Flow Accounting: a new hybrid of hydrogeological and tracer test techniques to improve site characterization in fractured crystalline rocks

摘要

Many nuclear waste repository proposals are based on demonstrating that they will retain leached radionuclides within a kilometer-scale block for tens of thousands of years. Although interference-style pumping tests can measure suitable rock volumes within time frames of a few months, tracer tests can only be performed either within very limited size scales or on inappropriately transmissive regions (fault zones) of the repository volume. The "flow accounting" technique is a hybrid. Two pumping tests are performed consecutively. The first is a conventional interference-style pumping test. The second involves injecting into the observation points of the first test in order to retain equilibrium head. The technique is successful when the abstraction is matched by the total of the various injections: the flow is "accounted for". Within limits, the distribution of specific storage and its link to transmissivity is then known and the system is at or approaching pressure equilibrium. Injected water is still far from the abstraction point but the steady-state flow field is a highly suitable environment for smaller-scale, fully conserving, directly linked, tracer tests. The technique also demonstrates that there are no unforeseen highly transmissive regions within the proposed repository volume. The technique has been fully simulated in fractured rock suitable for a repository.