Turbidity of Stormwater Runoff from Highway Construction Sites

The US Environmental Protection Agency (EPA) proposed a new “Effluent Limitations Guidelines” for construction site runoff management in 2009, which required that stormwater effluents be sampled and that the daily average turbidity must not exceed 280 NTU. Although this numeric limitation was removed from its final amendment in 2014, the EPA still reserves the right to reconsider it in future regulations. To better understand the limits of the turbidity in the effluent, stormwater runoff samples were collected at different times during storms from five highway construction sites in Wisconsin. Measured turbidity in grab samples taken during or after storms ranged from 20 to 2,300 NTU. Sample analyses showed that a power law relation can well describe the correlation between turbidity and the total suspended solids in construction site effluent. Automated turbidity sampling devices were deployed at outfalls of four selected sites to continuously record the time series of turbidity. Statistical analyses were conducted on turbidity data and the precipitation time series. Both the peak and the maximum daily average turbidity values were found to be linearly correlated with the total precipitation, while they are not correlated with either the average or the peak rainfall intensity. It is also demonstrated that a characteristic unit runoff turbidity function, which represents the response of turbidity with respect to a unit precipitation depth, can be reconstructed from a measured time series. Reconstruction of the turbidity response function and the observed statistical correlations suggest that it is possible to develop models to predict the daily maximum turbidity and the total turbidity load of construction site runoff for designated storm events. Models of this kind are valuable for future management practice and for evaluating new regulation policies.