Turbidity monitoring on construction sites: insight into the factors influencing the turbidity and TSS relationship

Abstract

Stormwater runoff from construction sites can transport eroded sediment to nearby water bodies degrading water quality and impairing biotic communities. The United States Environmental Protection Agency (EPA) is considering a turbidity limit for effluent stormwater on construction and demolition sites and is requesting data to support that limit (EPA, 2011). Laboratory protocols have been developed herein for studying the factors that impact turbidity from construction site soils. Experimental procedures include the use of a rainfall simulator to generate runoff and turbidity values from soils carefully packed in appropriate test boxes. Turbidity characteristics of fourteen different soils in Minnesota were investigated using the laboratory protocols. Trends in turbidity with sediment concentrations were well represented by power functions. The exponent of these power functions was relatively constant between soils and the log-intercept, or scaling parameter varied substantially among the different soils. Multiple soil properties were evaluated for each soil. An extensive regression analysis resulted in a model using percent silt, interrill erodibility, and maximum abstraction that best represented the intercept term. A power value of 7/5 was chosen to represent all soils. A second laboratory experiment was performed to determine how particle settling affects the coefficients of the turbidity -TSS relationship. The scaling parameter increased with sediment deposition and the power value decreased. Field studies on two construction sites in the Twin Cities of metropolitan area of Minnesota were performed for this project. Grab samples and continuously monitored turbidity were collected for each site. The grab samples also exhibited a strong power relationship between turbidity and TSS with similar coefficients as the laboratory samples. The laboratory relationships were also applied to the observed field conditions to demonstrate their usefulness in estimating turbidity, concentration, sediment load, and load reduction.