Mass Balance of Phosphorus in the Drainfield of Onsite Wastewater Treatment System.

Elevated concentrations of phosphorus (P) beneath drainfield of onsite wastewater systems (OWTS) can impact shallow groundwater. Our objectives were to conduct mass balance of P and partition P in various media (soil, water, plant) in the drainfield of a conventional OWTS. Major P input source was septic tank effluent (STE), while the major P output was as P leaching. We built three mounds (1.5 m length x 0.9 m width x 0.9 m height) with 1:1 side slope using pressure treated wood. A hole was drilled at the bottom of each mound to facilitate collection of leachate. Each mound was packed to 7.5 cm depth with a mixture of sand and pea gravel (to facilitate free drainage), followed by 30 cm depth of soil, and 30 cm depth of sand. Then, a drip line placed on top of the sand layer dispersed 9 liter of STE per day (maximum allowable rate in Florida). Drip line was covered with 15 cm depth of sand and turf grass (St. Augustine) was planted to mimic a residential OWTS. Each mound was instrumented with 10 Campbell CS650 sensors to measure moisture, electrical conductivity, and temperature at different parts of the drainfield. Collected STE, leachate, plant and soil samples during January-July 2013 were analyzed for pH, EC, chloride, dissolved reactive P (DRP), and total P. As most of the P applied with STE remained in the drainfield, we took soil cores after several months of STE application to understand distribution and accumulation of P at various depths. For this, soil P was fractionated into various P pools of lability using Hedley fractionation scheme and P sorption capacity was quantified. Spectroscopic techniques such as ³¹P Nuclear Magnetic Resonance (NMR) and X-ray Adsorption Near Edge Spectroscopy (XANES) were employed to understand the nature of P accumulation whether organic or inorganic P.