Fosnrs 4: Water and Nitrogen Balance for Mounded, Drip Irrigation Systems Receiving Septic Tank Effluent.

Critical for understanding OWS performance is the water and nitrogen (N) budget below a soil treatment unit (STU), or drainfield. The objective of this FOSNRS project investigation was to determine the mass balance of water and N below mounded drip irrigation STUs and determine how the data could be scaled up to site and regional impacts.  To achieve this, bench scale mound systems were established at the FOSNRS test facility at the UF Gulf Coast Research and Education Center (GCREC). Three small mound systems were constructed in treated wood boxes that were 5 ft. long, 3 ft. wide and 3 ft. high with 1:1 side slope (hereafter referred to as micro-mounds). Each micro-mound was 33 inches high from bottom to top and included 3 inches of washed gravel and sand at the base; followed by 12 inches of natural soil; 12 inches of sand; a single drip line with 3 emitters, and 6 inches of sand on top of the drip line.  St. Augustine grass (19.2 square ft) was planted on the top and sides of each micro-mound. A hole was drilled at the bottom of each micro-mound box to which a floor drain strainer was attached to collect the percolate. Each micro-mound received 2.4 gallons per day (gpd) of septic tank effluent (STE, equivalent to maximum allowable rate 0.8 gpd/square ft. for Florida sandy soils), dosed 6 times per day via the drip emitters. Each micro-mound was instrumented with 10 multi-probes (CS 650, Campbell Scientific Inc.) to measure volumetric moisture content, electrical conductivity (EC), and soil temperature in different layers and sides. Results show that major input of water during January through September 2013 was from STE (52%) and rainfall (47%), while major water output was percolate (47% of total input) and evapotranspiration (29% of total input). About 24% of the added water (STE and rainfall) was stored in the mound. Concentrations of total N in the percolate, sampled during 50 events, from January through September ranged from 2 to 60 mg N/L, with nitrate-N being the dominant N fraction, followed by dissolved organic N.  As the study is ongoing, the mass balance of N has not yet been computed. However, preliminary data shows that as expected, the major input of N is from STE (98.3%), followed by rainfall (1.7%), while about 30% of applied N was recovered in the percolate and about 13% of N was removed by plant uptake.  This implies that about 57% of applied N is either stored in the soil and/or otherwise lost (denitrification, anammox) from the micro-mounds. Future plans include destructively sampling the micro-mounds in early 2014 to determine N stored in soil to update the N mass balance and calculate the proportion of N lost. This presentation will discuss the mass balance of water and nitrogen in micro-mounds and how the data from this study can be scaled up to present water and N budget for mounded OWS. Further, this data can be scaled up to a regional level to estimate the amount of water and N loading to groundwater below mounded STUs in sandy soils.