Evaluation of gravel-filled drainfield hydraulics and nitrogen processes in an onsite wastewater teatment system
Bradshaw, James Kenneth
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The function of onsite wastewater treatment systems (OWTSs) in clay-textured soils has not been studied extensively and the relationships between N transport and hydraulic function of 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 a conventional OWTS. This study was composed of a field study where soil pressure heads and N concentrations were measured and a modeling study where a numerical model was fit to the data collected in the field study. The field study indicated that the soil near trench bottoms cycled between saturated and unsaturated conditions when effluent was dosed to the system at regular intervals. Mean NH4-N concentrations decreased to < 1 mg L-1 within 90 cm of trench bottoms likely due to adsorption and nitrification. Mean NO3-N concentrations increased in the drainfield and ranged from 10 to 25 mg L-1 near the end of the study period. N:Cl ratios suggested 74% of N was lost as volatile N species, possibly from denitrification. The 30-cm zone beneath the trench bottoms had high biomass N and C contents (126 mg kg-1 C and 6.50 mg kg-1 N) and presumably was the most bioactive. The fate and transport of N was dynamic in the clay soil due to variable moisture conditions near the trench-soil interface which may have intermittently stimulated denitrification. A calibrated HYDRUS model was used to estimate N losses to groundwater in the OWTS. The overall predicted soil pressure heads and solute concentrations were similar to data collected from the field experiment. Using the calibrated model, the estimated annual steady-state N loss from leaching from the experimental drainfield was 3.43 kg yr-1. Scaled up to an OWTS zoning density of 5 homes ha-1 with full-size systems, the N load to groundwater would be 52.5 kg ha-1 yr-1, which is comparable to losses due to intensive agriculture. Our model predicted 57% of the N removal in the system was from denitrification. These estimates were specific to clay textured soils, and should be valuable for modeling N transport from OWTSs on a regional scale in the Piedmont of the southeastern U.S.