By Brenna Mosher Clarkson University '16
Brenna Mosher is a Clarkson University environmental engineering major who will graduate in Spring 2016. This post was composed for Biological Systems & Global Environmental Change, taught by Dr. Tom Langen.
Water diversion, detention in reservoirs, increased channelization, and changes in rainfall and snowmelt have all played a role in drastically altering water quality and ecosystem function. A new generation of sensors could enable the development of real-time warning systems that will allow us to manage and monitor these conditions before they become catastrophes.
In the past, the collection and analysis of water samples was laborious, time-consuming work. Recent advancements in commercially available in situ sensors, data platforms, and new techniques for data analysis provide an opportunity to monitor water quality in rivers, lakes, and estuaries on the time scales in which changes occur (Pellerin & Bergamaschi 2014). Some of the most useful improvements have been made in optical sensors. An optical sensor is a device that converts light rays into electronic signals that can measure concentrations of nitrogen and dissolved organic matter.
New sensor technologies, such as ultraviolet (UV) photometers, have gained ground in freshwater quality monitoring in the past few years. UV photometers measure the intensity of light in the ultraviolet-visible spectral region (10 to 400 nm). Nitrate is the largest component of total nitrogen in most freshwater systems. Nitrates stimulate the growth of plankton, which provide food for many aquatic organisms. However, if a lake becomes inundated with nitrate, algal blooms will grow rapidly, thereby reducing the oxygen levels. These devices provide continuous nitrate measurements.
USGS scientists Brian A. Pellerin and Brian A. Bergamaschi wrote that the current generation of optical nitrate sensors operate on the principle that nitrate ions absorb UV light at wavelengths around 220 nanometers (nm). This property is utilized by converting spectral absorption measured by a photometer to a nitrate concentration. This allows for calculating real-time nitrate concentrations without the need for chemical reagents that degrade over time and present a source of waste. Better analysis will lead to a deeper understanding of nitrate fate and transport within the environment.
Fluorescence-based optical sensors have helped to offer a better understanding of dissolved organic matter (DOM) in lakes, rivers, and streams. DOM includes a range of organic molecules that are released by plants and animals, both living and dead. DOM has multiple effects on water quality, contaminate transport, and ecosystem health. For example, a high concentration of DOM in a lake will lower the available concentration of dissolved oxygen (DO). Lower DO can lead to fish and plant death.
Fluorescence-based sensors, also called FDOM sensors, work by measuring the fraction of DOM that absorbs light at specific wavelengths and subsequently releases it at longer wavelengths. This is a diagnostic of DOM type and amount. Studies have often used the excitation and emission at 370 and 460 nm, respectively, to quantify the fluorescent fraction of colored DOM (Pellerin & Bergamaschi 2014). FDOM sensors have also been used to understand the transport of dissolved organic carbon (DOC) through watersheds as well as its internal sources in drinking water reservoirs.
Improved sensors have increased sampling rates over conventional methods, and they have low detection limits, low power consumption, no chemicals, easy applicability to field studies, and long-term deployment capabilities. While these devices offer numerous advantages, there are some drawbacks. These sensors were originally developed to be used in entirely different conditions (coastal oceans).So when relocated to freshwater habitats, parameters such as appropriate optical path length and anti-fouling techniques must be considered. Also, these sensors are not exactly cheap. At an initial cost of $2,000-5,000 for a florescence-based sensor and $15,000-25000 for a UV nitrate sensor, plus ongoing operation and maintenance costs, it is imperative that the user consider what actually needs to be studied. If long-term, continuous and high temporal resolution data are needed for study, these sensors will be more than adequate.
The future of sensor technology does not end here. Advancements in LED technology, wet chemical sensors, and data transmission and communication continue to improve our ability to conduct real-time studies which could ultimately provide an early warning of water quality issues, allow for adaptive sampling, and increase the public’s awareness of water issues. These optical sensors are only the beginning of what will be a long road of environmental surveillance and progress.