Monitoring Pressure Heads and Nitrogen Concentrations in An Onsite Wastewater Treatment System: Year 1.

A conventional onsite wastewater management system (i.e. septic system) was installed in a typical Piedmont soil in Griffin, GA.  Three 10m trenches were monitored for 1 yr using automated tensiometers installed at 3.3 m and 6.6 m from the septic tank outlet.  The tensiometers were installed at 4 points beneath each trench to monitor the hydraulic head of soil water beneath the drainfield.  Suction lysimeters were also installed beneath and adjacent to trenches to collect soil water for nitrogen analysis.  Each trench was dosed 3x per day at the design loading rate of 2.5 cm d^-1.  Tensiometers installed 10 and 15 cm directly below the trench bottoms indicated the soil was saturated (h>0) at all times when the system was being dosed.  Tensiometers installed down gradient of trenches, at 1 cm above and 15 cm below trench bottoms, indicated the soil was unsaturated (h<0) except for wet periods (i.e. after precipitation).  Unsaturated conditions down gradient from trenches indicate biomat formation and reductions in saturated hydraulic conductivity at the gravel-soil interface in trench bottoms and sidewalls.  Analysis of lysimeter data showed that nitrate concentrations increased beneath the drainfield from 0 to 50 mg L^-1 over the first year of operation.  These data will be used to calibrate a HYDRUS 2D model of the trenches during the biomat development period.    We hope to use the model to compare the differences in water movement beneath the drainfield pre- and post-biomat.