Factors influencing the behavior of hydrocarbons in stormwater runoff during quiescent settling

A laboratory investigation was conducted to identify the factors responsible for influencing the sedimentation of hydrocarbons in stormwater runoff during quiescent settling. Bulk runoff water samples were generated by simulation of a typical rainstorm. An area of pavement at a gasoline filling station was washed down with tapwater and the resultant runoff was collected and transported to the laboratory. Four large, clear Plexiglass cylinders were filled with well mixed bulk runoff and then allowed to quiescently settle. Interval composite samples were collected at specified times and at four depths from each of these columns. Samples were analyzed for total hydrocarbons (THC), total organic carbon (TOC), total suspended solids (TSS), and turbidity. Analytical results were statistically evaluated using multiple linear regression techniques. Relationships between the dependent variable (THC) and the independent variables (TOC, TSS, turbidity, and time) were determined. The entire experiment was repeated three times. Sedimentation of the majority of the hydrocarbons in the runoff occurred within 18 hours. Statistical results indicate that the behavior of THC during quiescent settling is highly dependent upon TSS concentration and time, and less dependent upon the concentration of TOC. Results of this investigation may provide guidance for improvements in the design of facilities for handling urban stormwater to minimize contamination of receiving waters and sediments due to the hydrocarbon load transported by runoff.