Insignificant Groundwater Discharge to Lakes in a Large Fractured Rock Watershed

Understanding the rate and pattern of fresh groundwater discharge to lakes and rivers is critical for watershed budgets and for protecting the ecological integrity of lake and river ecosystems. A 900 km2 study watershed contains a river and over 3000 lakes and wetlands, mostly underlain by exposed crystalline bedrock or a thin veneer of coarse-grained sediments. 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 river and lakes using temperature, conductivity and radon-222 tracers. Surface water samples from representative lakes were analyzed for (delta)2H, (delta)18O, radon- 222 and chloride for three consecutive summers. Radon and chloride concentrations are used in a steady- state advective model to determine groundwater fluxes to the representative lakes. The detailed transects identified minor and highly localized groundwater discharge locations to the river and lakes 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. The steady-state model indicates that the groundwater flux to lakes is generally less than 0.1 percent of the total input. This integrated thermal, chemical, isotopic and hydraulic dataset indicates that the rate of groundwater discharge to lakes in this crystalline bedrock 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 watershed groundwater and surface water is largely decoupled, which has significant ecological and water management implications.