Geochemical modelling of secondary calcite formation and its composition in deep saline groundwaters : a case study

This section describes the work carried out for WP4 of PADAMOT by UK partners Intellisci and British Geological Survey (BGS). The work comprised the simulation of hydrogeochemical processes that are involved in the precipitation of late-stage secondary calcite. Simulations were carried out using batch-reaction modelling with the geochemical speciation and equilibrium computer program PHREEQC2 (Parkhurst and Appelo, 1999) and fully-coupled flow, transport and reaction modelling with the BGS program PRECIP (Noy, 1998) and the Universidade da Coruña program CORE2D (Samper et al., 2000). The overall purpose of doing the simulations was to mimic the geochemical reactions that might have accounted for precipitation of late-stage calcites and to study the ways that variations in the contents of the redox-sensitive trace elements Fe and Mn in the calcites are related to variations in Fe and Mn in co-existing groundwaters and thus to the redox conditions. The insights gained from this modelling should provide understanding that improves confidence in the interpretations of data acquired in WP2 of PADAMOT. Work was carried out in several stages between early- 2002 and end-2004.