Seasonal variation in nitrogen leaching from shallow-narrow drainfields

Document Type

Conference Proceeding

Date of Original Version



Nitrogen removal from septic tank effluent is one of the most pressing issues in coastal areas undergoing growth and development. Seven home-sites using onsite wastewater treatment systems were monitored in coastal Rhode Island to examine N treatment and leaching. The primary treatment units at these sites include: geo-textile filters; recirculating sand filters; single pass sand filters; a fixed activated sludge treatment system; and a modular peat filter. The final treatment step of all of these systems is a pressure-dosed shallow-narrow drainfield (SND). This paper focuses on N-removal by the SND serving these sites (treatment performance of the secondary treatment units will be delivered in a separate paper). Sites vary in age from four to six years. Five suction-cup lysimeters were installed at each site, three within the SND and two within a control plot (i.e., outside the drainfield area). In the SND, lysimeters were installed in the undisturbed soils adjacent to each trench at a depth of 30 cm below the drainfield lines. Control lysimeters were placed at 70 cm below the soil surface. Soil porewater samples were collected through the lysimeters twice seasonally from the winter of 2001 until the summer of 2003 and analyzed for total N. Average concentrations of N entering the groundwater for these seven sites ranged from 2 to 41 mg/L (ppm). Six of the seven sites showed a 33 to 73% overall reduction in N levels as a result of treatment in the SND. Seasonal effects were recognized for inputs of N into the groundwater for two of the sites. There were no observed seasonal effects on the amount that N levels were reduced as a result of treatment in the SND. Porewater samples collected from the control area of two sites had considerably higher levels of total nitrogen (TN) than those below the SND. The higher N levels outside the SND are likely the result of excess fertilizer additions to the lawns.

Publication Title, e.g., Journal

Proceedings of the 10th National Symposium on Individual and Small Community Sewage Systems - On-Site Wastewater Treatment X