Abstract
Literature sources suggest that the particle size distribution (PSD) in highway runoff changes during sample storage. The change is believed to be due to auto-flocculation of particles, causing aggregation of small particles to form large particles. The measurement error caused by this process would require samples to be measured very soon after collection, making it difficult to accurately analyze samples collected at remote sites and composite samples which are collected over time. In this study, the effect of sample holding time on the measurement of PSDs in highway runoff is investigated.
Five highway runoff samples were collected in five different storm events in Sacramento, California. To observe the changes in the particle size distribution over time, a series of PSD measurements in the size range of 0.8 to 20 µm were made at different time intervals up to four days after sample collection. A novel method of using Sodium Dodecyl Sulfate (SDS) to prevent particle aggregation and to extend the allowable sample holding time was also attempted.
The results of the study were that the number of particles measured decreased as storage time increased for samples without SOS. The difference in total numbers of particles was statistically significant after only 15 hours of holding time. In contrast, samples with an SDS dose of 7.5 mL/L did not show statistically significant changes in particle counts for up to 97 hours after sample collection. When dosed with the same concentration of SDS, samples from the Tahoe Basin showed an increase in particle numbers with time. The different responses of different samples to the same SDS dose suggests that the optimal SDS dose depends on site-specific factors.