although individual borehole yields are low, there are at least 900 abstractions from the Lower Palaeozoic aquifer across the region, primarily for domestic and farm use. The quality of the water is usually good and potable, although the chemistry can be variable with, for example, higher calcium and bicarbonate in areas where the sandstones have a calcite cement. In a few places, highly mineralised groundwater can be related to mudstone beds within the Lower Palaeozoic succession.
During the 19th century, groundwater from the Lower Palaeozoic aquifer, including some mineralised groundwaters, provided the basis for thriving spa industries at Innerleithen and Moffat. St Ronan’s Well at Innerleithen reputedly provided sulphurous, saline and freshwater springs, though the saline springs cannot be located today. Near Moffat, Hartfell Spa is a chalybeate spring whilst the Moffat Well is sulphurous.
Devonian and Carboniferous rocks
The Devonian sandstones are well cemented and have generally low intergranular permeability, but relatively high productivity may be provided locally by fracture flow. Groundwater flow paths are likely to be longer and deeper than those found in the Lower Palaeozoic aquifer. There are approximately 100 small abstractions from the Devonian aquifer in southern Scotland, mostly from boreholes for domestic and farm use in Berwickshire and the southern Borders; there are likely to be many more unrecorded shallow wells and springs.
The lithologically variably Carboniferous successions may form moderately productive aquifers but with a relatively complex, multilayered hydrogeology that is at present only poorly understood; water quality and physical aquifer properties are likely to be highly variable. In general, the sandstone units are likely to act as effectively discrete aquifers, in which fracture flow dominates, separated by finer-grained units of lower permeability. There are at least 50 abstractions from the Carboniferous aquifer in southern Scotland, mostly farm boreholes in the Tweed Basin.
Permian and Triassic rocks
Within the Permian to Triassic basins of Annandale, Lochmaben, Dumfries, Stranraer and Carlisle–Solway, the red sandstones are a well-known aquifer and have been used extensively in the past for industrial enterprises with large water demand such as creameries. These rocks have good potential for public water supply, especially in isolated communities where demand is relatively low and the cost of reservoirs high. At the beginning of the 21st century, the Dumfries Basin, Thornhill and Annandale Permo-Triassic aquifers all support groundwater abstraction for public supply, agriculture and, in some cases, industrial use. Only one borehole is used for public supply in the Thornhill Basin, and three boreholes are currently used for public supply on a standby basis in the Annandale Basin. The Dumfries Basin is the most heavily exploited for public water supply and industrial use, with groundwater abstraction concentrated in the western part of the basin where aquifer productivity is highest (see below). There is some evidence that locally falling groundwater levels here reflect the effects of intense pumping. Also of concern is the rise in nitrate concentration caused by leaching of agricultural fertilisers.
Two main aquifers can be identified in the Dumfries Basin. In the west of the basin lies the breccio-conglomerate and interbedded sandstone of the Doweel Breccia Formation, which interfingers eastwards with the finer-grained Locharbriggs Sandstone Formation. The Doweel Formation breccias have a matrix of lithified clay minerals and have very low intergranular permeability. However, the formation is cut by subhorizontal fissures, commonly along the breaks between sandstone and breccia beds, which significantly increase the rock permeability. Consequently, the Doweel Breccia Formation is one of the most highly transmissive bedrock aquifers in Scotland. Fractures dominate groundwater flow, providing more than 90 per cent of the influx into boreholes. The fracture network also acts as a rapid conduit for groundwater transport towards the River Nith. As a result, the Doweel Breccia Formation aquifer has generally younger water, more active recharge, and higher nitrate concentrations than the Locharbriggs Sandstone Formation aquifer in the eastern sector of the Dumfries Basin. This sandstone has significant intergranular porosity and permeability but few fractures or joints of any hydraulic significance. Consequently, borehole yields are generally lower here than in the west of the basin; storage capacity is higher and groundwater residence times are longer.
The Dumfries Basin is bounded by weakly permeable Silurian sandstones, and is largely concealed by variable superficial deposits. Recharge is either by inflow of surface water from the surrounding catchment via the River Nith and the Lochar Water, or directly from rainfall percolating through overlying superficial sand and gravel. Discharge is predominantly to the rivers in the central part of the basin, rather than directly southward to the sea. Across the basin as a whole, there is a seasonal variation in groundwater levels of about 2 m, with the highest levels in late winter followed by a steady natural decline through spring. The lowest lying parts of the basin, south of Dumfries, are less than 10 m above sea level, and boreholes located within these low-lying areas can be artesian where silty clay deposits overlie the bedrock aquifer. Elsewhere, individual fractures and sandstone horizons may have different groundwater levels.
The natural groundwater chemistry in the Dumfries Basin is a product of maritime rainfall modified by the dissolution of carbonate material in the breccia, sandstone, and superficial deposits. The groundwater is typically hard, moderately mineralised and of calcium-magnesium-bicarbonate type, with bicarbonate concentrations greater than 100 mg/l. Distributions of major ions throughout the basin are related to rock and superficial deposit geochemistry, such as the lower concentrations of calcium, magnesium and bicarbonate in groundwater within the Locharbriggs Sandstone Formation, reflecting the aeolian origin of these sediments and the presence of silica cement. Concentrations of sulphate are highest in the south of the basin beneath raised beach and marine superficial deposits, but may also be related to the presence of thin gypsum lenses or sulphide oxidation in sporadic mudstone horizons.
Across the south of Scotland, the hydrogeological properties of superficial deposits are highly deposits variable. Some of the more extensive spreads of glaciofluvial and alluvial deposits form highly productive, local aquifers which are exploited by public supply boreholes in a number of places in the Borders, including Innerleithen and Ettrickbridge. There has also been some commercial exploitation of superficial aquifers as a source of bottled mineral water. Aquifers within glaciofluvial sand and gravel units may be confined by impermeable, interbedded silts, clays or glacial till, and supplies are likely to fluctuate rapidly in response to precipitation. The water is generally relatively hard compared to surface waters, due to bicarbonate or sulphate concentration, and may contain significant concentrations of iron and manganese locally. Some of the alluvial gravels along the major valleys have also proved highly productive, with high flows of water from boreholes, though for these aquifers it is more difficult to predict the quality of the water and the quantities available. Aquifers within the permeable superficial deposits are particularly vulnerable to pollution, for example from agricultural and industrial discharges. As well as forming productive local aquifers in their own right, these deposits also play a significant role regionally in controlling the volume, timing and chemistry of recharge to underlying bedrock aquifers.
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