OR/17/006 Hydrogeology

Monaghan, A A, Dochartaigh, B O, Fordyce, F, Loveless, S, Entwisle, D, Quinn, M, Smith, K, Ellen, R, Arkley, S, Kearsey, T, Campbell, S D G, Fellgett, M, and Mosca, I. 2017. UKGEOS - Glasgow geothermal Energy Research Field Site (GGERFS): initial summary of the geological platform. British Geological Survey Open Report, OR/17/006.

Introduction

Background

Current knowledge of the hydrogeology of the Clyde Gateway area (through which the River Clyde flows) and its surrounding area (Figure 35), including knowledge of aquifer properties and groundwater processes and systems, is summarised below, including:

- a review of existing hydrogeological information (e.g. aquifer properties and groundwater levels, quality and temperature);

- a preliminary 3D conceptual model of the hydrogeology of the Clyde Gateway area; and

- a discussion of the requirements for monitoring to establish baseline conditions and detect environmental changes resulting directly or indirectly from future development.

Figure 35    Glasgow City Council boundary in black; Clyde Gateway area including part of South Lanarkshire Council area; River Clyde and tributaries; location of River Clyde stage gauges (river level monitoring gauges, operated by SEPA) and nominal tidal limit. Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Surface water

The River Clyde is the main surface water course in the area, flowing approximately east to west through the Clyde Gateway area towards the Clyde estuary (Figure 35). The Clyde Gateway is upstream of the weir that marks the nominal tidal limit of the River Clyde. At least two river stage gauges are operated by SEPA in Glasgow: one upstream at Daldowie and one downstream at Renfrew (Figure 35). A number of tributaries flow into the River Clyde in the Clyde Gateway; these are largely culverted (Figure 35).

Geology — summary

The Clyde Gateway area, as with most of Glasgow, is underlain by a heterogeneous sequence of Quaternary sediments of glacial, estuarine/marine and alluvial origin, which are up to ~35 m thick. Below this, the uppermost bedrock unit is the sedimentary Carboniferous Scottish Coal Measures Group, which comprises a repetitive sequence of sandstone, siltstone, mudstone, shale, coal and ironstone.

The natural geological sequence in the Clyde Gateway area is overlain by extensive anthropogenic deposits, which are highly variable in origin, nature and thickness, including worked, infilled, landscaped and disturbed ground. Artificial ground in Glasgow is generally less than 2.5 m thick, but in the most industrialised areas are frequently up to 10 m thick, and less frequently more than 20 m thick (Monaghan et al., 2014).

Groundwater bodies

SEPA classify and regularly assess the status of bedrock and superficial groundwater bodies. The Clyde Gateway area lies on the Glasgow and Motherwell bedrock groundwater body; and three superficial groundwater bodies: the Glasgow Sand and Gravel; Carmyle and Tollcross Sand and Gravel; and Govan Sand and Gravel groundwater bodies. The current status of these groundwater bodies is shown in Table 8 (source: www.environment.scotland.gov.uk/get-interactive/data/groundwater/).

Table 8    Groundwater bodies in the Clyde Gateway area and their current status.

Groundwater body	Bedrock/Superficial	Quantitative Status (2016)	Chemical Status (2016)
Glasgow and Motherwell	Bedrock	Poor	Poor
Glasgow Sand and Gravel	Superficial	Good	Good
Carmyle and Tollcross Sand and Gravel	Superficial	Good	Good
Govan Sand and Gravel	Superficial	Good	Poor

Bedrock (carboniferous sedimentary) hydrogeology

This section describes the hydrogeology of Carboniferous sedimentary aquifers in general in the Glasgow area. There is limited evidence to characterise the bedrock hydrogeology of Glasgow, and it remains poorly understood. It is expected to be internally complex and to show complex interaction with groundwater in overlying superficial deposits and surface waters.

Background information (not specifically for the Clyde Gateway area)

Carboniferous sedimentary rocks in the Central Belt typically form multi-layered and vertically segmented aquifers. The typically fine-grained, well-cemented rocks have low intergranular porosity and permeability, and groundwater flow and storage dominantly occur in fractures in the rock. Hydraulic aquifer properties therefore depend largely on the local nature of fracturing in the rock (Ó Dochartaigh et al., 2015). The rocks tend to form moderately productive aquifers (Ó Dochartaigh et al., 2015). Measured matrix porosity values are in the range 12–17%; hydraulic conductivity (permeability) values are in the range 0.003–0.1 m/d; and transmissivity values are in the range of 10–1000 m²/d (Table 9).

Sandstone units within the sedimentary sequence generally have the highest transmissivity and storage capacity, and therefore tend to act as discrete aquifer units, interspersed by lower permeability siltstones, mudstones and (undisturbed) coal seams. Limestone beds have variable permeability, but are generally thin in comparison with the whole aquifer sequence, and so their overall impact on groundwater flow is generally only significant on a local scale (Ó Dochartaigh et al. 2015).

Groundwater can be present in the aquifer under unconfined or confined conditions, which can vary between different sandstone and other sedimentary units and at different depths. Groundwater heads likewise vary between different aquifer layers (Ó Dochartaigh et al. 2015).

Groundwater flow paths through the aquifer are thought to be complex, due to their naturally layered nature, which tends to promote preferential horizontal flow, and the predominance of fracture flow (Figure 37). Flow paths are likely to be relatively deep (100s of metres) and long (1–10 km) (Figure 37). Previous assessments have thought that Glasgow acts as the focal point for much of the groundwater discharge from Carboniferous aquifers from the Central Coalfield area, with prevailing groundwater flow paths from the east, north-east and south-east (Hall et al., 1998). There is, however, a lack of measured hydrogeological data from Glasgow to support this hypothesis.

Faults can divide the sedimentary sequence vertically. Little direct evidence is available for the hydraulic nature of the faults: some may be permeable, acting as a preferential flow pathways; others may act as barriers to groundwater flow (Ó Dochartaigh et al., 2015; Figure 37).

Impacts of mining

Mining in Carboniferous sedimentary rocks has significantly changed natural hydrogeological conditions. Mine voids (shafts and tunnels) can artificially and greatly increase aquifer transmissivity, sometimes across large areas and depths (to ~1 km), and can link formerly separate groundwater flow systems both laterally and vertically (Figure 38). Aquifer storage can also be locally increased. Even where mine voids have subsequently collapsed, deformation of the surrounding rock mass is likely to cause further changes in transmissivity and, to a lesser degree, storage (Younger and Robins, 2002). Parts of former mine workings have been infilled, which may cause further diversions in groundwater flow, leading to groundwater discharge and/or chemical degradation in unexpected places.

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; and there are many records of yields from mine dewatering boreholes (Table 9). 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 in mined aquifers than in undisturbed Carboniferous aquifers.

Mine dewatering — by abstraction of groundwater from boreholes penetrating mineworkings — continued throughout mining activities, which finally ended in the 1980s in the Glasgow area (BGS data). After mine dewatering ceased, it is likely that groundwater levels in former workings will have risen. As far as BGS is aware, there are few or no recorded problems caused by rising groundwater levels in Glasgow, but this may be partly a consequence of the lack of monitoring of, or data on, groundwater levels in bedrock.

Groundwater chemistry

There is little recent information on groundwater chemistry in the Carboniferous sedimentary aquifer in Glasgow. Some information is available from across central and southern Scotland from the Baseline Scotland project (Table 10). The natural chemistry of groundwater in Carboniferous sedimentary aquifers is often moderately to highly mineralised. Groundwaters in the Coal Measures and Clackmannan groups are typically more mineralised than in the Inverclyde and Strathclyde groups. A detailed description of the chemistry of groundwater in these different Carboniferous groups across the Midland Valley is given in Ó Dochartaigh et al. (2011). A summary for all Carboniferous aquifers is provided in Table 10 and for individual groups in Box 1.

Groundwater quality is also affected by mining. Groundwater discharges from mine workings are typically strongly mineralised, with high specific electrical conductivity (SEC) and particularly high concentrations of HCO₃, Ca, SO₄, Fe and Mn, and low in dissolved oxygen. The pH values are generally well buffered and alkalinity is high, indicating significant reaction with carbonate material in the aquifers (Ó Dochartaigh et al., 2011). Acid mine water discharge is not currently a known problem in Glasgow, and investigations at a number of sites showed good quality groundwater in abandoned mine workings (Glasgow City Council, pers. comm.).

Groundwater residence times are often in excess of 60 years (Ó Dochartaigh et al., 2011).

Figure 36    Bedrock aquifer productivity in Scotland. Carboniferous sedimentary rocks in the Central Belt are dominantly moderately productive, with mixed fracture/intergranular flow.

Table 9    Summary of available aquifer properties data for Carboniferous sedimentary aquifers: (top) not extensively mined for coal; (bottom) extensively mined for coal. From Ó Dochartaigh et al. (2015).

Figure 37    Overview of the hydrogeology of Carboniferous aquifers in Scotland that have not been extensively mined for coal: (top) schematic cross-section; (bottom) summary of aquifer characteristics. From Ó Dochartaigh et al. (2015).

Figure 38    Overview of the hydrogeology of Carboniferous aquifers in Scotland that have been extensively mined for coal: (top) schematic cross-section; (bottom) summary of aquifer characteristics. From Ó Dochartaigh et al. (2015).

Table 10    Summary of baseline chemistry of Carboniferous sedimentary aquifers in Scotland: (top) not extensively mined for coal; (bottom) extensively mined for coal. From Ó Dochartaigh et al. (2015).

Box 1    A summary of groundwater chemistry in Carboniferous sedimentary
aquifer units in the Midland Valley (from Ó Dochartaigh et al. 2011).

Coal Measures Group Groundwaters from the Coal Measures Group are generally of bicarbonate type, with cations either dominated by Na or with no dominant cation. Average alkalinity values are the highest of all the hydrogeological units sampled, including the 'Mine' waters. There is a large range in SEC values and the median value is the highest for all the groups except 'Mine' waters. The waters are generally anoxic and slightly acidic to near-neutral. The groundwaters typically have moderate concentrations of the major cations Ca, Mg, Cl and SO4, and high concentrations of Na relative to the other aquifer groups. In most cases the calcite saturation index showed the groundwaters were close to saturation. There is a large range in Fe and Mn concentrations but concentrations are usually high, with the highest average of any group except the 'Mine' waters. Clackmannan Group Groundwaters from the Clackmannan Group are generally either of Ca-Mg-HCO3 type Na or have no dominant anion; a few show a cationic dominance of Na. The groundwaters typically have high HCO3 concentrations, a near-neutral pH, and generally low dissolved oxygen. SEC values show a wide range but a moderate average. The waters generally have moderate to high concentrations of the major cations Ca, Mg, Ba and Cl. Concentrations of SO4 were the highest of all the hydrogeological units except 'Mine' waters. Most of the samples were significantly undersaturated with respect to calcite. Concentrations of Fe and Mn were moderate to high, with variability due to the redox conditions in the aquifer. Inverclyde Group Most of the groundwaters from the Inverclyde Group were of Ca-HCO3 type. They typically have moderate HCO3 with a near-neutral pH. Dissolved oxygen concentrations are generally low but most of the waters are not anoxic. SEC values are typically moderate. The groundwaters typically have relatively low concentrations of the cations Na, Cl and SO4 and moderate concentrations of Ca and Mg. In most cases the calcite saturation index showed the groundwaters were close to saturation. Concentrations of Fe and Mn are typically relatively low, reflecting the generally oxic nature of the groundwaters from this group. Strathclyde Group Most of the groundwaters sampled from the Strathclyde Group were of Ca-Mg-HCO3. Some samples are more dominated by Na with no anionic dominance. The groundwaters typically have moderate HCO3 concentrations with near-neutral pH. Dissolved oxygen concentrations are usually low but most of the waters are not anoxic. SEC values are generally moderate to high. The waters typically have moderate concentrations of the cations Ca, SO4, Na, Cl and Mg. In about half of the samples the calcite saturation index showed the groundwaters were close to saturation or saturated with respect to calcite; the other half were undersaturated. Iron concentrations are relatively low on average but show a wide range. Mn concentrations are typically moderate.

Bedrock hydrogeology: data specifically for Glasgow/Clyde Gateway

The known availability of bedrock aquifer properties data is summarised in Table 11 and Figure 39. The known availability of bedrock groundwater level data is summarised in Table 12.

There are no analyses of Carboniferous groundwater chemistry measurements in Glasgow in the Baseline Scotland (O Dochartaigh et al., 2011) dataset.

• There are two known analyses within ~5 km of the city boundary.
• There are no known existing boreholes in bedrock in Glasgow that are monitoring groundwater levels or chemistry. SEPA have indicated the presence of a limited number of local site monitoring of bedrock hydrogeology and this will be investigated further.
• BGS has no records of historical mine dewatering from boreholes in the Clyde Gateway. We have 14 records within ~20 km of the Clyde Gateway, all but two from boreholes that finished abstracting from collieries between 1948 and 1968; and two that continued until 1982 and 1985.
• BGS has had sight of a database cataloguing discharges from abandoned mines from the Coal Authority, via SEPA. This includes some data on flows and chemistry. There are no records in the Clyde Gateway area but ~2 in Glasgow, and ~15 within ~10 km of the city boundary to the east. This information does not appear to be publically available on SEPA or The Coal Athority websites; BGS will request these data in due course.

Table 11    Availability of aquifer properties data for Carboniferous bedrock in Glasgow. Permeability (field test) data are from BGS Engineering Properties database.

Parameter	No. of values – in Clyde Gateway?	No. of values – within ~5 km of CG boundary
Core permeability (horizontal)	N	1
Core permeability (vertical)	N	N
Permeability (from field tests?)	~25	~35
Hydraulic conductivity	N	N
Storativity	N	N
Porosity	N	1
Specific capacity	N	3
Transmissivity	N	N
Borehole yield	~5	~25

Figure 39    Known permeability measurements for bedrock in Glasgow, from BGS Engineering Properties Database. Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Table 12    Availability of groundwater level data for Carboniferous bedrock in Glasgow.

No. of values – in Clyde Gateway?	No. of values – within ~5 km of CG boundary	Notes
~4	~20	All measurements are very old (decades)

Figure 40    Known groundwater level measurements for bedrock in Glasgow, from various sources GWL mbd = groundwater level, metres below datum (datum is normally approximately ground level). Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Superficial deposits hydrogeology

Significantly more has been researched and is known about the hydrogeology of superficial deposits than about bedrock in Glasgow and the Clyde Gateway area.

The known hydrogeology and a conceptual model of groundwater in superficial deposits in Glasgow are described in Ó Dochartaigh et al. (in review). A simple numerical model simulating groundwater flow through the superficial deposits in central Glasgow is described in Turner et al. (2015). A brief summary of the known hydrogeology and hydrogeological data is given here.

The Quaternary geological sequence in the central Clyde valley in Glasgow, including the Clyde Gateway area, forms a shallow complex aquifer system with a sequence of hydrogeologically heterogeneous lithostratigraphic units. Three Quaternary lithostratigraphic units — the Bridgeton Sand, Gourock Sand and Paisley Clay members — together form a linear aquifer approximately 2 to 3 km wide and typically between 10 and 30 m thick beneath central Glasgow (Figure 41, Figure 42). This aquifer is highly heterogeneous both naturally, due to varying lithology within aquifer units and to the varying influence of the tidal River Clyde with distance from the river; and due to urban influences, such as altered surface permeability, subsurface flowpaths, and urban recharge (Dochartaigh et al., in review).

The uppermost of the aquifer units, the Gourock Sand Member, and the lowermost Bridgeton Sand Member, are dominated by coarse-grained sediment (gravel and/or sand), and are moderately to locally highly permeable. The Paisley Clay Member usually lies between the Gourock and Bridgeton members, but sometimes overlies the Bridgeton Sand Member with no overlying Gourock Sand Member (Figure 42). It is dominated by fine-grained sediment (silt and/or clay), but contains significant layers and/or lenses of coarser-grained sediment, and as a consequence shows locally moderately high hydraulic conductivity. It appears to allow groundwater flow locally, but at a city scale it has relatively low hydraulic conductivity. All three units are formed largely of marine sediment, although the Gourock Sand Member also includes some alluvium in its uppermost parts. At a city scale, the similarities and differences in hydraulic conductivity between the units are largely controlled by lithological differences, which are in turn controlled by depositional environment and process. However, there are likely to be strong influences on hydraulic conductivity, at scales of <1 to ~10s of metres, resulting from the known local variability in depositional and post-depositional environment (Ó Dochartaigh et al. in review).

A conceptual model of groundwater occurrence and flow within the Quaternary aquifer is described in the Conceptual model of groundwater system in the Clyde Gateway area section.

The vulnerability of groundwater at the uppermost water table (i.e. in the Quaternary aquifer) in the Clyde Gateway area is indicated by the national map of groundwater vulnerability (Scotland) (Figure 45). This indicates that groundwater in the uppermost Quaternary aquifer is highly vulnerable across much of the area, with zones of low vulnerability. However, this national-scale map is not likely to provide an accurate assessment of the actual vulnerability of groundwater in the small urban Clyde Gateway area. The widespread presence of anthropogenically altered ground is likely to have a major impact on local groundwater vulnerability that cannot be accurately assessed without detailed local investigations.

Available hydrogeological data for superficial deposits in Glasgow:

• The known availability of superficial deposits aquifer properties data is summarised in Table 13 and Figure 43.
• The known availability of superficial deposits groundwater level data is summarised in Table 14 and Figure 44.
• There are five known currently active superficial groundwater monitoring boreholes in the Clyde Gateway: all set up by BGS and currently managed by SEPA, and being monitored for groundwater level, temperature, and in one or two cases, SEC. Another ~15 boreholes in Glasgow, within ~5 km of the Clyde Gateway, were monitored at some point in the last fifteen years for groundwater levels, but are not currently being monitored.
• There are a number of chemistry analyses for superficial deposits in Glasgow, all from contaminated land/development sites, and focussed on contaminants. All were collected between 2003 and 2013, and most from 2007 to 2010 in eastern Glasgow. A dataset of 250 water chemistry analyses from 68 boreholes in three Quaternary aquifer units (the Gourock Sand, Bridgeton Sand and Paisley Clay members), and 440 water chemistry analyses from 68 boreholes in artificial ground deposits in Glasgow, has been collated by BGS (Ó Dochartaigh et al., in review). Only a subset of these sample analyses has been assessed as reliable data, and full inorganic chemistry is not available for all of these (Ó Dochartaigh et al., in review RSE). Further details of the groundwater chemistry as assessed from this dataset are provided in Ó Dochartaigh et al. (in review).

Figure 41     Simplified surface Quaternary geology of Glasgow showing key hydrogeological units and lines of cross-sections in Figure 43. Geology exported from 3D geological model (see Monaghan et al., 2014) (from Ó Dochartaigh et al., in review). Geological Data, BGS Copyright, NERC. Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Figure 42     Lithostratigraphic cross-sections along the lines shown in Figure 42, from 3D Quaternary geological model (Monaghan et al., 2014) (from Ó Dochartaigh et al., in review). Note variable vertical and horizontal scales.

Table 13    Availability of aquifer properties data
for superficial deposits in Glasgow.

Parameter	No. of values – in Clyde Gateway?	No. of values – within ~5 km of CG boundary
Permeability (from field tests?)	~25	~10

Figure 43    Known permeability measurements for superficial deposits in Glasgow, from BGS Engineering Properties Database given in metres/second (m/s). Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Table 14    Availability of groundwater level data
for superficial deposits in Glasgow.

No. of values – in Clyde Gateway?	No. of values – within ~5 km of CG boundary	Notes
~135	~240	Almost all spot measurements

Figure 44    Known groundwater level measurements for superficial deposits aquifers in Glasgow, from various sources. Includes mapping data licensed from Ordnance Survey. © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

Figure 45     Groundwater vulnerability (at the uppermost water table) in the Clyde Gateway. Key: 5 = highest vulnerability; 1 = lowest vulnerability.

Conceptual model of groundwater system in the Clyde Gateway area

The simple 3D conceptual model of the groundwater system (superficial and bedrock) described below is largely taken from Ó Dochartaigh et al. (in review). It is based on the available data and information, which are reviewed above. Far more is known of the groundwater system in the Quaternary aquifer. The conceptual model includes an assessment of geological structure, aquifer properties, groundwater levels and chemistry, and groundwater flow paths. It forms the basis for identifying potential pollutant pathways and receptors, and will help to determine the optimum design for groundwater monitoring activities, and in interpreting acquired monitoring data.

Quaternary aquifer

A simple conceptual model of the groundwater system in the Quaternary aquifer in central Glasgow, including the Clyde Gateway area, is shown in Figure 47 (from Ó Dochartaigh et al., in review).

The Gourock Sand and Bridgeton Sand members appear to form a single hydraulically connected aquifer unit where the former directly overlies the latter. The Paisley Clay Member is also hydraulically active, although it has lower permeability overall than the other two aquifer units, and where it occurs between the two other aquifer units, groundwater in all three is likely to be hydraulically connected (Figure 46).

Figure 46     Conceptual model of groundwater occurrence and flow in the Quaternary aquifer in Glasgow, indicating groundwater flow directions; hydraulic connections between aquifer units and river; and groundwater level in the Gourock and Bridgeton Sand members from Ó Dochartaigh et al., in review. Geological Data, BGS © NERC.

Other Quaternary geological units in Glasgow are likely to be less significant hydrogeologically, because they are typically thinner, laterally restricted and/or have relatively low permeability. Where minor permeable (gravel and sand-dominated) units occur they may allow local groundwater storage and flow, and where they directly over- or under-lie one of the main three aquifer units they may contribute to overall groundwater storage and flow in the main aquifer, but they are not likely to be significant at a city scale.

The Quaternary aquifer is overlain by widespread, highly heterogeneous anthropogenic deposits, and the surface land cover is heavily urbanised. The generally high permeability and widely unconfined nature of the upper parts of the aquifer mean that it is likely to accept potential recharge reaching its upper surface. Preliminary modelling estimated average long term recharge from rainfall to the aquifer at 275 mm/year (Turner et al., 2015). This model took into account some urban processes, specifically runoff from paved surfaces, and sewer leakages, but did not take all urban processes into account. Groundwater is likely to recharge to the Quaternary aquifer from a number of sources as well as directly from rainfall: by groundwater inflow from the upstream Quaternary aquifer; from lateral shallow groundwater flow from adjacent Quaternary units, including the Wilderness Till Formation; from leakage from mains water pipes; and potentially by upward flowing groundwater from the underlying Carboniferous bedrock.

At the city scale, groundwater level elevations show that overall groundwater flow directions through the Quaternary aquifer are down-valley, from south-east to north-west. There is also evidence from groundwater-river level relationships of a component of flow convergent towards the River Clyde from the edge of the aquifer. This is likely to include groundwater discharge to the Clyde. Local reversal of this convergent flow in one area in centre-west Glasgow has also been observed, driven by a groundwater level gradient from the river into the aquifer for at least 50 m distance away from the river. On a very local scale, extensive buried infrastructure (e.g. building and quay walls, engineered river banks) is also likely to influence groundwater flow (Ó Dochartaigh et al., in review).

Two mechanisms drive the strong observed relationship between groundwater level response, and rainfall and river stage fluctuations. Rapid groundwater level change — on a scale of hours — is driven primarily by pressure changes in the aquifer as it responds to rising and falling piezometric head in the River Clyde. Slower, longer lasting groundwater level changes — on a scale of days to weeks — occur in response to the physical flow of water through the aquifer, from the infiltration of rainfall at the ground surface and/or the infiltration of river water through the river bed. The rapid, pressure head driven connection between River Clyde stage and Quaternary groundwater levels is clearly demonstrated by tidal forcing of groundwater levels. This effect is evident throughout the tidal zone of the River Clyde in Glasgow, in all the monitored aquifer units, and at distances of up to 200 m from the river, although the greatest effects are seen closest to the river. A small tidal influence on groundwater levels is also seen some 3 km (straightline distance) upstream of the navigational tidal limit of the River Clyde, indicating that tidal impacts on groundwater extend further upstream than this nominal limit (Ó Dochartaigh et al. in review).

Consistently more mineralised groundwaters are seen in Glasgow than in non-urban areas, with higher concentrations of most major and trace ions, suggest widespread urban and/or industrial contamination. Significant increases in groundwater conductivity over relatively short timescales, as seen in one monitoring borehole, indicate that there is active flow of contaminated groundwater through some parts of the aquifer. Widespread elevated concentrations of major ions such as Ca, K and SO₄ are likely to be linked to contamination from urban waste material, such as cement, metals, mine spoil or chemicals from activities such as building, manufacturing, mining and industrial processes such as chromite ore processing. The elevation of Cl and Na concentrations across the study area, not just close to the tidal section of the river, is also likely to be linked to pollution, particularly as there is not a strong correlation, even in natural Quaternary deposits, between Na and Cl. However, the relatively low NO₃ values in urban groundwater indicate that sewage contamination of groundwater is not significant or widespread. The highly elevated concentrations of trace metals, including As, Cr and Pb, in specific areas of the city, in particular an area in east Glasgow, indicates that historical industrial contamination at specific sites, from activities including mining, chromite ore processing and the manufacture of iron and steel, is still impacting on the quality of groundwater in the Quaternary aquifer system (Ó Dochartaigh et al., in review).

The spatial distribution of groundwater chemistry provides no evidence of evolution of groundwater chemistry down-valley within Glasgow. The effects of contamination on groundwater chemistry are likely to be far stronger than any evolutionary changes with groundwater flow. There is, however, strong evidence for specific areas of greater contaminant impact, related to historical industrial activity, such as elevated chromium and lead at particular sites (Ó Dochartaigh et al., in review).

Carboniferous aquifer

No numerical modelling of groundwater in bedrock aquifers in Glasgow has been undertaken by BGS or as documented in published literature. The limited available information suggests that the hydrogeology of the Carboniferous sedimentary aquifers is complex. The aquifers are likely to be moderately productive, to contain significant amounts of groundwater, and to be dominated by fracture flow, and it is likely that groundwater flow paths are relatively deep and long. Hall et al. (1998) concluded that Glasgow is the focal point for much of the groundwater discharge from the Central Coalfield, with prevailing groundwater flow from the east, north-east and south-east. However, this hypothesis was not well constrained by hydrogeological data, because of the lack of groundwater level measurements for groundwater in the Carboniferous aquifers in the Glasgow area. The limited available data on groundwater chemistry in the Carboniferous indicates that groundwater is often naturally moderately to highly mineralised, with abundant iron and manganese in solution (Hall et al., 1998; Robins, 1986; Ball, 1999). Extensive mining and post-mining measures in Glasgow are likely to have led to significant changes in the natural groundwater regime, increasing the complexity of groundwater flow and possibly reducing natural groundwater quality. Mine dewatering declined over the 20th century and ended in the 1980s (Ó Dochartaigh, 2005), and it is probable that groundwater levels rose over and after this period, but there is no modern monitoring of groundwater levels in the bedrock. Anecdotally, rising groundwater levels, poor groundwater quality, and other mining-related issues are not a problem in Glasgow (Ó Dochartaigh, 2005).

The likely active flow of groundwater in the Carboniferous bedrock aquifer means there is likely to be hydraulic connection between it and the overlying Quaternary aquifer system. However, there are no observed data to characterise or quantify any such connection.