Swimming Pool Blog

Old Pool Water in Drought Conditions

by Mark Garcia, February 7, 2017

Does pool water need to be changed? I used to advise customers to change the pool water every five years, because old pool water gets 'choked-up' or thick with dissolved material that gets in the way of sanitation and filtration.

However, In areas of severe drought, water restrictions can make it expensive or impossible to drain and refill a pool. Indoor pools, and year round pools in dry, arid, and hard water regions that don't winterize, are most affected by old or aging pool water.

In cases of extreme neglect, algae and staining can be hard to fight without emptying the pool and starting over. In other cases, TDS (total dissolved solids) can reach such a high level that water quality becomes difficult to maintain. In still other cases, accumulated minerals or chemicals, like high levels of calcium and cyanuric acid, can present problems.

How can you maintain a pool in drought conditions, when you have high TDS, CYA, and CH creating high levels of DBP compounds? :-)



Everything that has ever been dissolved by your pool water makes up the TDS measurement. 

  • Pool treatment chemicals
  • Metallic Ions
  • Salts like sodium and calcium
  • Pollen, dust, dirt
  • Phosphates and Nitrates
  • Bodily Fluids, Skin/Hair Products
  • Airborne pollutants, smog and oils
  • Dissolved carbon from plant debris

In pools with old, aging pool water, the build-up of Total Dissolved Solids can create difficulty maintaining water balance, controlling algae, and issues with cloudy water. Pools that have TDS levels higher than 1500 ppm (not including salt used for a salt system), may start to develop problems, unless certain steps are taken. Pools with good water balance can often operate well with TDS levels up to 2500 ppm, when necessary due to drought and water use restrictions. 


Another issue with TDS is that pool sanitizers like chlorine react with components of TDS to produce disinfection byproducts (DBP); forms of combined chlorine that linger and cannot be removed by breakpoint chlorination. For pools with high levels of TDS, from a variety of sources, that are not regularly diluted, with frequent rain or lowering the water level for winter, persistent chloramine levels can become a problem.

The signs of Disinfection Byproduct (DBP) or chlorine compound build-up are the same as for regular chloramines - red eyes, itchy skin, dry hair. In pools with lower TDS levels, shocking the pool to breakpoint chlorination (10x level of combined chlorine) may not lower the tested level of combined chlorine. High levels of calcium hardness and total alkalinity can compound the problem.


Cyanuric Acid, also known as Conditioner or Stabilizer, and abbreviated CYA, is added to sunny pools to protect chlorine from degradation by the sun's rays. For pools that use stabilized chlorine tablets, which is most people, the cya level can eventually grow too high -  especially for pools in desert areas with little rain, and those that don't lower the water level for winterization.

When CYA levels rise above 100 ppm +/-, there are several problems that can result.

  • Chlorine activity is overly suppressed, resulting in algae and bacteria growth
  • Contributes to total alkalinity measurements, giving false readings
  • Disinfection Byproducts result from 'lazy' chlorine molecules
  • Chlorine test results can be faulty, or non-existent
  • Cyanuric test results can be faulty, or non-existent

Cyanuric acid levels in pools were traditionally only lowered by dilution, for example replacing 30% of the water and refilling with fresh water, to lower your cyanuric acid level by 30%. In the new drought sensitive environment that many pools find themselves in, draining even a portion of the pool is discouraged. Fortunately, there is a new chemical, called Bio-Active cyanuric acid reducer. I've not used it myself, but reviews are mostly positive, though mixed. I'm sure it probably works better in a pool with normal TDS levels.


Calcium Hardness (CH) is a measurement of the levels of calcium carbonate in a pool, or how hard or soft your water is, approximated with a measurement of dissolved calcium and other minerals. It's no coincidence that hard water pools, or pools with high levels of calcium hardness, tend to also have high levels of TDS. Recommended levels for pools is 200-400 ppm, or 150 ppm for vinyl pools. If your water is too soft, just add calcium chloride to raise near 200 ppm, but what do you do when your fill water (tap water) is over 500 ppm? Hard water in pools can make certain water management issues.

  • More potential for scaling, liming or crystal deposits
  • Low pH and alkalinity levels to control scale upsets water balance
  • More potential for cloudy and turbid water conditions
  • Contributes to DBP compound creation

Calcium Hardness levels are difficult to lower with chemicals, although there is a product by United Chemicals called Cal-Treat that has mixed results among users. But again, the normal method for reducing calcium hardness in pools is to drain a portion of the pool water and refill, with softer water.

HOW TO TREAT OLD POOL WATER (When You Can't Drain the Pool):

So, what do you do when TDS, CYA or CH levels get so high that you start to have trouble with DBP's, Scale, Cloudy water and water balance - but you can't drain the pool? 

  • Home water softening units can be installed, and then you can fill your pool from the kitchen or laundry room
  • Reverse Osmosis or Nanofiltration trucks can visit your house, although they waste about 20% of the water
  • Reduce chemical usage by using UV, Ozone and/or Mineral water treatment, along with efficient filtration
  • Keep your water very well-balanced, and perform a regular Langelier Saturation Index. Try the Pentair LSI
  • Keep the pool clean to reduce the amount of organic solids introduced  by rain, wind and swimmers.



Thanks for Reading!


Mark Garcia