ABSTRACT: Evaluation of glaciofluvial aquifer heterogeneity and sedimentary architecture using multi-frequency ground-penetrating radar.

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Abstract

Glaciofluvial deposits are host to highly productive aquifers but their intrinsic heterogeneity and stratigraphic complexity can be a challenge for estimating hydrogeologic properties. In this study, spatial trends in heterogeneity and sedimentary architecture of a coarse-grained glaciofluvial deposit were investigated in a gravel pit near Limehouse, Ontario using multi-frequency ground-penetrating radar (GPR). The gravel pit is one of several sites in Halton Region being considered as possible locations for artificial groundwater recharge. The well-exposed outcrops in the pit (> 10 m) allow direct comparison of radar and outcrop lithofacies, and provide an ‘outcrop analogue’ for regional glaciofluvial aquifers present in the shallow subsurface. GPR surveys were acquired using four different frequencies (25, 50, 100 and 250 MHz) over a 300 × 22 m grid on the gravel pit floor and along an adjacent outcrop and roadway. Profiles were acquired at 2 m line spacing using D-GPS positioning and processed to a quasi-3D volume in GPR-SLICE software. Selected profiles for each frequency were assembled in a single 2-D profile (> 30 m depth) to optimize radar resolution and penetration depth. Radar reflection patterns were grouped into radar facies and interpreted using architectural element analysis (AEA) with reference to a well-exposed outcrop and nearby geophysically logged borehole (MW-22). Using this approach several distinctive lithosomes were identified and their geometries mapped out over the pit base. The lithosomes include accretionary elements (AC), consisting of both horizontal and low-angle (2-5°) clinoformal reflectors and incised channel elements (CH), indicated by concave up reflectors that truncate underlying reflection patterns. The lithosomes and outcrop lithofacies are characteristic of deposition within a high-energy glaciofluvial environment such as a sub-aerial braided river (e.g. outwash plain) or sub-aqueous fan-delta.Comparison of the radar profiles with borehole and outcrop data indicates that lithosome bounding surfaces are associated with major changes in sediment texture. Lithosomes defined using radar reflection patterns can therefore be used to predict changes in sediment grain size and associated hydrogeologic parameters (e.g. hydraulic conductivity). For example, spatial trends in hydraulic conductivity in the Limehouse deposit are likely to be strongly controlled by coarse gravel-filled channel elements (CH), and can be predicted from the spatial distribution and subsurface geometry of these elements. The results demonstrate that multi-frequency GPR is a viable approach for evaluating spatial trends in sedimentary heterogeneity and can be applied at other sites under consideration for artificial recharge projects.

Bibliography of Canadian Geomorphology