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Wastewater soil absorption systems (WSASs) have the potential to achieve high treatment efficiencies, yet the understanding and predictability of performance as well as the risk of inadequate function has not been fully quantified. This has been due to the complex and dynamic relationships between hydraulic and purification processes and the factors that control their behaviors. This paper provides an overview of WSAS process principles and performance, and then describes initial experimental research completed as part of a long-term program to elucidate the fundamental relationships between performance and WSAS process design and environmental conditions. Laboratory research has employed 3-D physical models of full-scale WSASs to explore the effects of infiltrative surface character (e.g., soil clogging development; aggregate-laden vs. aggregate-free) and vadose zone soil depth (e.g., 60 vs. 90 cm). Factorial design experiments have included monitoring of flow and transport behavior and treatment efficiency for nearly one year, periodic multicomponent surrogate and tracer tests using chemical and microbial agents, and soil core sampling and biogeochemical analyses. Field investigations have been completed at 16 WSASs located in Colorado including those of aggregate-free or aggregate-laden designs. At each site, wastewater characteristics were monitored and soil cores were collected to a depth of 60 to 75 cm with analyses made for chemical and microbial properties. At one site, a multicomponent surrogate and tracer test was completed to assess virus treatment. The observations made to date have demonstrated the advanced treatment potential of WSASs due to dynamic and interactive hydraulic and purification processes as affected by system design and environmental conditions.