Under the National Primary Drinking Water Regulations, which substance is NOT a primary contaminant?

Under the National Primary Drinking Water Regulations, which substance is NOT a primary contaminant?

• A. Barium
• B. Cadmium
• C. Selenium
• D. Zinc

Answer: (D)

The key idea is the distinction between health-based primary standards and aesthetics-based secondary standards. Primary contaminants are those that pose direct health risks and have enforceable maximum contaminant levels (MCLs). Secondary contaminants are not tied to health but affect taste, odor, color, or corrosion; they are addressed by non-enforceable guidelines called secondary maximum contaminant levels (SMCLs). Zinc fits the secondary category, so it is not a primary contaminant under the National Primary Drinking Water Regulations. Barium, cadmium, and selenium have health-based MCLs and are treated as primary contaminants. Zinc is typically regulated only as a secondary contaminant (with a SMCL, not an MCL).

The key idea is the distinction between health-based primary standards and aesthetics-based secondary standards. Primary contaminants are those that pose direct health risks and have enforceable maximum contaminant levels (MCLs). Secondary contaminants are not tied to health but affect taste, odor, color, or corrosion; they are addressed by non-enforceable guidelines called secondary maximum contaminant levels (SMCLs). Zinc fits the secondary category, so it is not a primary contaminant under the National Primary Drinking Water Regulations. Barium, cadmium, and selenium have health-based MCLs and are treated as primary contaminants. Zinc is typically regulated only as a secondary contaminant (with a SMCL, not an MCL).