Which unit is used to measure the concentration of contaminants in soil and sediment?

The correct answer is PPM.

To understand why, let us break down what each of the given units represents and see how they apply to measuring contaminant concentration in soil and sediment:
1. PPM (Parts Per Million): This is a unit of concentration that expresses the mass of a contaminant relative to the total mass of the soil or sediment. For example, 1 PPM is equivalent to 1 milligram of contaminant per kilogram of soil (1 mg/kg). Because contaminants in soil are typically present in very small, trace amounts, PPM is the standard unit of measurement used in environmental science and geology.
2. PPBB: This is not a standard scientific unit of measurement.
3. TDS (Total Dissolved Solids): This refers to the concentration of dissolved inorganic and organic substances in a liquid (usually water), rather than solid soil or sediment.
4. DPMO (Defects Per Million Opportunities): This is a metric used in process improvement and quality control (such as Six Sigma) to measure product defect rates, and is entirely unrelated to environmental chemistry.

Therefore, PPM is the appropriate unit used to measure the concentration of contaminants in soil and sediment.