Transport of Phosphorus From Drainfields of Onsite Wastewater Treatment Systems to Groundwater.

The contribution of phosphorus (P) from onsite wastewater treatment systems (OWTS) to groundwater pollution is largely not quantified in most aquifers and watersheds in the world. In areas with sandy soils and shallow water table such as in Florida, the drainfield of OWTS is constructed in raised beds (commonly known as mounds). Our objective was to investigate the P dynamics in three types of OWTS that have 6 m x 0.6 m drainfield. Wastewater (hereafter referred to as septic tank effluent, STE) was dispersed at the maximum allowable rate of 3 liter/ft²/day in each of the mounds (equivalent to 120 liter per mound). These OWTS included two most common systems (drip dispersal, gravel trench) and an advanced system with in-built aerobic (sand and ligno-cellulosic) and anaerobic (sulfur and oyster shells) media. During May-April 2012, soil-water samples were collected (n=13 at 3 days interval; n =25 at 7 days interval; n=15 at 14 day intervals) from the vadose zone using suction cup lysimeters installed at 0.30, 0.60, and 1.05 m depth below the drainfield. While groundwater samples were collected from piezometers installed at 3-3.30 m depth below the drainfield. Collected STE, soil-water, groundwater were analyzed for pH, EC, chloride, dissolved reactive P (DRP), and total P. This presentation will discuss our research on the phosphorus fate and transport in OWTS.