PHOSPHORUS RETENTION IN ON-SITE WASTEWATER SYSTEMS IN NOVA SCOTIA

Abstract

The use of residential on-site wastewater systems (OWS) is one of the main non-point sources of phosphorus (P) loading to aquatic systems, which is the primary cause of accelerated blue-green algal growth and eutrophication of surrounding aquatic systems. Reseach is lacking on the long term treatment performance of the OWSs currently used in Nova Scotia. This study examines how P is removed and retained within OWS disposal fields to gain a better understanding of how to limit P loading to surrounding aquatic systems and aid in the management of OWSs across Nova Scotia in regards to P loading. The first phase utilized an existing set of field scale OWSs at the Bio-Environmental Engineering Center in Truro, Nova Scotia. Two types of disposal fields, which had been receiving wastewater for 4-8 years, were sampled. These included C2 raised contour trench systems (C2R) and lateral flow sand filter (LFSF) systems. The second phase of the sampling was conducted in the Thomas Brook Watershed (TBW) located in Berwick, Nova Scotia. Three full-scale on-site wastewater systems that range in age from 10-20 years old were selected for sampling. Soil samples were taken at various depths, and at various spatial locations, within each disposal field to assess the spatial distribution of the P in the disposal field. The three full-scale systems in Berwick each had 6 piezometers installed in the disposal field for the collection of water samples on a monthly basis for one year to capture seasonal variations in P concentrations. Electron microprobe analysis and back scattered electron images were used to identify P-rich solids on soil grains to determine if mineral precipitation reactions involving P could be occurring within the disposal field as a possible long term treatment mechanism. This study highlights the importance of the physical and chemical characteristics of the sand media used in the disposal fields to increase P treatment. The soil media used in the OWSs observed in this study showed a minimal amount of P treatment as they mature, which should be taken into account particularly in watersheds that are currently experiencing water quality issues and are at risk for eutrophication of surrounding surface water.