The knowledge of soil and water movement in vadose zone is essential for a wide range of scientific and practical fields. One of the main but less studied media is the soil under the Undrained On-Site Treatment Systems (UOSTS). In situ monitoring is necessary to interpret most physical and hydric properties of soil and to determine the chemical composition of seepage water as a key aspect in sustainable management of soils. In this paper the design and instrumentation of different hydropedologic sensors in a real UOSTS, for the fist time in France, is presented. One of the main objectives is to extract the soil water samples in different stages of depth under the treatment system. Besides by performing the in situ hydraulic conductivity measurements, monitoring the recorded moisture and matrix potential data of the soil and determining soil physical properties in laboratory, we identify the existence of preferential flow in the heterogeneous soil of the site. As a main result, the executed pilot site is operational from March 2012. Soil water sample collecting and data recording are done regularly from this date. Also In situ permeameter tests give values of the conductivity which are averagely 135 times higher than values deduced from pedotransfer functions. This suggests that preferential flows occur within the soil via macropores shortcutting the matrix. Analysis of the recorded data shows the rapid reaction of the water table to the rainfall and the non-uniform reaction of the tensiometers according to their position. These results together suggest the existence of preferential pathway flux through the sand pack and the underlying soil.