Water and micro-particles transfer through unsaturated intact soil cores

Solute behaviour in soils under unsaturated conditions might be different than under saturated conditions as different pore sizes are active. Classical theory of water transfer through porous media sees pores as a junction of tubes of different sizes and uses Jurin’s law to determine the size of active pores depending of water tension. In reality, flow pathways more complex and also depend on pore connectivity and other pore network morphology characteristics. Well-connected macropores enable short-circuiting natural soil retention processes resulting in fast transport of contaminants through soils. To date our ability to predict such preferential flow processes is poor.

Combining measurements of preferential flow processes through well-structured soils under unsaturated conditions with visualizing and quantifying soils’ macropore networks might contribute to improved process understanding that can lead to the development of a new generation of flow models. Microbial transport through NZ soils is an issue of concern because most livestock is grazing during the entire year plus dairy effluents are commonly applied to land. Both practices harbor the risk of groundwater contamination with fecal coliforms. Microspheres have been shown to be useful surrogates for microbial transport through soils. There is some evidence that drying and wetting processes of soils between intermittent rainfall events might rapidly mobilize bacteria through soil macropores.
In this project, the fate of microspheres after application to soils from two vineyards with different management systems (organic vs. conventional) will be analysed in laboratory experiments. The student will conduct state-of-the-art solute transport experiments to investigate the transport behavior of microspheres under steady-state flow conditions after their application to intact soil cores. The experiments will use PFR’s recently developed fully-automated solute transport apparatus, which allows to study solute transfer in intact cores under unsaturated conditions, by applying a tension at the top and bottom of the cores with tension disc infiltrometers. The student with investigate the behavior of a tracer and a fluorescent microsphere under different tensions and intermittent unsaturated flow regimes, i.e., after the pulse application, additional flow events will be simulated to observe potential remobilizing of previously sequestered microspheres. After completion of the experiments, the same cores will be either scanned with an X-ray Computer Tomograph to obtain the 3D pore network at around 100 µm, or cut into vertical slices to image the distribution of the microspheres by photography.

The objectives of this study are (i) to determine the mobility of microspheres, used as surrogates for bacteria, during wetting and drying cycles in NZ soils, and (ii) to assess the impact of different vineyard management practices on macropore networks and transport of microspheres.

Pour candidater : Adresser un CV et une lettre de motivation par email à l’adresse ci-dessous
karin.mueller plantandfood.co.nz
celine.duwig ird.fr