EFFICIENCIES OF TEMPORARY SEDIMENT TRAPS ON TWO NORTH CAROLINA CONSTRUCTION SITES

Abstract

Sediment export from construction sites is receiving increasing scrutiny, and correspondingly the efficiencies of sediment controls are being questioned. Sediment or total suspended solids (TSS) and total phosphorus (TP) concentrations in outflow from, as well as sediment accumulation, in three temporary sediment traps located on North Carolina construction sites were monitored to assess the efficiencies of the traps. The trapping efficiency of the trap located on a Coastal Plain site (Woodsong) was 69%, while the efficiencies of two traps located on a Piedmont site (Carpenter) averaged 59%. In addition, the Carpenter trap retained 30% of the TP coming off the site, while the Woodsong trap retained 9%. Sediment size analyses of a limited number of samples indicated that the Woodsong trap retained 91%, 43%, and 21% of the sand, silt, and clay primary particles entering the trap, while the Carpenter traps retained 68%, 72%, and 40% of the sand, silt, and clay particles entering it. The turbidity of outflow samples was also measured and correlated to TSS concentrations. A relatively strong linear correlation was found for data from the Carpenter traps (r2 = 0.96), and a weaker correlation was documented for the Woodsong trap (r2 = 0.64). These data indicate that for sites with high TSS concentrations in runoff and relatively little organic matter left on the site, TSS may be computed from turbidity; however, more data is needed to confirm this assertion.