Consideration of boreholes in modeling of the regional-scale groundwater flow in a fractured rock

To evaluate the necessity of considering borehole structures in modeling regional groundwater flow in a fractured rock, the groundwater flow in natural and long-term pumping conditions at the Olkiluoto island, which is the selected site for a radioactive waste repository in Finland, was simulated. The structures of boreholes installed in the site were considered in modeling using 1-D finite elements. The recharge rate and the hydraulic conductivity of the overburden were calibrated using observed hydraulic heads at the boreholes, and the results were regarded as the basic case. To examine the influences of boreholes, two cases were simulated: one where the sizes of the fractured zones were expanded to infinity and the other where the borehole structures were ignored. The results of each simulation showed that the effect of borehole structures, which could be influential conduits of groundwater in fractured rocks, were definitely demonstrated in all cases, especially in the simulation of pumping test, and it was necessary to consider the borehole structures when the groundwater flow system in a fractured rock was modeled although it is a regional-scale one.

► The necessity of considering borehole in modeling groundwater flow is evaluated.
► We simulate the regional groundwater flows in natural and pumping conditions.
► We compare the effect of borehole structures on simulation to that of fracture zones.
► Results show boreholes are influential conduits for groundwater in fractured rocks.
► The borehole should be considered even in modeling regional groundwater flow.