Phosphate plume persistence at two decommissioned septic system sites

Long-term monitoring of two well-characterized, oxidizing septic system plu mes (Langton and Long Point 2 sites) over periods extending two to four gea rs after decommissioning, has revealed that ground water POP 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 PO43- plume at the Long Point 2 site can be observed to continue to advanc e during the decommissioned period. At the Langton site, where an active re gional ground water flow system is present, all major plume solutes (Na+, C a2+, Cl-, NO3-) returned to background values within one year of decommissi oning, with the exception of PO43-. This evidence suggests that phosphate behavior in the ground mater zone at these sites is dominated by sorption reactions that are both rapid and reve rsible. Thus, if septic system phosphorus is not retained in the vadose zon e, but is transported into the ground water zone, it has the potential to b e persistent and to be mobile enough to constitute a threat to downgradient surface mater environments. This evidence also shows that when a septic sy stem is decommissioned, if an oxidizing PO43- plume is present, downgradien t P loading is not likely to diminish for many years thereafter.