Assesment of preferential flow path connectivity and hydraulic properties at single-borehole and cross-borehole scales in a fractured aquifer

Preferential flow path connectivity is generally cited to explain scaling effects in hydraulic properties [Hsieh, P.A., 1998. Scale effects in fluid flow through fractured geological media, Scale dependence and scale invariance in hydrology. Cambridge University Press, pp. 335­353; Illman, W.A., in press. Strong evidence of directional permeability scale effect in fractured rock. Journal of Hydrology]. However, this information is rarely available in the field. In this study, we present a characterization of flow paths connectivity at the Ploemeur fractured crystalline aquifer from cross-borehole flowmeter tests. We show that high transmissivity zones are connected over distances of at least 150 m all over the site. In parallel, we synthesize hydraulic properties estimates obtained at this site from field techniques having distinct scales of investigation: single borehole flowmeter experiments, cross borehole flowmeter experiments and long term pumping tests. We find that borehole scale variability of transmissivity estimates vanishes at larger scale and that the transmissivity converges towards the high values of the transmissivity distribution. This effect may be explained by the organization of the flow field in the subsurface, and particularly the good connectivity of the permeable zones all over the site.