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

Potential impacts on groundwater resources of deep CO2 storage

Research into carbon dioxide (CO2) geological storage has been carried out over two decades, as part of studies to evaluate the feasibility of Carbon Dioxide Capture and Storage (CCS). Recently, there has been an increasing focus on potential impacts to surrounding geological formations and particularly, shallow aquifers and associated potable groundwater resources. Potential leakage of CO2 in supercritical and gaseous form from onshore or nearshore deep saline formations (DSF), or of the associated formation brines, is frequently cited as a key risk scenario for the overlying or neighbouring shallower groundwater. To date, the impact of CO2 storage has mainly been studied at near-well and reservoir scale, whereas risks in the context of regional multilayered groundwater systems have not yet been systematically assessed. Recent studies have begun to address this topic, using hydrodynamic and geochemical modelling approaches, and have identified several potential mechanisms that can lead to negative impact on groundwater quantity and/or quality. The IEA Greenhouse Gas R&D Programme (IEAGHG) recently commissioned the CO2GeoNet Association to undertake a literature review and unpublished original work on this topic, with the aim of summarising ‘state of the art’ knowledge and identifying knowledge gaps and research priorities in this field. This paper is a summary of this report