Electrical conductivity (EC) measures a liquid’s capacity to pass an electrical current, which is a fundamental property used to assess water quality. While pure water, consisting only of H\(_{2}\)O molecules, is an extremely poor conductor of electricity, the water flowing from your tap is not pure. The ability of tap water to conduct a current stems entirely from the substances dissolved within it. Understanding the electrical conductivity of tap water involves examining the nature of these dissolved materials.
Understanding Electrical Conductivity
Electrical conductivity is the inverse of electrical resistivity, representing how readily a solution allows the flow of an electrical current. This measurement is directly related to the concentration of mobile, electrically charged particles, or ions, within the water sample. The movement of these ions facilitates the transfer of electrical charge through the liquid.
The standard unit of measurement for water conductivity is the microsiemens per centimeter (\(\mu S/cm\)). For accurate comparison, readings are typically temperature-compensated to a reference temperature of 25 degrees Celsius. Measurement is performed using a conductivity meter, which applies a known voltage between two electrodes submerged in the water sample. The meter then calculates the conductivity based on the resistance encountered by the current passing between the electrodes.
The Source of Tap Water’s Conductivity
The electrical current in tap water is carried by dissolved inorganic salts and minerals, collectively known as Total Dissolved Solids (TDS). These substances dissociate, or break apart, into positively and negatively charged ions when they dissolve in water. The quantity and mobility of these ions directly determine the water’s conductivity level.
Common positively charged ions (cations) include calcium, magnesium, sodium, and potassium, often picked up as water flows over rock and soil. Negatively charged ions (anions), such as chloride, sulfate, and bicarbonate, also contribute significantly to the total conductivity. These ions are either naturally present in the source water or introduced during municipal water treatment processes. Organic compounds, like sugar or oil, do not break down into mobile ions and therefore contribute very little to the electrical conductivity.
Typical Measured Values and Geographic Variation
The electrical conductivity of municipal tap water is highly variable, but it generally falls within a broad range. Most potable drinking water supplies in the United States have values ranging from approximately 50 \(\mu S/cm\) to 1,500 \(\mu S/cm\). A more typical range for many systems is between 200 \(\mu S/cm\) and 800 \(\mu S/cm\), reflecting a moderate concentration of dissolved minerals.
This wide range is primarily due to the geological composition of the region supplying the water. Areas with limestone or chalk bedrock tend to have higher concentrations of calcium and magnesium, resulting in higher conductivity levels and what is commonly termed “hard water.” Conversely, water sourced from areas with granite or other non-soluble rock may have a naturally lower conductivity. Local water treatment practices also influence the final reading, as processes like demineralization lower the EC, while remineralization or the presence of salt-based water softeners can increase it.
Why This Measurement Matters
Measuring electrical conductivity serves as a simple and fast indicator of overall water quality, giving a proxy measurement for the total concentration of dissolved ionic material. A sudden, significant change in EC from an established baseline can signal an issue, such as a pollution event, sewage leak, or industrial discharge entering the water supply. This makes EC monitoring a valuable tool for environmental surveillance and municipal water safety checks.
In the home, high conductivity is often an indicator of water hardness. Water with a high mineral content can lead to scale buildup in boilers, water heaters, humidifiers, and coffee makers, reducing their efficiency and lifespan. For those using water filtration systems, such as reverse osmosis (RO) units, monitoring EC helps assess the system’s performance by measuring the difference in ion concentration before and after treatment. Specialized applications, like hydroponic gardening or maintaining an aquarium, rely on specific EC levels to ensure proper nutrient delivery to plants or a healthy environment for aquatic life.