FATE AND TRANSPORT OF WASTEWATER PHOSPHORUS IN THE NORTH CAROLINA COASTAL PLAIN : A COMPARISON OF WATERSHEDS SERVED BY MUNICIPAL AND ONSITE WASTEWATER SYSTEMS

Abstract

This study compares the phosphorus (P) treatment efficiency of on-site wastewater systems (OWS) and municipal wastewater treatment systems (MWS) and quantifies impacts each technology has on ground and surface water. Stream and groundwater samples were collected and analyzed for Phosphate (PO₄), Total Dissolved P (TDP), Particulate P (PP), pH, turbidity, electrical conductivity, temperature, and dissolved oxygen in watersheds served by OWS and MWS. Networks of piezometers were established at MWS (n=5) and OWS (n=5) sites to determine what influence OWS have on groundwater. Groundwater P loads were calculated using Darcy's equation for groundwater discharge and the measured P concentrations. The phosphorus index - a measure of soil's tendency to leach P - was determined from soil cores taken during instrumentation at MWS and OWS sites. Streams in catchments exclusively containing MWS (n=3) or OWS (n=3) were sampled monthly and during 2 storms for nutrients and environmental parameters such as pH dissolved oxygen (DO), and turbidity for use support. The P concentrations and loads were calculated using two methods to estimate stream discharge. The P loads from the OWS streams were compared to P loads from the MWS streams. Groundwater PO₄ and TDP concentration and loading and soil P-index were elevated down-gradient of OWS in comparison to up-gradient indicating significant mobility of P in groundwater. Groundwater plumes at OWS sites enriched with P were found to reach the stream/groundwater interface leading to the conclusion that elevated P in streams is influenced by OWS. Surface waters in OWS catchments had elevated P concentrations and P exports. The wastewater P loads generated within the MWS catchments and discharged to the Tar River after undergoing MWS treatment, were add to the stream P exports for each MWS catchment to determine the overall surface water P load. The overall surface water P load calculations for the MWS catchments were greater than the OWS surface water loads. Therefore, the wastewater P loads were reduced in a greater percentage in OWS catchments (about 90%) relative to the MWS treatment plant. (54%). Analysis of P concentrations during storms indicates a positive correlation between discharge and both PO₄ and TDP.