GEOCHEMISTRY OF A SEPTIC-SYSTEM PLUME IN A COASTAL BARRIER BAR, POINTPELEE, ONTARIO, CANADA

The major ion and trace metal geochemistry of a septic system plume in a shallow sand aquifer was characterized to assess geochemical proces ses controlling the transport of nutrients and their release to a near by wetland. The plume was generated from a 16-year-old tile bed, and i s 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 unsatura ted zone. The plume can be divided into distinct redox zones, which sh ow differences in nutrient mobility. Proximal to the tile bed, there i s a shallow suboxic zone, with intermediate Eh values (> 400 mV), low concentrations of dissolved oxygen (< 1.0 mg/l), and elevated concentr ations 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 ba se 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 concent rations of NO3 (<0.01 mg/l N). Distal from the tile bed, the shallow g roundwater is suboxic to oxic, and has elevated concentrations of NO3 and NH3, but negligible PO4. In the lower reduced zone, elevated conce ntrations of PO4 occur up to 60 m away. The release of groundwater con taining even very low concentrations of PO4 (< 0.02 mg/l P) can lead t o the development of eutrophic conditions in surface water bodies. Geo chemical calculations indicate that, in the Mn-rich zone, the groundwa ter is close to saturation or supersaturated with respect to hydroxyap atite, rhodochrosite, calcite and ferrihydrite. In the reduced zone, t he groundwater is close to saturation or supersaturated with respect t o hydroxyapatite, vivianite, calcite and siderite. Formation of these phases, or related phases, are likely Limiting the concentrations of d issolved PO4, Fe and Mn and controlling the geochemical evolution of t he plume. (C) 1998 Elsevier Science B.V. All rights reserved.