Phosphorus loading from onsite wastewater systems to a lake (at long time scales)

Abstract

Schellenger FL, Hellweger FL. Phosphorus loading from onsite wastewater systems to a lake (at long time scales). Lake Reserv Manage. 35:90–101.Phosphorus from onsite wastewater systems (OWS, septic systems, cesspools) enters the groundwater and may migrate to and contribute to eutrophication of down-gradient lakes. Since phosphorus sorbs relatively strongly to soil solids, this source may be assumed negligible in lake management. However, phosphorus adsorption is reversible, and given enough time, some phosphorus will eventually reach the lake. We investigated the magnitude and timing of phosphorus loading to a case study lake in southeastern Massachusetts, using a model that accounts for cesspools and septic systems installed over a 270-yr time frame, the 1994 laundry detergent phosphorus ban, loss of phosphorus in the vadose zone, and transport and adsorption/desorption (retardation) in the surficial aquifer. We parameterized the model using literature and local information and applied it to all the OWS in a the lake watershed to estimate annual wastewater total phosphorus loading over a long time. Model results were compared to previous phosphorus budgets based on limited field measurements. The model predicts that groundwater transport of phosphorus from OWS by itself can account for the current eutrophic state of the lake and will continue to increase. Our results show that the OWS source can be significant and should not be ignored. We performed parameter sensitivity and Monte Carlo uncertainty analyses and illustrated the effects of removal in the vadose zone and travel time in the aquifer. Our model suggests that past management actions (cesspool to septic system conversion, laundry detergent phosphorus ban) effectively reduced the 2018 load by 31.4%. A centralized wastewater treatment facility would eliminate the OWS loading, but the half-life due to the existing phosphorus reservoir in the aquifer is long (20 yr).