United States Department of Transportation
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Edition:Final.
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Abstract:In this study, the Regional Transportation District’s (RTD’s) light rail operations were examined for pollutant production and runoff. To
accomplish this, a laboratory study utilizing a rainfall-runoff facility was conducted. Input to this laboratory model was provided by
using RTD’s design criteria, data from existing installations, and a field study to sample surface materials along ballasted tracks. A
rainfall-runoff physical model of the light rail system was constructed at the Colorado State University Hydraulics Laboratory to study
the effectiveness of the as-built ballasted tracks in the railroad environment. This model was subjected to Denver hydrology and
environmental conditions using the available local rainfall information and pollution data. A 1-to-1 model of an 8-foot railroad segment
was constructed using RTD’s design criteria and materials. A rainfall simulator was designed to vary rainfall duration and intensity. The
model had the capability of capturing all of the runoff for volumetric measurement of the quantity and quality of the runoff.
Potential sources of pollutants from a light rail system are: i) metal introduced from track abrasion; ii) metal from wheel abrasion; iii)
material from disk brakes; and iv) material from overhead power lines, etc. These quantities were computed using RTD’s maintenance
records for wheel truing, brake rotor maintenance, track replacement, copper power line replacement, and field sampling of light rail
tracks. For field measurements, toe regions of tracks near the most heavily travelled sections of the light-rail system were sampled at
RTD’s Broadway Light Rail Station. The measured iron and aluminum concentrations from the field samples were introduced into the
laboratory ballasted-track model and were subjected to various rainfall events. Runoff water and soil samples collected during and after
different frequency events were analyzed to trace the effectiveness of ballasted tracks for capturing pollutants.
Conclusions from the study:
Water leaving ballasted tracks carries only a small fraction of the heavy metals that are introduced into tracks from the light rail
operations.
Heavy metal concentrations at the most heavily travelled light rail station showed that pollutant concentrations were far below
the regulatory limits.
Implementation:
It is recommended that the CDOT use the findings of the study in the CDOT Drainage Design Manual.
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