Groundwater Recharge, Flow and Stable Isotope Attenuation in Sedimentary and Crystalline Fractured Rocks: Spatiotemporal Monitoring from Multi-Level Wells

Thumbnail Image
Wright, Stephanie N.
Novakowski, Kent
Fractured Rock , Stable Isotopes , Snowmelt Recharge , Solute Transport , Regional Groundwater Flow , Field Characterization
The complexities of fractured rock systems necessitate the use of multi-level monitoring wells for the measurement of isotopic tracers with depth. These methods, however, are rarely employed in groundwater flow studies, particularly involving temporal isotope measurements in multiple rock types. To understand the use of isotopic snowmelt tracers for exploring surface connections and recharge, an investigation was undertaken at three geologically-distinct field sites in south-eastern Ontario, Canada. Results of constant head injection tests at 2-m intervals were used to identify and isolate high transmissivity zones (2–3 in each well) for the construction of eight multi-level wells (22 intervals) for groundwater sampling. Deuterium and oxygen-18 were measured in rain (n=64), snow (n=67), soil water (n=118) and groundwater (n=594) on a bi-weekly to monthly basis over two seasonal snowmelt cycles (> 1.5 years). In combination with stable isotopes, measurements of hydraulic head, groundwater temperature, specific conductance and two rounds of tritium sampling contributed to the development of a conceptual model for snowmelt recharge and flow in sandstone, limestone and crystalline rock. Seasonal variation of ẟ2H and ẟ18O was observed in all rock types indicating recent recharge and strong surface connections. The dampening and residence time of the snowmelt tracer increased with depth in crystalline rock. Attenuation was highest in higher porosity sedimentary rock and strong confining conditions were observed to inhibit recent recharge. Substantial isotopic attenuation measured in the overburden indicated that the strong snowmelt signal measured in groundwater stems from water infiltrating through areas with thin or no overburden. Identification of the periodic trends and distinct isotopic horizons in this study would not have been possible without the use of multi-level wells.