Nitrogen (N) is the main pollutant of concern below drainfields of onsite wastewater systems (OWTS), especially in areas with porous sandy soils and shallow groundwater such as Florida. Our objective was to determine mass balance of N in septic drainfield by accounting N inputs from St. Augustine grass roots and soil, septic tank effluent (STE), and rainfall and N outputs as leaching, and plant uptake. We hypothesized that variability in soil moisture, temperature, and carbon during different growing seasons may drive changes in soil N that may result in different concentrations and forms of N leaving drainfield. Three mounds (1.5 m length×0.9 m width×0.9 m height) with 1:1 side slope were constructed using pressure treated wood. A hole was drilled at the bottom of each mound to collect leachate. Each mound included a 7.5 cm depth mixture of sand and pea gravel to facilitate free drainage, followed by 30 cm depth of soil and 30 cm depth of sand. A drip line placed on top of the sand layer dispersed 9 L of STE per day. A sand layer of 15 cm was placed on the drip line and turf grass (St. Augustine) was planted to mimic a residential OWTS. Each mound was instrumented with 10 Campbell CS 650 sensors that measured moisture, electrical conductivity (EC), and soil temperature at different parts of the drainfield. Leachate was collected from micro-mounds each day in the beginning of the study (n=30) and then a weekly flow-weighted sample (n=75) was obtained. Collected STE, leachate, rainfall samples were analyzed for pH, EC, chloride, and organic and inorganic N forms. Soil cores at end of the study will be taken to understand the distribution and accumulation of various N pools at various depths. This abstract will be updated with the results in the next few months.