Time-Variable Hydraulic Potential Surrounding An Onsite Wastewater Drainfield.

A conventional onsite wastewater management system (i.e. septic system) was installed in a typical Piedmont soil in Griffin, GA.  Tensiometers were installed at 3.3 m and 6.6 m from the septic tank outlet and at 3 distances down gradient of each trench (3 trenches x 10 m each) in order to monitor hydraulic potential of wastewater.  Each trench was dosed 3x per day at the design loading rate of 2.5 cm d^-1.  Tensiometers were recorded monthly for 4h following the morning wastewater dose and contour plots were developed for several sampling dates to show time-variable hydraulic potential.  Over the 4h monitoring period we observed decreasing hydraulic pressure heads just below trench bottoms as water infiltrated and moved away from the drainfield.  Pressure heads were positive at most of the tensiometers installed below the trench bottoms during the 4h monitoring period, indicating saturation for some period greater than 4h as water moved away.  Two months after wastewater application started, we began to observe a lower ratio of tensiometer pressure head to trench ponding depth at the deeper tensiometers, for a given tensiometer.  The reduction of hydraulic pressure head is indicative of a biomat forming and beginning to alter the infiltration rate.  Monitoring will continue over the next eighteen months to aid in determining the rate of biomat formation in a newly installed system.