USING MULTIPLE ENVIRONMENTAL TRACERS AND LUMPED PARAMETER MODELS TO CHARACTERIZE FRACTURE FLOW SYSTEMS

The characterization of fractured groundwater systems remains one of the most complex problems facing groundwater scientists and managers. Techniques for investigating fracture system properties vary from detailed borehole investigations and discrete fracture network models to rough fracture network property estimation and effective porous media modeling. Highly detailed investigation and modeling may be cost prohibitive and are usually limited in spatial scale. Commonly, water resource management and protection studies require characterization at regional scales with tight budget constraints. Environmental tracers and their interpretation through Lumped Parameter Models (LPMs) can be used constrain groundwater flow and transport properties at scales applicable to resource management and protection. However, the use of LPMs in fracture flow systems is complicated by matrix diffusion and low storage volumes resulting in significant flow heterogeneity and transience. Here, new LPM models are developed which account for matrix diffusion in fractured systems and are used to investigate the transit time distribution in fracture networks discharging to the Bedrichov tunnel in the Czech Republic. These models are used to provide constraints on the transit time distribution, the mean age and seasonality of recharge by fitting stable isotopes, CFCs and tritium measured in fracture discharge. We find that the mean transit time is reasonably robust to structural model uncertainty in the transit time distribution, seasonality of recharge, and flow transience. Our results indicate LPM can provide useful constraints on fracture flow systems at a scale and cost reasonable for water resource management and groundwater safety assessment purposes.