Geochemistry of a septic-system plume in a coastal barrier bar, Point Pelee, Ontario, Canada

The major ion and trace metal geochemistry of a septic system plume in a shallow sand aquifer was characterized to assess geochemical processes controlling the transport of nutrients and their release to a nearby wetland. The plume was generated from a 16-year-old tile bed, and is more than 60 m long, 40 m wide and 7 m thick. The groundwater pH at the site is near neutral, but up to 0.4 units lower in the plume core as a result of H+ generated from NH3 and DOC oxidation in the unsaturated zone. The plume can be divided into distinct redox zones, which show differences in nutrient mobility. Proximal to the tile bed, there is a shallow suboxic zone, with intermediate Eh values (>400 mV), low concentrations of dissolved oxygen (<1.0 mg/l), and elevated concentrations of Mn (1–3 mg/l) and nutrients (10–80 mg/l NO3–N, 1–15 mg/l NH3–N, 0.1–1.5 mg/l PO4–P, 6–13 mg/l dissolved organic carbon). At the base of the aquifer, there is a reduced zone (Eh<200 mV) with elevated concentrations of Fe (1–14 mg/l), PO4 and NH3, but negligible concentrations of NO3 (<0.01 mg/l N). Distal from the tile bed, the shallow groundwater is suboxic to oxic, and has elevated concentrations of NO3 and NH3, but negligible PO4. In the lower reduced zone, elevated concentrations of PO4 occur up to 60 m away. The release of groundwater containing even very low concentrations of PO4 (<0.02 mg/l P) can lead to the development of eutrophic conditions in surface water bodies. Geochemical calculations indicate that, in the Mn-rich zone, the groundwater is close to saturation or supersaturated with respect to hydroxyapatite, rhodochrosite, calcite and ferrihydrite. In the reduced zone, the groundwater is close to saturation or supersaturated with respect to hydroxyapatite, vivianite, calcite and siderite. Formation of these phases, or related phases, are likely limiting the concentrations of dissolved PO4, Fe and Mn and controlling the geochemical evolution of the plume.