ESTIMATING UNSATURATED SOIL HYDRAULIC PROPERTIES IN WASTEWATER SOIL ABSORPTION SYSTEMS

Wastewater soil absorption systems (WSAS) are commonly used in the United States to manage and treat domestic wastewater. In a typical system, septic tank effluent (STE) is applied daily via intermittent dosing to a subsurface drainfield. As the STE percolates through the unsaturated soil, treatment occurs through filtration, sorption, biodegradation, and chemical processes. Soil clogging at the infiltrative surface of WSAS due to the accumulation of suspended solids, organic matter, and chemical precipitates is a phenomenon known to occur as a result of continued wastewater application. This clogging zone creates an impedance to flow, restricting the hydraulic conductivity and rate of infiltration. A certain degree of clogging may improve wastewater treatment by causing more widely distributed unsaturated flow and increased hydraulic residence times. However, excessive clogging can lead to eventual system failure if the infiltrative surface becomes essentially impermeable. Clogging zones strongly influence the unsaturated flow regime within WSAS and therefore play a critical role in the treatment of wastewater pollutants. However, few methods exist to rigorously quantify or predict the impact of system operation on clogging development and the impact of clogging development on treatment. One-dimensional laboratory columns have been developed to study the unsaturated flow and solute transport processes associated with soil clogging genesis under several wastewater loading conditions. Measured transient hydraulic data and STE mass-loading rates are coupled with the unsaturated flow and transport model, HYRDRUS-1D, to estimate the hydraulic parameters of the clogging zone and subsoil regions over time. This analysis provides a better understanding of the influence of cumulative wastewater loading on clogging zone development, as well as associated influence of the clogging zone on important biological, chemical, and hydraulic processes relevant to pollutant treatment. Calibrated models can describe the treatment and hydraulic processes within a simplified system, and can often be used for predictive purposes beyond the temporal domain of the data set.