adulterate
Definitions
To render something poorer in quality by adding another substance, typically an inferior one.
To corrupt, debase, or make impure by mixing in foreign or inferior elements.
Examples
The unscrupulous merchant tried to adulterate the expensive olive oil with cheaper vegetable oil.
The chemist warned that adding impurities would adulterate the purity of the final compound.
They were accused of attempting to adulterate the city's water supply with harmful chemicals.