On the Alert

Biosensors are being used as early warning systems to detect chemical contaminants in drinking water

Drinking water professionals know that the quality of raw or finished water supplies may be adversely impacted by a number of contaminants, including petroleum products from leaking tanks or pipelines, insecticides and herbicides from agricultural runoff, pathogens from untreated sewage discharges and others.

Since September 11, 2001, an increased focus has been put on the potential threats to drinking water from terrorism and acts of sabotage. While these threats can include physical destruction of key operational parts of a system or cyber terrorism such as hacking into a SCADA system, threats from intentional contamination using chemical or biological (biotoxins or pathogens) agents must also be addressed.

Intentional contamination of the drinking water distribution system provides a great detection challenge because of the wide variety of chemical contaminants, biotoxins or combinations of threat agents that could be used. There have been a number of recent studies to identify the agents that could be used against drinking water. These studies have identified a number of compounds that are stable and soluble in water, have little odor or color and are highly toxic to humans at low concentrations. These agents range from military chemical warfare agents (e.g., sarin, VX, etc.), biological toxins (e.g., aflatoxin, ricin, botulinum toxin) and pathogens (e.g., anthrax, e.coli, salmonella, listeria) to easily obtainable industrial chemicals (e.g., cyanides, heavy metals) and pesticides (e.g., aldicarb, DDT). Many experts are also issuing warnings about the potential use of street drugs (e.g., LSD) because they are easily obtained, relatively inexpensive, and could have a potentially devastating effect.

An Overview of Biotoxicity Monitoring
One of the most useful technologies currently being used for drinking water monitoring is biotoxicity monitoring. Toxicity testing is currently being used by a number of drinking water utilities to detect a contamination event. Toxicity monitoring is used in conjunction with conventional monitoring to supplement the information available and provide drinking water engineers with the time needed to make crucial decisions in the event of an actual contamination event.

Toxicity testing is based on exposing an organism to a water or wastewater sample and measuring the effect. Standard methods involve exposing species of freshwater invertebrates or fish (fathead minnows) to the water sample -- but these methods are far too slow for counter-terrorism monitoring.

There is a more effective method that uses a bacteria-based biosensor. These systems combine a bacterial species with an electronic measuring device to produce a sensitive and rapid test method in a single instrument. This technology has demonstrated the potential to quickly detect a broad range of toxic agents. The metabolism of bacterial cells, like all living cells, is supported by a broad range of continuous biochemical reactions that can be disrupted by toxic chemicals. By coupling a cell suspension with the appropriate instrument that can measure the metabolic rate of the cells, an operational biosensor can be developed.

Bacteria-based biosensors are superior to tests using other higher organisms because bacteria show a very rapid response to toxicity in water. Bacterial cells have a much higher metabolic rate than mammalian cells and they lack the complex organ systems of higher organisms. This means that toxic substances disrupt a bacterial cell's metabolic systems much more rapidly. Furthermore, millions of cells can be introduced to a small water sample, increasing the number of test organisms and reducing the effect of biological variability. And bacteria-based biosensors can be incorporated into a portable instrument, making rapid response and field-testing practical; something not possible with toxicity test formats using other organisms.

This rapid response is key to a utility's ability to take response actions. The time between sampling and the time potentially contaminated water reaches a consumer is influenced by the sampling locations and the hydraulic characteristics that determine the water's fate and transport. The faster an early warning system responds, the greater the time available for the utility to respond, producing a greater margin of safety.

Instruments employing bacteria-based biosensors are capable of detecting a wide range of chemical contaminants and toxins. This is because they detect the effect (toxicity) of the compounds and not specific compounds. Because of the diverse group of chemicals that could be used to contaminate a water supply, this type of broad screen is necessary.

Bacteria-based biosensors also produce reliable and reproducible results and offer a high level of sensitivity but a very low incidence of false alarms. Best results are achieved by normalizing the sensitivity of the system to the local water quality by establishing an adequate baseline of data. It is against this baseline that future sample test data will be compared to determine if there has been any significant change in water quality.

A commercially available bacteria-based monitoring system that that is being used by more than 70 drinking water systems throughout the United States and Canada is the Microtox toxicity test system. Microtox technology uses a bacterial species known as Vibrio fischeri, a strain of luminescent bacteria specially selected for its sensitivity to many chemicals. This microorganism is uniquely suited for this application because it produces light during its normal metabolism. Toxic compounds in a sample that adversely affect the metabolism of the organisms cause a decreased rate of luminescence -- that is, they cause the organisms to produce less light. The light output from these organisms under normal conditions is relatively constant and can be easily detected by a properly designed luminometer. The higher the level of toxicity in a water sample, however, the greater and faster the reduction in light.

Microtox systems have been in continuous use in a number of high profile situations and locations in recent years. Prior to September 2001, Microtox Test Systems were used during the Olympic Games in three U.S. cities, the 2000 Democratic National Convention and during periods of heightened security such as the 1991 Gulf War. Following the attacks of September 11, the U.S. Army Corps of Engineers began employing the system to monitor the drinking water supplied to the Pentagon. Many other public and private drinking water utilities across the country followed suit.

A typical application of the technology involves collecting samples at regular intervals from numerous points in the water system such as the intake from the water source (reservoir, river, etc.), various points throughout the treatment process, before and after chlorination and at key points in the distribution system. If required, a portable biosensor test system can be used to analyze samples at remote locations without having to bring samples back to the lab. This is particularly important for remote and rural water systems.

The data from the toxicity analyses are compared to the baseline toxicity data that has been developed from previous analyses. Each utility uses a pre-determined "action level" (i.e., the increase in toxicity above a baseline) that defines a possible contamination event. Any sample data that shows an incursion from the usual baseline data above the action level indicates a change in water quality and triggers a pre-planned response by the drinking water utility. The response will range, depending upon the magnitude of the incursion, from additional testing to confirm the initial result to more extreme actions such as changes in treatment, stopping intake from the contaminated source and immediate notification of appropriate authorities, media and consumers.

As an added benefit, toxicity monitoring can also detect other contamination events not related to an intentional act, such as cross-connections with sewerage lines, backflow contamination, or source contamination from leaking pipelines or tanks.

Conclusions
Current concerns demand increased vigilance to assure that our drinking water supplies remain safe and secure. Each and every drinking water treatment plant across the country and around the world needs to institute an effective security program to guard against tampering. Physical security is important, but it is not enough. Drinking water utilities need to include in their safety and security plans rapid analytical techniques for both routine monitoring and emergency response operations. These technologies are a key part of a comprehensive security plan and should be used in conjunction with procedures to stop the supply of water and notify residents and authorities if any contamination is confirmed.

e-sources

  • American Water Works Association -- www.awwa.org
  • Awwa Research Foundation -- www.awwarf.com
  • U.S. Environmental Protection Agency's Safe Drinking Water program -- www.epa.gov/safewater
  • U.S. Geological Survey's Drinking Water Programs -- water.usgs.gov/owq/dwi
  • Agency for Toxic Substances and Disease Registry Hazardous Substances Release and Health Effects Database -- www.atsdr.cdc.gov/hazdat.html

This article originally appeared in the 07/01/2003 issue of Environmental Protection.

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