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(KDPST) and ln(Kgrain 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 Kgrain size estimates that were significantly smaller than the KDPST 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.