UK Geoenergy Observatories : Glasgow baseline groundwater and surface water chemistry dataset release September 2020 - May 2021

This report describes baseline water chemistry sampling and analysis results for groundwater and surface water at the United Kingdom Geoenergy Observatory (UKGEOS) in Glasgow between September 2020 and May 2021. The report accompanies the Glasgow Observatory groundwater chemistry data release and the Glasgow Observatory surface water chemistry data release for the same periods. While the reporting period is nine months long, the global Covid-19 pandemic meant sampling was not always possible during this time. The groundwater data release contains data from six monthly sampling rounds, and the surface water data release contains data from three to five monthly sampling rounds (site dependant). The Glasgow Observatory comprises twelve boreholes drilled into the main hydrogeological units, known as target units. These are the superficial deposits, bedrock, Glasgow Upper mine workings and Glasgow Main mine workings. The ten boreholes used for groundwater sampling are located at the Cuningar Loop in South Lanarkshire. There are two additional boreholes in the Observatory, one seismic monitoring borehole in Dalmarnock in the east end of Glasgow, and one borehole used for sensor testing. Three boreholes are drilled into the superficial deposits, two into the unmined bedrock, three into the Glasgow Upper mine workings and two into the Glasgow Main mine workings. The boreholes are designed to assist geological and hydrogeological characterisation, including baseline water chemistry monitoring, and to act as mine water abstraction and reinjection wells. The aims of the Observatory are to: 1) provide baseline environmental characterisation, 2) assess changes in ambient conditions induced by mine water abstraction/re-injection cycles and, 3) provide data and evidence to de-risk low-temperature shallow mine water heat energy and heat storage in former coal mine workings. Groundwater sampling was conducted using either a submersible or bladder pump. Field parameters (pH, specific electrical conductance (SEC), redox potential (Eh) and dissolved oxygen (DO)) were measured in a flow-through cell. The flow-through cell was discharged to a plastic beaker containing a thermometer probe. Field parameters were measured for a period of 20 minutes and at least three readings were taken five minutes apart. After field parameters were taken, the flow cell was disconnected and samples were taken directly from the pump discharge tube. Field alkalinity was measured by titration against H2SO4. Groundwater samples were analysed for: major, minor, and trace elements, chromium speciation (Cr (III) and Cr (VI)), non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC), polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), volatile organic compounds (VOC), stable isotopes - deuterium (δ2H), oxygen 18 (δ18O) and carbon 13 of dissolved inorganic carbon (DIC) (δ13CDIC), ammonium (NH4), methane, ethane and carbon dioxide (CH4, C2H6, CO2), chlorofluorocarbons (CFC-12 and CFC-11), sulphur hexafluoride (SF6), and sulphide (S2-). The pH of groundwater samples (range 6.78 – 7.81) is circum-neutral to alkaline, with a similar range across all target units. Groundwater from all four lithologies is highly mineralised with median SEC values >1470 μS/cm. GGA01, installed in the Glasgow Upper mine working, had the most highly mineralised groundwater with a range of 2697 μS/cm – 3002 μS/cm. This range is significantly higher than those found in the groundwater of other boreholes screened into the Glasgow Upper mine workings: GGA04 (1597 μS/cm – 1669 μS/cm) and GGA07 (1664 μS/cm – 1756 μS/cm). The Glasgow Main mine workings boreholes, GGA05 and GGA08, had a combined SEC range of 1570 μS/cm – 1658 μS/cm. The range of recorded groundwater temperatures is largest in the superficial deposits (10.8°C – 15.1°C), reflecting the near-surface environment. The bedrock and mine workings all have similar ranges (bedrock 10.3°C – 12.6 °C, Glasgow Upper mine workings 10.6 °C – 13.3 °C, Glasgow Main mine workings 10.5 °C – 13.6 °C). In all target units the dissolved oxygen concentration is very low, the medians range from 0.23 mg/L to 0.31 mg/L. In general major elements and physio-chemical parameters measured in the groundwater samples have concentration ranges similar to those found in bedrock and mine workings across the Carboniferous sedimentary aquifers of the Midland Valley (Ó Dochartaigh et al., 2011). The water from most groundwater samples is unchanged from pumping tests conducted in early 2020 (Palumbo-Roe et al., 2021). Groundwaters are HCO3 type, with no dominant cation. However, groundwater in GGA01 has evolved since the pumping test and now has Ca-SO4 type water. Oxidation of iron sulphide minerals (e.g. pyrite) could have caused the dominance of the SO4 anion in GGA01 groundwaters. Dissolved organic carbon (as NPOC) has the largest range and the highest concentrations in the superficial deposits’ groundwaters (3.44 mg/L – 16.49 mg/L), the highest concentrations were all found in the groundwater at GGB04 (4.51 mg/L – 16.49 mg/L). Broadly similar concentrations were recorded in the bedrock (1.03 mg/L – 3.37 mg/L) and mine workings (Glasgow Upper mine workings: 1.91mg/L – 3.16 mg/L, Glasgow Main mine workings: 2.10 mg/L – 3.04 mg/L). The Ammonium (NH4) concentrations are high in all Glasgow Observatory groundwaters, with similar median values in all target units (12.9 mg/L – 13.5 mg/L). There was a large concentration range of trace elements in the Glasgow Observatory groundwaters, but there was no clear distribution. The lowest concentrations, and smallest ranges of each trace element tended to be in the Glasgow Main groundwaters, while the highest median and max concentrations of each trace element were found across the superficial deposits, bedrock and Glasgow Upper groundwaters. Some PAHs were detected in the superficial and bedrock boreholes, with TPH detected in low concentrations in all units at least once during the sampling period. VOCs were detected in the superficial deposits and Glasgow Main mine workings. Water stable isotopes, carbon 13 of DIC and residence time data were consistent with findings from the pumping test results (Palumbo-Roe et al., 2021). Groundwaters are recharged by modern recharge from local rainfall. Median concentrations of dissolved CH4 in the groundwaters range from 21 μg/L in the superficial deposits to 202 μg/L in the Glasgow Upper mine workings. These values lie within the upper range of groundwaters reported in other studies from Carboniferous sedimentary rocks in the Midland Valley of Scotland (Ó Dochartaigh et al., 2011). The highest concentration was found in borehole GGA04 (991 μg/L) and the superficial deposit borehole GGA09r, the latter being consistent with previous data (Palumbo-Roe et al., 2021). Dissolved CO2 has a median value of 116 mg/L with little variation between units. Cluster analysis revealed that the superficial deposits, the bedrock, the mine workings and the surface water samples are each clustered into statistically distinct groups. Borehole GGB04 generally clusters separately from the other boreholes drilled into the superficial deposits. GGB04 displays clear differences in concentrations and trends during the monitoring for a number of trace elements including Co, Mn, Fe, Ni and also in concentrations of NH4. It is possible that these differences are driven by migration of elements from the made ground at the site into the superficial deposits. Borehole GGA01, drilled into the Glasgow Upper mine workings is also distinct. This can be explained by the behaviour and concentrations of major and trace elements during the monitoring period. Trace elements As, Ba, Co, Fe, Li, Mn, Mo, Ni, Rb, Si, Sr, U and W, all have different concentrations to those observed in the other Glasgow Upper mine working boreholes. The packed waste that GGA01 is drilled into may act as a source for the elevated trace elements. Surface water samples were taken using an angular beaker and telescopic rod. Samples were taken from the River Clyde and the Tollcross Burn. Surface water samples were analysed for: major, minor, and trace elements, chromium species (Cr (III) and Cr (VI)), non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC), polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), and stable isotopes - deuterium (δ2H), oxygen 18 (δ18O) and carbon 13 of dissolved inorganic carbon (DIC) (δ13CDIC). While both the Clyde and Tollcross Burn have a near-neutral to alkaline pH and show a narrow range in pH values, the pH values measured at the Tollcross Burn tend to be higher (median 8.4 from the Tollcross Burn and 7.9 from the Clyde). The waters all follow a similar temporal trend, with the highest value each month measured at the Tollcross Burn. The SEC measurements are higher in the Tollcross Burn samples (median 953 μs/cm) than those measured in the River Clyde samples (median 385 μs/cm). The surface water samples all have similar major ion proportions. The River Clyde samples are Ca-HCO3 type. The samples taken from the Tollcross Burn are also Ca-HCO3 type, but can be distinguished from River Clyde samples as they have lower Ca concentrations and higher HCO3 concentrations. Most trace elements are present in higher concentrations in the River Clyde than in the Tollcross Burn; exceptions to this are Sr, B, and Rb, which are higher in the Tollcross Burn, and Co, Ni, Zn, As, Y, and Sb, which have similar concentrations in both watercourses. With the exception of Cr, the trace element concentrations are similar between all sites on the River Clyde, which would be expected given these sites are all on a relatively short stretch of the same river. The Cr concentrations are much higher at the sampling site closest to a former chemical works than elsewhere within the observatory. In general, the surface water results are consistent with findings from previous work (Fordyce et al., 2021)