To address the objectives detailed above, the INFILTRON project combines experimental investigations and modelling at different scales and relies on four scientific tasks.
Two tasks address the development of INFILTRON infiltrometer, INFILTRON-exp, and related numerical model, INFILTRON-mod. The other two tasks deal with the design and characterization of nano-tracers and their validation regarding their capability to mimic several emerging pollutants and bacterial taxa.
Task 1 – INFILTRON-exp
In more details, task 1 will design and develop a new infiltrometer INFILTRON-exp for water infiltration and nano-tracer injection into the soils. The device will be much larger than usual devices to address the meter scale, which seems much more appropriate for investigating preferential flows than usual devices. A specific protocol will be designed to monitor water infiltration at surface, around and below the infiltrometer, using photographs and geophysical methods (i.e. Ground Penetrating Radar – GPR). This data will allow to quantify the infiltration function and understand flow processes. Similarly, a specific protocol will be designed for the injection and the monitoring of the nano-tracer at surface and below the infiltrometer, using GPR, with the final aim to propose quantitative indicators of the filtration function of soils.
(See task 1 section for more details)
Task 2 – High-tech nano-tracers
Task 2 will design and develop the nano-tracers for INFILTRON-exp in line with several criteria (representativeness, ease-of-use, non-toxicity and detection with geophysical methods). The nano-tracer will be silica nanoparticles (dissolution in water and no toxicity), adjusted in shape, size and electrical charge to those of the targeted emerging pollutant and bacteria, and grafted with chemical functions to act as contrasting agent for GPR for the field and MRI for the lab.
(See task 2 section for more details)
Task 3 – Validation of nano-tracers
Task 3 will first investigate the characteristics of bacteria and anthropic nanoparticles in terms of size and electrical charge in stormwater and infiltration basins, as a guide to produce the nano-tracers (task 2). The resulting nano-tracers will be tested in lab (column experiments) and on the field (infiltration experiments) regarding their ability to mimic the targeted anthropic nanoparticles and bacteria. Laboratory column experiments will allow to test a large panel of contrasting experimental conditions (several hydric and hydraulic solicitations) and several kinds of synthetic porous media (homogeneous versus heterogeneous). This step will perfectly complete the field injection tests conducted under real conditions.
(See task 3 section for more details)
Task 4 – INFILTRON-mod
Finally, task 4 will aim at developing INFILTRON-mod, for modelling experimental results provided by INFILTRON-exp, considering the whole complexity of physical mechanisms (preferential flow and mass transfers) and for building the best quantitative indicators of the infiltration and the filtration functions of soils. This task will also propose a methodology to use INFILTRON-mod for the modelling of processes at larger scales including the work scale (infiltration basins as a whole) and the catchment scale.
(See task 4 section for more details)
Schedule for the scientific programme
The tasks were designed to be well integrated, but at the same time realizable regardless of the success of the other tasks (to minimize project risk). The schedule for the scientific programme is detailed below.
The INFILTRON project contains several challenging objectives, in relation with the development of the nanoparticles (NPs) as nano-tracer for bacteria or emerging pollutants. These nano-tracers (NTs) will also have to behave as contracting agent for non-intrusive techniques like Ground Penetrating Radar (GPR) to be detectable in urban soils on the field. This part is novel and, to the consortium knowledge, no study has dealt with such objective to date. This constitutes a risk of failure to a certain extent since the design of nanoparticles as contrasting agent for GPR is totally new.
However, several options are conceivable, given the previous works by ILM team for the design of chemical to trace oil spills  and the sensitivity of GPR methods on the chemical properties of soil water [31, 79]. GPR uses high-frequency radio waves (from 10 MHz to 2.6 GHz). A GPR transmitter emits radio waves into the ground while as a receiving antenna record the variations in the return signal and detects the reflections between zones of different properties regarding wave propagation. The main goal consists here in injecting water with chemical properties that contrast with those of interstitial water to create artificially a reflector between the two types of water. The nano-tracer could be engineered to supply this contrast in chemical properties of water. If this objective is too challenging, the consortium will use salts for modifying water properties and increase contrasts in GPR data .
A second challenge lies in the use of nanoparticles as nano-tracers and contrasting agent for MRI. This is mandatory for column experiments to allow the detection of nano-tracers trapped into the columns and compare their distribution with those of the targeted anthropic nanoparticles and bacteria. However, engineering nanoparticles to act as contrasting agent for MRI is routine, with a large expertise on this topic in the consortium (ILM & IFSTTAR).
Regarding the other aspects of INFILTRON project, the other risks and contingency plans are detailed below (see task description). The consortium has all the expertise on water infiltration measurements and on related modelling approaches [1, 47, 48] including the modelling of preferential flow in heterogeneous soils [8, 46, 53]. In addition, the consortium has strengthened its expertise by inviting internationally recognized teams from Italy for water infiltration measurements. The necessity to link the INFILTRON approach and results to hydrology at larger scales including the catchment scale was raised by one of the reviewer during the first round of evaluation of the project. The invitation of the Australian team (University of Melbourne, led by Prof. Tim Fletcher), specialized in hydrology and stormwater management at catchment scale, will allow the consortium to strengthen this line of the proposed research (see task 4).