CWA §316(a)


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Services | CWA §316(a) - THERMAL PLUME



Normandeau has been providing thermal and dilution plume surveys for cooling water discharges for utility clients for over 40 years. Many national or State Pollution Discharge Elimination System Permits (NPDES or SPDES) require that permittees demonstrate compliance with specific water quality standards for mixing and dissipating heated discharges or dilution of target pollutants within receiving water bodies. Normandeau has proven expertise in providing accurate thermal surveys of thermal plumes or dilution characteristics of effluent borne pollutants.

Normandeau has conducted original research on temperature preference and avoidance of numerous fish species, field determinations of aquatic organism distribution in relation to thermal discharges, thermal plume distribution, and other related studies. Normandeau has developed analyses and computer modeling of thermal plume distribution under various environmental and discharge conditions.

Our senior consultants have been involved in the entire 316(a) process which includes:

  • identification of desired operational and thermal limitation objectives with clients
  • negotiation of study designs and Representative Important Species with NPDES-permitting authority
  • coordination with key natural resources agencies at the state and federal levels
  • perform field studies, data analysis and literature review
  • coordinate with hydrothermal consultants
  • prepare Demonstration reports
  • provide support for adjudicatory proceedings

Normandeau uses a variety of state-of-the-art equipment for monitoring discharge plumes. For thermal discharges, surveys may be conducted using mobile or fixed equipment. For mobile surveys, we have available Falmouth Scientific, Inc. Excell 2 micro CTDs for in situ measurements of temperature to the nearest 0.1ºC. The Excell 2 probe is also used to measure depth below the surface to the nearest 0.03% (nearest 1.5 cm in water <50 m deep).

More recently, we have been using rapid response thermistors (24k ohm with 30 msec time constant) rigged to simultaneously measure temperature at up to six different depths. Probes for both measurement techniques are attached to either a 30-ft. long vertical boom mounted amidship of the sampling vessels or cable equipped with weights to maintain a constant depth. Both the Excell CTD and the rapid-response thermistors are coupled with DGPS (sub-meter accuracy) using HydroPro software to allow water temperature, depth, and geographic position to be logged and displayed on the on-board computer to provide real-time observations. For fixed temperature surveys, Normandeau frequently uses arrays of Onset Tidbit® v2 or HOBO Water Temp temperature data loggers which operate over a range of 0ºC to +50ºC at an accuracy of ±0.2ºC. We include in the array HOBO Water Level Loggers to measure both temperature and depth. If greater temperature accuracy is required, we may use Star-oddi Data Storage Tags (DTSs) which are mini-temperature and depth loggers with a higher temperature resolution (±0.032ºC) and the ability to measure depth (water pressure@±0.8%).

Measurement of dispersion of pollutants in a dilution plume may be accomplished directly by sampling and laboratory analysis or more commonly by dye injection into the discharge and subsequent monitoring of dye concentrations within the discharge plume. Monitoring equipment would be Turner fluorometers (capable of measuring Rhodamine WT to concentrations as low as 10 parts per trillion), generally operated in a continuous flow mode, coupled with GPS, for realtime measurement of location, depth and dye concentration.

Normandeau staff is experienced in representing clients in regulatory hearings to address hydrodynamics of thermal effluents and impacts of thermal plumes on aquatic biota. Call us today to find out how we can help you meet your 316(a) requirements.