Infiltration basins are frequently used for stormwater drainage. Because stormwater is polluted in highly toxic compounds, assessment of pollution retention by infiltration basins is necessary. Indeed, if basins are not effective in trapping pollution, deep soil and groundwater may be contaminated. This study's objective is to investigate soil pollution in infiltration basins: spatial distribution of soil pollution, optimisation of the number of soil samples and a contamination indicator are presented. It is part of a global project on long-term impact of stormwater infiltration on groundwater. Soil sampling was done on a basin in suburban Lyon (France). Samples were collected at different depths and analysed for nutrients, heavy metals, hydrocarbons and grain size. Pollutant concentrations decrease rapidly with depth while pH, mineralisation and grain size increase. Sustainable metal concentrations are reached at a 30-cm depth, even after 14 years of operation; hydrocarbon pollution is deeper. Principal component analysis shows how pollutants affect each level. The topsoil is different from other levels. Three specifically located points are enough to estimate the mass of pollution trapped by the basin with a 26% error. The proposed contamination indicator is calculated using either average level concentrations or maximum level concentrations. In both cases, the topsoil layer appears polluted but evaluation of lower levels is dependent on the choice of input concentrations.