Summary of the hydrogeology of the Glasgow area

Existing available hydrogeological, hydrogeochemical and groundwater temperature data have been collated for the Glasgow area and a general description is given first. Site-specific hydrogeological information from the observatory is then summarised.

Neither surface water nor groundwater in this area are used as a drinking water resource.

Hydrogeology of the superficial deposits

Much more is known about the hydrogeology of the superficial deposits than that of the bedrock in Glasgow area. The Quaternary geological sequence in the central Clyde valley in Glasgow forms a shallow, complex aquifer system with a sequence of hydrogeologically heterogeneous lithostratigraphical units. Three Quaternary lithostratigraphical 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. This aquifer is highly heterogeneous both naturally, due to varying lithologies within aquifer units and the varying influence of the tidal River Clyde with distance from the river; and due to urban influences, such as altered surface permeability, subsurface flow paths and urban recharge.

The national map of groundwater vulnerability 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 at specific sites. The widespread presence of anthropogenically altered ground not accounted for in the national-scale map is likely to have a major impact on local groundwater vulnerability and this has been considered in environmental assessments for the Glasgow Observatory site.

Hydrogeology of the bedrock

Unmined Carboniferous sedimentary rocks in the Central Belt of Scotland typically form multilayered 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. Overall, the unmined rocks tend to form moderately productive aquifers; available data on aquifer properties is given in Table 1.

Sandstone units within the sedimentary sequence generally have the highest transmissivity and storage capacity and therefore tend to act as discrete aquifer units. They are interspersed with lower permeability siltstones, mudstones and (undisturbed) coal seams.

Groundwater can be present in the aquifer under unconfined or confined conditions, which can vary between different sandstones and other sedimentary units, and at different depths. Groundwater heads therefore vary between different aquifer layers.

Groundwater flow paths through the aquifer are thought to be complex due to their naturally layered nature, the predominance of fracture flow and, potentially, the influence of faults. This may tend to promote preferential subhorizontal flow, such as within sandstone units, and subvertical flow, such as via transmissive fault zones. Flow paths are likely to be relatively deep (hundreds of metres) and long (1 to 10 km). Previous assessments suggested 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), but there is little measured hydrogeological data to support this hypothesis.

Impacts of mining

Figure 1 Conceptual model of the hydrogeology of extensively mined Carboniferous bedrock in central Scotland. From Ó Dochartaigh et al. (2015).

Mining in Carboniferous sedimentary rocks can significantly change natural hydrogeological conditions. Groundwater flow paths are likely to be even more complex in mined aquifers than in undisturbed Carboniferous aquifers. Mine voids (shafts and tunnels) can artificially and greatly increase aquifer transmissivity and can link formerly separate groundwater flow systems both laterally and vertically. 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.

Quantitative data on aquifer properties from borehole pumping tests are relatively rare for formerly mined aquifer zones in Carboniferous rocks in Scotland. However, records of specific capacity from boreholes drilled in aquifers that 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. There are also many records of yields from mine dewatering boreholes. Table 1 summarises the available data from these sources: in general, yield values are higher in aquifers that have experienced extensive coal mining.

Temperature

Figure 2 Measured temperatures at different depths within 20 km of the central to eastern greater Glasgow area. 10 data points from Burley et al. (1984), one from Rollin et al. (1987), two from the DECC Onshore Well Archive. The red data point is temperature measured in the Highhouse Colliery. The geothermal gradient is 30.2°C/km with a surface temperature of 7.3°C.

Measured temperature and depth within 20 km of the central-eastern Greater Glasgow area; 10 data points from Burley et al. (1984), one from Rollin et al. (1987), two from the DECC Onshore Well archive. The red data point is temperature measured in the Highhouse Colliery. The geothermal gradient is 30.2 °C/km with a surface temperature of 7.3 °C.

Hydrogeochemistry

Data on the hydrogeochemistry within superficial and artificial deposits is contained within site investigation reports and summarised in reports for the observatory planning applications (Ramboll, 2018).

Results are highly variable dependent on location, but some sites close to the planned borehole locations show exceedances of resource protection values in both the shallow (superficial deposits) groundwater and deeper (near top bedrock) groundwater.

There is little recent information on groundwater chemistry in the Carboniferous sedimentary aquifer in Glasgow. Some regional bedrock hydrochemistry information is available from the Baseline Scotland dataset (Ó Dochartaigh et al., 2011). The natural chemistry of groundwater in Carboniferous sedimentary aquifers is often moderately to highly mineralised.

Groundwater quality can be significantly affected by mining. Groundwater discharges from mine workings are often strongly mineralised, with high specific electrical conductivity (SEC) and particularly high concentrations of bicarbonate, calcium, sulphate, iron and magnesium, and low dissolved oxygen. pH is generally well buffered and alkalinity is high, indicating significant reaction with carbonate material in the aquifers.

Acid mine-water discharge is not currently a known problem in Glasgow and past investigations at a number of sites have indicated good-quality groundwater in abandoned mine workings.

Hydrogeology at the Glasgow Observatory site

Initial test pumping and monitoring has produced a wealth of data that indicates promising hydrogeology for a mine-water heat energy-research infrastructure, within typical ranges of conductivity, temperature and chemistry for mined Carboniferous bedrock in Scotland (more details in Shorter et al. (2021) and Palumbo-Roe et al. (2021)). For example, during constant-rate test pumping, up to 20 L/s was abstracted for five hours, which resulted in minimal drawdown (1.34 to 2.27 m in two of the boreholes in the Glasgow Upper mine working (GGA04 excluded) and 0.3 to 0.35 m in the Glasgow Main mine working boreholes).

The test pumping demonstrated clear connectivity between the bedrock boreholes and the Glasgow Upper mine workings, and within the individual mine workings. There is evidence of some connectivity between the Glasgow Main mine workings and the Glasgow Upper mine workings. This characterisation of measurable change on experimental timescales is important for future mine-water heat research.

Three boreholes in the Glasgow Upper mine workings gave a consistent transmissivity estimate (median 1020 m2/d, range 950 to 1020 m2/d) and the two boreholes intersecting the Glasgow Main mine workings give transmissivity estimates of 2000 and 2100 m2/d).

Temperature measurements show that the groundwater in the deeper Glasgow Main mine workings is warmer (12.4 to 12.8°C) than in the Glasgow Upper mine workings and the overlying bedrock (11.5 to 12°C).

Figure 3 Example plot from test pumping Borehole GGA01 (Glasgow Upper mine working). Borehole water levels (from data logger) in pumping and observation boreholes during the step drawdown test on GGA01. From Shorter et al., 2021.

The hydrochemistry of the groundwaters sampled during test pumping is variable. Three hydrochemical facies are identified: the principal facies is associated with the mine waters while the other two facies are associated with the superficial deposits and the bedrock aquifers. All the groundwaters are mineralised, near-neutral pH, bicarbonate-type waters with sodium as the dominant cation, except for two boreholes with calcium as the major cation. The mine-water composition is typical of Scottish mine waters reported in other studies. More details are given in Palumbo-Roe et al. (2021).