Erosion and Sediment Control
When Waters Are Muddy: A Case Study for Bridging the Divide Between Application and Research
The South Carolina Department of Transportation (SCDOT) currently implements a TSS design criteria for stormwater runoff from land disturbing activities. In 2009, the US Environmental Protection Agency (USEPA) proposed an additional numeric effluent standard for a turbidity limit of 280 NTU. Such a change would result in South Carolina moving from being a design standard state to being a performance standard state, which would likely have considerable financial consequences. For SCDOT to be capable of complying with potential new numeric turbidity standards, assessment of current Best Management Practices (BMPs) and the development of potential new BMPs may be necessary.
To be in position to meet this proposed rule, SCDOT proactively implemented a Federal Highway Administration funded research project with Clemson University to measure and evaluate turbidity in stormwater discharges from active SCDOT construction sites and in controlled experimental field testing of current SCDOT specified sediment control BMPs. The research assessed the application of passive treatment BMPs such as polymers and chemical flocculants. Understanding basic physical and chemical characteristics of polymers and other flocculants associated with SCDOT-specific applications was essential to optimize technical specifications and standard details.
Studies on SCDOT construction sites investigated reducing turbidity with and without a passive granular PAM application at three active roadway construction sites in South Carolina. Results of this study indicate that PAM application may be necessary for effective turbidity reduction. This study shows that granular PAM applied directly to sediment control BMPs can reduce turbidity below a target level of 280 NTU. Under designed controlled conditions, reapplication of granular PAM to sediment control BMPs after periods of dry weather and before storm events consistently reduced turbidity below 280 NTU. PAM longevity is critical in deciding when to reapply as a flocculant for turbidity and TSS reduction. This study recommends that PAM be reapplied at least once every seven days to ensure proper turbidity reductions. Results of this study also showed that proper maintenance and regular inspections should be a priority in reducing turbidity. Infrequent maintenance often corresponded to higher NTU values.
Another goal of the study was to evaluate SCDOT sediment basin design and assess conditions with various surface skimmers and baffle configurations. Controlled experimental results showed that 80% reduction in turbidity could be achieved with either skimmers alone or through a combination of skimmer and baffle arrangements. With addition of PAM, turbidity reduction could be greater than 90%.
Another element of this study compared sediment basin performance using only one baffle with the performance achieved using three baffles. Statistical analysis of controlled experimental results confirmed the 3-baffle configuration performed better than 1 baffle for reducing turbidity discharged from the sediment basin. While there was a statistical difference between 1 and 3 baffles, both resulted in turbidity reductions greater than 90%. For TSS, no statistical difference between 1 baffle and 3 baffles was found. TSS reduction for all configurations were greater than 95%.
The final element of this study included a series of laboratory bioassays to evaluate acute and chronic toxicological effects resulting from exposure to commercially available PAM formulae. The vertebrate Fathead Minnow species P. promelas showed to be the least sensitive in comparison to D. magna in acute exposures as described by LC50 values. The order of toxicity for PAM flocculants was similar for P. promelas and D. magna for acute exposures. Cationic PAM flocculants appeared to be the most toxic. Anionic PAM flocculants showed the least toxicity for all species – except for C. dubia under chronic exposure conditions. Toxicities reported from the study are well above dosage recommendations made by the manufacturers.