Turbidity in highway construction site runoff: Preparing for numerical effluent limits

A numerical turbidity limit that will affect highway construction sites to meet a standard turbidity value in the runoff was to be implemented in 2013. Although the limit appears to be deferred to future permits, this study addresses multiple turbidity issues that are relevant in implementations of numerical limits for construction storm water quality. Background turbidity maps are created to graphically present the natural turbidity background levels in the state of Texas, and those maps are compared with the proposed numerical limit; the median value map is interpreted as supportive of a regionally adjusted numerical limit. The water and solids color effect on turbidity measurement is evaluated, in addition to the use of sample dilution to extend instrument range. Substantial turbidity measurement differences between two different types of turbidimeters (field portable and bench) is explored, and a cause is postulated. Two construction sites were monitored to develop a reasonable protocol for self-reporting, should future permits require such activity. The results suggest that adjustable numerical limits should be used in future permits if such permits require monitoring; water and solids color effects are negligible for intended application; sample dilution can extend instrument range, but diluted samples under-report turbidity; and the selection of instrumentation is nontrivial.