Effluent tracing and the transport of contaminants from a domestic septic system.

A domestic soil absorption system in a coastal location was instrumented with suction lysimeters and piezometers and monitored between August and December 2002. Using the sandy soils from the site, column leaching experiments were also undertaken and these suggested that bromide would be a suitable conservative tracer which could be added to the wastewater system to determine the direction and rate of groundwater flow. The septic system plume boundaries were identified from the monitoring results and the subsurface fate of the inorganic nutrients determines using ion ratios. The tracing results indicated that groundwater was moving at 0.4 m/day towards a nearby drain. The ion ratios indicated that total inorganic nitrogen and orthophosphate were not substantially lost or diluted in the sandy soils downgradient from the soil absorption system, and that without riparian vegetarian lining the drain, these nutrients would have been largely unattenuated in transport. In the absence of adequate vertical and horizontal setback distances, riparian vegetation is regarded as very important in limiting the subsurface transport of inorganic nutrients from domestic septic systems.