Evaluating the cause of TDIP signals in woochips-filled infiltration trenches for treated wastewater

Summary Soil infiltration is regularly practised downstream of wastewater treatment plants, particularly in areas without surface water systems. In France, stakeholders are considering the use of biodegradable and renewable material such as woodchips. This material has two main advantages: it has a high capillarity, thus a great capacity for water retention, and it amends the soil with carbon improving its structure. Moreover, woodchips have been told to promote biodiversity in particular for bioturbating species like earthworms, which play a major role regarding soil structure and organic matter transport into the soil. Such macrofaune is expected to promote water infiltration for poorly infiltrating capacity soils and to limit soil clogging caused by the infiltration of treated wastewater. In this study, we evaluate the interaction between woodchips and loamy soils on woodchip-filled trenches operated for 2 to 6 years using a non-destructive geophysical approach: Electrical Resistivity Tomography (ERT) and Time Domain Induced Polarisation (TDIP). ERT provided information of the structure of the soil under the woodchip-filled trenches, but anything could be said about the impact of such system in the soil. By contrast, TDIP showed significant variations of the signal around the trenches which may result from the presence of organic matter.