Impacts on a sand aquifer from an old septic system: nitrate and phosphate

Four hundred ground-water sampling points were used to delineate a plume in an unconfined sand aquifer at a 44 year old septic system servicing a school in Ontario, Canada. A bromide tracer test indicated a residence time of one to two weeks for sewage effluent in the 1.6 meter unsaturated zone beneath the tile bed. This is sufficient time for the oxidation of all nitrogen to nitrate to be complete and for the content of dissolved organic carbon to decrease from about 19 mg/l in the septic tank to about 1 mg/l at the water table. The 15 m wide plume core emanates more than 110 meters downgradient of the tile bed, has detectable dissolved oxygen, high nitrate (20-120 mg/l as N), chloride (42-209 mg/l), sodium (34-101 mg/l), calcium (120-249 mg/l), and above background sulphate, and potassium. Ground-water flow velocity at this site is rapid (100 m/yr); thus the mapped extent of the plume (110 m) represents about one year effluent loading. Phosphate (PO43-) concentrations at the water table (approximately 1-2 mg/l as P) appear to have reached steady state at values significantly lower than that of the effluent (9 mg/l as P). Steady-state concentrations suggest that mineral precipitation reactions control attenuation in the unsaturated zone. A comparison of phosphate sorbed (74 mg/kg) and total P in the soil (1000 mg/kg) suggests that precipitation is a more important process in the unsaturated zone than is sorption. PO43- levels in the plume, however, remain elevated (0.3-1.8 mg/l as P) relative to background levels in ground water (<0.01 mg/l as P) up to 75 meters away from the tile bed. This migration distance of PO43- in ground water is greater than that observed at other younger septic system sites. The extent of the plume at this site suggests that long-term PO43- migration in the ground-water zone may be controlled by adsorption processes that allow slow but progressive advancement of PO43-.