Open AccessNitrogen Fate and Transport in a Conventional Onsite Wastewater Treatment System Installed in a Clay Soil: Experimental Results
Author(s) -
Bradshaw James K.,
Radcliffe David E.
Publication year - 2013
Publication title -
vadose zone journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.036
H-Index - 81
ISSN - 1539-1663
DOI - 10.2136/vzj2012.0149
Subject(s) - vadose zone , nitrification , soil water , wastewater , trench , nitrogen , denitrification , environmental chemistry , chemistry , environmental science , hydrology (agriculture) , soil science , geology , environmental engineering , geotechnical engineering , organic chemistry , layer (electronics)
Relationships between N transport and hydraulic function of onsite wastewater treatment systems (OWTSs) in clay‐textured soils are largely unknown. The objectives of this study were to quantify N concentrations in a clay soil and estimate denitrification in the vadose zone beneath a conventional OWTS. An OWTS installed in the Piedmont region of Georgia was dosed with residential strength wastewater three times per day for 2 yr. Soil near trench bottoms cycled between saturated and unsaturated conditions under the dosing schedule we used. Mean NH 4 –N concentrations decreased to <1 mg L −1 within 90 cm of trench bottoms due to adsorption, immobilization, and nitrification. Mean NO 3 –N concentrations increased as the drainfield matured and ranged from 10 to 25 mg L −1 near the end of the two year study period. Mean pressure heads and Cl − concentrations measured at 3.3 m and 6.6 m from trench inlets indicated that significantly more water infiltrated the drainfield at the 3.3 m position. Wet conditions at the 3.3 m position may have restricted nitrification, while dry conditions at the 6.6 m caused rapid nitrification. Nitrogen:chloride ratios suggested that 61% of N entering the drainfield was lost, potentially as volatile N‐species. The 30‐cm zone directly beneath the trenches had high biomass N and C contents (15 mg kg −1 N and 60 mg kg −1 C) and was presumably the most biologically active zone. The fate and transport of N was dynamic in this system due to variable moisture conditions near the trench‐soil interface which may have intermittently stimulated denitrification.