The Muddy Waters of the NTU

Andrew Der
March 27, 2012 — 1,164 views  
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The U. S. EPA’s recent withdrawal of its effluent limitation guidelines and associated turbidity criteria continues to show the dynamic changes of regulatory compliance associated with construction and stormwater.  Given that data shows new construction provides a fraction of the muddy water in the Bay and the unreliability of some US. EPA modeling and proposed compliance procedures, this decision is appropriate and equitable.  Presently evolving (acronym warning) TMDL’s, Executive Orders, NPDES MS4 permits, Maryland’s NPDES Construction Permit, SWM ordinances and E/S regulations reminds decision makers not to overlook basic technical concepts in regulatory criteria.

So, what exactly is an NTU and turbidity – and how can (or should it be) applied?  Turbidity is merely the optical transparency of water and can be determined with an instrument called a nephelometer that measures transparency with a probe in Nephelometric Turbidity Units.  The problem with this easy method of measuring turbidity is – well – that it is easy.  Monitoring turbidity appropriately can be a valuable tool for our toolbox but with caution.  For example, the originally EPA proposed NTU of 13 was unrealistic and technically unsubstantiated because it is easily exceeded naturally and in preconstruction conditions.

Further, direct proportional relationships cannot reliably be drawn between the level of turbidity and total suspended solids (TSS) – the more appropriate constituent of concern and is what is most relevant to stream health and water quality monitoring.  NTUs have their place but nature, streams and water chemistry can be complex and making a regulatory leap in the field based entirely on a turbidity measurement alone is not so simple.  The Maryland State standard of 50/150 NTU was originally developed for point source discharges and its application to diffuse measurements of land disturbance around a receiving stream can be challenging.

Transparency is not a pollutant.  Sometimes water can be turbid naturally or from phytoplankton (algae).  Much of the northern Atlantic and the Bay and its tributaries are turbid without our help due to the valuable abundance of phytoplankton and appropriate levels of nutrients and is the basis of our food chains as well as 70 percent of the oxygen we breathe.  The clear water of the Caribbean is actually a desert from a relative lack of life and food chains and is why reefs will pop up as oases.  A certain level of nutrients is naturally desirable - just not too much – and therein lies our challenge.  Less is not necessarily more and while excess sediments and particles always reduce light transmission, the opposite does not always follow.  Clear swimming pools do not make hospitable natural habitats.

All this leaves a lot of questions.  Measuring turbidity at the BMP could indicate relative performance, but the NPDES provisions of the Clean Water Act are applicable to waters of the U. S., so would measuring turbidity at the point it enters such waters better indicate receiving stream conditions?  How would turbidity monitoring correlate to a receiving stream that may already have a preconstruction turbidity in excess of a desired limit due to a pre-existing property use or offsite influence?  Or, if the goal is interpreted to preserve the stream as it is before new construction, what if the water entering a pre-existing muddy stream is less turbid to the point of a “shock”?  This may not seem as farfetched as it sounds with potential active treatment requirements for BMPs.  The use of flocculants could potentially attain such cleaner water – but what is their true net environmental benefit?  Some of the flocculent approaches referenced in the current Maryland E/S manual could allow the use of some compounds, which themselves may have adverse environmental effects or may need to be disposed of when settled out in special waste sites.

Turbidity standards for receiving waters originated for point source discharges.  Turbidity measurements can be constructive under certain conditions, but as regulatory definitions of discharges and applicability have broadened over time into the stormwater realm, we need to be technically cautious when substantiating the generic and wholesale transference of such compliance monitoring protocols to a nonpoint source scenario in an urbanizing – or agricultural - setting.  If something seems too simple, it usually is.

Andrew Der

Whitman Requardt & Associates, LLP

Andrew Der is an Associate and Director of Environmental Services at Whitman Requardt & Associates, LLP head office in Baltimore and practiced in the consulting industry since 2001 previously completing 17 years of service at the Maryland Department