Maintaining a hygienic and inviting swimming pool requires constant attention to water chemistry, which involves monitoring several factors beyond just sanitizer levels. Total Dissolved Solids (TDS) represents a fundamental metric for assessing the overall quality and age of the water in a pool system. TDS is a measurement of all the solid substances that have fully dissolved into the water, and its accumulation over time can significantly impact a pool’s operations.
Defining Total Dissolved Solids
Total Dissolved Solids is a cumulative measurement of every organic and inorganic substance present in the water that is not the water molecule itself and is not a suspended particulate. This includes a wide array of compounds such as salts, minerals, metals, and various organic materials. The dissolved materials are typically categorized into inorganic components, like calcium, sodium, and magnesium ions, and organic components, such as oils, nitrogen compounds, and cosmetic residues. This concentration is most commonly measured in parts per million (ppm) or milligrams per liter (mg/l). The measurement is often obtained using a digital conductivity meter, which assesses the water’s ability to conduct an electrical current. Since charged particles, or ions, increase conductivity, this method provides a practical proxy for the total amount of dissolved matter.
Sources of TDS Accumulation
The accumulation of dissolved solids begins the moment a pool is filled, as the source or “make-up” water itself contains minerals and salts. Evaporation is a significant factor in concentration, as only the pure water escapes as vapor, leaving all the solids behind to increase in density. This process ensures that TDS levels will steadily climb in any pool over time.
A substantial portion of TDS comes directly from the people using the pool, collectively termed bather waste. This includes sweat, body oils, skin flakes, sunscreen, hair products, and urine, all of which dissolve into the water. Environmental contaminants also contribute, with windblown dust, pollen, rain, and runoff introducing various organic and inorganic particles that dissolve over time.
Pool maintenance chemicals are another source, as many products break down into dissolved salts and ions after they have performed their function. Chlorine, algaecides, pH adjusters, and shock treatments all contribute to the cumulative TDS load. The use of chlorine stabilizers like cyanuric acid or products containing sodium salts can add significantly to the total.
Consequences of Elevated TDS Levels
An excess of dissolved solids can lead to several problems that compromise both the pool structure and the water quality. High TDS levels reduce the efficiency of chemical sanitizers, particularly chlorine. High TDS levels are theorized to suppress the activity of free chlorine, potentially requiring pool operators to use higher doses to maintain adequate disinfection.
When mineral content is high, elevated TDS can lead to scaling, which is the formation of white or gray deposits on pool surfaces, plumbing, and heaters. Conversely, if the dissolved solids are primarily chlorides or sulfates, the increased conductivity of the water can accelerate the corrosion of metal fixtures, such as stainless steel ladders and heat exchangers. This heightened conductivity means the water is more capable of carrying an electric current, increasing the risk of galvanic corrosion.
The physical appearance of the water may suffer, resulting in a dull, cloudy, or less sparkling look. High TDS levels can interfere with the water balance, making it more challenging to keep parameters like pH and alkalinity stable. Swimmers may notice the water feeling less pleasant, sometimes described as having an “oily” feel, and they may experience increased eye and skin irritation.
Monitoring and Management Strategies
Regularly testing the water is the first step in managing Total Dissolved Solids, often performed using a dedicated TDS meter or specialized test strips. For freshwater pools, the general guideline suggests that the TDS level should not exceed 1,500 ppm above the TDS level of the original source water. This baseline accounts for the minerals naturally present in the water used to fill the pool.
A common recommended maximum for a standard freshwater pool is generally between 1,500 and 2,000 ppm. Exceeding this threshold indicates that the water has aged significantly and is likely accumulating problematic contaminants. For pools utilizing salt chlorine generators, the TDS baseline is much higher, typically ranging from 3,000 to 6,000 ppm due to the required salt concentration.
The only effective and practical way for a typical pool owner to reduce an overly high TDS level is through dilution, which involves partially draining the pool and refilling it with fresh water. This process physically removes the concentrated dissolved solids, thereby lowering the ppm reading. For residential pools, periodic water replacement remains the standard management practice.