Total Dissolved Solids (TDS) and pH

What are TDS?

Total dissolved solids (TDS) represent the total concentration of dissolved inorganic and organic substances in a body of water. Inorganic salts include cations like magnesium (Mg2+), calcium (Ca2+), potassium (K+), or sodium (Na+) and an anion such as carbonates (CO32-), nitrates (NO3-), bicarbonates (HCO3-), chlorides (Cl-), and sulfates (SO32-).1 Cations are ions with a positive charge and anions are ions with a negative charge.

How do these substances become dissolved in water?

Both inorganic and organic substances can enter the water via both natural and human sources. Some mineral springs can contain high levels of dissolved solids because the water ran through an area of rocks with high salt content. Human activities like urban runoff and agriculture can put excess amounts of minerals and nutrients into the streams. Other good examples include wash off of the salt used to de-ice roads and industrial waste water discharge.

What is happening to the water when TDS are high?

Varying TDS levels are responsible for the mineral taste water can have when we drink. When water has very low TDS levels, it can be tasteless and many people find it undesirable. More dissolved solids in water can produce "hard water" and this can lead to technical issues like mineral deposits on fixtures and pipes. Such TDS levels can also stain or corrode pipes or fixtures and leave a metallic taste in drinking water.

While TDS isn't a huge factor monitored by the Environmental Protection Agency (EPA), it can be used as a warning sign for possible contamination. With High dissolved solid concentrations can indicate large amounts of harmful substances like iron, manganese, bromide, sulfate, and arsenic. Under natural conditions, these substances are usually present only at very small concentrations. High levels are usually the result of runoff and human pollution.

How are TDS levels lowered by water treatment facilities?

By utilizing reverse osmosis, water treatment facilities can remove most dissolved substances from high TDS waters. In reverse osmosis, pressure and a semipermeable membrane are used to remove dissolved solids from the water. If a water source is lacking essential minerals, it can be passed over a mineral bed made of calcium or magnesium. Such a bed can also be used to adjust the pH.

What is pH?

The pH of water is a measure of its acidity or alkalinity. When we measure pH, we are measuring the concentration of hydrogen ions (H) in the water. These measurements can be understood in the pH scale, which ranges from 0 to 14. A lower pH (0 - 6) indicates a high concentration of H+ ions and a more acidic water source. If a water source has a high pH (8.0 - 14.0) it is said to be basic, or alkaline, and contains much fewer H+ ions. On the pH scale, 7.0 indicates neutral pure water but having a pH of 7.0 is rare in nature. Because water sources also contain naturally dissolved solids, its pH varies. Most biologic process can survive in the 6.0 - 8.0 pH range. This scale is logarithmic and one step, say from pH of 7.0 to 6.0, would represent a ten-fold increase in acidity. Water with a pH of 2.0 is 100 times more acidic than a pH of 4.0.

The pH of a water resource can change naturally. Plants release carbon dioxide when the die and decompose in water. When the carbon dioxide combines with water, it forms weak acid known as carbonic acid and this decreases the water pH. Types of soil and minerals like limestone can help neutralize acidity. In terms of human activity, releasing large quantities of nitrogen and sulfur oxides from power plants and transportation vehicles can mix with water vapor and result in acid rain.

Coal Combustion and Acid Rain

pH Scale

What happens when the pH of the water changes?

When water pH changes outside the biologically preferred 6.0 - 8.0 region, harmful and even fatal affects can be seen on the environment. Many aquatic species are very sensitive to subtle changes in pH. These species are usually good indicators of chemical changes in the environmental. Other species are more adapted and can handle wider ranges of change. Ultimately, fewer organisms can survive when the water pH continues to decreases (more acidic).

Conductivity and Total Dissolved Solids

The United States Environmental Protection Agency (USEPA) has set secondary standards for drinking water that include maximum concentration limits (MCLs) for certain water constituents. The MCL for Total Dissolved Solids (TDS) is 500 mg/L. While our current testing involves only surface water samples, drinking water standards cannot be applied here. Although, this is still a good foundation to compare our field data upon. The graph below provides an overall view of our samples and the TDS compared to EPA's MCL.

Sample pH and Maximum Concentration Limits

Another USEPA secondary standard is pH, and for drinking water the USEPA recommends a pH of 6.5 - 8.5. There are many cases in natural settings in which the water pH is outside of this range. Water outside of this MCL range may cause corrosion in piping infrastructure. In the environment, water pH can have a great effect on the presence of aquatic life.