Technical Articles
Technical Bulletin: Metal Mischief
Ever seen a pool with reddish water? Have you ever heard of a pool that turned the swimmers’ hair green, even though the free chlorine level was in the proper range? These and other symptoms of elevated metal content can drive pool owners crazy. Metals can create serious mischief in swimming pools and spas, causing unsightly stains and discolored water.
Copper and iron are the two most common problem metals for pool and spa owners. They are essential micronutrients in an adult’s diet (not that we recommend drinking the pool water to get your copper and iron intake). Contrary to the human diet, we try to reduce or eliminate copper and iron levels in swimming pools. Let’s take a look at how they get there in the first place.
How Metals Occur in Water
The most common origin of copper and iron in pools and spas is the source water itself. Water percolating through soil and rock can dissolve minerals containing copper or iron. Eventually the water--now a solution containing iron and copper--makes its way into wells or other water supplies. While source water usually accounts for the metal content, there are other possible sources:
- ·Pipes or other metal equipment can corrode, particularly if the pH or hardness level of the water is too low.
·Some algaecides use copper as an active ingredient.
·Mineral purification systems for pools and hot tubs are increasing in popularity. Copper often plays a part in these systems since it has sanitizing abilities. If you have a mineral purification system, you must keep the copper level low (at or below 0.2 ppm). Otherwise, it can form scale deposits or stains.
Once metals have found their way into your pool, several water chemistry factors can contribute to discoloration and scaling. High total alkalinity, above 180 parts per million (ppm), can promote stains and scale formation from metals. Similarly, a pH above 7.8 can cause these problems.
Understanding Oxidation
To understand how metals behave in your water, you need to know a little bit about oxidation. When a metal compound is exposed to the air, it combines with oxygen or oxidizes, creating a new form of the metal. In deep wells or piping systems where the oxygen content is low, the water containing copper and iron is clear and colorless. Water from the tap containing these metals may be clear, but when exposed to the air, the metal content can oxidize into the colored, solid form.
Oxidation changes the iron in your water into a yellow or reddish-brown solid. Copper tends to turn into a blue color. When these metals settle out of water in their solid form, they cause stains and rust on pool surfaces, fixtures and equipment. Smaller particles remain suspended in the water, causing a reddish color in the water with iron and a blue coloration for copper.
As you may know, free chlorine is a powerful oxidizing agent. (It is this same oxidizing process that enables chlorine to “burn up” or destroy unwanted debris and contaminants in the water.) The free chlorine and available oxygen in your pool actually speed up the rate at which elevated levels of copper and iron oxidize. Other sanitizers or shock agents may also help oxidize the metals into the forms that cause stains and scale buildup.
Assessing and Controlling Metals
Remember, most metal compounds enter the pool or hot tub with the source water. If you suspect that metals are present in the source water, be sure to test the water before making any other adjustments to the water chemistry. You may need to reduce the metal content first. You should do this before adding any oxidizer such as chlorine, since it can oxidize the metals into a more problematic form.
If you are using a copper-based algaecide or a mineral purification system, you will need to test the water frequently. Keep the copper below 0.2 ppm, or else you may have to deal with stained water and scale deposits.
Dip-and-read test strips can provide a quick and easy analysis of the metals in solution. Test strips can test both iron and copper accurately in seconds. If you prefer, you can use liquid drop kits or test tablets. These methods can also provide reliable analysis in a short enough period of time to be used at poolside.
With occasional testing, you should have no trouble keeping the metal level in your water under control. Once you have detected metals, the typical way to eliminate or reduce the metal content in your water is with a sequestering agent, also known as a chelating agent. This is a chemical compound that can bind the metals into large particles. Once the chelating agent does its work, your filter can intercept the large particles containing the metals, allowing them to be backwashed out.
Keep in mind that your sequestering or chelating agent is more efficient if your pool water is properly balanced for pH, alkalinity and hardness. Otherwise the chelating agent will have to work harder to do its job. Be careful to follow the manufacturer’s directions with this or any other pool and spa chemical.
© 2001 Environmental Test Systems, Inc.
Joe Sweazy, Environmental Test Systems, Inc.
