Field evaluation of methods for determining hydraulic conductivity from grain size data

Determination of hydraulic conductivity (K) and its variation in space is often a major objective of hydrogeological site investigations. However, measurement of K at a high spatial resolution in sedimentary aquifers is a challenge. There are a number of field methods that can be used to determine K, although they differ greatly in terms of their spatial resolution.

One commonly used approach is to estimate K from grain size analyses, but the reliability of the resulting K estimates is unclear. The aims of this study are to compare frequently used formulas for the determination of K from grain size data for a broad range of sediment types and to evaluate how well these methods predict K. Sonic sampling was used to obtain minimally disturbed cores in a highly heterogeneous sedimentary aquifer and K values of grain size analyses from 108 core samples were calculated. Despite the high correlation of calculated K derived from different formulas, mean K values differed by several orders of magnitude between the formulas. For the evaluation of the reliability of the K estimates, high resolution direct push slug tests (DPSTs) were also performed in the close vicinity of the cores. A high correlation between ln(K_DPST) and ln(K_{grain size}) was found for most of the applied formulas. Nevertheless, sample heterogeneity, i.e. the presence of small clay layers in a generally highly permeable sample, led to K_{grain size} estimates that were significantly smaller than the K_DPST estimates. Based on these results, the applied formulas appear to be suitable for an initial assessment of aquifer K. However, considering the difference in calculated K mean values, results are not sufficiently reliable for the high resolution analyses of K variations needed for flow or transport modeling.