OR/15/028 Aquifer characteristics continued

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Ó Dochartaigh, B É, MacDonald, A M, Fitzsimons, V, and Ward, R. 2015. Scotland’s aquifers and groundwater bodies . British Geological Survey Internal Report, OR/15/028.

Old Red Sandstone (Devonian)

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Geological summary

Rocks of the Old Red Sandstone sedimentary succession crop out across large parts of north, north-east, central and south-east Scotland. It is a terrestrial red-bed sequence, dating from the late Silurian to the early Carboniferous but dominantly Devonian, formed predominantly of fluvial and alluvial fan sandstones, with variable proportions of conglomerates, siltstones and mudstones. Volcanic rocks, including lavas and volcaniclastic rocks, are significant in parts of the otherwise dominantly sedimentary succession, forming a distinct aquifer type, described in Silurian and Ordovician.

Old Red Sandstone rocks in Scotland were deposited in two distinct basins: a northern, ‘Orcadian’, basin, including outcrops in Shetland, Orkney, Caithness, Morayshire and Aberdeenshire; and a southern basin including outcrops in Aberdeen; the Vale of Strathmore, from Loch Lomond to Stonehaven; in Fife; and in the Scottish Borders. The maximum total thickness of Old Red Sandstone rocks can exceed 2000 m (Trewin and Thirlwall, 2002[1]).

Physical aquifer properties and groundwater flow

Sandstones form the most productive aquifers within the Old Red Sandstone succession. The youngest sandstones in the sequence are found in Fife, and form some of the most highly productive bedrock aquifers in Scotland. These sandstones are typically well cemented, with relatively low intergranular porosity and permeability (Table 3). Fracture flow dominates ground- water movement (Figure 12), with at least 80 per cent of borehole inflows from fracture flow (Ó Dochartaigh, 2004[2]).

Away from Fife, sandstones within the Upper Old Red Sandstone, and older sandstones and conglomerates of the Middle and Lower Old Red Sandstone, generally form moderately, and occasionally highly, productive aquifers (Table 10). In Caithness and Orkney there are extensive fine grained flagstones, within which groundwater flow is concentrated along bedding planes, and which tend to form only moderately productive aquifers at best. Where lower permeability siltstones or mudstones separate sandstone beds, they tend to act as barriers to groundwater flow, except where permeable faults cut through the sequence.

Figure 12 Schematic cross-section of the hydrogeology of Old Red Sandstone aquifers.
Table 10 Summary of available aquifer properties data for Old Red Sandstone sedimentary aquifers.
Porosity (%) Hydraulic conductivity (m/d) Transmissivity (m2/d) Specific capacity (m3/d/m) Storativity Operational yield (m3/d)
Upper Old Red Sandstone (Fife) 20 (7) 0.5 (7) 200–300 (12) 90–175 (13) ~0.002–0.01 (8) >1000 (34)
Upper Old Red Sandstone (away from Fife); Middle Old Red Sandstone ~10% (16) 150–200 (12) 50–120 (13) ~0.001 (8) 800–1000 (Moray and Aberdeenshire) (19)
Lower Old Red Sandstone (largely Strathmore; some Moray) ~10% (2) 0.01–2 (2) 50–150 (6) 40–100 (23) ~0.0001 (8) 200–400 (Strathmore and Midland Valley) (183)

Ranges of values refer to mean and median values
Number of values indicated in brackets
Data from the British Geological Survey

The predominance of fracture flow means groundwater transport can be rapid, and groundwater is generally well mixed in the top 50 m or so, but groundwater residence times of several decades are typical.

Summary of baseline chemistry

The baseline chemistry of groundwater in Old Red Sandstone aquifers in the Moray Basin is described in detail in Ó Dochartaigh et al. (2010)[3]. A summary is provided in Table 11. The baseline chemistry of groundwater in Old Red Sandstone aquifers in Fife is described in detail in Ó Dochartaigh et al. (2006)[4], and of Old Red Sandstone aquifers in southern Scotland in MacDonald et al. (2008)[5]. A summary is provided in Table 12.

Table 11 Summary of baseline chemistry of Old Red Sandstone North aquifers in the Moray Basin.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 99 0 26.6 47.3 61.6 79.5 99.1
Cl mg/L 99 0 17.5 26.9 46 72.7 92.6
DO2 mg/L 49 14 NA NA 1.22 5.59 8.16
Fe µg/L 91 19 2.1 4 28.8 300 857
HCO3 mg/L 99 1 77 127 193 250 327
K mg/L 98 1 1.3 1.8 2.6 3.8 5.8
Mg mg/L 99 0 2.3 3.67 8.1 17.4 23.8
Na mg/L 99 0 11.7 15.3 25.5 41.2 56.9
NO3 as NO3 mg/L 82 20 NA 0.18 2.92 15.91 27.54
pH 97 0 6.5 6.8 7.2 7.45 7.7
SEC µS/cm 94 0 240 374 547 708 890
SO4 mg/L 99 0 6.13 10.6 19.6 35.9 54.5

a number of samples
b number below detection limit

Table 12 Summary of baseline chemistry of Old Red Sandstone South aquifers.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 125 0 25.5 38 47.5 60.9 84.3
Cl mg/L 125 0 13.3 17.6 25.3 43.6 68.4
DO2 mg/L 64 2 1.4 3.2 6.64 8.2 9.02
Fe µg/L 105 46 2 2 11 44 158
HCO3 mg/L 124 0 89.8 128 162 214 257
K mg/L 125 1 0.9 1.45 2.1 3.22 5.1
Mg mg/L 125 0 5.72 9.82 14.4 22.2 34
Na mg/L 125 0 7.48 11.5 16 25.3 66.3
NO3 as NO3 mg/L 93 3 0.88 5.75 17.68 45.08 78.23
pH 113 0 6.77 7.02 7.41 7.7 7.8
SEC µS/cm 108 0 296 386 498 610 821
SO4 mg/L 124 0 9.01 13.3 21.2 33.2 60.3

a number of samples
b number below detection limit

Silurian and Ordovician

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Geological summary

Rocks of Silurian and Ordovician (early Palaeozoic) age dominate southern Scotland but are essentially absent from the rest of the country. They comprise mainly turbiditic sandstones (called greywackes) and siltstones, with variable proportions of conglomerates, mudstones, cherts, volcaniclastic rocks and marine lavas. The total sequence is many thousands of metres thick, and reflects varying marine depositional environments associated with an oceanic subduction zone. The rocks are divided into structural tracts by major north-east to south-west trending faults. Their northern boundary is marked generally by the Southern Uplands Fault, although smaller outcrops of Ordovician rocks occur to the north of this into the Midland Valley.

Physical aquifer properties and groundwater flow

The dominantly fine-grained, well-cemented nature of the rocks means intergranular permeability is generally low. Apart from a sometimes well-developed weathered zone at rockhead, in which intergranular permeability is enhanced, fractures in the rocks are thought to contribute virtually all groundwater storage and flow (Figure 13). There are no transmissivity or hydraulic conductivity data for the aquifer, reflecting the dominance of low yielding, private supply use — the aquifer has never been investigated in detail for public water or large scale industrial supply. Analysis of recorded operational borehole yields suggests that borehole yields are typically low, with an overall median of 0.3 l/s and a mean of 0.6 l/s.

Groundwater flow paths are likely to be controlled by fracture patterns and to be generally relatively shallow, with groundwater appearing to be well mixed in the top 50 metres or so. In general, flow paths are likely to be relatively short and localised, but in some cases there may be connectivity over several kilometres from higher ground to valleys, and groundwater is often resident in the aquifer for several decades (MacDonald et al., 2008)[5].

Figure 13 Schematic cross-section of the hydrogeology of Silurian and Ordovician aquifers in Scotland.

Summary of baseline chemistry

The baseline chemistry of groundwater from Silurian and Ordovician aquifers in Scotland is described in detail in MacDonald et al. (2008)[5]. A summary is provided in Table 13.

Table 13 Summary of baseline chemistry data for Silurian and Ordovician aquifers.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 73 0 10 19.7 39.1 54.6 70
Cl mg/L 72 0 7.05 10.5 14.4 28.7 49.6
DO2 mg/L 48 1 0.14 1.5 3.68 5.1 8.1
Fe µg/L 66 27 7.5 7.5 13 41 419
HCO3 mg/L 73 1 13 56.7 134 207 273
K mg/L 72 2 0.5 0.86 1.2 2.27 3.4
Mg mg/L 73 0 2.64 5.66 12.5 17.9 25.1
Na mg/L 73 0 5.45 7.4 10.5 15.7 23.9
NO3 as NO3 mg/L 70 1 0.51 1.54 11.09 21.30 43.93
pH 72 0 6.04 6.7 7.09 7.48 7.76
SEC µS/cm 73 0 116 190 340 536 655
SO4 mg/L 73 0 0.71 5.45 9.38 14.7 28.3

a number of samples
b number below detection limit

Highland calcareous: dominantly calcareous Precambrian and Cambro-Ordovician

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Geological summary

Most rocks in Scotland are dominantly siliceous, but a few, mostly Precambrian and Cambro-Ordovician in age and in the Highlands, are dominantly calcareous, which has significant impacts on the chemistry of groundwater stored in and flowing through the rocks. Calcareous rocks occur within the Appin, Argyll and Southern Highland Groups of the Dalradian Super- group (within the Precambrian South aquifer group), and include calcareous pelites, calcsilicates, and metalimestones (Strachan et al., 2002[6]). The calcareous horizons are often relatively thin — sometimes only a few metres thick — and are interbedded with greater thicknesses of other, noncalcareous metamorphic rocks.

The younger Durness Group, of Cambro-Ordovician age, is a dominantly carbonate sequence which crops out along a narrow belt some 250 km long just inland from the north-west coast. It can be seen as a moderate productivity, fracture flow aquifer in this area in Figure 3. It is usually no more than a few hundred metres thick, although reaches 1300 m at its thickest (Robins, 1990[7]). Only part of the group is dominated by the limestones in this aquifer group; the rest is dominated by dolostones with intervals of subordinate chert (Park et al., 2002[8]).

Physical aquifer properties and groundwater flow

Calcareous Precambrian rocks have low intergranular porosity and permeability, but dissolution of carbonate along fractures in the rock can produce secondary karstic permeability (Figure 14). Where karstic permeability becomes very well developed, these rocks can have higher transmissivity than noncalcareous Precambrian aquifers (Robins, 1990[9]).

Cambro-Ordovician limestones show significant karst development in places, thought to be concentrated along fault planes, and including cave sequences at least 3 km long, through which large flows of up to 00 l/s discharge (Robins, 1990[9]). However, the scarce available data indicate that where karst is poorly developed, the aquifer has relatively low productivity, similar to Precambrian aquifers.

Figure 14 Schematic cross-section of the hydrogeology of calcareous aquifers (dominantly Precambrian and Cambrian) in Scotland.

Summary of baseline chemistry

A summary of the baseline chemistry of groundwater in Highland Calcareous aquifers is provided in Table 14.

Table 14 Summary of baseline chemistry of Highland Calcareous aquifers.
Element Units na n <dlb P<sub>0.1 P</sub>0.25 P0.5 P0.75 P0.9
Ca mg/L 18 0 12.5 25 62.8 78.1 121
Cl mg/L 18 0 6.27 10.5 16.2 29.8 38.1
DO2 mg/L 18 0 3.59 5.56 7.86 9.28 10.5
Fe µg/L 18 3 1 2 10 69 271
HCO3 mg/L 18 0 46.4 54.3 194 344 377
K mg/L 18 0 0.877 1.02 2.35 3.48 4.4
Mg mg/L 18 0 2.89 3.51 4.8 10.4 28.4
Na mg/L 18 0 4.47 6.88 8.8 14.4 23.2
NO3 as NO3 mg/L 18 0 0.93 1.51 6.90 26.56 83.98
pH 18 0 6.46 6.7 7.2 7.42 7.72
SEC µS/cm 18 0 86.1 279 471 576 656

a number of samples
b number below detection limit

Precambrian

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Geological summary

Precambrian rocks cover most of Scotland north and west of the Highland Boundary Fault, geologically divided into a number of tectonostratigraphical terranes divided by major faults. From a hydrogeological point of view they can be divided into a northern and a southern division. They are divided here into Precambrian North and Precambrian South.

Precambrian North aquifers are typified by massive metamorphic rocks, including highly metamorphosed Lewisian gneiss, largely unmetamorphosed Torridonian sandstone, metamorphosed Moine schists, and Dalradian Grampian Group schists.

Precambrian South aquifers are typified by Dalradian metasedimentary schists of the Southern Highland and Argyll groups, which are typically more layered and less massive than Precambrian north aquifers.

Both Precambrian aquifer groups are variously intruded by igneous rocks.

Physical aquifer properties and groundwater flow

Scottish Precambrian rocks, both North and South, typically form low or very low productivity aquifers, with negligible intergranular porosity (less than 1 per cent — Robins, 1990[9]) and low permeability. Weathering of the uppermost few metres of rock, often most pronounced in areas of intensive fracturing, can create enhanced intergranular permeability, but in general groundwater flow and storage is entirely within fractures (Figure 15). These fractures are generally more common at depths of up to approximately 100 m. Often a single fracture, or at most three or four fractures, provide all of the inflow to a borehole, with individual fracture flows up to approximately 0.3 l/s, although typically lower than this.

Very little quantitative aquifer properties data are available for Precambrian aquifers in Scotland. For the Precambrian North, scarce records of operational borehole yields indicate that the largely unmetamorphosed Torridonian sandstone typically has the highest yields, transmissivity and specific capacity of the different Precambrian geological units (Table 15).

In parts of Aberdeenshire, the rare preservation of deep Tertiary weathering products — unlike in the rest of Scotland, where glacial erosion removed most pre-Quaternary weathering debris — has left tens of metres thickness of moderately to highly weathered Precambrian met- amorphic, and in some cases intrusive igneous, rock, with enhanced permeability. This special case is treated as a form of regolith — weathered, or superficial, aquifer — and is discussed in superficial aquifers.

Table 15 Summary of available aquifer properties data for Precambrian aquifers.
Transmissivity (m2/d) Specific capacity (m3/d/m) Operational yield (m3/d)
Precambrian North Lewisian 0.2–1.3 median 0.2 (3) 0.3–0.7 median 0.49 (4) 5–11 median 10.5 (4)
Torridonian 13–37 median 16 (3) 2.7–64 median 8 (5) 60–135 median 86 (5)
Moine 0.2 (1) 0.7–1.8 (2) 23–328 median 38 (4)
Precambrian South Dalradian 1.4 (1) 0.3–1.3 median 0.8 (2) 0–864 median 51 (88)

Ranges refer to total range in values
Number of values indicated in brackets
Data from the British Geological Survey

Figure 15 Schematic cross-section of the hydrogeology of Precambrian aquifers in Scotland.

Summary of baseline chemistry

A summary of the baseline chemistry of groundwater in Precambrian North aquifers is provided in Table 16. The baseline chemistry of groundwater in Precambrian South aquifers in Aberdeenshire is described in detail in Smedley et al. (2009)[10]. A summary is provided in Table 17.

Table 16 Summary of baseline chemistry of Precambrian North aquifers.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 41 0 9 13.3 20.9 34 64.6
Cl mg/L 41 0 10.1 14 22 37 67.1
DO2 mg/L 23 0 0.114 0.34 2.06 6.12 9.11
Fe µg/L 40 1 2 9 41 208 1100
HCO3 mg/L 41 0 27 46.9 72 116 221
K mg/L 41 0 0.95 1.33 1.91 2.6 4.1
Mg mg/L 41 0 1.18 2.1 3.73 5.9 7
Na mg/L 41 0 7.4 11.3 18 29 35
NO3 as NO3 mg/L 38 4 0.15 0.18 1.25 9.90 17.46
pH 38 0 6.07 6.27 6.68 7.28 8.09
SEC µS/cm 33 0 110 161 227 317 544
SO4 mg/L 41 0 2.27 4.54 7.67 10.5 12

a number of samples
b number below detection limit

Table 17 Summary of baseline chemistry of Precambrian South aquifers.
Element Units na n < dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 39 0 6.4 10.8 17.4 28 44.3
Cl mg/L 39 0 3.76 4.73 15.4 30.1 39.3
DO2 mg/L 32 2 1.09 3.32 5 7.76 9.78
Fe µg/L 39 5 5 10 35 120 508
HCO3 mg/L 38 0 10.8 18.2 37 108 155
K mg/L 39 0 0.7 1.04 1.51 2.45 3.34
Mg mg/L 39 0 1.2 2.41 4.53 7.72 11
Na mg/L 39 0 3.06 5.51 13.7 22.6 28.9
NO3 as NO3 mg/L 34 2 0.18 1.99 7.96 26.70 45.08
pH 39 0 5.71 6.06 6.65 7.54 8.13
SEC µS/cm 37 0 83.5 149 241 330 449
SO4 mg/L 39 0 4.53 5.86 10.9 15.9 25

a number of samples
b number below detection limit

Igneous volcanic

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Geological summary

There are innumerable outcrops of volcanic rocks across Scotland, from all time periods in the Phanerozoic. They were dominantly formed as lava flows that range from a few centimetres to many kilometres in size. Three periods in particular saw large volumes of volcanic lavas extruded: the Devonian (within the Old Red Sandstone sequence), Carboniferous and Tertiary. Volcaniclastic and, rarely, pyroclastic deposits are sometimes interlayered with the lavas; weathered layers and palaeosols within the sequence are also common.

Physical aquifer properties and groundwater flow

Volcanic rocks in Scotland typically form low productivity aquifers. In their unweathered state they have negligible intergranular porosity and permeability. The main controls on aquifer permeability are the degree and nature of rock fracturing, and the degree of weathering along junctions between individual lava flows (Figure 16). Highly weathered zones can have relatively highintergranular permeability, transmitting significant amounts of groundwater. Fractures in the intervening more massive rock can connect these zones and increase the overall productivity of the aquifer. Water bearing fractures are generally more common in the near surface, and are thought to become less common with depth: fracture inflows to boreholes at depths of more than 100 m do occur, but are rare.

The fractured and therefore heterogeneous nature of volcanic rock aquifers means there is a wide range in recorded borehole yields. Recorded yields range from <1 m3/d to >1300 m3/d, based on a total of 93 recorded values, with a mean of 145 m3/d and a median of 50 m3/d. The lowest recorded yields are from Devonian lavas (median 17 m3/d) and the highest from Tertiary (median 65 m3/d) and Carboniferous (median 57 m3/d) lavas. Specific capacity values for all volcanic rocks range from less than 1 to nearly 400 m3/d/m, with a median of 5 m3/d/m, based on a total of 17 values.

There are several notable abstractions for mineral water from Scottish volcanic rocks, and the aquifer is also used locally for private water supply. A number of springs from Tertiary lavas in the Highlands, particularly on Skye, are used for public water supply.

Figure 16 Schematic cross-section of the hydrogeology of volcanic igneous aquifers in Scotland.

Summary of baseline chemistry

A summary of the baseline chemistry of groundwater in Igneous Volcanic aquifers is provided in Table 18.

Table 18 Summary of baseline chemistry data for igneous volcanic aquifers.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 43 0 14.4 22.8 42.2 69 116
Cl mg/L 43 0 7.59 10.2 22 44.1 77.7
DO2 mg/L 24 1 1.48 2.31 5.46 6.9 9.17
Fe µg/L 39 3 4 5 26 92 300
HCO3 mg/L 43 0 55.7 94 158 211 245
K mg/L 43 2 0.51 0.71 1.1 2.18 6.9
Mg mg/L 42 0 2.31 7.57 11.5 19.3 36
Na mg/L 43 0 9.17 12 22.5 38.1 83.4
NO3 as NO3 mg/L 35 0 0.72 1.69 16.00 54.81 141.44
pH 37 0 6.62 6.94 7.3 7.71 8.14
SEC µS/cm 37 0 178 254 383 605 704

a number of samples
b number below detection limit

Igneous intrusive

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Geological summary

There are many major and innumerable minor igneous intrusions across Scotland, ranging from linear dykes a few millimetres wide to granitic batholiths many kilometres across. They date from all time periods in the Phanerozoic, but certain eras saw major periods of igneous activity, in particular the Devonian and Carboniferous.

Physical aquifer properties and groundwater flow

Intrusive igneous rocks typically form low or very low productivity aquifers. In their unweathered state, primary intergranular porosity and permeability are negligible, and groundwater flow and storage occurs entirely in fractures (Figure 17). In near-surface zones where the rocks are weathered, particularly for granitic rocks, intergranular permeability can be enhanced. Across most of Scotland, glacial erosion has removed most pre-Quaternary weathering, and these zones are rarely more than few metres thick, but in parts of Aberdeenshire and the Cairngorms, many metres, sometimes tens of metres, of weathered intrusive igneous rocks have been preserved. This special case is treated as a form of regolith — weathered, or superficial, aquifer — and is discussed in Superficial aquifers.

Few good quantitative aquifer properties data are available. The range in recorded opera- tional yields from all intrusive igneous rocks, based on 47 recorded values, is from <1 m3/d to 600 m3/d, with a mean of 110 m3/d and a median of 44 m3/d.

Figure 17 Schematic cross-sections of the hydrogeology of intrusive igneous aquifers in Scotland.

Summary of baseline chemistry

The baseline chemistry of groundwater in igneous intrusive aquifers in Aberdeenshire is described in Smedley et al. (2009)[11]. A summary is provided in Table 19.

Table 19 Summary of baseline chemistry data for igneous intrusive aquifers.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 34 0 3.93 10.9 20 33.9 45.2
Cl mg/L 34 0 7.4 13.5 25 41.4 68.1
DO2 mg/L 23 0 3.26 4.55 7.6 8.39 9.7
Fe µg/L 34 15 NA NA 16 100 250
HCO3 mg/L 34 1 8 17.1 37 106 204
K mg/L 34 2 0.4 1.02 1.4 2.84 4.19
Mg mg/L 33 0 1.08 3.6 6.69 9.29 13.1
Na mg/L 34 0 5.21 10.5 16.5 24.7 37.6
NO3 as NO3 mg/L 31 3 0.04 0.75 13.30 54.81 64.09
pH 33 0 5.69 6.19 6.63 7.04 7.32
SEC µS/cm 29 0 63.5 168 271 334 430
SO4 mg/L 34 0 1.03 7.25 11.8 18 22.3

a number of samples
b number below detection limit

Igneous/sedimentary

Figure 18 Schematic cross-section of the hydrogeology of mixed igneous–sedimentary aquifers in Scotland.

Summary of baseline chemistry

The chemistry of groundwater in this aquifer depends on the local proportion of sedimentary and igneous rocks. For the range of expected chemical parameters see the relevant sections: Igneous Volcanic aquifers; Carboniferous sedimentary aquifers not extensively mined for coal; Old Red Sandstone sedimentary aquifers and, where relevant, Aquifer characteristics for Permo-Triassic sedimentary aquifers.

Superficial aquifers in Scotland

15028 superficial aquifers.jpg

Some of the most productive aquifers in Scotland are formed of superficial deposits. However, superficial aquifers vary widely in productivity, due to their highly variable lithology, depositional history and therefore parameters such as sediment sorting, and also, importantly, 3D geometry (the lateral extent and thickness of the deposit). The thickest superficial aquifers in Scotland exceed 80 m, where thick sequences of gravels and sands infill deep valleys eroded by glaciers from bedrock. Superficial aquifers have been divided into three categories according to their age, provenance and whether they crop out at the ground surface or are hidden, described in more detail in Bedrock aquifers: Quaternary deposits mapped at the surface; buried aquifers; and deeply weathered bedrock (regolith).

Quaternary aquifers that crop out at the ground surface

The distribution and productivity of Quaternary aquifers that crop out at the ground surface is shown in the map above. The dominant control on aquifer properties is the mode of sediment deposition, which determines the grain size, sediment sorting and density, which in turn control permeability (MacDonald et al., 2012[12]). The 3D geometry of the aquifers is also a key control on the hydrogeology: the thickness and extent of Quaternary deposits can be highly variable, which means that aquifers do not persist over large areas. The highest productivity aquifers are usually glaciofluvial deposits; alluvium typically forms moderate to highly productive aquifers; and coastal deposits of various kinds typically form low to moderate productivity aquifers. Key characteristics of each of these are summarised in Table 20.

Table 20 Summary of key features of the main Quaternary aquifer types (Boulton et al., 2002[13]).
Aquifer type Key features
Glaciofluvial deposits Deposited by glacial meltwater rivers towards the end of the last glacial period.

Widespread in occurrence, commonly infilling deeply eroded bedrock valleys, to more than 80 m deep (e.g. in the lower Spey valley), or forming broad outwash spreads across many kilometres (e.g. along the Moray coast, around Forfar in Angus, and in the Nith valley north of Dumfries).

Dominated by gravel and sand but can have variable composition. Gravel channels can be very thick and highly permeable; but can be interbedded with finer grained, lower permeability deposits.
Alluvial deposits Deposited by rivers since the end of the last glacial period, forming floodplains Typically mixed sequences with interbedded silt, clay, sand and gravel.

Often overlie glaciofluvial deposits. Groundwater flows naturally towards rivers; groundwater is often in hydraulic contact with river.
Raised marine deposits Deposited by marine action around the end of the last glacial period, when sea levels were higher.

Variable composition, from coarse shingle to thick clay. Gravel beds tend to be thin and irregular, but highly permeable. Often thick clay beds.

Low lying and shallow water table.
Blown sand Typically forming sand dunes along coasts.

Well-sorted sand, which can be moderately to highly permeable, but can be thin and laterally restricted

Buried Quaternary aquifers

Buried Quaternary aquifers are those that do not crop out at the ground surface, but are overlain by nonaquifer deposits such as clays and silts. The locations of significant buried Quaternary aquifers are shown in Figure 6. They are all dominantly formed of glaciofluvial deposits, laid down by glacial meltwater rivers in deep, pre-existing eroded bedrock valleys towards the end of the last glacial period (Table 19). Little exploration of the deposits has been done, but they are known to exceed 80 m thickness in places. They may form significant local aquifers, with large volumes of groundwater storage and flows.

Weathered bedrock aquifers

Significant weathered bedrock aquifers occur in lowland parts of north-east Scotland (Figure 6). The deep weathering, of both intrusive igneous (granitic) and Precambrian South rocks is thought to have occurred under the humid temperate environments of the Pliocene and warmer periods of the Pleistocene. It is common in some parts of north-east Scotland to find extensive areas with few, if any, unweathered rock outcrops. The depth of weathering commonly extends to between 10 and 20 m, and can exceed 50 m. Significant variation in weathering thickness occurs laterally (Merritt et al., 2003[14]). Weathered rock has often been wrongly identified as superficial deposits in borehole logs, and its true extent may be much greater than is shown on this map.

The regolith — weathered rocks — varies widely in grain size, geochemistry and clay mineralogy, controlled largely by parent rock type and the degree of chemical alteration during weathering. Two distinct types are identified. The most common is dominantly sandy, with limited fine-grained material. Clay mineralogy is closely controlled by rock type, with granitic saprolites containing kaolinite-mica mineral assemblages, basic igneous saprolites containing a wide range of clay minerals, acid metamorphic rocks containing kaolinite and mica clays, and metalimestones dominated by smectite. Less common is a more evolved saprolite that has elevated clay content (more than 6 per cent) with clay mineralogy dominated by kaolinite and haematite (Merritt et al., 2003[14]).

The regolith has significantly higher permeability and storage than the unweathered parent bedrock. Although few systematic hydrogeological studies have been done, the aquifer supports many private water supplies in Aberdeenshire.

Baseline chemistry of superficial aquifers

Fewer data are available on the chemistry of groundwater from superficial aquifers in Scotland, as the Baseline Scotland project collected data only on bedrock aquifers. However, a total of 111 samples of groundwater from Quaternary aquifers have been collected by BGS and analysed at BGS laboratories since 1984. These data have been subject to preliminary quality assurance, and the results have been interpreted to characterise key chemical parameters of groundwater from superficial aquifers across Scotland (Table 21). Threshold values were calculated using the 5th and 95th percentile of the dataset, and the median is also quoted.

Superficial aquifers are highly heterogeneous lithologically, in their size and distribution across Scotland, and the baseline chemistry of groundwater in the aquifers is also highly variable. These summarised results are therefore only a general description of the chemistry of groundwater in Scotland’s superficial aquifers, but do provide threshold values within which most superficial groundwater is likely to fall.

Table 21 Summary chemistry data for all superficial aquifers in Scotland. Data from the Baseline Scotland project.
Parameter na P0.05 P0.5 P0.95
Ca mg/L 106 6.4 35.1 77.5
Cl mg/L 98 8.5 27.4 120.2
K mg/L 97 0.3 2.23 4.29
Mg mg/L 63 1.6 4.2 20.45
Na mg/L 95 6.19 14.5 71.7
NO3 as NO3 mg/L 61 0.44 9.68 70.4
SO4 mg/L 61 2.4 13.8 140
HCO3 mg/L 109 10.4 76.9 210.6
Fe mg/L 47 0.005 0.04 0.31
Mn mg/L 47 0.001 0.04 0.36
SEC uS/cm 107 105 316 737
Dissolved oxygen mg/L 62 0.4 4.8 11.3
pH 106 5.8 6.71 7.74
Temperature °C 98 7.38 9.95 12.1
Eh mV 69 12.2 296 458

a number of samples above detection limit

References

  1. Trewin, N H, and Thirlwall, M F. 2002. Old Red Sandstone. 213–249 in The Geology of Scotland. Trewin, N H (editor). (London: The Geological Society.)
  2. Ó Dochartaigh, B É. 2004. The physical properties of the Upper Devonian/Lower Carboniferous aquifer in Fife. British Geological Survey Internal Report, IR/04/003. Available at http://nora.nerc.ac.uk/12638/
  3. Ó Dochartaigh, B É, Smedley, P L, MacDonald, A M, and Darling, W G. 2010. Baseline Scotland: groundwater chemistry of the Old Red Sandstone aquifers of the Moray Firth area. British Geological Survey Open Report, OR/10/031. Available at http://nora.nerc.ac.uk/10293/
  4. Ó Dochartaigh, B É, Smedley, P L, MacDonald, A M, and Darling, W G. 2006. Baseline Scotland: the Lower Devonian aquifer of Strathmore. British Geological Survey Commissioned Report, CR/06/250N. Available at http://nora.nerc.ac.uk/10299/
  5. 5.0 5.1 5.2 MacDonald, A M, Ó Dochartaigh, B É, Kinneburgh, D G, and Darling, W G. 2008. Baseline Scotland: groundwater chemistry of southern Scotland. British Geological Survey Open Report, OR/08/62. Available at http://nora.nerc.ac.uk/9164/
  6. Strachan, R A, Smith, M, Harris, A L, and Fettes, D J. 2002. The Northern Highland and Grampian terranes. 81–147 in The Geology of Scotland. Trewin, N H (editor). (London: The Geological Society.)
  7. Robins, N S. 1990. Hydrogeology of Scotland. (London: HMSO Publications Centre.)
  8. Park, R G, Stewart, A D, and Wright, D T. 2002. The Hebridean terrane. 45–80 in The Geology of Scotland. Trewin, N H (editor). (London: The Geological Society.)
  9. 9.0 9.1 9.2 Robins, N S. 1990. Hydrogeology of Scotland. (London: HMSO Publications Centre.)
  10. Smedley, P L, Ó Dochartaigh, B É, MacDonald, A M, and Darling, W G. 2009. Baseline Scotland: groundwater chemistry of Aberdeenshire. British Geological Survey Open Report OR/09/065. Available at http://nora.nerc.ac.uk/9161/
  11. Smedley, P L, Ó Dochartaigh, B É, MacDonald, A M, and Darling, W G. 2009. Baseline Scotland: groundwater chemistry of Aberdeenshire. British Geological Survey Open Report OR/09/065. Available at http://nora.nerc.ac.uk/9161/
  12. MacDonald, A M, Maurice, L, Dobbs, M R, Reeves, H J, and Auton, C A. 2012. Relating in situ hydraulic conductivity, particle size and relative density of superficial deposits in a heterogeneous catchment. Journal of Hydrology, Vol. 130–141, 434–435. doi:10.1016/j.jhydrol.2012.01.018
  13. Boulton, G S, Peacock, J D, and Sutherland, D G. 2002. Quaternary. 409–430 in The Geology of Scotland. Trewin, N H (editor). (London: The Geological Society.)
  14. 14.0 14.1 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).