The rock core and the boreholes of 7 drillsites, each one penetrating about 15 metres into the bedrock at a contaminated site, are used in a field and laboratory investigation of the fluid flow properties of the fractured orthoquartzite bedrock formation. Porosity and hydraulic conductivity estimates were obtained from a variety of hydraulic tests in the boreholes and different laboratory measurements on rock cores. Conclusions are drawn on the variation of porosity and hydraulic conductivity with the scale of measurement.
In most fractured rocks, the fracture system constitutes the primary flowpath for groundwater flow. However, the storage capacity of a rock mass is largely controlled by the rock matrix porosity. Groundwater storage, together with the flow processes, determine in a large measure the mass transport properties of a fractured rock mass. The evaluation of the relative importance of the fractures and the matrix in the groundwater flow system, and in the storage capacity, is a critical step in the assessment of the mass transport properties of fractured rocks. Recent compilations of data from different sites have documented the effect of scale on the estimation of the permeability of geologic media (Clauser, 1992). Possible scaling rules have also been proposed (e.g. Neuman, 1990). Only a few studies have documented the variation of permeability with scale of measurement at a single site (Rovey and Cherkauer, 1995), and to our knowledge none has considered the scale variation in both the porosity and the permeability at the same site. We describe a site study based on field and laboratory investigations of an orthoquartzite formation at a contaminated site, at Mercier, Quebec. Hydraulic testing in the boreholes at this site was described by Denis and Rouleau (199Ib). This paper presents the analysis of the fracture system, the porosity and the hydraulic conductivity of the rock mass based on both the laboratory and the field data, focusing on the variability of the results with the type of test and with the scale of measurement.
The Mercier site is located approximately 20 km south of Montreal, in a relatively flat area of the St-Lawrence Lowlands. The substratum consists of gently folded meta sedimentary rocks. The site is located in a very open synclinal structure plunging to the north-east (Globensky, 1986; Figure 1). The site is underlain by the Theresa Formation, which is about 80 m thick and generally comprises quartzitic sandstone and dolomite. At the contaminated site, the bedrock is completely covered by about 30 m of unconsolidated Quaternary deposits. In contact with the bedrock, a compact silty till ranges from 0 to 3 m in thickness. It is overlaid by about 30 m of heterogeneous fluvio-glacial deposits consisting of sand and gravel, with a few pebbles and blocks. The regional groundwater system flows to the southwest. Under natural conditions, groundwater flowing through the contamination site must circulate through the bedrock, mostly the Theresa Formation, before discharging to the Turgeon River or to the Chateauguay River.