GRAIN SIZE CHARACTERISTICS & POTENTIAL MOBILITY OF ROAD TRACTION SAND IN WORCESTER, MASSACHUSETTS

We hypothesized that stormwater runoff may transport traction sand, applied to roads by municipalities where snow and ice are prevalent during the winter months, into local streams, thereby increasing sediment loads with potentially negative environmental effects. We compiled municipal records of road sand application and removal for the 13.4 km², urban Beaver Brook Watershed, located in Worcester, Massachusetts. We also monitored precipitation events, stream discharge, and suspended sediment concentration in Beaver Brook.

Annually, the City of Worcester deposits ~1.9x10⁴ Kg of sand on city roads, ~2.6x10³ Kg of which is applied in the Beaver Brook Watershed. The “sand” purchased by the city also contains 6.1 ± 0.2 % silt and clay (N=2). Grain size analysis shows that road sand applied during the winter months may have a narrower grain size distribution than road sand collected in the spring, suggesting that road sand breaks down after abrasive wear and the silt and clay fraction may be washed into streams. Silt and clay may contribute as much as 5.44 ± 2.44 mg/L of suspended sediment to the Blackstone River downstream if it all runs off during spring snowmelt. Because suspended sediment values in Beaver Brook range between 2.1 mg/L to 50.1 mg/L during the summer, the contribution of fine particles from road sand is probably not a significant environmental concern. The majority of coarse sand applied during the winter is likely to be recovered from storm drain catch basins and in the city street sweeping program. Sand tracer experiments show that a flow of 0.004 cms (0.141 cfs) is sufficient to entrain and remove sand in the ≤ 2 mm fraction, and a flow of 0.031 cms is sufficient to entrain and remove sand in the 4 mm fraction. Entrainment of 4 mm sand occurs when there is at least 2.3 cm of daily rainfall, an amount equaled or exceeded approximately once a week on average. Therefore coarse sand that does reach the channel is rapidly flushed from our relatively steep study reaches. While the City of Worcester applies ~19,000 Kg of sand to local roads annually, it appears that the city does a sufficient job maintaining road sand to minimize negative environmental consequences.