Accumulation of Organic Matter Components in Soil under Conditions Imposed by Wastewater Infiltration

Abstract

Onsite wastewater systems that use subsurface infiltration of septic tank effluent are used widely in the United States and abroad; however, the processes that govern effluent flow and pollutant transport during treatment in the subsurface have only recently begun to be elucidated. The conditions that often occur during intermittent wastewater application can lead to the formation and deposition of organic molecules, which can cause reduced permeability and soil pore clogging. To enhance the fundamental understanding of organic matter accumulation as affected by effluent infiltration, field research was performed at the Mines Park Test Site located at the Colorado School of Mines. Four pilot‐scale infiltration cells established in a sandy loam soil profile and used for infiltration of domestic septic tank effluent were dismantled to study the nature and extent of organic matter accumulation. Macro‐ and micromorphological observations were made, and soil samples were taken at and below the infiltrative surface and analyzed for total organic C, polysaccharides, and humic substances. Additionally, dissolved O2 concentrations were measured with time in the septic tank effluent within a cell as it infiltrated into the soil. Compared with background controls, soil samples collected at the soil infiltrative surface were characterized by high water contents and high concentrations of organic matter. The high values measured at the infiltrative surface declined sharply, however, from 1 to 2 cm of soil depth. An infiltrative surface exposed to microaerophilic concentrations (∼2 mg/L) of dissolved O2 accumulated 400 to 800% (w/w) more organic material than the mass accumulated under more continuously anaerobic conditions. The microaerophilic conditions also appeared to favor the growth of polyphosphate‐accumulating microorganisms.