In domestic water well supplies, which two elements are primarily responsible for hardness?

In domestic water well supplies, the primary elements responsible for hardness are calcium (Ca) and magnesium (Mg). These two minerals contribute to what is known as "hard water." Hardness in water is typically measured by the concentration of calcium and magnesium ions. When water passes through natural deposits of limestone or chalk, it can dissolve calcium and magnesium, leading to increased hardness.

Calcium typically appears in water as dissolved ions, which can originate from geological formations in the area surrounding the water supply. Magnesium is found in similar geological contexts and contributes to hardness as well. Water with a high concentration of these minerals may cause scale buildup in pipes and appliances, leading to operational inefficiency and increased maintenance needs.

While other elements such as sodium (Na) and zinc (Zn) exist in water, they do not have the same effect on hardness as calcium and magnesium. Sodium is usually associated with water salinity rather than hardness, and zinc is generally present in trace amounts and does not significantly affect the hardness level of water. Thus, the combined presence of calcium and magnesium is what classifies water as "hard" and is effectively why this answer is correct.