Evaluation of stormwater quality associated with milling of HMA surfaces [hot milled asphalt]

Stormwater quality was examined for roadway runoff collected from milled and unaltered sections of hot milled asphalt roadway. Time series of total suspended solids, and total and dissolved copper, zinc and lead concentrations were examined for two co-located pairs of milled and unaltered roadway sections, one co-located pair of freshly paved and unaltered roadway sections, two milled roadway sections and four replicate observations for an unaltered roadway section. Comparison of event mean concentrations between milled and unaltered roadway sections showed runoff from milled sections to have more suspended solids and higher lead concentrations, but no difference in copper and zinc concentrations from unaltered roadway runoff. Generally, the flushing of materials from the road surface at all locations was flow-driven with incomplete wash-off of roadway constituents during the short-duration convective storm events. Storm precipitation intensity was not an indicator of stormwater quality. Milled and unaltered road surface runoff water quality parameter concentrations in this study were within the ranges reported previously for unaltered rural and urban roadways. The presence of lead in runoff from milled roadway sections in this study indicated a roadside soil source of the solids resulting from overland flow from adjacent land surfaces onto the roadway. Future milling operations should proceed with consideration not to create new hydraulic connections with adjacent roadside areas that do not drain onto roadway surfaces.