CHARACTERIZATION OF FLUID FLOW AND WASTEWATER TRANSPORT PROCESSES WITHIN A WISCONSIN-TYPE MOUND
Abstract
The Wisconsin-type mound has gained widespread recognition as an alternative onsite system for problematic soils yet much remains to be learned concerning its use under severe conditions. This study characterized the fluid flow and waste transport processes of an operating mound under seasonal variations. To investigate fluid transport, a finite element model of the mound's unsaturated-saturated flow field was calibrated and verified through tracer studies and history matching. Waste transport processes within the mound were characterized through a long-term monitoring program of the wastewater, renovated mound water, and surrounding groundwater. Monitoring data and model studies revealed that the lower regions of the fill become saturated during periods of high water table movement resulting in seepage at the mound toe. Sensitivity analysis of the overflow rate indicated this situation could be prevented by reducing the design loading. Dissipation of mound effluent occurred primarily through the upper soil horizons underscoring the importance of horizontal conductivity for systems located on slowly permeable soils. Simulation of remedial action to improve the mound's performance through perimeter drain installation showed that such action would lower the operating water table, increase soil tension within the fill and upper soil horizons, and reduce Darcian velocities. Transient-state studies indicated that the "drained" mound would maintain an unsaturated fill under the ten-year, 24-hour rainfall event. Redox potentials and unsaturated flow conditions demonstrated that the mound's fill was aerobic. Influent COD was reduced substantially in passage through the bed and fill, whereas ammonium entering the system was nitrified at the bed-fill interface. Denitrification occurred periodically at the fill-soil interface. Evidence that a clogging mat had formed at the bed-fill interface was obtained with the tracer study. In general, based upon a statistical analysis of the monitoring data, there was no deterioration of local groundwater quality due to mound operation. Under conditions of saturated flow, however, fecal transport within the subsurface was observed.
Degree
Ph.D.
Subject Area
Civil engineering
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