Transient characteristics of effective porosity and specific yield in bedrock aquifers

Abstract

Studies of bedrock aquifers have often treated effective porosity and specific yield as having a similar, time-invariant value. Here, we apply a series of tests of specific yield and effective porosity at multiple temporal scales to the Chalk aquifer of east Hampshire, England. These include analysis of natural water level, discharge and water quality variations, numerical model calibrations, hydrograph recession analysis, pumping tests, and tracer tests using both injected and environmental tracers. Values of both effective porosity and specific yield are shown to be transient phenomena that are dependent upon the temporal test scale, an effect which results from the dual-porosity structure of the aquifer. Over short time periods, aquifer response is dominated by preferential flow through the low-storage, high-permeability fracture network; effective porosities on a timescale of hours to days are 0.0001–0.001. Over longer time periods, aquifer response becomes increasingly dominated by the high-storage, low-permeability matrix, with the effective porosity rising to approximate total porosity (0.39) on a timescale of decades. Specific yield also increases as a function of timescale, though there is much less variability than with effective porosity. Our findings demonstrate that it is critical to use an effective porosity value appropriate to the timescale when making transport calculations. Preferential flow through fracture networks is common in bedrock aquifers. Consequently, the transient nature of both effective porosity and specific yield that has been demonstrated in the Chalk aquifer is likely to be broadly representative of most bedrock aquifers.