Insignificant Groundwater Discharge in a Fractured Rock Watershed

Understanding the rate and pattern of groundwater discharge to surface water bodies is critical for watershed budgets, as a proxy for recharge rates, and for protecting the ecological integrity of lake and river ecosystems. The Tay River is a low gradient, warm-water river which flows over exposed and fractured bedrock or a thin veneer of coarse-grained sediments. Previous studies indicated that groundwater discharge to a 2 km section of the Tay River is insignificant. The objective of this study is to constrain the rate and pattern of groundwater discharge at the watershed-scale. Groundwater discharge points were identified by conducting detailed transects of the Tay River and other water bodies using a temperature, conductivity and radon-222 tracers. Surface water samples from twenty representative surface water bodies and the Tay River were analyzed for ä2H and ä18O. Daily flow rate data collected at three different locations along the Tay River were analyzed to determine if flow rate increases downstream during baseflow conditions. Results show that detailed transects identified minor and highly localized groundwater discharge locations to the Tay River or other surface water bodies even in regions underlain by potentially significant geological structures or exposed bedrock fractures. Stable isotope, temperature and conductivity data identified only one subsidiary stream with significant groundwater discharge. Tay River flow rate data indicate that flow rates do not increase downstream during baseflow. This integrated thermal, chemical, isotopic and hydraulic dataset indicates that the rate of groundwater discharge to the Tay River watershed is not significant and that discharge is localized but not focused at exposed geological structures or bedrock fractures. This conclusion implies that in the Tay River watershed groundwater and surface water is largely decoupled, which has significant ecological and water management implications.