UPDATES: October 2012


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Evaluation of the Factors Impacting the Turbidity of Construction Site Runoff

October 2012 (volume 7 - issue 9)

Contributed by Rebekah Perkins, Graduate Research Assistant, Biosystems and Bioproducts Engineering, University of Minnesota (Advisors: Bruce N. Wilson and John S. Gulliver)

Funded by Minnesota Department of Transportation (Dwayne Stenlund, Technical Liaison)

Stormwater runoff from construction sites (Figure 1) is a substantial source of non-point source pollution. This runoff contains eroded sediment from the exposed, barren ground that is often transported to nearby water bodies causing water quality impairment, degrading their biotic communities and filling up the water body with sediment. To mitigate these negative impacts, it is important to determine the amount of eroded sediment in runoff. The quickest and most cost effective method of doing so is to measure the turbidity of the runoff. Turbidity is an optical property of water associated with the light scattering properties of the particles suspended in water. This measurement can be used as a surrogate to determine the actual concentration of sediment in construction site runoff. If addition, high turbidity can be the primary pollutant for visibility in the water body, etc.


Figure 1: Runoff off of a local Twin Cities construction site. (Photo: Brad Hanson)

The United States Environmental Protection Agency (USEPA) is working toward setting a turbidity limit for construction and demolition sites. Suitable guidelines are needed for contractors to be able to accurately and cost-effectively monitor the turbidity of the runoff from their construction sites. Research is being conducted at the University of Minnesota evaluating construction site turbidity both in the field and laboratory. An important goal of this research is to develop protocol for turbidity monitoring that is easily understandable, repeatable and adaptable to a majority of construction sites. The research is also being done to gain insight into the factors that affect turbidity in construction site runoff. Both of these activities will be useful in establishing a reasonable turbidity standard limit for Minnesota.

Sampling of turbidity on a highly mobile construction site is a challenge. We discovered that a new measurement apparatus was needed that can be put into place without generating scour and can facilitate the turbidity instrumentation, yet can be moved easily and placed in a new location when needed. The development and features of this apparatus will be described in a later issue of UPDATES.

While our field work has provided knowledge about the benefits and shortcomings of the current monitoring techniques, we also quickly realized that our ability to attain insight into the factors affecting turbidity field work was limited due to the unpredictable site conditions and limited quality of field data. We thus incorporated a laboratory experiment that allowed for a controlled setting and a repeatable process for many different soils. The laboratory experiment relied on the acquisition of 14 soils from all over Minnesota. Synthetic runoff was created using a rainfall simulator (Figure 2) that rained on the soil for 30 minutes at an intensity corresponding to the peak intensity of a 2 year, 24 hour storm. The runoff from these soils was collected and thoroughly examined so that trends in turbidity with soil characteristics can be determined and used to develop a predictive relationship between them.


Figure 2: Laboratory apparatus including rainfall simulator, soil box, and collection basin. (Photo taken by Rebekah Perkins)

During the 30 minute rainfall, 50 mL samples of runoff were collected every 5 minutes and analyzed to understand how turbidity and concentration change over time during a rainfall. Each sample was then diluted, and concentration and turbidity values were recorded for each dilution to create time dependent concentration vs. turbidity curves, such as seen in Figure 3. After performing a regression analysis with different possible functions, it was seen that a power fit best represents the data.


Figure 3: Example of dilution curves. Regression equations are reported under the legend for each time sample.

From the regression equations in Figure 3, it is seen that the exponent values are close, but the constants before the exponent are different. The variation in this constant indicates that the relationship between turbidity and concentration varies with time. However, similar exponents suggest that these differences can largely be represented by properly selecting the constant before the exponent.

All of the dilution curves for the 14 soils and several replicates are plotted in Figure 4. The soils followed a pattern from left to right: the most silty soil, OV S, has the largest exponent on the left side of Figure 3. Moving to the right, the soil’s sand content increases and the silt content decreases.


Figure 4: Dilution curves for all 14 soils plotted together.

Figure 4 shows the dramatic difference among soils in the relationship between turbidity and suspended sediment concentration. For a given sediment concentration, the turbidity can clearly vary greatly depending on the type of soil. For some soils, only a few grams of soil per liter will skyrocket turbidity well over 1000 NTU, the highest readable turbidity value for many field meters. This trend as well as many soil characteristics will be considered in an analysis of the data.
Laboratory data are more useful if they are representative of runoff on construction sites. Similarities between laboratory and field data were evaluated by comparing results obtained at a construction site with those gathered in the laboratory for the same soil. Results for these conditions are shown in Figure 5. Three field samples collected at different times for the same runoff event and twelve 50 mL samples from the laboratory experiment are shown in this figure. The average exponent value for the field and laboratory samples is 1.38 and 1.39 respectively. The constant before the exponent values for the field samples are larger than those of the laboratory data. The runoff from the site flows over a considerable distance. Over this distance, the deposition of larger particles is likely, resulting in a finer distribution of sediment in the field sample. Larger constants before the exponent for finer distributions of sediment are consistent with observed trends in Figure 4.


Figure 5: Field samples 3, 12, and 20 and laboratory dilution curves from two replicates of AHS, soil from the same location, plotted together.

A more rigorous analysis of laboratory data is currently being conducted. From the analysis of the 50 mL samples, we are investigating relationships between turbidity and concentration for the power function defined as:

Turbidity = α*(Concentration)β

where α and β will be determined based on data analysis and soil characteristics.

Our next steps include a detailed regression analysis to understand how parameters of α and β change with runoff samples and soil characteristics. An understanding of the α and β values will be useful in extending the laboratory work to conditions at construction sites. These steps are necessary to predict the effectiveness of sedimentation ponds and other practices in reducing turbidity from construction sites. The results will also be useful in establishing turbidity standards for Minnesota.

References

  • Ankcorn, P. D. “Clarifying Turbidity – The Potential and Limitations of Turbidity as a Surrogate for Water-Quality Monitoring.” Proceedings of the 2003 Georgia Water Resources Conference, Athens, GA, 23-24 April 2003.
  • Jastram, J. D. , Zipper, C. E., Zelazny, L. W., and Hyer, K. E. “Increasing Precision of Turbidity-Based Suspended Sediment Concentration and Load Estimates.” J. Environ. Qual., 39, 1306-1326. 2009.
  • Hach. “Determining the Relationship Between Turbidity and Total Suspended Solids”. Analytical Procedures. Method 8366.
  • Patil, S. S., Barfield, B. J., and Wilber, G. G. “Turbidity Modeling Based on the Concentration of Total Suspended Solids for Stormwater Runoff from Construction and Development Sites.” World Environmental and Water Resources Congress 2011. Palm Springs, CA, 22-26 May, 2011.

We want to hear from you!!!

Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions:

  • What challenges have you encountered when monitoring runoff on construction sites?
  • Based on your experience, is a turbidity effluent limit realistic? Why or why not?

I like where you are going with this. One of the problems that i have encountered is a dramatic difference in readings of the same sample with different NTU meters. I had three meters on a site (including Dwayne Stenlund's) and found a significant differences. I think that we also need to be thinking about how this is going to work in the real world where most folks don't have and can't afford expensive NTU meters that may or may not give an accurate reading. On this same site, we also were using a T-tube, which was pretty simple to use, yet not very accurate in extrapolating NTU's from the T-tube reading.

Great point Jay. Turbidity meters are expensive and often times do not read turbidity accurately because the water being sampled is not sufficiently deep or the meter isn’t installed properly. It is hard to say what the best method for measuring turbidity on a construction site is because all methods have pros and cons and the sites themselves are so dynamic. We are currently comparing multiple meters with sample turbidities to see what the actual differences between meters are. From that study, we should know what differences you should expect between meters which should help in knowing if the meter is the problem or if the installation is the problem. The simplicity and cost of the T-tube obviously makes it attractive. However, the possibility of very high turbidities on a construction site may not make it the best option. Tools like that are usually better suited for lakes and rivers. If nothing else, taking grab samples on a site and analyzing them with a bench top turbidity meter may be an option if cost is the issue. You still need to pay for the meter upfront, but it eliminates the chance of a meter being lost, stolen, or damaged on site.