Turbidity Monitoring and Overcoming Error
As an industry, we are all perched on the edge of our seats, waiting for what could be the most impactful regulation to be refined: the EPA Construction & Development Effluent Limitation Guideline. The ELG rule has been finalized, though the final numeric limit has been vacated. We await EPA’s final determination of a scientifically sound and feasible number. But the bigger issue may not be the actual number. The bigger issue may in fact be the implementation of that number by developers and regulators and compliance monitoring by regulators.
In an effort to understand how to implement a numeric limit, the Tennessee Department of Transportation (TDOT) began turbidity monitoring when it seemed imminent that Tennessee’s construction general permit would contain the ELG numeric limit. Initial monitoring included grab samples, using hand held or bench top equipment. When the numeric limit was vacated, the Tennessee construction general permit was issued with the narrative criteria and the numeric limit was, of course, left out. However, a substantial amount of resources had been put toward establishing basic monitoring procedures by both the Tennessee Department of Environment and Conservation (TDEC) and TDOT. TDOT decided to continue its monitoring at select sites in the four regions of Tennessee to establish sampling protocols and to better understand how to effectively monitor its projects.
During monitoring, many very valuable lessons were learned, and all have been shared with EPA. The data collected have been shared with EPA as well. This project quickly identified hurdles with implementation. While achieving the daily maximum average turbidity value will be the ultimate goal of the regulation, the site operator must determine sampling/monitoring equipment type, establish sampling procedures, identify sampling locations, and show compliance. The following sections describe some of the hurdles of doing just that.
- Determining equipment type. At this point, EPA has not mandated the type of equipment or sampling method to be used. Portable or automated turbidity meters can be used. Portable meters require a field person to actually grab a stormwater sample and test it either there at the outfall location or back in the job trailer. The number of samples taken is determined by the field personnel or by the standard operating procedure for monitoring established for the project. A grab sample is taken during flow, but represents only the value for that time. Automated meters can be set to read turbidity at established intervals, such as every 15 minutes during flow. The automated meter can capture readings throughout the storm event, providing a larger data set for determining the daily maximum average.
- Each meter also can have a different light source. Since turbidity readings are based upon the amount of light scatter that is detected through a column of water, the type of light source a meter has can affect the turbidity number. So, for example, in our lab testing, a sample may read 29.9 nephelometric turbidity units (NTUs) on a Hach 2100Q (tungsten lamp light source) and 60.7 NTU on a 9500 (LED light source). The same column of water had two different turbidity readings simply due to the type of equipment chosen. Which number is correct?
- Understanding equipment limitations. Every turbidity meter has limitations. For example, the Troll 9500 turbidimeter can read turbidity in the range of 0-2,000 NTU with an accuracy of ±5% or 2 NTU, whichever is greater. The Hach 2100Q has a range of 0-1,000 NTU with an accuracy of ±2%. Considering accuracy, when the Troll meter reads 1,200 NTU, the true turbidity is somewhere between 1,140 NTU and 1,260 NTU. From a compliance perspective, what is the true turbidity reading?
- Another issue to consider about equipment limitations is the range of turbidity readings. The Hach 2100Q only reads from 0 to 1,000 NTU. If this meter is used in the field, each sample that exceeds the 1,000-NTU limit will have to be brought into the lab (or construction trailer) and diluted. Based upon the EPA’s Guidance Turbidity Provisions, Chapter 3, “samples should be measured expeditiously after being secured to prevent changes in particle characteristics due to temperature and settling.” This implies that grab samples should not be held but should be immediately read, causing conflict with the need to continue grabbing and reading samples during discharge, and the need to dilute samples that read higher than the meter’s range.
So, What’s the Solution?
Considering the inherent limitations of the equipment noted above—and not even considering the human error in sampling, such as correct equipment calibration, collection procedure infractions, and sample collection—what is the solution? How can a regulator determine compliance? Perhaps it is more appropriate to take a different approach to a turbidity limit. Establishing a range for turbidity may be more appropriate, given all the error being introduced into sampling. The compliance strategy could be a turbidity range based upon equipment accuracy or established through a tiered approach, such as the action levels included in California’s construction permit. One thing that seems painfully clear is that a specific number will be very difficult to verify and to regulate, given the uncertainties in both sample equipment and sample techniques.
Beth Chesson, CPESC, CPSWQ, serves on the IECA Board of Directors and is a member of the Executive Committee and Government Relations Committee.