Comparative effects of drying and disturbance on the hydraulic parameters of a recent lapilli deposit overlying fine andosols case of the Hudson Volcano, Chilean Patagonia = Efecto comparativo del secado y perturbaciones en los parámetros hidráulicos de un lapilli reciente que cubre andosols fino: caso del Volcán Hudson, Patagonia Chilena

Abstract

Explosive volcanic eruptions have significant long lasting effects on the landscape due to the deposition of tephra and subsequent formation of andosols. Usually, tephra has high water retention capacity, high permeability, as well as low bulk density when compared to other soil types, and these properties may be irreversibly affected by natural or human disturbance. Together with the high spatial and temporal heterogeneity from successive eruptions, these factors taken in concert can be significant in terms of hydrological change, and risk scenarios for soil stability/erosion. However, accurate hydraulic parameters for tephra, considering the broad range of grain sizes and the effects of disturbance, are scarce. Bulk density (ρt), porosity (η), volumetric water content at field capacity (ϴfc) and saturated hydraulic conductivity (Ks) were measured for minimally disturbed cores for two contrasting soils in the plume of the Hudson Volcano in southern Chile (46⁰ S): a recent, coarse tephra, and an old, fine andosol. Cores were then physically reworked, subjected to two successive treatments of drying at 45⁰ and 105⁰ C, and the parameters were re-measured for each case. Coarse tephra and fine andosol bulk density (0.78 ± 0.03 and 0.53 ± 0.04 g cm-3, respectively) were least affected by the manipulation, with a 17% decrease for the fine material following only the 45⁰ C treatment. The respective porosities (60 ± 1.1%; 75 ± 1.2 %) and moistures at field capacity (32 ± 0.8%; 60 ± 1 %) for the coarse and fine materials decreased progressively with each treatment, with 2x stronger effect on the coarse (-32% porosity, -34% field capacity) as compared to the fine soil (-19% and -17%, respectively). Ks (coarse: 20.6 ± 14.8 cm h-1; fine: 3.8 ± 4.4 cm h-1) was substantially affected by disturbance (overall increase by 1.5x and 3.1x, respectively) for the 45⁰ C treatment. Our results are the as yet southern-most characterization for these volcanic soil parameters. In addition, we provide evidence of natural reworking of both coarse and fine horizons at the field site, together with laboratory-demonstrated effects of disturbance on properties of volcanic soils of widely ranging clast sizes and age. We emphasize the need for a more comprehensive understanding of the hydraulic parameters of tephra, their sensitivity to alteration following disturbance, and the corresponding implications for engineering applications and catchment hydrology.