Evaluating Groundwater In a Permafrost Watershed Using Seasonal Geochemical and Isotope Discharge Trends, Ogilvie River, Yukon

Abstract

This study focuses on a mid-sized watershed of upper Ogilvie River (~4,500 km2 at the western extent of the 77,000 km2 Peel River basin) located at the northern extent of the discontinuous permafrost in central Yukon. Annual hydrograph is analyzed by characterizing the river flow components using geochemical tracers (anions, cations, and dissolved organic and inorganic carbon), stable isotopes (δ2H, δ18O, and δ13C) and radioactive isotopes (14C and 3H). The 2014 and 2015 flows are characterized by high weathering solute concentrations (Ca2+, Mg2+, SO42-, and DIC) in the winter that spike just prior to freshet, diluted to the lowest concentrations at the peak of freshet, and slowly recover throughout the summer and fall responding to overall flow fluctuations. Biogenic solutes (K+ and DOC) are lowest in the winter time and spike at the start of freshet. Two groundwater components, shallow and deep, were identified with corresponding tritium-based residence times of less than a year and three years, respectively. The local meteoric water line, developed based on the local precipitation samples δ2H=6.77·δ18O-24.54 (R2=0.98), was used to determine the recharge mix of the groundwaters: 65% rain and 35% snowmelt. The recharge waters interact greatly with soil (open system weathering) and carbonate bedrock (closed system weathering) as evident via the δ13C and F14C signatures of the active layer waters, shallow groundwater and deep groundwater. Radiocarbon of DIC proved to be an important tool to differentiate the weathering conditions by demonstrating F14C carbonate dilution resulting in groundwater F14C as low as 0.57. Based on the δ18O based hydrograph separation, deep groundwater comprised almost half of the annual discharge. The groundwater discharge reached its maximum flow of 129 m3/s during freshet when it contributed 44% to the flow. The potential groundwater recharge pathways include thermal contraction cracks in permafrost and bedrock fractures. The recharge is thought to occur over vegetated areas. In the summer, likely only large precipitation events result in recharge. In comparison to discharge, the annual DIC exports from the study catchment (5.6 gC/m2/yr) were disproportionally high in the winter time and the DOC exports (2.1 gC/m2/yr) varied proportionally with discharge.