EVALUATING THE INFLUENCE OF ONSITE WASTEWATER TREATMENT SYSTEMS ON STREAM NITROGEN CONCENTRATIONS IN A PIEDMONT WATER SUPPLY WATERSHED, FALLS LAKE, NC

In the southeastern Piedmont region, the majority of lakes are manmade reservoirs, and the growing population is heavily reliant on these for water supplies. Falls Lake, North Carolina, a multi-use reservoir in the Research Triangle region, is experiencing nutrient-related impairment. Recent efforts have improved estimates of various nutrient inputs, however, assessments of nutrient inputs from onsite wastewater treatment systems (OWTS or septic systems) are limited. This research focused on the influence of OWTS wastewater inputs on baseflow stream nitrogen concentrations in the Falls Lake watershed. Sites were selected in (26) sub-watersheds across a gradient of septic system density (0-2.3 systems/ha). Baseflow stream water N concentration and streamflow data were collected monthly (2020-2021) along with additional N-15 in nitrate samples. Median annual nitrate-N (0.78 mg/l) and total dissolved nitrogen (TDN: 1.67 mg/l) concentrations were elevated for the sub-watersheds with high densities (>1 system/ha) of septic systems relative to a sewered control watershed (nitrate-N: 0.25 mg/l, TDN: 1.35 mg/l). Stream nitrate concentrations and N-15 in nitrate had positive correlations with septic system density, indicating an influence of onsite wastewater contributions to stream nitrogen concentrations. The highest stream TDN concentrations were observed in sub-watersheds in the Triassic Basin region of the watershed, where poorly drained soils are common, suggesting that soils and hydrogeologic setting can influence onsite wastewater nitrogen contributions to streams.

Presentation Preference: Oral

Primary Presenter: Mike O'Driscoll, East Carolina University (odriscollm@ecu.edu)

Authors:

Michael O'Driscoll, East Carolina University  (odriscollm@ecu.edu)

Jennifer Richardson, East Carolina University  (richardsonj16@ecualumni.ecu.edu)

Charles Humphrey, East Carolina University  (humphreyc@ecu.edu)

Guy Iverson, East Carolina University  (iversong18@ecu.edu)

John Hoben, Virginia Tech  (jphoben@vt.edu)

Natasha Bell, Virginia Tech  (natashabell@vt.edu)