Guide to reference groundwater and porewater compositions in support of the UK GDF Programme

This guide provides a summary of available data and information relating to the chemical compositions of groundwaters/porewaters within a defined group of Lower-Strength Sedimentary Rocks (LSSRs) across England. The available data have been collated as a first step towards characterisation of the chemical compositions of water and to define realistic “reference” compositions for porewater/groundwater from the defined LSSRs. These are intended to encompass the compositional ranges likely to be encountered in a GDF construction, operational and closure context.
The LSSRs of interest in this study are defined as the Kimmeridge Clay Formation (KCF), Oxford Clay Formation (OCF), Mercia Mudstone Group (MMG) and thick evaporites within the MMG. These rock types of interest are present in localities where Nuclear Waste Services (NWS) is in formal discussion as at the date of this report with communities about the development of a geological disposal facility (GDF) for higher-activity radioactive waste for the UK.
“Credible”, internally-consistent groundwater and porewater compositions are models based on limited analytical data and caution should be applied when using them. The requirement for these credible compositions has arisen from the needs of the advancing UK GDF programme, but in the absence of specific sites or site-specific data. The intended uses of the data include technical development activities for the GDF programme, including for example radiochemistry safety assessments, materials investigations and engineering design.
Investigations of data have been made from existing national databases including pre-existing data from the published literature and BGS archives (including former Nirex studies). Summaries are also made of reference porewater compositions from suitable potential analogues from other British and overseas studies, including for English Lias, Callovo-Oxfordian Clay of Bure, East Paris Basin, France, and the Opalinus Clay from Mont Terri, Switzerland.
Investigations have highlighted that available data for groundwater/porewater from the English LSSRs are limited, reflecting both a lack of previous studies and the challenges of acquisition of reliable data. In particular, recovery of porewaters from highly consolidated clays requires lengthy procedures using bespoke apparatus, such as high-pressure squeezing cells. The investigation also highlights the strong limitations on quality of available data, with many problems associated with sampling artefacts for both collected groundwater and porewater.
Chemical compositions have been investigated for LSSRs in areas defined as the East Irish Sea Basin, east Lincolnshire, East Anglia and the Wessex Basin, to be reflective of variable rock compositions in varying hydrogeological settings and with variable evolutionary histories which are encountered across England. These areas do not reflect any preference in location for siting a GDF
At this early stage of LSSR investigations, the reference porewater/groundwater compositions are derived by a combination of lines of reasoning, including available raw data, rock mineralogy, hydrogeochemical evolutionary history and equilibrium geochemical modelling. They are not defined by real observed groundwater/porewater data, although in some cases, they may be close.
The reference waters are internally consistent, meaning that:
• values of all chemical parameters could feasibly occur together in situ in the rock;
• the composition is charge-balanced;
• the water chemistry is consistent with the mineralogy of the rock.
The reported porewater and groundwater analyses have been used as inputs to geochemical models that adjust the concentrations of certain constituents to attain internal consistency (e.g. balance charge on Cl- concentration) and use mineral solubility constraints to fix the concentrations of aqueous constituents for which data are lacking (e.g. constrain dissolved Al concentrations by equilibrium with gibbsite and dissolved SiO2(aq) concentrations by equilibrium with amorphous silica). The results are a reference water chemical composition in terms of the major cations and anions, total dissolved solids (TDS) and pH.
The reference porewaters/groundwaters compositions derived in this study are summarised in Section 2. The reference data provided are based on limited evidence and are not a substitute for detailed field observational data, sampled according to well-designed field experiments, which would form a next step in site-specific data gathering and investigation.
The selection of a suitable reference porewater or groundwater will depend on site-specific considerations and the intended use case (e.g., modelling, experimentation etc). It is recommended to consult an NWS Groundwater Chemistry expert before using any compositions described herein.