The importance of monitoring the transport of organic contaminants in soil and groundwater, and the pros and cons of existing sampling methods, are outlined. A new, alternative sampling method is proposed, using a passive sampler that functions as a water-permeable, semi-infinite sink for passing solutes of interest. Tracers integrated in the device store information on the volume of water passing through the sampler during the installation period. The conceptual basis of the sampling method is described. This device enables flux-proportional monitoring of the concentrations of mobile contaminants in the soil and groundwater. 14C-labeled phenanthrene (PHEN) and glyphosate (GLY) are used as case study compounds in laboratory experiments. The sorption capacities and uptake kinetics of 13 adsorbents are screened and compared, as well as the dissolution kinetics of three tracer salts: calcium citrate, calcium fluoride (CaF2), and calcium hydrogen phosphate (CaHPO4). The application of the passive sampler is then demonstrated in long-term laboratory experiments, using large soil columns under steady-state hydraulic conditions. The accumulated flux of PHEN was sampled with an accuracy of 3.6%-17.8%, using graphitized carbon, hexagonal mesoporous silica, and cross-linked polymers as adsorbents. The accumulated flux of GLY was sampled with an accuracy of 12.4%, using gamma-alumina as an adsorbent. The advantages and limitations of this new environmental monitoring method are discussed.