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.
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
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).
