HOMOGENEOUS BORDEN? A NEW LOOK AT THIS CLASSIC AQUIFER

Classic experiments conducted in the Borden Aquifer have often been cited as examples of transport in a relatively homogeneous setting (e.g., a “sand box” with limited range in K); however, recent work (e.g., Ritzi and Allen-King 2007) indicates that a greater range in physical heterogeneity exists at the site. Additionally, it has been demonstrated that the geochemical properties of different facies may vary, thus creating heterogeneity in reactivity (such as sorption) that is also correlated with sedimentary units. To explore the physical and geochemical heterogeneity at this classic site, we exposed 17 approximately 20 x 1.5 m panels in a sand quarry located approximately 2 km west of the Stanford-Waterloo natural tracer experiment study site. This gave us an approximately 10 m vertical exposure with 20-40 m lateral exposures at several orientations from which we can interpret the 3D sedimentology of the site. Sediments at this quarry appear to be stratigraphically equivalent to those at the Stanford-Waterloo test site. To evaluate and quantify the facies distributions, we acquired high-resolution terrestrial lidar scans and photographs of each exposure. Facies were mapped onto the photographs, which in turn will be draped onto the lidar point clouds to produce a map of the 3D spatial distribution of facies. The facies observed are consistent with a relatively high-energy, rapidly aggrading glacio-lacustrine setting, including moderately-sorted medium sand with gravel found in broad scour fills, horizontally-laminated to low-angle bedded medium to fine sand with some horizons consisting of climbing ripples, laterally-extensive contorted silt beds, and laterally-extensive silty clay beds. By using lidar in combination with photographic facies architecture interpretations, we will produce quantified facies distributions for developing a new generation of numerical groundwater flow and contaminant transport models that more realistically capture geologic geometries. Based on the facies observed at this site, the variability in K is much greater than previously described for the Borden Aquifer.