Using CFCs and SF6 for groundwater dating : a SWOT analysis

A knowledge of the residence time of groundwater is of importance in understanding key issues in the evolution of water quality. Chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) offer a convenient way of dating waters up to ~60 yrs old. In contrast to tritium, these gases are well-mixed in the atmosphere so their input functions are much less problematic. While any one of these gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple endmember mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Mainly for SF6, the phenomenon of ‘excess air’ also requires consideration. Mainly for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. This paper attempts a balanced look at the pros and cons of the trace-gas dating method

Baseline Scotland : groundwater chemistry of the Carboniferous sedimentary aquifers of the Midland Valley

This report describes the baseline groundwater chemistry of the sedimentary aquifers of Carboniferous age in the Midland Valley of Scotland. Groundwater is an important resource in the Midland Valley, largely for agriculture, but also for industry – including food and drink processing and mineral water bottling, and for domestic use. A large but unquantified volume of groundwater is also still pumped from former mine workings, largely coal mines, in order to maintain water levels and for quality treatment. Analyses for 62 groundwater samples were interpreted for the purposes of this study. Of these, 36 samples were collected for the Baseline Scotland project between September and December 2008. These were augmented with a further 25 samples collected during separate BGS projects since 2001. The sites were chosen largely to be representative of groundwater in the area, and sources that were very poorly constructed were avoided. A small number of samples were deliberately targeted from mines, either from adits, shafts or boreholes. The data were classed in one of five different hydrogeological units (or aquifer groups): four chronostratigraphic groups, which in decreasing order of age are the Inverclyde, Strathclyde, Clackmannan and Coal Measures groups; and a fifth group incorporating waters sampled from mine discharges. An estimate of the baseline groundwater chemistry conditions in the four chronostratigraphic hydrogeological units is presented, based on a statistical summary of the chemical data, which represents values between the 10th and 90th A summary of the conclusions arising from this study follows. percentiles of the full dataset range. This statistical approach to estimating baseline compositions was complemented by selecting 11 analyses of groundwater from sources where there is little or no indication of direct contamination, including likely impact from mining. The chemistry of these samples represents the typical groundwater conditions in the four non-mine hydrogeological units in the sedimentary Carboniferous aquifers of the Midland Valley

The baseline concentrations of methane in Great British groundwater : the National Methane Baseline Survey

This report describes the BGS research programme evaluating the baseline concentration of methane in groundwater which ran from November 2011 to completion in March 2016. The aim of the survey has been to improve knowledge of conditions in aquifers overlying potential shale gas source rocks present at depth, thus providing baseline knowledge relevant to the management of future exploration of new hydrocarbon sources. The necessity for doing this has been prompted by evidence from elsewhere (notably the USA) which has revealed very high methane concentrations in groundwater in some areas of shale gas extraction. Although this has often been directly attributed to shale gas operations, there have generally been no pre-development data on methane concentrations available to test this. Before the start of the current survey, BGS held some 170 analyses of methane in groundwaters from aquifers across Great Britain, acquired from the 1980s onwards. These data have been combined with new survey data, to give a total of 439 methane data points. In this combined dataset, 96% of samples show methane concentrations of less than 100µg/l and indicate that methane is rarely present at concentrations high enough to be potentially explosive (there are no health limits for methane in groundwaters). In the minority of samples with elevated concentrations, this was generally considered to be due to the proximity of organic rich coal seams or peats. The highest concentrations were found in the Cretaceous aquifers of the Weald Basin, a known area for occurrences of methane gas in the shallow subsurface. The new survey covers the majority of principal aquifers in Great Britain, including the Chalk, Permo-Triassic Sandstones, Carboniferous Limestone, and the Lower Greensand. Samples from over twenty different aquifers have been collected. In general, methane concentrations in carbonate aquifers (Chalk, limestones, Oolites etc.) are low, similar to those seen in the Permo-Triassic sandstone aquifers, although methane is widely present above detection limit (approximately 1µg/L) in all these aquifers. The Coal Measures of South Wales have the highest median value across Great Britain and this aquifer also shows the greatest temporal variability. Methane concentrations in the Carboniferous sediments of Scotland are also elevated, likely due to the impact of mining and the presence of coal seams. While little temporal variability is generally seen in aquifers used for public water supply or otherwise regularly pumped, further work is required to understand the impact of borehole use, pumping regime and aquifer type on the variability of the methane baseline of Great British aquifers. It should be noted that the Survey is not intended to replace any oil and gas operator’s local monitoring as required by the regulators. This is a national scale survey to enable a broad understanding of the distribution of methane in aquifers across relevant areas of Britain and cannot replace an understanding of groundwater quality at a local scale

Baseline Scotland : groundwater chemistry of the Old Red Sandstone aquifers of the Moray Firth area

The groundwater chemistry of Old Red Sandstone aquifers in the Moray Firth area has been characterised based on new chemistry analyses generated during the Baseline Scotland project, combined with existing analyses from earlier projects. A total of 39 groundwater sample analyses were interpreted for the purposes of this study. Of these, 17 were collected in 2007 specifically for the Baseline Scotland project. These were augmented with a further 22 samples collected during separate BGS projects since 2001. The sites were chosen to be representative of groundwater in the area, and sources that were poorly constructed were avoided. A summary of the conclusions arising from this study follows. 1. Groundwater in the Old Red Sandstone aquifers of the Moray Firth is generally moderately mineralised, with a median SEC of 469 μS/cm (interquartile range 341–591 μS/cm). The pH is variable: median pH is slightly alkaline (7.31), but values range from 5.33 to 8.06. The major ion chemistry appears to be dominated by the dissolution of carbonate cements within the aquifer and overlying deposits, and the variable influence of seawater (either directly as saline intrusion or as aerosols). 2. The dominant cation is Ca, with a median concentration of 54.8 mg/L (interquartile range 42.5 – 74 mg/L). Mg and K concentrations are generally low (median 4.21 and 3.7 mg/L respectively). The median Na is 20.6 mg/L (interquartile range 12.6–28 mg/L); however, a few samples have been affected by proximity to the sea and have much higher concentrations, as illustrated by the 95th percentile (68 mg/L) and maximum concentration (153 mg/L). 3. The dominant anion is bicarbonate, with a median concentration of 183 mg/L (interquartile range 183–230 mg/L). Around one third of the samples are saturated with respect to calcite. Sulphate concentrations are generally low (median 14.6 mg/L, interquartile range 7.7–36.4 mg/L), although higher concentrations are encountered in samples affected by seawater, and/or possibly by gypsum bands within the aquifer. Chloride concentrations follow broadly the same distribution as Na and have a median of 38.8 mg/L and interquartile range of 19.5–49.5 mg/L; the same few samples show high Cl concentrations as do Na. 4. Concentrations of minor and trace elements in the groundwater are dominated by the redox conditions. Measured values of dissolved oxygen indicate a large range in redox conditions across the aquifer. Concentrations of DO close to 10 mg/L indicate fully oxic conditions. By contrast, groundwaters with DO concentrations <1 mg/L are indicative of sub-oxic or mildly reducing conditions, and appear to be prevalent in much of the Upper Old Red Sandstone outcrop, and parts of the Middle Old Red Sandstone. Reducing conditions may reflect the presence of low permeability layers (often marine in origin) within the thick superficial deposits overlying the Old Red Sandstone aquifer. The effect of reducing conditions is to increase concentrations of Fe and Mn, which show median concentrations of 38 and 43 μg/L respectively, and 75th percentile values of 354 and 227 μg/L respectively. 5. The majority of samples (21) were collected from the Upper Old Red Sandstone aquifer; 14 samples were collected from the Middle Old Red Sandstone and only 4 samples from the Lower Old Red Sandstone. The samples show broadly similar chemistry across the three aquifer units; however, there are several notable differences. Samples from the Upper Old Sandstone aquifer show very similar cation distribution, dominated by Ca, while the Lower and Middle Old Red Sandstone aquifers show a wider cation distribution and appear less affected by calcite dissolution. The pH of the Lower and Middle Old Red Sandstone aquifers is slightly lower (more acidic), generally less than 7.0. Groundwaters within the Upper Old Red Sandstone aquifer are generally more reducing, probably reflecting their location close to the coast and hence the influence of the sea and/or overlying marine superficial deposits. 6. Nitrate concentrations are variable across the aquifer units, although median concentrations are low (1.45 mg/L TON-N or less in each aquifer). The prevalence of low oxygen conditions in the sampled groundwaters has led to denitrification, which means the relationship between land use and nitrate concentrations is less obvious than for other parts of Scotland (MacDonald et al., 2005a). However, there is a clear relationship between nitrate concentrations and the Nitrate Vulnerable Zone (NVZ) that covers much of the study area, with the seven highest groundwater nitrate concentrations, ranging from 5.98 to 22.1 mg/L TON-N, all from samples taken within the NVZ. The highest median concentrations were from samples collected on land known to be used for dairy, pig or poultry farming. 7. Phosphorus concentrations in Moray Firth groundwaters range from less than detection limit up to 172 μg P/L, with an overall median of 36 μg P/L, which is in the eutrophic range for surface waters. Concentrations are generally low in the western part of the study area, and an observed relationship with the spatial pattern of F suggests that both elements may be in part derived from the dissolution of phosphate minerals, such as apatite, from the aquifer rocks. Concentrations in the eastern part of the study area are generally higher, usually in the mesotrophic or eutrophic range for surface waters. The higher values may be related to land use, with P inputs from agricultural activity. 8. An estimate of the baseline groundwater chemistry conditions in the Old Red Sandstone aquifers has been presented, based on a statistical summary of the chemical data. This represents data between the 10th and 90th percentiles, with the exception of NO3-N and P, where the influence of anthropogenic activity is likely to have distorted baseline conditions throughout much of the study area. This statistical approach to estimating baseline was complemented by selecting ten analyses of groundwater samples collected from high quality groundwater sources, which are unlikely to have been impacted by any agricultural contamination, and which represent the general the groundwater conditions in the Old Red Sandstone aquifers in the Moray Firth area

Baseline Scotland : the Lower Devonian aquifer of Strathmore

This report presents a summary of the groundwater chemistry of the Devonian sedimentary aquifer in Strathmore, eastern Scotland. The area covered by this study extends from Perth in the southwest to Stonehaven in the northeast. The survey forms part of the ongoing Baseline Scotland project. The Devonian sedimentary rocks of Strathmore form an important regional aquifer in an area of some of the most fertile agricultural land in Scotland, with a number of major urban settlements. The aquifer provides water for agriculture, industry, recreation and domestic use. The aquifer can be divided into six main geological units, largely sandstones but with significant conglomerates and, less commonly, mudstones. All of these units are classed as moderately or highly productive aquifers, but too few data are available to allow a detailed analysis of the hydrogeological variations between the formations. A total of 35 new groundwater samples were collected during this project, and the resulting chemistry data combined with data from 13 additional samples collected during a BGS sampling programme in 2001. The samples were analysed at BGS laboratories for a wide range of chemical constituents. The collection and interpretation of new groundwater chemistry data for the Strathmore area has led to the following conclusions. • The groundwaters of the Devonian aquifer in Strathmore are mainly weakly mineralised, with TDS concentrations mostly less than 400 mg l–1. Groundwaters have near-neutral to slightly alkaline pH values and are for the most part oxygenated, with detectable dissolved oxygen and high redox potentials. As a result, dissolved iron, manganese and ammonium (NH4-N) concentrations are usually low. Water from shallow boreholes and springs is often undersaturated with calcite, but in deeper boreholes, reaction with carbonate minerals in the aquifer is more usual and most of these are saturated with respect to calcite. • Nitrate concentrations are often high, with an interquartile range of 2.6 mg l–1 to 11.7 mg l–1 as NO3-N. Nearly one third of the samples exceeded the EC drinkingwater limit for nitrate of 11.4 mg l-1 as NO3-N. An anomalously high nitrate concentration of 81 mg l–1 as NO3-N was observed in one sample, and appears to result from direct downhole contamination by nitrogen fertiliser. Under the oxidising conditions, nitrate is a stable solute species across the aquifer and is found at depths in excess of 100 m. • Concentrations of phosphorous in groundwater across the aquifer are typically less than 0.1 mg l-1 (the 90th percentile), with a median of 0.03 mg l-1. Given the importance of P in controlling eutrophication in surface water, these concentrations in groundwater may be significant. • Increased salinity occurs in groundwater in some near-coastal boreholes, most likely as a result of mixing with seawater. • Most cationic trace elements have low concentrations, in accordance with the neutralpH groundwater conditions. Concentrations of arsenic are relatively high in some groundwaters (up to 8.8 μg l-1) though none exceeds the EC maximum permissible value for drinking water of 10 μg l-1. Concentrations of uranium reach up to 15.4 μg l–1, with the highest concentration just exceeding the WHO provisional guideline value for drinking water of 15 μg l–1. Two other exceedances above maximum permissible values for drinking were observed, for nitrite (highest concentration 0.194 mg l-1) and fluoride (highest concentration 3.7 mg l-1). The chemistry and residence time indicators (CFC and stable isotopes) indicate that the groundwaters are largely of young age, being mostly recharged within the last 40 years, with mixing throughout the top 100 m of the aqufier. The samples show little evidence of the presence of palaeowaters. The young age of the groundwaters means they are vulnerable to contamination. • A first estimate of the baseline groundwater chemistry conditions in the Strathmore Lower Devonian aquifer can be given by the statistical summary of the data collected in this study (with the exception of nitrogen (N) and phosphorous (P), where the influence of anthropogenic activity is likely to have affected concentrations across the aquifer). To compliment this approach, six good quality sites have been chosen which represent the majority of groundwaters found in Strathmore. • The most significant groundwater-quality problems identified by this survey are: o the widespread presence of elevated nitrate concentrations in groundwater, which is strongly linked to agricultural activity; o elevated phosphate concentrations which may affect the quality of surface water when discharged to rivers as baseflow. o the presence of saline water in some near-coastal boreholes, indicating localised saline intrusion. It is not clear to what degree the saline intrusion is natural and to what degree it has been enhanced by over-pumping

A baseline survey of dissolved methane in aquifers in Great Britain

Interest in dissolved methane (CH4) concentrations in aquifers in England, Scotland and Wales (‘Great Britain’ or GB) has grown concurrently with interest in the exploitation of unconventional gas sources (UGS). Experience, mainly from North America, has shown the importance of a pre-production baseline against which changes possibly due to UGS extraction can be compared. The British Geological Survey, aided by water utilities, private users and regulators, has compiled a unique dataset for CH4 in groundwaters of GB. This focuses principally on areas where UGS exploration is considered more likely, as indicated by the underlying geology. All the main water supply aquifers (Principal aquifers) were targeted, plus Secondary aquifers where locally important. The average dissolved CH4 concentration across GB in the aquifers sampled was 45 μg/l. Out of a total of 343 sites, 96% showed dissolved CH4 concentrations b100 μg/l, 80% b10 μg/l, and 43% b 1 μg/l. No site had a CH4 concentration above the US Department of the Interior suggested risk action level of 10,000 μg/l. While most sites were sampled only once, a sub set was monitored quarterly to determine the magnitude of seasonal or other variations. Generally these variations were minor, with 84% of sites showing variations within the range 0.5–37 μg/l, but some aquifers where the porosity was primarily fracture-related showed larger changes (0.5–264 μg/l). This may have been due to the nature of sampling at these sites which, unlike the others, did not have installed pumps. Since the regulatory compliance monitoring attending UGS operations will include the measurement of parameters such as dissolved CH4, it is essential that sampling methods are tested to ensure that reliable and comparable datasets can be obtained

Groundwater : meltwater interaction in a proglacial aquifer

Groundwater plays a significant role in the hydrology of active glacial catchments, with evidence that it may buffer changes in meltwater river flow and partially compensate for glacial loss. However, to date there has been little direct research into the hydrogeology and groundwater dynamics of proglacial aquifers. Here we directly investigate the three dimensional nature of a proglacial sandur (floodplain) aquifer in SE Iceland, using hydrogeological, geophysical, hydrological and stable isotopic techniques, and provide evidence of groundwater-melt water dynamics over three years. We show that the proglacial sandur forms a thick (at least 50-100 m), high permeability (transmissivity up to 2500 m2/day) aquifer, extending over an area of approximately 6 km2. At least 35 million m3 of groundwater is stored in the aquifer, equivalent to ~23-28% of total annual river flow through the catchment. The volume of mean annual groundwater flow through the aquifer is at least 0.1-1 m3/sec, equivalent to ~10-20% of mean annual river flow. Groundwater across the aquifer is actively recharged from local precipitation and strongly influenced by individual rainfall events and seasonal precipitation. Glacial meltwater influence on groundwater also occurs in a zone extending from 20-500 m away from the meltwater river, for at least 3km down-sandur, and to at least 15 m deep. Within this zone summer recharge from the river to groundwater occurs when meltwater river flows are high, maintaining high summer groundwater levels compared to winter levels; and groundwater temperature and chemistry are strongly influenced by meltwater. Beyond this zone there is no substantial meltwater influence on groundwater. From ~2 km down-sandur there is extensive groundwater discharge via springs, supporting semi-perennial streams that form distinct local ecosystems, and providing baseflow to the main meltwater river. This research indicates that predicted continued climate change-related reductions in glacier coverage and increases in precipitation are likely to increase the significance of groundwater storage as a water resource, and of groundwater discharges in maintaining environmental river flows in glacier catchments

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)

UK Geoenergy Observatories : Glasgow baseline groundwater and surface water chemistry dataset release June 2021 - January 2022

This report follows the Glasgow baseline groundwater and surface-water chemistry dataset release report September 2020 – May 2021 (data release/ monitoring period 1) (Bearcock et al., 2022), and describes baseline water chemistry sampling and analysis results for groundwater and surface water at the United Kingdom Geoenergy Observatory (UKGEOS) in Glasgow between June 2021 and January 2022. The report accompanies the Glasgow Observatory groundwater chemistry data release and the Glasgow Observatory surface water chemistry data release for the same periods (data release/ monitoring period 2). The monitoring period is eight months long, with six rounds of surface water and groundwater sampling during this time. Sampling during non-consecutive months was a result of COVID-19 restrictions and construction activities at site. The Glasgow Observatory comprises twelve boreholes drilled into the main hydrogeological units, known as target horizons. 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, reduced iron (Fe), non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC), polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), volatile organic compounds (VOC), 2H and 18O abundance in water (δ2H and δ18O), 13C abundance in dissolved inorganic carbon (DIC) (δ13CDIC), ammonium (NH4), dissolved gases (methane, ethane and carbon dioxide (CH4, C2H6, CO2)), noble gases (helium, neon, argon, krypton, and xenon (He, Ne, Ar, Kr and Xe)), chlorofluorocarbons (CFC-12 and CFC-11), sulphur hexafluoride (SF6), and sulphide (S2-). The pH of groundwater samples (range 6.6 – 7.4) is circum-neutral, with a similar range across all target horizons. Groundwater from all four horizons is highly mineralised with median SEC values 1440 μS/cm - 1670 μS/cm. GGA01, installed in the Glasgow Upper mine working, had the most highly mineralised groundwater with a range of 2930 μS/cm – 3140 μS/cm. The range of recorded groundwater temperatures is largest in the superficial deposits (11.0°C – 16.6°C). The bedrock and mine workings groundwaters all have similar temperatures, and a narrower range of 10.1°C – 13.4 °C. In all target horizons the dissolved oxygen concentration is very low, all DO values are ≤0.81 mg/L. In general major elements and physico-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 chemistry of most groundwater samples is unchanged from pumping tests conducted in early 2020 (Palumbo-Roe et al., 2021), and the previous period of baseline monitoring that spanned the period from September 2020 to May 2021 (Bearcock et al., 2022). Groundwaters are generally HCO3 type, with no dominant cation. The exception is GGA01, where groundwaters changed from HCO3 type during the pumping test to Ca-SO4 type at the start of the previous round of monitoringThe Glasgow Observatory comprises twelve boreholes drilled into the main hydrogeological units, known as target horizons. 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, reduced iron (Fe), non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC), polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), volatile organic compounds (VOC), 2H and 18O abundance in water (δ2H and δ18O), 13C abundance in dissolved inorganic carbon (DIC) (δ13CDIC), ammonium (NH4), dissolved gases (methane, ethane and carbon dioxide (CH4, C2H6, CO2)), noble gases (helium, neon, argon, krypton, and xenon (He, Ne, Ar, Kr and Xe)), chlorofluorocarbons (CFC-12 and CFC-11), sulphur hexafluoride (SF6), and sulphide (S2-). The pH of groundwater samples (range 6.6 – 7.4) is circum-neutral, with a similar range across all target horizons. Groundwater from all four horizons is highly mineralised with median SEC values 1440 μS/cm - 1670 μS/cm. GGA01, installed in the Glasgow Upper mine working, had the most highly mineralised groundwater with a range of 2930 μS/cm – 3140 μS/cm. The range of recorded groundwater temperatures is largest in the superficial deposits (11.0°C – 16.6°C). The bedrock and mine workings groundwaters all have similar temperatures, and a narrower range of 10.1°C – 13.4 °C. In all target horizons the dissolved oxygen concentration is very low, all DO values are ≤0.81 mg/L. In general major elements and physico-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 chemistry of most groundwater samples is unchanged from pumping tests conducted in early 2020 (Palumbo-Roe et al., 2021), and the previous period of baseline monitoring that spanned the period from September 2020 to May 2021 (Bearcock et al., 2022). Groundwaters are generally HCO3 type, with no dominant cation. The exception is GGA01, where groundwaters changed from HCO3 type during the pumping test to Ca-SO4 type at the start of the previous round of monitoring (September 2020). During this monitoring period the concentrations of Ca and SO4 in GGA01 have continued to increase, albeit at a slowing rate, while HCO3 concentrations, which had initially fallen, are slowly increasing. Oxidation of sulphide minerals (e.g. pyrite) could have caused the dominance of the SO4 anion in GGA01 groundwaters. Dissolved organic carbon (as NPOC) is present in the range 1.05 mg/L to 5.46 mg/L, except for one outlier of 23.5 mg/L at GGA01. NPOC concentrations in the superficial deposits, with a median 4.39 mg/L, are at the upper end of this range, while all other target horizons have similar, lower, median values (medians from 2.32 mg/L to 2.54 mg/L). Groundwater samples from two boreholes have low ammonium (NH4) concentrations throughout the monitoring period (GGB04 in the superficial deposits and bedrock borehole GGA03r have a combined median of 3.15 mg/L). The remaining groundwaters in the Glasgow Observatory have high NH4 concentrations (combined median 23.2 mg/L). There was a large concentration range of trace elements in the Glasgow Observatory groundwaters. In general, the lowest concentrations were found in groundwaters from the Glasgow Main mine workings. In contrast the highest trace element concentrations were found in the groundwaters from the Glasgow Upper mine workings and the superficial deposits. TPH was detected in low concentrations in all units at some point during the sampling period. VOCs were not detected in any groundwater sample. Water stable isotopes (δ2H and δ18O), inorganic carbon δ13C and groundwater residence time data were consistent with findings from the pumping test results and previous monitoring period (Bearcock et al., 2022; Palumbo-Roe et al., 2021). Groundwaters are recharged by modern recharge from local rainfall. Median concentrations of dissolved methane (CH4) in the groundwaters range from 16.6 μg/L in the superficial deposits to 224 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). Dissolved ethane (C2H6) was below detection limits in all but one sample (5.2 μg/L at GGA01). Dissolved CO2 has a median value of 141 mg/L with little variation between units. Water chemistry cluster analysis shows that the superficial deposits, bedrock, mine working, and surface water samples cluster into statistically distinct groups. Notable exceptions are groundwaters from GGA01 which form their own separate group, and GGB05 groundwaters from the bedrock horizon which group with the mine workings. 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, non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC), polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), 2H and 18O abundance in water (δ2H and δ18O) and carbon 13 abundance in dissolved inorganic carbon (DIC) (δ13CDIC), . While both the Clyde and Tollcross Burn have a near-neutral to alkaline pH (7.3 – 8.7), the pH values measured at the Tollcross Burn tend to be higher (median 8.3 from the Tollcross Burn and 7.7 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 872 μs/cm) than those measured in the River Clyde samples (median 372 μs/cm). Surface water samples are all generally Ca-HCO3 type. The samples taken from the Tollcross Burn tend to have a greater HCO3 proportion than the river Clyde samples. Most detected trace elements are present in higher concentrations in the River Clyde than in the Tollcross Burn. 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 (median 4.18 μg/L) than elsewhere within the Glasgow Observatory (median 0.4 μg/L). In general, the surface water results are consistent with findings from previous work (Bearcock et al., 2022; Fordyce et al., 2021). (September 2020). During this monitoring period the concentrations of Ca and SO4 in GGA01 have continued to increase, albeit at a slowing rate, while HCO3 concentrations, which had initially fallen, are slowly increasing. Oxidation of sulphide minerals (e.g. pyrite) could have caused the dominance of the SO4 anion in GGA01 groundwaters. Dissolved organic carbon (as NPOC) is present in the range 1.05 mg/L to 5.46 mg/L, except for one outlier of 23.5 mg/L at GGA01. NPOC concentrations in the superficial deposits, with a median 4.39 mg/L, are at the upper end of this range, while all other target horizons have similar, lower, median values (medians from 2.32 mg/L to 2.54 mg/L). Groundwater samples from two boreholes have low ammonium (NH4) concentrations throughout the monitoring period (GGB04 in the superficial deposits and bedrock borehole GGA03r have a combined median of 3.15 mg/L). The remaining groundwaters in the Glasgow Observatory have high NH4 concentrations (combined median 23.2 mg/L). There was a large concentration range of trace elements in the Glasgow Observatory groundwaters. In general, the lowest concentrations were found in groundwaters from the Glasgow Main mine workings. In contrast the highest trace element concentrations were found in the groundwaters from the Glasgow Upper mine workings and the superficial deposits. TPH was detected in low concentrations in all units at some point during the sampling period. VOCs were not detected in any groundwater sample. Water stable isotopes (δ2H and δ18O), inorganic carbon δ13C and groundwater residence time data were consistent with findings from the pumping test results and previous monitoring period (Bearcock et al., 2022; Palumbo-Roe et al., 2021). Groundwaters are recharged by modern recharge from local rainfall. Median concentrations of dissolved methane (CH4) in the groundwaters range from 16.6 μg/L in the superficial deposits to 224 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). Dissolved ethane (C2H6) was below detection limits in all but one sample (5.2 μg/L at GGA01). Dissolved CO2 has a median value of 141 mg/L with little variation between units. Water chemistry cluster analysis shows that the superficial deposits, bedrock, mine working, and surface water samples cluster into statistically distinct groups. Notable exceptions are groundwaters from GGA01 which form their own separate group, and GGB05 groundwaters from the bedrock horizon which group with the mine workings. 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, non-purgeable organic carbon (NPOC) and total inorganic carbon (TIC), polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), 2H and 18O abundance in water (δ2H and δ18O) and carbon 13 abundance in dissolved inorganic carbon (DIC) (δ13CDIC), . While both the Clyde and Tollcross Burn have a near-neutral to alkaline pH (7.3 – 8.7), the pH values measured at the Tollcross Burn tend to be higher (median 8.3 from the Tollcross Burn and 7.7 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 872 μs/cm) than those measured in the River Clyde samples (median 372 μs/cm). Surface water samples are all generally Ca-HCO3 type. The samples taken from the Tollcross Burn tend to have a greater HCO3 proportion than the river Clyde samples. Most detected trace elements are present in higher concentrations in the River Clyde than in the Tollcross Burn. 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 (median 4.18 μg/L) than elsewhere within the Glasgow Observatory (median 0.4 μg/L). In general, the surface water results are consistent with findings from previous work (Bearcock et al., 2022; Fordyce et al., 2021)

UK Geoenergy Observatories : Glasgow borehole test pumping - groundwater chemistry

In 2015, the British Geological Survey (BGS) and the Natural Environment Research Council (NERC) were tasked with developing new centres for research into the subsurface environment to aid the responsible development of new low-carbon energy technologies in the United Kingdom (UK) and internationally. Glasgow is one of two UK Geoenergy Observatories (UKGEOS) (Figure 1). The Glasgow Observatory comprises a network of boreholes across five sites into the superficial deposits, mined and unmined bedrock in the Dalmarnock area in the east of Glasgow City (Site 10 on Figure 1b) and at the Cuningar Loop on the River Clyde in Rutherglen, South Lanarkshire (Sites 1, 2, 3 and 5 on Figure 1b). These were designed to characterise the geological and hydrogeological setting as a research infrastructure to de-risk key technical barriers to lowtemperature shallow mine water heat/storage in an urbanised former mine setting (Monaghan et al. 2017, 2019). The borehole network is intended also for baseline monitoring to assess the environmental status before and during the lifetime of the project. Figure 1c shows the details of the 11 boreholes located at the Cuningar Loop. Test pumping was carried out at nine of the Cuningar Loop boreholes in January and February 2020 to characterise the hydraulic properties of the target aquifer horizons (mine workings, bedrock, and superficial deposits), and to determine the extent to which these are hydraulically connected (Shorter et al. 2021; Figure 1c). Groundwater samples were collected during constantrate pumping tests and analysed to provide an initial hydrochemical characterisation of the aquifers and, where more than one sample was obtained during the test, to measure changes in selected constituents during pumping, to complement observed hydraulic responses. Fifteen groundwater samples were obtained and were analysed to determine the concentrations of selected chemical parameters at the BGS and associated laboratories. This report details the groundwater sampling protocols used during the test pumping, the analysis methods, and the groundwater hydrochemistry. The report accompanies the dataset: UKGEOS Glasgow Test Pumping Groundwater Chemistry Data Release