California Wastewater Treatment Operator Practice Exam 2025 – All-in-One Guide to Master Your Certification

The correct answer is 1 ppm (parts per million), which is equivalent to 1 mg/L (milligram per liter). This equivalence arises because both terms are used to describe the concentration of a substance in water or other solutions. Specifically, 1 mg of a substance dissolved in 1 liter of water results in a concentration of 1 mg/L, and this is also understood as 1 part per million due to the way concentrations are often quantified in water.

In water, 1 ppm is functionally the same as 1 mg/L when considering dilute aqueous solutions, as 1 liter of water has a mass of approximately 1,000 grams. Therefore, 1 mg of a substance in that liter represents 1 part of that substance per million parts of water.

The other choices—1 ppt, 1 ppb, and 1 g/L—reflect different concentration measurements that do not equate to 1 mg/L. Parts per trillion (ppt) and parts per billion (ppb) represent much lower concentrations, while grams per liter (g/L) implies a significantly higher concentration, demonstrating that the context and units of measurement are crucial when discussing concentration equivalences.