Sediment Monitoring Bias by Autosampler in Comparison with Whole Volume Sampling for Parking Lot Runoff

摘要

A field study was conducted to assess biases of suspended sediment concentration (SSC) analyses(ASTM standard D 3977-97) performed on discrete samples obtained by auto-sampler in comparisonwith actual sediment concentrations from whole volume sampling. Research results indicate that thebiases attributed to monitoring of sediment event mean concentration (EMC) and particle sizedistribution (PSD) in parking lot runoff by automated samplers (non-isokinetic) can be insignificant.Large volume samples (~15,000 L ) of the full pipe cross-section were taken from a stormwater sewersystem for eighteen storm events over two years to monitor for EMCs and PSDs. Concurrently, flowweighted grab samples were obtained by automatic samplers throughout the entire large volumesampling period. Thus characteristics of sediments were compared from a “whole storm” sample withthat of “sub-samples” obtained by an autosampler using non-isokinetic sampling. SSCs and particlesize distributions were compared for the two respective field sampling methods. The two methodsshowed a strong correlation for median sediment EMCs (R2=.9801, n=18). Biases to particle sizedistributions were found to be largely confined to the large particles (>160 um) representing less than25% of the load by mass. Moreover, sediment particles captured by the whole volume sampling andautosampler were not significantly different (α=0.05) for D50’s=0.067 and 0.047 microns respectively(particle size at which 50% of the sample mass is less).