THE INFLUENCE OF WATERSHED WASTEWATER MANAGEMENT APPROACH ON SHALLOW GROUNDWATER AND SURFACE WATER NITROGEN LOADING, PITT COUNTY, NC

Excess nitrogen (N) poses a threat to surface water and groundwater quality in the Coastal Plain of North Carolina. A better understanding of N contributions from on-site wastewater treatment systems (OWTS) and centralized sewer systems (CSS) to surface waters is needed to determine the role that domestic wastewater management has on water quality. The goal of this study is to determine if significant differences exist in watershed N concentrations and loads between residential areas served by OWTS relative to CSS. Piezometers (65) were installed at 5 residential sites in one watershed with OWTS, 3 of which are monitored intensively, and 5 sites in another watershed with CSS. Staff gauges were installed in 6 streams, 3 in CSS-served watersheds, and 3 in OWTS-served watersheds. Groundwater and surface water samples were collected and analyzed for N concentrations and speciation, pH, temperature, specific conductance, dissolved oxygen, and turbidity. Surface water samples from baseflow were collected monthly whereas groundwater samples were collected quarterly for a year. Watersheds served by OWTS had greater dissolved N concentrations (mg/L) in background groundwater (3.63 ± 2.65) and surface water (1.07 ± 0.32) than watersheds served by CSS (background groundwater=1.40 ± 3.14, surface water= 0.63 ± 0.33). Dissolved N concentrations in groundwater at the intensively monitored sites, declined from the septic tank (55.3 ± 25.9) to the drainfield (11.9 ± 15.2) and to the near-stream (6.18 ± 3.83). Dissolved N concentrations of groundwater from the near-stream were significantly elevated relative to background groundwater within OWTS watersheds, but were similar to the Greenville Utilities Corporation wastewater treatment plant effluent N concentrations (6.9 mg/L). TN surface water loads (kg/yr/ha) in the OWTS watershed (5.33 ± 5.23) were greater than the CSS watershed loads (1.69 ± 1.49). Preliminary results indicate that N loads and concentrations of groundwater and surface water in OWTS-served watersheds were significantly higher than in CSS-served watersheds. However, wastewater generated within the CSS watersheds was not treated and dispersed within the CSS watersheds. This study highlights the importance of wastewater management strategies that can influence water quality at the watershed-scale.