GEOTOMOGRAPHY OF A SHALLOW SAND AQUIFER IN NORTH-EASTERN ONTARIO

Ground-probing radar, seismic and resistivity surveys were used to identify subsurface structures and topography that may correlate with strong chemical gradients in a shallow glaciofluvial sand aquifer, 10km NW of Deep River, Ontario. Iron oxide springs led to the discovery of the small aquifer, and yearly analysis of ground water chemistry at 47 Piezometers have identified persistent, localized chemical gradients of ferrous, ferric, sulfate, and sulfide ions (Ferris, 2006). Microbial organisms have been identified as the redox protagonists (Shirokova, 2011). The radar grams collected along nine profiles (200 and 400 MHz antennae) show a subsurface boundary between 0.8m and 3.4m thick, distinguishable by its many hyperbola shaped reflections, and varying topography. This boundary can be mapped to an outcrop of glacial till, and it is believed that the hyperbolas are reflection off cobble and boulders in the till. Furthermore, decreases in the elevation of this boundary correspond to an increase in ionic concentration of sulfide, sulfate, ferric, and ferrous ions. The layer above the suspected till is identified to be glaciofluvial sand from outcrops along the surveys. Resistivity values from eight different lines (24 or 48 electrodes at 1m spacing) vary in general from 2000-6000 Ωm at 0 to 2 m depth, to less than 1000 Ωm from 2 to 5m depths. A permanent line of 24 electrodes spaced 1m apart, crossing the highest concentration of iron ions, was monitored over 8 weeks, closely observing weather to see how the resistivity of the ground water varied over time. These surveys consistently show low resistivity values in areas of high ionic concentration and high resistivity values to the top of the vadose zone including below dry sand outcrops and within tree root systems of trees that can be seen at the surface. Weather, including hot spells and torrential downpours have a lag time before difference in resistivity is observed; also, it seem to have very little effect on resistivity values over time. Seismic survey was conducted using 24-channel geometrics geode with 40 Hz vertical geophones at 1 m spacing and 8 m shot spacing, for a total of 120 m in length. Data show clear slope changes that suggest three boundaries, consistent with earlier interpretation of wet sand, till and crystalline rock, determined using GPR and resistivity survey.