Pore network and water retention characteristics of volcanic porous media

Aggregate media are often characterized by multi-porous systems, which have structural and water retention characteristics that depend on the complex interaction between intra- and inter-aggregate pores. Here we investigate the structure and water retention dynamics of rigid aggregate volcanic materials. In particular, we focus on commercially used pumices, lapilli and zeolites. The aim was to estimate the air and water content through complex dual-porous systems, and thus to evaluate their suitability for vegetation growth. Both inter- and intra-aggregate characteristics were determined by means of mercury intrusion porosimetry, X-ray microtomography and water retention curves. The wilting point was determined with pressure plates, a dew point hygrometer and the sunflower method to assess their reliability at small matric potentials. Results indicate that aggregate porous media were bimodal and their heterogeneous pore network affected the water retention dynamics because (i) the large inter-aggregate pores allowed a rapid drainage near saturation and (ii) the intra-aggregate porosity held water available for root uptake and plant growth. In contrast, volcanic powders were less affected by the inter- and intra-aggregate dual-porosity. The use of a dew point hygrometer instead of pressure plates for determining small matric potentials is also suggested because pressure plates might over-estimate the water content because of poor plate and soil conductance. However, the reference potential at wilting point should be set at values greater than -1471.5 kPa (-784.8 kPa) to consider the interaction between plant roots and porous media with small hydraulic conductivity. Results from this work indicate that aggregate multi-porous media allow the simultaneous supply of oxygen and available water for plants, although the heterogeneous nature of the pore network involves uncertainties regarding water balance and root-matrix interactions.