Nitrogen and Phosphorus Loading from Septic Systems in Small Piedmont Watersheds in North Carolina Estimated from Stream Monitoring Data.

Septic systems are under increasing scrutiny for their potential to contribute significant nutrient loads to downstream nutrient-sensitive water bodies.  Of particular concern in North Carolina has been from systems in water supply watersheds in the Piedmont region of the state (e.g., Falls and Jordan Lakes, which serve Raleigh, Cary, and other municipalities). Nutrient loads from septic systems and their potential for reduction are included in evolving management strategies being devised for existing and new residential development.  As with other sources, it is important to determine as accurately as possible actual and relative load contributions so that efforts and ultimately expenditures for modifying system management result in truly beneficial reductions in nutrients reaching receiving waters.  Estimates to date have been primarily made by applying large-scale calibrated water quality models (e.g., WARMF, GWLF, SWAT, HSPF).  These require assumptions to be made that quantify transport processes from the septic tank to the receiving waters.  While field research exists relevant to transport assessments for systems in the Coastal Plain, work in the Piedmont region has been much more limited. 

Stream monitoring from small Piedmont watersheds dominated by homes with septic systems provides the best source of data for more accurately determining their potential nutrient load contributions.  Sources evaluated include water quality and flow data collected by the United States Geological Survey (USGS), North Carolina State University (NCSU), and local stormwater programs from small watersheds in Durham, Wake and Orange counties in the Falls and Jordan Lake basins. Nitrogen and Phosphorus loads from small undeveloped watersheds are compared to data collected from subwatersheds with low to high densities of septic systems. Data indicate that three potential contributing conditions can be characterized – base flow, storm-water flow, and after-storm recession flows. Data presented do not substantiate a significant contribution of N and P attributable to septic systems during base flow conditions. Continued attention, however, appears warranted on the relative potential impacts during peak storm flow periods from malfunctioning systems and from storm water recession periods, perhaps even from properly functioning systems, when groundwater potentially transporting concentrated septic-generated loads may be highest.  Unfortunately, partitioning the septic system contribution from other sources during the latter two conditions is difficult. Using data from small septic-dominated watersheds such as those reported on herein should be considered for future monitoring and modeling efforts. It is hoped that the information presented will help advance our understanding and ability to more accurately quantify septic system nutrient contributions in Piedmont settings, and point to where more quantification research may be most productively directed.