DGT is a passive sampling technique for labile metals. It consists of the assembling of several layers of a chelating resin, a diffusive hydrogel and protective membranes. Once immerged in the water body of interest, a diffusion gradient of metal progressively establishes itself between the solution and the resin layer and metal species are transported through the diffusion hydrogel toward the resin. Now, as any solid immersed in the water, there is a region around DGT where the tangential velocity of water decreases, creating a diffusive boundary layer (DBL) at the surface of the device, that increases the actual thickness of the diffusion layer that needs to be known to interpret DGT measurement. In well-stirred solutions, the diffusive boundary layer thickness  has long been assumed to be negligible compared to the hydrogel precisely defined thickness g. However, in extreme unstirred cases or in low-flowing rivers,  could be significant and biased the concentration if not considered in the calculations. Recent papers based on laboratory and field experiments suggested a method to assess diffusive boundary layer (DBL) in front of DGT devices when they are immersed in water. Our study focuses on the robustness of the assessing DBL method in laboratory and in situ controlled conditions, and aims to provide insights on DGT measurement interpretation thanks to the data obtained with the method. Assessing the DBL requires to deploy DGT devices with a range of gel layer thicknesses (0.39-1.93 mm). Two types of diffusive gel were used. Deployments took place in laboratory with a range of stirring speed (0-400 rpm) and in a canal receiving treated wastewater with increasing controlled water velocity. No difference in the DBL values was noticed between the two types of gel, and we were able to calculate a DBL value in every case. Nevertheless results showed high uncertainties, and did not give the opportunity to discriminate the water velocity in the studied range. In addition to that, the several gel thicknesses deployment method provides a lot of data, which can be treated on various ways to obtain information about speciation and complexes sampled by DGT. This study, carried in a canal receiving effluent from a wastewater treatment plant at a known flow velocity, was the opportunity to go further into the interpretation of data obtained with DBL estimation method. It highlights one possible impact of DBL on the measurement: no-penetrating may be able to dissociate within the DBL. The method also provides information about the species sampled by DGT with open or restricted pores.