National Drinking Water Clearinghouse
West Virginia University
PO Box 6893
Morgantown, WV

How old is your water?

By Kathy JespersonOn Tap Associate Editor

Water age doesn’t seem like something that anyone would have to worry about. But the truth is that water quality will deteriorate the longer water stands in storage facilities or distribution lines. Water age refers to the time it takes for water to reach customers after it’s been treated. The longer it takes to get to the faucet, the more likely there will be problems. Water age is highly system specific, and many utilities have never calculated the age of water in their systems.

How does water age?
Two of the biggest contributors to water age are system size and storage design. Typically, design engineers try to figure future needs into a water system’s plan, making pipes and storage facilities larger than they need to be to accommodate future growth and construction. However, these oversized facilities may result in longer detention times, loss of chlorine residual, taste and odor problems, and other water quality concerns.
According to the U.S. Environmental Protection Agency (EPA), there are two ways that water age contributes to water quality deterioration:
1. if an interaction takes place between the pipe or tank wall and the water, or
2. if chemical reactions occur within the bulk water, such as a reaction between organic material and chlorine that creates disinfection byproducts (DBP).

Water Storage Tanks Are Culprits
Water storage tanks can have a real effect on water quality. Water can stagnate in a tank for a long time before it gets used. When it does eventually make its way to a customer’s faucet, the water probably isn’t going to taste or smell very good. And, it may even have become a health risk.

According to the Association of State Drinking Water Administrators (ASDWA), design engineers typically emphasize hydraulic concerns when they plan a storage facility, meaning they have worried about how much water can get from one place to another in a given amount of time. But by making sure that there is enough water for emergencies, many storage tanks are much larger than they need to be for everyday use. And many systems keep their tanks full, despite not needing the water.

A full tank does provide constant system pressure and helps systems to be better prepared for emergencies. But problems occur when the water isn’t used or continuously mixed. Dead zones can develop inside the tank. That means that the water does not move from its place of residence and stagnates. Continuously mixing the water and making sure that fresh water replaces stagnant water can help control dead zones inside tanks.

Besides over designing, some storage facilities have been built so that the high water level is below the hydraulic grade line of the system, making it more difficult to turn the water over. The hydraulic grade line is an imaginary line that, when plotted, represents the sum of pressure head plus elevation head for various positions along a given fluid flow path, such as along a pipeline or a groundwater streamline. If the hydraulic grade of the system drops significantly, very old water may enter the distribution lines.

Frequently exchanging the water in the distribution system and storage facilities can make water age less of a concern. EPA says, “some studies suggest a shelf life of three, five, or seven days, depending on water quality parameters. When possible, the water level in standpipes should fluctuate widely through withdrawal of a larger amount of water than normal one day and refilling the next.”

For Groundwater, Age Makes a Difference

“We can’t go in the ground and see everything, so we have to get snippets of information to learn more about the system,” said David L. Nelms, a hydrologist with U.S. Geological Survey (USGS) .

Researchers from the U.S. Geological Survey (USGS) have been busy trying to decipher the age of groundwater, according to a story that appeared in the August 18, 2003, Winchester (Virginia) Star. The answers they come up with could help them to better define the way groundwater flows and patterns that affect water quality, as well as potential for contamination.

“We can’t go in the ground and see everything, so we have to get snippets of information to learn more about the system,” said David L. Nelms, a hydrologist with USGS.

The age-dating process that Nelms and hydrologists George E. Harlow Jr. and Roger Moberg are using is much more complicated than counting rings to get the age of a tree. The process involves considerable data collection, according to Nelms, who must fill in a 30-column spreadsheet with data from multiple sources.

The research team must collect data about the water’s temperature, pH level, amount of dissolved oxygen, and specific conductance. In addition, they must gather samples to test for freon (CFC 11, 12, and 113), sulfur hexafluoride, and tritium. The tritium analysis measures helium-3—hydrogen ions that can occur naturally in low levels but can be detected in greater amounts in many areas because of the extensive hydrogen bomb testing during the 1950s and 1960s.

“By measuring these compounds like freon [and helium-3] that have only been around for the last 50 to 60 years, you can determine whether you’ve got old or young water,” Nelms said.
And it appears that older is better, at least in this instance.

Younger water has only been in the ground a short time and has more recently come into contact with contaminants on the surface. Older water, on the other hand, has been in the subsurface for much longer—hundreds, maybe even thousand of years—where it’s been isolated and protected from contaminants.

The hydrologists also can use any data they gather to speculate about how fast water moves through the groundwater network, providing data about flow patterns.

“That information just scratches the surface, and the information we get from the lab provides more advanced information about the water’s age,” he said.

For more information about groundwater age, visit, from Environmental Health Perspectives. More information may be found at Science and Technology’s Web site at USGS’s Web site has information at

Biofilms Form and Grow
Microbial survival and growth can occur when water moves slowly or becomes entrapped in dead-end sections of the distribution system, notes EPA. If any organisms have entered the system, biofilms can form within the network, including storage tanks. Biofilms usually appear as a patchy mass in pipes or as a uniform layer along inner walls of a storage tank. Coliform bacteria may colonize within the biofilm layer, causing taste and odor problems.

Factors that provide optimal growth conditions for microorganisms include long water-detention times in tanks and lines, adequate nutrient levels, and warm temperatures. In addition, long detention times increase the likelihood that opportunistic pathogens will re-grow.

Opportunistic pathogens are any disease-causing organism, bacterium, virus, helminth, or protozoan that slips through the treatment processes or enters the system during times of pressure loss and finds the opportunity or favorable circumstances to lodge or reproduce in organic material, bacterial slime, or other material that it finds attractive. (See the article “A Lesson in Microbiology.")

Reactions May Occur
Disinfection byproduct (DBP) formation occurs when disinfectants react with organic matter. Reaction time
is a key variable in DBP production. Higher temperatures during the summer months can increase DBPs as the chemical reactions proceed faster and go farther at higher temperatures. Also, higher temperatures increase chlorine demand, increasing the likelihood that DBPs will form.

Nitrification May Take Place
Nitrification refers to a microbial process in which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate. Bacteria capable of nitrification grow slowly. Therefore, nitrification problems usually occur in large reservoirs or low-flow sections of the distribution system.
Using processes that reduce residence time and circulation within the distribution system can minimize nitrification problems. Low circulation areas of the distribution system are where nitrification takes place because detention time and sedimentation buildup can be much greater there than in other parts of the system.

Pipes Can Corrode
Long detention times can greatly reduce corrosion control effectiveness, says EPA. The longer water is detained, the more likely phosphate inhibitors and pH management will be impacted. If pipes corrode, toxic metals, such as lead and copper, can leach into the water. Corroded pipes also can impart a metallic taste to the water, stain plumbing fixtures, harbor pathogenic microorganisms, reduce the pipe’s hydraulic carrying capacity, and cause leaks and clogs within the distribution and storage systems.

Signs of Water Age
EPA says that there are several indicators of excessive water age, including poor aesthetic conditions, such as:
• Customer taste and odor complaints—Stale, aged water provides an environment conducive to the growth and formation of taste- and odor-causing microorganisms.
• Discoloration—Water in low flow and dead-end areas often accumulates sediment, and during demand periods, these deposits are stirred up and degrade the clarity and color of the water.
• Water temperature—Stagnant water provides a nice, warm place for microorganisms to grow.
Monitoring indicators also provide clues that water has aged beyond its expiration date. Indicators include:
• depressed disinfectant residual—chlorine and chloramines decay over time,
• elevated DBP levels—the reaction between disinfectants and organic precursors occur over long periods,
• elevated bacterial counts—bacteria can grow in stagnant water, and
• elevated nitrite or nitrate levels—nitrification.

Dealing with Water Age
Often, operations practices have an impact on water age particularly with flow direction and flow velocity. Valve settings and pump station operations have a direct impact on water’s hydraulic path and detention time. System operators can modify pressure within the distribution system to help alleviate standing water. Also, flushing programs can help displace stale, stagnant water. (See the Summer 2002 On Tap insert “How to Flush Distribution Lines” for more about distribution system flushing.)

Systems also can clean or replace deteriorated pipelines, which are notorious
for contributing to finished water decay. There are a variety of cleaning methods that systems can use, including:
• swabbing,
• scraping,
• pigging,
• chemical cleaning, and
• jet flowing.

Swabbing, scrapping, and pigging refer to methods that remove scale and deposits from the inside of the pipes. Chemical cleaning is just what is says it is: cleaning out the pipes using chemical cleaners. Jet flowing typically uses a high-pressure method to wash down the inside of the pipe. Each technique has its benefits and drawbacks and should be tailored to the specific site. In addition, systems may want to consider relining the pipe to prevent future corrosion.

Using the proper treatment methods for the kind of source the system draws from also can ease problems. The right treatment methods improve the biochemical stability of the finished water. Bio-chemical stability is closely related to the amount and kind of organic matter present in the water. The more organic matter left in the water, the more problems will arise—usually in the form of DBPs. Treatment to remove organics, inorganics, and turbidity also will curb chlorine decay. The most effective treatment methods for maintaining biochemical stability include:
• enhanced coagulation,
• biological filtration,
• ultra- and nanofiltration, and
• granular activated carbon.

On Tap Editor Kathy Jesperson is always looking for story ideas. If you have any ideas, please contact her at or call 1-800-624-8301, ext. 5533.

For more information about water age, visit EPA’s Web site at You may download the National Drinking Water Clearing-house’s Fall 2002, Tech Brief, “Water Quality in Distribution Systems,” at ASDWA also has information about water age at