Mathematical modelling of groundwater flow at Sellafield, UK

Sellafield in West Cumbria was a potential site for the location of the UK' s first underground repository for radioactive, intermediate level waste (I LW). The repository was to lie around 650 m beneath the ground surface with in rocks of the Borrowdale volcanic group (BVG), a thick suite of SW dippin g, fractured, folded and metamorphosed Ordovician meta-andesites and ignimb rites. These are overlain by an onlapping sequence of Carboniferous and Per mo-Triassic sediments. In situ borehole measurements showed that upward tre nding fluid pressure gradients exist in the area of the potential repositor y site, and that there are three distinct fluid types in the subsurface; fr esh, saline and brine (at depth, to the west of the site). Simulations of f luid flow in the Sellafield region were undertaken with a 2D, steady-state, coupled fluid and heat flow simulation code (OILGEN). In both simplified a nd geologically complex models, topographically driven flow dominated the r egional hydrogeology. Fluids trended persistently upwards through the poten tial repository site. The dense brine to the west of the site promoted upwa rd deflection of topographically driven groundwaters. The inclusion in hydr ogeological models of faults and variably saline sub-surface fluids was ess ential to the accurate reproduction of regional hydraulic head variations. Sensitivity analyses of geological variables showed that the rate of ground water flow through the potential repository site was dependent upon the hyd raulic conductivity of the BVG, and was unaffected by the hydraulic conduct ivity of other hydrostratigraphic units. Calibration of the model was achie ved by matching simulated subsurface pressures to those measured in situ. S imulations performed with BVG hydraulic conductivity 100 times the base cas e median value provided the "best-fit" comparison between the calculated eq uivalent freshwater head and that measured in situ, regardless of the hydra ulic conductivity of other hydrostratigraphic units. Transient mass transpo rt simulations utilising the hydraulic conductivities of this "best fit" si mulation showed that fluids passing through the potential repository site c ould reach the surface m 15 000 years. Simple safety case implications draw n from the results of the study showed that the measured BVG hydraulic cond uctivity must be less than 0.03 m year(-1) to be simply declared safe. Rece nt BVG hydraulic conductivity measurements showed that the maximum BVG hydr aulic conductivity is around 1000 times this safety limit. (C) 1999 Elsevie r Science B.V. All rights reserved.