THE TRANSPORT AND FATE OF PHOSPHORUS FROM AN ON-SITE TERTIARY WASTEWATER TREATMENT UNIT AT A THIN-SOIL LOCATION

Near-shore water bodies with limited circulation are susceptible to eutrophication when excess nutrient loading occurs. In areas of high density residential development, it is possible that septic systems may result in release of phosphate that contributes to this loading. However, this study shows that interactions of phosphate with native soils can play a vital role in its mobility and subsequent transportation, resulting in less phosphate in receiving waters than would be predicted. This study examines an eight year-old domestic septic system that incorporates an extra process step utilizing a P-reactive media (steel mill blast furnace slag) that reduced phosphate by 45% before it is loaded onto a leaching bed. This resulted in the leaching bed received 425 g of P during the study season. Of the total P load to the leaching bed, only 15% left the leaching bed and was found in the wastewater/groundwater plume. The groundwater plume has a low PO₄-P concentration (up to 0.35 mg/L). Sediments located within one meter of the filter bed infiltration pipes, have substantially elevated desorbable P (up to 50 mg/kg) which is enrichment by a factor of up to 5000% compared to background values. This zone also has substantially elevated acid-extractable P. Microprobe images show that there are authigenic mineral coatings present on primary apatite grains found in the imported sand. This suggests that P-mineral precipitation reactions continue to occur beyond the primary treatment module and contribute substantially to overall P removal observed in the treatment system. This P retention has resulted in much lower P concentrations in the groundwater plume and may indicate that septic systems do not contribute significant quantities of nutrients to near shore water bodies.