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Despite the widespread use of soil treatment units (STUs) for wastewater reclamation, the fundamental physical, chemical, and biological processes that occur at the infiltrative surface are not entirely understood. It is known that effluent infiltration into soil can result in a reduction of the hydraulic capacity of a STU and, in extreme cases, can cause hydraulic failure of the system. Several studies suggest that the combination of cyclical aerobic/anaerobic conditions, changing redox potential, and high moisture content inherent in subsurface soil environments during wastewater infiltration may lead to the synthesis of polysaccharides and humic substances consisting of fulvic acid (FA), humic acid (HA), and humin. These organic compounds are not very biodegradable and may accumulate over extended periods of time, potentially causing long-term soil pore clogging. In order to examine the processes occurring at the infiltrative surface and at depth in a STU, soil samples were collected from pilot-scale STUs installed at the Mines Park Test Site, which had been loaded with domestic septic tank effluent for 3.2 years. In November 2006, two pilot-scale STUs, or cells, were dismantled and cores and shallow soil samples were collected to compare organic matter concentrations with depth. Soil samples collected at and immediately beneath the infiltrative surface were generally correlated with higher concentrations of humic substances and labile polysaccharides. The differences in concentrations over depth were most apparent at the infiltrative surface, which is generally accepted as the location where most soil clogging occurs during wastewater infiltration. The high concentrations at the infiltrative surface declined sharply over several centimeters of depth, which is quite different than the relatively consistent concentrations exhibited by the background cores over a similar depth interval. The accumulation of these organic materials at the infiltrative surface is consistent with current understanding of wastewater-induced soil clogging.