IMPACTS ON A SAND AQUIFER FROM AN OLD SEPTIC SYSTEM - NITRATE AND PHOSPHATE

Four hundred ground-water sampling points were used to delineate a plu me in an unconfined sand aquifer at a 44 year old septic system servic ing a school in Ontario, Canada. A bromide tracer test indicated a res idence time of one to two weeks for sewage effluent in the 1.6 meter u nsaturated zone beneath the tile bed. This is sufficient time for the oxidation of all nitrogen to nitrate to be complete and for the conten t of dissolved organic carbon to decrease from about 19 mg/l in the se ptic 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 detec table 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 bac kground sulphate, and potassium. Ground-water flow velocity at this si te is rapid (100 m/yr); thus the mapped extent of the plume (110 m) re presents about one year effluent loading. Phosphate (PO43-) concentrat ions at the water table (similar to 1-2 mg/l as P) appear to have reac hed steady state at values significantly lower than that of the efflue nt (9 mg/l as P). Steady-state concentrations suggest that mineral pre cipitation reactions control attenuation in the unsaturated zone. A co mparison 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 g round 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-wa ter zone may be controlled by adsorption processes that allow slow but progressive advancement of PO43-.