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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Modeling Effluent Distribution and Nitrate Transport through an On-Site Wastewater System

^a Crop and Soil Environmental Sciences Dep., Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061-0404
^b Div. of Onsite Sewage and Water Services, Virginia Dep. of Health, 109 Governor St., Richmond, VA 23219

^* Corresponding author (ghassan{at}vt.edu).

Received for publication September 26, 2007. Properly functioning on-site wastewater systems (OWS) are an integral component of the wastewater system infrastructure necessary to renovate wastewater before it reaches surface or ground waters. There are a large number of factors, including soil hydraulic properties, effluent quality and dispersal, and system design, that affect OWS function. The ability to evaluate these factors using a simulation model would improve the capability to determine the impact of wastewater application on the subsurface soil environment. An existing subsurface drip irrigation system (SDIS) dosed with sequential batch reactor effluent (SBRE) was used in this study. This system has the potential to solve soil and site problems that limit OWS and to reduce the potential for environmental degradation. Soil water potentials (_s) and nitrate (NO₃) migration were simulated at 55- and 120-cm depths within and downslope of the SDIS using a two-dimensional code in HYDRUS-3D. Results show that the average measured _s were –121 and –319 cm, whereas simulated values were –121 and –322 cm at 55- and 120-cm depths, respectively, indicating unsaturated conditions. Average measured NO₃ concentrations were 0.248 and 0.176 mmol N L^–1, whereas simulated values were 0.237 and 0.152 mmol N L^–1 at 55- and 120-cm depths, respectively. Observed unsaturated conditions decreased the potential for NO₃ to migrate in more concentrated plumes away from the SDIS. The agreement (high R² values 0.97) between the measured and simulated _s and NO₃ concentrations indicate that HYDRUS-3D adequately simulated SBRE flow and NO₃ transport through the soil domain under a range of environmental and effluent application conditions.

Abbreviations: ET_p, potential evapotranspiration • HTE, highly treated effluent • OWS, onsite wastewater systems • SBRE, sequential batch reactor effluent • SDIS, subsurface drip irrigation systems • _s, soil water potential