Septic Systems Contribution to Phosphorus in Shallow Groundwater: Field-Scale Studies Using Conventional Drainfield Designs.

PloS one

PubMedID: 28107505

Mechtensimer S, Toor GS. Septic Systems Contribution to Phosphorus in Shallow Groundwater: Field-Scale Studies Using Conventional Drainfield Designs. PLoS ONE. 2017;12(1):e0170304.
Septic systems can be a potential source of phosphorus (P) in groundwater and contribute to eutrophication in aquatic systems. Our objective was to investigate P transport from two conventional septic systems (drip dispersal and gravel trench) to shallow groundwater. Two new in-situ drainfields (6. 1 m long by 0. 61 m wide) with a 3. 72 m2 infiltrative surface were constructed. The drip dispersal drainfield was constructed by placing 30. 5 cm commercial sand on top of natural soil and the gravel trench drainfield was constructed by placing 30. 5 cm of gravel on top of 30. 5 cm commercial sand and natural soil. Suction cup lysimeters were installed in the drainfields (at 30. 5, 61, 106. 7 cm below infiltrative surface) and piezometers were installed in the groundwater (>300 cm below infiltrative surface) to capture P dynamics from the continuum of unsaturated to saturated zones in the septic systems. Septic tank effluent (STE), soil-water, and groundwater samples were collected for 64 events (May 2012-Dec 2013) at 2 to 3 days (n = 13), weekly (n = 29), biweekly (n = 17), and monthly (n = 5) intervals. One piezometer was installed up-gradient of the drainfields to monitor background groundwater (n = 15). Samples were analyzed for total P (TP), orthophosphate-P (PO4-P), and other-P (TP-PO4-P). The gravel trench drainfield removed significantly (p<0. 0001) greater TP (~20%) than the drip dispersal in the first 30. 5 cm of the drainfield. However, when STE reached >300 cm in the groundwater, both systems had similar TP reductions of >97%. After 18 months of STE application, there was no significant increase in groundwater TP concentrations in both systems. We conclude that both drainfield designs are effective at reducing P transport to shallow groundwater.