Performance of Riparian Buffers Around Onsite Systems in Suburban Settings.

Nonpoint source (NPS) pollution has been recognized as the leading source of water quality problems in the United States.  Onsite wastewater management systems are among the nonpoint sources that may cause water quality degradation.  In a properly functioning septic system, wastewater infiltrating the soil receives treatment in the unsaturated zone before reaching a saturated zone (i.e., groundwater) or a slowly permeable layer where a saturated zone or perched water table may form.  In this study we assessed the movement of water and selected constituents of domestic wastewater through the soil in the buffer areas between septic system drainfields and natural streams.  Three septic systems serving single-family homes located in the NC Piedmont were selected for the study.  The drainfields of two of the systems were near a small running creek, and the third system was more than 150 m from a major creek.  Buffer areas were in flood plains adjacent to the creeks. The soil at each site was characterized and a series of wells, piezometers, tensiometers and time domain reflectometry (TDR) rods were installed at different locations inside and outside the drainfield of each system for collecting groundwater samples and monitoring water level and soil water status. Soil water content and pressure head of the unsaturated zone at different depths, and depth to water table and submergence potential at two depths were measured biweekly.  Water samples from the groundwater were collected once a month using wells and piezometers.  In addition, soil solution samples were collected using tension samplers, and water samples from different locations along the neighboring creeks were collected for analysis.  On average, the amount of wastewater applied to the drainfield of each of the systems was less than 40% of the daily design flow for that system.  Although soil water content under the drainfield of each system was relatively high during part of the year, the soil remained unsaturated allowing the systems to hydraulically function properly.  With few exceptions, the concentrations of both nitrate-nitrogen (NO₃-N) and ammonium-nitrogen (NH₄-N) in the creek water were less than 0.5 mg/L.  Ammonium-N concentrations as high as 5 mg/L were occasionally measured in samples of well water and soil solution collected from the drainfield areas of the systems.  Nitrate-N concentrations greater than 5 mg/L, however, were observed frequently in groundwater and soil solution samples.  Based on these results, it appears that denitrification and dilution within the saturated zone were the primary mechanisms for low concentration of nitrogen compounds in the creek water at these sites.