Summary of the hydrogeology of the Glasgow area

Existing available hydrogeological, hydrogeochemical and groundwater temperature data have been collated for the research area before borehole drilling starts.

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 superficial deposits than about bedrock in Glasgow and the Clyde Gateway area. 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. 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.

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 in the small urban Clyde Gateway area. 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 research site.

Hydrogeology of the bedrock

Unmined Carboniferous sedimentary rocks in the Central Belt of Scotland 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. Overall, the unmined rocks tend to form moderately productive aquifers – available aquifer properties values are given below. 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.

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 therefore vary between different aquifer layers.

Groundwater flow paths through the aquifer are thought to be complex, due to their naturally layered nature and the predominance of fracture flow, and potentially to the influence of faults. This may tend to promote preferential sub-horizontal flow, such as within sandstone units, and sub-vertical flow, such as via transmissive fault zones. Flow paths are likely to be relatively deep (100s of metres) and long (1–10 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.

  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) 0.003-0.1 (37) 10-1000* (5) 48-132* (46)
(minimum 0.43; maximum 1320)*
131-418 (348)
  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)

Impacts of mining

Mined hydrogeology conceptual model for central Scotland

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 aquifer properties data from borehole test pumping are relatively rare for formerly mined aquifer zones in Carboniferous rocks in Scotland. 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, and fewer specific capacity values, from mine dewatering boreholes, which are typically higher than for the same aquifers that have not experienced extensive coal mining.

Temperature

Though there are no direct borehole temperature measurements within the Clyde Gateway area, there are 13 measurements from within 20 km. This existing dataset indicates that minewater temperature in the planned boreholes is likely to be around 12°C.

Measured temperature and depth within 20 km of the Clyde Gateway area; 10 data points from Burley et al. (1984)

Hydrogeochemistry

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

Results are highly variable dependent on location, but some sites close to the planned borehole locations show exceedances to 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 Baseline Scotland dataset. 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 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. 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.

Additional site characterisation information

Groundwater / hydrogeology

Ó Dochartaigh, B E, Smedley, P L; Macdonald, A M, Darling W G, Homoncik, S. 2011. Baseline Scotland : groundwater chemistry of the Carboniferous sedimentary aquifers of the Midland Valley. British Geological Survey, 91pp. British Geological Survey Open Report OR/11/021. http://nora.nerc.ac.uk/id/eprint/14314/

Ó Dochartaigh, B E, Macdonald, A M, Fitzsimons V, Ward, R. 2015. Scotland's aquifers and groundwater bodies.Nottingham, UK, British Geological Survey, 63pp. British Geological Survey open Report OR/15/028. http://nora.nerc.ac.uk/id/eprint/511413/

Ó Dochartaigh, B E, Bonsor, H and Bricker, S. In press (2018). Improving understanding of shallow urban groundwater: the Quaternary groundwater system in Glasgow, UK. In: The Geosciences in Europe’s Urban Sustainability: Lessons from Glasgow and Beyond (CUSP), Fordyce F (ed), Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

Hydrogeological modelling

Bianchi, M, Kearsey, T and Kingdon, A. 2015 Integrating deterministic lithostratigraphic models in stochastic realizations of subsurface heterogeneity. Impact on predictions of lithology, hydraulic heads and groundwater fluxes. Journal of Hydrology, 531 (3). 557-573. 10.1016/j.jhydrol.2015.10.072

Turner, R J, Mansour, M M, Dearden, R., Ó Dochartaigh, B E and Hughes, A G. 2015. Improved understanding of groundwater flow in complex superficial deposits using three-dimensional geological-framework and groundwater models: an example from Glasgow, Scotland (UK). Hydrogeology Journal Vol 23 (3), 493-506. https://doi.org/10.1007/s10040-014-1207-0

Ramboll reports

Ramboll for GGERFS planning application for South Lanarkshire Council, 2018a. Glasgow Geothermal Energy Research Field Site: Cuningar Loop, GGERFS01-05 Phase I Environmental Review. Available online at https://publicaccess.southlanarkshire.gov.uk/online-applications/caseDetails.do?caseType=Application&keyVal=P7OTP9OPHQJ00

Ramboll for GGERFS planning application for South Lanarkshire Council, 2018b. UK Geoenergy Observatories: Glasgow Geothermal Energy Research Field Site Environmental Report. Available online at https://publicaccess.southlanarkshire.gov.uk/online-applications/caseDetails.do?caseType=Application&keyVal=P7OTP9OPHQJ00

Ramboll for GGERFS planning application for Glasgow City Council, 2018c. Glasgow Geothermal Energy Research Field Site: Dalmarnock, GGERFS10 Phase I Environmental Review. Available online at https://publicaccess.glasgow.gov.uk/online-applications/applicationDetails.do?keyVal=P70V2DEXFH600&activeTab=summary

Ramboll for GGERFS planning application for Glasgow City Council, 2018d. UK Geoenergy Observatories: Glasgow Geothermal Energy Research Field Site Environmental Report. Available online at https://publicaccess.glasgow.gov.uk/online-applications/applicationDetails.do?keyVal=P70V2DEXFH600&amactiveTab=summary

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