Nirex Safety Assessment Research Programme. A natural analolgue study of cement-buffered, hyperalkaline groundwaters and their interaction with a repository host rock

This report is the final compiled report of work undertaken in the Jordan Natural Analogue Project (Phase II). It describes, in the main, a programme of field work and interpretation based on the Maqarin site in Northern Jordan, with additional data from sites in Central Jordan. The sites comprise metamorphosed zones with a mineralogy resembling 'cement' surrounded by clay biomicrite. At Maqarin, there is an active hyperalkaline groundwater system. The sites provide natural analogues of key processes that might occur within the 'alkaline disturbed zone' of a cementitious repository. The main aim of Phase II was to investigate the interaction between hyperalkaline waters and unaltered clay biomicrite. The nature and sequences of secondary alteration products forming within fractures are described, along with evidence of wallrock alteration adjacent to fractures. Detailed geochemical analyses were carried out to determine the distribution of major and trace elements both within fracture infill, and in profiles within the fracture wallrock. Groundwater samples were collected in order to improve on the extent and quality of data obtained during Phase I (reported in Nagra NTB 91-10). These data were used as input to geochemical modelling studies of trace element solubility and speciation, and also modelling of water-rock interaction using coupled chemistry/transport codes. Supporting studies of isotope hydrogeology provided information on the age of the system and a further study of microbiological activity at the site was undertaken. The Maqarin project has provided much insight into the reaction of high pH water with carbonate rock in a system that has evolved over a geological timescale. This understanding can be used to guide the performance assessment of repository systems involving cementitious materials. The main technical work reported here was carried out between April 1992 and March 1993. Phase II of the Maqarin project was funded jointly by Swiss Nationale Genossenschaft fuer die Lagerung radioaktiver Abfaelle (NAGRA), United Kingdom Nirex Limited (Nirex), and the Swedish Svensk Kaernbraenslehantering AB (SKB). (author)