The technology of environmental monitoring reflects the technology of the times (Figure 1). At the beginning of the 20th century, flow and water-quality measurements in the aquatic environment were collected using the technology of the Industrial Revolution. Mechanical devices employed floats, pulleys, wire drums, rubber bladders, and spinning vanes to measure depth and flow. Ink pens operating on spring-drive paper strip charts recorded the measurements, and water sampling at depth was accomplished with a lead “messenger” tripping spring-loaded caps on bottles. Water quality in samples was measured using “wet” chemistry.Electromechanical devices appeared in various forms in the 1920s and ’30s, allowing greater flexibility and a widening range of water-quality parameters that could be measured in-situ. The leading edge of data recording moved to punched tape and then to magnetic tape by the late 1960s. Figure 1. The Evolution of Technology
The power of microcomputer technology emerged in the 1980s and ’90s, and monitoring equipment became smaller, more reliable, and increasingly automatic in every function.With the dawn of the 21st century, advances in biotechnology are beginning to be applied to environmental biosensors, with capabilities that were only dreamed about by field scientists just a decade before.This article highlights examples of the current state-of-the-art in automated flow and water-quality monitoring in the aquatic environment. Some of the technologies discussed are refinements of products that have been in the field for a decade or more, while others are at the leading edge of practical application or still in the research and development stage.Acoustic Doppler Current Profiler Acoustic Doppler Current Profilers, or ADCPs, utilize frequency shifts in transmitted sound reflected off suspended particles in the water column to estimate water velocity. Velocities at specific depths in the water column can be measured through comparison of reflected sound from a number of transmitters in an array. Using this capability, the ADCP associates measured velocities with discrete cells within the water column to estimate flow through each cell (see Figure 2). Summing the measured flows in all cells along a transect provides a high-resolution spatially distributed “picture” of water movement (Figure 3). ADCP technology is applied in bottom-mounted, boat-mounted, and side-looking configurations, depending on the nature of the data being sought.Figure 2.
Velocity in the water column is measured in discrete “cells” by ADCP. The velocity and known cross-sectional area are then used to calculate flow through each cell.Figure 3.
The ADCP output is a high-resolution “picture” of water flow.
ADCPs can be deployed in boat-mounted or fixed configurations.Specific advantages of ADCP technology include:“¢ high-resolution flow measurement throughout the water column,“¢ simultaneous collection of bathymetry (i.e., depth), “¢ flexible applications (e.g., down-, side-, and up-looking)“¢ the ability to be coupled with a differential global positioning system (GPS) to provide a continuous record of positional and discharge measurements.Limitations of the technology include:“¢ The equipment requirements for deployment can be substantial.“¢ There is a “dead zone” in measurements at the surface and in shallow areas.“¢ High flow and moving bed conditions tend to result in erroneous measurements.“¢ Cost starts at $10,000 for the simplest unit.Automatic Samplers
Example of autosampler applicationAutomatic samplers have become invaluable tools for the unattended collection of water-quality samples in a variety of environments ranging from storm sewers to river systems. Advances in automatic sampler design within the past few years have resulted in samplers that also collect and log chemical and physical monitoring data and can be programmed to use these data to control sample collection.Samplers on the market from Isco and American Sigma allow the user to program a sampling routine for time- or flow-paced sampling and to have that program initiated through the specification of flow, water-quality, and/or rainfall criterion. Flow measurement is accomplished using add-on modules that use ultrasonic, pressure transducer, bubbler, or Doppler technology. Monitoring of water-quality parameters is accomplished using add-on temperature, pH, or dissolved oxygen (DO) modules or through the use of a YSI 600 or Hydrolab Series 4a multiparameter sonde. Rainfall is monitored using a tipping bucket–style rain gauge. The autosampler also acts as a data logger, storing measurements from the flow, water quality, or rain-gauge devices. Automatic samplers can be powered using a rechargeable 12V battery or direct 120V AC.Key advantages of the latest autosampler technology include:“¢ reduced staffing requirements for intense or long-term monitoring,“¢ recording of flow and water-quality data,“¢ the ability to vary sampling timing as a function of environmental parameters,“¢ user-friendly programming interfaces.The primary constraint of this technology is the potential requirement for supporting infrastructure with utilities and/or protection from vandalism and extreme weather exposure.Temperature Logger
Temperature loggers and associated readers are small and self-contained.Water temperature is a fundamental parameter in aquatic monitoring because it drives chemical processes and is a key characteristic of the physical environment. The development of inexpensive automatic temperature monitoring technology grew out of the food transportation industry, where refrigerator trucks must be kept at critical temperatures during transit to ensure food safety. This need was addressed through entirely self-contained programmable temperature loggers that can be placed in refrigerator compartments and generate a continuous record of temperature. The records are downloaded at the destination and examined to confirm that proper temperatures were maintained throughout the shipment’s journey.The environmental application of this technology takes the form of hermetically sealed temperature loggers that can be deployed in the field and programmed to record temperature at intervals ranging from fractions of a second to many hours. Programming and data transfer are accomplished in the field through an optical interface that uses light pulses to communicate between the logger and a base station or data shuttle device.Depending on the recording interval and available memory installed in the logger, the units can be left unattended to record and store up to several years of data. Key advantages of this technology:“¢ extended unattended logging of temperature,“¢ relatively inexpensive ($100-$190 per logger, and another $400 for support equipment and software),“¢ factory calibrated,“¢ a 10-year (approximately) battery.The primary limitation of temperature loggers is that they are serviceable only by the factory, which includes battery changes.Portable Fluorometer
Example of a field fluorometer in use
SCUFA-Hydrolab applicationFluorometers use the principle of selective fluorescence to detect and measure concentrations of fluorescent dyes, chlorophyll, or hydrocarbons in water. New optical kits also allow for the measurement of cyanobacterial pigments, ammonium, and colored dissolved organic material. Each of these materials has a characteristic fluorescence that serves as the basis for its detection. Field fluorometers are commonly used for the measurement of algae in water supplies, aromatic hydrocarbons in discharges, and flow and mixing characteristics in time of travel and dispersion studies. Field fluorometers can be used for discreet sample analysis or can be operated in a flow-through mode to provide a real-time record of the fluorescent material during a study. A relatively common application for a flow-though setup is onboard a boat. In this application, the fluorometer output can be linked with a differential GPS to provide real-time concentration data integrated with positional information for simplified conversion to a geographical information system format.The fluorometer has a user-friendly interface that allows for rapid setup and calibration of the unit. The field fluorometer can be equipped with data-logging capability to record observations on a nearly continuous basis. A temperature probe allows for the automatic, user-specified compensation of fluorescence measurements as they are collected. Depending on the length and water quality of the reach of interest, the sensitivity of the instrument (0.001 ug/l rhodamine WT minimum detection) allows the user to perform studies using quantities of dye that are easily detectable but remain below the visible threshold. The fluorometer interfaces with a personal computer for the downloading of data in the field. The fluorometer can be powered using a 12V rechargeable battery or direct 120V AC.
Global Water’s eight-channel Datalogger GL-400 has a rechargeable battery. A recent addition to the field of fluorometric analysis is the Turner Designs Self-Contained Underwater Fluorescence apparatus (SCUFA). The SCUFA is a submersible fluorometer that can be used for chlorophyll and dye tracing applications. While not as sensitive as conventional portable units, tests indicate that the SCUFA is as accurate as other field fluorometers from 0.5 to 200 ug/l. The SCUFA can be equipped with data-logging and temperature compensation capabilities and be deployed to a maximum depth of 600 m. An optional solid calibration standard allows the user to quickly check calibration stability. The SCUFA can also be used in concert with a Hydrolab data sonde.Key advantages of portable fluorometric technology include:“¢ extended unattended logging capability,“¢ a user-friendly programming interface,“¢ a temperature probe to provide automated compensation of fluorescence readings,“¢ sensitivity to fluorescent materials, allowing for accurate low-concentration readings.The primary limitation of portable fluorometric technology is the cost; for example, a Turner Model 10AU is in the range of $12,000-$13,000.Figure 4.
Example of temperature and DO data collected by a Hydrolab Recorder data sondeData Sondes
YSI and Hydrolab data sondes: Hydrolab Series 4 (above), YSI 6000 Series (below)
YSI data displayA data sonde or data logger is a self-contained programmable measuring/logging platform with sensors for one or more water-quality parameters. Available sensors support measurement of temperature, DO, pH, conductivity, salinity, turbidity, depth, oxidation reduction potential, nitrate, ammonium, ammonia, and fluorescence.User-specified configuration options include internal battery packs and data-logging capability to allow for remote, unattended data collection. As mentioned earlier, YSI and hydrolab data sondes can be used in conjunction with Isco autosamplers to conduct water-quality sampling regimes based on water chemistry. Hydrolab data displays can interface with the Turner Designs SCUFA for the collection of fluorescence data. Logged data can be downloaded using a portable display unit or a personal computer.Data sonde technology provides the following advantages and capabilities that have become integral to environmental monitoring programs:“¢ It has an extended unattended logging capability.“¢ Parameters can be added as monitoring needs change.“¢ Field-hardened units survive rough handling and severe environments.“¢ It has user-friendly programming interfaces.“¢ Many sensors hold calibration well (at least five to seven days before recalibration is necessary, depending on environment).Again, the primary constraint on this technology is cost, which can exceed $10,000 for a fully outfitted data sonde with a wide suite of parameters, internal batteries, and extended data-logging capacity.Remote Underwater Sampling StationOne of the invariable compromises in deploying an automatic monitoring package is location in the water column, especially in systems where water quality changes significantly with depth. Historically, the approach to this dilemma has been to deploy multiple sensors at different depths or, if funds for equipment were limited, to pick a depth that was judged to be reasonably representative of the entire water column.
Remote Underwater Sampling Station components
An innovative solution to this problem has recently been developed that allows in-situ monitoring throughout the water column using a single sensor package mounted on a variable buoyancy platform.The Remote Underwater Sampling Station consists of two integrated modules: an anchored flotation module (or buoy) and a tethered profiler module. The flotation module rides on the surface and contains a computer controller, a telemetry module, and the power system. The latter includes solar panels for self-contained operation for extended periods. The heart of the profiler module is a pair of cylinders filled with or drained of water by an onboard pump to adjust buoyancy. In-situ sensors are mounted on the profiler module to measure water quality and other parameters.In operation, the computer controller is programmed to move the profiler through the water column and collect measurements at whatever depth and time intervals are required. The result is a continuous record of water quality throughout the water column. In tidally influenced systems, the profiler can be programmed to maintain a fixed depth or distance above the bottom.Key advantages of the remote underwater sampling station:“¢ It allows unattended water-column logging.“¢ It provides real-time access to the monitoring data through telemetry links.“¢ Virtually any type of environmental sensor can be installed on the profiler module.“¢ The profiler is unaffected by wave action because its position in the water column is independently controlled.The primary constraints of this technology are the logistical requirements for deploying and retrieving the modules, as well as the cost of the equipment, which starts at $30,000.
Biochemical Oxygen Demand SensorBiochemical oxygen demand (BOD) is a fundamental parameter for measuring the potential for oxygen depletion in discharges and receiving waters. The traditional methods for BOD measurement involve preparation of a series of dilutions of sample water, inoculation with a standard “seed” of microorganisms, and incubation for a period of five days. The difference in DO at the beginning and end of the incubation period is used to calculate the five-day BOD (BOD5). A significant limitation of this approach is that results are only available five or more days after the sample is collected. Historically, surrogate parameters with short turnaround times, such as chemical oxygen demand or total organic carbon, were used in situations where real-time results are required for process control, compliance evaluation, and decision-making. The correlations between surrogate parameters and BOD is imperfect, and there are regulatory issues in using surrogates for permit compliance assessment and reporting.Isco/Stip has developed a true BOD sensor that uses a small bioreactor coupled to a computer controller to measure and report BOD concentrations. The unit works by drawing a sample from the wastestream or receiving water, diluting the sample stream with zero-BOD water, and passing the stream through a fluidized bed bioreactor filled with plastic media. A constant difference in DO between the inlet and outlet of the bioreactor is maintained by the computer varying the amount of dilution. The dilution required to maintain the constant DO “drop” is correlated to BOD5 as part of the instrument calibration process.Operational specifications of the unit include a response time of three to 15 minutes, a 5-100,000 mg/l range, and ±3% precision.The BOD meter’s key advantages are that it supports real-time monitoring and control and it captures short-term variations.The constraints associated with the BOD meter technology include:“¢ The calibration is based on a specific matrix of oxygen-demanding substances, so variability in the sampled stream constituents might affect results.“¢ Deployment requires an installation with utilities and protection from the environment.“¢ Maintenance can be significant.“¢ Cost falls into the range of $35,000-$50,000 for a reasonably permanent installation.Emerging Bioengineering Technologies Recent breakthroughs in the application of biotechnology provide a glimpse of what the future holds for environmental monitoring. Researchers Jing Li and Yi Lu of the University of Illinois developed a sensor that uses a catalytic DNA sequence sensitive to a specific lead ion; for example, Pb2+ (Li and Lu, 2000). A fluorescent tag is linked to the DNA sequence via a strand of RNA. The DNA is also bound to a “quencher” that inhibits the fluorescence of the tag. In the presence of a lead ion, the RNA strand is cleaved, separating the tag and quencher, and resulting in fluorescence that can be detected and measured.Unique DNA sequences can be isolated to react to different ions and molecules, with the potential for creating extremely specific sensors. Applications for the technology, which is still in the research and development stage, are envisioned to include industrial process control, clinical toxicology, and environmental monitoring.The anticipated advantages of DNA-based biosensors include real-time, in-situ monitoring of toxic metal ions; high selectivity for particular chemical structures; and a wide range of sensitivity (three orders of magnitude).
Biochemical Oxygen Demand SensorBiochemical oxygen demand (BOD) is a fundamental parameter for measuring the potential for oxygen depletion in discharges and receiving waters. The traditional methods for BOD measurement involve preparation of a series of dilutions of sample water, inoculation with a standard “seed” of microorganisms, and incubation for a period of five days. The difference in DO at the beginning and end of the incubation period is used to calculate the five-day BOD (BOD5). A significant limitation of this approach is that results are only available five or more days after the sample is collected. Historically, surrogate parameters with short turnaround times, such as chemical oxygen demand or total organic carbon, were used in situations where real-time results are required for process control, compliance evaluation, and decision-making. The correlations between surrogate parameters and BOD is imperfect, and there are regulatory issues in using surrogates for permit compliance assessment and reporting.Isco/Stip has developed a true BOD sensor that uses a small bioreactor coupled to a computer controller to measure and report BOD concentrations. The unit works by drawing a sample from the wastestream or receiving water, diluting the sample stream with zero-BOD water, and passing the stream through a fluidized bed bioreactor filled with plastic media. A constant difference in DO between the inlet and outlet of the bioreactor is maintained by the computer varying the amount of dilution. The dilution required to maintain the constant DO “drop” is correlated to BOD5 as part of the instrument calibration process.Operational specifications of the unit include a response time of three to 15 minutes, a 5-100,000 mg/l range, and ±3% precision.The BOD meter’s key advantages are that it supports real-time monitoring and control and it captures short-term variations.The constraints associated with the BOD meter technology include:“¢ The calibration is based on a specific matrix of oxygen-demanding substances, so variability in the sampled stream constituents might affect results.“¢ Deployment requires an installation with utilities and protection from the environment.“¢ Maintenance can be significant.“¢ Cost falls into the range of $35,000-$50,000 for a reasonably permanent installation.Emerging Bioengineering Technologies Recent breakthroughs in the application of biotechnology provide a glimpse of what the future holds for environmental monitoring. Researchers Jing Li and Yi Lu of the University of Illinois developed a sensor that uses a catalytic DNA sequence sensitive to a specific lead ion; for example, Pb2+ (Li and Lu, 2000). A fluorescent tag is linked to the DNA sequence via a strand of RNA. The DNA is also bound to a “quencher” that inhibits the fluorescence of the tag. In the presence of a lead ion, the RNA strand is cleaved, separating the tag and quencher, and resulting in fluorescence that can be detected and measured.Unique DNA sequences can be isolated to react to different ions and molecules, with the potential for creating extremely specific sensors. Applications for the technology, which is still in the research and development stage, are envisioned to include industrial process control, clinical toxicology, and environmental monitoring.The anticipated advantages of DNA-based biosensors include real-time, in-situ monitoring of toxic metal ions; high selectivity for particular chemical structures; and a wide range of sensitivity (three orders of magnitude).
Isco/Stip BOD meterWhat Will Tomorrow Bring?In 1946, Chester Gould gave his crime-fighting comic-strip hero Dick Tracy a two-way wrist radio, an unimaginably futuristic device in the minds of the postwar reading audience. In 2001, cell-phone watches are available at discount prices through any number of Web retailers. Just as we’ve seen monitoring tools advance with the technology of our day, we can look forward to new devices and tools in the future, reflecting advances in bioengineering, nanotechnology, and technical disciplines that we have not yet imagined.
