Phosphate Plume Persistence at Two Decommissioned Septic System Sites

Long‐term monitoring of two well‐characterized, oxidizing septic system plumes (Langton and Long Point 2 sites) over periods extending two to four years after decommissioning, has revealed that ground water PO 4 3− concentrations (0.4 to 5 mg/L P) have persisted at levels virtually unchanged from those observed during active sewage loading. In addition, the frontal part of the PO 4 3− plume at the Long Point 2 site can be observed to continue to advance during the decommissioned period. At the Langton site, where an active regional ground water flow system is present, all major plume solutes (Na+, Ca 2+ , Cl − , NO 3 − ) returned to background values within one year of decommissioning, with the exception of PO 4 3− . This evidence suggests that phosphate behavior in the ground water zone at these sites is dominated by sorption reactions that are both rapid and reversible. Thus, if septic system phosphorus is not retained in the vadose zone, but is transported into the ground water zone, it has the potential to be persistent and to be mobile enough to constitute a threat to downgradient surface water environments. This evidence also shows that when a septic system is decommissioned, if an oxidizing PO 4 3− plume is present, downgradient P loading is not likely to diminish for many years thereafter.