OR/15/028 Aquifer characteristics

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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.

Introduction

This chapter provides a summary of the hydrogeology of the main aquifer groups in Scotland.

The bedrock aquifers are shown in the map in Figure 5 and listed in Table 2, and their physical and chemical characteristics are summarised in Table 3. Also in Table 3 is a general summary of the dominant strata overlying each of the bedrock aquifers, related to their impact on infiltration to, and the vulnerability of groundwater in, the bedrock aquifers.

The superficial aquifers are shown in the map in Figure 6 and summarised in Table 3.

The hydrogeology of each aquifer group is summarised in Permo-triassic to superficial aquifers in Scotland, including geology, aquifer properties, groundwater flow characteristics, and groundwater chemistry.


Table 3 Summary of aquifer characteristics. For more detail see individual descriptions in Permo-Triassic to igneous/sedimentary.
Aquifer Dominant groundwater flow type Dominant aquifer productivity Dominant1 groundwater flow path length Typical groundwater flow depth Dominant groundwater age Dominant baseline groundwater chemical type Dominant overlying strata
Permo-Triassic Significantly intergranular (sandstone); Fracture (breccia) Moderate to very high 1 km +
Geological control usually dominates over catchments
100s m Years to millennia Moderately mineralised; oxic Ca-Mg-HCO3 dominated Variable.
Carboniferous–not extensively mined for coal Fracture (minor intergranular), except Passage Formation — significantly intergranular Moderate (except Passage Formation) to high 1–10 km
Geological control usually dominates
100s m Years to millennia Often anoxic; Moderately mineralised; often anoxic; wide range of water chemistry types Generally thick and low permeability.
Carboniferous–extensively mined for coal Fracture (minor intergranular) and through mined voids Moderate 1–10 km
Dominated by impacts of historical mining
100s m
+
Months to millennia Moderately to highly mineralised; often anoxic; wide range of water chemistry types; often elevated iron. Generally low permeability. Thick in valleys, thinner elsewhere.
Old Red Sandstone North Fracture (minor intergranular) Low to high 1 km +
Usually follows main river body catchments
10s to 100s m Decades to centuries Moderately mineralised; often anoxic; Ca-HCO3 dominated Variable: thick and low permeability in Caithness; generally higher permeability elsewhere.
Old Red Sandstone South Fracture (minor intergranular) Moderate to very high 1 km +
Usually follows main river body catchments
10s to 100s m Decades to centuries Moderately mineralised; generally oxic; Ca-Mg-HCO3 dominated Generally thick, moderate to high permeability
Silurian–Ordovician Fracture Low 0.1–1 km +
Usually follows local surface water catchments
10s m Years to centuries Weakly to moderately mineralised; oxic; Ca-Mg- HCO3 dominated. Some notable highly mineralised springs (Na-Cl-SO4 type) Thin, low permeability on hillslopes; thicker and permeable in valleys.
Highland calcareous Fracture Low to moderate 0.1–1 km +
Karstic. Flow paths highly unpredictable
10s to 100s m Months to decades Oxic, weakly to moderately mineralised a-HCO3 Generally thin and highly permeable.
Precambrian North Fracture Very low to low –1 km
Usually follows local surface water catchments
10s m Years to decades Weakly mineralised; variable redox conditions; Ca-Na-HCO3-Cl Thin, low to moderately permeable on hillslopes; thicker and permeable in valleys.
Precambrian South Fracture Very low to low 0.1–1 km
Usually follows local surface water catchments
10s m Years to decades Weakly mineralised; variable redox conditions; Ca-Na-HCO3-Cl type Thin, low-moderately permeable on hillslopes; thicker and permeable in valleys.
Igneous Volcanic Fracture Low 0.1–1 km
Usually follows local surface water catchments
10s to 100s m Months to decades Weakly to moderately mineralised; oxic; Ca- HCO3 dominated Generally thin or absent.
Igneous Intrusive Fracture; sometimes weathered intergranular Low 100s m
Usually follows local surface water catchments
10s m Years to decades Generally weakly mineralised and oxic; variable chemistry but often with low SO4 Generally thin and permeable, or absent.
Igneous/sedimentary Fracture (minor intergranular) Low to moderate 0.1–1 km +
Sometimes geologically controlled
10s to 100s m Years to centuries See relevant sections above Variable, generally low to moderate permeability.
Shetland low permeability Fracture Very low to low 0.1–1 km
Usually follows local surface water catchments
10s m Years to decades No data Generally thin and low permeability.
Ayrshire basic Fracture Very low to low 0.1–1 km
Usually follows local surface water catchments
10s m Years to decades No data
Superficial aquifers Intergranular Moderate to high 0.01–1 km
Usually follows main river body catchments
10s m Weeks to years Variable redox conditions, weakly to moderately mineralised with a wide range of water types, but often Ca (Na) HCO3 (Cl) Generally absent.

Permo-triassic

15028 permo-triassic.jpg

Geological summary

The main onshore surface outcrops of Permo-Triassic aquifers in Scotland are in the west and south-west, most of which take the form of basin infills surrounded by older rocks. Away from the south-west, there are small Triassic outcrops in the Hebrides and along the western coast of the Highlands, and on the Moray Firth coast near Elgin. Virtually all Permo-Triassic outcrops in Scotland comprise terrestrial sedimentary rocks, and most of the main basins are dominated by aeolian sandstones, interbedded with breccias representing more fluvial deposition. The main basins are generally thought to be many hundreds of metres thick. In the Dumfries Basin, the maximum aquifer thickness is inferred to be between 1.1 and 1.4 km (Robins and Ball, 2006[1]), although the Permo-Triassic aquifer in the Annan basin is only approximately 100 m thick.

Physical aquifer properties and groundwater flow

Permo-Triassic rocks form some of the most productive aquifers in Scotland. They are often characterised by the complex layering of two different geological units — sandstone and breccia, which can lead to a dual aquifer system dominated by fracture flow but intergranular storage (Figure 9). The hydrogeology can be further complicated by the nature and thickness of any overlying Quaternary deposits, which can be a significant control on the interaction between groundwater in the bedrock aquifer and surface water, such as acting as a source of delayed recharge to the underlying bedrock aquifer, or confining it to various degrees. Multiple groundwater abstractions and discharges in some of the aquifer basins also impact on the local hydrogeology.

Permo-Triassic breccias — such as the Doweel Breccia Formation in the west of the Dumfries basin — typically have very low intergranular permeability and porosity (Table 4), and high secondary (fracture) permeability. Sandstones typically have relatively higher intergranular porosity and permeability, but much less fracture development. Fracture flow is generally concentrated along subhorizontal discontinuities: in particular, bedding-plane fractures along the breaks between breccia and sandstone layers. Horizontal permeability is therefore commonly much greater than vertical permeability. In some areas, though, there is significant subvertical fracturing associated with basin-marginal fault trends, which can allow rapid recharge deep into the aquifer. In breccias transmissivity is significantly higher, groundwater flow more rapid and groundwater storage more limited, than in sandstones, in which groundwater flow is slower and storage higher (Table 4). Groundwater age reflects this, with the bulk of water within sandstones typically older — more than 50 years old — than the bulk of water in breccias, which is dominated by water recharged since the 1990s. However, small volumes of much older groundwater — possibly more than 10000 years old — are stored in pores in the matrix of sandstones interbedded with breccia in the west of the Dumfries basin.

Significant fracture inflows occur at up to 100 m below ground level, and smaller inflows have been measured to at least 13 m depth. Groundwater can flow many kilometres laterally along continuous subhorizontal fracture systems (Robins and Ball, 2006[1]).

Figure 9 Schematic cross-section of the hydrogeology of a Permo-Triassic basin in Scotland.
Table 4 Summary of available aquifer properties data for Permo-Triassic aquifers.
Porosity (%) Horizontal matrix hydraulic conductivity (m/d) Vertical matrix hydraulic conductivity (m/d) Transmissivity (m2/d) Specific capacity (m3/d/m) Storativity Operational yield (m3/d)
Sandstone (Dumfries) 26 (5) 1.9 (5) 0.95 (5) 55–320 (5)1 60–360 (3)1 0.00005–0.0003 (4) <500 (<15)
Breccia (Dumfries) 12 (5) <0.009 (4) <0.0002 (3) 10–40001
(14)
25–15001(15) 0.0002–0.003 (7) >2000 (~30)
Other mainland Permian basins 18–25 (11) 0.4–2.5 (10) 0.1–4.5 (8) 500–800 125–210 (18) 0.0005–0.001 (8) 600–900(<20)
Triassic (Annan–Gretna) 19–20 (5) 0.02–0.03 (5) 0.015–0.012 (5) 50 (2) 30–50 (3) 80–1901 (4)
Arran 20 (2) 55–65 (9) 50–60 (9)

1 Total range in measured values Number of values indicated in brackets
Single values are averages; ranges generally indicate mean and median values except where indicated
Data from the British Geological Survey

Summary of baseline chemistry

The chemistry of groundwater in the Dumfries Basin aquifer is described in detail in British Geological Survey (2006)[2] and MacDonald et al. (2000)[3]

Table 5 Summary of baseline chemistry of Permo-Triassic aquifers.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 62 0 19.4 26.4 37.5 58.6 82.2
Cl mg/L 62 0 11 14.2 18 22.9 46.7
DO2 mg/L 20 2 NA 1.22 4.19 7.43 10
Fe µg/L 61 16 1.7 3 11 74 226
HCO3 mg/L 62 0 48.8 83.4 139 200 241
K mg/L 62 0 1.1 1.2 1.65 2.26 3.59
Mg mg/L 62 0 6.89 10.7 17.2 21.8 29
Na mg/L 62 0 7.64 9.45 11.8 17 27.5
NO3 as NO3 mg/L 57 2 1.59 7.07 17.24 28.73 43.80
pH 57 0 6.18 6.8 7.18 7.46 7.78
SEC µS/cm 58 0 196 326 405 510 654
SO4 mg/L 62 0 6.98 11.1 20.2 31.6 73.1

a number of samples
b number below detection limit

Carboniferous sedimentary aquifers, not extensively mined for coal

15028 carboniferous.jpg

Geological summary

These Carboniferous sedimentary rocks contain little or no coal, although some contain other minerals such as oil shale. They crop out dominantly in Scotland’s Central Belt (known as the Midland Valley in geological terms), with smaller outcrops in southern Scotland, particularly along the border with England. The main geological formations are the Strathclyde, Inverclyde (both largely in the Midland Valley), Border and Yoredale (both largely in southern Scotland) groups. They generally comprise repetitive sequences of sandstone and siltstone beds with thinner interbedded mudstones and, more rarely, limestones and coals. The thickness of the sedimentary sequences varies from generally 600 to 1000 m in the various outcrops in southern Scotland, up to 2000 to 3000 m in the Midland Valley (Read et al., 2002[4]). These Carboniferous rocks have not been extensively mined for coal. Some areas have seen localised coal mining and/or mining for oil shale, limestone, fireclay or metals (MacDonald et al., 2003[5]).

Physical aquifer properties and groundwater flow

Carboniferous sedimentary rocks typically form multilayered and vertically segmented aquifers. Sandstone units tend to act as discrete aquifer units, separated by lower permeability siltstones, mudstones and coals; limestone beds have variable permeability, but are generally thin in comparison with the whole aquifer sequence, so their overall impact on groundwater flow is typically only significant on a local scale. Faults divide the sedimentary sequence vertically: some are permeable, acting as preferential flow pathways, and others act as barriers to groundwater flow (Figure 10). The exception is the Passage Formation, which forms an extensive productive aquifer within the Carboniferous rocks (Ball, 1999[6]).

The sandstones tend to be fine grained and well cemented, with relatively low intergranular porosity and permeability (Table 3, Table 6), and therefore fracture flow dominates groundwater movement throughout the aquifer, and aquifer permeability depends on the local nature of natural fracturing. The Passage Formation, by contrast, has higher porosity and a larger proportion of groundwater moves by intergranular flow.

Carboniferous sedimentary rocks tend to form moderately productive aquifers (Table 6). However, the higher transmissivity and specific capacity values recorded may relate to aquifers which have been impacted by mining.

Groundwater flow paths are likely to be complex, due to the naturally layered nature of the aquifers, which tends to impart preferential horizontal flow, and the predominance of fracture flow. Groundwater may be present under unconfined or confined conditions, at various depths, and different groundwater heads are seen in different aquifer layers. Groundwater residence times are often in excess of 60 years (Ó Dochartaigh et al., 2011[7]

Figure 10 Schematic cross-section of the hydrogeology of Carboniferous aquifers in Scotland not extensively mined for coal.
Table 6 Summary of available aquifer properties data for Carboniferous sedimentary aquifers not extensively mined for coal.
Porosity (%) Matrix hydraulic conductivity (m/d) Transmissivity (m2/d) Specific capacity (m3/d/m) Operational yield (m3/d)
Carboniferous aquifers — not extensively mined for coal 12–17 (34) 0.0003–0.1 (37) 10–1000 * (5) 48–132 * (46) (minimum 0.43; maximum 1320) * 131–418 (348)

* May refer to both mined and nonmined aquifers
Ranges of values refer to mean and median values except where indicated
Number of values indicated in brackets
Data from the British Geological Survey

Summary of baseline chemistry

The baseline chemistry of groundwater in Carboniferous aquifers which have not been extensively mined for coal is described in detail in Ó Dochartaigh et al. (2011)[7]

Table 7 Summary of baseline chemistry of Carboniferous sedimentary aquifers not extensively mined for coal.
Element Units na n <dlb P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 55 0 35.5 43.8 58.7 76.2 154
Cl mg/L 54 0 9.67 18.1 36.8 52 98.8
DO2 mg/L 21 0 0.8 1.85 2.76 6.11 9.3
Fe µg/L 52 9 3 6 46 300 921
HCO3 mg/L 54 0 166 213 256 318 382
K mg/L 54 0 1.4 2.08 4.42 7.75 9.48
Mg mg/L 55 0 12.2 18.9 27.2 36.7 53.3
Na mg/L 55 0 6.58 12.4 27.9 51.8 146
NO3 as NO3 mg/L 36 8 0.04 0.07 3.21 17.24 30.98
pH 50 0 6.69 7.02 7.3 7.68 8.04
SEC µS/cm 50 0 353 516 694 976 1450
SO4 mg/L 55 0 5.03 22 38.5 74.6 175

a number of samples
b number below detection limit

Carboniferous sedimentary aquifers—extensively mined for coal

15028 carboniferous EM.jpg

Geological summary

Carboniferous sedimentary rocks which have been extensively mined for coal in Scotland are largely found in the Midland Valley, with minor outcrops in southern Scotland. The aquifers have often been significantly altered by mining, with remnant voids and/or enhanced fracturing causing hydraulic connections over potentially large volumes of aquifer.

The main geological formations which have been mined for coal are the Scottish Coal Measures Group and parts of the Clackmannan Group (with the significant exception of the Passage Formation, which does not contain significant coal seams and is not extensively mined for coal. They comprise repetitive sequences dominated by sandstone and siltstone beds, interbedded with thinner mudstones, limestones and coals. The thickness of the formations varies from less than 500 m in south-west Scotland up to 2000 to 3000 m in the Midland Valley.

Physical aquifer properties and groundwater flow

In their unmined state, these coal bearing rocks form essentially the same kinds of aquifers as Carboniferous sedimentary rocks which have not been extensively mined for coal: the main difference is that unmined coal seams act as additional low permeability layers, restricting groundwater flow between more permeable sandstone aquifer units. However, the productivity of mined aquifers depends not only on the nature of natural fracturing but often more significantly on the extent and nature of mining impacts. The generally moderate productivity of Scottish Carboniferous sedimentary aquifers not extensively mined for coal is the lower limit of the productivity of Carboniferous aquifers subject to extensive coal mining (Table 7). Mine voids (shafts and tunnels) can artificially and greatly increase aquifer transmissivity, sometimes across large areas and depths, and can link formerly separate groundwater flow systems, both laterally and vertically (Figure 11). Aquifer storage can also be locally increased. Even where mine voids (e.g. tunnels) have subsequently collapsed, deformation of the surrounding rock mass is likely to have caused further changes in transmissivity and, to a lesser degree, storage (Younger and Robins, 2002[8]).

Quantitative aquifer properties data from test pumping are rare for boreholes intercepting former mines. However, records of specific capacity from boreholes drilled in aquifers which have been extensively mined, many of which intercept mine workings, give an indication of the range in aquifer properties and how this varies from the unmined aquifers (Table 8). There are also many records of yields from mine dewatering boreholes (Table 8). The higher yield and specific capacity values for these boreholes are likely to reflect the productivity of Carboniferous aquifers subject to extensive coal mining.

Groundwater flow paths are likely to be even more complex than in Carboniferous aquifers not extensively mined for coal, due partly to preferential flow in the naturally layered aquifers, and partly to mining impacts. Groundwater may be present under unconfined or confined conditions, at various depths, and different groundwater heads are seen in different aquifer layers and/or mining levels. Groundwater residence times are often in excess of 60 years (Ó Dochartaigh et al., 2011[7]).

Figure 11 Schematic cross-section of the hydrogeology of Carboniferous aquifers in Scotland which have been extensively mined for coal.
Table 8 Summary of available aquifer properties data for Carboniferous sedimentary aquifers extensively mined for coal.
Porosity (%) Matrix hydraulic conductivity (m/d) Transmissivity (m2/d) Specific capacity (m3/d/m) Operational yield (m3/d)
Carboniferous aquifers — extensively mined for coal 10–1000 * (5) 48–132 * (46) (minimum 0.43; maximum 1320) * 1987–3279 (171) (minimum 41; maximum 22 248)

* May refer to both mined and nonmined aquifers
Ranges of values refer to mean and median values except where indicated
Number of values indicated in brackets
Data from the British Geological Survey

Summary of baseline chemistry

The chemistry of some mining-impacted groundwaters from Carboniferous aquifers in the Midland Valley which have been extensively mined for coal is described in detail in Ó Dochartaigh et al. (2011)[7]. A summary is provided in Table 9.

Table 9 Summary of baseline chemistry of Carboniferous sedimentary aquifers in Scotland that have been extensively mined for coal.
Element Units na n <dla P0.1 P0.25 P0.5 P0.75 P0.9
Ca mg/L 56 0 30.9 41.5 71 113 245
Cl mg/L 56 0 10.7 14.4 23.4 63.4 1140
DO2 mg/L 36 4 NA 0.3 1.2 2.99 5.96
Fe µg/L 54 0 31.3 106 675 3730 9360
HCO3 mg/L 54 0 100 206 324 467 581
K mg/L 54 0 1.51 2.52 4.26 11.3 26.3
Mg mg/L 56 0 7.42 15 28.2 48 102
Na mg/L 56 0 11 13.2 27.6 132 519
NO3 as NO3 mg/L 52 24 0.04 0.09 0.13 1.33 8.84
pH 52 0 6.3 6.58 7 7.46 7.69
SEC µS/cm 52 0 311 470 740 1240 1700
SO4 mg/L 56 0 10.8 32.9 73 118 270

a number of samples
b number below detection limit

References

  1. 1.0 1.1 Robins, N S, and Ball, D F (editors).2006. The Dumfries Basin aquifer. British Geological Survey Research Report, RR/06/02.
  2. British Geological Survey. 2006. The Dumfries Basin Aquifer. British Geological Survey Research Report, RR/06/002. Available at http://nora.nerc.ac.uk/3685/
  3. MacDonald, A M, Ball, D F, and Darling, W G. 2000. The Permian aquifer of Dumfries: groundwater chemistry and age. British Geological Survey Technical Report, WD/00/24. Available at http://nora.nerc.ac.uk/12698/
  4. Read, W A, Browne, M A E, Stephenson, D, and Upton, B G J. 2002. Carboniferous. 251–299 in The Geology of Scotland. Trewin, N H (editor). (London: The Geological Society.)
  5. MacDonald, A M, Browne, M A E, Smith, N A, Colman, T, and McMillan, A A. 2003. A GIS of the extent of historical mining activities in Scotland: explanatory notes. British Geological Survey Commissioned Report, CR/03/331N. Available at http://nora.nerc.ac.uk/504763/
  6. Ball, D F. 1999. An overview of groundwater in Scotland. British Geological Survey Technical Report, WD/99/44.
  7. 7.0 7.1 7.2 7.3 Ó Dochartaigh, B É, Smedley, P L, MacDonald, A M, Darling, W G, and Homoncik, S. 2011. Baseline Scotland: groundwater chemistry of the Carboniferous sedimentary aquifers of the Midland Valley. British Geological Survey Commissioned Report, OR/11/021. Available at http://nora.nerc.ac.uk/14314/
  8. Younger, P L, and Robins, N S. 2002.Challenges in the characterisation and prediction of the hydrogeology and geochemistry of mined ground. 1–16 in Mine Water Hydrogeology and Geochemistry. Younger, P L, and Robins, N S (editors). Geological Society of London Special Publication, No. 198.