Characterization and Estimation of Hydraulic Properties in an Alluvial Gravel Vadose Zone

Understanding the physical and hydraulic properties of the vadose zone is important for modeling land use effects on groundwater quality. This study used a variety of characterization methods to derive conceptual understanding and estimates of hydraulic properties of a coarse alluvial gravel vadose zone in New Zealand. Sandy gravel (SG) material constituted approximately 90% of the vadose zone, with the remainder comprising sand lenses and open-framework gravels. The gravel content of the SG material was approximately 70% (v/v) (range 68–73%). The water content of the bulk SG material (43 samples) ranged from 3.5 to 13.9%. The average bulk density of the SG material was 2.20 g/cm³ (range 2.00–2.33 g/cm³) giving an average calculated porosity of 17%. The average porosity of the open-framework gravels was 34% and these gravels were often coated with 2- to 3-mm-thick deposits of amorphous Fe and Mn oxides. Neutron probe (NP) depth profiles indicated unsteady conditions, with variable water contents with depth and time reflecting the vertical heterogeneity and the variably saturated state of the vadose zone. Time series NP data to 3 m indicated water content in the alluvial gravels responded quickly to soil drainage events, and saturation variability was greater in the sand lenses and the SG material immediately underlying. When compared with derived water retention curves, variability in the water content equated to significant fluctuation in unsaturated hydraulic conductivity (K ᵤₙₛₐₜ). Tension infiltrometer measurements were variable but were within the range of the K ᵤₙₛₐₜ estimates from site-average particle size distribution data. The gravel-transformed, texture-based models used to estimate saturated water content values in this study appeared to underestimate the measured values.