Drinking-water safety—mercury threshold limit value (TLV) The approximate permissible concentration (TLV) of mercury in potable water is closest to which value (ppm)?

Introduction / Context:
Mercury is a potent neurotoxin with strict regulatory limits in drinking water. Process engineers working on water treatment and environmental compliance must know typical threshold values to design appropriate removal systems and monitoring programs.

Given Data / Assumptions:

• Units are parts per million (ppm), where 1 ppm = 1 mg/L.
• Question seeks an order-of-magnitude TLV appropriate for safe potable water.
• Conservative health-based guidelines target very low concentrations.

Concept / Approach:
Public health guidelines set mercury limits in the microgram per litre (μg/L) range. A value of 0.001 ppm equals 1 μg/L, which aligns with stringent international standards depending on the mercury species and jurisdiction. Higher options (0.1–5 ppm) are orders of magnitude too high for potable water.

Step-by-Step Solution:

Verification / Alternative check:
Many jurisdictions specify mercury limits at or near 1–2 μg/L for total mercury in drinking water, consistent with 0.001–0.002 ppm guidance levels.

Why Other Options Are Wrong:

• 0.1, 1, 5 ppm: far above accepted health-based limits; such levels would trigger immediate advisories.

Common Pitfalls:
Confusing ppm with ppb; remember that 0.001 ppm equals 1 ppb (μg/L), which is within the accepted safe range.

Final Answer:
0.001 ppm