OR/15/028 Aquifer characteristics

5.1 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 infil- tration 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 Sections 5.2 to 5.12, including geol- ogy, aquifer properties, groundwater flow characteristics, and groundwater chemistry.

Table 3 Summary of aquifer characteristics. For more detail see individual descriptions in Sections 5.2 to 5.11.

Aquifer Dominant groundwater flow type Dominant aquifer productivity Dominant1 groundwater flow path length Typical ground- water 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.

Aquifer Dominant groundwater flow type Dominant aquifer productivity Dominant1 groundwater flow path length Typical ground- water flow depth Dominant groundwater age Dominant baseline groundwater chemical type Dominant overlying strata 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. 1Dominant here refers to the typical/modal range of flow path lengths. Shorter and longer outliers also occur.

5.2 PERMO-TRIASSIC

5.2.1 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 Ba- sin, the maximum aquifer thickness is inferred to be between 1.1 and 1.4 km (Robins and Ball, 2006), although the Permo-Triassic aquifer in the Annan basin is only approximately 100 m thick.

5.2.2 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 brec- cia, 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 con- centrated 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 subver- tical 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 brec- cias, 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).

Runoff from low permeability surrounding hill slopes into basin

Recharge where superficial deposits are thin or absent (on higher ground) or permeable (in valleys) Little or no recharge through low permeability superficial deposits (eg. peat)

1 1

2 4

	3 	

LEGEND Thin superficial deposits on hill slopes Thick superficial deposits infilling valleys Sandstone with moderate porosity and significant intergranular permeability Breccia with low intergranular permeability and porosity Low permeability Silurian-Ordovician (or other basement rock)

Groundwater flow lines Groundwater level (water table) Shallow groundwater flow over 1 to 10s years Deeper groundwater flow over 10s to 1000s years Rapid groundwater flow in horizontal fractures between interfingering breccia and sandsone layers Fault controlled fracturing parallel to basin edge acts as conduit for groundwater flow

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 (%) Horizon- tal matrix hydraulic conductivi- ty (m/d) Vertical matrix hydraulic conductivity (m/d) Trans- missivity (m2/d) Specific capacity (m3/d/m) Storativ- ity Oper- ational 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

5.2.3 Summary of baseline chemistry The chemistry of groundwater in the Dumfries Basin aquifer is described in detail in British Geological Survey (2006) and MacDonald et al. (2000). A summary of the baseline chemistry is provided in Table 5.

Table 5 Summary of baseline chemistry of Permo-Triassic aquifers.

Element Units na n <dlb P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.3 CARBONIFEROUS SEDIMENTARY AQUIFERS, NOT EXTENSIVELY MINED FOR COAL

5.3.1 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 south- ern Scotland, up to 2000 to 3000 m in the Midland Valley (Read et al., 2002). These Carbon- iferous 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).

5.3.2 Physical aquifer properties and groundwater flow Carboniferous sedimentary rocks typically form multilayered and vertically segmented aqui- fers. 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 verti- cally: 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 exten- sive productive aquifer within the Carboniferous rocks (Ball, 1999). 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 groundwa-

ter 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). How- ever, the higher transmissivity and specific capacity values recorded may relate to aquifers which have been impacted by mining (Section 5.4). 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 frac- ture 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).

recharge to sandstone aquifer units through thin, absent or permeable superficial deposits

LEGEND Thin superficial deposits on hill slopes Thick superficial deposits infilling valleys Low permeability siltstones and mudstones Moderate permeability sandstone dominated by fracture permeability High permeability sandstone with significant intergranular flow

Groundwater flow lines

	  Groundwater level (water table)

Mineral seam (worked) eg. ironstone, limestone

Poor quality, sometimes saline water, can be present at depth

Shallow groundwater flow over 1 to 100 years Deeper groundwater flow over 10s to 1000s years Occasionally fracturing allows connection between two sandstone units through a generally lower permeability layer Some faults are permeable with fracture zones around them, allowing groundwater to flow across and along them

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

5.3.3 Summary of baseline chemistry The baseline chemistry of groundwater in Carboniferous aquifers which have not been exten- sively mined for coal is described in detail in Ó Dochartaigh et al. (2011) for the Midland Valley and MacDonald et al. (2008) for southern Scotland. A summary for groundwater from these aquifers across both the Midland Valley and southern Scotland is provided in Table 7.

Table 7 Summary of baseline chemistry of Carboniferous sedimentary aquifers not extensively mined for coal.

Element Units na n <dlb P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.4 CARBONIFEROUS SEDIMENTARY AQUIFERS — EXTENSIVELY MINED FOR COAL

5.4.1 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 fractur- ing causing hydraulic connections over potentially large volumes of aquifer. The main geological formations which have been mined for coal are the Scottish Coal Mea- sures Group and parts of the Clackmannan Group (with the significant exception of the Pas- sage Formation, which does not contain significant coal seams and is not extensively mined for coal; see Section 5.3). 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.

5.4.2 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 (Section 5.3): the main difference is that unmined coal seams act as additional low permeability lay- ers, 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 trans- missivity, sometimes across large areas and depths, and can link formerly separate groundwa-

ter 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). 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 Carbonifer- ous 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).

LEGEND Superficial deposits infilling valleys

Siltstone and mudstone with low permeability Sandstone with moderate permeability Groundwater flow lines Groundwater level (water table)

Contaminated groundwater flow

Coal seam (worked) Coal seam (unworked) Other mineral seam (worked) eg. Ironstone, limestone

Contaminated groundwater from mineworkings entering aquifer

Shaft connecting mine workings pumping contaminated groundwater from coal workings for treatment

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

5.4.3 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 Dochar- taigh et al. (2011). 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 P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.5 OLD RED SANDST

Old Red Sandstone North

Groundwater flow type Aquifer productivity Groundwater flow path length

Groundwater flow depth

Groundwater age

Baseline groundwater chemistry

Fracture (minor intergranular) Low to High 1 km +; usually follows major surface water catchments Tens to hundreds of metres Decades to centuries Often anoxic; moderately mineralised; Ca HCO3 dominated

Overlying strata Variable: thick & low permeability in Caithness; generally higher permeability elsewhere

5.5.1 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 Section 5.6, below.

Old Red Sandstone rocks in Scotland were deposited in two distinct basins: a northern, ‘Orca- dian’, basin, including outcrops in Shetland, Orkney, Caithness, Morayshire and Aberdeenshire; and a southern basin including outcrops in Aberdeen; the Vale of Strathmore, from Loch Lo- mond 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).

5.5.2 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 pro- ductive bedrock aquifers in Scotland. These sandstones are typically well cemented, with rela- tively 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). Aquifer properties data for these and other Old Red Sandstone aquifers are summarised in Table 9. 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.

Unconfined Aquifer Zone

Confined Aquifer Zone in lower sandstone

LEGEND Superficial deposits on hill slopes

Thick superficial deposits infilling valleys

Groundwater flow lines

Low permeability mudstone/siltstone, limited Groundwater level (water table) groundwater flow except where faults act as conduits

Well cemented sandstone, groundwater moves dominantly by fracture flow Very well cemented flagstone, groundwater flow is dominantly along parallel bedding planes Conglomerate

Shallow groundwater flow over 10s years Deeper groundwater flow over 100s years Groundwater in sandstones is well mixed at least in the uppermost 150 metres.

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 ground- water is generally well mixed in the top 50 m or so, but groundwater residence times of several decades are typical.

5.5.3 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). 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), and of Old Red Sandstone aquifers in southern Scotland in MacDonald et al. (2008). 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 P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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 P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.6 SILURIAN AND ORDOVICIAN

5.6.1 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 sub- duction 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.

5.6.2 Physical aquifer properties and groundwater flow The dominantly fine-grained, well-cemented nature of the rocks means intergranular permea- bility 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 typ- ically 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).

Relatively unimpeded recharge through absent, thin, and/or permeable superficial deposits on hill slopes

LEGEND Thin superficial deposits on hill slopes

Thick superficial deposits infilling valleys Highly deformed, low permeability rocks (folded & faulted) Shale band

Groundwater flow lines

Shallow groundwater flow through near-surface zone of enhanced fracturing/weathering over 1 to 10s of years

Occaional deeper groundwater flow through scarcer fractures at depth over 10s to 100s years

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

5.6.3 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). A summary is provided in Table 13.

Table 13 Summary of baseline chemistry data for Silurian and Ordovician aquifers.

Element Units na n <dlb P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.7 HIGHLAND CALCAREOUS: DOMINANTLY CALCAREOUS PRECAMBRIAN AND CAMBRO-ORDOVICIAN

5.7.1 Geological summary Most rocks in Scotland are dominantly siliceous, but a few, mostly Precambrian and Cam- bro-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, calcsili- cates, and metalimestones (Strachan et al., 2002). 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). 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).

5.7.2 Physical aquifer properties and groundwater flow Calcareous Precambrian rocks have low intergranular porosity and permeability, but dissolu- tion of carbonate along fractures in the rock can produce secondary karstic permeability (Fig- ure 14). Where karstic permeability becomes very well developed, these rocks can have higher transmissivity than noncalcareous Precambrian aquifers (Robins, 1990). 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). However, the scarce available data indicate that where karst is poorly developed, the aquifer has relatively low productivity, sim- ilar to Precambrian aquifers (Section 5.8).

Recharge where superficial cover is absent, thin and/or permeable. Focused recharge through swallow holes

Minor spring fed by fracture system

Major spring fed 2 by karstic system 1

River

LEGEND Thin, often permeable superficial deposits on hill slopes

Calcareous rock

	  Groundwater flow lines

Rapid groundwater flow often over months rather than years Groundwater flow over 1 to 10s years

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

5.7.3 Summary of baseline chemistry A summary of the baseline chemistry of groundwater in Highland Calcareous aquifers is pro- vided in Table 14.

Table 14 Summary of baseline chemistry of Highland Calcareous aquifers.

Element Units na n <dlb P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.8 PRECAMBRIAN

Precambrian North

Groundwater flow type Aquifer productivity Groundwater flow path length Groundwater flow depth Groundwater age Baseline groundwater chemistry

Fracture

Very Low to Low

0.1–1 km; usually follows local surface water catchment

Tens of metres Years to decades Variable redox conditions, weakly mineralised, Ca-(Na) HCO3 (Cl)

Overlying strata Thin, low-moderately permeable on hillslopes; thicker and permeable in valleys

5.8.1 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 di- vision. 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, metamor- phosed Moine schists, and Dalradian Grampian Group schists. Precambrian South aquifers are typified by Dalradian metasedimentary schists of the South- ern Highland and Argyll groups, which are typically more layered and less massive than Pre- cambrian north aquifers. Both Precambrian aquifer groups are variously intruded by igneous rocks.

5.8.2 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) 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 Scot- land. For the Precambrian North, scarce records of operational borehole yields indicate that the largely unmetamorphosed Torridonian sandstone typically has the highest yields, trans- missivity 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 de- bris — 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 Section 5.12.

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

LEGEND Thin superficial deposits on hill slopes Thick superficial deposits infilling valleys

	  Groundwater flow lines

Groundwater level (water table)

Psammites Pelites

Metasedimentary rocks: minor groundwater flow through isolated fracture zones. Pelites often more fractured than Psammites

Steep sided valleys infilled with permeable glacial deposits

Small amounts of shallow groundwater flow through

Massive metamorphic rocks (eg gneiss) Groundwater flow focused in zones of intensive fracturing (often below faulted valleys or along intruded sills or dykes)

Intruded sill or dyke

weathered near-surface zone over 1 to 10s years

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

5.8.3 Summary of baseline chemistry A summary of the baseline chemistry of groundwater in Precambrian North aquifers is provid- ed in Table 16. The baseline chemistry of groundwater in Precambrian South aquifers in Ab- erdeenshire is described in detail in Smedley et al. (2009). A summary is provided in Table 17.

Table 16 Summary of baseline chemistry of Precambrian North aquifers.

Element Units na n <dlb P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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 P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.9 IGNEOUS VOLCANIC

5.9.1 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 Tertia- ry. Volcaniclastic and, rarely, pyroclastic deposits are sometimes interlayered with the lavas; weathered layers and palaeosols within the sequence are also common.

5.9.2 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 rela- tively highintergranular permeability, transmitting significant amounts of groundwater. Frac- tures 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.

Recharge through absent, thin and/or permeable superficial deposits

LEGEND

Volcanic rock/lava flows

Highly fractured/weathered zones

Thick superficial deposits infilling valleys Groundwater flow lines

Groundwater flow dominantly through highly fractured/weathered zones at the junctions of lava flows

Small amounts of groundwater flow in near-surface fractured/weathered zone over months to 10s of years

	  Groundwater level (water table)

Spring discharging where fracture zone meets the ground surface

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

5.9.3 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 P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.10 IGNEOUS INTRUSIVE

5.10.1 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.

5.10.2 Physical aquifer properties and groundwater flow Intrusive igneous rocks typically form low or very low productivity aquifers. In their unweath- ered 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 superfi- cial, aquifer — and is discussed in Section 5.12. 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.

Unrestricted recharge through absent, thin and/or permeable superficial deposits

1

DEEP WEATHERING OFTEN TO GRAVEL

ZONE OF WELL DEVELOPED 2 FRACTURING

UNWEATHERED ROCK

LEGEND

Thin superficial deposits Highly weathered rock Base rock

Groundwater flow lines Groundwater level (water table) Shallow groundwater flow through highly weathered near-surface zone over 1 to10s years Deeper groundwater flow path through fractured zone over 10s years

Small groundwater flows in shallow weathered zone a few metres thick at most Intruded sills or dykes can increase local permeability and allow (usually small) groundwater flows to deeper levels

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

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

Table 19 Summary of baseline chemistry data for igneous intrusive aquifers.

Element Units na n <dlb P 0.1 P 0.25 P 0.5 P 0.75 P 0.9 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

5.11 IGNEOUS/SEDIMENTARY

5.11.1 Geological summary In some parts of Scotland, particularly within Carboniferous and Old Red Sandstone rocks, there are thick sequences— often many hundreds of metres in total— of interbedded sedi- mentary and igneous volcanic rocks. The individual volcanic or sedimentary rock layers are no different from those described in other relevant sections here (e.g. Sections 5.3, 5.5 and 5.9), but the combined sequence can behave differently as an aquifer.

5.11.2 Physical aquifer properties and groundwater flow The individual properties of the sedimentary and volcanic layers are generally the same as for the relevant rock where it occurs separately, but as a whole sequence there are differences. Sandstone beds tend to have the highest permeability and storage, with fracture and inter- granular permeability throughout them, and can therefore contain larger volumes of ground- water than volcanic layers. However, groundwater flow that does occur through volcanic rocks can be faster, restricted to narrow, highly weathered zones (Figure 18).

Relatively unimpeded recharge through absent, thin and/or permeable superficial deposits on hillslopes

LEGEND Thin superficial deposits on hill slopes Thick superficial deposits infilling valleys Sandstone Conglomerate Volcanic rock with highly weathered /fractured zones

Groundwater flow lines Spring Shallow groundwater flow over 1 to 10s years Deeper groundwater flow over 10s to 100s years

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

5.11.3 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: Section 5.9 for Igneous Volcanic aquifers; Section 5.3 for Carboniferous sedimentary aquifers not extensively mined for coal; Section 5.5 for Old Red Sandstone sedimentary aquifers and, where relevant, Section 5.1 for Permo-Triassic sedimentary aquifers.

5.12 SUPERFICIAL AQUIFERS IN SCOTLAND

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, deposi- tional 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 80m, where thick sequences of gravels and sands infill deep valleys eroded by glaciers from bedrock. Superficial aquifers have been divided into three categories accord- ing to their age, provenance and whether they crop out at the ground surface or are hidden, described in more detail in Section 3.2: Quaternary deposits mapped at the surface; buried aquifers; and deeply weathered bedrock (regolith).

5.12.1 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 con- trol permeability (MacDonald et al., 2012). 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 aqui- fers 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).

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

5.12.2 Buried Quaternary aquifers Buried Quaternary aquifers are those that do not crop out at the ground surface, but are over- lain by nonaquifer deposits such as clays and silts. The locations of significant buried Quater- nary 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.

5.12.3 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-eastScotland 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). 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 miner- alogy, 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). The regolith has significantly higher permeability and storage than the unweathered parent bedrock. Although few systematic hydrogeological studies have been done, the aquifer sup- ports many private water supplies in Aberdeenshire.

5.12.4 Baseline chemistry of superficial aquifers Fewer data are available on the chemistry of groundwater from superficial aquifers in Scot- land, 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 calcu- lated 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 ground- water 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 P 0.05 P 0.5 P 0.95 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 oC 98 7.38 9.95 12.1 Eh mV 69 12.2 296 458

a number of samples above detection limit