Fate, mass balance, and transport of phosphorus in the septic system drainfields

• Less than 1% of effluent applied P leached from drainfield of a septic system.
• Approximately 95% of the effluent applied P remained in drainfield.
• Water extractable P increased in drainfield from <5 to >10 mg kg−1 after effluent dispersal.
• Effluent application for a year saturated 18% of P sorption capacity in drainfield.

Septic systems can be a potential source of phosphorus (P) in shallow groundwater. Our objective was to investigate the fate, mass balance, and transport of P in the drainfield of a drip-dispersal septic system. Drainfields were replicated in lysimeters (152.4 cm long, 91.4 cm wide, and 91.4 cm high). Leachate and effluent samples were collected over 67 events (n = 15 daily; n = 52 weekly flow-weighted) and analyzed for total P (TP), orthophosphate (PO4P), and other P (TP – PO4P). Mean TP was 15 mg L−1 (84% PO4P; 16% other P) in the effluent and 0.16 mg L−1 (47% PO4P, 53% other P) in the leachate. After one year, 46.8 g of TP was added with effluent and rainfall to each drainfield, of which, <1% leached, 3.8% was taken up by St. Augustine grass, leaving >95% in the drainfield. Effluent dispersal increased water extractable P (WEP) in the drainfield from <5 to >10 mg kg−1. Using the P sorption maxima of sand (118 mg kg−1) and soil (260 mg kg−1), we estimated that ∼18% of the drainfield P sorption capacity was saturated after one year of effluent dispersal. We conclude that despite the low leaching potential of P dispersed with effluent in the first year of drainfield operation, a growing WEP pool in the drainfield and low P sorption capacity of Florida’s sandy soils may have the potential to transport P to shallow groundwater in long-running septic systems.