Palaeohydrogeological Data Analysis and Model Testing (PADAMOT)

Why is palaeohydrogeology relevant for safety of radioactive waste repositories?

How groundwaters have changed in the last million (10⁶) years is relevant to the safety of radioactive wastes in underground repositories ('geological disposal') because this mirrors the maximum time into the future for which the safety must be considered. The last 2 million years is known in geology as the Quaternary period - this is the time when cycles of the earth's climate caused glacial periods or 'ice ages' in northern latitudes and alternations of wet ('pluvial') and dry ('arid') conditions in southern regions. The last ice age in northern Europe ended about 10,000 years ago. These fluctuations in climate are analogues for climate states in the future, though these will additionally have impacts from man's activities (e.g. CO2 increases).

By investigating how groundwaters have evolved in the past, palaeohydrogeology increases our confidence in modelling future groundwater movements:

"Understanding the past is the key to predicting the future"

Though the future evolution of climate is unlikely to be a repetition of the past, the processes that link climate change with groundwater remain the same and are the key to having confidence in how groundwater systems will evolve in the future.

One of the ways that the safety of a store or repository for radioactive wastes is assessed is by computer modelling of potential leakage of radionuclides and the impact that this would have on our environment (the 'biosphere'). This is called Performance Assessment (or 'PA'). For wastes in an underground repository, the main pathway for leakage would be via groundwaters that occur in all rock formations.

PA bases predictions of future safety on a model of the present state of the groundwater system: the 'base scenario' or 'reference case'. This may not be an adequate model for all possible future states, so a number of 'variant scenarios' are also considered that test the sensitivity of PA to environmental changes that might occur in the future. These scenarios are built from information on 'FEPs' (features, events and processes) that comes from palaeohydrogeology and many other types of groundwater investigations.

The issue for Performance Assessment is essentially whether the present-day groundwaters give a representative 'snapshot' of how the system will be, and has been, for a long period, or whether the groundwaters around the proposed repository have changed significantly in response to episodic changes in climate or other external factors.

Some general scenarios for how movements and compositions of shallow groundwaters in a coastal region in northern Europe might change through the different stages of a glacial climate cycle are illustrated here:

[©2003 Adrian Bath, Intellisci Ltd., reproduced by permission]

Comparable scenarios for changes in shallow groundwaters in southern Europe through a glacial - interglacial climate cycle are illustrated here:

[©2003 Adrian Bath, Intellisci Ltd., reproduced by permission]

The types of rock formations that are being considered as hosts for radioactive waste repositories are those with low permeabilities. Groundwater moves very slowly in such rocks, and therefore it would take a long time for contamination leaking from a repository to travel to the biosphere where it could cause harm. Typical rock formations having low permeabilities are crystalline rocks and clay rocks. Crystalline rocks have igneous or metamorphic origins deep in the earth - the small amounts of groundwater in them is mostly contained in networks of fractures. Clay rocks have sedimentary origins, laid down as mud and silts in ancient oceans or lakes and since buried and compacted - water is now contained in microscopic pores.

In addition to the 'geosphere' barrier against leakage which is provided by rocks with low permeability, a repository is designed and built to contain the wastes and thus to prevent leakage even to the deep geosphere for a long time - the 'engineered' barrier. The high level of safety for a very long time into the future which is provided by this design for long-term waste storage or disposal is called 'multi-barrier containment'. We can be more confident about multi-barrier containment working well for a long time into the future if the conditions, especially the groundwater movements and compositions, around a repository are known to be stable.

Investigating groundwater stability is another way of describing the significance of palaeohydrogeology for assessing the safety of proposed locations for radioactive waste repositories. A major reason for constructing repositories deep (e.g. many hundreds of metres) underground is that stable rock and groundwater conditions at these depths will eliminate the threats to safety due to climate change that would occur if wastes were stored at the ground surface.

Deep groundwaters in rocks with lower permeabilities, such as those in which radioactive waste repositories are proposed, move very slowly and may contain waters that infiltrated into the ground ('recharged') at various times in the Quaternary or even earlier. Understanding these types of groundwater systems is a scientific challenge for palaeohydrogeology, in terms of sampling and testing the groundwaters and interpreting how they have evolved. The EC Euratom Community Research Programme has recently funded a number of projects: 'EQUIP' (Bath et al, 2000), 'PAGEPA' (Boulton et al, 2001), 'PHYMOL' (Marivoet et al, 2000), and studies at Palmottu (Blomqvist et al, 1998), which are summarised in a paper by Bath et al, 2000, and is presently funding the PADAMOT project to continue this research in support of radioactive waste management.

Palaeohydrogeology research for safety assessment of radioactive waste repositories has been discussed at international conferences:

Palaeohydrogeological Methods and Their Applications for Radioactive Waste Disposal. NEA Symposium, November 1992, Paris. OECD-Nuclear Energy Agency, Paris.

Glaciation and Hydrogeology: Workshop on the Impact of Climate Change and Glaciations on Rock Stresses, Groundwater Flow and Hydrochemistry - Past, Present and Future. NKS/SKI/SKB Symposium, April 1996, Stockholm. Report SKI 97:13. Swedish Nuclear Safety Inspectorate, Stockholm.

Use of Hydrogeochemical Information in Testing Groundwater Flow Models. NEA Workshop, September 1997, Borgholm, Sweden. OECD-Nuclear Energy Agency, Paris.