Technical Report NTB 87-34

Hydrogeological Characterization of the Stripa Site

This study was initiated in January, 1986, to determine a) if the permeability of the rock mass in the immediate mine area was anisotropic, b) the effective and total fracture porosity distributions based on field and laboratory data and c) the three dimensional configuration of the groundwater flow system at Stripa in order to properly interpret the hydrogeological, geochemical and isotopic data.

The borehole packer test data show that on average SBH1 and SBH2 have lower permeabilities than SBH3. This is consistent with the pattern that one would expect for the orientation of the boreholes with respect to in-situ stresses. Laboratory studies showed a strong decrease in fracture permeability with increase in normal stress in core samples containing natural fractures suggesting that anisotropy to flow in the vertical direction must exist, since in-situ stresses increase with depth. The contribution of fracture geometry to the rock mass flow anisotropy was analyzed using a fracture network generator to simulate fracture networks in three orthogonal planes. In the horizontal plane the relative flow rates indicate an anisotropy factor of 1.5 with the principal direction oriented North-Northwest. Similar degrees of anisotropy were determined for the two vertical planes.

The total and flow porosities of single fractures from Stripa were determined in the laboratory using a resin impregnation technique. The equivalent uniform apertures for two samples, computed using the measured variation in fracture aperture and resin thickness, were consistent with apertures computed from the hydraulic data. The mean effective porosity contributed by the fractures in the rock mass calculated by combining the aperture data from the field packer tests with the fracture statistics for trace length and spacing was about an order of magnitude less than the porosity computed using the hydraulic data from the laboratory tests on single fractures in the core samples. More important, the porosity calculated using resin thickness data was almost a factor of 100 greater than that computed using the field data.

The three-dimensional numerical model gave mine inflows that were consistent with the measured mine inflows with perturbations extending to at least 3'000 m of depth. Transit times predicted from the flow tube calculations were much shorter than those predicted from the existing geochemical and isotopic data for porosities developed from field data. Corrections for the higher porosities determined from laboratory studies gave transit times that were more consistent with those inferred from isotope studies.