Tracing Seasonal Snowmelt Recharge in Three Distinct Fractured Rock Field Sites with Thin Overburden Using Oxygen and Hydrogen Isotopes

The unique isotopic signature of oxygen and hydrogen in snowmelt is being used as a tracer for recharge to depth in a fractured rock aquifer system. Arrival time and residence time of the isotopic tracer may reveal rates of recharge as well as transport mechanisms for surface contaminants. Three geologically distinct field sites in eastern Ontario are being used to compare these mechanisms in an Ordovician limestone, Precambrian crystalline rock and a gneissic formation. These sites are overlain with thin or no overburden and range in topography from flat-lying to artesian conditions. A total of seven multi-level wells across the sites are being used to trace two snowmelt events that occurred in February and April, 2017. The wells range in total depth from 90 feet to 180 feet. Lysimeters are installed at the soil-bedrock interface to obtain isotope samples of recharge entering the bedrock system after undergoing soil attenuation. Snow and rain were also collected to quantify attenuation of the seasonal isotopic inputs. Current trends in the wells show rapid, but highly attenuated signals following the melting events. However, some wells in the gneissic and Precambrian formations show prolonged response to snowmelt initially in the shallow intervals and two months later in the middle and deep intervals. As of mid-July, these wells continue to decrease in isotopic value, indicating long-term response to snow melt. Potential mechanisms for the observed responses continue to be explored, but may include overburden thickness, local versus regional recharge and multiple fracture contributions at varying quantities and rates. Sampling will continue bi-weekly with updated results and conceptual models presented at the conference.