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ON-SITE DOMESTIC SYSTEM EFFLUENT TRACING IN A COASTAL CATCHMENT

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  701P0104,  . (doi: 10.13031/2013.15832)
Authors:   P. M. Geary
Keywords:   Nitrate, nitrogen, groundwater, tracers, lithium, septic systems, contamination

Nitrate contamination, which is often associated with septic wastewater treatment and disposal, is a major problem in some shallow aquifers and is becoming a threat in some groundwater supplies. In unsewered coastal communities, where there are commonly sandy soils, nitrate may move rapidly into the groundwater system. In this paper, the results of a monitoring project at one individual property in the Port Stephens area, N.S.W (Australia) are presented. A septic tank effluent disposal field was instrumented with suction lysimeters and piezometers and monitoring undertaken during 2002 to determine the plume boundaries and the subsurface fate of the effluent, in particular nitrogen. A tracer was also added to the wastewater system to determine the direction and rate of subsurface flow through the disposal field towards an adjacent dish drain.

Suction lysimeters collected effluent samples from the capillary fringe and the surface of the groundwater table. The highly oxidised septic system plume, which was approximately 10 m wide, had generally well-defined boundaries and the lateral movement of nitrate was rapid through the subsurface towards a nearby dish drain. Nitrogen losses were minor within the effluent disposal field, however, substantial nitrate loss occurred within meters of the drain, due primarily to plant uptake (riparian vegetation was lining the drain), denitrification (which has also previously been shown to be an important removal mechanism in soil-stream interfaces and shallow groundwater systems), and dilution. The study demonstrates that stream riparian zones are important in reducing the environmental impacts associated with the subsurface migration of nitrate from a septic tank effluent disposal system in a coastal location with a shallow groundwater table.

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