Understanding of wastewater flow and transport processes through soil-based onsite wastewater treatment systems (OWTS) remains a challenge. An extensive field research program was executed in Alberta, Canada to investigate the treatment effectiveness of a new, litter-fermented-humic layer (LFH), at-grade, soil-based OWTS. The site had been receiving secondarily treated and ultraviolet (UV)-disinfected effluent via pressurized, at-grade laterals for 4 yr before the start of this research. The objective of this study was to investigate the hydrologic response of a shallow groundwater table to effluent infiltration. Following a detailed site characterization of groundwater flow direction and wastewater plume extent, daily groundwater fluctuation, effluent and atmospheric fluxes were monitored for more than 2 yr. Considering the local existing regulatory requirement of 7-d effluent travel depth through the vadose zone to achieve a treatment objective, weekly cycle hydrologic responses were interpreted using signal analysis tools. Findings indicated (i) a significant hydrologic response to effluent infiltration from the at-grade laterals at a weekly scale, and (ii) effluent reaches the groundwater within 1 wk of application approximately 15% of the time in the spring and summer periods when effluent loading rates are ≥5 cm3 cm−2 d−1 and the water table is ≤0.5 m below the ground surface. These conditions also coincided with the significant 7-d cycle of the effluent input function, which is a function of the weekly cycle of facility use, that is, more intensive facility use on weekends. The results presented in here contribute significantly to the understanding of wastewater flow and transport under boundary conditions typical of OWTS and shallow groundwater conditions.