Depth dependence and exponential models of permeability in alluvial-fan gravel deposits

To determine depth dependence of permeability in various geologic deposits, exponential models have often been proposed. However, spatial variability in hydraulic conductivity, K, rarely fits this trend in coarse alluvial aquifers, where complex stratigraphic sequences follow unique trends due to depositional and post-depositional processes. This paper analyzes K of alluvial-fan gravel deposits in several boreholes, and finds exponential decay in K with depth. Relatively undisturbed gravel cores obtained in the Toyohira River alluvial fan, Sapporo, Japan, are categorized by four levels of fine-sediment packing between gravel grains. Grain size is also analyzed in cores from two boreholes in the mid-fan and one in the fan-toe. Profiles of estimated conductivity, \( \overline{K} \), are constructed from profiles of core properties through a well-defined relation between slug-test results and core properties. Errors in \( \overline{K} \) are eliminated by a moving-average method, and regression analysis provides the decay exponents of \( \overline{K} \) with depth. Moving-average results show a similar decreasing trend in only the mid-fan above ∼30-m depth, and the decay exponent is estimated as ≈0.11 m⁻¹, which is 10- to 1,000-fold that in consolidated rocks. A longitudinal cross section is also generated by using the profiles to establish hydrogeologic boundaries in the fan.