Transient characteristics of effective porosity and specific yield in bedrock aquifers

Abstract

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