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What the Heck is a Nephelometric Turbidity Unit? If You Discharge Stormwater, You Better Find Out Soon

This article was originally published in the February 2010 edition of the Chester County Bar Association's monthly newsletter "New Matter."

A Nephelometric Turbidity Unit (“NTU”) is a means of measuring the amount of suspended solids in a water sample. The word nephelometric is derived from a Greek work meaning cloud or cloudy. Cloudy water has suspended solids (as well as other dissolved materials possibly) in it.

The conventional manner of measuring suspended solids (“Total Suspended Solids” or “TSS”) in a water sample involves filtering the sample and weighing the filtered material after it has dried. In this way the actual weight of the filterable material is determined, and the solid material can be tested further to determine its precise chemical characteristics.

The device that measures NTUs is called a nephelometer (or turbidimeter), and it measures the amount of light that is deflected or scattered by particles in the water. The measuring device must be calibrated using a chemical standard (in this case a chemical preparation containing Formazine that has predictable light scattering properties).

Both techniques evaluate suspended solids in a water sample, but do not exactly correlate with one another. Some commentators have observed that the nephelometer is a quick and inexpensive way of determining how much sediment is in a water sample. Indeed, when used to measure the success of stormwater control methods (which act to reduce the amount of sediment in runoff) this seems to be suitable, as the regulators are largely interested in the amount of sediment in the water, and not its precise chemical constituents (as might be the case in an industrial waste water discharge).

It is also true that the size and nature of particles in water will affect nephelometer readings. For example, very heavy and coarse particles will quickly settle out of the water sample, and will not be captured by the measurement device. Further, particles of sediment behave differently than particles of algae, and even gas bubbles entrained in the water sample will affect nephelometer readings.

Why is this important? On December 1, 2009, the U.S. EPA issued a final rule which sets monitoring standards for stormwater discharges from construction sites that disturb surface area of 10 or more acres. For the first time, EPA also set enforceable numeric discharge limits on suspended solids in construction site runoff. EPA estimated that the imposition of the rule would ultimately reduce the amount of sediment (and probably other pollutants) discharged from construction sites by an estimated 4 billion pounds per year.

EPA chose turbidity as the water quality parameter that will be measured, and selected the NTU as the measurement yardstick. EPA has set the NTU standard at 280 NTUs, expressed as a maximum daily discharge limitation (EPA originally proposed to set 13 NTUs as the discharge limit (close to drinking water standards for turbidity)! – for example, lakes usually have natural turbidity levels between 0 to 20 NTUs, and streams and wetlands anywhere between 0 to 100 NTUs).

The new stormwater discharge requirements will be phased in over a four year period, and many regulatees likely will be able to comply using passive controls, such as those described in Pennsylvania’s Stormwater Best Management Practices Manual.

However, monitoring and compliance with numeric discharge standards will add costs and increase the level of construction site management and supervision in order to meet the new stormwater discharge requirements.

In the past, EPA has not hesitated to enforce stormwater requirements. Several prominent big box retailers and large home building companies have been fined millions of dollars for stormwater violations.

Accordingly, we think contractors and developers should be working now to develop plans to meet the new stormwater discharge rules.

For more information, please contact John R. Embick at 215.640.8530 or jembick@thorpreed.com.

This Thorp Reed & Armstrong, LLP article is prepared in summary form and is not to be construed as legal advice or opinion on any specific fact or circumstance. We do not assume any responsibility to revise the Communiqué if there are subsequent changes in the law.

#1085916 February 2010