Suspended sediments in water, such as clay, silt, and algae, can have many negative effects on aquatic life. Suspended materials reduce water clarity and can block light to aquatic plants, smother aquatic organisms, and carry contaminants and pathogens, such as lead, mercury, and bacteria. Suspended sediments can be caused by runoff from construction sites, agriculture, and logging sites; runoff from urban areas with paved and impermeable surfaces; eroding stream banks; bottom-dwelling fish and burrowing animals; excessive algae growth; high-velocity water, including storm water; and windy conditions in shallow-water areas.
While measuring total suspended solids (TSS) directly is the ideal method to evaluate sediment suspension, it is not feasible for real-time or continuous monitoring applications. TSS can presently only be evaluated by collecting water samples and performing laboratory tests, which involve separating the sediment from the water and weighing it.
Thus, turbidity, a measurement of water cloudiness, is typically used to provide real-time data that assists with monitoring suspended sediment.
Turbidity is commonly reported in nephelometric turbidity units (NTU). An instrument called a nephelometer, also named a turbidimeter, is the most common device used present-day to measure turbidity. It does so by shining a light beam through the water and then measuring how much light is scattered to the side at a 90-degree angle.
Particle density in the water is a function of how much light is scattered. This is in part a qualitative test, however, as the properties of the suspended sediment particles — shape, color, and distribution — can affect the measurement of turbidity. This means that two samples of water with the same level of suspended solids but different particle composition could potentially yield varying turbidity readings.
Although nephelometers are by far the most commonly used means for measuring turbidity levels in field deployments, these measurements are also possible with backscatter sensors and transmissometers.
A sensor based on the backscatter technique measures turbidity as a function of how much light bounces back to a sensing diode adjacent to the light emitter. A transmissometer measures how much light emitted though an area of water strikes a light sensor on the opposite end. Turbidity is then measured by the degree of light attenuation caused by particles in the water. Transmissometers are typically more cumbersome and are most useful in waters with extremely low turbidity levels.
The model 6136 turbidity sensor from YSI has become the preferred sensor for many real-time suspended sediment monitoring projects. Before taking a reading, a mechanical wiper cleans the sensing optics to ensure the measurement is not affected by fouling sediment debris. The YSI 6136 sensor measures turbidity using the ISO 7027 method. Extended deployments with this sensor, a component of YSI’s multi-parameter water quality monitoring sondes, have exhibited stable and accurate results for long deployment periods.
Turbidity sensors such as the YSI model 6136 can be incorporated into real-time suspended sediment monitoring systems. Fondriest Environmental provides fixed and buoy-based platforms and data loggers that have been refined over the years to accommodate the specific needs of sediment monitoring systems.
The fixed platform option includes a one of YSI’s multi-parameter sondes housed within a perforated PVC pipe that can mount along bridges, piers, dams, railroad trestles, other structures in the river, or river banks. A solar panel for charging the data logger can also be mounted to the structure.
The floating platform option consists of a data buoy with a cross-linked polyethylene foam hull with a tough polymer skin coating. A round center housing accommodates a NexSens SDL500 submersible data logger. Three NexSens 5-watt solar power packs are designed to mount to the top of the buoy to provide continuous power to the data logger and communications module. Top and bottom mounted stainless steel eye-nuts accommodate moorings and lifting rigs for quick and easy deployment. The buoy is moored to the bottom via anchors, chain, and shackles. A Fondriest Environmental application engineer can offer recommendations for deployment based on site conditions to ensure the systems will remain in location. The buoy can house a multi-parameter sonde within the counterweight, or the sonde can be suspended within the water along a stainless steel mooring cable.
NexSens data loggers are configured with sensor ports for connection to sensors that provide industry-standard digital and analog interfaces including RS-485, SDI-12, 1-wire temp string, 0-2.5 VDC, pulse count, and more. Moreover, if sensors are equipped with a unique or proprietary interface, custom built connections and control software can be developed. These data loggers can be outfitted with radio, cellular, landline telephone, Ethernet, WiFi, or satellite telemetry technology, making real-time suspended sediment monitoring from anywhere in the world possible.
Once the suspended sediment monitoring systems are set up, data is logged at a user-defined interval (minimum 1 minute). The user also sets the interval at which data is transmitted via radio or cellular telemetry. A typical monitoring system logs turbidity data every 10 minutes and transmits data every 30 minutes.
Data is transmitted to a nearby (or remote) computer running iChart Software. iChart is a user-friendly software package that serves as the centralized interface and database for all incoming data. All data and sensor configuration settings are also stored in a single iChart database.
The software offers a unique historical report creation tool that can generate customized reports with data from all sensors in an iChart database. When creating a report, users can include specific information about the monitoring site, location, sensors, and project. After creation, reports can be converted to PDF, exported to Microsoft Excel, sent to interested parties via e-mail, uploaded to a web server, and more. The report template can also be saved and automatically generated, further automating the reporting process.
The NexSens WQData web datacenter is an optional service that automatically generates an online graphical interface for viewing, analyzing, and downloading data in real-time. This datacenter allows project members and stakeholders to remotely experience the project information and data in order to monitor turbidity during remediation activities.
While it often makes sense to purchase systems outright, there are many short-term projects that make it cost-prohibitive. Fondriest Environmental offers real-time Suspended Sediment Monitoring Systems with weekly and monthly rental rates to meet project requirements.
To ensure customers can begin using their systems as quickly and efficiently as possible, our application engineers are available for training and technical support, both by phone and in person. Our company offers both extensive field experience and a wide array of deployment hardware to facilitate seamless implementation of the project.