Watch Your Glass

• Mar 01, 2002

A continuous supply of clean drinking water is something that most Americans take for granted. Even those of us in the environmental profession, who know the effort and engineering it takes to provide clean drinking water, rarely go to the tap doubting that the water will be clean and safe.

Since September 2001, the general population and water supply professionals have been concerned about the safety and protection of drinking water supplies. It seems like every day since then that there have been advisories from federal and local government agencies issued to water utilities to take every precaution against a possible terrorist attack. Many important steps have been taken by drinking water utilities in the past months to prevent possible attacks on water utility infrastructure and to prevent any attempts to contaminate reservoirs or treated water supplies.

One of the principles repeated by security professionals is "Delay, Detect and Respond." This means that anything that can be done to slow down or delay an intruder trying to access a water utility increases his or her chances of being detected and of getting caught, which allows the utility to respond -- by arresting the intruder and, if needed, preventing that source water from entering the distribution system.

An intruder who tries to physically break in to a water utility can be detected by the type of security devices that we are all accustomed to - motion detectors, alarms, cameras, etc. But what devices are being used to detect the chemical agents that might be introduced into a water system?

One of the technologies being used is biotoxicity monitoring. Toxicity testing is based on exposing an organism to a water or a wastewater sample and measuring the effect. The sensing element can include a variety of biological elements that can range from antibodies to complex whole organisms. Standard methods involve exposing species of freshwater invertebrates or fish (Fathead minnows) to the water sample.

Another type of system, bacteria-based biosensors, combine a bacterial species with an electronic measuring device producing a sensitive and rapid test method in a single instrument. This 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 also unique in that they produce a very rapid response to toxicity in water. While toxicity methods using higher-level organisms, such as fish, may take many days to produce a measurable result, bacteria-based biosensors can usually produce measurable acute toxicity data in a little as 15 minutes. This makes these types of instruments uniquely suited as an early warning screening tool for drinking water systems. Also, 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.

The primary goal of a biosensor-based early warning monitoring system is to reliably identify low-probability, high-impact chemical contamination events in source water or distribution systems. Some of the key attributes of an ideal early warning system are summarized in Table 1.

Table 1: Requirements for a ideal useful early warning system

An important criterion for an effective biosensor monitoring system is that it must be capable of detecting contamination rapidly enough so that there is time to take response actions before the water is delivered to consumers. This factor is influenced by the sampling locations and the time required completing the test. Utilities need to determine the flow rate of their water and the time difference between the source, treatment and ultimate delivery of the water to consumers and what level of risk they feel is acceptable to determine how rapid a response they require. The faster an early warning system responds, the more time available for the utility to respond producing a greater margin of safety.

A biosensor-based early warning system must also be precise and reliable, with a high level of sensitivity but a very low incidence of false alarms. This is best 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.

Because of the number of water systems that need protection, biosensor systems must be affordable. An effective system will have a low daily testing cost. In addition, the sensitivity and specificity of the test should be such that frequency of false negatives and false positives must be low compared to the benefits of averting a true contamination event.

Test systems that can detect the presence of many chemical classes will be more desirable than a test system that provides narrow coverage or detects relatively few chemical parameters. Unfortunately for all of us, many of the chemicals that could be used to contaminate a water supply are relatively easy to obtain. Many of these compounds are commonly used in large quantities, inexpensive and are available with few purchase restrictions. Many pesticides, herbicides, rodenticides and certain industrial chemicals are available through relatively unrestricted commercial vendors. For example, sodium cyanide is highly toxic in small doses and is easily obtainable from numerous suppliers to the mining and metals industries. A simple search on the Internet will turn up many sources. While it is not possible to prevent a terrorist from gaining access to these and other common chemicals, it is possible to use a biosensor-based test system to detect acute concentrations of these chemicals if they have been introduced into a water system.

Biosensors are well suited for this application because they detect the effect (toxicity) of chemicals and are not limited to a few specific compounds. Because of the diverse group of chemicals that could be used to contaminate a water supply, it is not feasible to screen for all of them using traditional analytical methods.

A commercially available biosensor-based test system that is being widely used by many water utilities and meets the requirements of rapid detection and broad-spectrum sensitivity is known as the Microtox toxicity test system

Microtox technology uses a bacterial species known as Vibrio fisheri, 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, when suspended in clean water, is relatively constant and can be easily detected by a properly designed luminometer. The higher the level of toxicity in a water sample, the greater and faster the reduction in light.

Microtox systems have been used for more than 20 years. While the basic principal of its operation -- the sensitivity of Vibrio fisheri to toxic compounds - remains relatively unchanged, the sensitivity and accuracy of the associated instrumentation has been continually improved over time.

Microtox systems were used to monitor water supplies for contamination during the Olympic Games in Atlanta in 1996 and in Los Angeles in 1984. Microtox test systems were also used during other periods of heightened security, such as the 1991 Gulf War and during the 2000 Democratic National Convention in Los Angeles.

Following the attacks of September 11, 2001, the U.S. Army Corps of Engineers began employing the system to monitor the drinking water supplied to the Pentagon. The system is also currently being used by a large number of public and private drinking water utilities across the country.

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 analysis 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) which defines a possible contamination event prompting the utility to initiate previously defined actions. 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. These responses 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.

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 with drinking water. Physical security is important, but it is not enough. Drinking water utilities need to include in their safety and security plans early warning systems to test the water for evidence of tampering and use them in conjunction with procedures to stop the supply of water and notify residents if any contamination is found.

This article originally appeared in the 03/01/2002 issue of Environmental Protection.