BGR Bundesanstalt für Geowissenschaften und Rohstoffe

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Development of noninvasive methods for soil physical and hydraulic characterization of the vadose zone

Begin of project: January 1, 2014

End of project: December 31, 2017

Status of project: September 1, 2015

The hydraulic characterization of the vadose zone is of enormous importance for predicting groundwater recharge processes triggered by natural rainfall or artificial infiltration, for predicting solute transport processes in the near subsurface and for assessing the risk of groundwater pollution. Conventional soil physical methods are invasive as they require collecting samples, digging trenches, drilling boreholes or similar activities that may change the hydraulic properties of the investigated medium. Thus, noninvasive methods are needed (Fig. 1).

Fig. 1: Installation of a precipation device and electrode array for performing an infiltration experiment to be monitored by electrical resistivity tomographyFig. 1: Installation of a precipation device and electrode array for performing an infiltration experiment to be monitored by electrical resistivity tomography Source: BGR

Geophysical methods such as electrical resistivity tomography (ERT) and surface nuclear magnetic resonance (surface NMR) are applied with no or at least minimized invasiveness, but exhibit significant limitations regarding an accurate soil physical interpretation. The ERT method can provide the distribution of electrical resistivity in the subsurface with high spatial and temporal resolution. The corresponding estimates of water content, however, suffer from high uncertainties (Fig. 2) as resistivity methods are in principle ambiguous regarding content and mineralization of pore water. The surface NMR method on the other hand provides direct measurements of water content, but with much lesser spatial and temporal resolution compared to ERT. When applied alone, each of these two methods is not sufficient for identifying hydraulically effective parameters in the nearest subsurface.

Fig. 2: (a) to (c) Results of a 3-D ERT monitoring experiment, (d) estimation of the infiltrated amount of water from ERT data with corresponding uncertainty intervalsFig. 2: (a) to (c) Results of a 3-D ERT monitoring experiment: visualization of the infiltration front at different time steps after beginning of the precipation experiment, (d) estimation of the infiltrated amount of water from ERT data with corresponding uncertainty intervals

In this project, monitoring strategies are developed that combine both ERT and surface NMR to take advantage of the individual benefits of each method. In doing so, novel inversion strategies are developed that consider the physical basics of water transport through the vadose zone. In this way, hydraulically effective soil parameters and site-specific soil properties controlling infiltration processes are characterized. In addition, application strategies are developed to increase the measurement progress of the method in the field and to gather lateral information much faster. The final output of this project is of particular importance for (semi-)arid regions with a high potential for rare but heavy rainfall. When falling on dry soils, such rainfall events lead to preferential flow, clogging, and/or water repellency - effects that are seldom included in common modeling approaches as relevant data to characterize those effects are only available on small scales after intensive investigation. We expect that the combined use of ERT and surface NMR improves the observation and prediction of infiltration processes and supports the groundwater management regarding natural and artificial groundwater recharge, groundwater protection and precision farming.

Literature:

Relevant papers, posters, and presentations

Contact 1:

    
Dr. Stephan Costabel
Phone: +49-(0)30-36993-391
Fax: +49-(0)30-36993-100

Contact 2:

    
Dr. Ursula Noell
Phone: +49-(0)511-643-3489
Fax: +49-(0)511-643-3662

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