Nitrogen Fate and Transport in a Conventional Onsite Wastewater Treatment System Installed in a Clay Soil: Experimental Results

Abstract

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 NH4–N concentrations decreased to <1 mg L−1 within 90 cm of trench bottoms due to adsorption, immobilization, and nitrification. Mean NO3–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.

Fate of Effluent‐Borne Nitrogen in the Mounded Drainfield of an Onsite Wastewater Treatment System

“…Mean concentration of TN in the STE (70 mg L −1 ; Table 2) was within the reported values from other conventional OWTS of 47 to 51 mg L −1 (Bradshaw and Radcliffe, 2013a; Siegrist et al, 2014), 43 to 74 mg L −1 in the Wekiva Study area, Florida (Aley et al, 2007), ∼60 mg L −1 ( n = 61) based on a national study (Lowe et al, 2009), 81 mg L −1 (Robertson, 2012), and 26 to 75 mg L −1 (USEPA, 2007). The range of TN (45–80 mg L −1 ) in the STE was also consistent with the literature values of 50 to 90 mg L −1 (Crites and Tchobanoglous, 1998) and 26 to 124 mg L −1 (Lowe et al, 2009).…”

Nitrogen Treatment Efficiency of a Large Onsite Wastewater System in Relation to Water Table Dynamics

“…The residence time of wastewater in the unsaturated zone will increase with an increase in vadose zone thickness, allowing more time for microbial transformations of nitrogen such as nitrification and denitrification, to occur. Other factors such as the availability of carbon as an electron donor, anaerobic conditions, and availability of NO 3 as an electron acceptor are requirements for NO 3 removal via heterotrophic denitrification and changes in these factors will influence the nitrogen treatment efficiency of OSW. The hydraulic gradient beneath the system was positively correlated with groundwater table elevation ( r = 0.768; p = 0.026) (Figure ).…”

“…Groundwater pH beneath the OSW drainfield (4.2 ± 0.5) was significantly lower than in background groundwater away from the system (5.6 ± 0.7) and septic tank effluent (6.4 ± 0.6) (Figure A, Table ). Prior research has also shown that groundwater pH beneath OSW can be one or more pH units lower than wastewater because of the nitrification process beneath the drainfield. Nitrification, the conversion of NH 4 ‐N to NO 3 ‐N via microorganisms, increases the availability of H + , which lowers pH if the soil does not have sufficient buffering capacity or alkalinity .…”

“…Prior research has also shown that groundwater pH beneath OSW can be one or more pH units lower than wastewater because of the nitrification process beneath the drainfield. Nitrification, the conversion of NH 4 ‐N to NO 3 ‐N via microorganisms, increases the availability of H + , which lowers pH if the soil does not have sufficient buffering capacity or alkalinity . Septic tank effluent was mostly NH 4 ‐N (90% of total), but groundwater beneath the drainfield was predominantly NO 3 ‐N (84% of total) (Table ).…”

“…Nitrification requires aerobic conditions. The higher dissolved oxygen and NO 3 concentrations and lower pH and NH 4 ‐N values in groundwater beneath the OSW relative to septic tank effluent suggest nitrification . Sandy, coastal soils have relatively high effective porosities, which are conducive to oxygen diffusion and nitrification of effluent in drainfield trenches .…”