Lesson 7 of 6
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Module 14: Compliance

June 3, 2021

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Each dredging project site will be different regarding Turbidity Testing. Where will testing take place? How often are tests required? How many locations require testing? Those are a couple of questions that will be answered in the specs or when the project gets underway.

The most common monitoring setup relies on three measurement locations. The chosen measurement locations are known as points of compliance. At these points, the USACE suggests using stationary or towed turbidity sensors in conjunction with water column sampling.

Selecting the Location

Stationary systems are set at fixed locations. These systems can be placed on structure or on an anchored buoy in the water. Seawalls are a common example of a structure that can be used, as turbidity monitoring sensors can be mounted in a perforated PVC pipe along the seawall. However, site conditions often make it difficult or impractical to mount the monitoring equipment along the shore. In addition, relying solely on shoreline solutions may not accurately represent re-suspension levels and sediment transport due to the nature of water currents. For these reasons, buoy-based systems offer the most comprehensive and cost-effective solution. Data buoys provide a stable platform for turbidity monitoring, with the ability to house multiple sensors at different depths in the water column. In addition, they can carry a data logger, solar-powered battery pack, and telemetry (wireless communications) systems for extended deployments.

Anchored buoys are usually located both up and downstream of the dredging operation. The downstream buoy(s) measure re-suspended sediment and sediment transport in areas at risk, while the upstream (and off-site) buoy serves as a control for suitable turbidity levels. These buoys can come in different sizes based on the environmental conditions and the number of sensors suspended from the buoy. Data buoys with 150- to 450-lb. net buoyancy are generally adequate for monitoring projects on rivers, inland lakes, and protected waters. Larger platforms may be required in coastal and Great Lakes deployments that are subject to more extreme conditions and wave action.

Towed sensors are known as “floating” points of compliance, as they move with the dredging operation. While these can be attached directly to the dredging equipment, they can also be deployed on a buoy system a set distance away from the dredge location. The buoy systems then move with the dredge as the project progresses, remaining the same distance away from the active dredging site.


For accurate data, all sensors should be calibrated shortly before the project begins. If using a buoy-based system, the platforms should be fully assembled on shore prior to deployment. This includes attaching any sensors, towers, solar panels, and additional ballast weights if needed. Furthermore, the complete turbidity monitoring system (sensors, data logger, telemetry, software) should be tested before the buoy is put in the water. While this process ensures that all equipment is functioning within specifications, it also gives everyone the chance to familiarize themselves with the system prior to deployment. Issues are always easier to deal with before the buoy platform is deployed in the water.

Buoy Mooring

A buoy-based system must be moored to ensure that it remains stationary. It is recommended to moor the buoy in the deepest part of the waterway to ensure the most inclusive measurements. This allows for multiple measurement depths and will best reflect the characteristics of the water body as a whole. Buoy-based systems are typically moored as either a single-point or two-point mooring, based on environmental and application-specific factors.

Data Management

A major part of any monitoring project is collecting and accessing the data. While it is possible to simply record measurements onsite, the ability to log, send and view monitoring data in real time is far more efficient.

Quality Assurance

Accurate data ensure that dredging operations remain in compliance with set regulations. To maintain that accuracy and keep equipment functioning within specifications, best practice recommends cleaning and calibrating the instruments at regular intervals. It is also recommended to cross-check sensor accuracy against a separate instrument. This cross-checked data can then be used to show compliance against an EPA-approved method, if needed.

System Maintenance

Regardless of the turbidity equipment chosen to monitor a dredging site, periodic maintenance and calibration is required. Instrument maintenance includes cleaning the instruments (especially if a sensor is not self-wiping) and replacing any deteriorating O-rings to prevent water ingress. If using additional parameter sensors (such as conductivity), further field servicing may be required. Maintenance intervals are largely dependent on-site conditions and other variables, such as the potential for biofouling. Common calibration and maintenance intervals are on a weekly or bi-weekly basis.

Turbidity sensors usually require a two-point calibration — one at 0 NTU/FNU and another at a higher turbidity value (often 126 NTU/124 FNU). The 0 NTU value can be obtained using de-ionized water, though 0 NTU/FNU standard solutions do exist. The high range calibration should use an approved standard such as a commercial or prepared Formazin, or commercial AMCO-AEPA standards. For greater accuracy and a wider range, turbidity sensors can follow a three-point calibration (e.g. 0 NTU, 100 NTU, and 1000 NTU). Regardless of the number of points used, it is important to always include 0 NTU/FNU as the first point. For best results, use only freshly prepared or purchased turbidity standard, as they degrade over time. Any diluted standard should only be used on the day it is made.

In case of sensor failure or damage, it is useful to have spare sensors or sondes on hand. These can be field swapped during calibration or routine maintenance checks. Having a spare turbidity sensor available will reduce downtime due to unforeseen sensor failure, which could cause critical and costly interruptions to safe dredging operations.

For greater details regarding maintenance requirements and sensor calibration, the manufacturer’s user manual should be referenced.

Performance Verification

In addition to keeping turbidity equipment calibrated, it’s important to periodically verify that the sensors are providing accurate turbidity data. The best way to achieve this is by checking turbidity in the same area using a separate, portable monitoring system. This is also known as spot sampling.

There are two portable turbidity measurement methods available for spot sampling. The first method uses a sensor (much like the one used in the buoy-based monitoring system) and a handheld display. The turbidity sensor can be attached directly to a handheld display or through a sonde to the display to show real-time data. Sondes are useful instruments if other parameters, such as pH, dissolved oxygen, or conductivity should be recorded as well. The sonde or sensor can be lowered into the water column to a depth that matches the in-place sensors. While it is at that depth, a turbidity measurement can be taken. The data from this portable system can then be compared to the in-place monitoring system and checked for discrepancies. As with the buoy-based system, this method reports in FNU or FBU based on the sensor used. While FNU readings are approved by ISO 7027, neither option is approved by the EPA.

The second method uses a handheld turbidity meter. This method requires taking a water sample, and then inserting it into the meter for a reading. As this method uses a sample instead of an in-situ measurement, it is often not representative of the surface water as a whole. This is especially a hindrance when quality checking monitoring systems that measure turbidity at multiple depths along the water column. While handheld meters are not submersible, many styles are compliant with EPA method 180.1. This means that the long-term data can be cross-checked against an EPA approved method. However, while correlations can be made between data sets, care should be taken when comparing data between the meter and the buoy-based monitoring system. Turbidity instruments are inherently not comparable, and comparative models are only accurate for the specific sample location.

Recommended Equipment

While there are plenty of turbidity monitoring instruments on the market, some stand out above the rest. Fondriest Environmental has selected these products as the best in their field for their quality, reliability, and value. Together, they provide an advanced and powerful real-time turbidity monitoring system. For a sensor or sonde, accuracy is of utmost importance and the YSI EXO2 does not disappoint. The rugged NexSens CB-450 offers long-term durability with 30 W of solar power, 450 pounds of buoyancy and a choice of instrument mounts for multiple sensors. Alone, real-time data are good, but with remote data collection, 24/7 access from any computer, instant alarm notifications and trend tracking, the intuitive WQData LIVE takes real-time data to the next level.

Typical Turbidity Monitoring System

When dredging, it is necessary to monitor sediment re-suspension levels to prevent damage to the local ecosystem and minimize the spread of contaminants. This is usually done with a turbidity monitoring station upstream (for background data) and two stations downstream (at locations affected by the dredging).

To be effective, measurement data should be provided in real time. The easiest and most efficient way to do this is with a buoy-based turbidity monitoring system. A data buoy can house sensors at multiple depths in the deepest part of the waterway, providing more comprehensive data than a shore-based system. This system can then securely transmit the data to the Internet in real time for access from any computer.

Buoy systems are flexible and customizable based on the dredging project requirements. They can house as many sensors as needed, and all equipment is powered via a marine battery and recharging solar panel system. With multiple telemetry options to choose from, continuous real-time data are available from any computer. This ensures that the dredging operations progress smoothly, and any control measures can be implemented immediately if parameter limits are exceeded.