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A tale of two pools: when it comes to problems with pool water, where a customer lives can make a big difference. Find out how source water can determine its effect on the chemical balance in customers' pools.

Jim and John were good friends in high school, but college and jobs in different areas of the country eventually separated them. At their high school reunion, they found they had something more than friendship in common. They both had built new homes and installed inground pools.

Unfortunately, they had something else in common: a problem with the makeup water they were using in their pools. They learned more about chemistry at that reunion than they did in their days as lab partners in high school.

Jim's pool

Jim's housing development was located on an old farm. It was just north of the metropolitan area, so he was on the city water system. Rather than having his initial fill trucked in, Jim decided to pay a high bill and use water from his hose. He had paid close attention to the builder's advice about start-up, water testing and chlorine use (even though the builder had never analyzed the local source water).

Before christening the pool with his first swim, he added a 1 ppm dose of chlorine. He tested the water to see if his dosage was right, but he was surprised at the results. Not only did he have only 0.5 ppm free chlorine, but his combined chlorine was 1.5 ppm. Jim remembered that combined chlorine was the result of free chlorine reacting with ammonia-based products, such as urine and sweat. He was sweating all right, but not in the pool. Where did all of this combined chlorine come from?

That day, Jim's family enjoyed the pool, but everyone complained about the odor and eye burn. The next day Jim decided to take a sample to the local pool dealer. The problem was not new to other pool owners in the area nor to the store owner--she had seen it before.

The first time she encountered it, it took a bit of detective work to determine why these high combined chlorine readings were occurring in new pools. She finally phoned her chemical supplier, who suggested contacting the city water department. Sure enough, the city yeas using combined chlorine in the drinking water. This was puzzling to Jim. If combined chlorine was undesirable in water, why would the local utility department use it?

The store owner recently obtained an Information Bulletin on combined chlorine in drinking water from the National Spa & Pool Institute. The process is referred to as chloramination. She copied the bulletin and gave it to Jim. She also told Jim to superchlorinate his pool that evening with 15 ppm of chlorine (10 x 1.5 ppm combined chlorine) and test it the next day.

The bulletin explained that there were substances in the water (such as decaying plant matter often found in reservoirs) that can react with free chlorine to form carcinogens called trihalomethanes (THMs). The Environmental Protection Agency has limited the concentration of THMs to 60 ppb. Research found that when water was treated with combined chlorine, these carcinogens were less likely to form.

The combined chlorine also had another advantage. Because it is a slower reactant than free chlorine, it tends to last longer in water distribution systems, keeping them more sanitary, especially near the end.

Operators at water plants adjust the chlorine, pH and ammonia so that only monochloramine forms. Tap water concentrations of monochloramine can range from 1-4 ppm, depending on where the home is on the distribution system. The monochloramines usually do not present an objectionable odor problem until there's been further chlorination and they become dichloromine. The bulletin mentioned that a free chlorine dose equal to the combined concentration would be sufficient to eliminate the combined chlorine.

However, because ammonia, other chloramines and additional contaminants might be present, the store owner decided to recommend the usual dose of 10 times the combined chlorine. She also recommended maintaining a chlorine concentration of 3 ppm (or an ORP level of 650-750 mV) in the pool.

Future additions of makeup water would not offer as much combined chlorine as Jim had at start-up. It is hoped that the higher free chlorine residual would take care of the additions of combined chlorine in the makeup water. Jim's start-up problem was a one-time event, thanks to the store owner.

Though the store had an in-house lab, the owner used a modification of the DPD pool water rest to get an accurate reading in the presence of high chloramines. Once the concentration of chloramines exceeds about 0.5 ppm, they start to break through into the free chlorine reading.

An ingredient in the reagent called thioacetamide, or Stedifac, is used to prevent this from happening. Using a DPD pool water test, the reagent is added immediately after the free chlorine reagents and this freezes the reading. A separate total chlorine test must be run on these samples.

John's pool

John always wanted to live in the country. Like Jim, be built his home in a development that was formerly a farm, but John's property had well water and a septic tank. To enhance the "country" feel, John had a number of trees and bushes planted around the pool.

For his initial fill, John had his pool water trucked in and his first summer was great. The following spring, he opened the pool and over several days added about 1,000 gallons of makeup water from his well. He decided to have a professional take care of the landscaping, which meant a good fertilizing of the trees and bushes around the pool. Everything looked beautiful, so John added a 5 ppm chlorine dose on a Friday night. He looked forward to his first swim of the new season over the weekend.

Luckily, the next morning, John checked his chlorine. It registered zero. How could this be? Maybe the chlorine left over from last season was bad, he thought. So he added a 10 ppm dose and went in for a second cup of coffee. John's test an hour later still showed zero chlorine. He decided the chlorine that the dealer had sold him no longer worked, so he took a sample of it and the pool water to the dealer.

Again, this was nothing new to the store manager, but the solution to the problem was not going to be easy. The manager's in-store lab included tests for ammonia, nitrite, nitrate and phosphate. He performed these tests and discussed the results with John.

First, the chlorine John was adding was still good, but he was going to need a lot more. The test results revealed 0.5 ppm ammonia, 2.2 ppm nitrite, 6.1 ppm nitrate, 0.5 ppm phosphate and no chlorine. The store manager asked John about any landscaping near the pool. He thought this was a big part of the problem and due to all of the fertilizing.

The store manager asked John to return home and add a 25 ppm dose of chlorine. He also sold John some phosphate removal product. He asked that John return the next day with not only a pool sample, but also one with tap water. As if that weren't enough, while at the store, John's wife called and said the pool looked slightly green. Upon hearing this information, the store manager increased the chlorine dose to 35 ppm and added a bottle of algaecide to John's order.

John did everything the store manager recommended. No one got to swim that weekend, but things eventually improved. As instructed, John returned with his pool and tap water samples. The pool water had a free chlorine residual of 6.2 ppm, no ammonia, no nitrite, very low parts per billion levels of phosphate and 9 ppm nitrate. The pool was still a light green, but John was in the process of brushing the surfaces of the liner to dislodge the algae.

The tap water had 28 ppm nitrate. The store manager knew that well water pH was usually lower than the 6.8-8.4 range of phenol red and was again prepared with a pH meter to properly test the tap water. Though not a problem for his pool so far, the tap water pH was 5.8, and it also had a high hardness level.

The store manager offered his explanation of what he thought was happening: The tap water presented a slight problem in that it contained nitrates. The EPA limit for nitrates is 44 ppm. (Note: Some test kits read nitrate as nitrogen. In that case, the limit is 10 ppm as nitrogen).

Once nitrates are in a pool, there is no practical way to eliminate them other than draining and diluting. There are some promising ion exchange resins that will remove nitrate, but this is a bit painstaking for pool water.

The rest of John's problem was due to fertilizing his landscaping. The trees and bushes had been sprayed and the wind blew some of this into the pool. Most fertilizers typically contain ammonia, nitrite, nitrate and phosphate.

These chemicals can significantly increase chlorine demand. Each ppm of nitrite alone needs about 5 ppm of chlorine to convert it to nitrate. Nitrate and phosphate are the two key nutrients and they can lead to the start of the green algae that John saw. He was fortunate to be able to stop the growth early.

The store manager explained that because there's not much one can do about nitrates, the industry has turned to phosphate removal products to prevent algae growth. He recommended that John keep a good maintenance dose in the pool, as per the label instructions.

John's nitrates actually arose from three sources in the pool. Some was formed from oxidation of ammonia and nitrite; a little came from a fertilizer component; and still others originated from his makeup water. Most of the nitrate probably was due to the nitrogen-based fertilizer components, so better control of this in the future would mean much less nitrate in the pool.

The store manager gave John the number of a domestic water-treatment dealer in the area. He suggested that John have a system installed to increase his pH and remove the calcium hardness. They also might be able to investigate a nitrate removal resin. However, most home water systems usually bypass the outdoor spigots, so this would not help John's pool situation.

John and Jim exchanged e-mail addresses at the reunion, not to only renew their friendship, but also to stay up to date with their pool-chemistry situations. Thanks to their knowledgeable retailers, their makeup water problems are now behind them.
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Author:Seechuk, Tom
Publication:Pool & Spa News
Date:Aug 6, 2004
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